Rationales, evidence, and design considerations for fractionated radioimmunotherapy

$Page 1: Rationales, evidence, and design considerations for fractionated radioimmunotherapy$
Eighth Conference on Radioimmunodetection andRadioimmunotherapy of Cancer

Supplement to Cancer

Rationales, Evidence, and Design Considerations forFractionated Radioimmunotherapy

Gerald L. DeNardo, M.D.1

Jeffery Schlom, Ph.D.2

Donald J. Buchsbaum, Ph.D.3

Ruby F. Meredith, M.D., Ph.D.3

Joseph A. O’Donoghue, Ph.D.5

George Sgouros, Ph.D.4

John L. Humm, Ph.D.4

Sally J. DeNardo, M.D.1

1 Department of Internal Medicine, Division of He-matology and Oncology, Section of Radiodiagnosisand Therapy, University of California, Davis Medi-cal Center, Sacramento, Calfornia.

2 Laboratory of Tumor Immunology and Biology,National Institutes of Health, National Cancer In-stitute, Bethesda, Maryland.

3 Comprehensive Cancer Center, University of Al-abama, Birmingham, Alabama.

4 Department of Medical Physics, Memorial Sloan-Kettering Cancer Center, New York, New York.

5 Departments of Clinical Physics and RadiationOncology, CRC Beatson Laboratories, Glasgow,United Kingdom.

Presented at the Eighth Conference on Radioim-munodetection and Radioimmunotherapy of Can-cer, Princeton, New Jersey, October 12–14, 2000.

Supported by grants from the National CancerInstitute (PO1-CA47829) and the U.S. Departmentof Energy (DOE) (DE-FG03-84ER60233).

The authors thank Linda A. Kroger and Nona L.Simons for substantial assistance in preparing themanuscript.

Address for reprints: Gerald L. DeNardo, M.D.,Molecular Cancer Institute, Davis Medical Center,University of California, 1508 Alhambra Boulevard,Room 3100 Sacramento, CA 95816; Fax: (916)451-2857; E-mail: [email protected]

Received October 31, 2001; accepted November14, 2001.

Although fractionation can be used in a discrete radiobiologic sense, herein it is

generally used in the broader context of administration of multiple, rather than

single, doses of radionuclide for radioimmunotherapy (RIT) or other targeted

radionuclide therapies. Fractionation is a strategy for overcoming heterogeneity of

monoclonal antibody (MAb) distribution in the tumor and the consequent non-

uniformity of tumor radiation doses. Additional advantages of fractionated RIT are

the ability to 1) provide patient-specific radionuclide and radiation dosing, 2)

control toxicity by titration of the individual patient, 3) reduce toxicity, 4) increase

the maximum tolerated dose (MTD) for many patients, 5) increase tumor radiation

dose and efficacy, and 6) prolong tumor response by permitting treatment over

time. However, fractionated RIT has logistic and economic implications. Preclin-

ical and clinical data substantiate the advantages of fractionated RIT, although the

radiobiology for conventional external beam radiotherapy does not provide a

straightforward rationale for RIT unless fractionation leads to more uniform dis-

tribution of radiation dose throughout the tumor. Preclinical data have shown that

toxicity and mortality can be reduced while efficacy is increased, thereby providing

inferential evidence of greater uniformity of radiation dose. Direct evidence of

superior dosimetry and tumor activity distribution has also been found. Clinical

data have shown that toxicity can be better controlled and reduced and the MTD

extended for many patients. It is clear that fractionated RIT can only fulfill its

potential if the effects of critical issues, such as the number and amount of

radionuclide doses, the radionuclide physical and effective half-life, and the dose

interval, are better characterized. Cancer 2002;94:1332– 48.

© 2002 American Cancer Society.

DOI 10.1002/cncr.10304

KEYWORDS: fractionation, therapy, cancer, radioimmunotherapy, radiation, anti-body, radionuclide, radioisotope.

The benefit from fractionation of the total dose of radiation forexternal beam radiotherapy (EBRT) is well established; multiple

doses, usually given daily, extend the total radiation dose that can begiven to the malignancy by decreasing normal tissue toxicity.1,2 In-deed, hyperfractionation—that is, multiple doses of radiation eachday— has been shown to be superior to daily doses of EBRT,3–5 al-though it has logistic and economic disadvantages. Sealed radionu-clide source implant radiotherapy involves continuous, high-dose-rate radiation rather than the multiple, short bursts of radiationcharacteristic of EBRT. The critical importance of fractionation forEBRT depends on the steepness of the dose-response relationship fortherapeutic advantage.6

Radionuclide treatment, whether radioimmunotherapy (RIT) or

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© 2002 American Cancer Society

other, involves continuous and continuously decreas-ing low-dose-rate radiation that appears to destroycells primarily through apoptosis,7 rather thanthrough the necrosis characteristic of the cellular ef-fects of EBRT and chemotherapy. There is evidencesuggesting that RIT may have greater therapeutic effi-cacy for equivalent total radiation dose and dose rateswhen compared with EBRT.8 –10 There are two broadapproaches to radionuclide dose (activity; mCi) sched-ules in RIT. One of these is the administration of asingle, large dose of radionuclide (radiolabeled mono-clonal antibody [MAb]), which is often associated withbone marrow reconstitution (transplantation); this ap-proach has been used by Press et al.11 and, morerecently, in pivotal Phase III trials of Bexxar™ (CorixaCorp., Seattle, WA; SmithKline Beecham Corp., Phila-delphia, PA) and Zevalin™ (IDEC Pharmaceuticals,San Diego, CA). Potential advantages of a single largedose of radionuclide include less sublethal damagethat can be repaired and avoidance of treatment in-terruption because of antiglobulin, e.g., human anti-mouse MAb (HAMA), development. A second ap-proach to dose schedule is to divide the total dose ofradionuclide into multiple-doses, referred to as “frac-tionated”, RIT.12–14 Preclinical and clinical evidenceindicate that a larger total radionuclide dose can beadministered when fractionated, regardless ofwhether the individual doses are low,13 near the non-myeloablative maximum tolerated dose (MTD),14 ornear the myeloablative MTD.15

The fractionation of RIT was first formally de-scribed in 1985,16 although multiple doses were givenin earlier RIT trials. Although fractionation for conven-tional radiotherapy has come to have specific radio-biologic implications relative to tissues with acute(early) and delayed (late) response, the term is gener-ally used herein in the broader dictionary sense of“dividing into fractions or parts”—that is, the use ofmultiple, rather than single, radionuclide doses. Thereis a strong rationale for fractionated RIT because ofknown tumor biology that leads to significant nonuni-formity of the distribution of macromolecules in ma-lignant tissue.17

An advantage of RIT over immunotherapy is that aradionuclide can be chosen with emissions that have amulticellular range, thereby distributing the radiationdose more uniformly in the malignant tissue. How-ever, nonuniform distribution of radiation throughoutthe tumor persists, with some regions receiving higherand others lower radiation doses. When a single, largeradionuclide dose of radiolabeled MAb is given, someregions of the malignancy may be underirradiatedwhile other regions are overirradiated. The purposesof this publication are to provide rationales and data

favoring fractionation for RIT (Table 1), to identify thedisadvantages (Table 2), to assess its efficacy, to char-acterize preferred methods for implementation, andto further stimulate preclinical and clinical examina-tion of this strategy.

Heterogeneity of Macromolecules in TumorsRIT produces a less uniform radiation dose distribu-tion within tumor than EBRT. The inability of MAbsand other macromolecules to penetrate uniformlythroughout a tumor and bind to all cells results in aheterogeneous radiation dose deposition in the tu-mor.17–22 Tumor control is more difficult with a non-uniform dose distribution because some regions of thetumor may be underdosed.23–25

Terms such as “guided missile” and “magic bul-let” are occasionally used to describe MAb targeting. Amore realistic concept depends upon an understand-ing of the physiologic factors involved in tumor “tar-geting.” To achieve a greater MAb concentration intumor than in normal tissues, administered MAb mustfirst be distributed in a volume that includes the tu-mor cells. Then the MAb must react with and remainbound to antigen on tumor cells; unbound MAb mustclear its distribution volume prior to significant disso-ciation of tumor-bound MAb.

The factors that affect blood flow and macromol-ecule delivery to tumors have been described in de-tail.17,26 –28 Dvorak et al.28 have described tumor archi-tecture as consisting of tumor vessels, stroma, andparenchyma that serve as barriers to the delivery ofmacromolecules (Fig. 1). Vascular density and bloodflow varies widely within different regions of a tu-mor29,30 and affects the uniformity of macromoleculedelivery.31–33 In addition, elevated interstitial fluid

TABLE 1Advantages of Fractionated Radioimmunotherapy

1. More uniform distribution of MAb and radiation dose2. Patient-specific radionuclide and radiation dose3. Control toxicity by titration of an individual patient4. Reduced toxicity5. Increased MTD for many patients6. Increased tumor radiation and efficacy7. Prolongation of tumor response

TABLE 2Disadvantages of Fractionated Radioimmunotherapy

1. Lower radiation dose rate2. Complex strategy to implement3. Treatment interruption4. Increased cost5. Potential delay in tumor regression

Fractionated RIT/DeNardo et al. 1333

pressure inhibits the inward diffusion of MAbs fromthe periphery toward the center of the tumor.34 –36

Measurements in experimental tumors have shownthat the interstitial pressure is substantially lower inthe periphery and the hydrostatic pressures in thevascular and interstitial spaces are nearly equal, whichlimits the convective delivery of macromolecules tosolid tumors.36 –38 Heterogenous tumor uptake mayalso occur due to the existence of a “binding sitebarrier” in which a high ratio between tumor antigenconcentration and the concentration of MAb in thetumor milieu can lead to a MAb “sink,” which requiresprogressive saturation of antigen sites in parenchymalperivascular regions before MAb diffuses to deepertumor regions.19,20,39 – 41 Other factors contributing toheterogeneous uptake of macromolecules include an-tigenic heterogeneity and modulation.

The result of these phenomena is that MAbs havehighly nonuniform distributions. Despite radionu-clides with radiations that traverse many cell diame-ters, nonuniform radiation dose to different regions ofthe tumor continues to be a problem for RIT. Griffithet al.42 performed quantitative autoradiography of io-dine-131(131I)–Lym-1 MAb in Raji B-cell lymphoma

xenografts to obtain a correlation of film density withdose determined by sectioned microthermolumines-cent dosimeters implanted in tumors (Fig. 2). Themeasured absorbed dose heterogeneity varied up to400%. Roberson and Buchsbaum23 investigated thetumor uptake of 131I-labeled 17-1A MAb in subcuta-neous LS174T colon cancer xenografts as a function oftime after injection. Three-dimensional (3-D) activitydistributions were determined from serial section au-toradiographs and used to construct a mathematicdescription of spatial and temporal changes in doserate distributions. The characteristic pattern was hightumor surface deposition at early times after injectionand a slow diffusion of the activity toward the centerof the tumor at later times. The average 3-D dose ratedistributions for 1 and 4 days after injection clearlyillustrated the heterogeneity in dose rate throughoutthe tumor as a function of time after radiolabeled MAbinjection (Fig. 3).

Fractionation of the total amount of radionuclideinto a series of smaller doses represents a strategy forachieving more uniform distribution of the radiationdose throughout the tumor. Subsequent doses of ra-diolabeled MAb can access regions different fromthose accessed earlier, if blood flow has been im-proved and reductions in tumor size have led to re-ductions in interstitial pressure and redistribution ofblood flow.

FIGURE 1. Schematic representation of solid tumor structure. Solid tumors

consist of parenchymal (tumor cell) units (A, B, and C) enveloped in stroma.

Blood vessels and focal sites of vascular leakage from hyperpermeable blood

vessels are concentrated at the tumor-host interface but also traverse tumor

stroma. Parenchymal units may consist of (A) loosely packed tumor cells,

typical of lymphomas, melanomas, and poorly differentiated carcinomas, or (B)

tightly packed cells linked together by occlusive intercellular junctions and an

enveloping basement membrane, typical of well-differentiated carcinomas. In

(C), a well-differentiated carcinoma with occlusive junctions exhibits focal

invasion at a site of basement membrane dissolution. From: Abrams PG,

Fritzberg A, Editors. Radioimmunotherapy of Cancer. New York: Marcel Dekker,

Inc., 2000:107–35. Reprinted with permission from Marcel Dekker, Inc.

FIGURE 2. An autoradiographic image of a section from a Raji human B-cell

lymphoma tumor from a mouse given iodine-131(131I)–Lym-1 monoclonal

antibody. The straight cursored line of interest passed directly over two

micro-TLD providing an activity distribution, showing that radiation dose varied

greatly in different tumor regions. Griffith MH, Yorke, ED, Wessels BW, DeNardo

GL, Neacy WP. Direct dose confirmation of quantitative autoradiography with

micro-TLD measurements for radioimmunotherapy. J Nucl Med 1988;29:

1795–1809. Reprinted with permission from the Society of Nuclear Medicine,

Inc.

1334 CANCER February 15, 2002 / Volume 94 / Number 4

Under certain conditions, fractionated adminis-tration of radiolabeled MAbs and peptides has beenshown to be efficacious;43– 47 multiple administrationshave caused less toxicity than single administra-tions.45,46,48,49 Other methods of reducing the effects ofMAb heterogeneity include the use of radionuclideswith longer ranges, e.g., yttrium-90 (90Y) or rhenium-188 (188Re);24 combined modality RIT with EBRTadded; or other strategies.50,51

Preclinical Radioimmunotherapy EvidenceA number of preclinical studies demonstrate the ad-vantages of dose fractionation for RIT. A fractionatedRIT study of beagle dogs was conducted to assesstoxicity.52 90Y-B72.3 was administered as a single dose(range, 1–2.5 mCi/kg) or 2 doses of 0.9 mCi/kg on Days0 and 4 or 0 and 8 (total, 1.8 mCi/kg), or as 2 mCi/kgfor the first dose and 0.9 mCi/kg for the second doseon Day 8 (total, 2.9 mCi/kg). Bone marrow was thedose-limiting tissue, while liver was the second dose-limiting tissue. A 20% decrease in bone marrow toxic-ity was observed with fractionation; an even greaterdecrease in liver toxicity accompanied fractionatedRIT.

To determine whether fractionation of dose pro-vided an advantage for RIT, MAb B72.3 immunoglob-ulin (Ig)G labeled with 131I was given to athymic mice

FIGURE 3. Radial dependence of dose rate for 300 �Ci injection of

iodine-131 (131I)–labeled 17-1A monoclonal antibody in LS174T xenografts.

Plotted are histograms for each of 30 radial increments (tumor center

� 0; tumor surface � 30; for a 10-mm diameter tumor, each radial

increment represented 0.167 mm). Each histogram represents the num-

ber of cubic voxels experiencing dose rates within a radial interval

(dose rate-volume histograms). Voxel dimensions were 0.2 or 0.25 mm

on a side. Top and Bottom, 1 and 4 days after injection, respectively.

Roberson PL, Buchsbaum DJ. Reconciliation of tumor dose response to

external beam radiotherapy versus radioimmunotherapy with 131iodine-

labeled antibody for a colon cancer model. Cancer Res 1995;55(suppl):

5811– 6. Reprinted with permission from the American Association for

Cancer Research, Inc.

FIGURE 4. Percentage of mice demonstrating antitumor effect, defined as an

increase of less than 50% in tumor volume when compared with growth of the

untreated control mice, during the observation period (35–45 days) after one

to three doses of iodine-131 (131I)–B72.3 at weekly intervals. Mortality is the

percentage of mice that died during the observation period. Prepared from data

in: Schlom J, Molinolo A, Simpson JF, Siler K, Roselli M, Hinkle G, et al.

Advantage of dose fractionation in monoclonal antibody–targeted radioimmu-

notherapy. J Natl Cancer Inst 1990;82:763–71.


bearing LS174T human colon adenocarcinoma xeno-grafts.45 The LS174T xenograft, in which 30 – 60% ofcells express the TAG-72 antigen, was used to reflectthe heterogeneity of the TAG-72 antigen seen in tu-mors from patients. Otherwise lethal doses were frac-tionated to assess toxicity and the effect on tumorgrowth. In contrast to a single 600-�Ci dose of 131I-B72.3 IgG, after which 60% of the mice died of toxiceffects, two 300-�Ci doses of 131I-B72.3 IgG (total, 600�Ci) reduced or eliminated tumor growth in 90% ofmice, and only 10% of the mice died of toxic effects(Fig. 4). Furthermore, dose fractionation permitted es-calation to three weekly doses of 300 �Ci of 131I-B72.3IgG (total, 900 �Ci), resulting in even more extensivetumor reduction or elimination and minimal toxic

effects. In a further investigation of acute toxic effects,mice receiving multiple doses were sacrificed at 2weeks, the time of peak marrow toxicity,53 and at 7weeks after MAb administration. In all cases, normaltissues showed no evidence of toxicity upon histologicexamination. The bone marrow was normocellularwith all three cell lines present in normal proportion.

A study by Buchsbaum et al.46 to investigateshorter fractionation intervals also demonstrated thatfractionated RIT provided a therapeutic advantagewith increased tumor cures and regression and de-creased toxicity. For example, one dose of 600 �Ci131I-CC49 to LS174T tumor-bearing mice was lethal to25% of mice, and no tumors disappeared (Fig. 5).When three doses of 300 Ci 131I-CC49 (total, 900 �CI)were given within 1 week, tumors disappeared in 40%of the mice, accompanied by 30% mortality. More-over, three doses of 300 �Ci 131I-CC49 reduced thetumor recurrence rate dramatically. The data alsoshowed that a higher concentration of 125I-CC49 wasmaintained in the tumor for a longer period of timeafter fractionated treatments than after a single treat-ment with 131I-CC49. Fractionated dose and continu-ous infusion of 131I-CC49 (total radioactivity doseequal in both groups) were also compared in thismodel; mice that received three doses within 1 weekhad longer survival and tumor doubling times thanthe mice that received continuous infusion.54 Tumorradiation dose was higher and bone marrow doselower in the groups that received multiple doses. Ahigher concentration of 125I-CC49 was maintained inthe tumor periphery for a longer period of time fol-lowing two treatments with 131I-CC49 at a 3-day inter-val than after a single dose of 131I-CC49, so that theradiation time and the total radiation dose were in-creased in the tumor. Using serial section autoradiog-raphy to reconstruct tumor activity distributions,Buchsbaum et al.54 and Roberson et al.55 comparedthe 3-D dosimetry of LS174T human colon carcinomaxenographs in athymic mice for a single dose, threedose fractions, and continuous infusion of 131I-CC49over 7 days. Radiation dose rate nonuniformities werereduced by fractionated and continuous infusions;fractionated doses produced superior dosimetric re-sults when compared with single dose or continuousinfusion.

Beaumier et al.56 evaluated the use of 186Re-la-beled NR-LU-10 MAb in mice bearing SHT-1 small celllung cancer xenografts. When compared with a singledose of 430 �Ci 186Re-NR-LU-10, fractionation intotwo to four doses (total, 492– 603 �Ci) within 7–10 dayswas associated with significantly delayed tumorgrowth and reduced toxicity, and actually allowedmore radiolabeled MAb to be administered.

FIGURE 5. Percentage of mice demonstrating an antitumor effect (complete

response, defined as no visible or palpable tumor) after one to three doses of

iodine-131 (131I)–CC49 given within a 1-week interval. Mortality is the percent

of mice that died during the observation period (180 days). Prepared from data

in: Buchsbaum D, Khazaeli MB, Liu T, Bright S, Richardson K, Jones M, et al.

Fractionated radioimmunotherapy of human colon carcinoma xenografts with

131I-labeled monoclonal antibody CC49. Buchsbaum D, Khazaeli MB, Liu T,

Bright S, Richardson K, Jones M, et al. Fractionated radioimmunotherapy of

human colon carcinoma xenografts with 131I-labeled monoclonal antibody

CC49. Cancer Res 1995;55:5881s–5887s.


Studies by Blumenthal et al.,57,58 who found thattumor vascularity was profoundly altered followingradiolabeled MAb treatment, even when fractionated,are of some concern. Between 7 and 21 days after 150�Ci 131I-Mu-9, the number of blood vessels, vascularvolume, blood flow, and vascular permeability inGW-39 human colon cancer xenografts in athymicmice were profoundly reduced, as was the tumor up-take of a second dose of radiolabeled MAb. The mac-roscopic radiation dose to the tumor was about 4200centigrays (cGy), although that to the vessels wasmany times greater because of preferential perivascu-lar localization of radiolabeled MAb. The threshold fordecreased vascular permeability was a macroscopictumor dose of 800 cGy, at which other alterations oftumor vascularity were not observed. Subsequently,Blumenthal et al.59 reported that vascular permeabil-ity, the most sensitive of the vascular parameters stud-ied, was actually increased in 40% and unchanged inan additional 20% of 10 other tumor xenograft modelsat 14 days after a fixed 1500 cGy RIT dose to tumor.Doubling the tumor dose to 3000 cGy by doubling theradionuclide amount further produced mixed resultsin the vascular permeability measurements. Despitethe care with which these investigations were con-ducted, their relevance to the clinical circumstance isuncertain because of the variability of the results indifferent tumor models, the high radiation doses totumor vessels due to preferential perivascular local-ization, and the difficulties of delivering tumor dosesof these magnitudes to patients.

Studies of RIT fractionation have generally shownbeneficial effects in preclinical models. A higher totalradionuclide dose could be delivered in several frac-tions than could be tolerated as a single dose, andtumor control was improved with fractionation. Tu-mor concentrations of radiolabeled MAb were pre-served over multiple treatment doses, and there wasdirect and indirect evidence of more uniform radia-tion dose distribution in the tumor after multipledoses.

Clinical Radioimmunotherapy EvidenceAlthough most RIT trials have involved the use ofmultiple radionuclide doses, few, thus far, have beenintended to evaluate fractionation and only one, byMeredith et al.,60 has been designed to provide directcomparisons of single and multiple dosing. Severalconclusions can be drawn from the multiple-dose tri-als that have used a fractionation strategy (Tables 1and 2). First, fractionation is an effective method fortitrating the radionuclide dose and the associated tox-icity for an individual patient. In a sense, fractionationis a form of patient-specific radionuclide dosing,

wherein the total radionuclide dose is determined bythe accumulated amount of radionuclide that ulti-mately leads to Grade 3 or 4 toxicity in the patient.Second, fractionation provides an added level of safetywhen radionuclide doses are given at 2- to 8-weekintervals, because radionuclide dose reduction anddose delay techniques can be used to ameliorate fu-ture toxicity in response to the observed acute toxici-ties from the earlier radionuclide dose(s). In RIT trialsthat have used 131I-Lym-1 for non-Hodgkin lymphoma(NHL) and chronic lymphocytic leukemia, these strat-egies have allowed patients with extensive marrowmalignancy to be safely and successfully treated (Fig.6).13 Third, fractionation is difficult to execute in ma-lignancies associated with immunocompetence un-less humanized antibodies or immunosuppressantdrugs are used.61 On the other hand, in a RIT trialdesigned to determine the MTD of a minimum of twoand a maximum of four doses of 131I-Lym-1 mouse

FIGURE 6. A male age 45 years with non-Hodgkin lymphoma (NHL) unre-

sponsive to five chemotherapy regimens had normal blood counts prior to a

series of doses of iodine-131(131I)–Lym-1. Peripheral blood cells (left scale) are

expressed as the percentage of pretreatment baseline (platelets, 307 k/mm3

(squares); granulocytes, 4.5 k/mm3 (circles); white blood cells (WBC), 6.7

k/mm3 (triangles); Hematocrit (HCT), 40.5% (diamonds)); bars (right scale)

indicate accumulated 131I (total radioactivity dose 184 mCi/m2; 328 mCi).

G1–G4 indicate hematologic toxicity grade based on conventional criteria for

the nadir value after each treatment dose. Peripheral blood cells decreased

dramatically after the first treatment dose, likely due to diffuse marrow NHL

observed by marrow imaging and biopsy. The second and fourth treatment

doses were dose-delayed and dose-reduced due to hematologic toxicity. By

fractionating the 131I-Lym-1 dose, the patient’s treatment could be titrated

achieving substantial total doses of 131I and a durable partial remission.

Marrow eligibility criteria (less than 25% marrow NHL) often used for NHL

radioimmunotherapy would have precluded treatment or, if single-dose data

from maximum tolerated dose trials were used, the patient would have incurred

life-threatening hematologic toxicities.


MAb for NHL, HAMA interrupted treatment for only10% of the patients.14 Fourth, fractionated RIT canalso be used in association with bone marrow recon-stitution;15 greater lung radiation doses were toleratedby the patients given fractionated, high-dose RIT thanwere reported to be dose-limiting by Press et al. forsingle-dose RIT.11,62 Finally, tumor targeting is clearlypreserved over multiple therapeutic doses of radiola-beled MAb.

Thirty patients with Stage III or IV B-cell malig-nancies (25 NHL and 5 chronic lymphocytic leukemia[CLL]) who had progressed despite standard treat-ment entered a trial to assess 131I-Lym-1 toxicity and,secondarily, efficacy.13 Lym-1, a mouse MAb, binds anantigen that is expressed on malignant human Bcells.63 At trial entry, bone marrow from 12 of 14 NHLpatients showed substantial infiltration of the marrowby malignant cells on microscopic examination. Anadditional 7 of 11 NHL patients had historical evi-dence for extensive marrow malignancy, and all 5patients with CLL had diffuse involvement of the bonemarrow. Of the 30 patients, 15 had gradable hemato-logic abnormalities at trial entry, including 5 patientswho had Grade 3 or 4 abnormalities. Patients weretreated with doses of 30 or 60 mCi of 131I-Lym-1 at 2-to 6-week intervals; 11 of the 30 patients completedthe intended 300 mCi. Treatment was interrupted byhematologic toxicity in 3 patients and the develop-ment of HAMA in 3 patients. There were no deathsdue to toxicity, and no bleeding episodes or neutro-penic sepsis. Tumor regression was great enough toqualify 57% as responders (13 NHL and 4 CLL pa-tients). The responses from this low-dose, fractionatedRIT were remarkable because the patients’ hemato-logic status made them poor candidates. Otherwiseuntreatable patients were treated with fractionatedRIT, achieving responses while morbidity was con-trolled.

Based on this strategy of fractionating the total 131Iand radiation dose, a dose escalation trial was de-signed to define the MTD and efficacy of the first twoof a maximum of four doses of 131I-Lym-1 given 4weeks apart.14 131I was escalated from 40 to 100mCi/m2 of body surface area. The nonmyeloablativeMTD for each of two doses of 131I-Lym-1 given 4 weeksapart was 100 mCi/m2 in patients with not more than25% marrow NHL.14 All three entries in this patientcohort had complete responses. Two of three patientsin the 100 mCi/m2 cohort tolerated the study maxi-mum of four treatment doses of 131I-Lym-1. Total 131Ireceived by these three patients was 355, 626, and 810mCi, respectively, contributing 121, 207, and 275 cGyand 103, 194 and 275 cGy to the body and marrow(from the blood and body), respectively. Despite total

radionuclide doses at the MTD level approximatingthose reported to be dose-limiting in NHL for single-dose RIT with bone marrow reconstitution, fraction-ated RIT without bone marrow reconstitution was welltolerated; there were no instances of significant bleed-ing or neutropenic sepsis.

A noteworthy trial of fractionated RIT was imple-mented by Divgi et al.,64 in which 131I-chimeric G250MAb was given to patients with metastatic renal cellcarcinoma. As a strategy to avoid hospitalizing pa-tients, an initial dose of 30 mCi was given; then, atintervals of several days, the patient was given addi-tional doses, dependent upon measurements of thebody content of 131I, to bring the body content back to30 mCi. One might describe the approach as “toppingoff” the body content of 131I. Although the trial had notbeen completed at the time this article was written,tumor targeting was excellent for all doses up to thattime.

A study to determine the potential of fractionatedRIT for patients with recurrent Hodgkin disease wasconducted by Vriesendorp et al.65,66 Ninety patientsreceived 90Y-labeled polyclonal rabbit antihuman fer-ritin IgG, 57 patients received a single dose (0.3– 0.5mCi/kg), and 33 patients received 0.25 mCi/kg doseson Days 0 and 7. HAMA occurred in about 5% of thepatients. In this study, fractionation did not providethe expected decrease in hematologic toxicity or in-crease in tumor response.

Meredith et al.60 reported 12 patients with meta-static colon cancer who were treated with 131I– chi-meric B72.3 at total doses of 28 or 36 mCi/m2. TheMTD for a single dose was 36 mCi/m2 with marrowsuppression as the dose-limiting toxicity. The degreeof bone marrow suppression in response to a totaldose of 36 mCi/m2 was significantly less, when frac-tionated in two or three weekly fractions, than thatseen with the same amount given as a single dose (Fig.7). To our knowledge, this study represents the onlycontrolled trial of radiolabeled MAb fractionation inhumans.

Fractionated or Hyperfractionated Radiotherapy andRadioimmunotherapyMost modern radiation treatment is given in a frac-tionated manner in order to increase antitumor effi-cacy and spare normal tissues. Historically, the advan-tages of fractionation, due to differing effects betweentissues, were first noted when it was not possible tosterilize a ram with a single dose of radiation to thetestes without causing skin breakdown; sterilizationcould be accomplished with skin tolerance by frac-tionation of the radiation.1 Since then, it has beendetermined that dose per fraction, time between frac-


tions, dose rate, total dose, and overall treatment timeinfluence normal tissue and tumor effects.2 To pre-serve normal tissues, the larger the dose per fraction,the fewer fractions can be given and the lower the totaldose for equivalent risk of complications.

Although there is a spectrum of rates at whichnormal tissues respond to radiation, responses areroughly divided into acute and late responses.67 This“approximate division” is based on the proliferationkinetics of the cells, with tissues with acute responseundergoing rapid renewal versus late-responding tis-sues that have infrequent cell division and turnover.Tolerance-limiting early-responding cells includebasal cells of the skin, crypt cells of the intestines, andhematopoeitic stem cells. Examples of tolerance-lim-iting late-responding cells include renal tubular cells,fibroblasts, and smooth muscle cells in arterial walls.Rapidly growing tumors generally respond in a man-ner similar to early-responding normal tissues.

There are radiobiologic mechanisms that explain

differences between early- and late-responding nor-mal tissues and between tumors and normal tissues.67

These factors include repair of sublethal or potentiallylethal damage, redistribution in the cell cycle, andregeneration from surviving stem cells. For tumors,reoxygenation of hypoxic areas between fractions isimportant.

Several mechanisms favor fractionation for differ-ential tumor and normal tissue effects, including thedecreased ability of tumor cells to repair sublethal andpotentially lethal damage between fractions that areclosely timed. There is a wealth of information, accu-mulated over several decades, that has allowed exten-sive analysis of tumor control and normal tissue ef-fects resulting from EBRT dose fractionation schemes.Analysis of clinical, animal model, and in vitro studiesof radiation effects allows application of tables andequations for prediction of equivalence between var-ious schemes for early and late effects.68

Standard fractionation for EBRT is usually consid-ered as 1.8 or 2 Gy per fraction, one fraction per day,5 days per week. For simplicity of further discussionand examples, 2-Gy fractions will be considered thestandard, unless specifically stated otherwise.

Hyperfractionation, such as giving EBRT at leasttwice per day, decreases the radiation dose per frac-tion and the time interval between fractions. Becausehyperfractionation decreases late effects at the ex-pense of increasing acute effects, higher total dosescan be delivered with the same risk of late complica-tions. A general rule of thumb applicable to mosttissues is that 1.2 Gy twice daily can be given to a totaldose of about 115% of that for single daily 2-Gy frac-tions with the same risk of late complications. In tak-ing advantage of the differences between fast-growingtumors and decreased late effects by hyperfraction-ation, the higher total tumor doses usually result inimproved tumor control rates. Accelerated fraction-ation may use conventional doses per fraction, but itspaces fractions more closely together, such that over-all treatment time is reduced to deliver the same totaldose. This scheme can also increase tumor control,but normal tissue late effects are increased. Hypofrac-tionation uses larger doses per fraction, often spacedfurther apart and to a smaller total dose than standardfractionation.

Head and neck cancer is a disease for which frac-tionation has been extensively studied and principlesapplied to clinical trials. For these fast-growing, rela-tively radiosensitive tumors, fractionation increasestumor control rates. Results from the national coop-erative Radiation Therapy Oncology Group 9003 trialfor advanced head and neck cancer confirmed theoutcomes for various fractionation regimens, which

FIGURE 7. Fractionation versus nadir white blood cells (WBC) and platelet

grade. Comparison of mean toxicity score (sum of WBC and platelet grade) for

groups of patients treated with total doses of 28 mCi/m2 or 36 mCi/m2

iodine-131 (131I)–ch B72.3 given as one, two, or three weekly fractions. From:

Meredith RF, Khazaeli MB, Liu T, Plott G, Wheeler RH, Russell C, et al. Dose

fractionation of radiolabeled antibodies in patients with metastatic colon can-

cer. J Nucl Med 1992;33:1648–53. Reprinted with permission from the Society

of Nuclear Medicine, Inc.


were predicted from prior analyses.3 In this trial, hy-perfractionated and accelerated fractionation schemesoffered a control advantage over standard fraction orsplit course schemes, while late effects were similar. Aspredicted, the results showed the influence of treat-ment duration on tumor control. Due to tumor re-growth, higher doses were needed for equivalentcontrol if delivered over longer intervals.3–5 Afteradjustment for the dose per fraction (using an alpha/beta ratio of 15 Gy), 7 days’ shortening of the treat-ment duration for hyperfractionation or acceleratedfractionation increased tumor control rates for ad-vanced head and neck cancers treated with variousaltered fractionation schemes.4

Some RIT treatments have been given at a near-tolerance dose with repetition of that dose after aperiod of normal dose-limiting tissue recovery, ratherthan a true fractionation of the MTD. This scheme issimilar to that frequently used for chemotherapy thatcan be cycled every 3– 4 weeks due to bone marrowsuppression and recovery kinetics. For fast-growingtumors, this regimen may be suboptimal because itallows surviving tumor cell proliferation to occur be-tween treatments, whereas with true fractionation, ashort time interval prohibits tumor cells from prolif-erating while allowing recovery of normal tissues. Forexample, regrowth of malignancy has been reported ina lymphoma patient treated with RIT; initial regres-sion was followed by regrowth of some lymph nodeswithin 3 weeks after treatment, before the marrow hadrecovered sufficiently to allow the next treatment.44

This experience illustrates that the time between frac-tions is limited by normal tissue recovery, in the ab-sence of marrow support, even when the time be-tween treatments is suboptimal for tumor control. Forthis patient’s lymphoma, which was sensitive to treat-ment but had rapid growth kinetics, smaller dosesgiven more frequently may have provided better con-trol. This suggestion is supported by results reportedby DeNardo, et al.,13 after using dose fractionationschedules for 131I-Lym-1, and Shen et al., after mod-eling of marrow recovery for timing between frac-tions.69

RIT is inherently hyperfractionated because it de-livers continuously decreasing, low-dose-rate radia-tion as the radionuclide decays (often over days). Fur-thermore, fractionation regimens that have a greaterchance of tumor control than single dosing have beenachieved. Some theoretic advantages of RIT fraction-ation, in addition to those already discussed, includedecreasing the effects of heterogeneity, as discussed indetail by O’Donoghue et al.70 Parameters for applyingfractionation principles to RIT have been ana-lyzed.71,72 In addition to factors discussed above that

apply to EBRT, the relatively low-dose-rate effects ofRIT have also been considered. To compensate foranticipated decreased effectiveness of the low doserate of RIT, Fowler72 suggested that a total dose in-crease of 20% would be required to achieve antitumorefficacy comparable to that of EBRT. However, thisadjusted dose increase cannot be applied as an abso-lute rule, since the results of comparing the efficacy ofRIT with that of EBRT have been variable; some havereported that RIT was more efficacious than high-dose-rate radiation.8 –10 The variety of responses maybe indicative of different underlying mechanisms thataffect radiation sensitivity, as summarized by Knox etal.8 In their analysis, they found that the size of thesurvival curve shoulder [alpha/beta ratio] and the tu-mor doubling time were important determinants ofthe magnitude of dose rate effects in a given tumortype. Study of underlying mechanisms for some tu-mors revealed a correlation between tumor sensitivityto low-dose-rate radiation and G2/M block.73 Moulderet al.74 found a therapeutic gain for fractionated low-dose-rate when compared with conventional-dose-rate fractionated radiotherapy for gastrointestinal andrenal damage. Guidelines for the optimal fractionationof RIT have not been determined, since many of thedose rate experiments have not fractionated the low-dose-rate radiation or taken into account other factorsthat may be important for normal tissue toxicity aswell as tumor control.

Comparison of the Radiobiologic Aspects of Fractionationfor External Beam Radiotherapy andRadioimmunotherapyFrom the earliest days of radiotherapy, it has beenrecognized that radiation effects on biologic systemsare highly dependent on the temporal pattern of ex-posure. In any particular tissue, a series of small high-dose-rate fractions enables a larger total dose to betolerated than would be possible using a single, high-dose-rate exposure. Conversely, a radiation dose de-livered as a single acute exposure will be more dam-aging to normal tissues than the same total dosedivided into smaller fractions. This indicates that therelationship between dose and response is nonlinearfor high dose rates. By itself, however, it does notconstitute a rationale for fractionation. The other keyfactor is the dissociation of response in different tis-sues when the pattern of radiation exposure changes.The relative sensitivity of tissues to radiation is notinvariant but may be altered by manipulating thetreatment structure. This is the major rationale forfractionation of EBRT. For EBRT, the dose limitation isgenerally imposed by delayed reactions in late-re-


sponding normal tissues, often associated with thedevelopment of fibrosis.

Mechanistically, the major radiobiologic advan-tage associated with fractionation of EBRT is the dif-ferential increase in repair of radiation damage inlate-responding normal tissues compared with tu-mors. This appears to be a consequence of differencesin the shapes of the underlying dose-response rela-tionships and can be understood in terms of the lin-ear-quadratic (LQ) model. First advanced as a modelfor normal tissue and tumor responses to radiation inthe early 1980s,75,76 the LQ model has been of greatvalue in the analysis of clinical data and in the designof EBRT fractionation schemes. It is based on theassumption that cell survival and, more generally,dose-response relations for high-dose-rate exposurescan be decomposed into linear and quadratic compo-nents.

The clonogenic survival of mammalian cells inculture following single doses of radiation can gener-ally be described by the equation

Fs � exp � � �d � �d 2� (1)

where Fs is the surviving fraction, d is the dose deliv-ered, and � and � are parameters that describe theshape of the survival curve. One possible biophysicalinterpretation of these parameters is that � equals therate of cell kill by a single-hit mechanism and � equalsthe rate of cell kill caused by the interaction of twosublethal hits. However, this is probably an oversim-plification.77

Assuming that each fraction has an identical ef-fect, the surviving fraction after n fractions of size d is

Fs � �exp � � �d � �d 2��n � exp (��D � �Dd) (2)

where D � nd is the total dose.If the quantity E � -ln(Fs) is used as a metric for

biologic effect, we can write

E � �D �1 � d/��/�� (3)

In this notation, the quantity (1 � d/(�/�)) repre-sents the relative effectiveness (RE) of the total dose,D, when delivered at a high dose rate in fractions ofsize d.68,75

RE � 1 � d/��/�� (4)

The quantity D�1 � d/��/�� has units of doseand may be thought of as the dose required to pro-duce the biologic effect, E, if given as a very largenumber of very small fractions. This quantity has beencalled the “biologically effective dose,” or BED.72

BED � D�1 � d/��/�� (5)

For computational purposes, there is a linear re-lationship between BED and biologic response.

E � � BED (6)

This formalism is not the only one possible for theLQ model, but it has the advantage that for any treat-ment schedule the BED is given by

BED � RE � D (7)

By making some assumptions about the natureand kinetics of repair of radiation damage, it is possi-ble to calculate the RE for radiation delivered overprotracted times. In particular, this type of analysismay be extended to treatment with biologically tar-geted radionuclides.

Although numeric values of � and � may be de-rived from the analysis of survival data for tumor cellsgrown in culture, it is usually not possible to assignmeaningful values to these parameters in the contextof a normal tissue dose-response relationship. How-ever, it is possible to estimate the ratio �/�. The �/�ratio has emerged as an important concept in clinicalradiobiology because it characterizes the degree ofnonlinearity associated with the dose-response rela-tionship. Normal tissues fall into two broad categories:acutely responding and late-responding. Acute re-sponses develop over days to weeks; examples includeradiation damage in various epithelia and the hema-topoietic system. These tissue responses are generallyassociated with high values of (� 10 Gy), indicatingthat the dose-response relationship is relatively insen-sitive to changes in fraction size. Late effects, devel-oping typically over months to years, include radiationdamage to the liver, central nervous system, and fi-brotic changes in endothelial tissue. Generally, theseverity of late effects is not predictable on the basis ofacute effects. Late responses are associated with lowvalues of the �/� ratio (�3 Gy), indicating a pro-nounced dependency on fraction size. Data from ex-perimental tumors and from the analysis of clinicaldose-response relationships indicate that tumors usu-ally behave like acutely responding normal tissues.

It is well known that prolonging, or inserting timegaps into, conventional fractionated radiotherapy re-duces tumor control rates unless total doses are in-creased.78 – 83 This is usually attributed to allowingmore time for tumor cell proliferation.84 – 86 In terms ofthe LQ model, the simplest assumption is that cellularproliferation is exponential with a growth rate, . Thebiologic effect can then be written as

E�t� � �D �1 � d/��/��t (8)

where E(t) is now explicitly dependent on time, t.


This represents the effect of the radiation expo-sure modified by concurrent cellular proliferation. Al-though an oversimplification, this model may be ap-plicable to systems, such as tumors and acutelyresponding normal tissues, where rapid cellular pro-liferation is an important factor, but not late-respond-ing tissues, where proliferation over the course oftreatment is unlikely. The above equation indicatesthat the biologic effect produced in a proliferating cellpopulation is determined primarily by the total dose,modified by the radiobiologic effectiveness of thatdose and the time over which it is delivered.

Some of the radiobiologic factors that are appli-cable to fractionated EBRT may also be applicable toRIT. However, there are fundamental differences inthe patterns of radiation and toxicity between the twomodalities. Radionuclide therapy is customarily a sys-temic rather than a local treatment modality. Thedose-limiting toxicity is usually due to the hematopoi-etic system, unless bone marrow reconstitution is alsoused. The hematopoietic system is an acutely re-sponding system with a limited tolerance for radia-tion. In radionuclide therapy, organs are unlikely to beirradiated in relative isolation as occurs in EBRT, andit is also unlikely that one segment of an organ willexperience a very high dose while another segmenthas a dose of zero. Radiation dose rates have lowerorders of magnitude in radionuclide therapy than inEBRT. Another significant difference is the muchgreater level of microscopic nonuniformity of dose forradionuclide therapy. These contrasts mean that wehave to be selective in how the lessons derived fromfractionated EBRT are applied to RIT.

The major radiobiologic advantage of fraction-ation in EBRT is the differential increase in repairbetween late-responding normal tissues and tumorscaused by delivering radiation in smaller individualdoses. Does this rationale apply to radionuclide ther-apy?

For radiation delivered at a constant dose rate, r,over a period of time, T, Dale68 calculated that theequation for the relative effectiveness (RE) is

RE � 1 �2r

� ��/�� 1 �1

�T�1 � exp� � �T�� (9)

where � is a time constant that characterizes the rateof repair of radiation damage that is presumed to be amonoexponential process.

For radiation by an exponentially decaying doserate with an effective decay constant, , where theradiation time is long enough that the dose rate decaysall the way to zero, the analogous equation for RE is

RE � 1 �r0

�� /�(10)

where r0 is the initial dose rate.68

Other formulae for RE have been provided whenthe pattern of dose rate has been more complex. How-ever, a number of observations that remain valid forthe more complex versions may be made using thesimpler formula (Equation 10).

There is usually a significant difference betweenthe rates of repair and decay. Values derived for therepair half-time for mammalian cells in culture andnormal tissues in patients fall within a range of min-utes to hours.10,87 In contrast, most clinical applica-tions of radionuclide therapy deliver radiation dosewith an effective halftime of several days. In terms ofEquation 10, this means � . For a repair halftime of1.5 hours, a reasonable approximation, � is about 0.5hr�1. Comparing Equation 10 with the correspondingEquation 4 for the RE of fractionated EBRT, it can beseen that r0/� plays an analogous role to that of thefraction size, d. The sparing effect of treatment with adecaying dose rate will be equivalent to that of con-ventional fractionated EBRT if r0/� � 2 Gy or r0 � 2�Gy/hr. For � � 0.5 hr�1, this corresponds to an initialdose rate of 1 Gy/hr. If the initial dose rate producedby radionuclide therapy is less than this, the treatmentis already more sparing than conventional fraction-ated EBRT, suggesting that there is little to be gainedin terms of differential repair by delivering radionu-clide therapy in a fractionated manner. Contrast thiswith the sort of dose rates achievable in patients withtargeted radionuclide therapy. To provide an approx-imate value, we note that tumor concentrations ofradionuclide are typically 0.01– 0.02% injected doseper gram. For an administered activity of 100 mCi of131I, the activity per gram of tumor assuming instan-taneous uptake is 10 –20 �Ci/g. The absorbed doserate produced by this concentration is 4 – 8 cGy/hr,assuming electronic equilibrium and ignoring photonradiation. This is significantly less than the previouscalculation of the 2-Gy equivalent dose rate. The high-dose-rate fraction size that is theoretically equivalentto exponentially decaying dose rates of initial value r0

is given by

d � r0/�� _ � _r0/� if � �� (11)

For the initial dose rates quoted above, equivalentfraction sizes are generally less than 0.5 Gy for reason-able values of �. Moreover, the dose rates in normaltissues must be significantly less than the values fortumors if targeted radionuclide therapy is a rationalapproach. These considerations suggest that fraction-


ation of RIT will provide little further sparing due torepair of radiation damage.

Another element of the sparing effect of fraction-ated treatments is due to proliferative regeneration inrapidly dividing cell populations. We now considerhow this principle relates to the concept of RIT frac-tionation. It is apparent that moving from a singlelarge dose of radiolabeled MAb to a series of smallerdoses, each separated by a matter of some days, willresult in a lower average dose rate delivered over alonger time period. For similar total doses, uniformdose distributions, and cell populations with time-independent radiosensitivity, this pattern of radiationis expected to result in reduced biologic effective-ness.88 However, as fractionation also spares the dose-limiting hematopoietic system, it enables an increasein the total administered activity.45,46,89 The relativeproliferation rates of the targeted tumor and the he-matopoietic progenitor cells then become key factors.Fractionation would be advantageous if there wasfaster proliferative regeneration in the hematopoieticsystem than in the tumor. This may be applicable to aslowly growing tumor cell population, but, given theimportance of treatment duration in EBRT, it is un-likely to provide a significant advantage for many tu-mors.

After radiation, the progression of dividing cellsthrough the cell cycle is delayed. The delay dependson both dose and dose rate, occurs only at specificpoints in the cell cycle, and is similar for both surviv-ing and nonsurviving cells. The net result is that manycells accumulate in the G2/M and G1/S boundariesand alter the mitotic index. The length of the delay andthe decrease in mitotic index are both functions ofdose.90 It is well known that the radiosensitivity ofcells is a function of their position within the cell cycle,cells in late S phase being the most resistant and thosein G2/M phase being the most radiosensitive. Cellsthat are in the most sensitive phase, when radiationoccurs, will be preferentially killed. For acute radiationexposures, this produces a partial cell synchrony and achange in the overall sensitivity of the population. Inthe case of fractionated EBRT, it is believed that thissynchrony is rapidly lost because of natural variationin the rates at which cells pass through the cycle, in aprocess called redistribution. For continuous radia-tion exposures, it is possible that certain dose ratesenable limited cell cycle progression but produce acheck at the radiosensitive G2/M boundary. This maybe the mechanism underlying the inverse dose rateeffect observed in vitro for low-dose-rate radiation,although this is not absolutely established.91 Radio-sensitization due to induced cell-cycle blocks may berelevant for combined EBRT and low-dose-rate radia-

tion92 and, in principle, may render RIT more effectivethan otherwise expected. However, it is not clear whatadditional advantage this mechanism confers on frac-tionated RIT.

The oxygenation status of cells is a major deter-minant of their radiosensitivity. The oxygen effect isgreatest for sparsely ionizing radiation (e.g., beta par-ticles) and is absent for densely ionizing radiation(e.g., alpha particles). Fractionated EBRT is thought toallow previously hypoxic tumor regions to reoxygen-ate. Reoxygenation may also be of importance in thecontext of RIT fractionation. The effectiveness of sin-gle-dose RIT may be limited by hypoxic subpopula-tions of tumor cells, whereas multiple doses of RITmay enable tumor sensitization by reoxygenation pro-cesses. It would be very interesting to perform anexperiment comparing fractionated and single-dosealpha-emitter (e.g., bismuth-213) RIT. As the oxygeneffect is absent from densely ionizing radiation, anytherapeutic advantage seen with fractionated treat-ments must be due to other mechanisms.

One other mechanism that is highly significant forRIT is the nonuniformity of radiation dose within tu-mors. Nonuniformity of tumor uptake of MAb is analmost invariable finding when tumors are biopsiedfollowing RIT,93,94 even in cases where the target an-tigen is homogeneously expressed. This is a majorradiobiologic difference between RIT and EBRT.Mathematical modeling studies suggest that nonuni-form dose distributions become proportionately lesseffective as the mean dose increases.24 “Dose escala-tion” may not lead to a significant increase in tumorresponses. In addition, the negative impact of dosi-metric nonuniformity is expected to be most severefor radiosensitive tumors. In this context, fraction-ated RIT may confer a therapeutic benefit becausethe loss of effectiveness due to nonuniformity isminimized. The degree to which the pattern of ra-dionuclide uptake changes from fraction to fractionis a critical factor in the comparative effectiveness offractionated versus single-dose RIT. Fractionationshould have a therapeutic advantage if differentdoses target different subpopulations of tumor cells.This could come about through time-dependentchanges in tumor capillary blood flow or modifica-tions to tumor architecture caused by the effects ofpreceding doses. Experimental data with trace-labeledMAb indicate no change in the distribution for twosuccessive doses.95 However, experimentation withtherapeutic levels of radionuclide has clearly shownthat fractionated RIT produces superior dosimetricresults and tumor activity distributions.54,55 The re-sults of this experiment are of critical importance to


the question of the therapeutic advantage of frac-tionated RIT at a radiobiologic level.

DISCUSSIONBecause MAbs are macromolecules, they have diffi-culty penetrating the tumor.96,97 The mechanisms un-derlying poor penetration and heterogenous distribu-tion of MAbs in tumors have been elegantly examinedby Jain.98,99 Nonuniform and inadequate blood flow,elevated interstitial pressure, necrotic regions, and ab-sent antigenic targets on some cells contribute to het-erogenous distribution of MAbs. Radionuclides withradiation that traverses many cell diameters distributethe radiation dose more uniformly. Despite the use ofthese radionuclides, nonuniform dose to different re-gions of the tumor remains an obstacle for RIT.9,100

Press et al.62 have demonstrated that a single,large dose of 131I-labeled anti-CD20 MAb was remark-ably effective treatment for NHL when autologousmarrow reconstitution was used. However, this maynot be the best approach, because normal tissues andsome regions of the tumor are overradiated to assurethat all regions of the tumor are adequately radiated.In early trials, the strategy of fractionating RIT into aseries of smaller radionuclide doses was used. Frac-tionation of RIT provided an opportunity to exploretoxicity at a time when the toxicity was largely un-known.

Despite its logistic disadvantages, there are otherreasons for, and advantages to, fractionating RIT. Amajor purpose for fractionated RIT is its use as astrategy for overcoming underradiation of tumor re-gions. In the presence of nonuniform radiation dose,an increase in dose from an increase in radionuclideamount given as a single dose becomes progressivelyless effective. This is also true for multiple dosingunless the MAb, and therefore the radiation dose,distribution is different from dose to dose. Subse-quently administered doses of radiolabeled MAb canaccess regions different from those accessed earlier,because blood flow has improved and reductions intumor size have led to reductions in interstitial pres-sure. There is considerable evidence—namely, betterefficacy—to support this thesis, but no one has yetdeveloped a study design to fully document thispremise. Preclinical studies provide powerful evidenceof better radiation dose distribution from fraction-ation, reflected in increased therapeutic response toequivalent radionuclide doses and to higher but equi-toxic radionuclide doses when the doses are fraction-ated. Furthermore, larger amounts of radionuclide canbe given in multiple doses than as a single dose, withequal or reduced toxicity. Although bone marrow stemcells are generally felt to be less influenced by frac-

tionation than are other normal tissues, preclinicalstudies of fractionated RIT indicate that larger doses ofradiation can be administered with less marrow sup-pression than with single large doses. Clinical studieshave confirmed these preclinical observations60 andhave shown other advantages to fractionated RIT, in-cluding the opportunity to apply a patient-specificapproach to radionuclide and radiation dosing and anincrease in the duration of tumor responses. In addi-tion to the well-documented benefits of fractionatedEBRT, there is evidence of a need to fractionate radio-nuclide therapy. Experience over 50 years indicatesthat multiple doses of 131I-iodide are often required toeliminate differentiated thyroid cancer. Administra-tion of multiple doses of 131I-iodide for thyroid canceris based on observations that not all thyroid metasta-ses compete equally for an individual dose of 131I-iodide. A subsequent dose of 131I-iodide often revealsmetastases that were not apparent on images ob-tained after the earlier dose of 131I-iodide. Further-more, other targeted radionuclide therapies involvethe administration of multiple doses. This strategy hasbeen used for radiolabeled peptides101 and, more re-cently, for radionuclide therapy for bone pain pallia-tion.102

An important consideration in the use of dosefractionation of RIT is the stage of disease. Most trialsto date have involved patients with late-stage disease,placing dose fractionation at a disadvantage. Patientswith advanced cancer have usually received severalprior chemotherapeutic regimens. These have mostlikely damaged the bone marrow and, thus, the MTDwas most probably less. In patients with less advanceddisease, longer time intervals that permit greater nor-mal tissue recovery can be employed between doses ofradiolabeled MAb; moreover, more dose fractions canbe given.

In addition, the ability to employ dose fraction-ation of RIT could be facilitated through investigationsof the following: 1) the immunogenicity of the MAb orIg form used, and 2) the size of the Ig form. Themajority of clinical trials with radiolabeled MAbs haveemployed whole mouse IgG. This is perhaps the worstform of Ig to use for dose fractionation because of theimmunogenicity of the mouse constant regions. Thereare two major ways to reduce immunogenicity: alter-ing the amino acid sequences of the Ig to make themless immunogenic, and altering the size of the Ig.Initial modification of mouse MAbs involved con-struction of chimeric MAbs. These molecules consistof mouse variable regions, i.e., complementarity de-termining regions (CDRs) and human constant re-gions (Fc). The next generation of molecules in whichimmunogenicity was decreased were the so-called


“humanized” or CDR-grafted MAbs. In these mole-cules, the six CDRs (three of the heavy chain and threeof the light chain) of the mouse MAb were grafted ontoa human MAb so that the only mouse sequences werein the binding sites. It has since become apparent thateven these CDR-grafted MAbs may elicit binding sitehost immune responses.103 Recent studies have shownthat it is possible to identify specificity determiningresidues (SDRs) for a given MAb.104 These SDRs are, ofcourse, in the hypervariable region of the MAb and arethe most critical for antigen-MAb interactions. Indeed,studies have shown that all six CDRs are not critical toMAb binding to an antigen, and in some cases onlythree to five CDRs are essential. It is now possible togenerate variants with minimal potential immunoge-nicity that still maintain antigen-binding capabilities,by modification at the single amino acid level withinCDR regions. These constructs should be most usefulin dose fractionation strategies involving multiplefractions over a long period of time.

Another approach to the efficient use of dose frac-tionation may well be the use of smaller Ig forms. Inaddition to less immunogenicity, these forms pene-trate tumors more efficiently. Studies of the tumorpenetration of different Ig forms, including whole IgG,F(ab�)2, Fab� fragments, and single-chain sFv, havebeen conducted, using a human colon cancer xeno-graft in athymic mice.105 After systemic administra-tion, quantitative autoradiographic analyses revealedthat the whole IgG delivered to the tumor was con-centrated in the region of, or immediately adjacent to,vessels, whereas the smaller sFv was more evenly dis-tributed throughout the tumor. The distributions ofthe Fab� and F(ab�)2 fragments showed intermediatepenetration in a size-related manner. These findingshave clear implications for dose fractionation. Forlarger tumors and IgG forms, one can use the analogyof peeling the skin of an onion with each dose fraction.

Fractionation of RIT has precedence; EBRT is rou-tinely fractionated into 20 – 40 doses in an effort toameliorate toxicity.106 Classically, fractionation ofEBRT has been a means of increasing therapeutic gainby relatively sparing normal tissues compared withadjacent tumor. A rationale for fractionated RIT isbased on irrefutable evidence for EBRT that the radi-ation dose to the tumor and the dose tolerated bynormal tissues can be increased. Another advantage offractionating RIT into multiple doses is better distri-bution of the microscopic radiation dose because ofreduced heterogeneity of MAb targeting over severaldoses. It is important to appreciate the differencesbetween RIT and EBRT. Whereas the latter representshigh dose and high-dose-rate radiation, the formerrepresents low dose and low-dose-rate radiation that

seems to induce apoptosis, rather than reproductivecell death, as its primary mechanism of cytotoxicity.Tumor cells generally have greater propensity thannormal cells for apoptosis, perhaps explaining thesometimes remarkable efficacy of RIT, particularly inlymphomas, known to have high levels of inherentapoptosis. Meyn107 demonstrated that low doses ofradiation induced substantial apoptosis and that thedose response actually leveled off at doses higher than7.5 Gy, suggesting that only a subset of cells in thetumor have the propensity for radiation-induced ap-optosis at any discrete time. Meyn also showed thatmultiple smaller fractions of radiation produced ahigher total of apoptotic cells than larger doses ofradiation, suggesting that an apoptotic subpopulationof cells reemerged between doses in the fractionatedprotocols. He envisioned strategies (such as fraction-ated RIT) that capitalized on restoration of apoptoticpropensity to radioresistant tumor cells for therapeu-tic benefit. There is empiric evidence from studies ofmice45,46,108 and patients12–15 that fractionation is ef-fective for RIT. Using a colon tumor xenograft modelthat mimics the heterogeneity of antigen and MAbdistribution, Schlom et al.45 have shown the benefitsof fractionated RIT in mice. Fractionation into threedoses permitted dose escalation by 50% and greatertherapeutic benefit; similar results have been reportedby others.46,108 Trials involving patients have also pro-vided evidence that the fractionated dose strategy iseffective, increasing the tolerated radiation dose.12–15,60

Issues that need to be addressed if fractionatedRIT is to be optimized include 1) the number of ra-dionuclide doses; 2) the radionuclide dose amount,e.g., the use of multiple doses to achieve an MTD, orthe use of multiple doses at the MTD dose level withadequate recovery interval between doses; 3) the in-terval between radionuclide doses; and 4) the optimalradionuclide physical half-time for a specific treat-ment interval. Finally, the dosing method is signifi-cant; in theory, it would seem that a radionuclide dosefor each treatment based upon dose-limiting organradiation dose would be preferable to an empiricallydetermined radionuclide dose.

In summary, fractionation of RIT has been limitedby the immunogenicity of MAbs, most of which havebeen of mouse origin thus far. Technologic develop-ments and the increased availability of humanizedMAbs or fragments of decreased immunogenicity nowmake fractionated RIT even more attractive. Trialsinvolving humans should be designed with the goal ofdetermining a fractionation interval for a nonimmu-nogenic, genetically engineered, or human MAb thatreduces bone marrow toxicity while enhancing tumoruptake and distribution of each radiolabeled MAb


dose. Improved efficacy is predicted for future RIT-treated malignancies because of the advantages ofdose fractionation.

REFERENCES1. Thames HD, Hendry JH. Fractionation in radiotherapy. New

York: Taylor & Francis, 1987:234.2. Hall EJ. Radiation biology. Cancer 1985;55:2051–7.3. Fu KK, Pajak TF, Trotti A, Jones CU, Spencer SA, Phillips TL,

et al. A radiation therapy oncology group (RTOG) phase IIIrandomized study to compare hyperfractionation and twovariants of accelerated fractionation to standard fraction-ation radiotherapy for head and neck squamous cell carci-noas: First report of RTOG 9003. Int J Radiat Oncol Biol Phys2000;48:7–16.

4. Fu KK, Withers HR, Peters LJ. Transmutability of dose andtime commentary on the first report of RTOG 90003. Int JRadiat Oncol Biol Phys 2000;48:1–2.

5. Fowler JF, Harari PM. Confirmation of improved local-re-gional control with altered fractionation in head and neckcancer. Int J Radiat Oncol Biol Phys 2000;48:3– 6.

6. Fischer JJ, Moulder JE. The steepness of the dose-responsecurve in radiation therapy. Radiology 1975;117:179 – 84.

7. Kroger LA, DeNardo GL, Gumerlock PH, Xiong CY, WinthropMD, Shi XB, et al. Apoptosis related gene and protein ex-pression in human lymphoma xenografts (Raji) after lowdose rate radiation using 67Cu-2IT-BAT-Lym-1 radioimmu-notherapy. Cancer Biother Radiopharm 2001;16:247– 62.

8. Knox SJ, Goris ML, Wessels BW. Overview of animal studiescomparing radioimmunotherapy with dose equivalent ex-ternal beam irradiation. Radiother Oncol 1992;23:111–7.

9. Wessels BW, Vessella RL, Palme DF, Berkopec JM, Smith GK,Bradley EW. Radiobiological comparison of external beam ir-radiation and radioimmunotherapy in renal cell carcinomaxenografts. Int J Radiat Oncol Biol Phys 1989;17:1257–63.

10. Hall EJ, Brenner DJ. The dose-rate effect revisited: radiobi-ological considerations of importance in radiotherapy. Int JRadiat Oncol Biol Phys 1991;21:1403–14.

11. Press OW, Eary JF, Appelbaum FR, Martin PJ, Nelp WB,Glenn S, et al. Phase II trial of 131I-B1 (anti-CD20) antibodytherapy with autologous stem cell transplantation for re-lapsed B cell lymphomas. Lancet 1995;346:336 – 40.

12. DeNardo GL, DeNardo SJ, O’Grady LF, Levy NB, Adams GP,Mills SL. Fractionated radioimmunotherapy of B-cell malig-nancies with 131I- Lym-1. Cancer Res 1990; 50(Suppl):1014–6.

13. DeNardo GL, DeNardo SJ, Lamborn KR, Goldstein DS, LevyNB, Lewis JP, et al. Low-dose fractionated radioimmuno-therapy for B-cell malignancies using 131I-Lym-1 antibody.Cancer Biother Radiopharm 1998;13:239 –54.

14. DeNardo GL, DeNardo SJ, Goldstein DS, Kroger LA, Lam-born KR, Levy NB, et al. Maximum tolerated dose, toxicity,and efficacy of 131I-Lym-1 antibody for fractionated radio-immunotherapy of non-Hodgkin’s lymphoma. J Clin Oncol1998;16:3246 –56.

15. Richman CM, DeNardo SJ, O’Grady LF, DeNardo GL. Radio-immunotherapy for breast cancer using escalating fraction-ated doses of 131I-labeled chimeric L6 antibody with periph-eral blood progenitor cell transfusions. Cancer Res 1995;55(Suppl):5916 –20.

16. DeNardo SJ, DeNardo GL, O’Grady LF, et al. Radioimmuno-therapy of lymphoma with fractionated I-131 Lym-1: PhaseI/II study [abstract]. J Nucl Med 1987;28:1077.

17. Jain RK. Vascular and interstitial barriers to delivery of thera-peutic agents in tumors. Cancer Metastasis Rev 1990;9:253–66.

18. Humm JL. Dosimetric aspects of radiolabeled antibodies fortumor therapy. J Nucl Med 1986;27:1490 –7.

19. Fujimori K, Covell DG, Fletcher JE, Weinstein JN. Modelinganalysis of the global and microscopic distribution of im-munoglobulin G, F(ab�)2, and Fab in tumors. Cancer Res1989;49:5656 – 63.

20. Van Osdol W, Fujimori K, Weinstein JN. An analysis ofmonoclonal antibody distribution in microscopic tumornodules: consequences of a “binding site barrier.” CancerRes 1991;51:4776 – 84.

21. Baxter LT, Yuan F, Jain RK. Pharmacokinetic analysis of theperivascular distribution of bifunctional antibodies andhaptens: comparison with experimental data. Cancer Res1992;52:5838 – 44.

22. Buchsbaum DJ, Wessels BW. Introduction: radiolabeled an-tibody tumor dosimetry. Med Phys 1993;20:499 –501.

23. Roberson PL, Buchsbaum DJ. Reconciliation of tumor doseresponse to external beam radiotherapy versus radioimmu-notherapy with 131Iodine-labeled antibody for colon cancermodel. Cancer Res 1995;55(Suppl):5811s– 6s.

24. O’Donoghue JA. The implications of nonuniform tumor dosesfor radioimmunotherapy. J Nucl Med 1999;40:1337–41.

25. Meredith RF, Buchsbaum DJ, Knox SJ. Radioimmunotherapy.In: Gunderson L, Tepper J, editors. Clinical radiation oncology.New York: Churchill Livingstone, 2000;283–99.

26. Jain RK. Determinants of tumor blood flow: a review. CancerRes 1988;48:2641–58.

27. Weinstein JN, van Osdol W. The macroscopic and micro-scopic pharmacology of monoclonal antibodies. Int J Im-munopharmacol 1992;14:457– 63.

28. Dvorak HF, Nagy JA, Feng D, Dvorak AM. Tumor architec-ture and targeted delivery. In: Abrams PG, Fritzberg AR.,editors. Radioimmunotherapy of cancer. New York: MarcelDekker, 2000:107–35.

29. Eskey CJ, Koretsky AP, Domach MM, Jain RK. 2H-nuclearmagnetic resonance imaging of tumor blood flow: spacialand temporal heterogeneity in a tissue-isolated mammaryadenocarcinoma. Cancer Res 1992;52:6010 –9.

30. Weidner N, Folkman J. Tumoral vascularity as a prognosticfactor in cancer. In: DeVita VT, Hellman S, Rosenberg SA.,editors. Important advances in oncology. Philadelphia: Lip-pincott, 1996:167–90.

31. Shockley TR, Lin K, Nagy JA, Tompkins RG, Yarmush ML,Dvorak HF. Spatial distribution of tumor-specific monoclo-nal antibodies in human melanoma xenografts. Cancer Res1992;52:367–76.

32. Lin K, Nagy JA, Xu H, Shockley TR, Yarmush ML, Dvorak HF.Compartmental distribution of tumor-specific monoclonalantibodies in human melanoma xenografts. Cancer Res1994;54:2269 –77.

33. Brown RS, Kaminski MS, Fisher SJ, Chang AE, Wahl RL.Intratumoral microdistribution of [131I]MB-1 in patientswith B-cell lymphoma following radioimmunotherapy. NuclMed Biol 1997;24:657– 63.

34. Jain RK, Baxter LT. Mechanisms of heterogenious distibu-tion of monoclonal antibpdies and other macromolecules inturmors; significance of elevated interstitial pressure [ab-stract]. Cancer Res 1988;48:7022.

35. Netti PA, Baxter LT, Boucher Y, Skalak R, Jain RK. Time-de-pendent behavior of interstitial fluid pressure in solid tumors:implications for drug delivery. Cancer Res 1995;55:5451–8.

36. Stohrer M, Boucher Y, Stangassinger M, Jain RK. Oncoticpressure in solid tumors is elevated. Cancer Res 2000;60:4251–5.


37. Boucher Y, Jain RK. Microvascular pressure is the principaldriving force for interstitial hypertension in solid tumors: im-plications for vascular collapse. Cancer Res 1992;52:5110–4.

38. Boucher Y, Leunig M, Jain RK. Tumor angiogenesis andinterstitial hypertension. Cancer Res 1996;56:4264 – 6.

39. Fujimori K, Covell DG, Fletcher JE, Weinstein JN. A model-ing analysis of monoclonal antibody percolation throughtumors: a binding-site barrier. J Nucl Med 1990;31:1191– 8.

40. Juweid M, Neumann R, Paik C, Perez-Bacete MJ, Sato J, vanOsdol W, et al. Micropharmacology of monoclonal antibod-ies in solid tumors: direct experimental evidence for a bind-ing site barrier. Cancer Res 1992;52:5144 –53.

41. Saga T, Neumann R, Heya T, Sato J, Kinuya S, Le N, et al.Targeting cancer micrometastases with monoclonal anti-bodies: a binding-site barrier. Proc Natl Acad Sci 1995;92:8999 –9003.

42. Griffith MH, Yorke ED, Wessels BW, DeNardo GL, NeacyWP. Direct dose confirmation of quantitative autoradiogra-phy with micro-TLD measurements for radioimmuno-therapy. J Nucl Med 1988;29:1795– 809.

43. Buchsbaum DJ, Brubaker PG, Hanna DE, Glatfelter AA,Terry VH, Guilbault DM, et al. Comparative binding andpreclinical localization and therapy studies with radiola-beled human chimeric and murine 17-1A monoclonal anti-bodies. Cancer Res 1990;50:993–9.

44. Buchsbaum DJ, ten Haken RK, Heidorn DB, Lawrence TS,Glatfelter AA, Terry VH, et al. A comparison of 131I-labeledmonoclonal antibody 17-1A treatment to external beam ir-radiation on the growth of LS174T human colon carcinomaxenografts. Int J Radiat Oncol Biol Phys 1990;18:1033– 41.

45. Schlom J, Molinolo A, Simpson JF, Siler K, Roselli M, HinkleG, et al. Advantage of dose fractionation in monoclonalantibody-targeted radioimmunotherapy. J Natl Cancer Inst1990;82:763–71.

46. Buchsbaum DJ, Khazaeli MB, Liu TP, Bright S. Fractionatedradioimmunotherapy of human colon carcinoma xenograftswith I-131-labeled monoclonal antibody CC49. Cancer Res1995;55(Suppl):5881–7.

47. Anderson CJ, Jones LA, Bass LA, Sherman ELC, McCarthyDW, Cutler PD, et al. Radiotherapy, toxicity and dosimetryof copper-64-TETA-octreotide in tumor-bearing rats. J NuclMed 1998;39:1944 –51.

48. Vriesendorp HM, Quadri SM, Andersson BS, Dicke KA. He-matologic side effects of radiolabeled immunoglobulin ther-apy. Exp Hematol 1996;24:1183–90.

49. Blumenthal RD, Alisauskas R, Lew W, Sharkey RM, Golden-berg DM. Myelosuppressive changes from single or repeateddoses of radioantibody therapy: effect of bone marrowtransplantation, cytokines, and hematopoietic suppression.Exp Hematol 1998;26:859 – 68.

50. O’Donoghue JA. Dosimetric aspects of radioimmuno-therapy. Tumor Targeting 1998;3:105–11.

51. Ruan S, O’Donoghue JA, Larson SM, Finn RD, Jungbluth A,Welt S, et al. Optimizing the sequence of combination ther-apy with radiolabeled antibodies and fractionated externalbeam. J Nucl Med 2000;41:1905–12.

52. Vriesendorp HM, Shao Y, Blum JE, Quadri SM, Williams JR.Fractionated intravenous administration of 90Y-labeledB72.3 GYK-DTPA immunoconjugate in beagle dogs. NuclMed Biol 1993;20:571– 8.

53. Esteban JM, Schlom J, Mornex F, et al. Radioimmuno-therapy of athymic mice bearing human colon carcinomaswith monoclonal antibody B72.3: histological and autora-

diographic study of effects on tumors and normal organs.Eur J Cancer Clin Oncol 1987;23:643–55.

54. Buchsbaum DJ, Khazaeli MB, Mayo MS, Roberson PL. Com-parison of multiple bolus and continuous injections of 131I-labeled CC49 for therapy in a colon cancer xenograft model.Clin Cancer Res 1999;5:3153s–3159s.

55. Roberson PL, Dudek S, Buchsbaum DJ. Dosimetric compar-ison of bolus and continuous injections of CC49 monoclonalantibody in a colon cancer xenograft model. Cancer 1997;80:2567–75.

56. Beaumier PL, Venkatesan P, Vanderheyden JL, Burgua WD,Kunz LL, Fritzberg AR, et al. 186-Re radioimmunotherapy ofsmall cell lung carcinoma xenografts in nude mice. CancerRes 1991;51:676 – 81.

57. Blumenthal RD, Sharkey RM, Kashi R, Goldenberg DM. Sup-pression of tumor vascular activity by radioantibody ther-apy: Implications for multiple cycle treatments. Sel CancerTher 1991;7:9 –16.

58. Blumenthal RD, Kashi R, Sharkey RM, Goldenberg DM.Quantitative and qualitative effects of experimental radio-immunotherapy on tumor vascular permeability. Int J Can-cer 1995;61:557– 66.

59. Blumenthal RD, Sharkey RM, Kashi R, Sides K, Stein R,Goldenberg DM. Changes in tumor vascular permeability inresponse to experimental radioimmunotherapy: a compar-ative study of 11 xenografts. Tumor Biol 1997;18:367–77.

60. Meredith RF, Khazaeli MB, Liu T, Plott G, Wheeler RH,Russel C, et al. Dose fractionation of radiolabeled antibodiesin patients with metastatic colon cancer. J Nucl Med 1992;33:1648 –53.

61. Meredith RF, Khazaeli MB, Plott WE, Liu T, Russell CD,Wheeler RH, et al. Effect of human immune response onrepeat courses of 131I-chimeric B72.3 antibody therapy. An-tibody Immunoconj Radiopharm 1993;6:39 – 46.

62. Press OW, Eary JF, Appelbaum FR, Martin PJ, Badger CC,Nelp WB, et al. Radiolabeled-antibody therapy of B-celllymphoma with autologous bone marrow support. N EnglJ Med 1993;329:1219 –24.

63. Epstein AL, Marder RJ, Winter JN, Stathopoulos E, Chen FM,Parker JW, et al. Two new monoclonal antibodies, Lym-1and Lym-2, reactive with human-B-lymphocytes and de-rived tumors, with immunodiagnostic and immunothera-peutic potential. Cancer Res 1987;47:830 – 40.

64. Divgi CR, O’Donoghue JA, McGagh DA, et al. Fractionatedradioimmunotherapy with I-131-labeled chimeric antibodyG250 (CG250) in patients with metastatic renal cell carci-noma [Abstract 970]. J Nucl Med 1999;40(5 suppl):217P.

65. Vriesendorp HM, Quadri SM, Wyllie CT, Lai J, Borchardt PE,Harris L, et al. Fractionated radiolabeled antiferritin therapyfor patients with recurrent Hodgkin’s disease. Clin CancerRes 1999;5:3324 –9.

66. Vriesendorp HM, Quadri SM. Update: radiolabeled immu-noglobulin therapy in patients with Hodgkin’s disease. Can-cer Biother Radiopharm 2000;15:431– 46.

67. Withers HR, Peters LJ, Thames HD, Fletcher GH. Responsesof normal tissues to dose fractionation. Cancer Bull 1982;34:182–5.

68. Dale RG. The application of the linear-quadratic dose-effectequation to fractionated and protracted radiotherapy. Br JRadiol 1985;58:515–28.

69. Shen S, DeNardo GL, Jones TD, Wilder RB, O’Donnell RT,DeNardo SJ. A preliminary cell-kinetics model of thrombo-cytopenia after radioimmunotherapy [abstract]. J Nucl Med1998;39:1223–9.


70. O’Donoghue JA. Dosimetric principles of targeted radiother-apy. In: Fritzberg A, Abrams P, editors. Radioimmuno-therapy of cancer. New York: Marcel Dekker, 2000:1–20.

71. Rao DV, Howell RW. Time-dose-fractionation in radioim-munotherapy: implications for selecting radionuclides.J Nucl Med 1993;34:1801–10.

72. Fowler JF. Radiobiological aspects of low dose rates in ra-dioimmunotherapy. Int J Radiat Oncol Biol Phys 1990;18:1261–9.

73. Knox SJ, Sutherland W, Goris ML. Correlation of tumorsensitivity to low-dose-rate irradiation with G2/M-phaseblock and other radiobiological parameters. Radiat Res1993;135:24 –31.

74. Moulder JE, Fish BL, Wilson JF. Tumor and normal tissuetolerance for fractionated low-dose-rate radiotherapy. Int JRadiat Oncol Biol Phys 1990;19:341– 8.

75. Barendsen GW. Dose fractionation, dose-rate and isoeffectrelationships for normal tissue responses. Int J Radiat OncolBiol Phys 1982;8:1981–97.

76. Withers HR, Thames HD, Peters LJ. A new isoeffect curve forchanges in dose per fraction. Radiother Oncol 1983;1:187–91.

77. Kellerer AM, Rossi HH. A generalised definition of dosimet-ric quantities. Int J Radiat Biol1990;57:859 – 64.

78. Bentzen SM, Johansen LV, Overgaard J, Thames HD. Clinicalradiobiology of squamous cell carcinoma of the oropharynx.Int J Radiat Oncol Biol Phys 1991;20:1197–206.

79. Maciejewski B, Majewski S. Dose fractionation and tumourrepopulation in radiotherapy for bladder cancer. RadiotherOncol 1991;21:163–70.

80. Fowler JF, Lindstrom MJ. Loss of local control with prolon-gation in radiotherapy. Int J Radiat Oncol Biol Phys 1992;23:457– 67.

81. Skladowski K, Law MG, Maciejewski B, Steel GG. Plannedand unplanned gaps in radiotherapy: the importance of gapposition and gap duration. Radiother Oncol 1994;30:109 –20.

82. Chappell R, Nondahl DM, Rezvani M, Fowler JF. Furtheranalysis of radiobiological parameters from the first andsecond British-Institute-of-Radiology randomized studies oflarynx pharynx radiotherapy. Int J Radiat Oncol Biol Phys1995;33:509 –18.

83. Petereit DG, Sarkaria JN, Chappell R, Fowler JF, HartmannTJ, Kinsella TJ, et al. The adverse effect of treatment prolon-gation in cervical carcinoma. Int J Radiat Oncol Biol Phys1995;32:1301–7.

84. Withers HR, Taylor JM, Maciejewski B. The hazard of accel-erated tumor clonogen repopulation during radiotherapy.Acta Oncol 1988;27:131– 46.

85. Trott KR. Cell repopulation and overall treatment time. Int JRadiat Oncol Biol Phys 1990;19:1071–5.

86. Fowler JF. The phantom of tumor treatment-continually rapidproliferation unmasked. Radiother Oncol 1991;22:156–8.

87. Bentzen SM, Saunders M, Dische S. Repair halftimes esti-mated from observations of treatment-related morbidity af-ter CHART or conventional radiotherapy in head and neckcancer. Radiother Oncol 1999;53:219 –26.

88. O’Donoghue JA, Sgouros G, Divgi CR, Humm JL. Single-doseversus fractionated radioimmunotherapy: Model comparisonsfor uniform tumor dosimetry. J Nucl Med 2000;41:538–47.

89. Sun LQ, Vogel CA, Mirimanoff RO, et al. Timing effects ofcombined radioimmunotherapy and radiotherapy on a hu-man solid tumor in nude mice. Cancer Res 1997;57:1312–9.

90. Whitmore GF, Till JE, Gulyas S. Radiation-induced mitoticdelay in L-cells. Radiat Res 1967;30:155–71.

91. DeWeese TL, Shipman JM, Dillehay LE, Nelson WG. Sensi-tivity of human prostatic carcinoma cell lines to low doserate radiation exposure. J Urol 1998;159:591– 8.

92. Williams JA, Williams JR, Yuan X, Dillehay LE. Protractedexposure radiosensitization of experimental human malig-nant glioma. Radiat Oncol Invest 1998;6:255– 63.

93. Oosterwijk E, Bander NH, Divgi CR, Welt S, Wakka JC, FinnRD, et al. Antibody localization in human renal cell carci-noma: a phase I study of monoclonal antibody G250. J ClinOncol 1993;11:738 –50.

94. Finstad CL, LLoyd KO, Federici MG, Divgi C, VenkatramanE, Barakat RR, et al. Distribution of radiolabeled monoclonalantibody MX35 F(ab�)2 in tissue samples by storage phos-phor screen image analysis: evaluation of antibody localiza-tion to micrometastatic disease in epthelial ovarian cancer.Clin Cancer Res 1997;3:1433– 42.

95. Steffens MG, Boerman OC, Oyen WJ, Kniest PH, Witjes JA,Oosterhof GO, et al. Intratumoral distribution of two con-secutive injections of chimeric antibody G250 in primaryrenal cell carcinoma: implications for fractionated dose ra-dioimmunotherapy. Cancer Res 1999;59:1615–9.

96. DeNardo GL, DeNardo SJ. Overview of obstacles and oppor-tunities for radioimmunotherapy of cancer. In: GoldenbergDM, editor. Cancer therapy with radiolabeled antibodies.Boca Raton, Florida: CRC, 1994:141–54.

97. DeNardo GL, DeNardo SJ. Perspectives on the future ofradioimmunodiagnosis and radioimmunotherapy of cancer.In: Burchiel SW, Rhodes BA., editors. Radioimmunoimagingand radioimmunotherapy. New York: Elsevier, 1983:41– 62.

98. Jain RK. Haemodynamic and transport barriers to the treat-ment of solid tumors. Int J Radiat Biol 1991;60:85–100.

99. Jain RK. Barriers to drug delivery in solid tumors. Sci Am1994;271:58 – 65.

100. Langmuir VK, McGann JK, Buchegger F, Sutherland RM. Theeffect of antigen concentration, antibody valency and size,and tumor architecture on antibody binding in multicellspheroids. Nucl Med Biol 1991;18:753– 64.

101. Otte A, Herrmann R, Heppeler A, Behe M, Jermann E, PowellP, et al. Yttrium-90 DOTATOC: first clinical results. EurJ Nucl Med 1999;26:1439 – 47.

102. Sinzinger H, Granegger S, Meghdadi S, Kratzik C. Pain pal-liation with 153Sm-EDTMP using the Vienna protocol [ab-stract]. Eur J Nucl Med 2000;27:965.

103. Iwahashi M, Milenic DE, Padlan EA, Bei R, Schlom J, KashmiriSVS. CDR substitutions of a humanized monoclonal antibody(CC49): contributions of individual CDRs to antigen bindingand immunogenicity. Mol Immunol 1999;36:1079–91.

104. Tamura M, Milenic DE, Iwahashi M, Padlan EA, Schlom J,Kashmiri SVS. Structural correlates of an anticarcinoma Ab:identification of specificity-determining residues (SDRs) anddevelopment of a minimally immunogenic antibody variant byretention of SDRs only. J Immunol 2000;16:1432–41.

105. Yokota T, Milenic D, Whitlow M, Schlom J. Rapid tumorpenetration of a single-chain Fv and comparison with otherimmunoglobulin forms. Cancer Res 1992;52:3402– 8.

106. Nias AHW. Fractionated radiotherapy. In: Nias AHW, editor.An Introduction to Radiobiology. Chichester, England: JohnWiley & Sons, 1990:256 –76.

107. Meyn RE. Apoptosis and response to radiation: implicationsfor radiation therapy. Oncology 1997;11:349 –56.

108. Buchegger F, Vacca A, Carrel S, Schreyer M, Mach JP. Ra-dioimmunotherapy of human colon carcinoma by 131I-labelled monoclonal anti-CEA antibodies in a nude mousemodel. Int J Cancer 1988;41:127–34.


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Rationales, evidence, and design considerations for fractionated radioimmunotherapy