THE UNIVERSITY OF NEW MEXICOCOLLEGE OF PHARMACY

ALBUQUERQUE, NEW MEXICO

The University of New Mexico

Correspondence Continuing Education Coursesfor

Nuclear Pharmacists and Nuclear Medicine Professionals

VOLUME III, NUMBER 6

Pharmacologic Enhancement of Radioimmunotherapy

w:Kenneth T. Cheng, Ph. D., 13,C.N.P.

Co-sponsored by: mpi@nrmacy services incan ~mer$ham company

m The University of New Mexico College of Pharmacy is approved by the American Council on Pharrnaccutical

Education as a provider of continuing pharmaceutical education. Program No. 180-039-93-006. 2.5 Contact

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Edtior

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University of New Mexico

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University of New Mexico

While the advice and information in this publication are believed to bc true and accurate at press time, neither the author(s) northe editor nor the publisher can accept any legal responsibility for any errors or omissions that may bc mtide. The puhlishcrmakes no warranty, express or implied, with respect to the material contained herein.

Copyright 1994Universityof New Mexico

PharmacyContinuingEducati(]nAlbuquerque,New Mexico

PHARMACOLOGIC ENHANCEMENT OF RADIOIMMUNOT~~Y

STATEMENT OF OBJECTIVES

The goal of this correspondence continuing ducation lesson is to increase the reader’s knowledge in the use ofpharmacologic agents to enhance the efficacy of RADIOIMMU NOTHERAPY (RIT). This lesson is intended fornuclear pharmacists and nuclear mdicine professionals who have an interest in the use of pharmacologic agents toincrease the therapeutic efficacy of radiolabeled antibodies in treating cancer patients.

Upon successfti completion of this coufie, the reader shotid be able to:

1.

2.

3.

4.

5.

0 6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

●

List the four major causes of physiologic difference between normal tissue perfusion and tumor perfusion.

Explain the difficulty involved with the process of radiolabeled antibody crossing the interstitial fluid space.

List three pharmacologic agents which can increase tumor blood tlow and explain their mechanisms of action.

List two pharmacologic agents which can modulate microregional heterogeneity in tumor blood flow andexplain their mechanisms of action.

Name the pharmacologic agent which can enhance tumor antigen expression.

Explain the rationale of using pharmacologic agents to sensitize tumor to radiolabeled antibodies.

List the five classes of agents which can sensitize tumor cells to radiation.

Explain the importance of oxygen in radiation therapy.

Describe the mechanism of action of hypoxic cell sensitizer.

Explain the mechanism of radiation sensitization of thiol-depleting agents.

Explain the mechanism of radiation sensitization of potentially lethal damage @LD) repair inhibitors.

Explain the mechanism of radiation sensitization of halogenated pyrimidines.

Describe the mechanism of action of Taxol and how it sensitizes tumor cells.

Describe the mechanism of radiation sensitization of 5-fluorouracil (5-FU).

Describe the mechanism of radiation sensitization of hydroxyurea.

Describe the mechanism of radiation sensitization of cisplatin.

1

I.

11.

III.

IV.

v.

COURSE OUTLINE

INTRODUCTION

TUMOR PATHOPHYSIOLOGY

A. Tumor VasculatureB. Tumor Perfusionc. The Interstitial Fluid Space BarrierD. Tumor Antigen Expression

PHARMACOLOGIC ENHANCEMENT OFRADIOLABELED ANTIBODY LOCALIZA-TION IN THE TUMOR

A. Pharmacologic Agents Which ElicitIndirect Effects on Tumor Blood Flow

1. Angiotensin 112. Beta-adrenergic Blocking

Agents3. Anesthetics

B. Pharmacologic Agents WhichModulate Microregional Heterogeneityin Tumor Blood Flow

1. Nicotinamide2. Calcium Channel Blockers

c, Agents Which Enhance Tumor AntigenExpression

PHARMACOLOGIC ENHANCEMENT OFTUMOR SENSITIVITY TO THERADIATION DOSE DELIVERED BYTHREE RADIOLABELED ANTIBODIES

A. Hypoxic Cell SensitizersB. Thiol-Depleting Agentsc. PLD Repair InhibitorD. Halogenatd PyrimidinesE. Chemotherapeutic Agents

1.2.3.4.

SUMMARY

Paclitaxel (Taxol)5-Fluorouracil (5-FU)HydroxyureaCisplatin

PHARMACOLOGIC E~ANCEMENT OFRADIOIMMUNOTHERA PY

By:

Kenneth T. Cheng, Ph. D., B.C.N.P.Associate Professor

Coordinator of Radiologic PharmacyMedical University of South Carolina

Hospital Pharmacy Practice andAdministration/Radiology

171 Ashley AvenueCharleston, SC 29425

INTRODUCTION

Targeted cancer therapy using radiolabeledantibodies has been of great interest in nuclearmdicine for many years, and the production (Ifmonoclinal antibodies (MoAhs) using hybridomatechnology has led to a great surge of research in thisfield (1), The basic theory behind radioimmunotherapy(RIT) is that radiolabeled antibodies can selectivelyseek out antigen-positive cancer cells in vivo anddeliver a therapeutic dose of radiation to the wholetumor. As more experience has been gained from bothobasic and clinical research studies, it has become clearthat the in vivo behavior of radiolabeled MoAbs is verycomplex (2). The fact is that the development ofradiolabeled MoAbs for tumor imaging and therapy hasbeen hindered by complex problems associated withtumor pathophysiology and related radiolabeledantibody biodistribution (3).

The successful treatment of tumor with radiolabeledantibody depends largely on the achievement of a hightarget (tumor) to nontarget (normal tissue) ratio. Inorder to be effective, radiolabeled antibodies have todestroy tumor cells without harming the surroundingnormal tissues. Thus, the target-to-nontarget ratiomust be particularly high when dealing with normaltissue (non-target tissue) that include radiosensitiveorgans such as bone marrow. It is only by achievinga very high ratio that the lethal consequences ofradiation can be avoided. Unfortunately, such hightarget-to-nontarget ratios have been difficult to achievewith radiolabeld antibodies because of variouscomplex and unfavorable in vivo conditions related totumor pathophysiology,

The hiodistribution of radiolabeled MoAbs maydiffer from patient to patient depending on many @

factors (4). For example, one of these fdctors is thetumor mass. If the tumor mass is large and the anti-

2

tumor MoAtJ has some affinity for normal tissues, thenthe amount of radiolabeled antibody localized innormal tissues will vary inversely with tho tumor mass.Other obvious fidctors are the tumor vascularit y and

* ascular permeability. Uptake of radiolaheled antibodyby tumor is not only dependent on vascular supply butalso on vascular permeability. Thus, very small tumorsmay have relatively poor uptake of radiolabeledantibodies because of their poor vascular supply whilelarger tumors may show significantly higher uptake ofradiolabeled antibodies due to their increased vascularpermeability.

All of these complex tumor pathophysiologic fdctorshave contributed significantly to the inconsistent resultsof current clinical RIT studies. Since tumorpathophysiology is not controllable in patients, even aperfect radiolabeled antibody is therefore not likely toproduce consistent therapeutic efficacy in all patients.

Among different methods that have been studied toincrease the therapeutic efficacy of radiolabeledantibodies, one potential area that deserves ourattention is the utilization of pharmacologic agents toenhance RIT efficacy. Presently, there are two majorareas that are under active investigation. One area isthe utilization of pharmacologic agents to enhancelocalization of radiolabeled antibodies in tumors (5).For example, radiolabeled antibody delivery to tumor

●cells may be improved by exploiting the differencesbetween tumor blood vessels and normal blood vessels.The expression of cellular antigen may also beimproved by pharmacologic means to enhance thetumor binding of radioiabeled antibody.

Another area is the use of pharmacologic agents toenhance radiosensitivity of tumor cells to the radiationdose deliverd by RIT (6). Pharmacologic agents canrender tumor cells more sensitive to radiation thannormal cells based on the growth and cell cycle activitydifferences between tumor cells and normal cells.

TUMOR PATHOPHYSIOLOGY

Tumor VasculatureTumor physiology is very different from that of

normal tissue. Normal tissues are governed by naturalphysiologic laws and orders, but tumor is a place ofchaos. As tumor cells multiply to form a solid massthey become increasingly dependent on thedevelopment of a vascular system (3). The proximityof cells to the functional vasculature is very importantbecause it will determine their supply of oxygen andnutrients. During the progressive growth of a tumor,

● the host vessels are replaced by new nutrient vesselsunder stimulation by angiogenetic fdctors. Thesenmplastic vessels may develop anatomically, but theydo not retain normal physiological function.

Tumor PerfusionIn general, normal tissues have a good blood flow

in relation to their needs (3). On the otier hand, bloodflow to tumors is poor. Studies in animal tumors andhuman malignancies have shown the presence of low0, levels, acidic pH, and overt necrosis (7). It isevident that the vascular system does not adequatelysupply all the cells within a tumor. This vascularinsufficiency is also more pronounced in larger tumormasses for which response to RIT is often poor. Adeficiency in functional vasculature (and thus bloodsupply for antibody delivery) plays an important rolein RIT treatment failures.

Despite the growth of new n~)plastic vessels, therecan be a rapid and drastic rtiuction in the blood flowas the tumor mass increases. One reason for thereduction of flow is that the number of vessels in thetumor decreases proportionate y as the tumor enlarges.This is caused by the fact that tumor cells divideseveral times faster than endothclial cells. Thisdecrease in the number of tumor capillaries may varyfrom one tumor type to another and even within thesame tumor type.

In addition, as tumors enlarge, the actual diameterof the tumor capillaries increases dramatically. As aresu]t, there is a decredse in the total vascular cross-sectional area of the tumor and an incredse in theefferent flow resistance. Much of this blood is staticin tumor vessels, and does not exchange well withblood from the systemic vascular compartment. Underthese circumstances, some areas of the tumor will turnout to be fairly well perfused and others virtuallynonperfused. This poor profusion is especiallyprominent in the central portion of the tumor.

Arteriovenous shunts also occur in tumors (3). Theentering blood is shunted back into the systemiccirculation before it has a chance to go through acapillary bed. In this situation, the radiolabeledantibodies in the shuntti blood will not exchange withthe tumor cells during the pass.

Blood tlow is also affected by pH. The pH in atumor is relatively low ~acidic), causing the capillariesto lose their flexibility. There is also high pressurewithin tumors duc to the lack of lymphatic. The thinwalls of the tumor capillaries often collapse under thishigh pressure which further decreases perfusion (3).

Poorly perfusd tumor regions are much moreresistant to RIT than well-perfused regions. It is wellknown that hypoxic cells in poorly perfused areas arethree times more resistant to radiation treatment thanwell oxygenated ones (8). Such regions are alsopoorly accessible by radiolabeled antibodies.

The Interstitial Fluid Space BarritirOnce the radiolabeld antibody leaves the capillary,

3

it is met by the interstitial tluid surrounding the cells(3,4). At a few microns distance, two major forcesmove the antibody molecule to the cells. me first isdiffusion, and the second is convection. Convectionrefers to movement of the antibodies along withmovement of interstitial fluid. The effect of these twoforces on the molecule will depend on molecular size.Small molecules are highly affected by diffusion andpenetrate the interstitium of tissues primarily by thatmechanism. As the molecules get larger, diffusionbecomes less important, and convection becomes thedominant moving force. Antibodies have a largemolecular size and thus depend more on convectionthan diffusion. For the vast majority of theradiolabeled antibodies, penetration will be poor.Since tumors have an interstitial fluid space two to fourtimes greater than normal tissues, it is also a biggerobstacle to move the radiolabeled antibody from acapillary to a tumor cell.

In animal tumor models, the majority of capillariesin the tumor are found at the surface in the area of thehighly viable tissue. As expected, studies in humantumor models have shown that there is a great decreasein radioactivity in the interior of the tumor due to thepoor penetration power of the radiolabeled antibody.

Tumor Antigen Expr~sionA tumor antigen is an antigen expressed as a

consequence of a malignant transformation event (9).Tumor-specific antigens are antigens that are uniquelyexpressed by tumors and not by normal tissues. Cellsurface antigens are ideal targets for systemicallycirculating radiolabeled antibodies. Some antigens aremodulated and released from the cell surface as aconsequence of antibody binding. Many other cellsurface antigens are not affectd. There is someevidence that antigen concentration may vary withdifferent phases of the cell cycle. However, antigenproduction remains basically a mystery. In cell culture,the production of antigen can be very fast. Regardlessof how it is produced, the expression of antigen ontumor cells is critically important to the binding ofantibody.

PHARMACOLOGIC ENHANCEMENT OFRADIOLABELED ANTIBODY LOCALIZATIONIN THE TUMOR

Awareness of the unfavorable consequences ofvarious aspects of tumor pathophysiology has led toinvestigations to study methods of improving in vivolocal antibody concentration in tumors (10). In oneapproach to facilitate the localization of radiolabeledantibodies in tumors, a number of pharmacologicagents have been identified which produce selective

modulation of tumor blood flow. Studies havesuggested that neoplastic vessels lack the ability toreact to vasoactive agents, However, vasoactive agentscan be manipulated to exert an indirect influence toincrease the tumor blood flow. ●

In another approach, pharmacologic agents are usedto modulate the tumor surface antigens for radiolabeledantibody binding. Heterogeneity in the expression oftumor-associated antigens, as defined by the binding ofMoAbs, is a characteristic common to most humancarcinoma cell populations. Making a human tumorcell population more homogenwus for the expressionof an antigen could enhance antibody binding and localtumor concentration. Human natural and recombinantinterferon can change the surface antigenic phenotypeand increase the amount of tumor antigen expressed(11). Thus, concomitant administration of interferonand radiolabeled antibody may effectively enhance thein vivo tumor binding of radiolabeled antibody. Thiswill lead to increased antibody localization in the tumordue to enhanced tumor antigen expression.

Pharmacologic Agents which Elicit Indirect EffectsOn Tumor Blood Flow

The effects of vasoactive drugs on tumor blood flowhave been investigated extensively over the past 40years. Many agents have been identified whichproduce transient (minutes to hours) changes in tumorblood flow. These vasoactive drugs increase the ratio oof tumor blood tlow to surrounding normal tissueblood flow as a result of differences in vascularstructure and function between normal and tumor tissue(12).

Nwplastic vessels do not retain normal

physiological function. Regulation of blood flow

velocity, direction, pressure, and capacity is lost (10).

In addition, tumor tissue lacks vascular smooth muscleand possesses many areas in which interstitial tluidpressure is elevated. Tumor vessels are therefore notvery responsive to the pharmacologic effects ofvasoactive drugs directly. Instead, the influence ofvasoactive drugs on tumor circulation is most likely tobe mediated indirectly via effects on flow insurrounding normal tissue, or on systemic bloodpressure. Hence, vasodilators (such as papaverine)have not been found to be effective in increasing tumorblood flOW.In fact, some vasodilators (such ashydralazine) have been found to decrease tumor bloodtlow by selectively dilating surrounding normal tissuevessels and shunting blood away from the tumor (13).Conversely, vasoconstrictive agents hold much promise—as pharmacologic agents of choice in increasing tumorblood perfusion. o

Angiotensin II. Vasoconstrictive agents have beentested, with varying success, to determine their

4

capacity to inureasc blood tlow to tumors. Adrenaline,noradrenalirle, oxytocin and vasopressin have producedconflicting results in different animal models. This is

●probably a result of the presence of different receptorpopulations in different organ systems. Amongvasoconstrictors, angiotensin 11 (angiotensin amide,Hypertensin’rM), a vasoconstrictor peptide, has beenshown to he most promising and appears toconsistent y increase blood flow in experimental tumors(14-15). It increases the peripheral resistance, mainlyin cutaneous, splanchnic, and renal blood vessels, andacts both directly and via the sympathetic nervoussystem. The increased blood pressure is accompaniedby a reflex reduction in heart rate, and cardiac outputmay also be reduced.

Suzuki, et al. (16) described an approximately six-fold selective increase in blood flow to subcutaneoustumors without increasing blood flow to normal tissue.Angiotensin 11has been investigated as an adjunctiveagent in renal pharmacoangiography to reduce bloodflow in normal tissue, while allowing pooling ofcontrast medium in tumor vessels (17). Specifically,the effect of angiotensin II and tolazoline (avasodilator) was compared in 18 patients with bone andsoft-tissue tumors. Angiotensin was found to be thedrug of choice in increasing diagnostic information inangiographic procedures. Ten to 15 ug of angiotensin

●II appeared to be a convenient dose in the axillary,iliac, and femoral arteries. In the same study,tolazoline was found to be totally ineffective.

Mechanistically, angiotensin II causesvasoconstriction by binding to specific saturablereceptors present in vascular smooth muscle. Thesereceptors are primarily distributed in the precapillarysphincter arterioles, and their quantity is tissue-dependent. There are fewer arterioles incorporatedinto the tumor mass than are present in the surrounding

normal tissue. Angiotensin II acts more peripherally in

the vascular bed than other vasoconstrictors and,therefore, should have less effect on the arteriesfeeding the tumor while still providing a net decreasein flow to the smaller vessels of the normal tissue.The increase in tumor blood tlow following angiotensin11is most likely to result from the elevation in systemicblood pressure which it induces in the host animal.The absence of vascular smooth muscle and theoccurrence of high interstitial pressure in tumorsresults in increased perfusion pressure. Consequently,the infusion of angiotensin 11 will cause significantreduction in normal tissue blood flow, whereas tumorblood flow will either be decreased to a significantly

● lesser degree or actually increased if the arterialpressure is sufficient]y elevated. In all cases, the ratioof blood flow in the tumor versus that in the normaltissue will be significant y increased.

Although no study has been performed withradiolabeled antibody, Burton, et al. (15) have shownthat intravenous (IV) infusion of angiotensin J1increased the number of radioactive microspheregaining arterial access to the central portions ofexperimental hepatic tumors in rats and rabbits. Interms of radiolabeld antibody therapy, the resultswould indicate a subs~dntially enhanced dose reachingtumor tissue after angiotensin 11 infusion, whilerelatively sparing the surrounding normal tissues.

Beta-adrenergic blocking agetis. Bomber andcolleagues (18) have shown that the injection ofpropranolol HC1 (InderalTM)increased the uptake ofGa-67 in tumor relative to normal tissues. In thisstudy, the effect of propranolol hydrochloride on theblood perfusion of a mouse sarcoma and other tissueswas studied. The maximum increase in relative tumorperfusion (2X control) occurred 15 minutes after IVadministration of 10 mg/kg propranolol hydrochloride.The propranolol hydrochloride was also given 10mg/kg 10 minutes before administering Ga-67 citrate.Four hours after Ga-67 administration, the tumor-to-blood ratio increased from 1.16 in controls to 3.41 intest animals. Propranolol hydrochloride blocks thebeta adrenoreceptors to produce a decrease in heart rateand blood pressure. The increased tumor perfusion isthought to be due to the change in the relativeperfusion in muscle and of other tissues as a result ofthe decreased cardiac output with compensatorysympathomirnetic vasoconstriction to maintain bloodpressure. Tumor blood vessels lack smooth muscleand do not respond to the peripheral vasoconstrictionfeedback.

Gther nonselective and cardioselective heta-adrenergic blocking agents at therapeutic doses havealso been shown to increase twofold to fourfold tumor-to-blood and tumor-to-liver ratios of 1-125 labeled anti-Ly-2. 1 antibody uptake in mouse E3 thymoma (19).The increased ratio was caused by a decreasedaccumulation of I-125 labeled antibody in normalorgans (resulting from a reduced cardiac output) and arelative increased in tumor uptake. Using the sametumor animal model, propranolol, pindolol, andoxprenolol were also found to increase the antitumorefficacy of Ida-anti-Ly-2. 1 conjugate. By contrast,prazosin HCI ~alpha-adrenergic blocking agent) andcyclandelate (Cyclospasmo]n, peripheral vasodilator)did not enhance the tumor perfusion and antibodytarget-to-nontarget ratio.

Unlike angiotensin II, not all studies with betablockers have been positive. A study by Pimm (20)using nude mice with human tumor xenograftsindicated that both propranolol and pindolol might notgive highly effective or consistent increases in tumorblood flow, and therefore might not be very effective

5

in enhancing tumor localization of MoAbs for tumortherapy.

Anesthetics. Zanelli and colleagues(21) have shownthat the anesthetic agents pentobarbitone sodium andurethane increased the relative tumor perfusion inexperimental tumor in mice through a decrease inblood flow to muscle. These agents appeared to causea decrease in cardiac output, with compensatorysyrnpathomimetic vasoconstriction taking place in aneffort to maintain the blood pressure. Pentobarbitonewas found to increase the relative blood perfusion bya factor of 1.3 to 2.0 in tumors but muscle perfusionfell to 0.3 to 0.5 that of controls. The effects ofurethane were found to be smaller and dose dependent.Despite this positive report, no other studies haverepeated or confirmed the results of this study.

Pharmacologic Agents which Modulate Micro-regional Heterogeneity in Tumor Blood Flow

In tumors, hypoxia can result from transientalterations in blood flow (10). The mechanism for suchblood flow changes has not yet been clearly elucidated.Temporary plugging of blood vessels by circulatingcancer cells or white blood cells may play a role. Inaddition, if the interstitial tumor pressure exceeded theintravascular pressure in some tumor microregions,blood flow stasis would result. These cessations inmicroregional flow have been shown to involve up to10% of tumor vessels at any one time, and to last forat least several minutes. Moraver, this can affectseveral vessels in the same microregion, resulting inrelatively large patches of tissue hypoxia. Thus it hasbecome evident that agents which could prevent thetransient microregional alterations in tumor blood flow,could provide another class of agents with a defineduse in RIT.

Two classes of pharmacologic agents have beenfound to be active in reducing such subtle blood flowchanges (5): the calcium channel blockers andnicotinamide. These agents have been shown toimprove the response of tumors to radiation ifadministered prior to therapy and to provide smallimprovements in tumor blood flow. Studies have alsoshown that enhanced radiation response can beachieved by combining nicotinamide with therapiessuch as Fluosol DA and carbogen which improve theoxygen-carrying capacity of blood. Such an approachimproves the oxygenation status of hypoxic cellsresulting from either blood flow fluctuations (i.e.,perfusion limitations) or location relative to vasculature(diffusion limitations).

Nicotin.umtie. Chaplin and Trotter (22) havedemonstrated that prior treatment with nicotinamideprevents the opening and closing of blood vesselsknown to occur in the SCCVII tumor implanted in

C~H/He mice. Nicotinamide was found to be moreeffective in reducing tumor acute hypoxia thanflunarazine (a calcium channel blocker) and Fluosol-DA (20%). It has little or no significantradiosensitizing effect on hypoxic cells in vitro but●does modify tumor blood flow. The results areconsistent with other studies that nicotinarnide reducesthe dynamic changes in microregional tumor perfusionand, as a consequence,hypoxia in the tumor.

Nicotinamide alonevasoactive properties,which nicotinamide is

decreases the amount of acute

is not known to have anyalthough nicotinic acid, fora precursor, does have some

weak vasodilator effects. -Also of interest is the factthat nicotinamide, at a dose of 500 mg/kg, has beenreported to rduce mortality in endotoxin-inducedshock in rats. Lethality in endotoxin-induced shock isdue, at least in part, to ischemia in normal tissues.This phenomenon suggests that nicotinamide may havethe ability to reduce transient ischemia in tumor tissue.

Calcium Chunnel Blockers. Calcium channelblockers are a diverse group of compounds with thegeneral property of uncoupling calcium-mediatedcellular processes by blocking the uptidke of this ionthrough the calcium channels of thti plasma membrane(23). They are particularly effective on vascularsmooth muscle, and for this reason have foundwidespread use in the treatment of cardiovasculardisease. The calcium antagonists may be divided into @a number of subgroups, according to their chemicalstructure and preferential site of activity. They mayact upon the tumor vasculature in a mannerindependent of the systemic circulation.

Four different calcium channel mtagonists werestudied by Wood and colleagues (24) in SCCVII/Sttumor implanted in C~H/Km mice. Verapamil,diltiazem, nifedipine, and flunarizine, representing fourdifferent main subgroups of calcium blockers, werestudied. Verapamil is a phenylalkylamine andprimarily targets the cardiac conduction mechanism andcoronary blood vessels, It has activity at the largesystemic blood vessels at high doses. Diltiazem is abenzothiapine and has activity primarily on coronaryvessels. Nifdipine (1,4-dihydropyridine) targets thelarge systemic blood vessels. Flunarizine, adiphenylpiperazine, is active on peripheral vessels andon blood cellular components at concentrations whichhave little cardiac effect. All of these agents havedifferent effects on tumor perfusion over a large doserange.

Among these four agents, flunarizine was found tobe most effective in increasing tumor blood flow.Flunarizine sensitized tumors to external irradiation o

over the dose range 0.005-500 mg/kg I,P. Increases(20% to 30%) in tumor perfusion were seen at doses

6

of 0.05-5 mg/kg. Verapamil increased tumorradioresistance at doses of 20 mg/kg and above. Itactually reduced tumor perfusion at 50 mg/kg. Below

●10 mg/kg, verapamil sensitized tumors to externalirradiation, with little or no increase in tumor

●

●

perfusion. Niftiipine at 10 mg/kg and above producedvery radioresistant tumors, with correspondingly largereductions in turnor perfusion. At doses below 0.5mg/kg sensitization was seen, but no increased tumorperfusion. Diltiazem at 50 mg/kg also increased tumorradioresistance, with a reduction in tumor perfusion.At lower doses, it sensitizti tumors to irradiation, withsmall increases in tumor perfusion. Between 0.05 to5 mg/kg, flunarizine increased tumor perfusion by30%. All of these agents have activity over a largedose range. The similarity between the tumor radiationresponses among these four calcium blockers suggeststhat they may be acting upon the tumor vasculature ina manner independent of the systemic circulation.They may prevent the rigidification of red blood cellsand permit them to pass through more tortuous bloodvessels within the tumor, This phenomenon mayexplain why verapamil and dilti~em did not increasetumor blood flow yet sensitized tumors to externalirradiation.

Flunarizine shows the best potential to be usedclinically sinco it gives the flattest dose-response curvewithin the clinical range. Flunarizine is active onperipheral vessels and on blood cellular components atconcentrations which have little cardiac effect. Theincrease in tumor perfusion is probably mediatedthrough prevention of the naturally-occurringconstriction of small vessels in or around the tumor,and subsequently rduces the acutely hypoxic cellpopulation.

Agents Which Enhance Tumor Antigen ExpressionThe problems of insutiicient local concentration of

radiolabeled MoAbs in tumor may be partiallycompensated by enhancing the expression of tumor-associatd antigens. Heterogeneity of antigenexpression is characteristic of human carcinoma cellpopulations. This phenomenon has importantimplications for the application of radiolaheledantibodies for therapy. Rendering a human tumor cellpopulation more homogeneous for the expression of anantigen could result in an augmentation of radiolabeledantibody binding.

Human natural and recombinant interferon canalter the surface antigenic phenotype of various humantarget cells in vilro. They can modulate the level ofclass I and class II histocompability antigens, andcertain tumor-associated antigens (24-25). Both thetype I and type II interferon (IFNs) can induce oramplify expression of the major histocompability

complex (MHC) antigens. Human immune (gamma)IFN has been shown to be a potent inducer of de novoexpression of the class 11 human leukocyte antigens(HLA) and can also ampiify expression of the class IHLA surface glycoproteins. The type I leukocyte(alpha) and fibroblast (beta) human lFNs (Hu-IFNs)play a more restrictive regulatory role in themodulation of these antigens.

Hu-IFNs are potent regulators of antigenicexpression on the established human carcinoma celllines. Studies have shown that Hu-IFN treatment couldalso augment tumor antigen content, amplifying thesignal emitted by radiolabeled MoAbs localized to thetumor site. Among all Hu-lFNs tested, Hu-IFN-gamma appeard to be most potent in vitro. Animportant difference in the abilities of Hu-IFNs toaugment tumor antigen and normal HLA expressionseems to be the requirement for constitutive geneexpression for an Hu-IFN to amplify the surfaceantigen. Hu-IFN-gamma is an inducer of geneexpression for the class 11MHC antigens. In addition,Hu-IFN-gamma seems to amplify the expression ofactively transcribed genes, resulting in theaugmentation of the gene product, the tumor antigen.Studies have shown that the Hu-IFN treatment of avariety of human cancer cells resulted in an increase inantigen density per cell as well as increase in theproportion of the cell population that was antigenpositive.

Greiner and colleagues (26) have shown thatadministration of lFN-gamma effectively increased theamount of antigen expressed in patients with eithergastrointestinal or ovarian carcinoma. This enhancedantigen expression has demonstrated a subsequentlyincreased tumor localization of 1-131 B72. 3 MoAb.

PHARMACOLOGIC ENHANCEMENT OFTUMOR SENSITIVITY TO THE RAD1ATIONDOSE DELIVERED BY THE RADIOLABELEDANTIBODY

Another way to increase therapeutic efficacy of RITwould be making the tumor more sensitive to theradiation dose delivered by the antibody. There arecurrently five classes of pharmacologic agents that canenhance tumor sensitivity to radiation (6).

Hypoxic Cell SensitizersThe first class of agents consists of compounds

which sensitize hypoxic cancer cells to radiation. Theapplication of these agents was started approximately30 years ago with the discovery of hypoxic cells withintumors (27). ltis now recognized that 1%-30% cellsin rapidly growing tumors could become deprived ofoxygen through the abnormality of the tumor blood

7

supply and the rapidity of tumor cell growth versuscapillary proliferation. Hypoxic cells are very resistantto radiation therapy.

A therapeutic dose of ionizing radiation is ofsufficient energy to eject electrons from target tissues(28). After exposure to radiation, these free electronsinteract with various intracellular molecules to formmany very short-lived free radicals. These unstableand highly reactive radicals are formed in water, DNA,and other cellular molecules. When present, oxygencan interact with the DNA radical and form a morepermanent DNA-peroxy radical (Figure 1). TheseDNA-peroxy radicals can decay and produce DNAlesions by fragmenting DNA. The mechanismresponsible for cell death is often the presence ofcritical lesions of double-strand breaks in DNA inducedby either radiation direct]y or DNA-radicals indirectl y.A therapeutic dose of ionizing radiation can produce asufficient number of these lesions in DNA leading tocell death.

rate-k[tbl]PRO~ECTION

DNA ‘~ r ‘N’— DNA.

\\

\

~ DNA-00.o,

SENSITIZATION

~ ~NA-sensitizer.

Figure 1. The interactwn between oxygen and DNA radical(DNA) produced by radiation. In the presence of thwk, the DNAradkal can be chemica~ restored. In the absence of oxygen, ahypoxic cell sensitker can bind to the DNA radical to produceDNA kswns,

The presence of oxygen to produce peroxy-DNAradicals is extremely important for cell killing. In theabsence of oxygen (0.01%), it requires between twoand three times the radiation dose to produce the samefraction of cell killing as obtained in ambient air. Inthmry, an oxygen-mimetic hypoxic-cell sensitizer cantake the place of oxygen, leading to a stable DNAlesion. Since oxygen is much more reactive than thesensitizer, the sensitizer will not increase the damageformal in cells in the presence of oxygen. Instead, itwill sensitize cells that are lacking oxygen, Theprocess in which DNA radicals are stabilized by eitheroxygen or an oxygen-mimetic sensitizer @ypoxic cellsensitizer) is referred to as sensitization. The ratio ofradiation dose required to produce a certain fraction ofcell killing under hypoxia compared with dose required

to kill the same fraction in air is the oxygenenhancement ratio (OER). This ratio is known as thesensitizer enhancement ratio (SER) in the presence ofhypoxic cell sensitizers.

Many compounds are able to enhance the radiation ●response of hypoxic cells in virro. Most of thesebelong to the electron-afflnic class (29) because thereis a direct relationship between radiation sensitizingability and the electron affinities of the compound.However, only a few of these agents are active in vivo

due to either toxicity limitations or to poor tumorpenetration. The hypoxic sensitizers that have receivedthe most attention are the 2-nitroimidazole compounds.As with oxygen, the sensitizer must be present at thetime of irradiation to achieve its oxygen-mimeticsensitization. The efficacy of a 2-nitroimidazolesensitizer increases with increasing drug dose.

One compound that has been introduced for clinicaltesting is misonidazole. The dose-limiting toxicity ofthis drug is peripheral sensory neuropathy. Half of thepatients in one clinical trial had this toxicity at acumulative dose of 10 to 12 g/m2. Misonidazo]eproduces an SER of 1.5 at the dose of 2 g/m2. At thisdose level, only five to six doses of misonidmole couldbe administered. Given the toxicity limitations, mostof the clinical trials did not show an advantage usingmisonid~ole. The most encouraging results have beenreported from a head and neck cancer trial thatdemonstrated a statistically significant increase insurvival for men with pharyngeal squamous cellcancer.

A series of less lipid-soluble misonidazole analogsand nonnitrosensitizers have been produced in the past10-15 years. Some of their structures are presented inFigure 2. They are as potent as misonidazole but lessneurotoxic. There are present] y at least 20 differentgroups worldwide attempting to develop hypoxic cellradiosensitizers that are potent enough at clinicallyacceptable doses.

I CH2CHOMCH20CH3 M,sonfidazole

I CH2CONHCH*CH20H SR 2S8❑,,

CH2CHOHCH2.N~ RO03.8799

CH2CHOHCHZ.N~ RSU 1069

Figure 2. Structures of rnkontiazok and its analogs.

8

ThioI-Depleting AgentsThe second class of agents is composed of the thiol

depleting agents. All cells contain varying degrees ofnon-protein reducing species such as sulth ydryls (thiol,

●-SH) which act ‘0 protect the DNA when DNAradicals are produced by radiation, l.)NA can be

protected and chemically restored by naturally existingthiols inside the cell (Figure 1). These thiols act asfree radical scavengers and serve to protect cellsagainst radiation damages by competing with oxygenfor DNA radicals. Thus, as the level of these reducingspecies is reduced to low concentrations either throughchemical suppressors such as N-ethylmaleimide orthrough synthesis blockers such as L-buthione-sulfoximine (L-BSO), sensitization of the cell occurs(6).

The most promising agent in this class has been L-BSO. L-BSO acts to deplete cellular glutathione (GSH)which has been shown to play a crucial protective roleagainst cellular injury produced by ionizing radiation(30).

PLD Repair InhibitorThe PLD (potentially lethal damage) repair

inhibitors (6) comprise the third class of agents. It hasbeen shown in several in virro study systems thatincreased survival will occur under certain conditionsafter radiation exposure and before plating or otherassay of cell survival. These studies provide evidenceof the process of PLD repair. It is not known to whatextent PLD occurs in normal tissues and whether ornot compounds inhibiting it in tumors would alsoinhibit it to the same degree in normal tissues.

There are a number of mechanisms of action ofPLD repair inhibition, and a number of drugs shownto be active either in cells in culture or in vivo intumor systems. The best agents appear to be purineana!ogs such as 3-deoxyadenosine and 3-dmxyguanosine, which have been shown active both invivo and in vitro. These agents alter the intracellularnucleotide pool in cancer cells, block DNA synthesisand thus inhibit repair of radiation-induced lesions.

Halogenated Pyrimidin~The fourth class is the brominated and iodinated

pyrimidines. These halogenated pyrimidines weredesigned as thymidine analogues that would incorporateinto the DNA of cycling cells (31). The ease ofsubstitution of halogenated pyrimidines is due to thestereochemical similarity in atomic radius between themethyl group of thymidine (2.0 A) and the bromide(1.95 A) and iodine (2.15 A) atoms of the halogenatedpyrimidines.

The mechanism of sensitization is attributed toenhanced susceptibility of substituted DNA to the

induction of the radiation lesion and effects on repair.The degree of radiosensitization actually increases asthe percentage of thymidine replacement increases. Thethymidine substitution weakens the DNA chain. Asincorporation into DNA is important for sensitization,the cells must be exposed to halogenated pyrimidinesfor a sufficient period.

Studies have recently been published using 5-iododeoxyuridine (IUdR) and bromodeoxyuridine(BUdR) (Figure 3) to enhance the efficacy ofradioimmunotherapy (32-34). JUdR and BUdR arehalogenated pyrimidine analogues. These compoundscan be incorporated into DNA in place of thymidine.They are taken up preferentially by the more rapidlydividing tumor cells. Tumor Cells are sensitized to adegree dependent on the amount of analogueincorporated. However, conflicting results have beenreported from Santos, et al. (32) and Pedley, et al.(33). Pedley and colleagues reported that IUdR at avery low dose of 200 mg/kg (total) resulted inradioresistance of tumors trwdted with 1-131 anti-CEAantibodies. However, Santos et al. showed that IUdRat 1,200 mg/kg effectively increased therapeuticeffectiveness of RIT.

.;?./’:,”N

d’R d’RBUdR lUdR

(Bromodeoxyuridine) (Iododeoxyuridine)

Figure 3. Structures of lUdr and BUdr.

Chemotherapeutic AgentsThe fifth class of agents consists of currently

approved chemotherapeutic agents that are not onlycytotoxic to cancer cells, but also can sensitize cancercells to radiation. All of these agents have uniquemechanisms of killing cancer cells and sensitizing cellsto radiation (35).

Paclituel (Taxo/). Taxol, a novel antineoplasticagent, has recently been approval for use in ovarianand breast cancers (36). Taxol is a natural productderived from the needles and bark of the western yew(Figure 4), Taxus Brevifolia (37). It has a uniquemechanism of action. Taxol acts as a rnitotic inhibitorby inducing formation of microtubles and then preventsmicrotubular depolymerization (38). Research studiesperformed in our own research laboratories have

9

a very effective induced by Taxol, can provide an effective means toshown that paclitaxel could beradiosensitizer. (unpublished data)

HC— N—C=O

o~/I

/,\ \

Figure 4. Structure of Taxol.

Taxol’s mechanism of action essentially blocks cellsin the Gz/M phase of the cell cycle (Figure 5);therefore, cells cannot form a competent “ -spindle or dissociate a spindle.

M(Mitosis)

G2

\

S (DNA synthetic phase)

mltotlc

G1

Figure 5. The celt cyck.

Studies of radiosensitivity in mammalian cells havedetermined that the G2/M phase of mitosis in the cellcycle is most sensitive to radiation (39-40). Thesynchronization or blocking of cells in the Gz/M phase,

increase the efficacy of radiation treatment of cancercells (41-45).

In our first in vitro study using Taxol, we incubatdCEA secretingTS-174 human carcinoma cells with 1-131 labeled anti-CEA monoclinal antibody (1-131 anti-CEA MoAb actively binds to TS-174) without or withdifferent concentrations (1, 2, 5, 7, 10 nM) of Taxol.The fraction of cells surviving after the two differenttreatments aro plotted against Taxol concentrations(Figure 6). Taxol appeared to effectively enhance thecell killing effect of I-131 anti-CEA MoAb atconcentrations as low as 5 nM. We also repeated thestudy using different radioactivities (25, 50, 100, 200,and 300 uCi I-131 anti-CEA MoAb). In this study(Figure 7), Taxol effectively enhanced the cell killingeffect of I-131 anti-CEA MoAb at radioactivities aslow as 50 uCi.

2.5

Y<

1.5-

1-

0.5-

o~012345678 910”

Taxol Concentration (nM)

Figure 6. Survival curve of TS-174 human cobn carcinoma cekat various concentrattins of Taxol with and without 1-131 anti-CEA MoAb.

11

In our in vivo study, we treatd human coloniccarcinoma implanted in nude mice (N= 6 for eachtreatment group) with (a) 1-131 MoAb (500 uCi) alone,(b) Taxol alone, (c) Taxol + 1-131 MoAb, and (d)saline (control). The results are expressed as tumorgrowth ratios (tumor size increase/original tumor sizeat the beginning of treatment) versus time in days(Figure 8). The study shows Taxol increased theeffectiveness of 500 uCi of 1-131 anti-CEA MoAb insuppressing human colonic carcinoma growth in nudemice when compared to the results of animals treatedwith 1-131 anti-CEA MoAb alone.

The results of our study have supported thehypothesis of a synergistic effect between Taxol and o

radiolabeled antibody in colon cancer cells in vilro andin vivo. Taxol has shown evidence of antitumor

10

activity in a variety of solid tumors, such as ovarianand breast carcinoma and malignant melanoma.Clinical investigations are evaluating Taxol’s role in

2.5

2-

1.5-

11- 1

A0.5 I 1 1 1 1 I

0 50 100 150 200 250 300 350

I-B1 MoAb Activity (vCI)

Figure 7. Survival curve of TS-174 human cobn carcinoma cekat various radwactivities of 1-131 anti-CEA MoAb with andwithout Taxol.

●“~ ~ Control

~ Taxol Only

----0---- MoAb Only

~ Taxol + MoAb

0!? I01234 s 6789101112 131415

Day

Figure 8. Suppresswn of tumor gro wth in nude mice receivingdvferent treatments V=6).

refractory acute 1eukemias, non-small cell lungcarcinoma, as well as gastric, colon, and cervicalcarcinomas (37). Taxol’s iimitations in cancer therapyare its dose-related toxicities (rnyelosuppression,bradycardia, peripheral neuropathy, myalgia,arthralgia, nausea/vomiting, diarrhea, and alopecia)(38). The in vivo Taxol concentrations resulting inthese toxicities are much higher than those in ‘ourstudies. For example, a phase I study of Taxol

reported peak plasma concentrations ranging from 2-10UM with doses ranging from 175-275 mg/m2 (41).Toxicities reported most commonly from this regimenincluded bone marrow suppression and alopecia, withboth toxicities now commonly seen in clinical practice.The same study also reported a large volume ofdistribution (60 L/m’) and accumulation of Taxol in theascitic fluid of one patient, with maximum asciticconcentrations of 0.25 UM (250 nM), then stabilizingat a level of 40% above plasma concentrations forapproximately 12 hours. The use of low dose Taxol incombination with radiation or RIT (radiolabeledantibody) may therefore enhance tumor cell killingeffects without inducing the dose-related toxicities seenclinically.

5-FZuorouracd (5-FU). 5-FU has also emerged asone of the most promising clinical radiosensitizers nowavailable (46). The chemical structure of 5-FU isshown in Figure 9. 5-FU is an analogue of thenaturally occurring pyrimidine uracil with a fluorineatom substituted at the 5 carbon position of thepyrimidine ring in place of hydrogen. 5-FU isindicated for the palliative treatment of carcinoma ofcolon, rectum, breast, stomach, and pancreas that isnot amenable to surgery or irradiation. It is also usedas an adjunct to surgery for the treatment of varioussolid tumors (e.g., adenocarcinoma of the colon, rectalcarcinoma, breast cancer).

o

F

IA?0“

Figure 9. Structure of 5-FU,

The anti-cancer mechanism of action of 5-FU isrelatd to its antimetastatic activity. 5-FU is thought tofunction as an antimetabolite in at least threti differentways (47). It inhibits thymidylate synthetase,incorporates onto RNA, and blocks uracil phosphatase.Its radiation sensitization effect appears to come mainlyfrom the inhibition of thymidylate synthetase by 5-fluoro-2’deoxyuridylate, a metabolize of 5-FU. Thisinhibition results in a loss of the de novo source ofthymidine necessary for DNA synthesis. This causesimbalances in triphosphate pools and subsequent alteredDNA damage repair (48). Clinical trials with 5-FUand radiotherapyy are reporting impressive results witha variety of tumors.

11

Hydroxyurea. Hydroxyurea is the first clinicallyavailable derivative of urea to show antineoplasticactivity (Figure 10). The drug is structurally similar tourea and acetohydroxamic acid, and is also a ureaseinhibitor (49). Hydroxyurea is indicated in thetreatment of melanoma, resistant chronic myelocytic(granulocytic) leukemia, and recurrent, metastatic, orinoperable ovarian carcinoma. It is also used incombination with radiation therapy for local control ofprimary squamous cell (epidermoid) carcinoma of thehead and neck.

OH

II IH2N—C—N—OH

Figure 10, Structure of hydroxyurea.

The primary site of cytotoxic action forhydroxyurea is inhibition of the ribonucleotidereductase system. This highly regulated enzymesystem is responsible for the conversion ofribonucleotide diphosphates to the deoxyribonucleotideform, which can subsequently be utilized in either denovo DNA synthesis or DNA repair.

In 1965, hydroxyurea was shown to selectively killcells that were synthesizing DNA and to block cells atthe G,-S border (50). This capacity to sensitize cellsoccurred when hydroxyured was continually present orwas added after irradiation. It has been shown that thetime course of repair of radiation-induced, singlestrand DNA breaks is partially inhibited by exposure tohydroxyurea before and after irradiation (51).Hydroxyurea and radiotherapy have been used inpatients with advanced stages of cervical cancer. Asurvival advantage was noted in a small, prospective,randomized trial comparing radiotherapy /hydroxyureawith radiotherapy alone.

Cfiptiin. Cisplatin is a platinum-containingantinmplastic agent (52). The drug is an inorganiccomplex that contains a platinum atom surrounded ina plane by 2 chloride atoms and 2 ammonia moleculesin cis position. It is used for the treatment ofmetastatic testicular tumors, metastatic ovarian tumors,advartcd bladder carcinoma, and a wide variety ofother neoplasms. Cisplatin is oflen used as acomponent of combination chemotherapeutic regimens.

The exact mechanism of cytotoxic action of cisplatinhas not been conclusively determined. Cisplatin bindsto DNA and inhibits DNA synthesis. It also inhibitsprotein and RNA synthesis. The drug also producesintrastrand and interstrand crossl inks in DNA. DNA

with these crosslinks are more sensitive to radiationdamage (53). This is thought to be the mechanismresponsible for radiation sensitization. Unfortunately,the effect of radiation sensitization of cisplatin is stillinconclusive. ●Cisplatin has been shown to potentateradiation damage in some, but not all, in vitro culturecell lines and in some, hut not all, in vivo experimentaltumor models.

SUMMARY

Clinical applications of cancer therapy withradiolabeled monoclinal antibody are hinderedconsiderably by the complex in vivo behavior ofantibody and by the pathophysiology of tumor.Pharmacologic enhancement using vasoactive drugs,calcium channel blockers, nicotinamide and interferonhave offered some potential solutions to the problemsby enhancing the local concentration of radiolabeledantibodies. Alternative y, pharmacologic agents can beusd to sensitize tumor cells to radiation. The mostpromising group of drugs in this approach is thecurrently-approved chemotherapeutic agents includingpaclitaxel, 5-FU, hydroxyurea and cisplatin. In thefuture, it would seem logical to integrate these twodifferent approaches in a single treatment plan so thatthe efficacy of RIT could be effective]y improved.

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Ekelund L, Laurin S, Lunderquist A. Comparisonof a vasoconstrictor and a vasodilator inpharmacoangiography of bone and soft-tissuetumors. Radiology 1977; 122:95-99.

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1189.

14

QUESTIONS

●1. What is the major requirement for a

successful application of radiolabeledantibody therapy in cancer?

a. low target (tumor) to non-target(normal tissue) ratio

b. low chemical toxicityc. The capability of the radiolabeled

antibody to destroy tumor cellswithout harming normal tissue.

d. The presence of hypoxic cells inthe tumor.

2. What is causing the inconsistent resultsfrom current clinical studies withradiolabeled antibodies for cancer therapy?

a. complex tumor pathoph ysiologicfactors

b. The gamma isotopes are not ideal.c. complicated dosing schemesd. Radiolabeled antibodies are totally

ineffective.

3. What is the major hindrance of developingradiolabeled antibody for cancer therapy?

a. There is no acceptable therapeuticradioisotope,

b. The radio labed antibodypreparation is too expensive.

c. The tumor pathophysiology and in

vi vo radiolabeled antibodybiodistribution are verycomplex.

d. The radiolabeling techniques aretoo difficult.

4. How is the blood perfusion in tumordifferent from blood perfusion in normaltissue?

a. It is better and more rapid,b. It is poorer and less uniform.c. It is especially good in the central

portion of the tumor.d, There is not that much difference.

5. What is causing the rapid and drasticrduction in tumor blood flow as the tumormass increases?

a. There is not enough systemicpressure.

b. All tumor vessels are constricted bythe stimulation of excessive sympa-thomimetic stimulation.

c. The number of vessels in the tumordecreases proportionately as thetumor enlarges.

d. Tumor cells grow too slowly.

6. How does the pH in a tumor affect thetumor capillaries?

a. The pH is acidic and causes thecapillaries to lose their flexibility,

b. The pH is alkaline and causes thecapillaries to expand.

c. The pH is neutral and causes thecapillaries to grow faster than thesurrounding tissue.

d. The pH does not affect thecapillaries in the tumor.

7. What is the major method of transportingradiolabelqd antibody molecules across theinterstitial fluid barrier to reach the cancercells?

convection:: active transport

diffusion:: molecular linking

8. What is the basis for using vasoactive

drugs to increase tumor blood flow?

a. Tumor tissue lacks vascular smoothmuscle.

b. Normal tissue does not havereceptors to these drugs,

c. Tumor blood vessels respond tovasoconstriction only,

d. Tumor tissue has low interstitialpressure.

15

9. What is the beta-adrenergic blocking agentthat has b=n shown to be useful toenhance the tumor perfusion?

a. nembutib. hydraluine

c, isoproterenold. propranolol

10, How does pentobarbitone sodium increasetumor perfusion?

a. It causes vasolidation of tumorvessels.

b. It stimulates the production ofcatecholamines.

c. It causes compensatorysympathomimetic vasoconstriction.

d. It modulates microregionalperfusion,

11. Which of the following pharmacologicagents may prevent the transientmircroregional alterations in tumor bloodflow?

a. urethaneb. angiotensin IIc. calcium channel blockersd. epinephrine

12. How does nicotinamide improve tumorperfusion?

a. It decreases the amount of acutehypoxia in the tumor.

b, It increases mortality in endotoxin-induced shock in rats.

c. It can cause vasoconstriction.d. It can induce transient ischemia in

tumor tissue.

13. What is the pharmacologic effect offlunarizine:

a. It can enhance tumor antigenexpression.

b. It is a cytokine.c. It is a calcium channel blocker.d. It increases the acutel y hypoxic cell

population.

14, Which of the following statements is trueabout the expression of tumor antigen?

a. It can be modulated by therecombinant interferon. ●

b. It is not important for antibodybinding.

c. It is sensitive to verapamil,d. It is proportional to tumor blood

flow.

15. Which of the following agents has beenshown to be effective in increasing bloodflow in experimental tumors?

vasopressin;: epinephrinec. angiotensin IId, nifdipine

16. Which of the following agents is a hypoxiccell sensitizer?

benzamide:: bromodeoxyuridinec. misonidazoled. taxol

17. What is the mechanim of radiationsensitization of L-buthione-sulfoxi mine (L-BSO)?

a. It minimics oxygen.b, It depletes cellular glutathione.c. It promotes the synthesis of thiols.d. It acts as a free radical.

18. How does oxygen sensitize cells to ‘radiation?

a. It increases cellular tension.b. It interacts with the DNA radical to

form DNA-peroxyl radical,c. It produces interstrand and

intrastrand crosslinks in DNA.d. It impairs the PLD repair

mechanism.

16

19.

●

20,

21,

●

22,

23.

What kind of pharmacologic agent isTaxol?

a novel antineoplastic agent:: monoclinal antibody against CEAc. purine analogd. murine protein

The is the major mechanismTaxol?

a. a pyrimidine analog

of action of

to replaceDNA to cause cell toxicity -

b. a potential y lethal damage repairinhibitor

c. a nuclear ADP-ribosyl transferaseinhibitor

d. a mitotic inhibitor by inducingformation of microtubules

How does Taxol sensitize cancer cells:

It binds to hypoxic cells.:: It blocks cancer cells in the Gz/M

phase of the cell cycle.c. It causes synchronization of cells in

the G1/S phase of the cell cycle.d. It causes depolymerization of the

cell membrane.

What is the mechanism of radiationsensitization of halogenated pyrimidines?

They can inhibit RNA synthesis.:: They can be incorporated into

DNA in place of thyrnidine toweaken the DNA.

c. They block cells in the G, phase ofthe cell cycle.

d. They are preferentially taken up bythe cancer cell nuclei.

Which of the following chemotherapeuticagents can sensitize tumor cells toradiation?

methotrexate:. BCNUc. 5-FUd. Cytoxan

24. What is the mechanism of radiationsensitization of hydrox yurea?

a. It inhibits repair of single strandDNA breaks.

b. It promotes membrane synthesis,c. It replaces thymidine incorporation

into DNA.d. It sensitizes hypoxic cells to

radiation.

25. What is the probable radiation sensitizationmechanism of ci splatin?

It inhibits the thiols production.:: It produces intrastrand and

interstrand crosslinks in DNA,c. It protects the calcium channels.d. It binds up all RNA.

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