P olymers derived from the amino acid -tyrosine: glycol ...kyc/pdf/491/wonge/Bourke 2003.pdfdegradable implant materials. This need was ad- of one tyrosine molecule by desaminotyrosine

Advanced Drug Delivery Reviews 55 (2003) 447–466www.elsevier.com/ locate/addr

P olymers derived from the amino acidL-tyrosine:polycarbonates, polyarylates and copolymers with poly(ethylene

glycol)*Sharon L. Bourke, Joachim Kohn

Department of Chemistry and Chemical Biology, Rutgers University, 610 Taylor Rd., Piscataway, NJ 08854,USA

Received 15 November 2002; accepted 30 January 2003

Abstract

The natural amino acidL-tyrosine is a major nutrient having a phenolic hydroxyl group. This feature makes it possible touse derivatives of tyrosine dipeptide as a motif to generate diphenolic monomers, which are important building blocks for thedesign of biodegradable polymers. Particularly useful monomers are desaminotyrosyl-tyrosine alkyl esters (abbreviated asDTR, where R stands for the specific alkyl ester used). Using this approach, a wide variety of polymers have beensynthesized. Here, tyrosine-derived polycarbonates, polyarylates, and polyethers are reviewed with special emphasis onrecent developments relating to cellular and in vivo responses, sterilization techniques, surface characterization, drugdelivery, and processing and fabrication techniques. The commercial development of tyrosine-derived polycarbonates is mostadvanced, with one polymer, poly(DTE carbonate) (E5 ethyl), being under review by the USA Federal Drug Administra-tion. 2003 Elsevier Science B.V. All rights reserved.

Keywords: Tyrosine-derived polymers; Desaminotyrosyl-tyrosine alkyl esters; Biodegradable polymers; Polycarbonate; Polyarylate; Cell–material interactions; Sterilization techniques; Surface characterization; Drug delivery; Polymer processing

Contents

1 . An historic overview of amino-acid-derived polymers................................................................................................................ 4481 .1. Synthetic polymers with amino acid side chains ................................................................................................................. 4481 .2. Copolymers ofa-L-amino acids and non-amino acid monomers .......................................................................................... 4481 .3. Block copolymers containing peptide or poly(amino acid) blocks ........................................................................................ 4481 .4. Pseudo-poly(amino acid)s ................................................................................................................................................. 448

2 . Design and synthesis of tyrosine-derived diphenolic monomers .................................................................................................. 4493 . Properties of tyrosine-derived polymers .................................................................................................................................... 450

3 .1. Tyrosine-derived polycarbonates ....................................................................................................................................... 4503 .2. Tyrosine-derived polyarylates ........................................................................................................................................... 4513 .3. Tyrosine-containing poly(DTR–PEG carbonate)s and poly(DTR–PEG ether)s ..................................................................... 452

*Corresponding author. Tel.:11-732-445-3888; fax:11-732-445-5006.E-mail address: [email protected](J. Kohn).

0169-409X/03/$ – see front matter 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0169-409X(03)00038-3

mailto:[email protected]

448 S.L. Bourke, J. Kohn / Advanced Drug Delivery Reviews 55 (2003) 447–466

4 . Cellular and in vivo response ................................................................................................................................................... 4555 . Sterilization ............................................................................................................................................................................ 4586 . Surface characterization ........................................................................................................................................................... 4587 . Drug delivery.......................................................................................................................................................................... 4598 . Processing and fabrication ....................................................................................................................................................... 4609 . Conclusions and outlook .......................................................................................................................................................... 462References .................................................................................................................................................................................. 462

1 . An historic overview of amino-acid-derived with modified backbones’. These materials havepolymers previously been classified into four major groups by

Kenmitzer and Kohn [13], as described below.Over the last 25 years, significant efforts have

been devoted to the development of polymeric 1 .1. Synthetic polymers with amino acid sidebiomaterials. Historically, the vast majority of these chainsefforts were focused on identifying ‘off the shelf’polymers that were biologically inert and stable These polymers consist of amino acids or peptidesunder physiological conditions. These materials were which have been grafted as side chains onto aused as permanent prosthesis such as bone and joint synthetic polymer backbone. Some of these materialsreplacements, dental devices and cosmetic implants. exhibit polyelectrolyte and metal complexation be-However, the emerging field of tissue engineering havior [14–17].and the need for advanced drug and gene deliverysystems have resulted in an increasing need for 1 .2. Copolymers of a-L-amino acids and non-resorbable polymers. amino acid monomers

A new approach in the development of polymericbiomaterials is to custom design the polymer to tailor Copolymerization ofa-L-amino acids with non-its properties for the desired application. Since amino acid monomers has been achieved through apoly(amino acid)s are structurally related to natural variety of reaction schemes, leading to polymersproteins, the synthesis of amino-acid-based polymers with a wide range of structures and properties [18–was explored as a potential source of new biomateri- 23].als. Starting from about 1970, the use of both homo-and copolymers of amino acids was studied for a 1 .3. Block copolymers containing peptide orvariety of biomedical applications [1–5]. Several poly(amino acid) blocksexcellent, comprehensive reviews are available fordevelopments prior to 1987 [3,6–9]. The early These polymers mostly consist of A–B or A–B–Astudies revealed that most poly(amino acid)s could block copolymers, where A is poly(ethylene glycol)not be considered as potential biomaterials due to and B is a conventional poly(amino acid) or atheir immunogenicity and unfavorable mechanical peptide. Such systems represent promising materialsproperties. So far, only a small number of poly(g- for the delivery of therapeutic agents by control ofsubstituted glutamates) and copolymers thereof supramolecular solution structures [24–30].[3,10–12] have been identified as promising candi-date materials for biomedical applications. 1 .4. Pseudo-poly(amino acid)s

To improve the unfavorable physicomechanicalproperties of most poly(amino acid)s, amino acids Naturally occurring amino acids are linked byhave been used as monomeric building blocks in non-amide bonds, such as ester, iminocarbonate andpolymers that do not have the conventional backbone carbonate bonds. The resulting polymers contain thestructure found in peptides. Collectively, these ma- same monomeric building blocks as conventionalterials are referred to as ‘non-peptide amino-acid- poly(amino acid)s, but do not have a peptide-likebased polymers’ or as ‘amino-acid-derived polymers backbone structure. They were first described in

S.L. Bourke, J. Kohn / Advanced Drug Delivery Reviews 55 (2003) 447–466 449

1984 [31,32]. A number of tyrosine-derived pseudo-poly(amino acid)s have been extensively studied andhave found practical, biomedical applications.

This article focuses on the design, synthesis,characterization and applications of two families oftyrosine-derived polymers: the polycarbonates andpolyarylates. We will also briefly introduce newcopolymers with poly(ethylene glycol) (PEG), thetyrosine-containing poly(DTR–PEG carbonate)s andpoly(DTR–PEG ether)s. For more information onother types of amino-acid-derived polymers, thereader is referred to several publications [3,20,33–38] and to the many publications cited in the Fig. 1. Chemical structures of Bisphenol A and tyrosine di-preceding paragraphs. peptide. In the dipeptide structure, the amino terminal group and

the carboxylic acid terminal group are shown with appropriatechemical protecting groups attached (X and X ). The nature of1 2

these protecting groups affects the chemical synthesis of the2 . Design and synthesis of tyrosine-derived polymer as well as the final physicomechanical properties of thediphenolic monomers polymer.

Diphenols, such as Bisphenol A, are frequentlyused in industry, since their aromatic backbone non-toxic, fully degradable polymers with goodstructures can significantly increase the stiffness and engineering properties. The combination of thesemechanical strength of polymers. However, Bis- different properties within one single design provedphenol A and other industrially used diphenols are to be a difficult task and early investigations did notcytotoxic and can therefore not be used as monomers lead to readily processible materials [32,39]. Later, itin degradable biomaterials. There was a significant was recognized that the number of inter-chain hydro-need for a non-cytotoxic, diphenolic monomer that gen bonding sites per monomer unit had to becould be used as a building block in the design of minimized [40]. These studies led to the replacementdegradable implant materials. This need was ad- of one tyrosine molecule by desaminotyrosine [3-(49-dressed by the development of several tyrosine-based hydroxyphenyl)propionic acid] and the identificationmonomers. Tyrosine is the only major, natural of desaminotyrosyl-tyrosine alkyl esters (Fig. 2) asnutrient containing an aromatic hydroxyl group. In fully biocompatible replacements for Bisphenol Aview of the non-processibility of conventional poly-(L-tyrosine), which cannot be used as an engineeringplastic, the development of a tyrosine-based pseudo-poly(amino acid) was envisioned. In this context,derivatives of tyrosine dipeptide can be regarded asdiphenols and may be employed as replacements forthe industrially used diphenols in the design of

Fig. 2. Chemical structure of desaminotyrosyl-tyrosine alkylmedical implant materials (Fig. 1). This approach esters, abbreviated ‘DTR’. The carboxylic acid terminal group isled, for the first time, to tyrosine-derived polymers protected by an alkyl ester which can be regarded as a pendentwith favorable engineering properties. chain after polymerization. The structure of the alkyl esters is

indicated by the following nomenclature convention: DTE, de-The protecting groups used to block the N- andsaminotyrosyl-tyrosine ethyl ester; DTB, desaminotyrosyl-tyrosineC-termini of tyrosine dipeptide (Fig. 1) have abutyl ester; DTH, desaminotyrosyl-tyrosine hexyl ester; DTO,

significant impact on the properties of the resulting desaminotyrosyl-tyrosine octyl ester; DTD, desaminotyrosyl-polymers. The challenge of the early studies was to tyrosine dodecyl ester. These particular monomers are mostidentify suitable protecting groups that will lead to commonly used in the synthesis of tyrosine-derived polymers.


and other industrial diphenols in a wide range of These properties are therefore reviewed here onlypolymers [41–43]. very briefly.

Monomer synthesis from 3-(49-hydroxy-phenyl)propionic acid and tyrosine alkyl esters was 3 .1. Tyrosine-derived polycarbonatesaccomplished by carbodiimide-mediated couplingreactions, following known procedures of peptide Tyrosine-derived polycarbonates are a group ofsynthesis [44,45], giving typical yields of 70%. ‘homologous’ carbonate–amide copolymers differingMonomers carrying an ethyl, butyl, hexyl, or octyl only in the length of their respective alkyl esterester pendent chain have been investigated extensive- pendent chains (Fig. 3). The diphenolic monomersly [45,46]. were polymerized using either phosgene or the more

easily handled bis(chloromethyl) carbonate tri-phosgene. Polymers with weight-average molecular

3 . Properties of tyrosine-derived polymers weights (M ) of up to 400,000 [44,45] were ob-w

tained, although for practical applications,M valuesw

The basic design, synthesis, and material prop- around 100,000 are usually preferred. Polymer prop-erties of tyrosine-derived polycarbonates and poly- erties, such as glass transition temperature, surfacearylates have been reviewed in detail [13,47–49]. free energy, and mechanical properties, can be easily

Fig. 3. Chemical structures of (A) tyrosine-derived polycarbonates, (B) tyrosine-derived polyarylates, (C) tyrosine-containing poly(DTR–PEG carbonate), and (D) tyrosine-containing poly(DTR–PEG ether). See Fig. 2 for more details of the structure of ‘DTR’.


Table 1aPhysicomechanical properties of tyrosine-derived polycarbonates

Polycarbonate Molecular weight Poly- Glass transition Decomposition Contactderived from (weight average) dispersity temp. (8C) temp. (8C) angle (deg)

DTE 176,000 1.8 81 290 73DTB 120,000 1.4 66 290 77DTH 350,000 1.7 58 320 86DTO 450,000 1.7 53 300 90

a From Ertel and Kohn [45].

controlled by varying the length of the alkyl ester under very acidic conditions (pH#3) did the acid-pendent chain (Table 1). Surprisingly, the degra- catalyzed hydrolysis of the ester bond become adation rate is not a sensitive function of the length of dominant factor and pendent chain ester hydrolysisthe alkyl ester pendent chain, therefore all poly(DTR outpaced the rate of hydrolysis of the backbonecarbonate)s can be easily handled under ambient carbonate bonds. Increasing the length of the pendentconditions and degrade only slowly under physiolog- chain from ethyl to octyl reduced the rate of hy-ical conditions. In vivo studies confirmed the ab- drolysis of both the ester and carbonate bonds,sence of enzymatic involvement in the degradation possibly by hindering the access of water moleculesprocess [52–54]. to these bonds. The mechanism of polycarbonate

The physicomechanical properties and potential degradation is shown schematically in Fig. 4. Ac-applications of tyrosine-derived polycarbonates were cording to this mechanism, the final degradationstudied by Ertel and Kohn [45]. Briefly, the poly- products in vitro are desaminotyrosyl-tyrosine andcarbonates are amorphous polymers. Because of their the alcohol used to protect the carboxylic acid group.high hydrophobicity, they do not swell in aqueous In vivo, it is reasonable to expect the enzymaticmedia or during the degradation process. Their degradation of desaminotyrosyl-tyrosine to de-equilibrium water content is about 2 to 3% and saminotyrosine andL-tyrosine.remains below 5% even at advanced stages ofdegradation. Glass transition temperatures (T ) range 3 .2. Tyrosine-derived polyarylatesg

from 52 to 93 8C and decomposition temperaturesexceed 2908C, providing a wide temperature win- The combinatorial approach used in the design ofdow for thermal processing. Thorough evaluations of the polyarylate library was described in detail byenthalpy relaxation kinetics [50,51] determined that Brocchini et al. [57]. Briefly, a permutationallystorage of polycarbonates at a temperature ofT 2 designed monomer system was used for the synthesisg

15 8C for only a few hours is sufficient to bring the of strictly alternating A–B-type copolymers in whichphysical aging process to completion. Even in an the first monomer (A) contains a reactive group forunoriented stage (thin solvent cast or compression the attachment of a series of pendent chains, whilemolded films), tyrosine-derived polycarbonates are the second monomer (B) allows for systematiccharacterized by their high mechanical strength (50– variations in the polymer backbone structure. The70 MPa) and stiffness (1–2 GPa) [45]. These values copolymerization ofn different monomers A withmcan be further increased by processing conditions different monomers B gave rise to an array ofn 3mthat induce molecular orientation. structurally related copolymers. The first five struc-

Recently, the degradation mechanism was studied turally related polyarylates were reported by Fiordel-in detail by Tangpasuthadol et al. [55,56] utilizing a iso [58] and this initial group of polymers wasseries of small model compounds that mimic the further extended to 112 synthesized polyarylates byrepeat unit of the polymer, followed by a thorough Brocchini et al. [57]. Polymerization was achieved3-year degradation study. These results indicated that by reaction with 1,3-diisopropylcarbodiimide accord-the backbone carbonate bond is hydrolyzed at a ing to a procedure first published by Moore andfaster rate than the pendent chain ester bond. Only Stupp [59].


300 8C. The mechanical properties range from soft,elastomeric materials, e.g. poly(DTO sebacate), tofairly tough and strong materials, e.g. poly(DTEsuccinate). In vitro and in vivo degradation studies ofa limited number of polyarylates indicate that poly-arylates have a faster degradation rate than poly-carbonates [54].

3 .3. Tyrosine-containing poly(DTR–PEGcarbonate)s and poly(DTR–PEG ether)s

There is a growing need for hydrophilic, soft, andfast degrading biomaterials. Some applications thatmay benefit from these materials are drug delivery,non-thrombogenic coatings for stents and vasculargrafts, degradable membranes for the prevention ofsurgical adhesions, and scaffolds for wound healingand artificial skin. To address these needs, tyrosine-derived diphenolic monomers were copolymerizedwith blocks of poly(ethylene glycol) (PEG), resultingin a new class of poly(DTR–PEG carbonate)s (Fig.

Fig. 4. Schematic summary of the mechanism of degradation of 3C) [76]. The chemical structure of the copolymers,tyrosine-derived polycarbonates. Proceeding via two alternate

referred to as poly(DTR-co-f % PEG carbonate)s,Mpathways, the final degradation products in vitro are de- w

supports the optimization of material propertiessaminotyrosyl-tyrosine and the alkyl alcohol used to protect thecarboxylic acid group. In vivo, it is reasonable to expect that through variation of three independent structuredesaminotyrosyl-tyrosine will be further degraded into de- parameters: the percent mole fraction of PEG (f ),saminotyrosine andL-tyrosine.

the average molecular weight of the PEG blocks(M ), and the pendent alkyl group (R) present inw

A total of 18 publications [54,58,60–75] have each tyrosine-derived diphenol. Synthesis of thisheretofore been published describing the properties copolymer was achieved by reacting de-and potential applications of this first library of saminotyrosyl-tyrosine alkyl ester with a pre-deter-combinatorially designed biomaterials (Table 2). mined molar ratio of PEG in the presence ofOne of the most significant features of this library of phosgene at room temperature by a condensationpolymers is the fact that important polymer prop- copolymerization reaction. General structure–proper-erties and polymer performance characteristics fol- ty correlations for glass transition and melting tem-low predictable patterns. For example, changes in the peratures were established [76]. The introduction ofglass transition temperature (Fig. 5) increase in about PEG significantly increased water uptake. As the1 8C steps from 2 to 918C as the number of carbon PEG content was increased, the rate of water uptakeor oxygen atoms in the polymer backbone and and the equilibrium water content increased. At PEGpendent chain decreased [57]. Similarly, the air– contents above 15 mol%, the copolymers behavedwater contact angle ranged from 64 to 1018 and increasingly like hydrogels, and at PEG contentsincreased in steps of about 0.58 from one polymer to over 70 mol%, the polymers became water soluble.the next (Fig. 6). X-ray scattering and DSC data The incorporation of PEG into the backbone ofindicate that these tyrosine-derived polyarylates tyrosine-derived polycarbonates had a significantrange from amorphous to liquid crystalline. The impact on their mechanical properties. At low PEGpolyarylates are thermally stable polymers with content, the polymers were strong and tough and hadthermal decomposition temperatures in the range of tensile stiffness (1.8 GPa) and strength (36 MPa)


Table 2Overview of publications describing tyrosine-derived polyarylates

Authors Year Title

S. Bron and J. Kohn [60] 1993 The effect of small changes in the chemicalstructure on the chain mobility in the glassystate of a new series of polyarylates

J. Fiordeliso, S. Bron 1994 Design, synthesis, and preliminary characterizationand J. Kohn [58] of tyrosine-containing polyarylates: new

biomaterials for medical applicationsJ. Kohn [61] 1994 Tyrosine-based polyarylates: polymers

designed for the systematic study of structure–property correlations

J. Fiordeliso, S. Bron 1995 Design, synthesis, and preliminary characterizationand J. Kohn [62] of tyrosine-containing polyarylates: new

biomaterials for medical applicationsJ. Kohn and S. Brocchini [63] 1996 Pseudo-poly(amino acid)sV. Tangpasuthadol, A. Shefer, 1997 Thermal properties and enthalpy relaxationsC. Yu, J. Zhou and J. Kohn [64] of tyrosine-derived polyarylates

K.A. Hooper, N.D. Macon and 1998 Comparative histological evaluation of newJ. Kohn [54] tyrosine-derived polymers and poly(L-lactic acid)

as a function of polymer degradationM. Puma, N. Suarez and 1999 Conductivity and high-temperature relaxationJ. Kohn [65] of tyrosine-derived polyarylates measured

with thermal stimulated currentsD.M. Schachter and J. Kohn [66] 1999 A new approach to the control of peptide

drug release using novel polymer blendsA.M. Belu, S. Brocchini, 2000 Characterization of combinatorially designedJ. Kohn and B.D. Ratner [67] polyarylates by time-of-flight secondary ion

mass spectrometryF. Bouevich, S. Pulapura 2000 Microscopic analysis of porous biodegradableand J. Kohn [68] scaffolds for tissue engineering

E.A.B. Effah-Kaufmann 2000 Correlations of osteoblast activity and chemicaland J. Kohn [69] structure in the first combinatorial library of

degradable polymersJ. Kohn [70] 2000 The use of combinatorial approaches for the

design of biomaterialsJ. Kohn, E.A.B. Effah 2000 Combinatorial approaches in the design ofKaufmann, E. Tziampazis degradable polymers for use in tissue engineeringand P.V. Moghe [71]

D.M. Schachter and 2000 Design of a polymer matrix for the programmableJ. Kohn [72] delayed release of a water-soluble model peptide

E. Tziampazis, J.A. Cassaday, 2000 Dynamic control of cell adhesion and migrationJ. Kohn and P.V. Moghe [73] behavior on protein-adsorbed, PEG-variant

polymer surfacesN. Suarez, S. Brocchini 2001 Study of relaxation mechanisms in structurally relatedand J. Kohn [74] biomaterials by thermally stimulated depolarization

currentsD.M. Schachter and 2002 A synthetic polymer matrix for the delayed or pulsatileJ. Kohn [75] release of water-soluble peptides

within the range observed for the corresponding than 5 mol% of PEG were flexible and soft elastom-tyrosine-derived homopolymers. As the PEG content ers in the wet state (UTS|20 MPa, Young’s moduliwas increased, the polymers lost their stiffness and 37–500 MPa; up to 1000% strain) [76].strength. Generally, copolymers containing more Increasing PEG content also increased the degra-


Fig. 5. Pattern of glass transition temperatures in the library of polyarylates.

dation rate compared to films of the homopolymer. bonate)s are random copolymers, a correspondingThe hydrolytic cleavage of the homopolymer back- family of strictly alternating copolymers, poly(DTR–bone is limited by the low water content within the PEG ether)s (Fig. 3D), were synthesized by co-polymeric matrix. Thus, the introduction of PEG into polymerization of DTR monomers with methylsul-the copolymer structure increases the degradation fone-activated PEG. This strategy resulted in alter-rate by increasing the availability of water within the nating multiblock copolymers containing approxi-matrix [76]. An inherent property of the high PEG mately four to eight repeat units [78]. PEG blocks ofcontent, marginally water-soluble poly(DTR–PEG molecular weights ranging from 1000 to 8000 andcarbonate)s is that they exhibit inverse temperature alkyl pendent chains (R) ranging from ethyl (C2) totransitions, e.g. the polymers are more soluble in dodecyl (C12) were used to vary the molecularcold water and tend to precipitate when the tempera- structure of the multiblock copolymers. These co-ture is increased. By changing the polymer structure, polymers have a high tendency to self-assemble andthe transition temperature could be varied from 18 to to form polymeric micelles in aqueous solution [79].58 8C [77]. Since the polymer structure can be The increased tendency of the poly(DTR–PEGtailored to undergo an inverse temperature transition ether)s to form supramolecular structures in aqueousslightly below body temperature, these materials solutions is most probably a direct consequence ofhave a wide range of potential applications in their higher molecular order compared to the randommedicine. poly(DTR–PEG carbonate)s. In the future, these two

While the above described poly(DTR–PEG car- families of polymers may be used as an interesting


Fig. 6. Pattern of air–water contact angle in the library of polyarylates.

tool in studies relating molecular structure to various ether)s are also non-cytotoxic, but, at the currentperformance measures. time, these studies have not yet been completed.

The tissue response to fracture fixation pins madeof various tyrosine-derived polycarbonates, such aspoly(DTE carbonate), poly(DTB carbonate), poly-

4 . Cellular and in vivo response (DTH carbonate) and poly(DTO carbonate), wasevaluated in a rabbit transcortical model [53,81,82].

The cellular responses to polycarbonates and The test polymers had identical chemical backbonepolyarylates indicate no sign of cytotoxicity. The structures and varied only in the structure of theability of cells to attach and proliferate on these alkyl ester pendent chain which was increased inpolymer surfaces was strongly correlated with the length from ethyl (two carbons), to butyl (fourhydrophobicity of the polymers [45,61,80]. Tyrosine- carbons), hexyl (six carbons) and octyl (eight car-containing poly(DTR–PEG carbonate)s are also non- bons), as shown in Figs. 2 and 3. Identical, extrudedcytotoxic. However, the presence of PEG leads to pins of poly(DTE carbonate), poly(DTB carbonate),low or no cell attachment in short-term cell culture poly(DTH carbonate), and poly(DTO carbonate)[76]. It is reasonable to assume that poly(DTR–PEG were implanted transcortically in bone defects in the


rabbit’s distal femora and proximal tibiae permitting lated with fibrous tissue (an encapsulation response),histological comparisons of the bone–implant inter- whereas others exhibited predominantly direct appo-face as a function of incremental changes in polymer sition of bone to the implant surface. The encapsula-structure and time. The transcortical implants were tion response was distinguished by an organizedfollowed over a period of 1090 days (3 years). fibrous capsule that ranged between three and 30 cell

The experimental details of this study were recent- layers. The capsule lined the entire circumference ofly published [81]. Great care was taken to control the the implant and effectively separated the implantstudy and to account for possible artifacts or effects from the surrounding bone. In contrast, at thosethat were independent of the chemical structure. implant sites where bone apposition was observed,After implant retrieval from the hard tissue sites, the circumference of the implant was devoid of ansamples were prepared for undecalcified light micro- organized fibrous capsule.scopy analysis. The histological sections were evalu- Table 3 lists the frequency with which the en-ated for general material biocompatibility. Next, the capsulation or bone apposition responses were ob-bone–implant interface around the circumference of served. Most striking was the bone response toeach implanted pin was evaluated in light of whether poly(DTE carbonate) where direct bone apposition to(a) a fibrous tissue layer separated the implant from the implant was the defining feature in 73% of thethe surrounding bone or (b) direct bone apposition retrieved implants (22 of 30 pins). Those poly(DTEwas the dominant feature of the bone–implant carbonate) implants that did exhibit an encapsulationinterface. response tended to have thin capsules of less than 10

Upon histological evaluation of pin cross sections, cell layers. In contrast, as the length of the pendentat time points as early as 90 days (3 months) and as chain was increased to butyl and octyl, less bonelate as 1090 days (3 years), all the polymeric apposition was observed and the predominant re-implants were found to be surrounded by bone sponse was the formation of a fibrous capsule.without any obvious deleterious effects such as bone Particularly noteworthy is the dramatic differenceresorption or large concentrations of inflammatory between poly(DTE carbonate) and poly(DTB car-cells at the implant site. Poly(DTE carbonate), bonate) as these two polymers have very closelypoly(DTB carbonate), poly(DTH carbonate), and matched chemical structures and material properties.poly(DTO carbonate) were all osteocompatible ac- Poly(DTB carbonate) elicited direct bone appositioncording to traditional definitions [83,84] at only 21% of the implant sites. Clearly, in this

However, fundamental differences were observed family of tyrosine-derived polycarbonates, the pre-at the bone–pin interface—some pins were encapsu- dominant response elicited at the bone–implant

Table 3aFrequencies of direct bone apposition and encapsulation responses at the bone–implant interface

Poly(DTE carbonate) Poly(DTB carbonate) Poly(DTO carbonate)

Bone appos- Encapsul- Bone appos- Encapsul- Bone appos- Encapsul-b c b c b cition (%) ation (%) ition (%) ation (%) ition (%) ation (%)

dShort-term 60 40 30 70 20 80eLong-term 80 20 17 83 16 84

fTotal 73 27 21 79 17 83a From James et al. [82].b Bone apposition responses were reported when an organized fibrous tissue layer could not be identified at the light microscopic level at

the bone–implant interface.c Encapsulation responses were reported when a fibrous capsule encompassed the implanted pin.d 0–180 days (n510).e 270–1090 days (n526).f Overall results (n536).


interface was significantly influenced by a relatively responses to poly(DTE carbonate), poly(DTE adi-minor modification of the polymer structure. pate) (a representative member of the library of

The unique osteocompatibility of poly(DTE car- polyarylates), and poly(L-lactic acid).bonate) has been confirmed in a canine model [52]. The study employed extruded pins of the testThis particular polymer has a wide range of applica- polymers in a subcutaneous rat model. The tissuetions as orthopedic implant material and is currently response at the implant sites was followed histomor-under review by the US Food and Drug Administra- phometrically for 570 days. In this study, poly(DTEtion. adipate) consistently elicited the mildest tissue re-

In general, serum proteins such as fibrinogen, or sponse, as judged by the width of the fibrous layerextracellular matrix proteins such as fibronectin and the lack of cellularity of the fibrous tissueadsorb strongly onto surfaces of tyrosine-derived formed around the implant. The tissue response topolycarbonates. These polymers therefore tend to be poly(DTE carbonate) was mild throughout the 570strongly adhesive to cells in tissue culture. Fortunate- day study. However, the response to PLLA fluc-ly, it is possible to modulate protein and cell tuated as a function of degradation, exhibiting anadhesiveness by copolymerization with PEG. For increase in the intensity of inflammation as thetyrosine-containing poly(DTR–PEG carbonate)s con- implant began to lose mass. At the completion of thetaining more than 10% of PEG, dramatically reduced study, tissue ingrowth into the degrading poly(DTElevels of cell attachment were observed in short-term adipate) pins was evident, while no comparativecell culture [76]. The use of DTR–PEG copolymers ingrowth of tissue was seen for PLLA.as a tool for the study of cell–biomaterial interac- This study represents the first comprehensive,tions is particularly attractive. For example, Tziam- comparative study of the degradation and tissuepazis et al. [85] used a series of poly(DTE-co-f % compatibility of tyrosine-derived polycarbonates,PEG carbonate)s (f50, 2, 4, 6, 8 and 10) as a polyarylates and PLLA. The results of this study1000

model system to understand the role of small molar indicate that complete resorption of poly(DTE adi-fractions of PEG in the regulation of cellular re- pate) implanted in a non-load-bearing, soft tissue sitesponses in tissue culture. They analyzed the effect of will require 1 to 2 years depending on the devicelow concentrations of PEG on the amount, con- configuration. In correspondence with this resorptionformation and bioactivity of fibronectin, and then rate, poly(DTE adipate) pins have a useful devicerelated the data on protein adsorption to the attach- lifetime of about 200 days. The complete resorptionment, adhesion strength, and motility of L929 fi- of pins of poly(DTE carbonate) may require morebroblasts. It was observed that the ability of cells to than 3 years. This long resorption time is associatedattach to the polymer surface was related to fibro- with an extended ‘use life’ of over 1 year, makingnectin bioactivity and not to the totalamount of this material most suitable for long-term applicationsfibronectin present. However, the strength of cell similar to those of high-molecular-weight PLLA.adhesion was correlated with the total amount of There were significant differences between poly(L-fibronectin present on the polymer surface. Addition- lactic acid) and the tyrosine-derived polymers, poly-ally, the rate of cell migration was inversely corre- (DTE adipate) and poly(DTE carbonate). Thelated with PEG concentration over a narrow range of tyrosine-derived polymers did not take up more thanPEG concentrations. This work demonstrated the 5% of water even at the most advanced stages ofutility of incorporating very small amounts of PEG degradation. This is in contrast to the known ten-into the surface of a biomaterial to study cell dency of lactic- or glycolic-acid-derived polymers,attachment, adhesion strength and motility. These which swell significantly due to water uptake towardtypes of studies are relevant to the development of the end of their useful life time. The lack of watertissue engineered scaffolds. uptake also means that implants of tyrosine-derived

Heretofore, less work has been published on the polymers will maintain their shape for longer periodsresponses of cells and tissues to tyrosine-derived than PLLA. Another important difference betweenpolyarylates. The earliest study is a report by Hooper tyrosine-derived polymers and lactide- or glycolide-et al. [54] comparing the degradation and tissue derived polymers relates to the onset of mass loss:


since the tyrosine-derived monomers are not readily tyrosine-derived polycarbonates were significantlywater soluble, mass loss occurs very slowly and only more resistant tog-irradiation than PLLA. Thus,at the very end of the degradation process. In contrary to PLLA, tyrosine-derived polycarbonatescontrast, PLLA starts to lose mass when the molecu- can be sterilized byg-irradiation.lar weight decreases to about 20,000. Another key The effects of both ethylene oxide andg-irradia-difference between tyrosine-derived polymers and tion sterilization were also investigated for thelactide- or glycolide-derived polymers relates to the copolymers with PEG. Specifically, a series ofamount of acidic degradation products produced per copolymers from the group of poly(DTR-co-PEGgram of device. The values for poly(glycolic acid), carbonate)s were studied [76]. No noticeable changepoly(lactic acid), poly(DTE adipate), and poly(DTE in color or physical appearance was observed for anycarbonate) are 15.5, 11.4, 6.4, and 2.6 meq of acid of the copolymers after their exposure to ethyleneper gram of polymer, respectively. The dramatic oxide. However, for copolymers containing highreduction of the amount of acidic degradation prod- weight fractions of PEG (PEG content$70%), anucts being formed in tyrosine-derived polymers increase in the molecular weight after exposure torelative to the commonly used poly(lactic acid) and ethylene oxide was observed, possibly caused bypoly(glycolic acid) may contribute to their better crosslinking. This observation indicates that ethylenetissue compatibility, especially in bone. oxide should be used with caution when sterilizing

As described before [57,86], tyrosine-derived polymers containing a high PEG content.polyarylates were designed as part of an extended Three doses ofg-irradiation (0.3, 1.1, 3.9 Mrad)library of structurally related polymers. The develop- were used to evaluate the effect ofg-sterilization.ment of a combinatorial approach to biomaterials The copolymers retained their initial molecularresulted in a large number of polymeric biomaterials weight after exposure to 0.3 Mrad of irradiation but,becoming available for screening and exploration. when the irradiation dose was increased to 1.1 andThis, in turn, requires the development of new 3.9 Mrad, the molecular weight decreased. Thetechniques for the systematic study of correlations higher the PEG content of the copolymer, the morebetween polymer structures, material properties and pronounced was the decrease in molecular weightbiological performance. Studies are, therefore, under- following irradiation. Since for the sterilization ofway to develop rapid screening methods for the medical plants a 2.5 Mrad dose is required, onlyevaluation of biological properties and performance copolymers with PEG content below 30 mol%characteristics for multiple polymers. should be considered forg-irradiation. These ob-

servations are readily explained: the aromatic struc-ture of DTR is highly resistant to radiation damage,

5 . Sterilization while the aliphatic structure of PEG is highlysensitive to radiation damage. Therefore, the higher

To further evaluate tyrosine-derived polycarbo- the fraction of PEG within the copolymer com-nates for clinical use, the ability of selected poly- position, the more susceptible the polymer becomescarbonates and PLLA to maintain their properties to radiation damage.after ethylene oxide org-irradiation sterilization wasdetermined by Hooper et al. [87]. Ethylene oxidewas found to induce less structural damage to the 6 . Surface characterizationpolymers thang-irradiation based on measurementsof molecular weight, surface composition, mechani- The surface properties of polymeric biomaterialscal properties, and in vitro degradation rate. Ethylene are of interest because they play a significant role inoxide was determined to be a feasible method of the cell, blood, and tissue responses observed both in

´sterilization, except for poly(DTO carbonate), which vitro and in vivo. Therefore, Perez-Luna et al. [88]exhibited a higher degradation rate after exposure to characterized the surface of five tyrosine-derivedethylene oxide than the non-exposed control. Due to polycarbonates (pendent chain: DTE, DTB, DTH,the presence of aromatic groups in the backbone, DTO and DT-benzyl) by using contact angle mea-


surements, electron spectroscopy for chemical analy- drug and the polymer), the apparent protective actionsis (ESCA) and static secondary ion mass spec- of the polymeric matrix on dopamine, the prolongedtrometry (SIMS). release of only about 15% of the total load of

Results showed that the wettability, critical surface dopamine over about 180 days, and the high degreetension, and polarity of these polymers decreased of compatibility with brain tissue. Results indicatedwith increasing chain length of the pendent alkyl an average release of about 1 to 2mg/day, a dosinggroups. The surface elemental composition, as de- rate that falls within the therapeutically useful range.termined by ESCA, was consistent with the stoi- In Europe, tyrosine-derived polyarylates [58,91]chiometry of the repeat unit of the polymers. Addi- and polycarbonates [92] were tested as haemocom-tionally, high-resolution C 1s, O 1s, and N 1s ESCA patible coatings for blood-contacting devices. Tech-spectra also showed results consistent with the niques were developed to incorporate anticoagulantsspecific bonding states of these elements in the into coatings made of tyrosine-derived polyarylates,polymer repeat unit. SIMS experiments showed polycarbonates or lactide/glycolide copolymers.fragment ions characteristic of the pendent groups in These coatings were then applied to carbon fibers.the negative-ion SIMS spectra only, while the posi- Without coating, the fibers were covered withintive SIMS spectra provided a characteristic finger- minutes by a coagulation plug rich in fibrin andprint for each polymer. Because of the reproducibil- platelets. Degradable coatings without anticoagulantsity of the spectra and the high cleanliness of the reduced the thrombogenicity of the test materials, but

´polymer surface, Perez-Luna et al. suggested the use coatings releasing hirudin and prostacyclin inhibitorsof tyrosine-derived polycarbonates as reference stan- prevented the formation of thrombin at the coateddards for surface characterization studies. surfaces.

In a similar study, Belu et al. [67] used a series of In a low-molecular-weight drug release model16 tyrosine-derived polyarylates to test the ability of study, the adipic acid series of polyarylates consist-time-of-flight secondary ion mass spectrometry ing of poly(DTE adipate), poly(DTH adipate), and(TOF-SIMS) to identify structurally very closely poly(DTO adipate) [58,93] exhibited a diffusionrelated polymers. The tyrosine-derived polyarylates controlled release mechanism, as indicated by thewere selected for this study since they exhibit well- linear correlation between the cumulative release andcontrolled and systematically varying chemistry, and the square root of the release time.individual polymers are structurally identical except Schachter and Kohn [75] developed a releasefor the incremental additions of C H units to either formulation for the delayed or pulsatile release of2 4

the backbone and/or the side chain. From the water-soluble peptides. In general, water-solublespectra, peaks characteristic of all polyarylates could peptides are difficult to release without bursts. Thisbe identified. Furthermore, evaluation of the spectra problem was addressed by using polymers that haveand identification of unique signals allowed the a peptide-like backbone structure and that can pro-unambiguous classification of the polyarylates ac- vide strong hydrogen bonding interactions with thecording to side chain and backbone chemistry. peptide drug. Tyrosine-derived polyarylates were

particularly useful in this context, since it is possibleto screen the library of polyarylates for those poly-

7 . Drug delivery mers that provide a high level of peptide–polymerinteractions for any given target peptide. Using the

Poly(DTH carbonate) was selected for the design development of a delayed release system for theof a long-term controlled-release device for the heptapeptide Integrilin� as a ‘test study’, Schachterintracranial administration of dopamine [89,90]. The and Kohn screened the library of polyarylates andpotential advantages of poly(DTH carbonate) over identified poly(DTE adipate) as a polymer that hadother degradable polymers include the ease with particularly strong hydrogen bonding interactionswhich dopamine can be physically incorporated into with Integrilin�. In fact, when up to 30% by weightthe polymer (due to its relatively low processing of the water-soluble Integrilin� was incorporatedtemperature and the structural similarity between the into a poly(DTE adipate) matrix, virtually no peptide


was released during a 50 day incubation period under solvents allows for the use of solvent casting tophysiological conditions (pH 7.4, 378C). Next, small fabricate films, fibers, sponges and coatings. So far,amounts of fast degrading poly(lactide-co-glycolide) films [45], rods [94], porous scaffolds (sponges)(PLGA) were blended into the formulation. Upon [56,95], pins [56,81] and fibers [96–98] have beenincubation, PLGA degraded, releasing acidic degra- processed by one or more of the above-mentioneddation products within the poly(DTE adipate) matrix methods.which weakened the peptide–polymer hydrogen Conventional solvent casting/salt leaching tech-bonds. Consequently, the fast degrading PLGA niques produce scaffolds with low pore interconnec-copolymers acted as ‘delayed-action’ excipient. The tivity. Consequently, Levene et al. [95] developed ainitial molecular weight of PLGA controlled the fabrication process which creates porous scaffoldslength of time before degradation occurred. As the with novel architectures for guided bone regenerationinitial molecular weight of PLGA was varied from and other tissue engineering applications. The scaf-12,000 to 62,000, the duration of the delay period folds were fabricated from tyrosine-derived poly-prior to release increased from 5 to 28 days. These carbonates using a well-controlled phase separationresults demonstrate the development of a novel process added to the salt leaching technique. Thisapproach for the formulation of delayed-release method resulted in scaffolds containing a highlypeptide delivery systems. It is easy to envision interconnected pore network with porosity greatermultiple release phases or even pulsatile release than 90%. The morphology of these scaffolds isbehavior with multiple preprogrammed lag periods illustrated in Fig. 7, where a bimodal distribution ofwhen a combination of different film sets is used.The timing of the release phase was controlled to avery high degree of accuracy by the selection of theinitial molecular weight of PLGA. This system caneasily lend itself to such applications as antigendelivery (vaccination) where a pulsatile release rateis often more effective than a sustained release rate.

Tyrosine–PEG-derived poly(DTR–PEG carbon-ate)s have been explored for drug release from amicrosphere configuration [76]. p-Nitroaniline( pNA) and fluorescein isothiocyanate–dextran(FITC–dextran) were used as models for low-molec-ular-weight hydrophobic and high-molecular-weighthydrophilic drugs, respectively. With increasing PEGcontent in the polymer, a significant increase in therelease ofpNA was observed. For FITC–dextran, therelease was characterized by a short burst during thefirst hour, followed by a long lag period of about 14days during which very little additional FITC–dex-tran was released.

8 . Processing and fabricationFig. 7. Porous scaffold fabricated from poly(DTE carbonate). Abimodal distribution of macro- and micro-pores can be easilyPolycarbonates and polyarylates have sufficientdistinguished. The macropores (212–425mm) are the impressionsthermal stability to be processed by conventionalof the salts on which the solution is cast. The macropores

polymer fabrication techniques such as extrusion, (212–425mm) are the impressions of the salt crystals used ascompression molding and injection molding. Addi- porogens, while the micropores (about 1 to 10mm) are caused bytionally, high solubility in a wide range of organic the formation of frozen solvent crystals during cooling.


Fig. 8. A comparison of histologies from scaffolds without micropores (a) at 3 weeks and scaffolds with micropores (b) at 4 weeks. Thearea within the circle shows directed collagen ingrowth only in the scaffold with micropores. Both samples are viewed at 62.53 and stainedwith Stevenel’s Blue and Van Geison’s Picrofuchsin.


macro- and micro-pores can be easily distinguished. and self-assembly properties (copolymers with PEG).The macropores (212–425mm) are the impressions In particular, the tyrosine-derived polycarbonate,of the salt crystals used as porogens, while the poly(DTE carbonate), has been shown to have a highmicropores (about 1 to 10mm) are caused by the degree of tissue compatibility and is currently beingformation of frozen solvent crystals during cooling. evaluated for possible clinical uses by the USAThese highly porous scaffolds provide a large surface Federal Drug Administration (FDA). Poly(DTE car-area and internal volume which may be ideal for cell bonate) is expected to be the first tyrosine-derivedseeding, growth and production of extracellular polymer to become commercially available for clini-matrix by attached cells. The small pores, created in cal use.the walls of the larger pores, are oriented in linear The main driving forces for the development ofarrays. This very specific microstructure may affect new, degradable biomaterials are (i) the need of thecell–matrix interactions, as illustrated in Fig. 8, pharmaceutical industry to develop advanced drugwhere the micropores provide a template for directed delivery systems for the many new peptide andcollagen ingrowth. protein drugs that will become available through

Bourke et al. [99] produced poly(DTE carbonate) biotechnology and genomics, (ii) the need of thefibers with ultimate tensile strength (UTS) values of medical device industry to develop degradable im-230 MPa and Young Moduli of 3.1 GPa by a plants (scaffolds) for tissue regeneration and tissueone-step melt extrusion. When compared in strength engineering applications, and (iii) the need to im-retention to poly(L-lactic acid) fibers, poly(DTE prove the biocompatibility of biosensors and im-carbonate) performed significantly better. UTS val- plantable medical devices. This last application callsues for poly(DTE carbonate) remained above 200 for new materials with surfaces that prevent scarringMPa (87% strength retention) after 30 weeks of and/or protein adsorption at the implant / tissue inter-incubation, while UTS values for poly(L-lactic acid) face.dropping to 20 MPa (7% strength retention) within 2 While in the past the vast majority of all commer-weeks. cial research involving degradable polymers was

Recently, fibers having yield stress values above limited to the use of poly(lactic acid), poly(glycolic200 MPa combined with lower modulus values (1–2 acid) or copolymers thereof, it is obvious that, in theGPa) have been fabricated from poly(DTD future, a wider range of new materials will bedodecanoate) (Bourke and Kohn, unpublished re- needed. Tyrosine-derived pseudo-poly(amino acid)ssults). These fibers may be useful in applications represent one of many new ‘second generationwhere compliance with soft tissue is important. biomaterials’ that will enter into clinical use over the

next decade.

9 . Conclusions and outlook

R eferencesThe amino acidL-tyrosine was shown to be aversatile building block for biodegradable and

[1] L.L. Hench, E.C. Ethridge, Biomaterials—an interfacialbiocompatible polymers. The incorporation of de-approach, in: A. Noordergraaf (Ed.), Biophysics and Bioen-rivatives of tyrosine dipeptide, such as the de-gineering Series, Academic Press, New York, 1982.saminotyrosyl-tyrosine alkyl esters (DTR), into the

[2] D.J. Lyman, Polymers in medicine—An overview, in: E.backbone of different polymer systems results in Chiellini, P. Giusti (Eds.), Polymers in Medicine. Biomedicalversatile polymers with interesting properties. Con- and Pharmacological Applications, Plenum Press, New York,

1983, pp. 215–218.trary to most conventional poly(amino acid)s,[3] J.M. Anderson, K.L. Spilizewski, A. Hiltner, Poly-a aminotyrosine-derived pseudo-poly(amino acid)s exhibit

acids as biomedical polymers, in: D.F. Williams (Ed.),excellent engineering properties and polymer sys-Biocompatibility of Tissue Analogs,Vol. 1, CRC Press, Boca

tems can be designed whose members show excep- Raton, FL, 1985, pp. 67–88.tional strength (polycarbonates), flexibility and elas- [4] R.E. Marchant, T. Sugie, A. Hiltner, J.M. Anderson,tomeric behavior (polyarylates), or water-solubility Biocompatibility and an enhanced acute inflammatory phase


model, ASTM Spec. Tech. Publ. (Corros. Degrad. Implant [18] C. Samyn, M.v. Beylen, Polydespeptides: ring-opening poly-Mater.) 859 (1985) 251–266. merization of 3-methyl-2,5-morpholinedione, 3,6-dimethyl-

[5] F. Lescure, R. Gurny, E. Doelker, M.L. Pelaprat, D. Bichon, 2,5-morpholinedione and copolymerization thereof withD,L-J.M. Anderson, Acute histopathological response to a new lactide, Makromol. Chem., Macromol. Symp. 19 (1988)biodegradable, polypeptidic polymer for implantable drug 225–234.delivery system, J. Biomed. Mater. Res. 23 (1989) 1299– [19] M. Yoshida, M. Asano, M. Kumakura, R. Katakai, T.1313. Mashimo, H. Yuasa, H. Yamanaka, Sequential polydespep-

[6] E. Katchalski, M. Sela, Synthesis and chemical properties of tides containing tripeptide sequences anda-hydroxy acids aspoly-a-amino acids, in: C.B. Anfinsen, M.L. Anson, K. biodegradable carriers, Eur. Polym. J. 27 (3) (1991) 325–Bailey, J.T. Edsall (Eds.), Advances in Protein Chemistry, 329.Vol. 13, Academic Press, New York, 1958, pp. 243–492. [20] A. Staubli, E. Mathiowitz, M. Lucarelli, R. Langer, Charac-

[7] N. Lotan, A. Berger, E. Katchalski, Conformation and terization of hydrolytically degradable amino acid containingconformational transitions of poly-a-amino acids in solution, poly(anhydride-co-imides), Macromolecules 24 (1991)in: E.E. Snell, P.D. Boyer, A. Meister, R.L. Sinsheimer 2283–2290.(Eds.), Annual Review of Biochemistry, Vol. 41, Annual [21] D.A. Barrera, E. Zylstra, P.T. Lansbury, R. Langer, SynthesisReviews Inc, Palo Alto, CA, 1972, pp. 869–901. and RGD peptide modification of a new biodegradable

[8] E. Katchalski, Poly(amino acids): achievements and pros- copolymer: poly(lactic acid-co-lysine), J. Am. Chem. Soc.pects, in: E.R. Blout, F.A. Bovey, M. Goodman, N. Lotan 115 (1993) 11010–11011.(Eds.), Peptides, Polypeptides, and Proteins—Proceedings of [22] D.A. Barrera, E. Zylstra, P.T. Lansbury, R. Langer, Co-the Rehovot Symposium on Poly(Amino Acids), Polypep- polymerization and degradation of poly(lactic acid-co-tides, and Proteins and their Biological Implications, Wiley, lysine), Macromolecules 28 (1995) 425–432.New York, 1974, pp. 1–13. [23] H.R. Allcock, S.R. Pucher, A.G. Scopelianos, Poly[(amino

[9] G.D. Fasman, The road from poly(a-amino acids) to the acid ester)phosphazenes]: synthesis, crystallinity, and hydro-prediction of protein conformation, Biopolymers 26 (1987) lytic sensitivity in solution and the solid state, Macromole-S59–S79. cules 27 (1994) 1071–1075.

[10] K.R. Sidman, A.D. Schwope, W.D. Steber, S.E. Rudolph, [24] S. Cammas, K. Kataoka, Functional poly[(ethylene oxide)-S.B. Poulin, Biodegradable implantable sustained release co-(b-benzyl L-aspartate)] polymeric micelles: block copoly-systems based on glutamic acid copolymers, J. Membr. Sci. mer synthesis and micelles formation, Makromol. Chem.7 (1980) 277–291. Phys. 196 (1995) 1899–1905.

[11] K.R. Sidman, W.D. Steber, A.D. Schwope, G.R. Schnaper, [25] S.H. Jeon, S.M. Park, T. Ree, Preparation and complexationControlled release of macromolecules and pharmaceuticals of an A–B–A type triblock copolymer consisting of helicalfrom synthetic polypeptides based on glutamic acid, Bio- poly(L-proline) and random-coil poly(ethylene oxide), J.polymers 22 (1983) 547–556. Polym. Sci. Part A: Polym. Chem. 27 (1989) 1721–1730.

[12] R.K. Bhaskar, R.V. Sparer, K.J. Himmelstein, Effect of an [26] M. Yokoyama, S. Inoue, K. Kataoka, N. Yui, T. Okano, Y.applied electric field on liquid crystalline membranes: control Sakurai, Molecular design for missile drug: synthesis ofof permeability, J. Membr. Sci. 24 (1) (1985) 83–96. adriamycin conjugated with immunoglobulin G using poly-

[13] J. Kemnitzer, J. Kohn, Degradable polymers derived from (ethylene glycol)-block-poly(aspartic acid) as intermediatethe amino acidL-tyrosine, in: A.J. Domb, J. Kost, D.M. carrier, Macromol. Chem. 190 (1989) 2041–2054.Wiseman (Eds.), Drug Targeting and Delivery, Handbook of [27] M. Yokoyama, T. Okano, Y. Sakurai, K. Kataoka, S. Inoue,Biodegradable Polymers, Vol. 7, Harwood Academic, Am- Stabilization of disulfide linkage in drug–polymer–immuno-sterdam, 1997, pp. 251–272. globulin conjugate by microenvironmental control, Biochem.

[14] C. Methenitis, J. Morcellet, G. Pneumatikakis, M. Morcellet, Biophys. Res. Commun. 164 (3) (1989) 1234–1239.Polymers with amino acids in their side chain: conformation [28] M. Yokoyama, M. Miyauchi, N. Yamada, T. Okano, Y.of polymers derived from glycylglycine and phenylalanine, Sakurai, K. Kataoka, S. Inoue, Characterization and anti-Macromolecules 27 (1994) 1455–1460. cancer activity of the micelle-forming polymeric anticancer

[15] J. Morcellet-Sauvage, M. Morcellet, C. Loucheux, Poly- drug adriamycin-conjugated poly(ethylene glycol)–poly(as-methacrylic acid derivatives. 1. Preparation, characterization, partic acid) block copolymer, Cancer Res. 50 (1990) 1693–and potentiometric study of poly(N-methacryloyl-L-alanine- 1700.co-N-phenylmethacrylamide), Makromol. Chem. 182 (1981) [29] M. Yokoyama, H. Anazawa, A. Takahashi, S. Inoue, Syn-949–963. thesis and permeation behavior of membranes from seg-

[16] M. Morcellet, C. Loucheux, H. Daoust, Poly(methacrylic mented multiblock copolymers containing poly(ethyleneacid) derivatives. 5. Microcalorimetric study of poly(N- oxide) and poly(b-benzyl L-aspartate) blocks, Makromol.methacryloyl-L-alanine) and poly(N-methacrylol-L-alanine- Chem. 191 (1990) 301–311.co-N-phenylmethacrylamide) in aqueous solutions, Macro- [30] M. Yokoyama, M. Miyauchi, N. Yamada, T. Okano, Y.molecules 15 (1982) 890–894. Sakuri, K. Kataoka, S. Inoue, Polymer micelles as novel drug

[17] A. Lekchiri, J. Morcellet, M. Morcellet, Complex formation carrier: adriamycin-conjugated poly(ethylene glycol)–poly-between copper (II) and poly(N-methacryloyl-L-asparagine), (aspartic acid) block copolymer, J. Control. Release 11Macromolecules 20 (1987) 49–53. (1990) 269–278.


[31] J. Kohn, R. Langer, A new approach to the development of [46] K.A. Hooper, J. Kohn, Diphenolic monomers derived frombioerodible polymers for controlled release applications the natural amino acida-L-tyrosine: an evaluation of peptideemploying naturally occurring amino acids, in: Polymeric coupling techniques, J. Bioact. Compat. Polym. 10 (4)Materials, Science and Engineering, Vol. 51, American (1995) 327–340.Chemical Society, Washington, DC, 1984, pp. 119–121. [47] K. James, J. Kohn, Pseudo-poly(amino acid)s: examples for

[32] J. Kohn, R. Langer, Polymerization reactions involving the synthetic materials derived from natural metabolites, in: K.side chains ofa-L-amino acids, J. Am. Chem. Soc. 109 Park (Ed.), Controlled Drug Delivery: Challenges and Strate-(1987) 817–820. gies, American Chemical Society, Washington, DC, 1997,

pp. 389–403.[33] D.A. Tirrell, M.J. Fournier, T.L. Mason, New polymers fromartificial genes: a progress report, Polym. Prepr. 32 (3) [48] J. Kohn, The use of natural metabolites in the design of new(1991) 704–705. materials for tissue engineering, in: Y. Ikada, Y. Yamaoka

(Eds.), Organ Regeneration, Tissue Engineering for Thera-[34] X. Li, D.B. Bennett, N.W. Adams, S.W. Kim, Poly(a-aminopeutic Use, Vol. 1, Elsevier, New York, 1998, pp. 61–70.acid)–drug conjugates, in: R.L. Dunn, R.M. Ottenbrite

(Eds.), Polymeric Drug and Drug Delivery Systems, ACS [49] J.M. Pachence, J. Kohn, Biodegradable polymers, in: R.P.Symposium Series, Vol. 469, American Chemical Society, Lanza, R. Langer, J. Vacanti (Eds.), Principles of TissueWashington, DC, 1991, pp. 101–116. Engineering, 2nd Edition, Academic Press, San Diego, CA,

2000, pp. 263–277.[35] P. Caliceti, C. Monfardini, L. Sartore, O. Schiavon, F.Baccichetti, F. Carlassare, F.M. Veronese, Preparation and [50] V. Tangpasuthadol, A. Shefer, K.A. Hooper, J. Kohn,properties of monomethoxy poly(ethylene glycol) doxorubi- Evaluation of thermal properties and physical aging ascin conjugates linked by an amino acid or a peptide as function of the pendent chain length in tyrosine-derivedspacer, Il Farmaco 48 (7) (1993) 919–932. polycarbonates, a class of new biomaterials, in: A.G. Mikos,

K.W. Leong, M.J. Yaszemski, J.A. Tamada, M.L. Radomsky[36] D.B. Bennett, N.W. Adams, X. Li, J. Feijen, S.W. Kim,(Eds.), Spring Meeting of the Materials Research Society,Drug-coupled poly(amino acids) as polymeric prodrugs, J.San Fransisco, CA, Symposium Proceedings, Vol. 394,Bioact. Compat. Polym. 3 (1988) 44–52.Materials Research Society, Pittsburgh, PA, 1995, pp. 143–[37] T. Kumaki, M. Sisido, Y. Imanishi, Antithrombogenicity and148.oxygen permeability of block and graft copolymers of

[51] N. Suarez, E. Laredo, A. Bello, J. Kohn, Molecular relaxa-polydimethylsiloxane and poly(a-amino acid), J. Biomed.tion mechanisms of tyrosine-derived polycarbonates byMater. Res. 19 (1985) 785–811.thermally stimulated depolarization currents, J. Appl. Polym.[38] D.L. Wise, O. Midler, Poly(alkylamino acids) as sustainedSci. 63 (11) (1997) 1157–1466.release vehicles, in: D.L. Wise (Ed.), Biopolymers in Con-

[52] J. Choueka, J.L. Charvet, K.J. Koval, H. Alexander, K.S.trolled Release Systems, Vol. 2, CRC Press, Boca Raton, FL,James, K.A. Hooper, J. Kohn, Canine bone response to1984, pp. 219–229.tyrosine-derived polycarbonates and poly(L-lactic acid), J.[39] J. Kohn, R. Langer, Backbone modifications of syntheticBiomed. Mater. Res. 31 (1996) 35–41.poly-a-L-amino acids, in: G.R. Marshall (Ed.), Peptides—

[53] S.I. Ertel, J. Kohn, M.C. Zimmerman, J.R. Parsons, Evalua-Chemistry and Biology: Proceedings of the 10th Americantion of poly(DTH carbonate), a tyrosine-derived degradablePeptide Symposium, Escom Publishing, Leiden, 1988, pp.polymer, for orthopaedic applications, J. Biomed. Mater.658–661.Res. 29 (11) (1995) 1337–1348.[40] S. Pulapura, C. Li, J. Kohn, Structure–property relationships

[54] K.A. Hooper, N.D. Macon, J. Kohn, Comparative histologi-for the design of polyiminocarbonates, Biomaterials 11cal evaluation of new tyrosine-derived polymers and poly(L-(1990) 666–678.lactic acid) as a function of polymer degradation, J. Biomed.[41] J. Kohn, Pseudo-poly(amino acids), Drug News Perspect. 4Mater. Res. 41 (3) (1998) 443–454.(5) (1991) 289–294.

[55] V. Tangpasuthadol, S.M. Pendharkar, J. Kohn, Hydrolytic[42] J. Kohn, The use of natural metabolites in the design ofdegradation of tyrosine-derived polycarbonates, a class ofnon-toxic polymers for medical applications, Polym. Newsnew biomaterials. Part I: Study of model compounds,16 (11) (1991) 325–332.Biomaterials 21 (2000) 2371–2378.[43] J. Kohn, Desaminotyrosyl-tyrosine alkyl esters: new

[56] V. Tangpasuthadol, S.M. Pendharkar, R.C. Peterson, J. Kohn,diphenolic monomers for the design of tyrosine-derivedHydrolytic degradation of tyrosine-derived polycarbonates, apseudopoly(amino acids), in: R.L. Dunn, R.M. Ottenbriteclass of new biomaterials. Part II: 3-yr study of polymeric(Eds.), Polymeric Drugs and Drug Delivery Systems, ACSdevices, Biomaterials 21 (2000) 2379–2387.Symposium Series, Vol. 469, American Chemical Society,

Washington, DC, 1991, pp. 155–169. [57] S. Brocchini, K. James, V. Tangpasuthadol, J. Kohn, Acombinatorial approach for polymer design, J. Am. Chem.[44] S. Pulapura, J. Kohn, Tyrosine derived polycarbonates:Soc. 119 (19) (1997) 4553–4554.backbone modified, ‘pseudo’-poly(amino acids) designed for

biomedical applications, Biopolymers 32 (1992) 411–417. [58] J. Fiordeliso, S. Bron, J. Kohn, Design, synthesis, and[45] S.I. Ertel, J. Kohn, Evaluation of a series of tyrosine-derived preliminary characterization of tyrosine-containing poly-

polycarbonates for biomaterial applications, J. Biomed. arylates: new biomaterials for medical applications, J.Mater. Res. 28 (1994) 919–930. Biomater. Sci. (Polym. Ed.) 5 (6) (1994) 497–510.


[59] J.S. Moore, S.I. Stupp, Room temperature polyesterification, peptide, in: Transactions of the Sixth World BiomaterialsMacromolecules 23 (1) (1990) 65–70. Congress, Kamuela, HI, Society for Biomaterials, Min-

neapolis, MN, 2000, p. 301, Vol. 1.[60] S. Bron, J. Kohn, The effect of small changes in thechemical structure on the chain mobility in the glassy state of [73] E. Tziampazis, J.A. Cassaday, J. Kohn, P.V. Moghe, Dynamica new series of polyarylates, in: Polymeric Materials, Sci- control of cell adhesion and migration behavior on protein-ence and Engineering, Vol. 69, American Chemical Society, adsorbed, PEG-variant polymer surfaces, in: Transactions ofWashington, DC, 1993, pp. 37–39. the Sixth World Biomaterials Congress, Kamuela, HI, Socie-

ty for Biomaterials, Minneapolis, MN, 2000, p. 166, Vol. 1.[61] J. Kohn, Tyrosine-based polyarylates: polymers designed forthe systematic study of structure–property correlations, in: [74] N. Suarez, S. Brocchini, J. Kohn, Study of relaxationAnnual Meeting of the Society for Biomaterials, Boston, mechanisms in structurally related biomaterials by thermallyMA, Society for Biomaterials, Minneapolis, MN, 1994, p. stimulated depolarization currents, Polymer 42 (21) (2001)67. 8671–8680.

[62] J. Fiordeliso, S. Bron, J. Kohn, Design, synthesis, and [75] D.M. Schachter, J. Kohn, A synthetic polymer matrix for thepreliminary characterization of tyrosine-containing poly- delayed or pulsatile release of water-soluble peptides, J.arylates: new biomaterials for medical applications, in: S.L. Control. Release 78 (2002) 143–153.Cooper, C.H. Bamford, T. Tsuruta (Eds.), Polymer Bioma- [76] C. Yu, J. Kohn, Tyrosine-PEG-derived poly(ether carbonate)sterials in Solution, as Interfaces and as Solids, VSP, Utrecht, as new biomaterials. Part I: Synthesis and evaluation,1995, pp. 695–708. Biomaterials 20 (3) (1999) 253–264.

[63] J. Kohn, S. Brocchini, Pseudo-poly(amino acid)s, in: J.C. [77] C. Yu, S.S. Mielewczyk, K.J. Breslauer, J. Kohn, Tyrosine-Salamone (Ed.), Polymeric Materials Encyclopedia, Vol. 9, PEG-derived poly(ether carbonate)s as new biomaterials. PartCRC Press, Boca Raton, FL, 1996, pp. 7279–7290. II: Study of inverse temperature transitions, Biomaterials 20

[64] V. Tangpasuthadol, A. Shefer, C. Yu, J. Zhou, J. Kohn, (3) (1999) 265–272.Thermal properties and enthalpy relaxations of tyrosine- [78] F. d’Acunzo, J. Kohn, Alternating multiblock amphiphilicderived polyarylates, J. Appl. Polym. Sci. 63 (11) (1997) copolymers of PEG and tyrosine-derived diphenols. I. Syn-1441–1448. thesis and characterization, Macromolecules (in press).

[65] M. Puma, N. Suarez, J. Kohn, Conductivity and high-tem- [79] F. d’Acunzo, T.Q. Le, J. Kohn, Alternating multiblockperature relaxation of tyrosine-derived polyarylates measured amphiphilic copolymers of PEG and tyrosine-derivedwith thermal stimulated currents, J. Polym. Sci. Part B: diphenols. II. Self-assembly in aqueous solution and atPolym. Phys. 37 (1999) 3504–3511. hydrophobic surfaces, Macromolecules (in press).

[66] D.M. Schachter, J. Kohn, A new approach to the control of [80] J. Zhou, S.I. Ertel, H.M. Buettner, J. Kohn, Evaluation ofpeptide drug release using novel polymer blends, in: Annual tyrosine-derived pseudo-poly(amino acids): in vitro cellMeeting of the Society for Biomaterials, Providence, RI, interactions, in: Annual Meeting of the Society for Bioma-Society of Biomaterials, Minneapolis, MN, 1999, p. 119. terials, Boston, MA, Society for Biomaterials, Minneapolis,

[67] A.M. Belu, S. Brocchini, J. Kohn, B.D. Ratner, Characteriza- MN, 1994, p. 371.tion of combinatorially designed polyarylates by time-of- [81] K. James, H. Levene, J.R. Parsons, J. Kohn, Small changesflight secondary ion mass spectrometry, Rapid Commun. in polymer chemistry have a large effect on the bone–Mass Spectrom. 14 (2000) 564–571. implant interface: evaluation of a series of degradable

[68] F. Bouevich, S. Pulapura, J. Kohn, Microscopic analysis of tyrosine-derived polycarbonates in bone defects, Biomateri-porous biodegradable scaffolds for tissue engineering, in: als 20 (23/24) (1999) 2203–2212.Microscopy and Microanalysis 2000, Philadelphia, 2000. [82] K. James, H. Levene, E.E. Kaufmann, J.R. Parsons, J. Kohn,

[69] E.A.B. Effah-Kaufmann, J. Kohn, Correlations of osteoblast Small changes in chemical structure of a polymer can have aactivity and chemical structure in the first combinatorial significant effect on the hard-tissue response in vivo, in: J.E.library of degradable polymers, in: Transactions of the Sixth Davies (Ed.), Bone Engineering, EM Squared Incorporated,World Biomaterials Congress, Kamuela, HI, Society for Toronto, Canada, 2000, pp. 195–203.Biomaterials, Minneapolis, MN, 2000, p. 811, Vol. 2. [83] R.Z. LeGeros, R.G. Craig, Strategies to affect bone remodel-

ing: osteointegration, J. Bone Miner. Res. 8 (1993) S583–[70] J. Kohn, The use of combinatorial approaches for the designS596.of biomaterials, in: Transactions of the Sixth World Bioma-

terials Congress, Kamuela, HI, Society for Biomaterials, [84] H. Plenk, Prosthesis–bone interface, J. Biomed. Mater. Res.Minneapolis, MN, 2000, p. 78, Vol. 1. (Appl. Biomater.) 43 (1998) 350–355.

[71] J. Kohn, E.A.B. Effah Kaufmann, E. Tziampazis, P.V. [85] E. Tziampazis, J. Kohn, P.V. Moghe, PEG-variant biomateri-Moghe, Combinatorial approaches in the design of degrad- als as selectively adhesive protein templates: model surfacesable polymers for use in tissue engineering, in: Transactions for controlled cell adhesion and migration, Biomaterials 21of the Sixth World Biomaterials Congress, Kamuela, HI, (2000) 511–520.Society for Biomaterials, Minneapolis, MN, 2000, p. 84, Vol. [86] S. Brocchini, K. James, V. Tangpasuthadol, J. Kohn, Struc-1. ture–property correlations in a combinatorial library of

[72] D.M. Schachter, J. Kohn, Design of a polymer matrix for the degradable biomaterials, J. Biomed. Mater. Res. 42 (1998)programmable delayed release of a water-soluble model 66–75.


[87] K.A. Hooper, J.D. Cox, J. Kohn, Comparison of the effect of [94] K.A. Hooper, N.D. Macon, J. Kohn, Long-term in vitro andethylene oxide andg-irradiation on selected tyrosine-derived in vivo evaluations of degradation of new tyrosine-derivedpolycarbonates and poly(L-lactic acid), J. Appl. Polym. Sci. polymers and poly(L-lactic acid), in: Annual Meeting of the63 (11) (1997) 1499–1510. Society for Biomaterials, San Diego, CA, Society for Bioma-

[88] V.H. Perez-Luna, K.A. Hooper, J. Kohn, B.D. Ratner, terials, Minneapolis, MN, 1998, p. 387.Surface characterization of tyrosine-derived polycarbonates, [95] H.B. Levene, C.M. Lhommeau, J.B. Kohn, Porous polymerJ. Appl. Polym. Sci. 63 (11) (1997) 1467–1479. scaffolds for tissue engineering, US Patent 6,103,255, issued

[89] Z. Dong, Synthesis of four structurally related tyrosine- 2000, assigned to Rutgers University.derived polycarbonates and in vitro study of dopamine [96] S.L. Bourke, J. Kohn, M.G. Dunn, Comparative in vitrorelease from poly(desaminotyrosyl-tyrosine hexyl ester car- degradation study of selected tyrosine-based polymers andbonate), M.Sc. Thesis, Rutgers University, 1993. PLLA, in: Annual Meeting of the Society for Biomaterials,

[90] D. Coffey, Z. Dong, R. Goodman, A. Israni, J. Kohn, K.O. San Diego, CA, Society for Biomaterials, Minneapolis, MN,Schwarz, Evaluation of a tyrosine derived polycarbonate 1998, p. 8.device for the intracranial release of dopamine, in: Sym- [97] S.L. Bourke, N. Tovar, J. Kohn, M.G. Dunn, Effects of staticposium on Polymer Delivery Systems, Presented at the 203rd and cyclic loading in different environments on the strengthMeeting of the American Chemical Society, San Francisco, retention of resorbable synthetic polymer fiber scaffolds, in:CA, 1992, CELL 0058. Annual Meeting of the Society for Biomaterials, Providence,

¨[91] A. Stemberger, E. Alt, G. Schmidmaier, J. Kohn, G. Blumel, RI, Society for Biomaterials, Minneapolis, MN, 1999, p.Blood compatible biomaterials through resorbable antico- 222.agulant drugs with coatings, Ann. Hematol. 68 (Suppl. II) [98] S. Bourke, M. Dunn, J. Kohn, Evaluation of a new resorb-(1994) A48. able synthetic polymer fiber scaffold for ACL reconstruction,

¨[92] A. Stemberger, G. Schmidmaier, C. Forster, E. Alt, J. Kohn, in: 46th Annual Meeting of the Orthopaedic ResearchA. Calatzis, New antithrombin agents: potential for coating Society, Orlando, FL, Orthopaedic Research Society, 2000,biomaterials used in cardiopulmonary bypass, in: R. Pifarre p. 0023.(Ed.), New Anticoagulants for the Cardiovascular Patient, [99] S.L. Bourke, J. Kohn, M.G. Dunn, Development of a novelHanley and Belfus, Philadelphia, PA, 1997, pp. 377–386. resorbable synthetic polymer fiber scaffold for anterior

[93] J. Fiordeliso, Aliphatic polyarylates derived fromL-tyrosine: cruciate ligament reconstruction, Tissue Eng. (submitted fora new class of biomaterials for biomedical applications, publication).M.Sc. Thesis, Rutgers University, 1993.

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P olymers derived from the amino acid -tyrosine: glycol ...kyc/pdf/491/wonge/Bourke 2003.pdfdegradable implant materials. This need was ad- of one tyrosine molecule by desaminotyrosine