Immobilization of Aminothiols on Poly(oxyalkylene phosphates). Formation of Poly(oxyethylene phosphates)/Cysteamine Complexes and Their Radioprotective Efficiency Radostina Georgieva, † Raina Tsevi, § Krassimir Kossev, § Rossitsa Kusheva, † Mariana Balgjiska, ‡ Radostina Petrova, ‡ Violeta Tenchova, † Ivan Gitsov,* ,| and Kolio Troev* ,§ Department of Radiobiology, National Center of Radiobiology and Radiation Protection, 132 Kl. Ohridski Boulevard, Sofia 1756, Bulgaria; Institute of Polymers, Bulgarian Academy of Sciences, Sofia 1113; Department of Biophysics and Radiobiology, University of Sofia, 8 Dr. Tsankov Street, Sofia 1421; and Michael M. Szwarc Polymer Research Institute and Department of Chemistry, College of Environmental Science and Forestry, State University of New York, Syracuse, New York 13210 Received July 15, 2002 The necessity to apply near-toxic amounts of radioprotective drugs to achieve adequate protection during radiation treatments represents a major problem in human medicine. One of the promising strategies to suppress the toxicity of these drugs involves their incorporation into biocompatible polymers. In this study cysteamine (Cy) was attached to poly(oxyethylene phosphate), POEP, via an ionic bond. Radioprotection of E. coli B cells by this substance and its acute toxicity on male C57 BL mice were measured. The toxicity of Cy immobilized within the poly(oxyethylene phosphate) was significantly lower in comparison to pure Cy while its radioprotective efficiency remained high at half the maximum tolerable dose. The high radioprotective efficiency of the Cy/POEP complexes was further confirmed on mice at different polymer molecular weight characteristics, drug immobilization degrees, application times, and doses. It was found that POEP with molecular weight 4700 Da and containing 24% repeating units with attached Cy has the highest protection potential combined with a depot effect. Introduction Human and technological factors continue to contrib- ute toward the necessity for efficient protection against nuclear radiation. Cysteamine (Cy) hydrochloride is a well-known conventional chemical radioprotector. Bacq reported the first clinical applications of this substance several decades ago. 1 Cy was administered in single or multiple doses before or immediately after irradiation, and it protected against the symptoms of radiation sickness. Its protective effect was manifested mainly by the more rapid recovery after the termination of the therapy of treated patients. 2,3 The need to use near-toxic amounts of radioprotective drugs to achieve adequate protection, however, still represents a major problem in human medicine. 4 It is well-known that polymer- drug conjugates could improve drug localization in the target tissue, diminish drug exposure in potential sites of toxicity, and optimize drug release rate. 5 If Cy could be immobilized on biodegradable polymers, it would be possible to reduce its toxicity while preserving its good radioprotective capability. Poly(ethylene glycol), PEG, is a hydrophilic synthetic polymer, which has been extensively studied as biocompatible polymer drug carrier. 6 The fact that the polymer backbone is not biodegradable in vivo is one of the few potential short- comings of PEG. The other major disadvantage is the low drug-loading capacity of this polymer, limited by the availability of only two attachment sites at the termini of the linear PEG molecule. Promising candidates for an immobilization template are the poly(oxyalkylene phosphates), POAP, a family of biodegradable, hydro- philic, and nontoxic polymers that contain PEG moieties and suitable multifunctional sites for immobilization. 7 They possess several favorable features: (i) excellent solubility in aqueous media; (ii) existence of multiple anchoring positions that would extend their drug- loading capacity (P-OCH 3 and PdO groups in every repeating unit); (iii) broader molecular weight range of administration because after hydrolysis the individual segments (low molecular weight PEG) will be safely and efficiently excreted. The basic strategy for the synthesis of radioprotective substances involves a ‘molecule combination’ based on covalent or ionic bonds between the basic components. 8,9 Other alternatives are the absorption complexes where the formation is caused by donor-acceptor interactions and hydrogen bonds. 10 The major goals of the present study are the im- mobilization of the conventional chemical protective agent cysteamine (Cy) on POAP via ionic bonds and the evaluation of the radioprotective efficiency of the com- plexes formed. These experiments combine and extend our line of inquiry toward development of low-toxicity polymers for drug encapsulation and transport 11 and capable of binding highly efficient radiomodifying agents. 7g Chemistry. Synthesis of Poly(oxyethylene phos- phonate), 1. 1 was synthesized via transesterification of dimethyl hydrogen phosphonates by poly(ethylene * Corresponding author (in U.S.): tel. 1-315-470-6851; fax 1-315- 470-6856; e-mail igivanov@mailbox.syr.edu; (in Europe): tel. 359-2- 979-2203; fax 359-2-707-523; e-mail ktroev@polymer.bas.bg. † National Center of Radiobiology and Radiation Protection. § Bulgarian Academy of Sciences. ‡ University of Sofia. | State University of New York. 5797 J. Med. Chem. 2002, 45, 5797-5801 10.1021/jm020309o CCC: $22.00 © 2002 American Chemical Society Published on Web 11/23/2002