Indium-based and iodine-based labeling of HPMA copolymer–epirubicin conjugates: Impact of structure on the in vivo fate

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Recently, we developed 2nd generation backbone degradable N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer-drug conjugates which contain enzymatically cleavable sequences (GFLG) in both polymeric backbone and side-chains. This design allows using polymeric carriers with molecular weights above renal threshold without impairing their biocompatibility, thereby leading to significant improvement in therapeutic efficacy. For example, 2nd generation HPMA copolymer-epirubicin (EPI) conjugates (2P-EPI) demonstrated complete tumor regression in the treatment of mice bearing ovarian carcinoma. To obtain a better understanding of the in vivo fate of this system, we developed a dual-labeling strategy to simultaneously investigate the pharmacokinetics and biodistribution of the polymer carrier and drug EPI. First, we synthesized two different types of dual-radiolabeled conjugates, including 1) 111In-2P-EPI-125I (polymeric carrier 2P was radiolabeled with 111In and drug EPI with 125I), and 2) 125I-2P-EPI-111In (polymeric carrier 2P was radiolabeled with 125I and drug EPI with 111In). Then, we compared the pharmacokinetics and biodistribution of these two dual-labeled conjugates in female nude mice bearing A2780 human ovarian carcinoma. There was no significant difference in the blood circulation between polymeric carrier and payload; the carriers (111In-2P and 125I-2P) showed similar retention of radioactivity in both tumor and major organs except kidney. However, compared to 111In-labeled payload EPI, 125I-labeled EPI showed lower radioactivity in normal organs and tumor at 48 h and 144 h after intravenous administration of conjugates. This may be due to different drug release rates resulting from steric hindrance to the formation of enzyme-substrate complex as indicated by cleavage experiments with lysosomal enzymes (Tritosomes). A slower release rate of EPI(DTPA)111In than EPI(Tyr)125I was observed. It may be also due to in vivo catabolism and subsequent iodine loss as literature reported. Nevertheless, tumor-to-tissue uptake ratios of both radionuclides were comparable, indicating that drug-labeling strategy does not affect the tumor targeting ability of HPMA copolymer conjugates.

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