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

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) (111)In-2P-EPI-(125)I (polymeric carrier 2P was radiolabeled with (111)In and drug EPI with (125)I), and 2) (125)I-2P-EPI-(111)In (polymeric carrier 2P was radiolabeled with (125)I and drug EPI with (111)In). 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 ((111)In-2P and (125)I-2P) showed similar retention of radioactivity in both tumor and major organs except kidney. However, compared to (111)In-labeled payload EPI, (125)I-labeled EPI showed lower radioactivity in normal organs and tumor at 48h and 144h 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)(111)In than EPI(Tyr)(125)I 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.