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) co-polymer-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) In-111-2P-EPI-I-125 (polymeric carrier 2P was radiolabeled with In-111 and drug EPI with I-125), and 2) I-125-2P-EPI-In-111 (polymeric carrier 2P was radiolabeled with I-125 and drug EPI with In-111). 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 (In-111-2P and I-125-2P) showed similar retention of radioactivity in both tumor and major organs except kidney. However, compared to In-111-labeled payload EPI, I-125-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)In-111 than EPI(Tyr)I-125 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 radionuclideswere comparable, indicating that drug-labeling strategy does not affect the tumor targeting ability of HPMA copolymer conjugates. (C) 2016 Elsevier B.V. All rights reserved.