177Lu-labeled HPMA copolymers utilizing cathepsin B and S cleavable linkers: Synthesis, characterization and preliminary in vivo investigation in a pancreatic cancer model

Introduction: A major barrier to the advancement of therapeutic nanomedicines has been the non-target toxicity caused by the accumulation of the drug delivery systems in organs associated with the reticuloendothelial system, particularly the liver and spleen. Herein, we report the development of peptide based metabolically active linkers (MALs) that are enzymatically cleaved by cysteine cathepsin B and S, two proteases highly expressed in the liver and spleen. The overall goal of this approach is to utilize the MALs to lower the non-target retention and toxicity of radiolabeled drug delivery systems, thus resulting in higher diagnostic and radiotherapeutic efficacy. Methods: In this study three MALs (MAL0, MAL1 and MAL2) were investigated. MAL1 and MAL2 are composed of known substrates of cathepsin B and S, respectively, while MALO is a non-cleavable control. Both MAL1 and MAL2 were shown to undergo enzymatic cleavage with the appropriate cathepsin protease. Subsequent to conjugation to the HPMA copolymer and radiolabeling with Lu-177, the peptide-polymer conjugates were renamed Lu-177-metabolically active copolymers (Lu-177-MACs) with the corresponding designations: Lu-177-MAC0, Lu-177-MAC1 and Lu-177-MAC2. Results: In vivo evaluation of the Lu-177-MACs was performed in an HPAC human pancreatic cancer xenograft mouse model. Lu-177-MAC1 and Lu-177-MAC2 demonstrated 3.1 and 2.1 fold lower liver retention, respectively, compared to control (Lu-177-MAC0) at 72 h post-injection. With regard to spleen retention, Lu-177-MAC1 and Lu-177-MAC2 each exhibited a nearly fourfold lower retention, relative to control, at the 72 h time point. However, the tumor accumulation of the Lu-177-MAC0 was two to three times greater than Lu-177-MAC1 and Lu-177-MAC2 at the same time point. The MAL approach demonstrated the capability of substantially reducing the non-target retention of the Lu-177-labeled HPMA copolymers. Conclusions: While further studies are needed to optimize the pharmacokinetics of the Lu-177-MACs design, the ability of the MAL to significantly decrease non-target retention establishes the potential this avenue of research may have for the improvement of diagnostic and radiotherapeutic drug delivery systems. (C) 2013 Elsevier Inc. All rights reserved.