Long-term biodistribution study of HPMA-ran-LMA copolymers in vivo by means of 131I-labeling

Background: For the evaluation of macromolecular drug delivery systems suitable pre-clinical monitoring of potential nanocarrier systems is needed. In this regard, both short-term as well as long-term in vivo tracking is crucial to understand structure-property relationships of polymer carrier systems and their resulting pharmacokinetic profile. Based on former studies revealing favorable in vivo characteristics for F-18-labeled random (ran) copolymers consisting of N-(2-hydroxypropyl)methacrylamide (HPMA) and lauryl methacrylate (LMA) - including prolonged plasma half-life as well as enhanced tumor accumulation - the presented work focuses on their long-term investigation in the living organism. Methods: In this respect, four different HPMA-based polymers (homopolymers as well as random copolymers with LMA as hydrophobic segment) were synthesized and subsequent radioactive labeling was accomplished via the longer-lived radioisotope I-131. In vivo results, concentrating on the pharmacokinetics of a high molecular weight HPMA-ran-LMA copolymer, were obtained by means of biodistribution and metabolism studies in the Walker 256 mammary carcinoma model over a time-span of up to three days. Besides, a direct comparison with the F-18-radiolabeled polymer was drawn. To consider physico-chemical differences between the differently labeled polymer (F-18 or I-131) on the critical micelle concentration (CMC) and the size of the polymeric micelles, those properties were determined using the F-19- or I-127-functionalized polymer. Special emphasis was laid on the time-dependent correlation between blood circulation properties and corresponding tumor accumulation, particularly regarding the enhanced permeability and retention (EPR) effect. Results: Studies revealed, at first, differences in the short time (2 h) body distribution, despite the very similar properties (molecular structure, CMC and size of the micellar aggregates) of the non-radioactive F-19- and I-127-functionalized polymers. Long-term investigations with the I-131-labeled polymer demonstrated that, despite a polymer clearance from the blood within 72 h, there was still an increase in tumor uptake observed over time. Regarding the stability of the I-131-label, ex vivo biodistribution experiments, considering the uptake in the thyroid, indicated low metabolism rates. Conclusion: The observed in vivo characteristics strongly underline the EPR effect. The findings illustrate the need to combine information of different labeling approaches and in vivo evaluation techniques to generate an overall pharmacokinetic picture of potential nanocarriers in the pre-clinical setting. (C) 2017 Elsevier Inc. All rights reserved.