Thermal Cycling Enhances the Accumulation of a Temperature- Sensitive Biopolymer in Solid Tumors Matthew R. Dreher, 1 Wenge Liu, 1 Charles R. Michelich, 1,3 Mark W. Dewhirst, 2 and Ashutosh Chilkoti 1 1 Department of Biomedical Engineering, Duke University; 2 Department of Radiation Oncology, Duke University Medical Center, Durham, North Carolina; and 3 GE Healthcare, Waukesha, Wisconsin Abstract The delivery of anticancer therapeutics to solid tumors remains a critical problem in the treatment of cancer. This study reports a new methodology to target a temperature- responsive macromolecular drug carrier, an elastin-like polypeptide (ELP) to solid tumors. Using a dorsal skin fold window chamber model and intravital laser scanning confocal microscopy, we show that the ELP forms micron-sized aggregates that adhere to the tumor vasculature only when tumors are heated to 41.5°C. Upon return to normothermia, the vascular particles dissolve into the plasma, increasing the vascular concentration, which drives more ELPs across the tumor blood vessel and significantly increases its extravascu- lar accumulation. These observations suggested that thermal cycling of tumors would increase the exposure of tumor cells to ELP drug carriers. We investigated this hypothesis in this study by thermally cycling an implanted tumor in nude mice from body temperature to 41.5°C thrice within 1.5 h, and showed the repeated formation of adherent microparticles of ELP in the heated tumor vasculature in each thermal cycle. These results suggest that thermal cycling of tumors can be repeated multiple times to further increase the accumulation of a thermally responsive polymeric drug carrier in solid tumors over a single heat-cool cycle. More broadly, this study shows a new approach—tumor thermal cycling—to exploit stimuli-responsive polymers in vivo to target the tumor vas- culature or extravascular compartment with high specificity. [Cancer Res 2007;67(9):4418–24] Introduction The treatment of cancer with anticancer agents is typically limited by toxic side effects in normal tissues. The goal of drug delivery in the treatment of solid tumors is to increase the concentration of an anticancer agent within a tumor while limiting systemic exposure, thereby reducing normal tissue toxicity and increasing overall therapeutic efficacy (1, 2). Numerous drug delivery technologies have been developed to accomplish this goal, including liposomes (3), micelles (4), antibody-directed enzyme prodrug therapy (5), photodynamic therapy (6), affinity targeting (7), and macromolecular drug carriers (8, 9), the class of drug carriers that are the focus of this study. Macromolecular drug carriers typically consist of high–molec- ular weight polymers (>10 kDa) that are linked to a therapeutic agent and target solid tumors either ‘‘passively,’’ based on molecular weight and charge, or ‘‘actively,’’ due to a specific affinity or stimulus (9–11). The passive targeting of solid tumors by macromolecular carriers occurs through the enhanced permeabil- ity and retention effect (12–15), which is caused by the increased vascular permeability of tumors relative to normal tissues and a slower rate of clearance due to a lack of functional lymphatics (16). Combined, these two features of solid tumors result in the increased accumulation of macromolecules in tumors as compared with low–molecular weight drugs (12, 17). In addition to the enhanced permeability and retention effect, macromolecular drug carriers are an attractive drug delivery vehicle because they have longer plasma half-lives, reduced normal tissue toxicity, activity against multiple drug-resistant cell lines, and greater solubility than free drug (8, 9). These attributes often result in the better efficacy of macromolecular drug carriers as compared with low molecular weight drugs (8, 9). Recently, stimuli-responsive, ‘‘smart’’ polymers have been inves- tigated for drug and gene delivery because these polymers offer the opportunity to modulate their physicochemical properties within the tumor microenvironment (18–23), and therefore, enhance the delivery of their therapeutic payload within a specific tumor compartment. We are interested in a class of thermally responsive macromolecules called elastin-like polypeptides (ELPs) as drug carriers because ELPs combine the passive targeting afforded by the enhanced permeability and retention effect with active thermal targeting using externally focused hyperthermia (19, 24). ELPs are artificial repetitive polypeptides that undergo an inverse temper- ature phase transition (also called a lower critical solution temperature transition); they are soluble at temperatures below their transition temperature (T t ) but become insoluble and aggregate at temperatures above their T t (25–27). ELPs are composed of a Val-Pro-Gly-Xaa-Gly pentapeptide repeat (in which the ‘‘guest residue’’ Xaa is any amino acid except Pro) derived from a structural motif found in mammalian elastin (28, 29). The inverse temperature transition is fully reversible, such that the aggregated ELP becomes soluble when the temperature is decreased below its T t . We have previously shown that a thermally responsive ELP exhibits a 2-fold greater accumulation in solid tumors (24) that are heated to 42jC and an f2-fold increase in cellular uptake (30) under hyperthermic conditions when compared with an identical ELP under normothermia. In this study, we show a new method of tumor cell targeting that we term ‘‘tumor thermal cycling’’ that can further increase the accumulation of thermally responsive polymer drug carriers in solid tumors. In this strategy, the ELP is first administered i.v. and concentrated as immobile ELP particles that adhere only to tumor vasculature that is heated using externally Note: Supplementary data for this article are available at Cancer Research Online (http://cancerres.aacrjournals.org/). Requests for reprints: Ashutosh Chilkoti, Department of Biomedical Engineering, Duke University, Box 90281, Durham, NC 27708. E-mail: chilkoti@duke.edu. I2007 American Association for Cancer Research. doi:10.1158/0008-5472.CAN-06-4444 Cancer Res 2007; 67: (9). May 1, 2007 4418 www.aacrjournals.org Research Article Research. on February 6, 2016. © 2007 American Association for Cancer cancerres.aacrjournals.org Downloaded from