Controlled Cellular Uptake of Elastin-Like Polypeptide Diblock Copolymers for Thermally Targeted Drug Delivery

Targeted drug delivery to solid tumors aims to increase the accumulation of drug at the site of disease while limiting accumulation in healthy tissues. Thus, targeted delivery serves to enhance therapeutic efficacy while minimizing off-target side effects. Targeting drug to the site of disease is especially important for many current anti-cancer therapeutics whose cytotoxic effects are not exclusive to cancer cells. Drug carriers can improve tumor targeting of drug cargo by either passive or active mechanisms. Passive targeting of drug carriers occurs by the enhanced permeability and retention effect, whereby long circulating drug carriers can accumulate in the tumor by extravasation from the tumor's leaky vasculature and be retained in the tumor due to the lack of an organized tumor lymphatic system. Alternatively, active targeting can improve drug delivery to the tumor by means of functionalizing a drug carrier such that it interacts specifically with the tumor tissue. Traditionally, actively targeted drug carriers rely on intrinsic features of the tumor such as upregulated cell receptors, overexpressed extracellular enzymes, or depressed tissue pH. These intrinsic targets, however, are heterogeneous across cancer classes and between patients with a single tumor type. Therefore traditional active targeting cannot be applied to a breadth of cancers or patients without prior knowledge of the cancer phenotype.

Active targeting can alternatively be achieved by an extrinsic trigger, independent of the characteristics of the tumor. This approach could thereby achieve targeted drug delivery in a breadth of tumor types and cancer patients. This dissertation describes one such approach that exploits cell-penetrating peptides (CPPs) to achieve receptor-independent and non-specific uptake in a variety of cancer cells. The function of this non-specific CPP is controlled by an extrinsic trigger by means of the modulation of its local interfacial density with temperature-triggered micelle assembly. Elastin-like polypeptide diblock copolymers (ELP_BCs) were used as the drug carrier platform, as their lower critical solution temperature phase transition behavior permits their controlled self-assembly from unimer to micelle in response to a thermal stimulus. CPP-ELP_BCs were recombinantly synthesized in E. coli with CPP-functionalization at their hydrophilic terminus, such that temperature-triggered micelle assembly would result in the decoration of CPP on the micelle corona. The CPP-ELP_BC design was carefully optimized to permit micelle self-assembly in response to the clinically relevant trigger of mild hyperthermia.

Temperature-triggered micelle assembly of CPP-ELP_BCs achieved controlled cellular uptake in vitro by means of their CPP density modulation, such that cellular uptake was minimized at physiologic temperature and was greatly enhanced at conditions of mild hyperthermia. This effect was achieved in multiple cell lines, albeit with variable magnitude. Controlled uptake of the CPP-ELP_BC carrier could control the intracellular delivery of appended drug cargo. This controlled intracellular delivery was translated to controlled therapeutic effect when the CPP-ELP_BC was genetically appended to a proapoptotic peptide drug cargo. These drug-loaded CPP-ELP_BCs achieved controlled cytotoxicity in cancer cells, whereby significant cell death was induced at conditions of mild hyperthermia, but cells at physiologic temperature were spared.

For use as targeted drug carriers in vivo, CPP-ELP_BCs would be systemically administered and circulate throughout the body in their soluble state. It would only be at the site of the solid tumor that local mild hyperthermia would be applied and induce the self-assembly of CPP-ELP_BC micelles that could induce internalization into cancer cells. Translation of CPP-ELP_BC function from in vitro to in vivo environments proved to be quite challenging. Issues such as perturbation of temperature-triggered assembly in serum and interference of CPP-ELP_BC internalization by serum proteins likely played a role in preventing the extrinsically targeted accumulation of CPP-ELP_BCs in hyperthermia treated tumors, as investigated by intravital tumor microscopy and biodistribution studies. Further optimization of the CPP-ELP_BC platform is thus required to achieve extrinsically targeted drug carrier delivery in vivo.