Injectable intratumoral depot of thermally responsive polypeptide–radionuclide conjugates delays tumor progression in a mouse model

This study evaluated a biodegradable drug delivery system for local cancer radiotherapy consisting of a thermally sensitive elastin-like polypeptide (ELP) conjugated to a therapeutic radionuclide. Two ELPs (49 kDa) were synthesized using genetic engineering to test the hypothesis that injectable biopolymeric depots can retain radionuclides locally and reduce the growth of tumors. A thermally sensitive polypeptide, ELP 1, was designed to spontaneously undergo a soluble–insoluble phase transition (forming viscous microparticles) between room temperature and body temperature upon intratumoral injection, while ELP 2 was designed to remain soluble upon injection and to serve as a negative control for the effect of aggregate assembly. After intratumoral administration of radionuclide conjugates of ELPs into implanted tumor xenografts in nude mice, their retention within the tumor, spatio-temporal distribution, and therapeutic effect were quantified. The residence time of the radionuclide–ELP 1 in the tumor was significantly longer than the thermally insensitive ELP 2 conjugate. In addition, the thermal transition of ELP 1 significantly protected the conjugated radionuclide from dehalogenation, whereas the conjugated radionuclide on ELP 2 was quickly eliminated from the tumor and cleaved from the biopolymer. These attributes of the thermally sensitive ELP 1 depot improved the antitumor efficacy of iodine-131 compared to the soluble ELP 2 control. This novel injectable and biodegradable depot has the potential to control advanced-stage cancers by reducing the bulk of inoperable tumors, enabling surgical removal of de-bulked tumors, and preserving healthy tissues. [Display omitted]