Abstract
Rapalogues are a unique class of drugs with both cytostatic and immunosuppressive properties. Two founding members, rapamycin (Rapa) and its chemical derivative everolimus (Eve), are extremely potent, but their clinical use presents multiple challenges. Being water-insoluble, administration is restricted to the oral route, which results in a low bioavailability of <10%. Human studies of rapalogues are reported to yield a high blood to plasma ratio and poor correlation between blood concentration and dose. Moreover, treatment results in dose-limiting toxicities such as stomatitis and pneumonitis, which often leads to discontinuation of therapy. We previously reported an elastin-like polypeptide decorated with two-headed FKBP rapalogue-binding domains. Called "FAF", this biomacromolecular drug-carrier solubilizes, retargets, and releases rapalogues within disease sites. FAF-rapalogue formulations are free of cosolvents or surfactants, which promotes their parenteral administration. In this study, subcutaneously given FAF-Rapa significantly suppressed tumor growth in a mouse model of hormone receptor positive (HR+) breast cancer, compared to an oral formulation of Eve (Affinitor). Additionally, mTOR, the pharmacological target of rapalogues, was inhibited to a greater extent in tumors of FAF-Rapa and FAF-Eve groups compared to mice that received oral Eve. No signaling suppression was detected in the liver and spleen, which were evaluated to represent off-target organs exposed to the circulating formulation.