- Elastin-like polypeptides are humanized, repetitive peptides that undergo temperature-dependent phase separation
- ELP fusion proteins can be designed to phase separate upon heating to physiological temperatures
- When fused with the FKBP protein, ELPs can specifically bind and carry rapamycin
- This formulation can be injected through a narrow-gauge needle and provide, while providing zero-order release for at least one month
- Through pharmacokinetic and efficacy studies, this approach has been evaluated in multiple mouse models of disease
The pharmacophore rapamycin is a potent macrolide that inhibits the mechanistic target of rapamycin complex 1 (mTORC1). Rapalogues exert clinically useful effects in diverse applications ranging from transplant rejection to cancer. Its oral formulation, called Sirolimus, has low bioavailability, poor solubility, and dose-limiting side effects that could benefit from advanced drug delivery. To do so, our group studies recombinant protein-polymers, derived from the human tropoelastin protein. These ‘elastin-like polypeptides’ (ELPs) consist of five amino acids repeated 24 to 192 times, which can be appended to fusion proteins in a number of ways. Of great interest, ELPs undergo a temperature-dependent phase separation into a secondary aqueous phase. Without solvents, this viscous phase assembles upon injection to the body, from which it elutes over a period of one month without toxicity. These ELPs can be expressed in direct fusion with other functional peptides and proteins, which gain the same thermal responsive assembly. ELPs linked to human fusion proteins appear tolerated by the immune system and biodegraded by lysosomal proteases upon cellular uptake. We harnessed this approach to specifically carry rapamycin by fusing ELPs to the human FKBP protein. FBKP is a 12 kDa cytosolic protein, which blocks mTORC1 activity upon complexation with rapamycin in the cytosol. FBKP-ELPs can complex with rapamycin, thus retaining the drug in a depot at the injection site until they are released into circulation. Molecular imaging studies and plasma sampling shows this formulation is absorbed by zero-order pharmacokinetics, which enables their steady release over extended periods. We have now developed single-injection formulations using this strategy that can both block breast cancer and graft-host rejection. This enables effective therapies after a single injection. If proven safe in humans, we propose that this strategy may be generally useful to develop sustained-release carriers for other drugs and indications.
Professor Andrew Mackay, Gavin Herbert Associate Professor of Pharmaceutical Sciences, University of Southern California School of Pharmacy
