Controlled Release Systems for Treating Type 2 Diabetes and Their Application Toward Multi-Agonist Combination Therapies

Over 30 million people in the United States suffer from type 2 diabetes (T2D), and this figure is rapidly increasing. Currently available glucose-lowering drugs largely treat the symptoms of diabetes and not the underlying pathology, leaving one third of diabetes patients with improperly managed disease. Thus, there exists an urgent need for novel drugs that slow T2D progression while posing a minimal burden on the patient.

The metabolic regulatory factor fibroblast growth factor 21 (FGF21) is under investigation as a T2D therapeutic due to its favorable effects on glycemic control and body weight. However, the feasibility of native FGF21 as a drug candidate is impeded by its rapid in vivo clearance and by costly production methods associated with poor protein solubility. To address these issues, FGF21 was recombinantly expressed in E. coli as a fusion with an elastin-like polypeptide (ELP), a repetitive peptide polymer with reversible thermal phase behavior. Below their transition temperature (Tt), ELPs exist as soluble unimers, while above their Tt, they aggregate into an insoluble coacervate. The thermal responsiveness of the ELP was retained when genetically fused to FGF21, with several notably positive impacts for the synthesis and efficacy of this protein drug. First, the ELP fusion partner acted as a solubility enhancer, yielding 50 mg/L of active FGF21 protein from the soluble cell lysate fraction in shaker flask culture, and eliminating the need for protein refolding. Second, the phase transition behavior of the ELP was exploited for chromatography-free FGF21 purification. Third, the Tt of the ELP was tuned to below body temperature, such that the phase transition was initiated solely by body heat. Indeed, in vivo injection of the fusion resulted in an immiscible viscous phase in the subcutaneous (s.c.) space that dissolved at a steady rate, temporally releasing fusion unimers into circulation. The injectable FGF21 drug depot was tested in diabetic ob/ob mice, and conferred dose-dependent glucose-and weight-lowering effects that were sustained out to 5 days following a single s.c. injection.

Once an optimized ELP-based FGF21 delivery strategy was established, the fusion concept was applied to a combination therapy to afford even greater metabolic benefits, while providing controlled release properties exclusive to the ELP platform. Recent evidence supports the development of combination drug treatments that incorporate complementary mechanisms of action to more effectively treat T2D. Thus, we developed a unimolecular dual agonist by combining the incretin glucagon-like peptide-1 (GLP1) with FGF21, hypothesizing that this agent would merge the insulinotropic and anorectic effects of GLP1 with the enhanced insulin sensitivity and energy expenditure associated with FGF21 signaling. The dual agonist was designed as a single polypeptide fusion, with GLP1 located at the N terminus and FGF21 at the C terminus. This orientation allowed each peptide to activate its endogenous receptor, while the linear architecture enabled facile synthesis in a bacterial expression system. An ELP was fused between GLP1 and FGF21 to serve as both a flexible linker and a depot-forming delivery scaffold. Indeed, a single s.c. injection of GLP1-ELP-FGF21 into diabetic db/db mice resulted in potent metabolic effects that were sustained at least 7 days, indicating formation of an ELP depot with a highly controlled rate of drug release. Furthermore, dual agonist treatment outperformed a long-acting GLP1 analog in restoring glycemic control and inducing weight loss, supporting the rationale for a GLP1/FGF21 combination therapy.

With a significant proportion of T2D patients failing to properly manage their disease, there is an urgent need for novel drug and drug combinations that effectively target disease pathophysiology, while posing a minimal burden on the patient. Meanwhile, the vast – and global – prevalence of metabolic disease argues for cost-effective and scalable manufacturing methods for new drugs. An ELP-based approach to therapeutics precisely addresses these needs by providing a streamlined method for production, as well as an innovative strategy for drug delivery to reduce the frequency of administration and thereby promote patient compliance. Furthermore, the ELP platform can be utilized to unite distinct drugs into one multi-functioning molecule to more effectively treat diabetes, altogether simplifying and improving metabolic disease management.