An engineered human follistatin variant: insights into the pharmacokinetic and pharmocodynamic relationships of a novel molecule with broad therapeutic potential.

Human follistatin is a regulatory glycoprotein with widespread biologic functions, including antiinflammatory activities, wound-healing properties, and muscle-stimulating effects. The role of follistatin in a wide range of biologic activities shows promise for potential clinical application, which has prompted considerable interest in the investigation of the protein as a potential disease-modifying agent. In spite of this potential, the development of follistatin as a broad use biotherapeutic has been severely hindered by a poor understanding and characterization of its pharmacokinetic/pharmacodynamic (PK/PD) relationships. Therefore, to better define these relationships, we performed in-depth analyses of the PK/PD relationships of native follistatin-315 (FST315). Our data indicate that the intrinsic PK/PD properties of native FST315 are poorly suited for acting as a parentally administered biotherapeutic with broad systemic effects. Here, we leveraged protein engineering to modify the PK characteristics of the native molecule by fusing FST315 to a murine IgG(1) Fc and removing the intrinsic heparan sulfate-binding activity of follistatin. The engineered variant molecule had ~100- and ~1600-fold improvements in terminal half-life and exposure, respectively. In contrast to the native FST315, the variant showed a robust, dose-dependent pharmacological effect when administered subcutaneously on a weekly basis in mouse models of muscle atrophy and degeneration. These studies highlight the underappreciated and critical relationship between optimizing multiple physical and chemical properties of follistatin on its overall PK/PD profile. Moreover, our findings provide the first documented strategy toward the development of a follistatin therapeutic with potential use in patients affected with skeletal muscle diseases.