Abstract
Follistatin (FS) is an important regulatory protein, a natural antagonist for transforming growth factor beta family members activin and myostatin. The diverse biological roles of the activin and myostatin signaling pathways make FS a promising therapeutic target for treating human diseases exhibiting inflammation, fibrosis and muscle disorders, such as Duchenne muscular dystrophy. However, rapid heparin-mediated hepatic clearance of FS limits its therapeutic potential. We targeted the heparin binding loop of FS for site-directed mutagenesis to improve clearance parameters. By generating a series of FS variants with one, two, or three negative amino acid substitutions, we demonstrated a direct and proportional relationship between the degree of heparin binding affinity in vitro and the exposure in vivo. The triple mutation K(76,81,82)E abolished heparin binding affinity, resulting in ~20-fold improved in vivo exposure. This triple mutant retains full functional activity and antibody-like pharmacokinetic profile, and shows a superior developability profile in physical stability and cell productivity compared to FS variants which substitute the entire heparin binding loop with alternative sequences. Our surgical approach to mutagenesis should also reduce the immunogenicity risk. To further lower this risk, we introduced a novel glycosylation site into the heparin-binding loop. This hyperglycosylated variant showed a 10-fold improved exposure and decreased clearance in mice compared an IgG1 Fc fusion protein containing the native FS sequence. Collectively, our data highlights the importance of improving pharmacokinetic properties by manipulating heparin binding affinity and glycosylation content and provide a valuable guideline to design desirable therapeutic FS molecules.
- The American Society for Pharmacology and Experimental Therapeutics