Protein mirror images could generate long-lasting biologics

Peptide- and protein-based biologic drugs have a problem. Enzymes in the body called proteases break down the linkages between L-amino acids in natural peptides and proteins. As a result, biologics typically have short lifetimes in the body and need to be injected or infused instead of taken as pills.

Synthetic analogs of natural peptides and proteins consisting of D-amino acids are safe from proteases. But drugmakers can’t just change L-amino acids to D-amino acids because this alters the orientation of the molecules’ sidechains, disrupting the way a peptide or protein drug binds to its target.

Two techniques, retroinversion and mirror-image phage display (MIPD), can create D-amino-acid analogs of natural bioactive peptides and proteins that bind targets effectively. But retroinversion—reversing a peptide’s sequence and changing its amino acids to D-versions—is ineffective for peptides with α-helical binding motifs because it doesn’t change the direction of helix rotation, causing helical binding features to be oriented differently than in natural peptides. And MIPD—screening libraries to find L-peptides that bind D-versions of drug targets and then synthesizing the corresponding D-peptides—doesn’t work for large protein targets such as G protein-coupled receptors because those targets are too hard to synthesize.

Philip M. Kim, postdoc Michael Garton, and coworkers at the University of Toronto have now devised a way to make D and D-protein analogs that can bind most biological targets (Proc. Natl. Acad. Sci. USA 2018, DOI: 10.1073/pnas.1711837115).

They computationally generated a D version of every protein in the Protein Data Bank (PDB), creating the D-PDB, and extracted the D-proteins’ α-helices into more than 2.8 million separate database files. They then used known drug-target interactions to screen the helix database for D-helices with binding features positioned similarly to those of natural peptide and protein drugs. To create matches for drugs that bind in complex ways, the researchers made short D-strands by retroinversion and used the strands to link D-helices into three-part D-analogs.

Kim and coworkers used the method to create D-analogs for GLP-1, a diabetes and obesity treatment that targets the GLP-1 receptor, and parathyroid hormone, an osteoporosis medication that hits the parathyroid receptor. The D-analogs had about the same efficacy as their natural counterparts in cells, although the GLP-1 replacement required a higher dose. And the D-analogs withstood the cells’ proteases for longer than the natural peptides.

Danny Hung-Chieh Chou of the University of Utah notes that the technique is limited to drugs with a known binding mechanism but calls it “a great concept that solves a problem existing technology couldn’t handle.” Bradley L. Pentelute of MIT adds that he believes the approach “will be a very useful tool for the biotech community.”