Getting in shape: controlling peptide bioactivity and bioavailability using conformational constraints

Abstract

Chemical biologists commonly seek out correlations between the physicochemical properties of molecules and their behavior in biological systems. However, a new paradigm is emerging for peptides in which conformation is recognized as the primary determinant of bioactivity and bioavailability. This review highlights an emerging body of work that directly addresses how a peptide's conformation controls its biological effects, cell penetration, and intestinal absorption. Based on this work, the dream of mimicking the potency and bioavailability of natural product peptides is getting closer to reality.

Figure 1

“To be steady as a rock and always trembling.” α-Amanitin and cyclosporine A provide contrasting lessons from natural products. Both are head-to-tail cyclic peptides, and deviations from the 20 proteinogenic amino acids are shown in red. α-Amanitin is locked into a single conformation by virtue of a sulfone-indole intramolecular cross-link. This protects it from proteolytic degradation despite having a largely unmodified peptide backbone and appears to promote gut absorption and transport into liver cells by organic anion transport proteins (OATPs). Cyclosporine, by contrast, survives digestive proteases by virtue of its highly N-methylated backbone. It can change conformations in order to form intramolecular hydrogen bonds in nonpolar environments. This is hypothesized to promote passive diffusion through plasma membranes.

Figure 2

Engineered peptides with high bioactivity and/or bioavailability. These constrained peptides vary greatly in size and hydrophobicity and employ different chemical cross-links, cyclizations, and folding topologies. (a) Somatostatin mimic with ~60 nM binding affinity to human somatostatin receptors sst2 and sst5. This compound permeates Caco-2 monolayers and has 7% oral bioavailability in rats. (b) Caco-2-penetrant cyclic peptide scaffold found by Kessler, Hoffman, and co-workers. (c) Cyclic peptide scaffold found by Lokey and co-workers to have 28% oral bioavailability in mice. (d) Cyclized α-conotoxin that targets GABA_B receptors and acts as an analgesic. Head-to-tail cyclization resulted in oral bioavailablility as judged by effects on pain-related phenotypes in rats. (e) Grafted analogue of kalata B that acts as an orally bioavailable analgesic that targets bradykinin receptors. (f) Stapled helix of Bid that is able to slow proliferation of leukemia xenografts. (g) Hydrogen-bond-surrogate helix that can penetrate cells and inhibit Ras.

Figure 3

New exceptions or new rules? Four head-to-tail cyclic peptides with significant oral bioavailability. Peptide backbones are shown in ball-and- stick, with side chains shown as thin lines; oxygens are in red, nitrogens are in blue, and hydrogens are omitted for clarity. All four peptides share similar turn conformations, despite having been developed via different strategies. (a) Three-dimensional structure of cyclosporine A in chloroform, thought to be representative of its structure when passing through lipid membranes.^,, (b) NMR structure of the optimized, orally bioavailable somatostatin mimic shown in Figure 2a. (c) NMR structure of an orally bioavailable scaffold found by Hoffman, Kessler, and co-workers, similar to the peptide shown in Figure 2b. (d) NMR structure of an orally bioavailable scaffold found by Lokey and co-workers, shown in Figure 2c.