Enhancing the oral bioavailability of peptide drug leads is a major challenge in drug design. As such, methods to address this challenge are highly sought after by the pharmaceutical industry. Here, we propose a strategy to identify appropriate amides for N-methylation using temperature coefficients measured by NMR to identify exposed amides in cyclic peptides. N-methylation effectively caps these amides, modifying the overall solvation properties of the peptides and making them more membrane permeable. The approach for identifying sites for N-methylation is a rapid alternative to the elucidation of 3D structures of peptide drug leads, which has been a commonly used structure-guided approach in the past. Five leucine-rich peptide scaffolds are reported with selectively designed N-methylated derivatives. In vitro membrane permeability was assessed by parallel artificial membrane permeability assay and Caco-2 assay. The most promising N-methylated peptide was then tested in vivo. Here we report a novel peptide (15), which displayed an oral bioavailability of 33% in a rat model, thus validating the design approach. We show that this approach can also be used to explain the notable increase in oral bioavailability of a somatostatin analog.cyclic peptide | permeability | N-methylation P eptides are potentially valuable compounds for drug development, offering many advantages over other molecular classes (1-4). Specifically, their ability to mimic endogenous bioactive molecules allows them to bind potently and selectively to "difficult" drug targets, including protein-protein interactions that are too challenging for small-molecule therapeutics. However, the widespread use of peptides in the clinic has been slow in coming, in large part because of their generally low stability in vivo, high clearance, and poor oral bioavailability.The low oral bioavailability of peptides is attributed to a disparity between their physicochemical properties and those traditionally expected for "drug-likeness" (5, 6), leading to a perception that peptides are good drug leads but poor drugs. However, this perception is being challenged by a growing number of peptides that seem to be stable (7) and well absorbed within the gastrointestinal tract (8) and examples in which cyclic peptides have shown orally delivered bioactivity in animal disease models, including inflammatory pain (9) and neuropathic pain (10), prompting us to devise new rules for predicting pharmacokinetic properties of this compound class. Arguably the most famous example of a peptide with poor drug-likeness but reasonable oral bioavailability is cyclosporin A, widely used as the immunosuppressant drug cyclosporine (11). Two structural features of cyclosporin A in particular are thought to contribute to its oral bioavailability, namely its macrocyclic architecture and backbone N-methylation.Cyclization imparts increased rigidity to a parent peptide, which not only improves its stability against proteolytic degradation but also directs it into specific conformations that m...

An increasing number of macrocyclic peptides that cross biological membranes are being reported, suggesting that it might be possible to develop peptides into orally bioavailable therapeutics; however, current understanding of what makes macrocyclic peptides cell permeable is still limited. Here, we synthesized 62 cyclic hexapeptides and characterized their permeability using in vitro assays commonly used to predict in vivo absorption rates, i.e. the Caco-2 and PAMPA assays. We correlated permeability with experimentally measured parameters of peptide conformation obtained using rapid methods based on chromatography and nuclear magnetic resonance spectroscopy. Based on these correlations, we propose a model describing the interplay between peptide permeability, lipophilicity and hydrogen bonding potential. Specifically, peptides with very high permeability have high lipophilicity and few solvent hydrogen bond interactions, whereas peptides with very low permeability have low lipophilicity or many solvent interactions. Our model is supported by molecular dynamics simulations of the cyclic peptides calculated in explicit solvent, providing a structural basis for the observed correlations. This prospective exploration into biomarkers of peptide permeability has the potential to unlock wider opportunities for development of peptides into drugs.3

Little is known about the molecular basis of somatosensory mechanotransduction in mammals. We screened a library of peptide toxins for effects on mechanically activated currents in cultured dorsal root ganglion neurons. One conopeptide analogue, termed NMB-1 for noxious mechanosensation blocker 1, selectively inhibits (IC50 1 µM) sustained mechanically activated currents in a subset of sensory neurons. Biotinylated NMB-1 retains activity and binds selectively to peripherin-positive nociceptive sensory neurons. The selectivity of NMB-1 was confirmed by the fact that it has no inhibitory effects on voltage-gated sodium and calcium channels, or ligand-gated channels such as acid-sensing ion channels or TRPA1 channels. Conversely, the tarantula toxin, GsMTx-4, which inhibits stretch-activated ion channels, had no effects on mechanically activated currents in sensory neurons. In behavioral assays, NMB-1 inhibits responses only to high intensity, painful mechanical stimulation and has no effects on low intensity mechanical stimulation or thermosensation. Unexpectedly, NMB-1 was found to also be an inhibitor of rapid FM1-43 loading (a measure of mechanotransduction) in cochlear hair cells. These data demonstrate that pharmacologically distinct channels respond to distinct types of mechanical stimuli and suggest that mechanically activated sustained currents underlie noxious mechanosensation. NMB-1 thus provides a novel diagnostic tool for the molecular definition of channels involved in hearing and pressure-evoked pain.

Norepinephrine (NE) amplifies the strength of descending pain inhibition, giving inhibitors of spinal NET clinical utility in the management of pain. chi-MrIA isolated from the venom of a predatory marine snail noncompetitively inhibits NET and reverses allodynia in rat models of neuropathic pain. An analogue of chi-MrIA has been found to be a suitable drug candidate. On the basis of the NMR solution structure of this related peptide, Xen2174 (3), and structure-activity relationships of analogues, a pharmacophore model for the allosteric binding of 3 to NET is proposed. It is shown that 3 interacts with NET predominantly through amino acids in the first loop, forming a tight inverse turn presenting amino acids Tyr7, Lys8, and Leu9 in an orientation allowing for high affinity interaction with NET. The second loop interacts with a large hydrophobic pocket within the transporter. Analogues based on the pharmacophore demonstrated activities that support the proposed model. On the basis of improved chemical stability and a wide therapeutic index, 3 was selected for further development and is currently in phase II clinical trials.

Development of peptide-based drugs has been severely limited by lack of oral bioavailability with less than a handful of peptides being truly orally bioavailable, mainly cyclic peptides with N-methyl amino acids and few hydrogen bond donors. Here we report that cyclic penta-and hexaleucine peptides, with no N-methylation and five or six amide NH protons, exhibit some degree of oral bioavailability (4− 17%) approaching that of the heavily N-methylated drug cyclosporine (22%) under the same conditions. These simple cyclic peptides demonstrate that oral bioavailability is achievable for peptides that fall outside of rule-of-five guidelines without the need for N-methylation or modified amino acids.