Enhancing Oral Bioavailability of Cyclic RGD Hexa-peptides by the Lipophilic Prodrug Charge Masking Approach: Redirection of Peptide Intestinal Permeability from a Paracellular to Transcellular Pathway

Hydrophilic peptides constitute most of the active peptides. They mostly permeate via tight junctions (paracellular pathway) in the intestine. This permeability mechanism restricts the magnitude of their oral absorption and bioavailability. We hypothesized that concealing the hydrophilic residues of the peptide using the lipophilic prodrug charge masking approach (LPCM) can improve the bioavailability of hydrophilic peptides. To test this hypothesis, a cyclic N-methylated hexapeptide containing Arg-Gly-Asp (RGD) and its prodrug derivatives, masking the Arg and Asp charged side chains, were synthesized. The library was evaluated for intestinal permeability in vitro using the Caco-2 model. Further investigation of metabolic stability ex vivo models in rat plasma, brush border membrane vesicles (BBMVs), and isolated CYP3A4 microsomes and pharmacokinetic studies was performed on a selected peptide and its prodrug (peptide 12). The parent drug analogues were found to have a low permeability rate in vitro, corresponding to atenolol, a marker for paracellular permeability. Moreover, palmitoyl carnitine increased the P_app of peptide 12 by 4-fold, indicating paracellular permeability. The P_app of the prodrug derivatives was much higher than that of their parent peptides. For instance, the P_app of the prodrug 12P was 20-fold higher than the P_app of peptide 12 in the apical to basolateral (AB) direction. Whereas the permeability in the opposite direction (BA of the Caco-2 model) was significantly faster than the P_app AB, indicating the involvement of an efflux system. These results were corroborated when verapamil, a P-gp inhibitor, was added to the Caco-2 model and increased the P_app AB of prodrug 12P by 3-fold. The prodrug 12P was stable in the BBMVs environment, yet degraded quickly (less than 5 min) in the plasma into the parent peptide 12. Pharmacokinetic studies in rats showed an increase in the bioavailability of peptide 12 > 70-fold (from 0.58 ± 0.11% to 43.8 ± 14.9%) after applying the LPCM method to peptide 12 and converting it to the prodrug 12P. To conclude, the LPCM approach converted the absorption mechanism of the polar peptides from a paracellular to transcellular pathway that tremendously affects their oral bioavailability. The LPCM method provides a solution for the poor bioavailability of RGD cyclohexapeptides and paves the way for other active hydrophilic and charged peptides with poor oral bioavailability.

A new and useful method for the macrocyclization of linear peptides

A new and useful procedure for the macrocyclization of linear peptides is described. The natural amino acid side chains of tyrosine (phenol), lysine (alkylamine), and histidine (imidazole) react in an intramolecular fashion with a pendent pyridine-N-oxide-carboxamide, which is selectively activated by the phosphonium salt, PyBroP. The reaction is mild, rapid, and efficient with a potentially large substrate scope. Multiple examples are provided with full characterization and analyses, including a novel aza-variant of the C-O-D ring system of vancomycin.

The synthesis and biological activity of linear and cyclic analogs of the two diuretic peptides of Diploptera punctata

We have investigated the effect of analogs of the two Dippu diuretic hormones, Dippu-DH(46) and Dippu-DH(31), on fluid secretion by Malpighian tubules of male Diploptera punctata. We synthesized analogs containing the amino acid methyl-homoserine, to replace methionine residues, to render these modified peptides less subject to oxidation. We have also synthesized C-terminal fragments and their corresponding cyclic analogs to determine their effect on fluid secretion in D. punctata. Our results indicate that the modified peptides retain significant activity in the Ramsay secretion assay. The linear fragments displayed no activity or some inhibitory activity whereas the cyclic analog fragments showed stimulatory activity, in the case of DH(46), or slight inhibitory activity, in the case of DH(31).

Total synthesis and revision of stereochemistry of the marine metabolite trunkamide A

The isolation of the cytotoxic Lissoclinum sp. metabolite trunkamide A was reported in 1996. After completion of a total synthesis in 1999, it became clear that the structure of this marine natural product had to be revised. We now report the first preparation of actual trunkamide A in a total synthesis that serves as an unambiguous structural and stereochemical proof. Highlights of our synthetic strategy are a Lewis acid assisted aziridine opening that was used for the preparation of the novel reverse-prenylated serine and threonine side chains as well as an efficient oxazoline-thiazoline interconversion on the macrocyclic skeleton. In addition, several stereoisomers prepared by complementary synthetic protocols serve to illustrate the general scope of our methodology and confirm the configurational assignment.