EVALUATION OF A VITAMIN-CLOAKING STRATEGY FOR OLIGOPEPTIDE THERAPEUTICS - BIOTINYLATED HIV-1 PROTEASE INHIBITORS

The outstanding limitations to the oligopeptide as a therapeutic agent are poor oral availability and rapid biliary clearance. To address th ese concerns a series of eight peptidic HIV-1 protease inhibitors cont aining the structural segment of the vitamin biotin have been prepared . These have been evaluated with regard to the hypothesis that this vi tamin would cloak the peptidic character of these oligopeptides, and t hus impart to these inhibitors the potential for absorption and distri bution via biotin transporters and receptors. By iterative optimizatio n about a -Chal psi[CH- (OH)CH(OH)]Val- core inhibitory insert, three particularly potent inhibitors (K-i less than or equal to 10 nM) of th e HIV-1 protease were obtained. Although excellent cell culture antivi ral activity is observed for other peptidic protease inhibitors of com parable affinity, none in this series exhibited satisfactory antiviral activity. This failure is-attributed to the incompatibility of the hy drophilic and hydrogen-bonding biotin segment, with the facile membran e permeability and intracellular access presumably required for antivi ral activity. The ability of the biotin to cloak the peptide, and thus render the overall appearance of the conjugate as that of a vitamin, was evaluated. Four of this series were evaluated for recognition by t he Caco-2 cell intestinal biotin transporter, None inhibited competiti vely biotin uptake, indicating a lack of recognition. A vitamin may bi nd to a specific protein carrier, and thus attain an improved serum pr ofile (by resistance to biliary clearance) and advantageous delivery t o cells. Therefore, the serum concentrations of three were evaluated f ollowing an iv bolus in a rat model for serum clearance. One of the th ree protease inhibitors (L-idonamide, l]amino]-N-[2-methyl-1-[[(2-pyri dinylmethyl)amino] carbonyl]butyl]-, [3aS-[3a alpha,4 beta(1R,2R*;3R* ),6a alpha]]-) sustained a more than 5-fold increase in serum concentr ation at all time points relative to the benchmark structure. The rema ining two had serum concentrations at least equal to the benchmark, su ggestive of improved resistance to clearance. One(L-idonamide, o]-2-(1 -methylethyl)-N-[2-methyl-1-[[(2-pyridinyl- methyl)amino]carbonyl]buty l]-, [3aS-[3a alpha,4 beta(1R,2R*),6a alpha]]-) was prepared as a com plex with the biotin-binding protein avidin. Avidin may resemble an en dogenous serum biotin carrier protein. The antiviral activity (evaluat ed in an H9-HTLV(IIIB) acute HIV-1 infection assay) of the inhibitor a nd the avidin complex was identical. This suggests that the avidin-inh ibitor complex is capable of cell internalization. Although the weak a ntiviral activity of these biotinylated inhibitors precludes considera tion as practical HIV therapeutics, the overall data remain suggestive of vitamin cloaking of oligopeptides as a strategy of potential value .