Binding of multivalent anionic porphyrins to V3 loop fragments of an HIV-1 envelope and their antiviral activity

Interactions of multivalent anionic porphyrins and their iron(III) complexes with cationic peptides, V3_Ba-L and V3_IIIB, which correspond to those of the V3 loop regions of the gp120 envelope proteins of the HIV-1_Ba-L and HIV-1_IIIB strains, respectively, are studied by UV/Vis, circular dichroism,¹H NMR, and EPR spectroscopy, a microcalorimetric titration method, and anti-HIV assays. Tetrakis(3,5- dicarboxylatophenyl)porphyrin (P1), tetrakis[4-(3,5-dicarboxylatophenylmethoxy) phenyl]porphyrin (P2), and their ferric complexes (Fe^IIIP1 and Fe^IIIP2) were used as the multivalent anionic porphyrins. P1 and Fe^IIIP1 formed stable complexes with both V3 peptides (binding constant K>10⁶m^-1) through combined electrostatic and van der Waals interactions. Coordination of the His residues in V3 _Ba-L to the iron center of Fe^IIIP1 also played an important role in the complex stabilization. As P2 and Fe^IIIP2 form self-aggregates in aqueous solution even at low concentrations, detailed analysis of their interactions with the V3 peptides could not be performed. To ascertain whether the results obtained in the model system are applicable to a real biological system, anti-HIV-1_BA-L and HIV-1_IIIB activity of the porphyrins is examined by multiple nuclear activation of a galactosidase indicator (MAGI) and 3-(4,5-dimethylthiazol-2-yl)-2,5- diphenyltetrazolium bromide (MTT) assays. There is little correlation between chemical analysis and actual anti-HIV activity, and the size rather than the number of the anionic groups of the porphyrin is important for anti-HIV activity. All the porphyrins show high selectivity, low cytotoxicity, and high viral activity. Fe^IIIP1 and Fe^IIIP2 are used for the pharmacokinetic study. Half-lives of these iron porphyrins in serum of male Wistar rats are around 4 to 6 h owing to strong interaction of these porphyrins with serum albumin.