Peptides 12-25
amino acids in length from the V1J1 region of the
CD4 molecule (residues 1-120) were synthesized as randomly derivatized, deliberately derivatized, or pure
peptide products, and tested for their ability to inhibit HIV-1-induced cell fusion, HIV-1 and SIV
infection of CD4-positive human cells, HIV-1 envelope
glycoprotein binding to the
CD4 molecule, CD4-neutralizing antibody binding to the CD4 holoreceptor, and CD4-dependent cellular immune function in the mixed lymphocyte and cytotoxic T-cell bioassays. Only
peptides derived from the
complementarity-determining region 3 (CDR3)-homologous domain of CD4, in particular
CD4(81-92) and CD4(81-101), were effective
antiviral agents. Within the
CD4(81-92) series, R-group derivatization of selective
amino acid residues was an absolute requirement for
biological activity. The prototype compound T1C4E5-tribenzyl-K10-acetyl-TYICEVEDQKEE inhibited HIV-1-induced cell fusion at 32 microM, HIV-1
infection of CEM-SS cells
at 10 microM, SIV
infection of CEM-174 cells at less than 125 microM, gp120/CD4 binding at 60 microM, and postinfection cell-mediated viral transmission
at 10-15 microM. Compounds of identical structure and derivatization, but of altered primary sequence, were substantially less active, or without activity, in these assays. These data indicate that the effect of
amino acid derivatization of the
CD4(81-92) peptide was most likely restriction of the flexible underivatized
peptide backbone to a conformation closely approximating that of the CDR3-homologous gp120 binding site of the native
CD4 molecule.
Peptide antiviral activity was specific, as judged by lack of cytotoxicity, lack of inhibition of HTLV-1-induced cell fusion, and lack of inhibition of CD4-dependent cellular immune function in vitro. Further derivatization of the prototype compound involving the production of cyclic congeners yielded
peptides with submicromolar potency to block HIV-1
infection, strengthening the hypothesis that previous
peptide derivations accomplished partial restriction of the conformation of
CD4(81-92) to one favorable for interaction with gp120. Concentrations of the original prototype compound T1C4E5-tribenzyl-CD4(81-92) that inhibited
infection in vitro more than 50% could be achieved for several hours by
intravenous infusion in primates and were well-tolerated at these levels. The
peptide was not efficacious to inhibit establishment of
viral infection at these doses; however,
peptide treatment did lower average viral antigenemia and delay the cumulative time to morbidity relative to the control group.