ObjectiveTo determine whether short peptides corresponding to the RGPGR motif of the V3 loop of gp120 have anti-human immunodeficiency virus type 1 (anti–HIV-1) activity.Design/MethodsShort peptides were tested against the HIV-1 laboratory strains and clinical isolates.ResultsThe tripeptide glycyl–prolyl–glycine amide (GPG-NH2) inhibited the replication of both laboratory strains and 47 clinical isolates, including 19 strains that were resistant to other drugs or that were from patients with failing therapy. The 50% inhibitory concentrations values were 2.7 to 37 &mgr;M. Phenotypic change of two isolates from nonsyncytia-inducing to syncytia-inducing did not change their sensitivity to GPG-NH2. The tripeptide added to the antiviral effect of both zidovudine and ritonavir.ConclusionsThe tripeptide GPG-NH2is a nontoxic compound that inhibits the replication of HIV-1 by an apparently new mode of action. Glycyl–prolyl–glycine-NH2might prove useful by itself or as a lead compound for the treatment of drug-resistant HIV-1. Glycyl–prolyl–glycine-NH2is currently undergoing phase I/II human clinical trials in Sweden.