Glycine-Amide Is an Active Metabolite of the Antiretroviral Tripeptide Glycyl-Prolyl-Glycine-Amide

The chemically modified tripeptide glycyl-prolyl-glycine-amide (GPG-NH 2 ) inhibits replication of human immunodeficiency virus (HIV) type 1 (HIV-1) in vitro, probably by interfering with capsid formation. The aim of the present study was to determine whether the metabolites glycyl-proline (GP-OH), glycine (G-OH), prolyl-glycine-amide (PG-NH 2 ), proline (P-OH), and glycine-amide (G-NH 2 ) from proteolytic cleavage may inhibit the replication of HIV-1 in vitro. PG-NH 2 has previously been shown to have a modest effect on HIV-1 replication. In the present study we show that G-NH 2 exhibits a pronounced inhibitory effect on HIV-1. This effect was not due to a decrease in cell proliferation or viability and could not be shown for herpes simplex virus type 1. The G-NH 2 concentration that inhibited virus replication by 50% (IC 50 ) was equimolar to that of GPG-NH 2 and ranged from 3 to 41 μM. Transmission electron microscopy revealed that the effect of G-NH 2 on HIV-1 morphology was equivalent to that of GPG-NH 2 and showed disarranged capsid structures, indicating interference with capsid formation. Serial passage of HIV-infected cells with G-NH 2 for more than 20 subcultivations did not decrease the susceptibility to the compound. The results from this study suggest that GPG-NH 2 might act as a prodrug and that G-NH 2 is an active antiretroviral metabolite.