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The Role of Thr139 in the Human Immunodeficiency Virus Type 1 Reverse Transcriptase Sensitivity to (+)-Calanolide A

Rega Institute for Medical Research, Katholieke Universiteit Leuven, Leuven, Belgium (J.A., E.D.C., J.B.); Department of Pharmacology, University of Alcalá, Alcalá de Henares, Spain (F.R.-B., F.G.); Department of Experimental Medicine, University of Rome "Tor Vergata", Rome, Italy (F.C.-S., C.-F.P.); and Instituto de Química Médica, Consejo Superior de Investigaciones Científicas, Madrid, Spain (A.S.-F., S.V., M.-J.C.)

The coumarins represent a unique class of non-nucleoside reverse transcriptase inhibitors (NNRTIs) that were isolated from tropical plants. (+)-Calanolide A, the most potent compound of this class, selects for the T139I resistance mutation in HIV-1 reverse transcriptase (RT). Seven RTs mutated at amino acid position 139 (Ala, Lys, Tyr, Asp, Ile, Ser, and Gln) were constructed by site-directed mutagenesis. The mutant T139Q enzyme retained full catalytic activity compared with wild-type RT, whereas the mutant T139I, T139S, and T139A RTs retained only 85 to 50% of the activity. Mutant T139K, T139D, and T139Y RTs had seriously impaired catalytic activities. The mutations in the T139I and T139D RTs were shown to destabilize the RT heterodimer. (+)-Calanolide A lost inhibitory activity (up to 20-fold) against the mutant T139Y, T139Q, T139K, and T139I enzymes. All of the mutant enzymes retained marked susceptibility toward the other NNRTIs, including nevirapine, delavirdine, efavirenz, thiocarboxanilide UC-781, quinoxaline GW867420X, TSAO [[2',5'-bis-O-(tert-butyldimethylsilyl)--D-ribofuranosyl]-3'-spiro-5''-(4''-amino-1'',2''-oxathiole-2'',2''-dioxide)] derivatives, and the nucleoside inhibitor, ddGTP. The fact that the T139I RT 1) proved to be resistant to (+)-calanolide A, 2) represents a catalytically efficient enzyme, and 3) requires only a single transition point mutation (ACAATA) in codon 139 seems to explain why mutant T139I RT virus strains, but not virus strains containing other amino acid changes at this position, predominantly emerge in cell cultures under (+)-calanolide A pressure.

Address correspondence to: Dr. J. Balzarini, Rega Institute for Medical Research, Minderbroedersstraat 10, B-3000 Leuven, Belgium. E-mail: jan.balzarini{at}rega.kuleuven.be