Isothiazolone derivatives selectively inhibit telomerase from human and rat cancer cells in vitro

The telomere hypothesis postulates stabilization of telomere length and tel omerase activation as key events in cellular immortalization and carcinogen eses. Accordingly, telomerase has been suggested as a novel and highly sele ctive target for design of antitumor drugs. Screening of a chemical library including 16 000 synthetic compounds yielded six that strongly inhibited t elomerase activity in extracts of cultured human cells, including four isot hiazolone derivatives and two unrelated compounds. The most potent inhibito r was 2-[3-(trifluoromethyl)phenyl]isothiazolin-3-one (TMPI), a concentrati on of 1.0 mu M inhibited telomerase activity by 50% according to a telomere repeat amplification protocol (TRAP) assay. Analysis using partially purif ied telomerase from AH7974 rat hepatoma cells demonstrated noncompetitive i nhibition with the telomere-repeat primer and mixed inhibition with the dNT Ps; the inhibition constant was 2.5 mu M. TMPI did not inhibit eukaryotic D NA polymerase alpha, beta, or human immunodeficiency virus reverse transcri ptase (HIV RT), Thus, inhibition by TMPI was highly selective for telomeras e, Inhibition by TMPI was quenched by 1 mM of dithiothreitol or glutathione , suggesting that TMPI inhibits telomerase by acting at a cystein residue. TMPI inhibition of this enzyme may find application as an antineoplastic ag ent.