Inhibitors of Delta(24(25)) sterol methyltransferase block sterol synthesis and cell proliferation in Pneumocystis carinii

Detailed analysis of the endogenous sterol content of purified Pneumocystis carinii preparations by gas-liquid chromatography coupled to mass spectrometry suggested that this parasite can both synthesize de novo steroid skeletons (to produce Delta(7) sterols) and take them from the infected host (lending to Delta(5) sterols). In both cases the final products are 24-alkyl sterols, resulting from the action of Delta(24(25)) and Delta(24(24')) sterol methyltransferases, enzymes not present in vertebrates, To investigate the physiological significance of these sterols, cultures of P. carinii in embryonic lung cells were exposed to 22,26-azasterol (20-piperidin-2-yl-5 alpha-pregnan-3 beta-20(R)-diol), a compound previously shown to inhibit both enzymes and to halt cell proliferation in fungi and protozoa, This compound produced a dose-dependent reduction in the parasite proliferation, with a 50% inhibitory concentration of 0.3 mu M and 80% reduction of growth after 96 h at 10 mu M. Correspondingly, parasites treated with the azasterol at 10 mu M for 48 h accumulated 24-desalkyl sterols such as zymosterol (cholesta-8,24-dien-3 beta-ol) and cholesta-8,14,24-trien-3 beta-ol to ca. 40% of the total mass of endogenous sterols, This is the first report on the antiproliferative effects of a sterol biosynthesis inhibitor on P. carinii and indicate that sterol methyltransferase inhibitors could be the basis of a novel and specific chemotherapeutic approach to the treatment of P. carinii infections.