Thioquinazolinones as antituberculosis agents targeting phosphopantetheinyl transferase
The essential mycobacterial enzyme phosphopantetheinyl transferase (PptT) is a potential target for anti-tuberculosis drugs. Following the identification of (pyridin-3-ylmethyl)-substituted thioquinazolinones as hits in a screen against PptT, we carried out a structure-activity relationship (SAR) campaign, focusing on modifications of the 6, 7, and 5'-positions of the thioquinazolinone ring and the N-methylpyridyl group. These studies led to analogs with improved drug-like physicochemical properties and whole-cell activity against Mtb. The resulting thioquinazolinones exert on-target antimycobacterial activity against Mtb in axenic culture and in macrophages with efficacy comparable to that of clinically used anti-tuberculosis drugs. Increased susceptibility of a pptT hypomorphic strain of Mtb, resistance of PptT Trp170 mutant Mtb strains and identification of relevant mutations in additional resistant mutants corroborated that the thioquinazolinones inhibit Mtb by virtue of their action on PptT. The thioquinazolinones studied did not inhibit other bacteria, were not cytotoxic to mammalian cells, and did not react with most mammalian receptors and ion channels tested or with glutathione in the presence or absence of liver microsomes. Nonetheless, Mtb defied our efforts to drive whole cell minimum inhibitory concentrations (MIC90) below 2.5 uM. Studies of intra-bacterial compound uptake and metabolism supplied an explanation. Mtb rapidly took up the thioquinazolinones but then rapidly methylated the thione and the 3'-N of the pyridylmethyl group. The mono- and di-methylated species that accumulated in Mtb were inactive against PptT. These findings illustrate Mtb's remarkable ability to metabolize xenobiotics that threaten its survival, complicating efforts to develop drugs for tuberculosis.