3-(4-fluoropiperidin-3-yl)-2-phenylindoles as high affinity, selective, and orally bioavailable h5-HT2A receptor antagonist

The development of very high affinity, selective, and bioavailable h5-HT2A receptor antagonists is described. By investigation of the optimal position for the basic nitrogen in a series of 2-phenyl-3-piperidylindoles, it was found that with the basic nitrogen at the S-position of the piperidine it was not necessary to further substitute the piperidine in order to obtain good binding at h5-HT2A receptors. This meant the compounds no longer had high affinity at the IKr potassium channel, an issue with previous series of 2-aryl-3-(4-piperidyl)indoles. Improvements could be made to oral bioavailability in this series by reduction of the pK(a) of the basic nitrogen, by adding a fluorine atom to the piperidine ring, leading to 3-(4-fluoropiperidin-3-yl)-2-phenyl-1H-indole (17). Metabolic studies with this compound identified oxidation at the B-position of the indole as a major route in vitro and in vivo in rats. Blocking this position with a fluorine atom led to 6-fluoro-3-(4-fluoropiperidin-3-yl)-2-phenyl-1H-indole (22), an antagonist with 0.06 nM affinity for h5-HT2A receptors, with bioavailability of 80% and half-life of 12 h in rats.