Discovery of an 8-methoxytetrahydroisoquinoline derivative as an orally active N-type calcium channel blocker for neuropathic pain without CYP inhibition liability

In lead optimization efforts starting from the tetrahydroisoquinoline (S)-1, we identified 2-{[(2R)-2-hydroxypropyl]amino}-1-[(1S)-8-methoxy-1-phenyl-3,4-dihydroisoquinolin-2(1H)-yl]ethanone ((1S)-8t) as a novel orally active small-molecule N-type calcium channel blocker without CYP inhibition liability. CYP3A4 inhibition profile was improved by reducing the lipophilicity of compound (S)-1. Moreover, introduction of a methoxy group to the C-8 position of tetrahydroisoquinoline led to identification of (1S)-8t, which eliminated CYP2D6 inhibition liability. Oral administration of (1S)-8t exerted efficacy in a rat spinal nerve ligation (SNL) model of neuropathic pain with an ED50 value of 2.8 mg/kg.