Synthesis and evaluation of orexin-1 receptor antagonists with improved solubility and CNS permeability

Orexins are hypothalamic neuropeptides playing important roles in many functions including the motivation of addictive behaviors. Blockade of the orexin-1 receptor has been suggested as a potential strategy for the treatment of drug addiction. We have previously reported OX1 receptor antagonists based on the tetrahydroisoquinoline scaffold with excellent OX1 potency and selectivity; however, these compounds had high lipophilicity (clogP > 5) and low to moderate solubility. In an effort to improve their properties, we have designed and synthesized a series of analogues where the 7-position substituents known to favor OX1 potency and selectivity were retained, and groups of different nature were introduced at the 1-position where substitution was generally tolerated as demonstrated in previous studies. Compound 44 with lower lipophilicity (clogP = 3.07) displayed excellent OX1 potency (K-e = 5.7 nM) and selectivity (>1,760-fold over OX2) in calcium mobilization assays. In preliminary ADME studies, 44 showed excellent kinetic solubility (>200 mu M), good CNS permeability (P-app = 14.7 X 10(-6) cm/sec in MDCK assay), and low drug efflux (efflux ratio = 3.3).