Synthesis and pharmacological evaluation of hydrophobic esters and ethers of butorphanol at opioid receptors.

We synthesized several hydrophobic esters and ethers of butorphanol and assessed their affinities at opioid receptors in CHO cell membranes. Tested compounds displayed moderate to high affinities to the mu and kappa receptors. The findings accord with previous evidence of a lipophilic binding pocket in the opioid receptors that can be accessed to afford good binding affinity without the need for a phenolic hydrogen-bond donor group. The most potent (K(i)=61 pM at mu and 48 pM at kappa) novel agent was (-)-N-cyclobutylmethylmorphinan-3-yl-14-ol phenoxyacetate (4d).