Healy, J.R., Bezawada, P., Shim, J., Jones, J.W., Kane, M.A., MacKerell, A.D., et al. (2013) Synthesis, modeling, and pharmacological evaluation of UMB 425, a mixed μ-agonist/δ-antagonist opioid analgesic with reduced tolerance liabilities. ACS Chemical Neuroscience, 4, 1256-1266.