Central and peripheral analgesic agents: chemical strategies for limiting brain penetration in kappa-opioid agonists belonging to different chemical classes.

Over the past decade there has been great interest by the pharmaceutical industry in the development of novel analgesics which act by activation of central kappa-opioid receptors. However, in view of the spectrum of unwanted CNS effects associated with such agents, recent efforts have been focused on peripherally-selective compounds with limited ability to cross the blood-brain barrier (BBB). In this review, the authors consider the chemical strategies and associated synthetic procedures employed by various research groups to produce hydrophilic compounds with retained kappa-opioid agonist activity. Physico-chemical (clogP, delta logP) and biological methods (antinociception following administration by peripheral and central routes; ex-vivo binding to detect plasma and brain levels of the drugs) utilized to assess brain penetration are described and compared. Overall, in-vivo ratios correlate better with delta logP values than with clogP.