The Effects of Herkinorin, the First -Selective Ligand from a Salvinorin A-Derived Scaffold, in a Neuroendocrine Biomarker Assay in Nonhuman Primates

Herkinorin is the first -opioid receptor-selective ligand from the salvinorin A diterpenoid scaffold. Herkinorin has relative binding selectivity, and it can act as an agonist at both and -receptors, in vitro. These studies were the first in vivo evaluation of the effects of herkinorin in nonhuman primates, using prolactin release, a neuroendocrine biomarker assay that is responsive to both and -agonists, as well as to compounds with limited ability to cross the blood-brain barrier. In cumulative dosing studies (0.01–0.32 mg/kg i.v.), herkinorin produced only small effects in gonadally intact males (n 4), but a more robust effect in females (n 4). Time course studies with herkinorin (0.32 mg/kg) confirmed this greater effectiveness in females and revealed a fast onset after i.v. administration (e.g., by 5–15 min). Antagonism experiments with different doses of nalmefene (0.01 and 0.1 mg/kg) caused dose-dependent and complete prevention of the effect of herkinorin in females. This is consistent with a principal -agonist effect of herkinorin, with likely partial contribution by -agonist effects. The peripherally selective antagonist quaternary naltrexone (1 mg/kg s.c.) caused approximately 70% reduction in the peak effect of herkinorin (0.32 mg/kg) in females, indicating that this effect of herkinorin is prominently mediated outside the blood-brain barrier. Salvinorin A, a plant-derived hallucinogenic diterpene, is a highly selective -opioid agonist, and a novel template for semisynthetic opioid analogs (Roth et al., 2002; Prisinzano and Rothman, 2008). One of these novel analogs is herkinorin, the first salvinorin-derived compound with over -selectivity reported in the literature (Harding et al., 2005). Herkinorin has approximately 8-fold selectivity for over -receptors and approximately 98-fold selectivity for over -receptors in competition binding assays (Harding et al., 2005). Herkinorin acts as a high-efficacy agonist at both and -receptors in the guanosine 5 -O-(3-thio)triphosphate assay (Harding et al., 2005), with greater relative potency at -receptors. Herkinorin also displays some unique features in its interactions at the -receptor, such as decreased agonist-induced internalization (Groer et al., 2007; Xu et al., 2007). To date, there is no information on the in vivo effects of herkinorin in primates. The present study focuses on an initial evaluation of the in vivo opioid agonist effects of herkinorin in rhesus monkeys, using a translationally viable neuroendocrine biomarker assay, release of the anterior pituitary hormone prolactin. Different factors render this neuroendocrine biomarker a practical approach for initial evaluation of herkinorin: 1) prolactin levels are increased by both and -agonists in mammals, including humans ( -agonists seem not be to be active) (Hoehe et al., 1988; Ur et al., 1997; Kreek et al., 1999; Bowen et al., 2002; Butelman et al., 2007); and 2) effects of these opioids are at least partially mediated by opioid receptors outside the blood-brain barrier (e.g., in particular hypothalamic nuclei) (Merchenthaler, 1991; Butelman et al., 2004, 2008; Zheng et al., 2005), and they may therefore be used to determine the pharmacodynamic effects of compounds with limited ability to cross the blood-brain barrier. This study presents the first direct evaluation of the pharmacodynamic effects of herkinorin in male and female nonhuman primates, and it also investigates potential -versus This work was funded by National Institutes of Health-National Institute on Drug Abuse Grants DA017369 (to E.R.B.), DA018151 (to T.E.P.), and DA05130 (to M.J.K.). Article, publication date, and citation information can be found at http://jpet.aspetjournals.org. doi:10.1124/jpet.108.140079. ABBREVIATIONS: ANOVA, analysis of variance. 0022-3565/08/3271-154–160$20.00 THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS Vol. 327, No. 1 Copyright © 2008 by The American Society for Pharmacology and Experimental Therapeutics 140079/3381513 JPET 327:154–160, 2008 Printed in U.S.A. 154 at U C L A B om eical ib S eials on D ecem er 1, 2008 jpet.asjournals.org D ow nladed fom -receptor effects of this novel compound, as well as activity outside the blood-brain barrier. Materials and Methods