Repeated systemic administration of the mixed inhibitor of enkephalin-degrading enzymes, RB101, does not induce either antinociceptive tolerance or cross-tolerance with morphine

Abstract

The potent analgesic responses elicited by systemic administration of RB101, N-[(R,S)-2-benzyl-3[(S)(2-amino-4-methylthio)butyldithio]-1-oxopropyl]-L-phenylalaninc benzyl ester, a prodrug able to inhibit enkephalin-degrading enzymes completely after in vivo bioactivation, has made it possible to investigate the development of antinociceptive tolerance after chronic potentiation of endogenous enkephalins. The ed 50 values of RB101 obtained 10 min after i.v. injection were not significantly different in mice treated for 4 days with i.p. administered vehicle (ED 50 = 9.50 (6.37–14.15) mg/kg), or with 80 mg/kg of RB101 twice daily (ED 50 = 9.50 (5.86–15.39) mg/kg). In contrast, a parallel rightwards shift of the dose-response curves, corresponding to a significant 1.92 (1.49–2.52)-fold decrease in analgesic potency, was observed after i.v. administration of morphine in mice chronically treated with morphine (3 mg/kg, twice daily for 4 days) (ED 50 = 3.10 (2.52–3.81) mg/kg) vs. saline (ED 50)= 1.60 (1.22–2.09) mg/kg). No tolerance to RB101 was observed even after a longer period (8 days) of chronic treatment with the prodrug. Moreover, no cross-tolerance between morphine and RB101 appeared to occur since the ED 50 values obtained after i.v. administration of RB101 were not significantly different in mice chronically pretreated with vehicle (ED 50 = 9.50 (6.37–14.15) mg/kg) or with morphine (ED 50= 10.00 (6.62–15.10) mg/kg). The analgesic effect of RB101 observed in morphine-tolerant mice was antagonized by prior injection of naloxone, but not naltrindole (δ-selective antagonist), supporting a preferential involvement of μ-opioid receptors in the antinociceptive effect of RB101, at least in mice in the hot-plate test. The present results suggest for the first time that the cellular events evoked by chronic stimulation of opioid receptors by morphine or by enkephalins, the endogenous opioid peptides, are different and emphasize the potential clinical interest of systemically active mixed inhibitors as new analgesics.