Gender and strain contributions to the variability of buprenorphine-related respiratory toxicity in mice.

While most deaths from asphyxia related to buprenorphine (BUP) overdose have been reported in males, higher plasma concentrations of BUP and its toxic metabolite norbuprenorphine (NBUP) have been observed in females. We previously demonstrated that P-glycoprotein (P-gp) modulation at the blood-brain barrier (BBB) contributes highly to BUP-related respiratory toxicity, by limiting NBUP entrance into the brain. In this work, we sought to investigate the role of P-gp-mediated transport at the BBB in gender and strain-related variability of BUP and NBUP-induced respiratory effects in mice. Ventilation was studied using plethysmography, P-gp expression using western blot, and transport at the BBB using in situ cerebral perfusion. In male Fvb and Swiss mice, BUP was responsible for ceiling respiratory effects. NBUP-related reduction in minute volume was dose-dependent but more marked in Fvb (p<0.01 at 1mg/kg NBUP and p<0.001 at 3 and 9mg/kg NBUP) than in Swiss mice (p<0.001 at 9mg/kg NBUP). Female Fvb mice were more susceptible to BUP than males with significantly increased inspiratory time (p<0.05) and to NBUP with significantly increased expiratory time (p<0.01). Following BUP administration, plasma BUP concentrations were significantly higher (p<0.01) and plasma NBUP concentrations significantly lower (p<0.001) in Fvb mice compared to Swiss mice. Plasma BUP concentrations were significantly higher (p<0.05) and plasma NBUP concentrations significantly lower (p<0.01) in male compared to female Fvb mice. In contrast, following NBUP administration, comparable plasma NBUP concentrations were observed in both genders and strains. No differences in P-gp expression or BUP and NBUP transport across the BBB were observed between male and female Fvb mice as well as between Swiss and Fvb mice. Our results suggest that P-gp-mediated transport across the BBB does not play a key-role in gender and strain-related variability in BUP and NBUP-induced respiratory toxicity in mice. Both gender- and strain-related differences in respiratory effects of BUP could be attributed to BUP itself rather than to its metabolite, NBUP.

Comparison of tolerance to morphine-induced respiratory and analgesic effects in mice.

Morphine is responsible for severe poisonings in chronically treated patients. We hypothesize that toxicity could be related to the development of weaker tolerance for morphine-induced deleterious respiratory effects in comparison to analgesic effects. Our objectives were to compare tolerance to both effects in mice and investigate possible mechanisms for such possible differences. Tolerance to morphine-induced analgesia and respiratory effects was assessed using hot plate response latencies and plethysmography, respectively. Mechanisms of tolerance were investigated using binding studies to mu-opioid receptors (MOR) and adenylate cyclase (AC) activity measurement in homogenates of cell membranes from the periaqueductal gray region (PAG) and brainstem. Morphine (2.5 mg/kg) was responsible for analgesia with significant increase in inspiratory time. Acute tolerance to analgesia (p<0.01) and effects on respiratory frequency (p<0.05) was observed in mice pre-treated with 100 mg/kg morphine in comparison to saline. Following repetitive administration (2.5 mg/kg/day during 10 days), we observed a 13-fold increase in the effective dose-50% (ED50) of morphine-induced analgesia in comparison to a 2- or 4-fold increase in the ED50 of its related increase in inspiratory time determined in air and 4% CO2, respectively. No significant alteration in MOR expression was observed in either PAG or brainstem following repeated morphine administration. However, in PAG, in contrast to brainstem, superactivation of AC was observed in morphine-treated mice in comparison to controls (p<0.05). In conclusion, tolerance to morphine-induced respiratory effects is much more limited than tolerance to its analgesic effects in repeatedly morphine-treated mice. The difference in morphine-induced AC activation between the brainstem and the PAG contributes to the observed difference in tolerance between both morphine effects.

Respiratory toxicity of buprenorphine results from the blockage of P-glycoprotein-mediated efflux of norbuprenorphine at the blood-brain barrier in mice.

OBJECTIVES: Deaths due to asphyxia as well as following acute poisoning with severe respiratory depression have been attributed to buprenorphine in opioid abusers. However, in human and animal studies, buprenorphine exhibited ceiling respiratory effects, whereas its metabolite, norbuprenorphine, was assessed as being a potent respiratory depressor in rodents. Recently, norbuprenorphine, in contrast to buprenorphine, was shown in vitro to be a substrate of human P-glycoprotein, a drug-transporter involved in all steps of pharmacokinetics including transport at the blood-brain barrier. Our objectives were to assess P-glycoprotein involvement in norbuprenorphine transport in vivo and study its role in the modulation of buprenorphine-related respiratory effects in mice. SETTING: University-affiliated research laboratory, INSERM U705, Paris, France. SUBJECTS: Wild-type and P-glycoprotein knockout female Friend virus B-type mice. INTERVENTIONS: Respiratory effects were studied using plethysmography and the P-glycoprotein role at the blood-brain barrier using in situ brain perfusion. MEASUREMENTS AND MAIN RESULTS: Norbuprenorphine(≥ 1 mg/kg) and to a lesser extent buprenorphine (≥ 10 mg/kg) were responsible for dose-dependent respiratory depression combining increased inspiratory (TI) and expiratory times (TE). PSC833, a powerful P-glycoprotein inhibitor, significantly enhanced buprenorphine-related effects on TI (p < .01) and TE (p < .05) and norbuprenorphine-related effects on minute volume (VE, p < .05), TI, and TE (p < .001). In P-glycoprotein-knockout mice, buprenorphine-related effects on VE (p < .01), TE (p < .001), and TI (p < .05) and norbuprenorphine-related effects on VE (p < .05) and TI (p < .001) were significantly enhanced. Plasma norbuprenorphine concentrations were significantly increased in PSC833-treated mice (p < .001), supporting a P-glycoprotein role in norbuprenorphine pharmacokinetics. Brain norbuprenorphine efflux was significantly reduced in PSC833-treated and P-glycoprotein-knockout mice (p < .001), supporting P-glycoprotein-mediated norbuprenorphine transport at the blood-brain barrier. CONCLUSIONS: P-glycoprotein plays a key-protective role in buprenorphine-related respiratory effects, by allowing norbuprenorphine efflux at the blood-brain barrier. Our findings suggest a major role for drug-drug interactions that lead to P-glycoprotein inhibition in buprenorphine-associated fatalities and respiratory depression.