Pharmacokinetics of Single-Dose Intramuscular and Subcutaneous Injections of Buprenorphine in Common Marmosets (Callithrix jacchus).

Although buprenorphine is the most frequently used opioid analgesic in common marmosets (Callithrix jacchus), there is limited information in the literature supporting current dosing regimens used for this species. The purpose of this study was to determine the pharmacokinetic profiles of single-dose buprenorphine HCl administered intramuscularly (IM) at 0.01 mg/kg in 6 adult marmosets (1.8 to 12.8 y old; 2 males, 4 females) and subcutaneously (SQ) at 0.01 mg/kg in 6 adult marmo- sets (2.3-4.4 y old; 3 males, 3 females) by mass spectrometry. Blood was collected at multiple time points from 0.25 to 24 h from unsedated animals following a hybrid sparse-serial sampling design. The maximal observed plasma concentration of buprenorphine (Cmax ) administered IM (2.57 ± 0.95 ng/mL) was significantly higher than administered SQ (1.47 ± 0.61 ng/mL). However, the time to Cmax (Tmax) was not statistically different between routes (17.4 ± 6 min for IM and 19.8 ± 7.8 min for SQ). The time of the last quantifiable concentration of buprenorphine was 5 ± 1.67 h for IM compared with 6.33 ± 1.51 h for SQ, which was not statistically different. The mean buprenorphine plasma concentration-time curves were used to propose a dosing frequency of 4 to 6 h for buprenorphine at 0.01 mg/kg IM or SQ based on a theoretical therapeutic plasma concentration threshold of 0.1 ng/mL. Based on the mean pharmacokinetic parameters and plasma-concentration time curves, both IM and SQ routes of buprenorphine at this dose provide a rapid increase in the plasma concentration of buprenorphine above the therapeutic threshold, and may be more effective for acute rather than long-lasting analgesia. Further studies are needed to examine repeated dosing regimens and the efficacy of buprenorphine in common marmosets.

Pharmacokinetics of Buprenorphine and Sustained-release Buprenorphine in Common Marmosets (Callithrix jacchus).

Buprenorphine is an essential component of analgesic protocols in common marmosets (Callithrix jacchus). The use of buprenorphine HCl (BUP) and sustained-release buprenorphine (BSR) formulations has become commonplace in this species, but the pharmacokinetics have not been evaluated. Healthy adult (age, 2.4 to 6.8 y; 6 female and 6 male) common marmosets were enrolled in this study to determine the pharmacokinetic parameters, plasma concentration-time curves, and any apparent adverse effects of these compounds. Equal numbers of each sex were randomly assigned to receive BUP (0.02 mg/kg IM) or BSR (0.2 mg/kg SC), resulting in peak plasma concentrations (mean ± 1 SD) of 15.2 ± 8.1 and 2.8 ± 1.2 ng/mL, terminal phase t1/2 of 2.2 ± 1.0 and 32.6 ± 9.6 h, and AUC0-last of 16.1 ± 3.7 and 98.6 ± 42.7 ng × h/mL. The plasma concentrations of buprenorphine exceeded the proposed minimal therapeutic threshold (0.1 ng/mL) at 5 and 15 min after BUP and BSR administration, showing that both compounds are rapid-acting, and remained above that threshold through the final time points of 8 and 72 h. Extrapolation of the terminal elimination phase of the mean concentration-time curves was used to develop the clinical dosing frequencies of 6 to 8 h for BUP and 3.0 to 3.5 d for BSR. Some adverse effects were observed after the administration of BUP to common marmosets in this study, thus mandating judicious use in clinical practice. BSR provided a safe, long-acting option for analgesia and therefore can be used to refine analgesic protocols in this species.

Varenicline treatment for methamphetamine dependence: A randomized, double-blind phase II clinical trial.

BACKGROUND: Previous studies have suggested that varenicline, an α4ß2 nicotinic receptor partial agonist, and α7 nicotinic receptor full agonist, may be effective for the treatment of methamphetamine (MA) dependence due to dopaminergic effects, relief of glutamatergic and cognitive dysfunction, and activation of nicotinic cholinergic systems. This study aimed to determine if varenicline (1 mg BID) resulted in reduced methamphetamine use compared to placebo among treatment-seeking MA-dependent volunteers. METHODS: Treatment-seeking MA-dependent volunteers were randomized to varenicline 1 mg twice daily (n = 27) or placebo (n = 25) and cognitive behavioral therapy for 9 weeks. The primary outcomes were the proportion of participants achieving end-of-treatment-abstinence (EOTA, MA-negative urine specimens during weeks 8 and 9) and the treatment effectiveness score (TES, number of MA-negative urine specimens) for varenicline versus placebo. RESULTS: There was no significant difference in EOTA between varenicline (15%, 4/27) and placebo (20%, 5/25; p = 0.9). There was some suggestion that urinary confirmed medication compliance corresponded with EOTA in the varenicline condition, though it did not reach statistical significance, OR = 1.57 for a 100 ng/ml increase in urine varenicline, p = 0.10, 95% CI (0.99, 3.02). There was no significant difference in mean TES in the varenicline condition (8.6) compared to the placebo condition (8.1), and treatment condition was not a statistically significant predictor of TES, IRR = 1.01, p = 0.9, 95% CI (0.39, 2.70). CONCLUSIONS: The results of this study indicate that 1 mg varenicline BID was not an effective treatment for MA dependence among treatment-seeking MA-dependent volunteers.

Inhibition of In Vitro Metabolism of Opioids by Skeletal Muscle Relaxants.

The purpose of this study was to test the hypothesis that skeletal muscle relaxants could inhibit the in vitro metabolism of common comedications opioids buprenorphine, methadone and oxycodone. The compounds [solubility-limited concentration (µM) studied] were as follows: baclofen (1000), carisoprodol (200), its metabolite meprobamate (1000), chlorzoxazone (200), cyclobenzaprine (1000), metaxalone (50), methocarbamol (1000), orphenadrine (1000) and tizanidine (1000). Compounds were first incubated with human liver microsomes ± pre-incubation, screened with pathway-specific cDNA-expressed cytochrome P450s (rCYP), and then IC50 values determined using either 8-concentration tests for those where the rCYP screen suggested an IC50 was achievable, or a 3-concentration test with downward extrapolation if screen suggested 50% inhibition was not achievable. These results were then extrapolated to determine an inhibitory potential. Six pathway inhibitor combinations were identified with a moderate inhibitory potential (≥2.0 < 5.0): five with chlorzoxazone, R-EDDP, S-EDDP and noroxycodone production by CYP3A4, and R- and S-EDDP production by CYP2B6; and one for the meprobamate effect on noroxycodone production by CYP3A4. An additional eleven combinations were found with a weak inhibitory potential (≥1.25 < 2.0): five with carisoprodol, two each with methocarbamol and meprobamate, and one each with metaxalone and orphenadrine. This represents the first comprehensive study of the inhibitory effect of this class of drugs and suggests that some of them may produce significant drug-drug interactions with opioids that are frequent comedications with skeletal muscle relaxants.

The Pharmacokinetics and Pharmacodynamics of Buprenorphine in Neonatal Abstinence Syndrome.

Neonatal abstinence syndrome (NAS) is a condition affecting newborns that are exposed to an opioid in utero. In a randomized, controlled trial assessing the efficacy of buprenorphine and morphine in NAS, blood samples were analyzed from a subset of patients receiving buprenorphine along with NAS scores. The data were used to validate and adapt an existing model of buprenorphine in neonates and to identify relationships between buprenorphine or norbuprenorphine pharmacokinetics (PK) and efficacy or safety. The time to NAS stabilization was found to decrease with increasing buprenorphine exposure. This pharmacokinetic-pharmacodynamic (PK-PD) relationship was able to be quantified and adequately described with a mathematical model. The findings confirm a previous PK model of buprenorphine and extend the model to describe the PK of norbuprenorphine and to identify a novel PK-PD relationship of buprenorphine in NAS. This model will allow optimization of dosing strategies in future clinical trials.

Azole antifungal inhibition of buprenorphine, methadone and oxycodone in vitro metabolism.

Opioid-related mortality rates have escalated. Drug interactions may increase blood concentrations of the opioid. We therefore used human liver microsomes (HLMs) and cDNA-expressed human cytochrome P450s (rCYPs) to study in vitro inhibition of buprenorphine metabolism to norbuprenorphine (CYP3A4 and 2C8), oxycodone metabolism to noroxycodone (CYP3A4 and 2C18) and oxymorphone (CYP2D6), and methadone metabolism to R- and S-2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP; CYP3A4 and 2B6). In this study, we have examined the inhibitory effect of 12 (mostly antifungal) azoles. These compounds have a wide range of solubility; to keep organic solvent ≤1%, there was an equally wide range of highest concentration tested (e.g., itraconazole 5 µM to fluconazole 1000 µM). Inhibitors were first incubated with HLMs at three concentrations with or without preincubation of inhibitor with reducing equivalents to also screen for time-dependent inhibition (TDI). Posaconazole displayed evidence of TDI; metronidazole and albendazole had no significant effect. Azoles were next screened at the highest achievable concentration for non-CYP3A4 pathways. IC50 values (µM) were determined for most CYP3A4 pathways (ranges) and other pathways as dictated by screen results: clotrimazole (0.30 - 0.35; others >30 µM); econazole (2.2 - 4.9; 2B6 R-EDDP - 9.5, S-EDDP - 6.8; 2C8 - 6.0; 2C18 - 1.0; 2D6 - 1.2); fluconazole (7.7 - 66; 2B6 - 313, 361; 2C8 - 1240; 2C18 - 17; 2D6 - 1000); itraconazole (2.5 to >5; others >5); ketoconazole (0.032 - 0.094; 2B6 - 12, 31; 2C8 - 78; 2C18 - 0.98; 2D6 - 182); miconazole (2.3 - 7.6; 2B6 - 2.8, 2.8; 2C8 - 5.3; 2C18 - 3.1; 2D6 - 5.9); posaconazole (3.4 - 20; 2C18 - 3.8; others >30); terconazole (0.48 to >10; 2C18 - 8.1; others >10) and voriconazole (0.40 - 15; 2B6 - 2.4, 2.5; 2C8 - 170; 2C18 - 13; 2D6 >300). Modeling based on estimated Ki values and plasma concentrations from the literature suggest that the orally administered azoles, particularly ketoconazole and voriconazole, have the greatest potential for inhibiting CYP3A4 pathways, as does voriconazole for the CYP2B6 pathways. Azoles used for mucosal and topical applications did not exceed the modeling threshold.

A Possible Mechanistic Link Between the CYP2C19 Genotype, the Methadone Metabolite Ethylidene-1,5-Dimethyl-3,3-Diphenylpyrrolidene (EDDP), and Methadone-Induced Corrected QT Interval Prolongation in a Pilot Study.

BACKGROUND: Methadone use and methadone-associated sudden cardiac death have increased dramatically. Prolongation of the QT interval of the cardiac cycle predisposes to arrhythmia and is common among methadone users. OBJECTIVE: We studied the relationship between pharmacogenetic variables and methadone metabolites and QT prolongation. METHODS: Blood was obtained on days 1, 7, and 21 from consenting individuals initiating methadone treatment. Plasma methadone and ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidene (EDDP) were measured using liquid chromatographic-electrospray ionization-tandem mass spectrometry. The corrected QT interval (QTc) from 12-lead electrocardiograms (ECGs) was obtained at baseline and at 21 days. RESULTS: Total plasma EDDP, (S)-EDDP, and (R)-EDDP concentrations were significantly higher for individuals carrying the CYP2C19*2 variant (n=8) than in 17 subjects carrying the CYP2C19 wild-type allele (p<0.004). Seventeen (68%) of 25 subjects experienced a QTc (Bazett) of 39.9±28.4 ms (mean±standard deviation). The methadone dose and the plasma EDDP concentration corrected for dose were both significantly associated with QTc at study termination and with change in QTc interval from baseline (∆QTc) (p<0.03 to p<0.0003). Based on a QTc increase, five subjects had a potentially increased arrhythmia risk. Compared with other participants, the mean dose for those individuals was higher (50.8 vs. 42.5 mg/day; p<0.04) as was the total plasma EDDP (36.8 vs. 21.0 ng/mL; p<0.002) and dose-corrected EDDP (0.27 vs. 0.16 ng/mL/mg; p<0.003). CONCLUSIONS: These results suggest that a notable change in the QTc interval was associated with both oral dose and increased methadone metabolism, as indicated by the higher plasma concentration of the principal methadone metabolite. The oral dose and plasma EDDP concentration may be useful in identifying individuals at risk for methadone-related arrhythmia.

In vitro inhibition of methadone and oxycodone cytochrome P450-dependent metabolism: reversible inhibition by H2-receptor agonists and proton-pump inhibitors.

In vitro inhibition of oxycodone metabolism to noroxycodone and oxymorphone and R- and S-methadone metabolism to R- and S-2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP) was measured for four H2-receptor antagonists and five proton-pump inhibitors (PPIs) using human liver microsomes (HLM) and cDNA-expressed human cytochrome P450s (rCYPs). Inhibitors were first incubated with HLM at three concentrations with and without preincubation of inhibitor, enzyme source and reducing equivalents to also screen for time-dependent inhibition (TDI). Cimetidine and famotidine (10-1,000 µM) inhibited all the four pathways >50%. Nizatidine and ranitidine did not. All the five PPIs (1-200 µM) inhibited one or more pathways >50%. Half maximal inhibitory concentrations (IC50s) were then determined using rCYPs. Cimetidine and famotidine both inhibited CYP3A4-mediated formation of noroxycodone and CYP2D6-mediated formation of oxymorphone, and famotidine inhibited CYP3A4-mediated formation of R- and S-EDDP, but IC50s were so high that only >10× therapeutic concentrations may have potential for reversible in vivo inhibition. The PPIs were more potent inhibitors; many have the potential for reversible in vivo inhibition at therapeutic concentrations. Omeprazole, esomeprazole and pantoprazole had greater effects on CYP3A4-mediated reactions, whereas lansoprazole was selective for CYP2D6-mediated formation of oxymorphone. Preincubation enhanced cimetidine inhibition of noroxycodone formation and rabeprazole inhibition of all pathways. Future studies will explore irreversible TDI.

Determination of oxycodone, noroxycodone and oxymorphone by high-performance liquid chromatography-electrospray ionization-tandem mass spectrometry in human matrices: in vivo and in vitro applications.

The opioid analgesic oxycodone is widely abused and increasingly associated with overdose deaths. A sensitive analytical method was developed for oxycodone and its metabolites, noroxycodone and oxymorphone, in human plasma, urine (±enzymatic hydrolysis at 50°C for 16 h) and liver microsomes (HLMs). Liquid-liquid extraction was followed by high-performance liquid chromatography-electrospray ionization-tandem mass spectrometry. The calibration range was 0.2-250 ng/mL for plasma and HLM and 10-5000 ng/mL for urine. Intra- and interrun accuracies were within 13.3% of target; precisions were within 12.8% for all matrices. Recoveries from plasma were: oxycodone, 75.6%; noroxycodone, 37.4% and oxymorphone, 18.2%. Analytes exhibited room temperature stability in plasma and urine up to 24 h, and freeze-thaw stability in plasma up to three cycles. In 24-h hydrolyzed urine from subjects administered intranasal oxycodone (30 mg/70 kg, n = 5), mean concentrations (ng/mL) and % daily doses excreted were: oxycodone, 1150, 6.53%; noroxycodone, 1330, 7.81% and oxymorphone, 3000, 17.1%. Oxycodone incubated with HLM produced more noroxycodone than oxymorphone. With a panel of recombinant human cytochrome P450s (CYPs), CYP2C18 and CYP3A4 produced the most noroxycodone, whereas CYP2D6 produced the most oxymorphone. These results demonstrate a new method suitable for both in vivo and in vitro metabolism and pharmacokinetic studies of oxycodone.

Pharmacokinetics of 2 formulations of buprenorphine in macaques (Macaca mulatta and Macaca fascicularis).

Buprenorphine is the cornerstone of pain management in nonhuman primates, but the pharmacokinetics of this widely used drug are unknown. The purpose of this study was to evaluate the pharmacokinetic profiles of buprenorphine (0.01 and 0.03 mg/kg IM) and sustained-release buprenorphine (0.2 mg/kg SC) in 2 macaque species (M. mulatta and M. fascicularis) by using mass spectrometry. The pharmacokinetics did not differ significantly between species, and buprenorphine was dose-proportional at the tested doses. The low and high doses of buprenorphine had elimination half-lives of 2.6 ± 0.7 and 5.3 ± 2.0 h, respectively, but the low-dose data were constrained by the sensitivity of the analytical method. Sustained-release buprenorphine had an elimination half-life of 42.6 ± 26.2 h. The AUC0-Tlast of buprenorphine were 9.1 ± 4.3 and 39.0 ± 25.1 ng × h/mL for the low and high doses, respectively, and sustained-release buprenorphine had an AUC0-Tlast of 177 ± 74 ng × h/mL. Assuming a hypothesized therapeutic buprenorphine plasma concentration threshold of 0.1 ng/mL in macaques, these results suggest that buprenorphine doses of 0.01 mg/kg IM should be administered every 6 to 8 h, whereas doses of 0.03 mg/kg IM can be administered every 12 h. These results further demonstrate that a single 0.2-mg/kg SC injection of sustained-release buprenorphine maintains plasma concentrations above 0.1 ng/mL for 5 d in macaques. These findings support a new dosing strategy using sustained-release buprenorphine to improve pain management, decrease animal stress, improve animal welfare, and simplify the postoperative management of nonhuman primates in laboratory animal and zoological settings.

Pharmacokinetic interactions between buprenorphine/naloxone and raltegravir in subjects receiving chronic buprenorphine/naloxone treatment.

BACKGROUND: Interactions between human immuno-deficiency virus (HIV) and opioid-dependence therapies can occur. OBJECTIVES: We sought to determine whether such interactions occurred between buprenorphine/naloxone and raltegravir. METHODS: We performed a within-subject open-labeled pharmacokinetic and pharmacodynamic study in 12 HIV-seronegative subjects stabilized on at least 3 weeks of buprenorphine/naloxone therapy. Subjects underwent baseline and steady-state evaluation of the effect of raltegravir 400 mg BID on buprenorphine/naloxone parameters. RESULTS: Compared with baseline values, buprenorphine AUC(0-24 h) (58.2 vs. 56.0 hr*ng/mL) and C(max) (7.37 vs. 6.60 ng/mL) did not differ significantly after achieving steady-state raltegravir. Similar analyses of norbuprenorphine, the primary metabolite of buprenorphine, demonstrated no significant difference after raltegravir administration. Naloxone concentrations were unchanged for AUC(0-24 h) (.595 vs. .581 hr*ng/mL), C(max) (.251 vs. .243 ng/mL) and T(max) (.75 vs.1.08 h). Objective opioid withdrawal was not observed. The AUC(0-12 h) and C(max) of raltegravir did not significantly differ from historical controls (5543 vs. 4428 h*ng/mL and 1070 vs. 1266 ng/mL), respectively. CONCLUSION: The addition of raltegravir to stabilized patients receiving buprenorphine/naloxone does not significantly affect buprenorphine/naloxone or raltegravir pharmacokinetic or pharmacodynamic parameters.

Gender differences in pharmacokinetics of maintenance dosed buprenorphine.

AIMS: Gender differences are known to occur in the pharmacokinetics of many drugs. Mechanisms may include differences in body composition, body weight, cardiac output, hormonal status, and use of different co-medications. Recently subtle gender-dependent differences in cytochrome P450 (CYP) 3A-dependent metabolism have been demonstrated. Buprenorphine N-dealkylation to norbuprenorphine is primarily performed by CYP3A. We therefore asked whether gender-dependent differences occur in the pharmacokinetics of buprenorphine. METHODS: A retrospective examination was made of control (buprenorphine/naloxone-only) sessions from a number of drug interaction studies between buprenorphine and antiretroviral drugs. Twenty males and eleven females were identified who had a negative cocaine urine test prior to participation in the control session and were all on the same maintenance dose (16/4 mg) of sublingual buprenorphine/naloxone. Pharmacokinetic data from their control sessions (buprenorphine/naloxone only) were sorted by gender and compared using the two-sample t-test. RESULTS: Females had significantly higher area under the plasma concentration curve (AUC) and maximum plasma concentrations for buprenorphine, norbuprenorphine and norbuprenorphine-3-glucuronide. AUCs relative to dose per body weight and surface area were significantly higher for only norbuprenorphine. AUCs relative to lean body mass were, however, not significantly different. CONCLUSIONS: Gender-related differences exist in the pharmacokinetics of buprenorphine; differences in body composition appear to have a major impact; differences in CYPA-dependent metabolism may also contribute.

Tipranavir/ritonavir induction of buprenorphine glucuronide metabolism in HIV-negative subjects chronically receiving buprenorphine/naloxone.

BACKGROUND: Previous reports on the pharmacokinetic of tipranavir (TPV) and buprenorphine (BUP)/ naloxone found that coadministration resulted in an 80% reduction in the area under the curve AUC of the primary BUP metabolite, norBUP, without any pharmacodynamic consequences. This study was conducted to characterize how tipranivir/ritonavir effects the glucuronide metabolites of BUP and may explain the reduction in the norBUP. METHODS: HIV-seronegative subjects stabilized on at least 3 weeks of BUP/naloxone sequentially underwent baseline and steady-state pharmacokinetic evaluation of twice daily TPV 500?mg coadministered with ritonavir 200?mg (TPV/r). RESULTS: Twelve subjects were enrolled and ten completed the study. The steady-state pharmacokinetics for BUP-3-glucuronide (BUP-3G) and norBUP-3-glucuronide (norBUP-3G) in the presence and absence of steady-state TPV/r were analyzed. The C(max) of BUP-3G was 8.78???5.23?ng/mL without TPV/r and increased to 12.7???11.7 after steady state of TPV/r was achieved. The AUC of BUP-3G was 31.1???19.4?(ng/mL)?(h) without TPV/r and increased to 58. 6???49.5 after steady state of TPV/r was achieved (p?=?.0966). In contrast, steady-state norBUP-3G AUC(0?24?h) (p?=?.0216) and C(max) (p?=?.0088) were significantly decreased in the presence of steady-state TPV/r. CONCLUSIONS AND SCIENTIFIC SIGNIFICANCE: This study further elucidates the effects of TPV/r on glucuronidation. The current evaluation of glucuronide metabolites of BUP and norBUP are suggestive of combined inhibition of Uridine diphosphate (UDP)-glucuronosyltransferase of the 1A family and cytochrome P450 3A4 that spares UGT2B7 leading to a shunting of BUP away from production of norBUP and toward BUP-3G as seen by a statistically significant increase in the AUC of BUP-3G.

Stereo-selective metabolism of methadone by human liver microsomes and cDNA-expressed cytochrome P450s: a reconciliation.

In vitro metabolism of methadone was investigated in cytochrome P450 (CYP) supersomes and phenotyped human liver microsomes (HLMs) to reconcile past findings on CYP involvement in stereo-selective metabolism of methadone. Racaemic methadone was used for incubations; (R)- and (S)-methadone turnover and (R)- and (S)-EDDP formation were determined using chiral liquid chromatography-tandem mass spectrometry. CYP supersome activity for methadone use and EDDP formation ranked CYP2B6 > 3A4 > 2C19 > 2D6 > 2C18, 3A7 > 2C8, 2C9, 3A5. After abundance scaling, CYP3A4, 2B6 and 2C19 accounted for 63-74, 12-32 and 1. 4-14% of respective activity. CYP2B6, 2D6 and 2C18 demonstrated a preference for (S)-EDDP formation; CYP2C19, 3A7 and 2C8 for (R)-EDDP; 3A4 none. Correlation analysis with 15 HLMs supported the involvement of CYP2B6 and 3A. The significant correlation of S/R ratio with CYP2B6 activity confirmed its stereo-selectivity. CYP2C19 and 2D6 inhibitors and monoclonal antibody (mAb) did not inhibit EDDP formation in HLM. Chemical and mAb inhibition of CYP3A in high 3A activity HLM reduced EDDP formation by 60-85%; inhibition of CYP2B6 in 2B6 high-activity HLM reduced (S)-EDDP formation by 80% and (R)-EDDP formation by 55%. Inhibition changed methadone metabolism in a stereo-selective manner. When CYP3A was inhibited, 2B6 mediated (S)-EDDP formation predominated; S/R stereo-selectivity increased. When 2B6 was inhibited (S)-EDDP formation fell and stereo-selectivity decreased. The results confirmed the primary roles of CYPs 3A4 and 2B6 in methadone metabolism; CYP2C8 and 2C9 did not appear involved; 2C19 and 2D6 have minimal roles. CYP2B6 is the primary determinant of stereo-selective metabolism; stereo-selective inhibition might play a role in varied plasma concentrations of the two enantiomers.

Pharmacokinetic interactions between buprenorphine/naloxone and once-daily lopinavir/ritonavir.

BACKGROUND: This study was conducted to examine the pharmacokinetic interactions between buprenorphine/naloxone (BUP/NLX) and lopinavir/ritonavir (LPV/r) in HIV-seronegative subjects chronically maintained on BUP/NLX. METHODS: This study was an open labeled pharmacokinetic study in twelve HIV-seronegative subjects stabilized on at least 3 weeks of BUP/NLX therapy. Subjects sequentially underwent baseline and steady-state pharmacokinetic evaluation of once-daily LPV/r (800/200 mg). RESULTS: Compared to baseline values, BUP AUC0-24h (46.8 vs. 46.2 ng*hr/mL) and Cmax (6.54 vs. 5.88 ng/mL) did not differ significantly after achieving steady-state LPV/r. Similar analyses of norBUP, the primary metabolite of BUP, demonstrated no significant difference in norBUP AUC0-24 hours (73.7 vs. 52.7 ng x h/mL); however, Cmax (5.29 vs. 3.11 ng/mL) levels were statistically different (P < 0.05) after LPV/r administration. Naloxone concentrations were similarly unchanged for AUC0-24 hours (0.421 vs. 0.374 ng x hr/mL) and Cmax (0.186 vs. 0.186 ng/mL). Using standardized measures, no objective opioid withdrawal was observed. The AUC0-24 hours and Cmin of LPV in this study did not significantly differ from historical controls (159.6 vs. 171.3 microg x hr/mL) and (2.3 vs. 1.3 microg/mL). CONCLUSIONS: The addition of LPV/r to stabilized patients receiving BUP/NLX did not affect buprenorphine pharmacokinetics but did increase the clearance of norbuprenorphine. Pharmacodynamic responses indicate that the altered norbuprenorphine clearance did not lead to opioid withdrawal. Buprenorphine/naloxone and LPV/r can be safely coadministered without need for dosage modification.

Determination of naloxone and nornaloxone (noroxymorphone) by high-performance liquid chromatography-electrospray ionization- tandem mass spectrometry.

A highly sensitive method was developed to measure naloxone and its metabolite nornaloxone in human plasma, urine, and human liver microsomes (HLM). Naltrexone-d(3) and oxymorphone-d(3) were used as respective internal standards. Solid-phase extraction, using mixed mode extraction columns and 0.1 M phosphate buffer (pH 5.9), was combined with high-performance liquid chromatography interfaced by electrospray ionization to tandem mass spectrometry. The calibration range in plasma was 0.025 to 2 ng/mL for naloxone and 0.5 to 20 ng/mL for nornaloxone. It was 10 to 2000 ng/mL in urine and 0.5 to 20 ng/mL in HLM for both. Enzymatic hydrolysis of urine was optimized for 4 h at 40 degrees C. Intra- and interrun accuracy was within 15% of target; precision within 13.4% for all matrices. The mean recoveries were 69.2% for naloxone and 32.0% for nornaloxone. Analytes were stable in plasma and urine for up to 24 h at room temperature and in plasma after three freeze-thaw cycles. In human subjects receiving 16 mg buprenorphine and 4 mg naloxone, naloxone was detected for up to 2 h in all three subjects and up to 4 h in one subject. Mean AUC(0-24) was 0.303 +/- 0.145 ng/mL.h; mean C(max) was 0.139 +/- 0.062 ng/mL; and T(max) was 0.5 h. In 24-h urine samples, about 55% of the daily dose was excreted in either conjugated or unconjugated forms of naloxone and nornaloxone in urine. When cDNA-expressed P450s were incubated with 20 ng of naloxone, nornaloxone formation was detected for P450s 2C18, 2C19, and 3A4. Naloxone utilization exceeded nornaloxone formation for 2C19 and 3A4, indicating they may produce products other than nornaloxone. These results demonstrate a new method suitable for both in vivo and in vitro metabolism and pharmacokinetic studies of naloxone.

Effect of rifampin and nelfinavir on the metabolism of methadone and buprenorphine in primary cultures of human hepatocytes.

We tested the hypothesis that primary cultures of human hepatocytes could predict potential drug interactions with methadone and buprenorphine. Hepatocytes (five donors) were preincubated with dimethyl sulfoxide (DMSO) (vehicle), rifampin, or nelfinavir before incubation with methadone or buprenorphine. Culture media (0-60 min) was analyzed by liquid chromatography-tandem mass spectrometry for R- and S-methadone and R- and S-2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP) or for buprenorphine, norbuprenorphine, and their glucuronides [buprenorphine-3-glucuronide (B-3-G) and norbuprenorphine-3-glucuronide (N-3-G)]. R- and S-EDDP were detected in three of five, four of five, and five of five media from cells pretreated with DMSO, nelfinavir, and rifampin. R-EDDP increased 3.1- and 26.5-fold, and S-EDDP increased 2.5- and 21.3-fold after nelfinavir and rifampin. The rifampin effect was significant. B-3-G production was detected in media of all cells incubated with buprenorphine and accounted for most of the buprenorphine loss from culture media; it was not significantly affected by either pretreatment. Norbuprenorphine and N-3-G together were detected in three of five, four of five, and five of five donors pretreated with DMSO, nelfinavir and rifampin, and norbuprenorphine in one of five, one of five, and two of five donors. Although there was a trend for norbuprenorphine (2.8- and 4.9-fold) and N-3-G (1.7- and 1.9-fold) to increase after nelfinavir and rifampin, none of the changes were significant. To investigate low norbuprenorphine production, buprenorphine was incubated with human liver and small intestine microsomes fortified to support both N-dealkylation and glucuronidation; N-dealkylation predominated in small intestine and glucuronidation in liver microsomes. These studies support the hypothesis that methadone metabolism and its potential for drug interactions can be predicted with cultured human hepatocytes, but for buprenorphine the combined effects of hepatic and small intestinal metabolism are probably involved.

Pharmacokinetic interactions between buprenorphine/naloxone and tipranavir/ritonavir in HIV-negative subjects chronically receiving buprenorphine/naloxone.

HIV-infected patients with opioid dependence often require opioid replacement therapy. Pharmacokinetic interactions between HIV therapy and opioid dependence treatment medications can occur. HIV-seronegative subjects stabilized on at least 3 weeks of buprenorphine/naloxone (BUP/NLX) therapy sequentially underwent baseline and steady-state pharmacokinetic evaluation of open-label, twice daily tipranavir 500 mg co-administered with ritonavir 200 mg (TPV/r). Twelve subjects were enrolled and 10 completed the study. Prior to starting TPV/r, the geometric mean BUP AUC(0-24h) and C(max) were 43.9 ng h/mL and 5.61 ng/mL, respectively. After achieving steady-state with TPV/r (> or = 7 days), these values were similar at 43.7 ng h/mL and 4.84 ng/mL, respectively. Similar analyses for norBUP, the primary metabolite of BUP, demonstrated a reduction in geometric mean for AUC(0-24h) [68.7-14.7 ng h/mL; ratio=0.21 (90% CI 0.19-0.25)] and C(max) [4.75-0.94 ng/mL; ratio=0.20 (90% CI 0.17-0.23)]. The last measurable NLX concentration (C(last)) in the concentration-time profile, never measured in previous BUP/NLX interaction studies with antiretroviral medications, was decreased by 20%. Despite these pharmacokinetic effects on BUP metabolites and NLX, no clinical opioid withdrawal symptoms were noted. TPV steady-state AUC(0-12h) and C(max) decreased 19% and 25%, respectively, and C(min) was relatively unchanged when compared to historical control subjects receiving TPV/r alone. No dosage modification of BUP/NLX is required when co-administered with TPV/r. Though mechanistically unclear, it is likely that decreased plasma RTV levels while on BUP/NLX contributed substantially to the decrease in TPV levels. BUP/NLX and TPV/r should therefore be used cautiously to avoid decreased efficacy of TPV in patients taking these agents concomitantly.

Pharmacokinetics of buprenorphine following intravenous and oral transmucosal administration in dogs.

Pharmacokinetic analysis of buprenorphine administered to six healthy dogs via the oral transmucosal (OTM) route at doses of 20 and 120 microg/kg was conducted using liquid chromatography-electrospray ionization-tandem mass spectroscopy (LC-ESI-MS/MS). Bioavailability was 38% plus or minus 12% for the 20 microg/kg dose and 47%+/-16% for the 120 microg/kg dose. Maximum plasma concentrations were similar for buprenorphine doses of 20 microg/kg IV and 120 microg/kg OTM. Sedation and salivation were common side effects, but no bradycardia, apnea, or cardiorespiratory depressive effects were seen. When the two OTM dosing rates were normalized to dose, LC-ESI-MS/MS analysis of buprenorphine and its metabolites detected no significant difference (P>.05), indicating dose proportionality. The results of this study suggest that OTM buprenorphine may be an alternative for pain management in dogs.

Quantitative analysis of naltrexone and 6beta-naltrexol in human, rat, and rabbit plasma by liquid chromatography-electrospray ionization tandem mass spectrometry with application to the pharmacokinetics of Depotrex in rabbits.

To improve the analysis of naltrexone and its primary metabolite 6beta-naltrexol, a sensitive and specific method for the analysis of subnanogram-per-milliliter concentrations of these analytes in human, rat, and rabbit plasma was developed utilizing liquid chromatography (LC) coupled to electrospray ionization (ESI) tandem mass spectrometry (MS-MS). Plasma samples were extracted utilizing a liquid-liquid extraction technique. Chromatographic separation was achieved using an isocratic solvent system consisting of dilute formic acid and methanol pumped through an ODS-AQ HPLC column. ESI-MS-MS was in the positive ion mode followed by collision-induced dissociation of the protonated molecular ions for naltrexone, 6beta-naltrexol, and their deuterated analogues. This method was validated using Good Laboratory Practice approved methods and was compared to an existing gas chromatography (GC)-MS method by analyzing plasma samples collected from a clinical study. Specificity determined from comparing blank plasma fortified with internal standard to samples fortified with internal standard and analyte at the lower limit of quantitation (LLOQ) from six different human, rat, and rabbit sources demonstrated sufficient signal-to-noise to set the LLOQ at 0.1 ng/mL. This assay has a quantitative range of 0.1-100 ng/mL. The inter- (human only) and intra-assay precision and accuracy in plasma varied by less than 13, 11, and 16% at the LLOQ for both analytes and by less than 10, 10, and 9% at higher concentrations for human, rat, and rabbit plasma, respectively. No loss of analyte was observed after 24 h of room temperature storage in human, rat, and rabbit plasma or three cycles of freezing and thawing of human plasma prior to extraction. Human samples that had been extracted were stable for at least five days when stored frozen at -20 degrees C or for at least two days when stored at room temperature on an autosampler. The GC-MS and LC-MS-MS methods correlated in the measured plasma concentrations of both naltrexone and 6beta-naltrexol. This method has been validated and subsequently used in the determination of the pharmacokinetics of Depotrex in rabbits. In rabbits, the parent compound shows dose-dependent pharmacokinetics as seen in humans, but rabbits have much lower unconjugated metabolite, 6beta-naltrexol, than that seen in humans.