Effect of zolpidem on human cytochrome P450 activity, and on transport mediated by P-glycoprotein

Effect of CYP2D6, 2C19, and 3A4 Phenoconversion in Drug-Related Deaths

Molecular autopsy is a very important tool in forensic toxicology. However, many determinants, such as co-medication and physiological parameters, should be considered for optimal results. These determinants could cause phenoconversion (PC), a discrepancy between the real metabolic profile after phenoconversion and the phenotype determined by the genotype. This study's objective was to assess the PC of drug-metabolizing enzymes, namely CYP2D6, 2C19, and 3A4, in 45 post-mortem cases where medications that are substrates, inducers, or inhibitors of these enzymes were detected. It also intended to evaluate how PC affected the drug's metabolic ratio (MR) in four cases. Blood samples from 45 cases of drug-related deaths were analyzed to detect and determine drug and metabolite concentrations. Moreover, all the samples underwent genotyping utilizing the HaloPlex Target Enrichment System for CYP2D6, 2C19, and 3A4. The results of the present study revealed a statistically significant rate of PC for the three investigated enzymes, with a higher frequency of poor metabolizers after PC. A compatibility was seen between the results of the genomic evaluation after PC and the observed MRs of venlafaxine, citalopram, and fentanyl. This leads us to focus on the determinants causing PC that may be mainly induced by drug interactions. This complex phenomenon can have a significant impact on the analysis, interpretation of genotypes, and accurate conclusions in forensic toxicology. Nevertheless, more research with more cases in the future is needed to confirm these results.

Addition of zolpidem to combination therapy with atomoxetine-oxybutynin increases sleep efficiency and the respiratory arousal threshold in obstructive sleep apnoea: A randomized trial

BACKGROUND AND OBJECTIVE

Atomoxetine combined with oxybutynin (Ato-Oxy) has recently been shown to reduce obstructive sleep apnoea (OSA) severity by >60%. However, Ato-Oxy also modestly reduced the respiratory arousal threshold, which may decrease sleep quality/efficiency. We sought to investigate the additional effect of zolpidem with Ato-Oxy on sleep efficiency (primary outcome), the arousal threshold, OSA severity, other standard polysomnography (PSG) parameters, next-day sleepiness and alertness (secondary outcomes).

METHODS

Twelve participants with OSA received 10 mg zolpidem plus Ato-Oxy (80-5 mg, respectively) or Ato-Oxy plus placebo prior to overnight in-laboratory PSG according to a double-blind, randomized, crossover design (1-week washout). Participants were fitted with an epiglottic catheter, a nasal mask and pneumotachograph to quantify arousal threshold and airflow. Next-day sleepiness and alertness were assessed via the Karolinska Sleepiness Scale and a driving simulation task.

RESULTS

The addition of zolpidem increased sleep efficiency by 9% ± 13% (80.9% ± 16.9% vs. 88.2% ± 8.2%, p = 0.037) and the respiratory arousal threshold by 17% ± 18% (-26.6 ± 14.5 vs. -33.8 ± 20.3 cm H₂ O, p = 0.004) versus Ato-Oxy + placebo. Zolpidem did not systematically change OSA severity. Combination therapy was well tolerated, and zolpidem did not worsen next-day sleepiness. However, median steering deviation during the driving simulator task increased following the zolpidem combination.

CONCLUSION

Zolpidem improves sleep efficiency via an increase in the respiratory arousal threshold to counteract potential wake-promoting properties of atomoxetine in OSA. These changes occur without altering the rate of respiratory events or overnight hypoxaemia. However, while the addition of zolpidem does not increase next-day perceived sleepiness, caution is warranted given the potential impact on next-morning objective alertness.

LC-MS/MS method for determining picogram-level of zolpidem and its main metabolites in hair using a zirconia-based sorbent

Although urine and blood samples have been conventionally used for testing zolpidem (ZPD), a sedative-hypnotic, these matrices have limited application because they have a relatively short detection period and can be used only in case of recent drug exposure. Therefore, it is necessary to use an alternative biological sample to obtain the evidence of ZPD misuse. Herein, a liquid chromatography-tandem mass spectrometric (LC-MS/MS) method was developed for the determination of ZPD and its metabolites, zolpidem phenyl-4-carboxylic acid (ZPCA) and zolpidem 6-carboxylic acid (ZCA), in hair to resolve the above-mentioned problems. Mechanical pulverization of hair, methanol extraction with sonication, and the zirconia-based hybrid solid-phase extraction technique were used for obtaining improved extraction efficiency and effective sample purification. The treated hair sample was analyzed using the LC-MS/MS method with the electrospray ionization source in positive and multiple-reaction monitoring modes. The target analytes were separated and detected within 8 min using an Xselect HSS T3 column. Gradient elution was performed using 5 mM ammonium formate and acetonitrile. The lower limit of quantification of ZPD, ZPCA, and ZCA were 1.0, 0.5, and 1.0 pg mg^-1, respectively. The calibration ranges were 1.0-1000.0 pg mg^-1 for ZPD, 0.5-200.0 pg mg^-1 for ZPCA, and 1.0-200.0 pg mg^-1 for ZCA, with the determination coefficients (r² ≥ 0.9986). The intraday accuracy and precision ranged from -7.1 to 9.0% and within 6.5%, respectively, and the interday accuracy and precision ranged from -6.1 to 7.9% and within 5.4%, respectively. The recovery, matrix effect, and process efficiency were 65.2-96.6%, 64.6-106.5%, and 44.3-100.5%, respectively, with the relative standard deviation of 4.0-5.0%. The developed method was successfully applied to analyze 13 forensic hair samples of ZPD abusers, and the concentration ratios of ZPD and its two main metabolites (ZPCA and ZCA) in the ZPD-positive samples were also presented. These results revealed that ZPCA and ZCA were not easily incorporated into hair, and demonstrated that their analysis in hair samples requires the employed method to have picogram-level sensitivity. Therefore, the developed method was suitable for simultaneous analysis of ZPD, ZPCA, and ZCA in hair samples, and it could provide clear evidence for illegal ZPD administration, including ZPD-facilitated sexual assault.

Renal Drug Transporters and Drug Interactions

Transporters in proximal renal tubules contribute to the disposition of numerous drugs. Furthermore, the molecular mechanisms of tubular secretion have been progressively elucidated during the past decades. Organic anions tend to be secreted by the transport proteins OAT1, OAT3 and OATP4C1 on the basolateral side of tubular cells, and multidrug resistance protein (MRP) 2, MRP4, OATP1A2 and breast cancer resistance protein (BCRP) on the apical side. Organic cations are secreted by organic cation transporter (OCT) 2 on the basolateral side, and multidrug and toxic compound extrusion (MATE) proteins MATE1, MATE2/2-K, P-glycoprotein, organic cation and carnitine transporter (OCTN) 1 and OCTN2 on the apical side. Significant drug-drug interactions (DDIs) may affect any of these transporters, altering the clearance and, consequently, the efficacy and/or toxicity of substrate drugs. Interactions at the level of basolateral transporters typically decrease the clearance of the victim drug, causing higher systemic exposure. Interactions at the apical level can also lower drug clearance, but may be associated with higher renal toxicity, due to intracellular accumulation. Whereas the importance of glomerular filtration in drug disposition is largely appreciated among clinicians, DDIs involving renal transporters are less well recognized. This review summarizes current knowledge on the roles, quantitative importance and clinical relevance of these transporters in drug therapy. It proposes an approach based on substrate-inhibitor associations for predicting potential tubular-based DDIs and preventing their adverse consequences. We provide a comprehensive list of known drug interactions with renally-expressed transporters. While many of these interactions have limited clinical consequences, some involving high-risk drugs (e.g. methotrexate) definitely deserve the attention of prescribers.

Risk of falling and hypnotic drugs: retrospective study of inpatients

Background

Falls and related injuries remain a concern for patient safety in many hospitals and nursing care facilities. In particular, reports examining the relationship between accidents and drugs with a sedative effect have been increasing; however, the analysis of correlation between the background factors of fall accidents and the detailed therapeutic category of drugs is insufficient.

Objectives

Our objective was to estimate fall risk following the administration of hypnotics in inpatients within an acute hospital. We assessed the relationship between falls and hypnotic drugs compared with other medicines.

Study Design and Setting

An inpatient population-based study was carried out at Gunma University Hospital, where all inpatients admitted between 1 October and 31 December 2007 were included. Over a 3-month follow-up period, all reports of falling accidents from ward medical staff were investigated.

Results and Discussion

Falls occurred in 1.8 % of males and 1.3 % of females in the study population (n = 3,683). The mean age of patients who experienced falls (64.7 ± 19.5 years) was significantly higher than that of patients who did not (56.2 ± 20.2 years). Multivariate analysis revealed the following drugs as high-risk factors for falling: hypnotics (odds ratio [OR] 2.17, 95 % CI 1.44–3.28), antiepileptics (OR 5.06, 95 % CI 2.70–9.46), opioids (OR 3.91, 95 % CI 2.16–7.10), anti-Alzheimer’s (OR 5.74, 95 % CI 1.62–20.3), anti-Parkinson’s (OR 5.06, 95 % CI 1.58–16.24), antidiabetics (OR 3.08, 95 % CI 1.63–5.84), antihypertensives (OR 2.24, 95 % CI 1.41–3.56), and antiarrhythmics (OR 2.82, 95 % CI 1.36–5.86). Multivariate logistic regression analysis of hypnotics, brotizolam, zopiclone, and estazolam revealed a significant association with an increased risk of inpatient falling accidents, while zolpidem, triazolam, flunitrazepam, and nitrazepam did not.

Conclusion

The present findings suggest that the risk of falling accidents in hospitals differs according to the type of hypnotic drug administered. The appropriate selection of hypnotic drugs, therefore, might be important for reducing the number of patient falls.

Electronic supplementary material

The online version of this article (doi:10.1007/s40268-013-0019-3) contains supplementary material, which is available to authorized users.

[Clinical survey of tizanidine-induced adverse effects--impact of concomitant drugs providing cytochrome P450 1A2 modification--]

The drug-drug interactions of tizanidine and cytochrome (CYP) P450 1A2 inhibitors, which potentially alter the hepatic metabolism of tizanidine, were investigated by retrospective survey of medical records with regard to prescription. One thousand five hundred sixty-three patients treated with tizanidine at University of Tsukuba Hospital were investigated. Of those, 713 patients (45.6%) were treated with coadministration of tizanidine and CYP1A2 inhibitors (37 drugs). The patients who received a combination of tizanidine and CYP1A2 inhibitors were characterized as elderly, having multiple diseases, and taking a large number of comedications (over 10 drugs) for a long period as compared with the patients who did not receive CYP1A2 inhibitors. Tizanidine-induced adverse effects were examined in 100 patients treated with coadministration of tizanidine and 8 CYP1A2 inhibitors. Adverse effects (e.g., drowsiness: 10 patients; low blood pressure: 9 patients; low heart rate: 9 patients) were observed in 23 patients (23%) 8±10 days after CYP1A2 inhibitors were coadministered. The patients with tizanidine-induced adverse effects were of older age (64.3±9.8 vs. 57.5±18.1 years, p<0.05) and received a higher daily dose of tizanidine (3.00±0.74 vs. 2.56±0.86 mg/day, p<0.05) than the patients without adverse effects. The present results suggest that coadministration of tizanidine and CYP1A2 inhibitors enhances tizanidine-induced adverse effects, especially in elderly patients treated with a higher dose of tizanidine.

Zolpidem for insomnia

INTRODUCTION

The imidazopyridine derivative zolpidem , which acts as a benzodiazepine (BZ) receptor agonist, is the most widely prescribed hypnotic drug in the US.

AREAS COVERED

This review addresses the neuroreceptor properties of zolpidem; clinical pharmacokinetics, pharmacodynamics and drug interactions; efficacy as a hypnotic; adverse effects; tolerance, dependence and withdrawal; relation to motor vehicle accidents and complex sleep behaviors; and new dosage forms.

EXPERT OPINION

Approved doses of zolpidem (10 mg for adults, 5 mg for the elderly) are consistently effective in reducing sleep latency and consequently increasing sleep duration in patients with insomnia. However, favorable effects on sleep maintenance are observed less consistently. Residual daytime effects are unlikely with recommended doses, and provided that at least 8 h elapse prior to arising. Hypnotic efficacy is maintained with repeated nightly use, and the risk of rebound insomnia is low. Dependence and abuse of zolpidem are no more likely to occur than with typical benzodiazepines. Newly available novel dosage forms of zolpidem have increased therapeutic options for patients with insomnia variants such as sleep maintenance insomnia and middle-of-the-night awakening.

Grapefruit juice, lyophilized grapefruit juice, and powdered whole grapefruit inhibit cytochrome P450-mediated triazolam hydroxylation by beagle dog liver microsomes

Coadministration of grapefruit juice (GFJ) has been proposed to enhance the systemic availability and decrease the required dose of drugs such as cyclosporine that are extensively metabolized in the intestine and liver. Although GFJ inhibits human cytochrome P450 (CYP) 3A, effects on dog CYP have not yet been reported. Consequently, we determined whether GFJ inhibits triazolam hydroxylation by Beagle dog liver microsomes (DLM) using human liver microsomes (HLM) as positive control. Results were compared with the effects of lyophilized GFJ and commercially-available powdered grapefruit capsules, which may be more convenient dosage forms. GFJ inhibited alpha-hydroxytriazolam formation in both DLM and HLM with similar IC(50) (inhibitor concentration producing a 50% decrease in reaction velocity) values of 0.56% and 0.52% (v/v), respectively. Lyophilized GFJ and powdered grapefruit also inhibited DLM alpha-hydroxytriazolam formation with IC(50) values of 0.76 and 1.2 mg/mL, respectively. Consistent with mechanism-based enzyme inhibition, preincubation of DLM with any of the grapefruit products for 20 min resulted in significant enhancement of inhibition of triazolam alpha-hydroxylation by 8-20%. The results indicate that 16 g of lyophilized GFJ or 23 g of powdered grapefruit would be equivalent to dosing 100 mL of GFJ. In vivo pharmacokinetic interaction studies are needed to confirm these in vitro findings.

Inhibition of human cytochromes P450 by components of Ginkgo biloba

The extraction, isolation and characterization of 29 natural products contained in Ginkgo biloba have been described, which we have now tested for their in-vitro capacity to inhibit the five major human cytochrome P450 (CYP) isoforms in human liver microsomes. Weak or negligible inhibitory activity was found for the terpene trilactones (ginkgolides A, B, C and J, and bilobalide), and the flavonol glycosides. However 50% inhibitory activity (IC50) was found at concentrations less than 10 μg mL−1 for the flavonol aglycones (kaempferol, quercetin, apigenin, myricetin, tamarixetin) with CYP1A2 and CYP3A. Quercetin, the biflavone amentoflavone, sesamin, as well as (Z,Z)-4,4′-(1,4-pentadiene-1,5-diyl)diphenol and 3-nonadec-8-enyl-benzene-1,2-diol, were also inhibitors of CYP2C9. The IC50 of amentoflavone for CYP2C9 was 0.019 μg mL−1 (0.035 μm). Thus, the principal components of Ginkgo biloba preparations in clinical use (terpene trilactones and flavonol glycosides) do not significantly inhibit these human CYPs in-vitro. However, flavonol aglycones, the biflavonol amentoflavone and several other non-glycosidic constituents are significant in-vitro inhibitors of CYP. The clinical importance of these potential inhibitors will depend on their amounts in ginkgo preparations sold to the public, and the extent to which their bioavailability allows them to reach the CYP enzymes in-situ.

Curcuminoids inhibit multiple human cytochromes P450, UDP-glucuronosyltransferase, and sulfotransferase enzymes, whereas piperine is a relatively selective CYP3A4 inhibitor

Curcuminoid extract and piperine are being evaluated for beneficial effects in Alzheimer's disease, among other intractable disorders. Consequently, we studied the potential for herb-drug interactions involving cytochrome P450 (P450), UDP-glucuronosyltransferase (UGT), and sulfotransferase (SULT) enzymes. The curcuminoid extract inhibited SULT > CYP2C19 > CYP2B6 > UGT > CYP2C9 > CYP3A activities with IC(50) values ranging from 0.99 +/- 0.04 to 25.3 +/- 1.3 microM, whereas CYP2D6, CYP1A2, and CYP2E1 activities were less affected (IC(50) values > 60 microM). Inhibition of CYP3A activity by curcuminoid extract was consistent with competitive inhibition (K(i) = 11.0 +/- 1.3 microM), whereas inhibition of both CYP2C9 and CYP2C19 activities were consistent with mixed competitive-noncompetitive inhibition (10.6 +/- 1.1 and 7.8 +/- 0.9 microM, respectively). Piperine was a relatively selective noncompetitive inhibitor of CYP3A (IC(50) 5.5 +/- 0.7 microM, K(i) = 5.4 +/- 0.3 microM) with less effect on other enzymes evaluated (IC(50) > 29 microM). Curcuminoid extract and piperine inhibited recombinant CYP3A4 much more potently (by >5-fold) than CYP3A5. Pure synthetic curcuminoids (curcumin, demethoxycurcumin, and bisdemethoxycurcumin) were also evaluated for their effects on CYP3A, CYP2C9, UGT, and SULT activities. All three curcuminoids had similar effects on CYP3A, UGT, and SULT activity, but demethoxycurcumin (IC(50) = 8.8 +/- 1.2 microM) was more active against CYP2C9 than either curcumin or bisdemethoxycurcumin (IC(50) > 50 microM). Based on these data and expected tissue concentrations of inhibitors, we predict that a p.o. administered curcuminoid/piperine combination is most likely to inhibit CYP3A, CYP2C9, UGT, and SULT metabolism within the intestinal mucosa.

The emerging importance of transporter proteins in the psychopharmacological treatment of the pregnant patient

P-glycoprotein, breast cancer resistance protein, and multidrug resistance proteins have physiological functions in placental tissue. Several antidepressants, antipsychotics, and anti-epileptic drugs have been found to be substrates of P-glycoprotein and other transporters. The extent that drugs pass through the placental barrier is likely influenced by drug transporters. The rational choice of psychoactive drugs to treat mental illness in women of child-bearing age should incorporate knowledge of both drug disposition as well as expected pharmacologic effects. This review summarizes the current data on drug transporters in the placental passage of medications, with a focus on medications used in clinical psychopharmacology.

3,4-Dehydrodebrisoquine, a novel debrisoquine metabolite formed from 4-hydroxydebrisoquine that affects the CYP2D6 metabolic ratio

Considerable unexplained intersubject variability in the debrisoquine metabolic ratio (urinary debrisoquine/4-hydroxydebrisoquine) exists within individual CYP2D6 genotypes. We speculated that debrisoquine was converted to as yet undisclosed metabolites. Thirteen healthy young volunteers, nine CYP2D6*1 homozygotes [extensive metabolizers (EMs)] and four CYP2D6*4 homozygotes [poor metabolizers (PMs)] took 12.8 mg of debrisoquine hemisulfate by mouth and collected 0- to 8- and 8- to 24-h urines, which were analyzed by gas chromatography-mass spectrometry (GCMS) before and after treatment with beta-glucuronidase. Authentic 3,4-dehydrodebrisoquine was synthesized and characterized by GCMS, liquid chromatography-tandem mass spectrometry, and (1)H NMR. 3,4-Dehydrodebrisoquine is a novel metabolite of debrisoquine excreted variably in 0- to 24-h urine, both in EMs (3.1-27.6% of dose) and PMs (0-2.1% of dose). This metabolite is produced from 4-hydroxydebrisoquine in vitro by human and rat liver microsomes. A previously unstudied CYP2D6*1 homozygote was administered 10.2 mg of 4-hydroxydebrisoquine orally and also excreted 3,4-dehydrodebrisoquine. EMs excreted 6-hydroxydebrisoquine (0-4.8%) and 8-hydroxydebrisoquine (0-1.3%), but these phenolic metabolites were not detected in PM urine. Debrisoquine and 4-hydroxydebrisoquine glucuronides were excreted in a highly genotype-dependent manner. A microsomal activity that probably does not involve cytochrome P450 participates in the further metabolism of 4-hydroxydebrisoquine, which we speculate may also lead to the formation of 1- and 3-hydroxydebrisoquine and their ring-opened products. In conclusion, this study suggests that the traditional metabolic ratio is not a true measure of the debrisoquine 4-hydroxylation capacity of an individual and thus may, in part, explain the wide intragenotype variation in metabolic ratio.

INTERACTION OF TRIAZOLAM AND KETOCONAZOLE IN P-GLYCOPROTEIN-DEFICIENT MICE

The role of P-glycoprotein (P-gp) on the distribution of the benzodiazepine triazolam (TRZ) and the azole antifungal agent ketoconazole (KET), and on the TRZ-KET interaction, was studied using mdr1a(-) or mdr1a/b(-/-) mice (P-gp-deficient mice) and matched controls. TRZ and KET also were studied in Caco-2 cells in Transwell culture. After single i.p. injections of TRZ or KET in separate groups of control mice, brain concentrations of TRZ exceeded those in serum [brain/serum area under the concentration curve (AUC) ratio, 5.0], whereas brain/serum AUC ratios for KET were approximately 0.5. On the basis of single time points, brain concentrations of TRZ, or brain/serum ratios, were similar in P-gp-deficient animals compared with controls, whereas P-gp-deficient animals had significantly higher KET brain concentrations and brain/serum ratios. Coadministration of KET with TRZ increased TRZ concentrations in serum, liver, and brain, both in controls and in P-gp-deficient animals, probably attributable to impairment by KET of CYP3A-mediated clearance of TRZ. However, KET did not increase brain/serum ratios of TRZ in either group. In Caco-2 cells, basal-to-apical flux of TRZ was higher than apical-to-basal flux. However, verapamil (100 microM) did not alter flux in either direction. KET inhibited basal-to-apical transport of rho-damine-123, with a 50% inhibitory concentration of 2.7 microM. Thus, TRZ does not appear to undergo measurable blood-brain barrier efflux transport by P-gp in this animal model. KET impairs clearance of TRZ but does not increase tissue uptake. However, KET itself may be a substrate for efflux transport at the blood-brain barrier.

Pharmacokinetic properties of zolpidem in elderly and young adults: possible modulation by testosterone in men

AIMS

The influence of ageing on the pharmacokinetics of zolpidem, an extensively prescribed hypnotic medication, was evaluated in healthy human volunteers.

METHODS

A series of 16 elderly (age: 61-85 years) and 24 young (age: 22-42 years) volunteers received single 5 mg oral doses of zolpidem tartrate. Serum zolpidem concentrations were determined by HPLC with fluorescence detection in samples drawn during 8 h after dosage. The effect of testosterone on zolpidem biotransformation was evaluated in vitro using human liver microsomes. Possible induction of CYP3A protein expression and function was studied in cultured human hepatocytes.

RESULTS

Among men, apparent oral clearance of zolpidem was decreased in elderly compared to young subjects (3.8 vs 11.0 ml min-1 kg-1, P < 0.01), Cmax was increased (93 vs 40 ng ml-1, P < 0.01), and half-life increased (2.7 vs 1.5 h, P < 0.03). Among women, zolpidem oral clearance was decreased in the elderly (3.0 vs 5.8 ml min-1 kg-1, P < 0.02), Cmax increased (108 vs 60 ng ml-1, P < 0.001), with no difference in t1/2 (2.3 vs 2.4 h). Among male subjects, free serum testosterone concentrations were lower in the elderly (10.5 vs 19.0 pg ml-1, P < 0.01), and were significantly correlated with zolpidem clearance (r2 = 0.46, P < 0.001). Multiple regression analysis indicated a greater relative contribution of serum testosterone than age to the oral clearance of zolpidem among men. In human liver microsomes, co-incubation of zolpidem (10 micro m) with varying concentrations of testosterone produced activation of biotransformation of zolpidem to its principal hydroxylated metabolite. Maximum activation was achieved at equimolar concentrations of testosterone (10 micro m). However, testosterone did not induce immunoactive CYP3A4 expression or catalytic function in cultured human hepatocytes.

CONCLUSIONS

The increased Cmax and lower oral clearance of zolpidem in the elderly are consistent with recommendations of lower clinical doses of zolpidem in the elderly. Our clinical and in vitro data both suggest that reduced free serum testosterone may have a modulatory role in age-dependent changes in zolpidem pharmacokinetics in men.

Apparent mechanism-based inhibition of human CYP3A in-vitro by lopinavir

The influence of the viral protease inhibitor lopinavir on the activity of six human cytochrome P450 (CYP) enzymes was evaluated in a model system using human liver microsomes. Column chromatography methodology was developed to separate lopinavir from ritonavir starting from the commercially available lopinavir-ritonavir combination dosage form. Lopinavir produced negligible or weak inhibition of human CYP1A2, 2B6, 2C9, 2C19 and 2D6. However, lopinavir was an inhibitor of CYP3A. At 250 microM triazolam (the CYP3A index substrate), the mean (+/- s.e., n = 4) IC50 versus triazolam alpha-hydroxylation (where IC50 is the concentration producing a 50% decrement in reaction velocity) was 7.3 (+/- 0.5) microM. Pre-incubation of lopinavir with microsomes prior to addition of triazolam yielded a significantly lower IC50 of 4.1 (+/- 0.5) microM. This is consistent with mechanism-based inhibition of human CYP3A by lopinavir. Although lopinavir is less potent than ritonavir as an inhibitor of CYP3A, lopinavir is nonetheless likely to contribute to net CYP3A inhibition in-vivo during treatment with the lopinavir-ritonavir combination.