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The caffeine metabolic ratio is an established marker for cytochrome P450 (CYP) 1A2 activity. Optimal sample size calculation for clinical pharmacokinetic xenobiotic–caffeine interaction studies requires robust estimates of interindividual and intraindividual variation in this ratio. Compared with interindividual variation, factors contributing to intraindividual variation are less defined. An exploratory analysis involving healthy nonsmoking non‐naïve caffeine drinkers (1–3 cups/day; 12 men, 12 women) administered caffeine (160 mg) on five occasions evaluated the effects of CYP1A2 induction status (based on genotype) and other factors on intraindividual variation in CYP1A2 activity. Results were compared with those from previous studies. Regardless of whether a hyperinducer (CYP1A2*1A/*1F or CYP1A2*1F/*1F) or normal metabolizer (CYP1A2*1A/*1A,CYP1A2*1C/*1F, or CYP1A2*1C*1F/*1C*1F), sex, age, oral contraceptive use by women, and smoking status, intraindividual variation was ≤30%. A value of 30% is proposed for optimal design of pharmacokinetic xenobiotic–caffeine interaction studies. Prospective studies are needed for confirmation. [ABSTRACT FROM AUTHOR]

Aldehyde oxidase (AO) contributes to the clearance of many approved and investigational small molecule drugs, which are often dual substrates of AO and drug‐metabolizing enzymes such as cytochrome P450s (CYPs). As such, the lack of established framework for quantitative translation of the clinical pharmacologic correlates of AO‐mediated clearance represents an unmet need. This study aimed to evaluate the utility of physiologically based pharmacokinetic (PBPK) modeling in the development of AO and dual AO‐CYP substrates. PBPK models were developed for capmatinib, idelalisib, lenvatinib, zaleplon, ziprasidone, and zoniporide, incorporating in vitro functional data from human liver subcellular fractions and human hepatocytes. Prediction of metabolic elimination with/without the additional empirical scaling factors (ESFs) was assessed. Clinical pharmacokinetics, human mass balance, and drug–drug interaction (DDI) studies with CYP3A4 modulators, where available, were used to refine/verify the models. Due to the lack of clinically significant AO‐DDIs with known AO inhibitors, the fraction metabolized by AO (fmAO) was verified indirectly. Clearance predictions were improved by using ESFs (GMFE ≤1.4‐fold versus up to fivefold with physiologically‐based scaling only). Observed fmi from mass balance studies were crucial for model verification/refinement, as illustrated by capmatinib, where the fmAO (40%) was otherwise underpredicted up to fourfold. Subsequently, independent DDI studies with ketoconazole, itraconazole, rifampicin, and carbamazepine verified the fmCYP3A4, with predicted ratios of the area under the concentration–time curve (AUCR) within 1.5‐fold of the observations. In conclusion, this study provides a novel PBPK‐based framework for predicting AO‐mediated pharmacokinetics and quantitative assessment of clinical DDI risks for dual AO‐CYP substrates within a totality‐of‐evidence approach. [ABSTRACT FROM AUTHOR]

Preparations from the leaves of the kratom plant ( Mitragyna speciosa ) are consumed for their opioid-like effects. Several deaths have been associated with kratom used concomitantly with some drugs. Pharmacokinetic interactions are potential underlying mechanisms of these fatalities. Accumulating in vitro evidence has demonstrated select kratom alkaloids, including the abundant indole alkaloid mitragynine, as reversible inhibitors of several cytochromes P450 (CYPs). The objective of this work was to refine the mechanistic understanding of potential kratom-drug interactions by considering both reversible and time-dependent inhibition (TDI) of CYPs in the liver and intestine. Mitragynine was tested against CYP2C9 (diclofenac 4'-hydroxylation), CYP2D6 (dextromethorphan O -demethylation), and CYP3A (midazolam 1'-hydroxylation) activities in human liver microsomes (HLMs) and CYP3A activity in human intestinal microsomes (HIMs). Comparing the absence to presence of NADPH during preincubation of mitragynine with HLMs or HIMs, an ∼7-fold leftward shift in IC 50 (∼20 to 3 μM) toward CYP3A resulted, prompting determination of TDI parameters (HLMs: K I , 4.1 ± 0.9 μM; k inact , 0.068 ± 0.01 min -1 ; HIMs: K I , 4.2 ± 2.5 μM; k inact , 0.079 ± 0.02 min -1 ). Mitragynine caused no leftward shift in IC 50 toward CYP2C9 (∼40 μM) and CYP2D6 (∼1 μM) but was a strong competitive inhibitor of CYP2D6 ( K i , 1.17 ± 0.07 μM). Using a recommended mechanistic static model, mitragynine (2-g kratom dose) was predicted to increase dextromethorphan and midazolam area under the plasma concentration-time curve by 1.06- and 5.69-fold, respectively. The predicted midazolam area under the plasma concentration-time curve ratio exceeded the recommended cutoff (1.25), which would have been missed if TDI was not considered. SIGNIFICANCE STATEMENT: Kratom, a botanical natural product increasingly consumed for its opioid-like effects, may precipitate potentially serious pharmacokinetic interactions with drugs. The abundant kratom indole alkaloid mitragynine was shown to be a time-dependent inhibitor of hepatic and intestinal cytochrome P450 3A activity. A mechanistic static model predicted mitragynine to increase systemic exposure to the probe drug substrate midazolam by 5.7-fold, necessitating further evaluation via dynamic models and clinical assessment to advance the understanding of consumer safety associated with kratom use., (Copyright © 2020 by The American Society for Pharmacology and Experimental Therapeutics.)