A quantitative framework and strategies for management and evaluation of metabolic drug-drug interactions in oncology drug development: new molecular entities as object drugs.

This article outlines general strategies for the management and evaluation of pharmacokinetic drug-drug interactions (DDIs) resulting from perturbation of clearance of investigational anticancer drug candidates by concomitantly administered agents in a drug development setting, with a focus on drug candidates that cannot be evaluated in first-in-human studies in healthy subjects. A risk level classification is proposed, based on quantitative integration of knowledge derived from preclinical drug-metabolism studies evaluating the projected percentage contribution [f(i)(%)] of individual molecular determinants (e.g. cytochrome P450 isoenzymes) to the overall human clearance of the investigational agent. The following classification is proposed with respect to susceptibility to DDIs with metabolic inhibitors: a projected maximum DDI expected to result in a ≤1.33-fold increase in exposure, representing a low level of risk; a projected maximum DDI expected to result in a >1.33-fold but <2-fold increase in exposure, representing a moderate level of risk; and a projected maximum DDI expected to result in a ≥2-fold increase in exposure, representing a potentially high level of risk. For DDIs with metabolic inducers, the following operational classification is proposed, based on the sum of the percentage contributions of enzymes that are inducible via a common mechanism to the overall clearance of the investigational drug: <<25%, representing a low level of risk; <50%, representing a moderate level of risk; and ≥50%, representing a potentially high level of risk. To ensure patient safety and to minimize bias in determination of the recommended phase II dose (RP2D), it is recommended that strong and moderate inhibitors and inducers of the major contributing enzyme are excluded in phase I dose-escalation studies of high-risk compounds, whereas exclusion of strong inhibitors and inducers of the contributing enzyme(s) is recommended as being sufficient for moderate-risk compounds. For drugs that will be investigated in diseases such as glioblastoma, where there may be relatively frequent use of enzyme-inducing antiepileptic agents (EIAEDs), a separate dose-escalation study in this subpopulation is recommended to define the RP2D. For compounds in the high-risk category, if genetic deficiencies in the activity of the major drug-metabolizing enzyme are known, it is recommended that poor metabolizers be studied separately to define the RP2D for this subpopulation. Whereas concomitant medication exclusion criteria that are utilized in the phase I dose-escalation studies will probably also need to be maintained for high-risk compounds in phase II studies unless the results of a clinical DDI study indicate the absence of a clinically relevant interaction, these exclusion criteria can potentially be relaxed beyond phase I for moderate-risk compounds, if supported by the nature of clinical toxicities and the understanding of the therapeutic index in phase I. Adequately designed clinical DDI studies will not only inform potential relaxation of concomitant medication exclusion criteria in later-phase studies but, importantly, will also inform the development of pharmacokinetically derived dose-modification guidelines for use in clinical practice when coupled with adequate safety monitoring, as illustrated in the prescribing guidance for many recently approved oncology therapeutics.

Predicting interactions between conventional medications and botanical products on the basis of in vitro investigations.

The potential for various natural products to perturb the metabolism and disposition of medications has been recognized for decades. There are numerous in vitro and in vivo methods available to screen botanical products for drug interaction potential. Although many normal volunteer botanical-drug interaction studies have been performed, clearly, in vitro studies assessing the potential for drug interactions with various natural products represent the predominant type of published research performed to date. In addition to the recognized limitations of in vitro screening methodologies to assess conventional drug interactions, further difficulties emerge when examining botanical products. Primary challenges include assigning hepatic concentrations and accounting for bioavailability, distribution, first-pass metabolism and active metabolites. Additionally, variability in the chemical composition of commercially available botanical supplements, the lack of analytical standards and the inability to accurately screen the entities as mixtures add to complexities in experimental design. This mini-review is intended to address the particular problems and challenges in evaluating botanical supplements using in vitro methods, and review what can and cannot be learned from such investigations.

In vitro interactions of water-soluble garlic components with human cytochromes p450.

Eight water-soluble components of aged garlic extract were evaluated to assess their potential to inhibit the activity of human cytochrome-P450 (CYP) enzymes. The in vitro model consisted of human liver microsomes with index reactions chosen to profile the activity of the following six CYP isoforms: CYP1A2, 2B6, 2C9, 2C19, 2D6, and 3A. With only 2 exceptions, none of the 8 garlic components produced >50% inhibition even at high concentrations (100 micromol/L). S-methyl-L-cysteine and S-allyl-L-cysteine at 100 micromol/L produced modest inhibition of CYP3A, reducing activity to 20-40% of control. However available clinical evidence does not indicate CYP3A inhibition in vivo. The findings suggest that drug interactions involving inhibition of CYP3A enzymes by aged garlic extract are very unlikely.

Interaction of flurbiprofen with cranberry juice, grape juice, tea, and fluconazole: in vitro and clinical studies.

OBJECTIVES: Recent anecdotal, unvalidated case reports have suggested potentiation of warfarin-induced anticoagulation by cranberry juice, possibly through inhibition of human cytochrome P450 (CYP) 2C9, the enzyme responsible for the clearance of the active S-enantiomer of warfarin. To address this question, the effect of cranberry juice and other beverages on CYP2C9 activity was evaluated in vitro and in vivo. METHODS: The effects of 4 beverages on CYP2C9 activity were studied in human liver microsomes, by use of flurbiprofen hydroxylation as the index reaction. In a clinical study 14 healthy volunteers received 100 mg flurbiprofen on 5 occasions in a crossover fashion, with at least 1 week separating the 5 trials. Flurbiprofen was preceded in random sequence by the following: (1) cranberry juice placebo (8 oz), (2) cranberry juice (8 oz), (3) brewed tea (8 oz), (4) grape juice (8 oz), and (5) fluconazole, a CYP2C9 inhibitor serving as a positive control, with 8 oz of water. RESULTS: Flubiprofen hydroxylation in vitro was reduced to 11% +/- 8% of control by 2.5% (vol/vol) brewed tea, to 10% +/- 7% of control by grape juice, to 56% +/- 16% of control by cranberry juice, to 85% +/- 5% of control by cranberry juice placebo, and to 21% +/- 6% of control by the index inhibitor sulfaphenazole (2.5 micromol/L) (P <.01 for all comparisons versus control). Flurbiprofen clearance (29-33 mL/min) and elimination half-life (3.3-3.4 hours) did not differ significantly among trials 1, 2, 3, and 4. However, clearance in the fluconazole treatment condition (trial 5) was significantly reduced compared with the placebo control (17 +/- 5 mL/min versus 31 +/- 8 mL/min, P <.05), and the half-life was prolonged (5.3 +/- 1.6 hours versus 3.3 +/- 0.8 hours, P <.05). Formation of 4-hydroxyflurbiprofen was correspondingly reduced by fluconazole (P <.05). CONCLUSIONS: Although grape juice and tea impaired CYP2C9 activity in vitro, none of the 3 beverages altered CYP2C9-mediated clearance of flurbiprofen in humans, making a pharmacokinetic interaction with warfarin highly unlikely.

Ginkgo biloba does not alter clearance of flurbiprofen, a cytochrome P450-2C9 substrate.

The effect of Ginkgo biloba on the activity of CYP2C9, the isoform responsible for S-warfarin clearance, was assessed in 11 healthy volunteers who received single 100-mg doses of flurbiprofen, a probe substrate for CYP2C9. Subjects also received either a standardized G biloba leaf preparation (Ginkgold, 3 doses of 120 mg) or matching placebo in a randomized, double-blind, 2-way crossover study. Mean kinetic variables for flurbiprofen with either placebo or G biloba were elimination half-life, 3.9 versus 3.5 hours; total AUC, 57 versus 55 microg/mL h; and oral clearance, 32.9 versus 31.6 mL/min. None of these differences was significant. Based on highperformance liquid chromatography analysis, each 60-mg Ginkgold tablet contained 6.6 mug of amentoflavone and 61.2 microg of quercetin, both previously identified as CYP2C9 inhibitors. These amounts were apparently too low to inhibit CYP2C9 function in vivo. The results confirm previous controlled clinical studies showing no effect of ginkgo on the kinetics or dynamics of warfarin.

Interaction of warfarin with drugs, natural substances, and foods.

Variability in the anticoagulant response to warfarin is an ongoing clinical dilemma. Fluctuations in dietary vitamin K are an important source of variance, and the need for constancy in vitamin K intake is routinely emphasized for warfarin-treated patients. Anticoagulant response is also influenced by a number of drugs that induce or inhibit warfarin metabolism, as well as by genetic polymorphisms that may modulate expression or activity of CYP2C9, the isoform mediating clearance of S-warfarin. The possible role of dietary factors other than vitamin K, as well as of herbal medicines or supplements as contributors to the instability of anticoagulation in warfarin-treated patients, has received recent attention. St. John's wort and possibly some ginseng formulations may have the potential to diminish warfarin anticoagulation, apparently by inducing CYP2C9 activity. Otherwise, there is no reliable evidence to indicate that any dietary component (other than vitamin K) or any herbal product has an effect on the anticoagulant response to warfarin. Scientific conclusions on this important therapeutic issue should be based on valid scientific data rather than unvalidated case reports.

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 microg L(-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 microg mL(-1) (0.035 microM). 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.

Evaluation of 5-hydroxytryptophol and other endogenous serotonin (5-hydroxytryptamine) analogs as substrates for UDP-glucuronosyltransferase 1A6.

Serotonin is a specific in vitro substrate for human UDP-glucuronosyltransferase (UGT) 1A6. In this study, the contribution of UGT1A6 to the glucuronidation of endogenous structural analogs of serotonin, including 5-hydroxytryptophol, N-acetylserotonin, and 6-hydroxymelatonin, was evaluated using available recombinant human UGT isoforms, human liver microsomes, and liver microsomes from animals that do not express functional UGT1A6 (Gunn rats and cats). Only UGT1A6 and UGT1A9 were found to glucuronidate 5-hydroxytryptophol at a concentration of 2 mM, although the glucuronidation rate with UGT1A6 was over 10 times that of UGT1A9. K(m) values for human liver microsomes (156, 141, and 134 microM) were most similar to that of expressed UGT1A6 (135 microM) but vastly different from that of UGT1A9 (3674 microM). 5-Hydroxytryptophol glucuronidation by human liver microsomes (n = 54) correlated well with serotonin glucuronidation (R(s) = 0.83) and UGT1A6 protein content (R(s) = 0.85). 5-Hydroxytryptophol also competitively inhibited serotonin glucuronidation by human liver microsomes (K(i) = 291 microM) and UGT1A6 (K(i) = 200 microM). N-acetylserotonin was glucuronidated most extensively by UGT1A6, although UGT1A9 and UGT1A10 showed moderate catalysis. 6-Hydroxymelatonin was glucuronidated largely by UGT1A9 and UGT1A10 but not at all by UGT1A6. Gunn rat liver glucuronidation rates for serotonin, 5-hydroxytryptophol, N-acetylserotonin, and 6-hydroxymelatonin were 11, 5, 32, and 3%, respectively, of that of normal rat liver. Cat liver microsomes did not glucuronidate serotonin, whereas relatively low activities were observed for the other indole substrates. In conclusion, these results indicate that human UGT1A6 plays a predominant role in the glucuronidation of 5-hydroxytryptophol and N-acetylserotonin, whereas 6-hydroxymelatonin is not a substrate for this enzyme.