Evaluation of the Effects of Repeat-Dose Dabrafenib on the Single-Dose Pharmacokinetics of Rosuvastatin (OATP1B1/1B3 Substrate) and Midazolam (CYP3A4 Substrate).

Dabrafenib is an oral BRAF kinase inhibitor approved for the treatment of various BRAF V600 mutation-positive solid tumors. In vitro observations suggesting cytochrome P450 (CYP) 3A induction and organic anion transporting polypeptide (OATP) inhibition prompted us to evaluate the effect of dabrafenib 150 mg twice daily on the pharmacokinetics of midazolam 3 mg (CYP3A substrate) and rosuvastatin 10 mg (OATP1B1/1B3 substrate) in a clinical phase 1, open-label, fixed-sequence study in patients with BRAF V600 mutation-positive tumors. Repeat dabrafenib dosing resulted in a 2.56-fold increase in rosuvastatin maximum observed concentration (Cmax ), an earlier time to Cmax , but only a 7% increase in area under the concentration-time curve from time 0 (predose) extrapolated to infinite time. Midazolam Cmax and AUC extrapolated to infinite time decreased by 47% and 65%, respectively, with little effect on time to Cmax . No new safety findings were reported. Exposure of drugs that are CYP3A4 substrates is likely to decrease when coadministered with dabrafenib. Concentrations of medicinal products that are sensitive OATP1B1/1B3 substrates may increase during the absorption phase.

Verification of a physiologically based pharmacokinetic model of ritonavir to estimate drug-drug interaction potential of CYP3A4 substrates.

Ritonavir is one of several ketoconazole alternatives used to evaluate strong CYP3A4 inhibition potential in clinical drug-drug interaction (DDI) studies. In this study, four physiologically based pharmacokinetic (PBPK) models of ritonavir as an in vivo time-dependent inhibitor of CYP3A4 were created and verified for oral doses of 20, 50, 100 and 200 mg using the fraction absorbed (Fa ) and oral clearance (CLoral ) values reported in the literature, because transporter and CYP enzyme reaction phenotyping data were not available. The models were used subsequently to predict and compare the magnitude of the AUC increase in nine reference DDI studies evaluating the effect of ritonavir at steady-state on midazolam (CYP3A4 substrate) exposure. Midazolam AUC and Cmax ratios were predicted within 2-fold of the respective observations in seven studies. Simulations of the hepatic and gut CYP3A4 abundance after multiple oral dosing of ritonavir indicated that a 3-day treatment with ritonavir 100 mg twice daily is sufficient to reach maximal CYP3A4 inhibition and subsequent systemic exposure increase of a CYP3A4 substrate, resulting in the reliable estimation of fm,CYP3A4 . The ritonavir model was submitted as part of the new drug application for Kisqali® (ribociclib) and accepted by health authorities.

Physiologically Based Pharmacokinetic Model Predictions of Panobinostat (LBH589) as a Victim and Perpetrator of Drug-Drug Interactions.

Panobinostat (Farydak) is an orally active hydroxamic acid-derived histone deacetylase inhibitor used for the treatment of relapsed or refractory multiple myeloma. Based on recombinant cytochrome P450 (P450) kinetic analyses in vitro, panobinostat oxidative metabolism in human liver microsomes was mediated primarily by CYP3A4 with lower contributions by CYP2D6 and CYP2C19. Panobinostat was also an in vitro reversible and time-dependent inhibitor of CYP3A4/5 and a reversible inhibitor of CYP2D6 and CYP2C19. Based on a previous clinical drug-drug interaction study with ketoconazole (KTZ), the contribution of CYP3A4 in vivo was estimated to be ∼40%. Using clinical pharmacokinetic (PK) data from several trials, including the KTZ drug-drug interaction (DDI) study, a physiologically based pharmacokinetic (PBPK) model was built to predict panobinostat PK after single and multiple doses (within 2-fold of observed values for most trials) and the clinical DDI with KTZ (predicted and observed area under the curve ratios of 1.8). The model was then applied to predict the drug interaction with the strong CYP3A4 inducer rifampin (RIF) and the sensitive CYP3A4 substrate midazolam (MDZ) in lieu of clinical trials. Panobinostat exposure was predicted to decrease in the presence of RIF (65%) and inconsequentially increase MDZ exposure (4%). Additionally, PBPK modeling was used to examine the effects of stomach pH on the absorption of panobinostat in humans and determined that absorption of panobinostat is not expected to be affected by increases in stomach pH. The results from these studies were incorporated into the Food and Drug Administration-approved product label, providing guidance for panobinostat dosing recommendations when it is combined with other drugs.

A Physiologically-Based Pharmacokinetic Modeling Approach To Predict Drug-Drug Interactions of Sonidegib (LDE225) with Perpetrators of CYP3A in Cancer Patients.

Sonidegib (Odomzo) is an orally available Smoothened inhibitor for the treatment of advanced basal cell carcinoma. Sonidegib was found to be metabolized primarily by cytochrome P450 (CYP)3A in vitro. The effect of multiple doses of the strong CYP3A perpetrators, ketoconazole (KTZ) and rifampin (RIF), on sonidegib pharmacokinetics (PK) after a single 800 mg dose in healthy subjects was therefore assessed. These data were used to verify a physiologically-based pharmacokinetic (PBPK) model developed to 1) bridge the clinical drug-drug interaction (DDI) study of sonidegib with KTZ and RIF in healthy subjects to the marketed dose (200 mg) in patients 2) predict acute (14 days) versus long-term dosing of the perpetrators with sonidegib at steady state and 3) predict the effect of moderate CYP3A perpetrators on sonidegib exposure in patients. Treatment of healthy subjects with KTZ resulted in an increased sonidegib exposure of 2.25- and 1.49-fold (area under the curve0-240h and maximal concentration respectively), and RIF decreased exposure by 72% and 54%, respectively. The model simulated the single- and/or multiple-dose PK of sonidegib (healthy subjects and patients) within ∼50% of observed values. The effect of KTZ and RIF on sonidegib in healthy subjects was also simulated well, and the predicted DDI in patients was slightly less and independent of sonidegib dose. At steady state, sonidegib was predicted to have a higher DDI magnitude with strong or moderate CYP3A perpetrators compared with a single dose. Different dosing regimens of sondigeb with the perpetrators were also simulated and provided guidance to the current dosing recommendations incorporated in the product label.

Prioritizing pharmacokinetic drug interaction precipitants in natural products: application to OATP inhibitors in grapefruit juice.

Natural products, including botanical dietary supplements and exotic drinks, represent an ever-increasing share of the health-care market. The parallel ever-increasing popularity of self-medicating with natural products increases the likelihood of co-consumption with conventional drugs, raising concerns for unwanted natural product-drug interactions. Assessing the drug interaction liability of natural products is challenging due to the complex and variable chemical composition inherent to these products, necessitating a streamlined preclinical testing approach to prioritize precipitant individual constituents for further investigation. Such an approach was evaluated in the current work to prioritize constituents in the model natural product, grapefruit juice, as inhibitors of intestinal organic anion-transporting peptide (OATP)-mediated uptake. Using OATP2B1-expressing MDCKII cells (Madin-Darby canine kidney type II) and the probe substrate estrone 3-sulfate, IC50s were determined for constituents representative of the flavanone (naringin, naringenin, hesperidin), furanocoumarin (bergamottin, 6',7'-dihydroxybergamottin) and polymethoxyflavone (nobiletin and tangeretin) classes contained in grapefruit juice. Nobiletin was the most potent (IC50 , 3.7 µm); 6',7'-dihydroxybergamottin, naringin, naringenin and tangeretin were moderately potent (IC50 , 20-50 µm); and bergamottin and hesperidin were the least potent (IC50 , >300 µm) OATP2B1 inhibitors. Intestinal absorption simulations based on physiochemical properties were used to determine the ratios of unbound concentration to IC50 for each constituent within enterocytes and to prioritize in order of pre-defined cut-off values. This streamlined approach could be applied to other natural products that contain multiple precipitants of natural product-drug interactions. Copyright © 2016 John Wiley & Sons, Ltd.

An Exposure-Response Modeling Approach to Examine the Relationship Between Potency of CYP3A Inducer and Plasma 4ß-Hydroxycholesterol in Healthy Subjects.

The objectives of this analysis were to establish the exposure-response relationship between plasma rifampicin and 4ß-hydroxycholesterol (4ßHC) concentration and to estimate the effect of weak, moderate, and potent CYP3A induction. Plasma rifampicin and 4ßHC concentration-time data from a drug-drug interaction study with rifampicin 600 mg were used for model development. An indirect response model with an effect compartment described the relationship between rifampicin and 4ßHC concentrations. The model predicted that the equilibration t1/2 and 4ßHC t1/2 were 72.8 and 142 hours, respectively. EM50 and Emax of rifampicin induction were 32.6 µg and 8.39-fold, respectively. The population PK-PD model was then used to simulate the effects of rifampicin 10, 20, and 100 mg on plasma 4ßHC for up to 21 days, in which rifampicin 10, 20, and 100 mg were used to represent weak, moderate, and strong inducers, respectively. The model-predicted median (5th, 95th percentiles) 1.13 (1.04, 1.44)-, 1.28 (1.10, 1.71)-, and 2.10 (1.45, 3.49)-fold increases in plasma 4ßHC after 14-day treatment with rifampicin 10, 20, and 100 mg, respectively. A new drug candidate can likely be classified as a weak, moderate, or strong inducer if baseline-normalized plasma 4ßHC increases by <1.13-, 1.13- to 2.10-, or >2.10-fold, respectively, after 14 days of dosing.

A modified grapefruit juice eliminates two compound classes as major mediators of the grapefruit juice-fexofenadine interaction: an in vitro-in vivo "connect".

The grapefruit juice (GFJ)-fexofenadine interaction involves inhibition of intestinal organic anion transporting polypeptide (OATP)-mediated uptake. Only naringin has been shown clinically to inhibit intestinal OATP; other constituents have not been evaluated. The effects of a modified GFJ devoid of furanocoumarins (~99%) and polymethoxyflavones (~90%) on fexofenadine disposition were compared to effects of the original juice. Extracts of both juices inhibited estrone 3-sulfate and fexofenadine uptake by similar extents in OATP-transfected cells (~50% and ~25%, respectively). Healthy volunteers (n = 18) were administered fexofenadine (120 mg) with water, GFJ, or modified GFJ (240 mL) by randomized, three-way crossover design. Compared to water, both juices decreased fexofenadine geometric mean AUC and C(max) by ~25% (P ≤ .008 and P ≤ .011, respectively), with no effect on terminal half-life (P = .11). Similar effects by both juices on fexofenadine pharmacokinetics indicate furanocoumarins and polymethoxyflavones are not major mediators of the GFJ-fexofenadine interaction.

Mechanisms underlying food-drug interactions: inhibition of intestinal metabolism and transport.

Food-drug interaction studies are critical to evaluate appropriate dosing, timing, and formulation of new drug candidates. These interactions often reflect prandial-associated changes in the extent and/or rate of systemic drug exposure. Physiologic and physicochemical mechanisms underlying food effects on drug disposition are well-characterized. However, biochemical mechanisms involving drug metabolizing enzymes and transport proteins remain underexplored. Several plant-derived beverages have been shown to modulate enzymes and transporters in the intestine, leading to altered pharmacokinetic (PK) and potentially negative pharmacodynamic (PD) outcomes. Commonly consumed fruit juices, teas, and alcoholic drinks contain phytochemicals that inhibit intestinal cytochrome P450 and phase II conjugation enzymes, as well as uptake and efflux transport proteins. Whereas myriad phytochemicals have been shown to inhibit these processes in vitro, translation to the clinic has been deemed insignificant or undetermined. An overlooked prerequisite for elucidating food effects on drug PK is thorough knowledge of causative bioactive ingredients. Substantial variability in bioactive ingredient composition and activity of a given dietary substance poses a challenge in conducting robust food-drug interaction studies. This confounding factor can be addressed by identifying and characterizing specific components, which could be used as marker compounds to improve clinical trial design and quantitatively predict food effects. Interpretation and integration of data from in vitro, in vivo, and in silico studies require collaborative expertise from multiple disciplines, from botany to clinical pharmacology (i.e., plant to patient). Development of more systematic methods and guidelines is needed to address the general lack of information on examining drug-dietary substance interactions prospectively.

Conversion of tryptophan to indole-3-acetic acid by TRYPTOPHAN AMINOTRANSFERASES OF ARABIDOPSIS and YUCCAs in Arabidopsis.

Auxin is an essential hormone, but its biosynthetic routes in plants have not been fully defined. In this paper, we show that the TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS (TAA) family of amino transferases converts tryptophan to indole-3-pyruvate (IPA) and that the YUCCA (YUC) family of flavin monooxygenases participates in converting IPA to indole-3-acetic acid, the main auxin in plants. Both the YUCs and the TAAs have been shown to play essential roles in auxin biosynthesis, but it has been suggested that they participate in two independent pathways. Here, we show that all of the taa mutant phenotypes, including defects in shade avoidance, root resistance to ethylene and N-1-naphthylphthalamic acid (NPA), are phenocopied by inactivating YUC genes. On the other hand, we show that the taa mutants in several known auxin mutant backgrounds, including pid and npy1, mimic all of the well-characterized developmental defects caused by combining yuc mutants with the auxin mutants. Furthermore, we show that overexpression of YUC1 partially suppresses the shade avoidance defects of taa1 and the sterile phenotypes of the weak but not the strong taa mutants. In addition, we discovered that the auxin overproduction phenotypes of YUC overexpression lines are dependent on active TAA genes. Our genetic data show that YUC and TAA work in the same pathway and that YUC is downstream of TAA. The yuc mutants accumulate IPA, and the taa mutants are partially IPA-deficient, indicating that TAAs are responsible for converting tryptophan to IPA, whereas YUCs play an important role in converting IPA to indole-3-acetic acid.

Influence of dietary substances on intestinal drug metabolism and transport.

Successful delivery of promising new chemical entities via the oral route is rife with challenges, some of which cannot be explained or foreseen during drug development. Further complicating an already multifaceted problem is the obvious, yet often overlooked, effect of dietary substances on drug disposition and response. Some dietary substances, particularly fruit juices, have been shown to inhibit biochemical processes in the intestine, leading to altered pharmacokinetic (PK), and potentially pharmacodynamic (PD), outcomes. Inhibition of intestinal CYP3Amediated metabolism is the major mechanism by which fruit juices, including grapefruit juice, enhances systemic exposure to new and already marketed drugs. Inhibition of intestinal non-CYP3A enzymes and apically-located transport proteins represent recently identified mechanisms that can alter PK and PD. Several fruit juices have been shown to inhibit these processes in vitro, but some interactions have not translated to the clinic. The lack of in vitroin vivo concordance is due largely to a lack of rigorous methods to elucidate causative ingredients prior to clinical testing. Identification of specific components and underlying mechanisms is challenging, as dietary substances frequently contain multiple, often unknown, bioactive ingredients that vary in composition and bioactivity. A translational research approach, combining expertise from clinical pharmacologists and natural products chemists, is needed to develop robust models describing PK/PD relationships between a given dietary substance and drug of interest. Validation of these models through well-designed clinical trials would facilitate development of common practice guidelines for managing drug-dietary substance interactions appropriately.