Uwe Fuhr

Drug Interactions with Grapefruit Juice Extent, Probable Mechanism and Clinical Relevance

SummaryConcomitant intake with grapefruit juice increases the concentrations of many drugs in humans. The effect seems to be mediated mainly by suppression of the cytochrome P450 enzyme CYP3A4 in the small intestine wall. This results in a diminished first pass metabolism with higher bioavailability and increased maximal plasma concentrations of substrates of this enzyme. The effect was most pronounced in drugs with a high first pass degradation and in many cases has the clear potential to reach clinical relevance, as shown by an occasional change in drug effects or tolerability. For felodipine, nitrendipine, nisoldipine and saquinavir, the interaction was most marked with median increases of area under the curve (AUC) and/or the maximum (peak) plasma drug concentration after singledose administration (Cmax) values exceeding 70% of respective control periods. Less pronounced, but possibly relevant, concentration increases were found for nifedipine, nimodipine, verapamil, cyclosporin, midazolam, triazolam and terfenadine. This list is not complete because many drugs have not been studied yet.The components of grapefruit juice which are the most probable causes of the interaction are psoralen derivatives, but the flavonoid naringenin may also contribute. Concomitant grapefruit juice intake does not generally decrease the variability of drug pharmacokinetic parameters. Therefore, it is recommended that patients refrain from drinking grapefruit juice when they are taking a drug that is extensively metabolised, unless a lack of interaction has already been demonstrated for that drug. It is also recommended that drugs possibly interacting with grapefruit juice should be appropriately labelled.A place for grapefruit juice as a drug-sparing agent in treatment involving expensive medicine cannot be derived from the information currently available on grapefruit juice interactions.

The fate of naringin in humans: A key to grapefruit juice‐drug interactions?

The increase of concentrations observed for many drugs when administered concomitantly with grapefruit juice was attributed to inhibition of cytochrome P450 enzymes by naringenin, the aglycone of the grapefruit flavonoid naringin. However, this explanation is equivocal, and formation of naringenin after ingestion of grapefruit juice has not been proved. We investigated renal excretion of naringin, naringenin, and its glucuronides after administration of 20 ml grapefruit juice (621 μmol/L naringin) per kilogram of body weight to six healthy adults. Urine was collected for 24 hours, and flavonoids were measured by HPLC in aliquots with and without glucuronidase pretreatment. Naringin or naringin glucuronides were not found. Naringenin and its glucuronides appeared in urine after a median lag‐time of 2 hours and reached 0.012% to 0.37% and 5.0% to 57%, respectively, of the molar naringin dose. In additional investigations, low concentrations (<4 μmol/L) of naringenin glucuronides, but neither naringin nor naringenin were found in plasma samples from previous grapefruit juice interaction studies, and metabolization of naringin to naringenin occurred during 24 hours of incubation (37° C) in three of five feces samples tested. The data suggest that cleavage of the sugar moiety, presumably by intestinal bacteria, is the first step of naringin metabolism. Naringenin formation is thought to be the crucial step in determination of bioavailability of the compound, which undergoes rapid glucuronidation. The pronounced interindividual variability of naringin kinetics provides a possible explanation for some of the apparently contradictory results of drug interaction studies with grapefruit and naringin.

Appropriate phenotyping procedures for drug metabolizing enzymes and transporters in humans and their simultaneous use in the cocktail approach

Phenotyping for drug metabolizing enzymes and transporters is used to assess quantitatively the effect of an intervention (e.g., drug therapy, diet) or a condition (e.g., genetic polymorphism, disease) on their activity. Appropriate selection of test drug and metric is essential to obtain results applicable for other substrates of the respective enzyme/transporter. The following phenotyping metrics are recommended based on the level of validation and on practicability: CYP1A2, paraxanthine/caffeine in plasma 6 h after 150 mg caffeine; CYP2C9, tolbutamide plasma concentration 24 h after 125 mg tolbutamide; CYP2C19, urinary excretion of 4′‐OH‐mephenytoin 0–12 h after 50 mg mephenytoin; CYP2D6, urinary molar ratio debrisoquine/4‐OH‐debrisoquine 0–8 h after 10 mg debrisoquine; and CYP3A4, plasma clearance of midazolam after 2 mg midazolam (all drugs given orally).

Time response of cytochrome P450 1A2 activity on cessation of heavy smoking.

Cytochrome P450 (CYP) 1A2 activity is induced by cigarette smoking. Thus smoking cessation in patients while they are undergoing therapy with a CYP1A2 substrate such as theophylline or clozapine increases its concentrations and may cause adverse effects. Our objective was to determine the time course of CYP1A2 activity changes after smoking cessation in heavy smokers as the basis for dosing adaptation schemes.

Pharmacogenetics-based therapeutic recommendations — ready for clinical practice?

Although considerable progress has been made in basic pharmacogenetic research, less has been demonstrated in the application of pharmacogenetics (PGx)-based diagnostics to drug development and in clinical practice. There are drugs that are currently used in the clinic for which individualized therapy could be beneficial based on PGx data. However, specific, actionable recommendations on how to implement individualized therapy — particularly with respect to dosage — still have to be developed. Moreover, to apply PGx efficiently in clinical drug development, and later in drug therapy, study designs and the generation and handling of PGx data need to become more standardized. Here, we argue for the development of concise guidelines for implementation of PGx analyses in drug development and therapy.

Should We Use N-Acetyltransferase Type 2 Genotyping To Personalize Isoniazid Doses?

ABSTRACT Isoniazid is metabolized by the genetically polymorphic arylamine N-acetyltransferase type 2 (NAT2). A greater number of high-activity alleles are related to increased acetylation capacity and in some reports to low efficacy and toxicity of isoniazid. The objective of this study was to assess individual isoniazid exposure based on NAT2 genotype to predict a personalized therapeutic dose. Isoniazid was administered to 18 healthy Caucasians (age 30 ± 6 years, body weight 74 ± 10 kg, five women) in random order as a 200-mg infusion, a 100-mg oral, and a 300-mg oral single dose. For the assessment of NAT2 genotype, common single nucleotide polymorphisms identifying 99.9% of variant alleles were characterized. Noncompartmental pharmacokinetics and compartmental population pharmacokinetics were estimated from isoniazid plasma concentrations until 24 h postdose by high-pressure liquid chromatography. The influence of NAT2 genotype, drug formulation, body weight, and sex on dose-normalized isoniazid pharmacokinetics was assessed by analysis of variance from noncompartmental data and confirmed by population pharmacokinetics. Eight high-activity NAT2*4 alleles were identified. Sex had no effect; the other factors explained 93% of the variability in apparent isoniazid clearance (analysis of variance). NAT2 genotype alone accounted for 88% of variability. Individual isoniazid clearance could be predicted as clearance (liters/hour) = 10 + 9 × (number of NAT2*4 alleles). To achieve similar isoniazid exposure, current standard doses presumably appropriate for patients with one high-activity NAT2 allele may be decreased or increased by approximately 50% for patients with no or two such alleles, respectively. Prospective clinical trials are required to assess the merits of this approach.

An amperometric biosensor with human CYP3A4 as a novel drug screening tool.

We developed a biosensor based on the redox properties of human CYP3A4 to directly monitor electron transfer to the heme protein. Enzyme films were assembled on gold electrodes by alternate adsorption of a CYP3A4 layer on top of a polycation layer. Direct, reversible electron transfer between the electrode and CYP3A4 was observed with voltammetry under anaerobic conditions. In the presence of oxygen, the oxidation peak of the hemoprotein disappeared, and the reduction peak increased 2- to 3-fold. Addition of CYP3A4 substrates (verapamil, midazolam, quinidine, and progesterone) to the oxygenated solution caused a concentration-dependent increase in the reduction current in cyclic voltammetric and amperometric experiments. Product analyses after electrolysis with the enzyme film showed catalytic activity of the biosensor depending on substrate concentration, its inhibition by ketoconazole, and a minor contribution of H(2)O(2) to the catalytic cycle. These results suggest that electron exchange between the electrode and the immobilized CYP3A4 occurred, and that metabolic activity of the enzyme was maintained. Thus, important requirements for the application of human CYP biosensors in order to identify drugs or drug candidates as substrates or inhibitors to the attached enzyme are fulfilled.

Induction of drug metabolising enzymes: pharmacokinetic and toxicological consequences in humans.

Currently, 5 different main mechanisms of induction are distinguished for drug-metabolising enzymes. The ethanol type of induction is mediated by ligand stabilisation of the enzyme, but the others appear to be mediated by intracellular ‘receptors’. These are the aryl hydrocarbon (Ah) receptor, the peroxisome proliferator activated receptor (PPAR), the constitutive androstane receptor (CAR, phenobarbital induction) and the pregnane X receptor [PXR, rifampicin (rifampin) induction].Enzyme induction has the net effect of increasing protein levels. However, many inducers are also inhibitors of the enzymes they induce, and the inductive effects of a single drug may be mediated by more than one mechanism. Therefore, it appears that every inducer has its own pattern of induction; knowledge of the main mechanism is often not sufficient to predict the extent and time course of induction, but may serve to make the clinician aware of potential dangers.The possible pharmacokinetic consequences of enzyme induction depend on the localisation of the enzyme. They include decreased or absent bioavailability for orally administered drugs, increased hepatic clearance or accelerated formation of reactive metabolites, which is usually related to local toxicity. Although some severe drug-drug interactions are caused by enzyme induction, most of the effects of inducers are not detected in the background of nonspecific variation. For any potent inducer, however, its addition to, or withdrawal from, an existing drug regimen may cause pronounced concentration changes and should be done gradually and with appropriate monitoring of therapeutic efficacy and adverse events.The toxicological consequences of enzyme induction in humans are rare, and appear to be mainly limited to hepatoxicity in ethanol-type induction.

Inhibitory potency of quinolone antibacterial agents against cytochrome P450IA2 activity in vivo and in vitro.

Inhibition of cytochrome P450IA2 activity is an important adverse effect of quinolone antibacterial agents. It results in a prolonged half-life for some drugs that are coadministered with quinolones, such as theophylline. The objective of the study described here was to define the parameters for quantifying the inhibitory potencies of quinolones against cytochrome P450IA2 in vivo and in vitro and to investigate the relationship between the results of both approaches. Cytochrome P450IA2 activity in vitro was measured by using the 3-demethylation rate of caffeine (500 microM) in human liver microsomes. The inhibitory potency of a quinolone in vitro was determined by calculating the decrease in the activity of cytochrome P450IA2 caused by addition of the quinolone (500 microM) into the incubation medium. The mean values (percent reduction of activity without quinolone) were as follows: enoxacin, 74.9%; ciprofloxacin, 70.4%; nalidixic acid, 66.6%; pipemidic acid, 59.3%; norfloxacin, 55.7%; lomefloxacin, 23.4%; pefloxacin, 22.0%; amifloxacin, 21.4%; difloxacin, 21.3%; ofloxacin, 11.8%; temafloxacin, 10.0%; fleroxacin, no effect. The inhibitory potency of a quinolone in vivo was defined by a dose- and bioavailability-normalized parameter calculated from changes of the elimination half-life of theophylline and/or caffeine reported in previously published studies. Taking the pharmacokinetics of the quinolones into account, it was possible to differentiate between substances with and without clinically relevant inhibitory effects by using results of in vitro investigations. The in vitro test described here may help to qualitatively predict the relevant drug interactions between quinolones and methylxanthines that occur during therapy.

Clinical Pharmacokinetics of Tyrosine Kinase Inhibitors

Pyrimidine (imatinib, dasatinib, nilotinib and pazopanib), pyridine (sorafenib) and pyrrole (sunitinib) tyrosine kinase inhibitors (TKIs) are multi-targeted TKIs with high activity towards several families of receptor and non-receptor tyrosine kinases involved in angiogenesis, tumour growth and metastatic progression of cancer. These orally administered TKIs have quite diverse characteristics with regard to absorption from the gastrointestinal tract. Absolute bioavailability in humans has been investigated only for imatinib (almost 100%) and pazopanib (14–39%; n = 3). On the basis of human radioactivity data, dasatinib is considered to be well absorbed after oral administration (19% and 0.1% of the total radioactivity were excreted as unchanged dasatinib in the faeces and urine, respectively). Quite low absolute bioavailability under fasted conditions is assumed for nilotinib (31%), sorafenib (50%) and sunitinib (50%). Imatinib, dasatinib and sunitinib exhibit dose-proportional increases in their area under the plasma concentration-time curve values over their therapeutic dose ranges. Less than dose-proportional increases were observed for nilotinib at doses ≥400 mg/day and for sorafenib and pazopanib at doses ≥800 mg/day. At steady state, the accumulation ratios are 1.5–2.5 (unchanged imatinib), 2.0 (nilotinib once-daily dosing), 3.4 (nilotinib twice-daily dosing), 1.2–4.5 (pazopanib), 5.7–6.4 (sorafenib) and 3.0–4.5 (sunitinib). Concomitant intake of a high-fat meal does not alter exposure to imatinib, dasatinib and sunitinib but leads to considerably increased bioavailability of nilotinib and pazopanib and decreased bioavailability of sorafenib. With the exception of pazopanib, the TKIs described here have large apparent volumes of distribution, exceeding the volume of body water by at least 4-fold.Very low penetration into the central nervous system in humans has been reported for imatinib and dasatinib, but there are currently no published human data for nilotinib, pazopanib, sorafenib or sunitinib. All TKIs that have been described are more than 90% bound to the plasma proteins: α1-acid glycoprotein and/or albumin. They are metabolized primarily via cytochrome P450 (CYP) 3A4, the only exception being sorafenib, for which uridine diphosphate glucuronosyltransferase 1A9 is the other main enzyme involved. Active metabolites of imatinib and sunitinib contribute to their antitumour activity. Although some patient demographics have been identified as significant co-factors that partly explain interindividual variability in exposure to TKIs, these findings have not been regarded as sufficient to recommend age-, sex-, bodyweight-or ethnicity-specific dose adjustment. Systemic exposure to imatinib, sorafenib and pazopanib increases in patients with hepatic impairment, and reduction of the initial therapeutic dose is recommended in this subpopulation. The starting dose of imatinib should also be reduced in renally impaired subjects. Because the solubility of dasatinib is pH dependent, co-administration of histamine H2-receptor antagonists and proton pump inhibitors with dasatinib should be avoided. With the exception of sorafenib, systemic exposure to TKIs is significantly decreased/increased by co-administration of potent CYP3A4 inducers/inhibitors, and so it is strongly recommended that the TKI dose is adjusted or that such co-administration is avoided. Caution is also recommended for co-administration of CYP3A4 substrates with TKIs, especially for those with a narrow therapeutic index. However, current recommendations with regard to dose adjustment of TKIs need to be validated in clinical studies. Further investigations are needed to explain the large interindividual variability in the pharmacokinetics of these drugs and to assess the clinical relevance of their interaction potential and inhibitory effects on metabolizing enzymes and transporters.