Oxana Doroshyenko

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.

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.

Pharmacokinetics and pharmacodynamics of rosiglitazone in relation to CYP2C8 genotype.

Rosiglitazone is metabolically inactivated predominantly via the cytochrome P450 (CYP) enzyme CYP2C8. The functional impact of the CYP2C8*3 allele coding for the Arg139Lys and Lys399Arg amino acid substitutions is controversial. The purpose of this was to clarify the role of this polymorphism with regard to the pharmacokinetics and clinical effects of rosiglitazone.

Clinical pharmacokinetics of tyrosine kinase inhibitors: focus on 4-anilinoquinazolines.

The 4-anilinoquinazolines (gefitinib, erlotinib and lapatinib) are members of a class of potent and selective inhibitors of the human epidermal growth factor receptor (HER) family of tyrosine kinases that have been developed to treat patients with tumours with defined genetic alterations of the HER tyrosine kinase domain. They are characterized by a moderate rate of absorption after oral administration with peak plasma concentrations at several hours post-dose. Absolute bioavailability of gefitinib and erlotinib is about 60%. Low bioavailability is assumed for lapatinib. The drugs are extensively distributed in human tissues, including tumour tissues, have a large volume of distribution at least 3-fold exceeding the volume of body water and are extensively (about 95%) protein bound to α1-acid glycoprotein and albumin.Existing human data for gefitinib and erlotinib indicate that these substances penetrate into the central nervous system and accumulate in brain tumours, possibly due to leaks in the blood-brain barrier. Gefitinib, erlotinib and the absorbed fraction of lapatinib undergo extensive metabolism — mainly via hepatic and intestinal cytochrome P450 (CYP) 3A4 and also via CYP2D6 (gefitinib) and CYP1A2 (erlotinib) — and are primarily eliminated by biotransformation. The excretion of unchanged gefitinib, erlotinib, lapatinib and their metabolites occurs predominantly in the faeces and only a minor fraction is excreted in the urine. No relevant effects of age, sex, bodyweight or race on their pharmacokinetics have been reported to date.Limited available data indicate that genetic polymorphisms in enzymes and transporters involved in the pharmacokinetics of gefitinib (CYP2D6) and erlotinib (CYP3A4, CYP3A5 and ABCG2 [breast cancer resistance protein]) alter the exposure to these drugs. Modification of drug dose should be considered in patients with severe hepatic impairment receiving these tyrosine kinase inhibitors and in current smokers receiving erlotinib. Existing recommendations for dose adjustment (i.e. a dose decrement or increment for gefitinib, erlotinib and lapatinib in the presence of CYP3A4 inhibitors or inducers, respectively; a dose increase for erlotinib in smoking patients) need to be validated in clinical studies. Further investigations are required to explain the large interindividual variability in the pharmacokinetics of these drugs and to assess the clinical relevance of interaction potential and inhibitory effects on the metabolizing enzymes and transporters.

Clinical pharmacokinetics of trospium chloride.

Trospium chloride, a quaternary amine with anticholinergic properties, is used for the treatment of overactive bladder with symptoms of urge urinary incontinence, urgency and urinary frequency. The pharmacokinetics of trospium chloride have been investigated in healthy volunteers, in patients with renal and hepatic impairment, and in those with symptoms of overactive bladder, after oral, intravenous and intravesical administration.After oral administration, absorption of the hydrophilic trospium chloride is slow and incomplete. Peak plasma concentrations (Cmax) of approximately 4 ng/mL are reached 4–5 hours after administration of a 20mg immediate-release preparation. The mean bioavailability is approximately 10% and decreases by concomitant food intake (to a mean of 26% of the fasting area under the plasma concentration-time curve [AUC]). Trospium chloride displays dose proportional increases in AUC and Cmax after a single dose within the clinically relevant dose range (20–60mg). The mean volume of distribution is approximately 350–800L. The drug is minimally (mean approximately 10%) metabolised to spiroalcohol by hydrolysis, is 50% plasma protein bound and does not cross the blood-brain barrier. Urinary excretion of the parent compound plays a major role in the disposition of the drug, with a mean renal clearance of 29 L/h (accounting for approximately 70% of total clearance) and a mean elimination half-life ranging from 10 to 20 hours. Elimination of the drug is slowed in patients with renal insufficiency, and population pharmacokinetic modelling has demonstrated that drug clearance is correlated with serum creatinine concentration. Thus, dose reduction is needed in patients with severe renal impairment (i.e. creatinine clearance <30 mL/min).To date, no clinically relevant pharmacokinetic drug-drug interactions have been identified; the drug does not bind to any of the drug metabolising cytochrome P450 enzymes.The pharmacokinetics of the drug are compatible with twice-daily administration. A once-daily schedule may also be appropriate, but this regimen needs formal clinical evaluation.

Absorption, Pharmacokinetics, and Safety of Triclosan after Dermal Administration

ABSTRACT We evaluated the pharmacokinetics and safety of the antimicrobial agent triclosan after dermal application of a 2% triclosan-containing cream to six volunteers. Percutaneous absorption calculated from urinary excretion was 5.9% ± 2.1% of the dose (mean ± standard deviation). The amount absorbed suggests that daily application of a standard adult dose would result in a systemic exposure 890 times lower than the relevant no-observed-adverse-effect level. Triclosan can be considered safe for use in hydrophobic creams.

Effect of an Antiretroviral Regimen Containing Ritonavir Boosted Lopinavir on Intestinal and Hepatic CYP3A, CYP2D6 and P‐glycoprotein in HIV‐infected Patients

This study aimed to quantify the inhibition of cytochrome P450 (CYP3A), CYP2D6, and P‐glycoprotein in human immunodeficiency virus (HIV)‐infected patients receiving an antiretroviral therapy (ART) containing ritonavir boosted lopinavir, and to identify factors influencing ritonavir and lopinavir pharmacokinetics. We measured activities of CYP3A, CYP2D6, and P‐glycoprotein in 28 patients before and during ART using a cocktail phenotyping approach. Activities, demographics, and genetic polymorphisms in CYP3A, CYP2D6, and P‐glycoprotein were tested as covariates. Oral midazolam clearance (overall CYP3A activity) decreased to 0.19‐fold (90% confidence interval (CI), 0.15–0.23), hepatic midazolam clearance and intestinal midazolam availability changed to 0.24‐fold (0.20–0.29) and 1.12‐fold (1.00–1.26), respectively. In CYP2D6 extensive metabolizers, the plasma ratio AUCdextromethorphan/AUCdextrorphan increased to 2.92‐fold (2.31–3.69). Digoxin area under the curve (AUC)0–12 (P‐glycoprotein activity) increased to 1.81‐fold (1.56–2.09). Covariates had no major influence on lopinavir and ritonavir pharmacokinetics. In conclusion, CYP3A, CYP2D6, and P‐glycoprotein are profoundly inhibited in patients receiving ritonavir boosted lopinavir. The covariates investigated are not useful for a priori dose selection.

In vivo Role of Cytochrome P450 2E1 and Glutathione-S-Transferase Activity for Acrylamide Toxicokinetics in Humans

Acrylamide, a potential food carcinogen in humans, is biotransformed to the epoxide glycidamide in vivo. Both acrylamide and glycidamide are conjugated with glutathione, possibly via glutathione-S-transferases (GST), and bind covalently to proteins and nucleic acids. We investigated acrylamide toxicokinetics in 16 healthy volunteers in a four-period change-over trial and evaluated the respective role of cytochrome P450 2E1 (CYP2E1) and GSTs. Participants ingested self-prepared potato chips containing acrylamide (1 mg) without comedication, after CYP2E1 inhibition (500 mg disulfiram, single dose) or induction (48 g/d ethanol for 1 week), and were phenotyped for CYP2E1 with chlorzoxazone (250 mg, single dose). Unchanged acrylamide and the mercapturic acids N-acetyl-S-(2-carbamoylethyl)-cysteine (AAMA) and N-acetyl-S-(2-hydroxy-2-carbamoylethyl)-cysteine (GAMA) accounted for urinary excretion [geometric mean (percent coefficient of variation)] of 2.9% (42), 65% (23), and 1.7% (65) of the acrylamide dose in the reference period. Hemoglobin adducts increased clearly following the acrylamide test-meal. The cumulative amounts of acrylamide, AAMA, and GAMA excreted and increases in AA adducts changed significantly during CYP2E1 blockade [point estimate (90% confidence interval)] to the 1.34-fold (1.14-1.58), 1.18-fold (1.02-1.36), 0.44-fold (0.31-0.61), and 1.08-fold (1.02-1.15) of the reference period, respectively, but were not changed significantly during moderate CYP2E1 induction. Individual baseline CYP2E1 activity, CYP2E1*6, GSTP1 313A>G and 341T>C single nucleotide polymorphisms, and GSTM1-and GSTT1-null genotypes had no major effect on acrylamide disposition. The changes in acrylamide toxicokinetics upon CYP2E1 blockade provide evidence that CYP2E1 is a major but not the only enzyme mediating acrylamide epoxidation in vivo to glycidamide in humans. No obvious genetic risks or protective factors in xenobiotic-metabolizing enzymes could be determined for exposed subjects. (Cancer Epidemiol Biomarkers Prev 2009;18(2):433–43)

Plasma 4β-Hydroxycholesterol: An Endogenous CYP3A Metric?

We assessed the suitability of 4β‐hydroxycholesterol (4βOH‐C) as an endogenous cytochrome P450 3A (CYP3A) phenotyping metric. 4βOH‐C and its ratio to cholesterol (4βOH‐C/C) were determined in five cocktail phenotyping studies, with and without co‐medication with a potential CYP3A inhibitor. These parameters were compared with established midazolam‐based CYP3A metrics: clearance after intravenous (i.v.) administration (M‐Cl) and apparent clearance after oral administration (M‐Cl/F), reflecting hepatic and overall activity, respectively. In a common evaluation of periods without co‐medication, there was a slight positive correlation of 4βOH‐C and 4βOH‐C/C with midazolam metrics: M‐Cl (r = 0.239 and 0.348, respectively) and M‐Cl/F (r = 0.267 and 0.353, respectively); P (one‐sided) < 0.05. Co‐medication with lopinavir/ritonavir caused a strong decrease in midazolam metrics and a mild decrease in cholesterol metrics. However, the intake of propiverine resulted in opposite trends for midazolam‐based and cholesterol‐based metrics. The information currently available does not justify the use of 4βOH‐C for estimation of basal CYP3A activity. Further studies to address the temporal variations in local CYP3A activity are needed to assess its role as a biomarker during CYP3A inhibition.

Effect of propiverine on cytochrome P450 enzymes: a cocktail interaction study in healthy volunteers*

The present study was conducted to assess a possible in vivo effect of propiverine, an anticholinergic drug to treat urinary incontinence and related disorders, on the activity of intestinal CYP3A4 and of hepatic CYP3A4, CYP2C9, CYP2C19, and CYP1A2. The activity of the respective cytochromes P450 was measured using the following metrics of selective substrates given as a tailored low-dose phenotyping cocktail: intestinal availability of midazolam (2 mg orally), clearance of midazolam (1 mg i.v.), apparent clearance of tolbutamide (125 mg orally), urinary excretion of 4′-hydroxymephenytoin 0 to 8 h postdose (50 mg of mephenytoin orally), and the paraxanthine/caffeine plasma ratio 6 h postdose (150 mg of caffeine orally). These metrics were determined in 16 healthy young men at the end of 7 days of treatment with 15 mg of propiverine (test) or placebo (reference) twice daily. All phenotyping drugs were quantified by liquid chromatography-tandem mass spectrometry. Chronic propiverine treatment reduced hepatic and intestinal CYP3A4 activity slightly to 0.89-fold and 0.80-fold, respectively [90% confidence interval (CI) for test/reference ratios 0.85–0.93 and 0.72–0.89], with the combined effect resulting in a 1.46-fold increase in area under the curve of oral midazolam (90% CI 1.36–1.57). Propiverine had no relevant effect on CYP2C9, CYP2C19, and CYP1A2 (90% CI for test/reference ratios 0.93–1.00, 0.84–0.96, and 0.97–1.07, respectively). All study drugs were well tolerated. In conclusion, propiverine has a minor potential to cause drug-drug interactions.