Hege Christensen

Cyclosporine A, but Not Tacrolimus, Shows Relevant Inhibition of Organic Anion-Transporting Protein 1B1-Mediated Transport of Atorvastatin

The aim of this study was to investigate the potential of calcineurin inhibitors [cyclosporine A (CsA) and tacrolimus (Tac)] to inhibit cellular uptake of atorvastatin mediated by the liver-specific organic anion-transporting polypeptide 1B1 (OATP1B1) in vitro. Patients with solid organ transplants are frequently treated with HMG-CoA reductase inhibitors (statins). CsA increases atorvastatin systemic exposure severalfold, an effect not observed with Tac. The effect of CsA and Tac on atorvastatin transport via OATP1B1 was investigated in transfected human embryonic kidney 293 cells. An in vitro-in vivo extrapolation (IVIVE) was performed to estimate the clinical potential for CsA and Tac to inhibit OATP1B1-mediated transport. CsA inhibited OATP1B1-mediated uptake of atorvastatin approximately 90-fold more efficiently than Tac, with half-maximal inhibitory concentration (IC50) values of 0.021 ± 0.004 and 1.99 ± 0.42 μM, respectively. Coincubation compared with preincubation with CsA showed a 20-fold lower inhibitory capacity, with an IC50 value of 0.47 ± 0.34 μM. The IVIVE showed that clinically obtainable concentrations of CsA, but not Tac, inhibit OATP1B1 transport of atorvastatin. CsA inhibition ranged from 28 to 77% within a dosing interval, whereas it was less than 1% for Tac, considering free concentrations and assuming competitive inhibition. This does not fully explain the clinically observed interaction with CsA, suggesting that a more complex inhibitory mechanism may be present. This is also supported by the decreased IC50 value of CsA after preincubation. This study provides evidence that OATP1B1 inhibition is a relevant mechanism for the interaction observed between CsA and atorvastatin.

Significantly altered systemic exposure to atorvastatin acid following gastric bypass surgery in morbidly obese patients.

The impact of gastric bypass on atorvastatin pharmacokinetics was investigated in 12 morbidly obese patients being treated with 20–80 mg atorvastatin each morning. Eight‐hour pharmacokinetic investigations were performed the day before the surgery and at a median of 5 weeks (range 3–6 weeks) after the surgery. Gastric bypass surgery produced a variable effect on individual systemic exposure to atorvastatin acid (area under the plasma concentration vs. time curve from 0 to 8 h postdose (AUC(0–8))), ranging from a threefold decrease to a twofold increase (median ratio = 1.1, P = 0.99). Patients with the highest systemic exposure to atorvastatin before surgery showed reduced exposure after surgery (n = 3, median ratio = 0.4, range = 0.3–0.5, P < 0.01), whereas those with lower systemic exposure before surgery showed a median 1.2‐fold increase in atorvastatin AUC(0–8) (n = 9, range = 0.8–2.3, P = 0.03) after surgery. This study indicates that the presurgical first‐pass metabolic capacity influences the effect of gastric bypass on atorvastatin bioavailability. Because individual first‐pass metabolic capacity is not readily assessable clinically, retitration up to the lowest effective dose should be performed after the surgery.

Substantially elevated levels of atorvastatin and metabolites in cyclosporine-treated renal transplant recipients

o the Editor: In the work of Hedman et al recently presented in the ournal, a 10-fold higher systemic exposure of pravastatin comined with unaffected terminal half-life was observed in pediatic patients undergoing immunosuppressive therapy compared ith hypercholesterolemic children. We here report similar reults for atorvastatin and cyclosporine (INN, cicloporin). Previously, we performed a study to investigate the pharmaokinetic interaction between atorvastatin and cyclosporine in enal transplant recipients, where analysis of atorvastatin was erformed by an enzyme inhibition assay. We have now perormed additional analyses by specific HPLC with tandem mass pectrometric detection and included a control group to enable urther investigation of the effect of cyclosporine on the pharacokinetics of atorvastatin and metabolites. In brief, 18 renal transplant recipients undergoing yclosporine-based immunosuppressive therapy were adminisered 10 mg atorvastatin daily for 4 weeks. At the end of 4 eeks of treatment, plasma samples for pharmacokinetic analyis were drawn at fixed time intervals from 0 hours (before dose dministration) to 24 hours after dosing. Eighteen age-matched, ealthy individuals (median age, 47 years [range, 30-60 years]; edian serum creatinine level, 93 mol/L [range, 70-115 mol/ ]) were included in the study to serve as a control group and ere administered 10 mg atorvastatin daily for 1 week. All articipants gave written informed consent. The mean plasma level versus time curves for both acid and actone forms of atorvastatin, oand p-hydroxyatorvastatin, are hown in Fig 1. Systemic exposure of atorvastatin and its meabolites was substantially higher in the patient group compared ith the control group, and the differences were about 2-fold reater for acid forms than for lactones (Table I). No statistically ignificant difference was observed for the terminal half-life etween the 2 groups (Table I). The mechanism of the interaction between atorvastatin and yclosporine is probably complex, as both drugs are metablized by cytochrome P450 (CYP) 3A4 and as cyclosporine as the potential to inhibit several drug transporters expected o be involved in the disposition of atorvastatin. The subtantial increase in systemic exposure combined with unalered terminal half-life could indicate an increased bioavailbility of atorvastatin by inhibition of intestinal efflux

Cyclosporine A- and Tacrolimus-Mediated Inhibition of CYP3A4 and CYP3A5 In Vitro

Cyclosporine A (CsA) and tacrolimus (Tac) are immunosuppressive drugs used in the majority of patients with solid organ transplants, generally in combination with a wide range of drugs. CsA and Tac seem not only to be substrates of CYP3A but have also been described as inhibitors of CYP3A. For CsA, in particular, inhibition of CYP3A has been suggested as the main mechanism of interactions seen clinically with various drugs. The aim of this study was to investigate the inhibitory effect and inhibition characteristics of CsA and Tac on CYP3A4 and CYP3A5 in vitro and to evaluate its clinical relevance. Inhibition by CsA and Tac was studied using midazolam as the probe substrate in coincubation and preincubation investigations using human liver microsomes (HLMs) as well as specific CYP3A4- and CYP3A5-expressing insect microsomes (Supersomes). In vitro-in vivo extrapolations (IVIVEs) were performed to evaluate the clinical relevance of the inhibition. Both CsA and Tac competitively inhibited CYP3A in HLMs, showing inhibition constants (Ki) of 0.98 and 0.61 μM, respectively. Experiments in Supersomes revealed that Tac inhibited both CYP3A4 and CYP3A5, whereas CsA only inhibited CYP3A4. In contrast to the HLM experiments, studies in Supersomes showed inhibition by Tac to be NADPH- and time-dependent, with a 5-fold reduction in IC50 after preincubation, supporting a time-dependent inhibition mechanism in recombinant microsomes. By application of HLM data, IVIVE estimated the area under the concentration versus time curve of midazolam to increase by 73 and 27% with CsA and Tac, respectively. The inhibitory effect was predominantly on the intestinal level, whereas hepatic intrinsic clearance seemed unaffected.

Influence of comedication on serum concentrations of aripiprazole and dehydroaripiprazole.

Aripiprazole, a relatively new antipsychotic drug, is metabolized by cytochrome P450 3A4 (CYP3A4) and CYP2D6 to an active metabolite, dehydroaripiprazole. As studies on pharmacokinetic drug interactions with aripiprazole are so far limited, the aim of the present study was to investigate the impact of comedication on serum concentrations of aripiprazole and dehydroaripiprazole in psychiatric patients in a clinical setting. A therapeutic drug monitoring database was screened for patients receiving aripiprazole tablets as part of their treatment. Of the 361 samples included, 78% were from patients receiving comedication. The remaining 79 samples constituted the control group. Steady-state dose-adjusted serum concentrations (concentration to dose ratios, C:D ratios) of aripiprazole, dehydroaripiprazole and the sum of aripiprazole and dehydroaripiprazole, and the metabolic ratio (dehydroaripiprazole/aripiprazole) in the different comedication groups were compared with controls. Coadministration of a CYP3A4 inducer resulted in approximately 60% lower mean C:D ratios of aripiprazole, dehydroaripiprazole, and the sum of aripiprazole and dehydroaripiprazole (P < 0.05, P < 0.01, and P < 0.01, respectively). Combination with a CYP2D6 inhibitor resulted in a 45% higher mean C:D ratio of aripiprazole (P < 0.05), with no effect on the C:D ratio of dehydroaripiprazole. When aripiprazole was coadministered with alimemazine or lithium, a 56% (P < 0.01) and 43% (P = 0.05) higher mean C:D ratio of aripiprazole, respectively, was observed. Olanzapine, risperidone injections, escitalopram, or lamotrigine also had statistically significant effects on aripiprazole disposition but to a lesser extent. In conclusion, concurrent treatment with CYP3A4 inducers, CYP2D6 inhibitors, alimemazine, or lithium resulted in changes in the systemic exposure of aripiprazole between 40% and 60%. This is of such a magnitude that dose adjustments of aripiprazole may be required.

Impact of OATP1B1, MDR1, and CYP3A4 Expression in Liver and Intestine on Interpatient Pharmacokinetic Variability of Atorvastatin in Obese Subjects

Individual variability in expression and function of organic anion‐transporting polypeptide 1B1 (OATP1B1), multidrug resistance protein 1 (MDR1), and/or cytochrome P450 3A4 (CYP3A4) may impact the clinical response of many drugs. We investigated the correlation between expression of these proteins and pharmacokinetics of atorvastatin, a substrate of all three, in 21 obese patients with paired biopsies from liver and intestinal segments. The patients were also screened for the SLCO1B1 c.521T→C variant alleles. Approximately 30% (r2 = 0.28) of the variation in oral clearance (CL/F) of atorvastatin was explained by hepatic OATP1B1 protein expression (P = 0.041). Patients carrying the SLCO1B1 c.521C variant allele (homozygous, n = 4; heterozygous, n = 2) exhibited 45% lower CL/F of atorvastatin than the c.521TT carriers (P = 0.067). No association between hepatic and intestinal expression of MDR1 or CYP3A4 and atorvastatin pharmacokinetics was found (P > 0.149). In conclusion, this study suggests that OATP1B1 phenotype is more important than CYP3A4 and MDR1 phenotypes for the individual pharmacokinetic variability of atorvastatin.

Pharmacokinetics of diltiazem and its metabolites in relation to CYP2D6 genotype

Recently, it was shown in vitro that the polymorphic enzyme cytochrome P450 (CYP) 2D6 mediates O‐demethylation of diltiazem. The aim of this study was to compare the pharmacokinetics of diltiazem and its major metabolites in healthy human volunteers representing different CYP2D6 genotypes.

Significant increase in systemic exposure of atorvastatin after biliopancreatic diversion with duodenal switch.

Biliopancreatic diversion with duodenal switch is a combined restrictive and malabsorptive surgical weight loss procedure. Given that this procedure introduces a bypass of the proximal small intestine, it is a suitable model for investigating the influence of the proximal intestine on drug bioavailability. Eight‐hour pharmacokinetic profiles were obtained the day before surgery and again after surgery at (median) 6 weeks (range, 4–8 weeks) in 10 morbidly obese patients who were receiving treatment with 20–80 mg atorvastatin each morning. The bioavailability of atorvastatin acid was significantly increased, with a mean twofold higher AUC0–8 after surgery (range 1.0–4.2, P = 0.001). Time to maximum plasma concentration (Cmax) increased from 1.2 h before surgery to 2.3 h after surgery (P = 0.03). The results emphasize the protective nature of the proximal small intestine against ingested exogenous compounds. Consequently, retitration to the lowest effective dose should be considered after biliopancreatic diversion with duodenal switch in the case of drugs with a high degree of intestinal first pass metabolism and a narrow therapeutic window.

Metabolism of quetiapine by CYP3A4 and CYP3A5 in presence or absence of cytochrome B5.

The antipsychotic drug quetiapine is extensively metabolized by CYP3A4, but little is known about the possible influence of the polymorphic enzyme CYP3A5. This in vitro study investigated the relative importance of CYP3A4 and CYP3A5 in the metabolism of quetiapine and compared the metabolic pattern by the two enzymes, in the presence or absence of cytochrome b5. Intrinsic clearance (CLint) of quetiapine was determined by the substrate depletion approach in CYP3A4 and CYP3A5 insect cell microsomes with or without coexpressed cytochrome b5. Formation of the metabolites quetiapine sulfoxide, N-desalkylquetiapine, O-desalkylquetiapine, and 7-hydroxyquetiapine by CYP3A4 and CYP3A5 were compared in the different microsomal preparations. CLint of quetiapine by CYP3A5 was less than 35% relative to CYP3A4. CLint was higher (3-fold) in CYP3A4 microsomes without cytochrome b5 compared with CYP3A4 microsomes with coexpressed cytochrome b5, whereas in CYP3A5 microsomes CLint was similar for both microsomal preparations. Metabolism of quetiapine by CYP3A5 revealed a different metabolic pattern compared with CYP3A4. The results indicated that O-desalkylquetiapine constituted a higher proportion of the formed metabolites by CYP3A5 compared with CYP3A4. In conclusion, the present study indicates that CYP3A5 is of minor importance for the overall metabolism of quetiapine, regardless of the presence of cytochrome b5. However, a different metabolic pattern by CYP3A5 compared with CYP3A4 could possibly result in different pharmacological and/or toxicological effects of quetiapine in patients expressing CYP3A5.

Immunological response as a source to variability in drug metabolism and transport

Through the last decades it has become increasingly evident that disease-states involving cytokines affect the pharmacokinetics of drugs through regulation of expression and activity of drug metabolizing enzymes, and more recently also drug transporters. The clinical implication is however difficult to predict, since these effects are dependent on the degree of inflammation and may be changed when the diseases are treated. This article will give an overview of the present understanding of the effects of cytokines on cytochrome P450 enzymes and drug transporters, and highlight the importance of considering these issues in regard to increasing use of the relatively new class of drugs, namely therapeutic proteins.