Pharmacokinetic interactions between microemulsion formulated cyclosporine A and diltiazem in renal transplant recipients

Diltiazem co treatment with cyclosporine for induction of disease remission in sight-threatening non-infectious intraocular inflammation

PURPOSE

To investigate the effects of diltiazem co-treatment with microemulsified cyclosporine-A (CsA) on dosage, blood concentrations of CsA, as well as therapeutic and side effects in patients with sight-threatening non-infectious intraocular inflammation.

METHODS

A prospective, randomized, placebo-controlled, open-label trial of 39 patients with sight-threatening non-infectious intraocular inflammation. The change in visual acuity (LogMAR), the grade of inflammatory activity, therapeutic concentration of CsA in the blood and systemic and renal toxicity were determined after comparing two groups, one treated with CsA and diltiazem (treatment group), and the other without diltiazem (control group).

RESULTS

Compared with the control group, the concentration of CsA in the blood of the treatment group significantly increased (P < 0.05) requiring a reduction in dosage of CsA (P < 0.05).

CONCLUSION

In patients with severe intraocular inflammation treated with CsA and diltiazem, blood concentrations of CsA increased as the dosage decreased. This efficient combination therapy reduced patient's economic burden, at the same time decreasing systemic and renal toxicity which in turn may promote the use of CsA for longer periods.

Management of Hypertension in Solid-Organ Transplantation

Posttransplant hypertension is a major risk factor for cardiovascular disease and chronic renal allograft dysfunction. A significant number of transplant recipients suffer from posttransplant hypertension in part because of corticosteroid and calcineurin inhibitor use. Although the optimal blood pressure range and the antihypertensive agents of choice in the transplant population have not been determined, the guidelines for blood pressure control in the general population can be extrapolated to the transplant population. The choice of an antihypertensive regimen should be tailored on the basis of the individual patient's risk factors and comorbidities.

The effects of diltiazem in renal transplantation patients treated with cyclosporine A

OBJECTIVE

To investigate the effects of diltiazem and cyclosporine A (CsA) combination therapy on protecting the kidney, promoting graft functioning and improving post-transplanted kidney recovery.

METHODS

The blood concentrations of CsA, the condition of the post-transplant kidney, the rate of acute rejection (AR), as well as hepatic and renal toxicity in 636 cases of renal transplant recipients were determined after being treated by CsA, with or without diltiazem.

RESULTS

Compared with the control group which received CsA, mycophenolate mofetil (MMF) and prednisolone (Pred) but lacked diltiazem, the group receiving these agents together with diltiazem required reduced dosage of CsA (P < 0.01), while blood concentrations of CsA were significantly increased (P < 0.01); the recovery time of graft function was reduced from (6.2±1.5) d to (3.9±1.4) d (P < 0.01), and the rate of AR was decreased from 13.2% to 7.9% (P < 0.01).

CONCLUSION

In renal transplantation patients treated with CsA and diltiazem, blood concentrations of CsA were increased while the dosage was decreased. This efficient combination therapy reduced patients' economic burden, at the same time retained kidney function, promoted graft function recovery and decreased hepatic and renal toxicity and the rate of AR.

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 (K(i)) 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 IC(50) 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.

Effect of genetic polymorphisms of CYP3A5 and MDR1 on cyclosporine concentration during the early stage after renal transplantation in Chinese patients co-treated with diltiazem

OBJECTIVE

The aim of this study was to assess the influence of the cytochrome (CYP450)3A5 and multidrug resistance (MDR1) gene polymorphisms on cyclosporine A (CsA) trough concentration during the early stage after renal transplantation in Chinese patients co-treated with diltiazem.

METHODS

CYP3A5*3 (A6986G) and MDR1 C1236T, G2677T/A and C3435T polymorphisms were determined by PCR followed by restriction fragment length polymorphism (RFLP) analysis. A total of 112 Chinese renal transplant patients were enrolled in the study. The whole blood trough concentration was measured at 7 days after transplantation, and the dose-adjusted trough levels were compared among the different genotypes.

RESULTS

The dose-adjusted trough levels of CsA were significantly higher in MDR1 2677TT carriers than in GG plus GT carriers (59.5 +/- 15.9 vs. 34.5 +/- 9.4 vs. 43.2 +/- 13.6 ng/mL per mg per kg; P < 0.0001). In patients who were co-treated with diltiazem, compared with carriers of haplotype T-T-C, the carriers of haplotype C-G-C and haplotype T-G-T had significantly lower dose-adjusted trough blood concentrations of CsA than the non-carrier group (P = 0.002, P = 0.000 and P = 0.000, respectively). However, no evidence was found that there was a relationship between the CYP3A5*3, MDR1 C1236T and MDR1 C3435T polymorphisms and CsA dose-adjusted trough concentrations.

CONCLUSION

This study demonstrated that the G2677T/A single nucleotide polymorphisms in MDR1 and MDR1 haplotypes C-G-C, T-G-T and T-T-C are associated with the CsA concentration during the very early post-transplant period in Chinese renal transplant patients co-treated with diltiazem. These polymorphisms may be useful for determining the appropriate initial dose of CsA after renal transplantation.

Effect of cyclosporine and tacrolimus on cytochrome p450 activities in human liver microsomes

The effects of cyclosporine and tacrolimus on cytochrome P450 (CYP) 1A2-mediated 7-ethoxyresorufin O-deethylation, CYP2C9-mediated tolbutamide hydroxylation, CYP2C19-mediated S-mephenytoin 4'-hydroxylation, CYP2D6-mediated debrisoquine 4-hydroxylation, CYP2E1-mediated chlorzoxazone 6-hydroxylation, CYP3A4-mediated nifedipine oxidation, and CYP3A4-mediated testosterone 6beta-hydroxylation activities in human liver microsomes were compared. Cyclosporine and tacrolimus, at concentrations of 0.2 or 2 muM, neither inhibited nor stimulated any of the metabolic activities except for those of CYP3A4. On the other hand, cyclosporine and tacrolimus competitively inhibited CYP3A4-mediated nifedipine oxidation activity, with inhibition constants (K(i)) of 1.42 and 0.36 muM, respectively. In addition, 20 muM cyclosporine inhibited CYP2C19 and CYP2D6 activities by 29% and 30%, respectively. These results suggest that tacrolimus would not cause clinically significant interactions with other drugs, which are metabolized by CYPs, via the inhibition of hepatic metabolism and that the reason why cyclosporine, but not tacrolimus, has a pharmacokinetic inhibitory effect might be that the dosage and/or the unbound concentrations around its metabolic enzymes are higher than those of tacrolimus, rather than the differences in the inhibition potential. Obvious substrate-dependent effects on CYP3A4-inhibition potential were not observed.

General Framework for the Quantitative Prediction of CYP3A4-Mediated Oral Drug Interactions Based on the AUC Increase by Coadministration of??Standard Drugs

BACKGROUND

Cytochrome P450 (CYP) 3A4 is the most prevalent metabolising enzyme in the human liver and is also a target for various drug interactions of significant clinical concern. Even though there are numerous reports regarding drug interactions involving CYP3A4, it is far from easy to estimate all potential interactions, since too many drugs are metabolised by CYP3A4. For this reason, a comprehensive framework for the prediction of CYP3A4-mediated drug interactions would be of considerable clinical importance.

OBJECTIVE

The objective of this study was to provide a robust and practical method for the prediction of drug interactions mediated by CYP3A4 using minimal in vivo information from drug-interaction studies, which are often carried out early in the course of drug development.

DATA SOURCES

The analysis was based on 113 drug-interaction studies reported in 78 published articles over the period 1983-2006. The articles were used if they contained sufficient information about drug interactions. Information on drug names, doses and the magnitude of the increase in the area under the concentration-time curve (AUC) were collected.

METHODS

The ratio of the contribution of CYP3A4 to oral clearance (CR(CYP)(3A4)) was calculated for 14 substrates (midazolam, alprazolam, buspirone, cerivastatin, atorvastatin, ciclosporin, felodipine, lovastatin, nifedipine, nisoldipine, simvastatin, triazolam, zolpidem and telithromycin) based on AUC increases observed in interaction studies with itraconazole or ketoconazole. Similarly, the time-averaged apparent inhibition ratio of CYP3A4 (IR(CYP)(3A4)) was calculated for 18 inhibitors (ketoconazole, voriconazole, itraconazole, telithromycin, clarithromycin, saquinavir, nefazodone, erythromycin, diltiazem, fluconazole, verapamil, cimetidine, ranitidine, roxithromycin, fluvoxamine, azithromycin, gatifloxacin and fluoxetine) primarily based on AUC increases observed in drug-interaction studies with midazolam. The increases in the AUC of a substrate associated with coadministration of an inhibitor were estimated using the equation 1/(1 - CR(CYP)(3A4) x IR(CYP)(3A4)), based on pharmacokinetic considerations.

RESULTS

The proposed method enabled predictions of the AUC increase by interactions with any combination of these substrates and inhibitors (total 251 matches). In order to validate the reliability of the method, the AUC increases in 60 additional studies were analysed. The method successfully predicted AUC increases within 67-150% of the observed increase for 50 studies (83%) and within 50-200% for 57 studies (95%). Midazolam is the most reliable standard substrate for evaluation of the in vivo inhibition of CYP3A4. The present analysis suggests that simvastatin, lovastatin and buspirone can be used as alternatives. To evaluate the in vivo contribution of CYP3A4, ketoconazole or itraconazole is the selective inhibitor of choice.

CONCLUSION

This method is applicable to (i) prioritize clinical trials for investigating drug interactions during the course of drug development and (ii) predict the clinical significance of unknown drug interactions. If a drug-interaction study is carefully designed using appropriate standard drugs, significant interactions involving CYP3A4 will not be missed. In addition, the extent of CYP3A4-mediated interactions between many other drugs can be predicted using the current method.

PREDICTION OF TIME-DEPENDENT CYP3A4 DRUG-DRUG INTERACTIONS: IMPACT OF ENZYME DEGRADATION, PARALLEL ELIMINATION PATHWAYS, AND INTESTINAL INHIBITION

Time-dependent inhibition of CYP3A4 often results in clinically significant drug-drug interactions. In the current study, 37 in vivo cases of irreversible inhibition were collated, focusing on macrolides (erythromycin, clarithromycin, and azithromycin) and diltiazem as inhibitors. The interactions included 17 different CYP3A substrates showing up to a 7-fold increase in AUC (13.5% of studies were in the range of potent inhibition). A systematic analysis of the impact of CYP3A4 degradation half-life (mean t1/2deg = 3 days, ranging from 1 to 6 days) on the prediction of the extent of interaction for compounds with a differential contribution from CYP3A4 to the overall elimination (defined by fmCYP3A4) was performed. Although the prediction accuracy was very sensitive to the CYP3A4 degradation rate for substrates mainly eliminated by this enzyme fm(CYP3A4 >or= 0.9), minimal effects are observed when CYP3A4 contributes less than 50% to the overall elimination in cases when the parallel elimination pathway is not subject to inhibition. Use of the mean CYP3A4 t1/2deg (3 days), average unbound systemic plasma concentration of the inhibitor, and the corresponding fm(CYP3A4) resulted in 89% of studies predicted within 2-fold of the in vivo value. The impact of the interaction in the gut wall was assessed by assuming maximal intestinal inhibition of CYP3A4. Although a reduced number of false-negative predictions was observed, there was an increased number of overpredictions, and generally, a loss of prediction accuracy was observed. The impact of the possible interplay between CYP3A4 and efflux transporters on the intestinal interaction requires further evaluation.

Effects of berberine on the blood concentration of cyclosporin A in renal transplanted recipients: clinical and pharmacokinetic study

OBJECTIVE

To study the effects of berberine (BBR) on the blood concentration and pharmacokinetics of cyclosporin A (CsA) in renal-transplant recipients.

METHODS

In a randomized and controlled clinical trial, 52 renal-transplant recipients were treated with CsA and 0.2 g BBR three times daily for 3 months, while another 52 subjects received CsA without BBR co-administration. Blood trough concentration of CsA and biochemistry indexes for hepatic and renal functions were determined. For the pharmacokinetic study, six renal-transplant recipients were included with a 3-mg/kg dosage of CsA twice daily before and after oral co-administration of 0.2 g BBR three times daily for 12 days.

RESULTS

The trough blood concentrations and the ratios of concentration/dose of CsA in the BBR-treated group increased by 88.9% and 98.4%, respectively, compared with those at baseline (P < 0.05). As for the BBR-free group, they rose by 64.5% and 69.4%, respectively, relative to those at baseline (P < 0.01). Nevertheless, the final blood concentrations and the ratios of concentration/dose of CsA in BBR-treated patients were still 29.3% and 27.8%, respectively, higher than those in BBR-free patients (P < 0.05). No significant effects on liver or renal functions were observed under coadministration of BBR. After co-administration of BBR in six patients for 12 days, the mean AUC of CsA was increased by 34.5% (P < 0.05). The mean time taken to reach the peak blood concentration (t(max)) and the mean half-life (t(1/2)) of CsA were increased by 1.7 h and 2.7 h, respectively (P < 0.05). The average percentage increases in the steady-state drug concentration (Css) and minimum blood concentration (Cmin) were 34.5% and 88.3%, respectively (P < 0.05). In addition, the average percentage decrease in CL/F was 40.4% (P < 0.05) and the peak-to-through fluctuation index was significantly reduced (P < 0.01).

CONCLUSION

The BBR can markedly elevate the blood concentration of CsA in renal-transplant recipients in both clinical and pharmacokinetic studies. This combination may allow a reduction of the CsA dosage. The mechanism for this interaction is most likely explained by inhibition of CYP3A4 by BBR in the liver and/or small intestine.

Co-prescription of cytochrome P450 2D6/3A4 inhibitor-substrate pairs in clinical practice. A retrospective analysis of data from Norwegian primary pharmacies

OBJECTIVE

Inhibition of cytochrome P (P450) (CYP) enzymes, in particular CYP3A4 and CYP2D6, is an important drug-interacting mechanism. The objective of our study was to assess how frequently CYP3A4 and CYP2D6 inhibitors are co-prescribed with substrates of the respective enzymes.

METHODS

Included inhibitors were clarithromycin, erythromycin, fluconazole, itraconazole, ketoconazole and nefazodone (CYP3A4 inhibitors) and bupropion, fluoxetine, paroxetine and terbinafine (CYP2D6 inhibitors). The inhibitors were combined with substrates shown to be pharmacokinetically sensitive towards inhibition (190 drug pairs in total). Lists of patients receiving inhibitors and substrates were drawn from prescription databases (approximately 43,500 patients) of three Norwegian primary pharmacies during a 6-month period (July 2002 to January 2003). The lists were matched on name and date of birth to identify patients using drug pairs. Concurrent use was made probable from dates of purchase and drug profiles.

RESULTS

Inhibitors were prescribed to 2,062 patients. Altogether, 369 events of substrate co-prescription were registered. The highest frequencies of co-prescribed substrates were found for paroxetine (101 events per 267 patients, 38%), fluoxetine (36 events per 110 patients, 33%) and clarithromycin (59 events per 242 patients, 24%). The drugs most often detected in combination with inhibitors were codeine (116 events) and metoprolol (38 events) for CYP2D6 and zopiclone (45 events) and simvastatin (26 events) for CYP3A4.

CONCLUSION

Several commonly used CYP2D6 and CYP3A4 inhibitors are frequently co-prescribed with substrates in Norwegian clinical practice. Alertness when inhibitors are prescribed would aid physicians and pharmacists to detect many drug combinations with potential interaction risk.

Diltiazem use in tacrolimus-treated renal transplant recipients

BACKGROUND

Calcium channel blockers are widely used in the treatment of post-transplant hypertension but have the potential for drug interaction with calcineurin inhibitors. Renal allograft outcomes when diltiazem is used with cyclosporine have been reported, but similar data with tacrolimus are not available.

METHODS

We performed a retrospective analysis of all our renal transplant recipients from March 1997 to March 2002 who were given tacrolimus, mycophenolate mofetil and prednisone. Patients were divided into two groups based on whether diltiazem was started in the first postoperative week. Outcome measures included renal function up to 2 years post-transplant, blood pressure (BP) control, tacrolimus exposure, and costs related to tacrolimus monitoring.

RESULTS

Sixty-four patients constituted the diltiazem group and 32 the control group. Their baseline characteristics were similar. The mean average daily dose of diltiazem used was 213.95 mg/day. There was no difference in renal function, graft survival, or patient survival over 2 years. BP control was similar although the diltiazem group required more medication. Diltiazem was discontinued in four patients due to side-effects. There was no difference in tacrolimus-related side-effects between the two groups. There was also no difference in tacrolimus exposure, cost related to tacrolimus monitoring, or combined costs when the expense of diltiazem was added.

CONCLUSION

Diltiazem use is acceptably safe and efficacious in renal transplant recipients treated with tacrolimus-based immunosuppressive therapy. It can be considered as a first-line antihypertensive in these patients and is cost neutral for tacrolimus use.

Antihypertensive therapy: special focus on drug interactions

Today, the lifetime risk of patients aged 55-65 years to receive antihypertensive drugs approaches 60%. Yet, recent trials suggest that hypertension is not adequately controlled in the majority of patients. The prevalence of hypertension increases with advancing age, as does the prevalence of comorbid conditions and the total number of medications taken. Multi-drug therapy, advancing age and comorbid conditions are also key risk factors for adverse drug reactions and drug interactions. In this review, the authors evaluate the most frequently used antihypertensive drugs (diuretics, beta-adrenergic blockers, angiotensin-converting enzyme inhibitors, calcium channel blockers, angiotensin II receptor Type 1 blockers and alpha-adrenergic blockers) with special reference to pharmacodynamic and pharmacokinetic drug interactions. As the spectrum of drugs prescribed is constantly changing, safety yesterday does not imply safety today and safety today does not imply safety tomorrow. Furthermore, therapeutic efficacy should not be neglected over concerns regarding drug interactions. Many patients are at risk of clinically relevant drug interactions involving antihypertensive drugs but, presently, even more patients may be at risk of suffering from the consequences of their inadequately treated hypertension. In this respect, the authors discuss controversial viewpoints on the overall clinical relevance of drug interactions occurring at the level of cytochrome P450 metabolism.

Diltiazem co-treatment in renal transplant patients receiving microemulsion cyclosporin

BACKGROUND

Usage of cyclosporin (the Hong Kong Hospital Authority's single largest item of drug expenditure) continues to increase, mainly due to increasing numbers of renal allograft patients taking it as long-term antirejection therapy. Diltiazem, an antihypertensive agent, interferes with the first pass extraction of oral cyclosporin, thus serving to conserve its dosage.

AIMS

In renal transplant patients, to assess whether diltiazem co-treatment could achieve worthwhile dosage conservation of Neoral (a relatively new microemulsified cyclosporin formulation), safely.

METHODS

A randomized, placebo-controlled, double-blind clinical trial was undertaken at three local hospitals. Renal transplant recipients receiving Neoral as prophylactic immunosuppression were randomized to two treatment arms. Active treatment consisted of diltiazem tablets 30 or 60 mg twice daily for patients weighing < 60 or >or= 60 kg, respectively. One hundred and ten eligible patients gave their informed consent, and were followed up for at least six months. The mean difference in the dollar cost in the sixth month was the primary outcome. Secondary/ancillary outcomes included changes in cyclosporin dosage and blood level, and untoward clinical events including rejection. Outcomes were evaluated by intention to treat analyses.

RESULTS

During weeks 23-26 (sixth month) post randomization, diltiazem co-treatment yielded an estimated average cost saving per patient on drugs of 15%[the 95% confidence interval (CI) of the difference being HK dollars 609 +/- 517 or pound 50 +/- 42], with no apparent excess of untoward or adverse events, complications, hospitalization, outpatient visits, or inferior quality of life.

CONCLUSIONS

This diltiazem co-treatment regime applied to the nearly 1800 surviving renal allograft patients followed up in Hospital Authority hospitals could have saved approximately HK dollars 14.3 million ( pound 1.17 million) annually, without adverse sequelae.

High-performance liquid chromatography-mass spectrometry analysis of diltiazem and 11 of its phase I metabolites in human plasma

The aim of the present work was to develop a high-performance liquid chromatography-mass spectrometry method for analysis of diltiazem (DTZ) and metabolites in human plasma after single dose administration (120 mg). Human plasma samples (1 ml) were cleaned up by a solid phase extraction procedure (C18 cartridges) using codeine as an internal standard. Reconstituted extracts were separated on a reversed-phase C8 column with a linear gradient mobile phase system. The run time per sample analysis was 11 min. Detection was performed using selected ion monitoring following atmospheric pressure chemical ionization. The lower limit of quantification was estimated to be 1 microg/l (in spiked plasma) for all available reference compounds (i.e. DTZ and five metabolites). Validation of the method showed good linearity, precision and accuracy for quantification of these six reference compounds. In addition, tandem MS analyses of human plasma sampled from healthy individuals after peroral intake of 120 mg DTZ revealed that the method enabled detection of six additional metabolites for which reference compounds were not available.

Cyclosporin: an updated review of the pharmacokinetic properties, clinical efficacy and tolerability of a microemulsion-based formulation (neoral)1 in organ transplantation

Cyclosporin is a lipophilic cyclic polypeptide immunosuppressant that interferes with the activity of T cells chiefly via calcineurin inhibition. The original oil-based oral formulation of this drug (Sandimmun)l was characterised by high intra- and interpatient pharmacokinetic variability, with poor bioavailability in many patients; a novel microemulsion formulation (Neoral)1 was therefore developed to circumvent these problems. Studies show increases, attributable chiefly to improved absorption in patients who absorb the drug only poorly from the original formulation, in mean systemic exposure to cyclosporin with the microemulsion, with no clinically significant differences in tolerability or drug interaction profiles. Cyclosporin microemulsion is at least as effective as the oil-based formulation in renal, liver and heart transplant recipients, with trends towards decreased incidence of acute rejection with the microemulsion formulation in some (statistically significant in a few) trials. Cyclosporin microemulsion and tacrolimus appear to have similar efficacy in preventing acute rejection episodes in most renal, pancreas-kidney, liver and heart transplant recipients. However, there are indications of superior efficacy for tacrolimus in some trials, particularly in the prevention of severe acute rejection and in Black transplant recipients. Current 12-month data also indicate equivalent efficacy of sirolimus in renal transplantation. Conversion from the oil-based to microemulsion formulation in stable renal, liver and heart transplant recipients is achievable with no change in acute rejection rates. The addition of an anti-interleukin-2 receptor monoclonal antibody and/or mycophenolate mofetil to cyclosporin microemulsion plus corticosteroids decreases rates of acute rejection; corticosteroid withdrawal without increased acute rejection rates was also achieved on the addition of these agents in some trials. Pharmacoeconomic analyses have shown savings in direct healthcare costs in kidney or liver transplantation when cyclosporin microemulsion is used in preference to the oil-based formulation, although studies incorporating indirect costs or expressing costs in terms of therapeutic outcomes are currently unavailable.

CONCLUSIONS

The introduction of cyclosporin microemulsion has consolidated the place of the drug as a mainstay of therapy in all types of solid organ transplantation; research into optimisation of outcomes through more effective therapeutic monitoring in patients receiving this formulation is ongoing. Several novel immunosuppressants have been introduced in recent years: further clinical and pharmacoeconomic research will be needed to clarify the relative positioning of these agents, particularly with respect to specific patient groups. Other new drugs (basiliximab/daclizumab and mycophenolate mofetil) offer particular advantages when used in combination with cyclosporin.

Desacetyl-diltiazem displays severalfold higher affinity to CYP2D6 compared with CYP3A4

It has earlier been shown that the isoenzymes CYP2D6 and CYP3A4 are involved in O- and N-demethylation of diltiazem (DTZ), respectively. Apparently, CYP3A4 plays a more prominent role than CYP2D6 in the overall metabolism of DTZ. However, previous observations indicate that the opposite might be true for the pharmacologically active metabolite desacetyl-DTZ (M1). Thus, the aim of the present in vitro investigation was to study the relative affinity of M1 to CYP2D6 and CYP3A4. Immortalized human liver epithelial cells transfected with either CYP2D6 or CYP3A4 were used as a model system, and the presence of M1 and its metabolites in the cell culture medium was analyzed by high-performance liquid chromatography/UV detection both before and following 90 min of incubation. The estimated K(m) value for the CYP2D6-mediated O-demethylation of M1 was approximately 5 microM. In comparison, the affinity of M1 to CYP3A4 (N-demethylation) was about 100 times lower (K(m), approximately 540 microM) than to CYP2D6. These in vitro data suggest that M1 metabolism via CYP2D6, in contrast to the parent drug, probably is the preferred pathway in vivo. Metabolism mediated through CYP2D6 is associated with a substantial interindividual variability, and since M1 expresses pharmacological activity, individual CYP2D6 metabolic capacity might be an aspect to consider when using DTZ.