Modification of the pharmacokinetics of cyclosporine A and metabolites by the concomitant use of Neoral and diltiazem or ketoconazol in stable adult kidney transplants

Intestinal permeation enhancers for oral peptide delivery

Intestinal permeation enhancers (PEs) are one of the most widely tested strategies to improve oral delivery of therapeutic peptides. This article assesses the intestinal permeation enhancement action of over 250 PEs that have been tested in intestinal delivery models. In depth analysis of pre-clinical data is presented for PEs as components of proprietary delivery systems that have progressed to clinical trials. Given the importance of co-presentation of sufficiently high concentrations of PE and peptide at the small intestinal epithelium, there is an emphasis on studies where PEs have been formulated with poorly permeable molecules in solid dosage forms and lipoidal dispersions.

Combining cytochrome P-450 3A4 modulators and cyclosporine or everolimus in transplantation is successful

AIM: To describe the long term follow-up of kidney allograft recipients receiving ketoconazole with calcineurin inhibitors (CNI) alone or combined with everolimus.

METHODS: This is an open-label, prospective observational clinical trial in low immunologic risk patients who, after signing an Institutional Review Board approved consent form, were included in one of two groups. The first one (n = 59) received everolimus (target blood level, 3-8 ng/mL) and the other (n = 114) azathioprine 2 mg/kg per day or mycophenolate mofetyl (MMF) 2 g/d. Both groups also received tapering steroids, the cytochrome P-450 3A4 (CYP3A4) modulator, ketoconazole 50-100 mg/d, and cyclosporine with C0 targets in the everolimus group of 200-250 ng/mL in 1 mo, 100-125 ng/mL in 2 mo, and 50-65 ng/mL thereafter, and in the azathioprine or MMF group of 250-300 ng/mL in 1 mo, 200-250 ng/mL in 2 mo, 180-200 ng/mL until 3-6 mo, and 100-125 ng/mL thereafter. Clinical visits were performed monthly the first year and quarterly thereafter by treating physicians and all data was extracted by the investigators.

RESULTS: The clinical characteristics of these two cohorts were similar. During the follow up (66 + 31 mo), both groups showed comparable clinical courses, but the biopsy proven acute rejection rate during the full follow-up period seemed to be lower in the everolimus group (20% vs 36%; P = 0.04). The everolimus group did not show a higher surgical complication rate than the other group. By the end of the follow-up period, the everolimus group tended to show a higher glomerular filtration rate. Nevertheless, we found no evidence of a consistent negative slope of the temporal allograft function estimated by the modification of the diet in renal disease formula in any of both groups. At 6 years of follow-up, the uncensored and death-censored graft survivals were 91% and 93%, and 91% and 83% in the everolimus plus cyclosporine, and cyclosporine alone groups, respectively. The addition of ketoconazole saved 80% of cyclosporine and 56% of everolimus doses.

CONCLUSION: Combining CYP3A4 modulators with CNI or mammalian target of rapamycin inhibitor, in low immunological risk kidney transplant recipients is feasible, effective, safe and affordable even in the long term.

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.

P-glycoprotein related drug interactions: clinical importance and a consideration of disease states

IMPORTANCE OF THE FIELD

P-glycoprotein (P-gp) is the most characterized drug transporter in terms of its clinical relevance for pharmacokinetic disposition and interaction with other medicines. Clinically significant P-gp related drug interactions appear restricted to digoxin. P-gp may act as a major barrier to current and effective drug treatment in a number of diseases including cancer, AIDS, Alzheimer's and epilepsy due to its expression in tumors, lymphocytes, cell membranes of brain capillaries and the choroid plexus.

AREAS COVERED IN THIS REVIEW

This review summarizes the current understanding of P-gp structure/function, clinical importance of P-gp related drug interactions and the modulatory role this transporter may contribute towards drug efficacy in disease states such as cancer, AIDS, Alzheimer's and epilepsy.

WHAT THE READER WILL GAIN

The reader will gain an understanding that the clinical relevance of P-gp in drug interactions is limited. In certain disease states, P-gp in barrier tissues can modulate changes in regional distribution.

TAKE HOME MESSAGE

P-gp inhibition in isolation will not result in clinically important alterations in systemic exposure; however, P-gp transport may be of significance in barrier tissues (tumors, lymphocytes, brain) resulting in attenuated efficacy.

Effect of micafungin on cytochrome P450 3A4 and multidrug resistance protein 1 activities, and its comparison with azole antifungal drugs

The effects of micafungin on cytochrome P450 3A4 (CYP3A4) metabolic and multidrug resistance protein 1 (MDR1) transport activities were investigated and compared with those of amphotericin B and four azole antifungal drugs (ketoconazole, itraconazole, fluconazole and miconazole). The effects on the metabolic activity of CYP3A4 were examined by measuring nifedipine oxidase activity in human liver microsomes and the effects on MDR1 transport activity were evaluated using [3H]digoxin in MDR1-overexpressing LLC-GA5-COL150 cells. An inhibitory effect on CYP3A4 activity was found for ketoconazole, itraconazole and miconazole, with 50% inhibitory concentrations of 11.7, 32.6 and 74.2 nM, respectively. Fluconazole and micafungin had only slight inhibitory effects and amphotericin B had no effect. The MDR1-mediated transport of [3H]digoxin was inhibited by ketoconazole and itraconazole, and slightly by miconazole. It is suggested that micafungin and amphotericin B would be unlikely to cause drug-drug interactions by inhibition of CYP3A4 and MDR1. A positive correlation between the inhibitory effects on CYP3A4 and MDR1 activities was observed, and the physicochemical mechanisms involved and impact on clinical treatment should be studied further.

A proposal for a pharmacokinetic interaction significance classification system (PISCS) based on predicted drug exposure changes and its potential application to alert classifications in product labelling

BACKGROUND AND OBJECTIVE

Pharmacokinetic drug-drug interactions (DDIs) are one of the major causes of adverse events in pharmacotherapy, and systematic prediction of the clinical relevance of DDIs is an issue of significant clinical importance. In a previous study, total exposure changes of many substrate drugs of cytochrome P450 (CYP) 3A4 caused by coadministration of inhibitor drugs were successfully predicted by using in vivo information. In order to exploit these predictions in daily pharmacotherapy, the clinical significance of the pharmacokinetic changes needs to be carefully evaluated. The aim of the present study was to construct a pharmacokinetic interaction significance classification system (PISCS) in which the clinical significance of DDIs was considered with pharmacokinetic changes in a systematic manner. Furthermore, the classifications proposed by PISCS were compared in a detailed manner with current alert classifications in the product labelling or the summary of product characteristics used in Japan, the US and the UK.

METHODS

A matrix table was composed by stratifying two basic parameters of the prediction: the contribution ratio of CYP3A4 to the oral clearance of substrates (CR), and the inhibition ratio of inhibitors (IR). The total exposure increase was estimated for each cell in the table by associating CR and IR values, and the cells were categorized into nine zones according to the magnitude of the exposure increase. Then, correspondences between the DDI significance and the zones were determined for each drug group considering the observed exposure changes and the current classification in the product labelling. Substrate drugs of CYP3A4 selected from three therapeutic groups, i.e. HMG-CoA reductase inhibitors (statins), calcium-channel antagonists/blockers (CCBs) and benzodiazepines (BZPs), were analysed as representative examples. The product labelling descriptions of drugs in Japan, US and UK were obtained from the websites of each regulatory body.

RESULTS

Among 220 combinations of drugs investigated, estimated exposure changes were more than 5-fold for 41 combinations in which ten combinations were not alerted in the product labelling at least in one country; these involved buspirone, nisoldipine and felodipine as substrates, and ketoconazole, voriconazole, telithromycin, clarithromycin and nefazodone as inhibitors. For those drug combinations, the alert classifications were anticipated as potentially inappropriate. In the current product labelling, many inter-country differences were also noted. Considering the relationships between previously observed exposure changes and the current alert classifications, the boundaries between 'contraindication' and 'warning/caution' were determined as a 7-fold exposure increase for statins and CCBs, and as a 4-fold increase for BZPs. PISCS clearly discriminated these drug combinations in accordance with the determined boundaries. Classifications by PISCS were expected to be valid even for future drugs because the classifications were made by zones, not by designating individual drugs.

CONCLUSION

The present analysis suggested that many current alert classifications were potentially inappropriate especially for drug combinations where pharmacokinetics had not been evaluated. It is expected that PISCS would contribute to constructing a leak-less alerting system for a broad range of pharmacokinetic DDIs. Further validation of PISCS is required in clinical studies with key drug combinations, and its extension to other CYP and metabolizing enzymes remains to be achieved.

Prediction of pharmacokinetic drug-drug interaction caused by changes in cytochrome P450 activity using in vivo information

The aim of the present paper was to present an overview of the current status of the methods used to predict the magnitude of pharmacokinetic drug-drug interactions (DDIs) which are caused by apparent changes in cytochrome P450 (CYP) activity with an emphasis on a method using in vivo information. In addition, more than a hundred representative CYP substrates, inhibitor and inducer drugs involved in significant pharmacokinetic DDIs were selected from the literature and are listed. Although the magnitude of DDIs has been conventionally predicted based on in vitro experiments, their predictability is restricted occasionally due to several difficulties, including a precise determination of the unbound inhibitor concentrations at the enzyme site and a reliable in vitro measurement of the inhibition constant (K(i)). Alternatively, a simple method has been recently proposed for the prediction of the magnitude of DDIs based on information fully available from in vivo clinical studies. The new in vivo-based method would be applicable to the adjustment of dose regimens in actual pharmacotherapy situations although it requires a prior clinical study for the prediction. In this review, theoretical and quantitative relationships between the in vivo- and the in vitro-based prediction methods are considered. One of the interesting outcomes of the consideration is that the K(i)-normalized dose (dose/in vitro K(i)) of larger than approximately 20L (2-200L, when variability is considered) may be a pragmatic index which predicts significant in vivo DDIs. In the last part of the article, the relevance of the inclusion of the in vivo-based method into the process of new drug development is discussed for good prediction of in vivo DDIs.

KDIGO clinical practice guideline for the care of kidney transplant recipients

The 2009 Kidney Disease: Improving Global Outcomes (KDIGO) clinical practice guideline on the monitoring, management, and treatment of kidney transplant recipients is intended to assist the practitioner caring for adults and children after kidney transplantation. The guideline development process followed an evidence-based approach, and management recommendations are based on systematic reviews of relevant treatment trials. Critical appraisal of the quality of the evidence and the strength of recommendations followed the Grades of Recommendation Assessment, Development, and Evaluation (GRADE) approach. The guideline makes recommendations for immunosuppression, graft monitoring, as well as prevention and treatment of infection, cardiovascular disease, malignancy, and other complications that are common in kidney transplant recipients, including hematological and bone disorders. Limitations of the evidence, especially on the lack of definitive clinical outcome trials, are discussed and suggestions are provided for future research.

[Drug-drug interactions in urology]

Many patients concomitantly receive multiple urological and nonurological medications. This practice can lead to drug-drug interactions (DDIs). These interactions can be pharmacodynamic (acting on the same body function) or pharmacokinetic (affecting each other's concentrations) and are a frequent cause of adverse drug reactions. Examples of pharmacodynamic DDIs include the use of overactive bladder drugs together with those prescribed for psychiatric or neurological indications that also have anticholinergic properties, or the use of PDE5 inhibitors together with vasodilating drugs, particularly nitrates. Pharmacokinetic DDIs are mainly due to effects on drug metabolism, specifically that involving CYP3A4 and 2D6, or on drug transporters. Drugs can both inhibit and induce the activity of many of these enzymes and transporters. CYP3A4 inducers can lower levels of cyclosporine or tacrolimus so much that transplant rejection occurs, and CYP3A4 inhibitors can increase their levels, leading to nephrotoxicity. Levels of the anticholinergic darifenacin can be increased so much by potent CYP3A4 inhibitors that this combination is contraindicated. These examples show that knowledge of DDI can help patients avoid undesirable side effects of drugs.

Effects of oral posaconazole on the pharmacokinetics of sirolimus

OBJECTIVES

Azole antifungal agents are often coadministered with immunosuppressants to recipients of solid organ and hematopoietic stem cell transplants. Posaconazole, an extended-spectrum triazole, is an inhibitor of the cytochrome P450 (CYP) isoenzyme CYP3A4, and sirolimus, an immunosuppressant, is a substrate of the enzyme. We evaluated the effects of posaconazole on sirolimus pharmacokinetics in an open-label, multiperiod, drug-interaction study.

METHODS

Twelve healthy subjects received one dose of sirolimus 2 mg on day 1. After a 28-day washout period, subjects received posaconazole 400 mg bid for 16 days (to day 45). On day 36, sirolimus 2 mg and posaconazole 400 mg were coadministered. Blood samples to determine sirolimus plasma concentrations were collected up to 216 hours post dose on days 1 and 36 and plasma pharmacokinetic parameters were calculated. Drug interactions were evaluated using one-way analysis of variance. Mean (% coefficient of variation) maximum plasma concentration (C(max)) and area under the curve (AUC) of sirolimus at day 1 were 4.9 ng/mL (38) and 145 h x ng/mL (45), respectively.

RESULTS

Coadministration with posaconazole increased sirolimus C(max) and AUC by 6.7- and 8.9-fold, respectively. These increases are consistent with CYP3A4 inhibition by posaconazole. Adverse events were reported by five subjects (42%) receiving posaconazole and sirolimus and by three (25%) and eight (67%) subjects receiving posaconazole only on days 30 to 35 (presirolimus) and days 37 to 45 (postsirolimus), respectively.

CONCLUSION

Because posaconazole has a clinically relevant effect on sirolimus exposure, the agents should probably not be coadministered. Although this was a descriptive study, one potential limitation was the small sample size. The conclusion could have been made stronger if the number of people enrolled in the study had been greater.

Past, present, and future technologies for oral delivery of therapeutic proteins

Biological drugs are usually complex proteins and cannot be orally delivered due to problems related to degradation in the acidic and protease-rich environment of the gastrointestinal (GI) tract. The high molecular weight of these drugs often results in poor absorption into the periphery when administered orally. The most common route of administration for these therapeutic proteins is injection. Most of these proteins have short serum half-lives and need to be administered frequently or in high doses to be effective. So, difficulties in the administration of protein-based drugs provides the motivation for developing drug delivery systems (DDSs) capable of maintaining therapeutic drug levels without side effects as well as traversing the deleterious mucosal environment. Employing a polymer as an entrapment matrix is a common feature among the different types of systems currently being pursued for protein delivery. Protein release from these matrices can occur through various mechanisms, such as diffusion through or erosion of the polymer matrix, and sometimes a combination of both. Encapsulation of proteins in liposomes has also been a widely investigated technology for protein delivery. All of these systems have merit and our worthy of pursuit.

Effect of Oral Posaconazole on the Pharmacokinetics of Cyclosporine and Tacrolimus

STUDY OBJECTIVE

To analyze the pharmacokinetic properties of the immunosuppressants cyclosporine and tacrolimus when either is coadministered with oral posaconazole.

DESIGN

Two single-center, open-label pharmacokinetic studies of cyclosporine in a multiple-dose design and of tacrolimus in a one-sequence, crossover, single- and multiple-dose design.

SETTING

One clinical investigative center in the United States and one in the United Kingdom.

SUBJECTS

Four adult heart transplant recipients in the cyclosporine study and 36 healthy adult volunteers in the tacrolimus study.

INTERVENTIONS

In the cyclosporine study, patients who took an established cyclosporine dose 3 times/day for 6 weeks or longer were given posaconazole 200 mg/day for 10 days. In the tacrolimus study, subjects received tacrolimus 0.05 mg/kg/day on days 1 and 14 and posaconazole 400 mg twice/day on days 7-14.

MEASUREMENTS AND MAIN RESULTS

In the cyclosporine study, blood samples were collected on day 1 to determine cyclosporine pharmacokinetics and on day 10 to measure the pharmacokinetics of cyclosporine and posaconazole. Coadministration of posaconazole increased cyclosporine exposure and necessitated dosage reductions of 14-29% for cyclosporine in three subjects. In the tacrolimus study, blood samples were collected on days 12-14 to assess posaconazole pharmacokinetics and on days 1 and 14 for as long 72 hours after dosing to evaluate tacrolimus pharmacokinetics. Posaconazole increased the maximum blood concentration and the area under the concentration-time curve for tacrolimus by 121% and 358%, respectively, on day 14 compared with day 1 (both p=0.001). In both studies, posaconazole pharmacokinetics were unaffected.

CONCLUSION

These findings suggest that the dosage of cyclosporine or tacrolimus should be reduced when posaconazole therapy is started and that plasma levels of the immunosuppressant should be monitored during and at the discontinuation of posaconazole therapy so that dosages are adjusted accordingly. This recommendation is consistent with current standard of care for patients receiving cyclosporine or tacrolimus with concomitant azole antifungal therapy.

Converting to a Generic Formulation of Mycophenolate Mofetil in Stable Kidney Transplant Recipients: 1 Year of Drug Surveillance and Outcome

OBJECTIVE

The objective of this study was to compare the pharmacokinetic profiles of a generic formulation with the innovator mycophenolate mofetil (MMF).

METHODS AND PATIENTS

Thirteen patients with stable renal grafts received MMF posttransplantation (mean, 32 months; confidence interval 95%: 51, 9-12, 36). Graft function improved significantly (serum creatinine levels 2.72 +/- 1.75 mg/dL to 1.9 +/- 0.66 and 1.79 +/- 0.86 mg/dL at 12 and 30 months, respectively; P < .05). Three point (basal, 2, and 6 hours) profiles were performed. Five patients were switched 1:1 to a generic formulation for 60 days and repeating the pharmacokinetics were repeated. Afterwards, they returned to the innovator MMF for 45 days. Finally, all patients received generic MMF, repeating the pharmacokinetics between 15 days and 6 months later.

RESULTS

The area under the curve (AUC 0-6) of MPA was 52.68 +/- 17.5 microg/mL*hr, with 61.5% of basal determinations (Co) under 5 microg/mL (4.08 +/- 0.74). The Pearson correlation coefficient was high between both MMF formulations regarding FMPA AUC 0.768 (P < .005), Co (0.774; P < 0.05), and C6 (0.996; P < .005). GMPA AUC were similar (Pearson) 0.7 (P < .01), Co 0.99 (P < .01), and C6 0.86 (P < .01). C2 MMF levels were variable and showed poor correlation. FMPA AUC of generic and original formulations after 60 days had tight correlation. (Pearson 0.883; P < .025) and comparing 15 and 60 days postconversion with generic drug in the same patients were similar (Pearson 0.58; P < .025). Renal function at 12 months remained stable postconversion.

CONCLUSIONS

Conversion to generic MMF in stable renal transplant recipients showed good clinical results.

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.

Severe Human Traumatic Brain Injury, but Not Cyclosporin A Treatment, Depresses Activated T Lymphocytes Early after Injury

Severe traumatic brain injury (TBI) leads to an immunocompromised state responsible for an increased morbidity and mortality. Our understanding of the mechanisms responsible for this brain damage is incomplete. Damage maybe mediated by a complex cascade of neuroinflammation, and cytokine activation. In addition, translocation and accumulation of T cells in the brain parenchyma could take place and be related to detrimental effects. Our aims in this prospective randomized pilot study, were to detect the early effect of severe TBI upon cell-mediated immunity, to verify if early immunologic impairment correlates with neurologic outcome, and finally, to test the effect of early administration of iv infusion of cyclosporin A upon cell-mediated immunologic function. Forty-nine patients with severe TBI were studied. Thirty-six of these patients received a 24-h intravenous infusion of Cyclosporin A, or two 24-h infusions of the drug. 10 patients were in the placebo group. Three patients, not enrolled in the cyclosporin trial, were studied only for the relationship between cellular immunity, neurological outcome, and infection rate. T cell counts and microbiological cultures were performed in all patients. Sixty-five percent of patients demonstrated reduced T lymphocyte counts on admission. Furthermore, reduction of T cell numbers was related with significantly worse neurologic outcome and an increase in pulmonary infection. There was no significant difference between the placebo and CsA treated patients for the studied immunological parameters, or for incidence of infection. We also observed sequestration/diapedesis of T cells into the brain parenchyma, around contusions, after human TBI and we speculate that this could be responsible for further brain damage.

Normal Glomerular Filtration Rate in Long-Term Follow-up of Children after Orthotopic Liver Transplantation

BACKGROUND

Cyclosporine A (CyA) and tacrolimus are the principal immunosuppressive agents used for OLT in children. However, progressive deterioration of renal function from calcineurin inhibitor toxicity after OLT has been widely reported. The aim of this study was to assess long-term renal function in children after OLT.

METHODS

We reviewed all OLT patients surviving >1 year at Sainte Justine Hospital from 1987 to 2003. The GFR (ml/min/1.73 m) was measured yearly by the Tc-99m DTPA single injection technique

RESULTS

In all, 101 OLT patients (27 tyrosinemia, 33 biliary atresia, 5 fulminant hepatic failure, 36 miscellaneous) were studied. Median age at OLT was 35 months (range 6-178 months) in tyrosinemia group and 58 months (range 1-226 months) in the "Others" (P = NS), median pediatric end-stage liver disease score was respectively 3 (range -9 to 21) and 15 (-9 to 35), (P=0.001), and median follow-up was 6 (range 1-14) and 6 (range 1-17) (P = NS) years, respectively. Median annual GFR values in tyrosinemia fluctuated between 61 and 104 ml/min/1.73 m, with an improving tendency, and 94-121 ml/min/1.73 m in the Others. GFR did not differ on CyA vs. tacrolimus treatment. The median duration of therapy with calcium-channel blocker in the tyrosinemia group was 5 (1-13) vs. 2 (1-13) years in the Others.

CONCLUSIONS

Median GFR remained normal in most nontyrosinemia patients (Others). Tyrosinemia patients remained stable at a lower GFR. CyA administration in three daily doses and prolonged calcium-channel blocker therapy may have contributed to this stability. Impairment of kidney function was associated with congenital kidney disease, toxic kidney injury, and portal hypertension.

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.

Hepatotoxicity associated with cyclosporine monitoring using C2 recommendations in adults renal recipients receiving ketoconazole

Following the change, in the way we monitored cyclosporine (CsA) levels in January 2000 namely from C0 to C2 concentrations, in renal "de novo" allograft recipients, some patients treated with concomitant ketoconazole experienced liver toxicity, a complication that had not been previously seen with CsA monitoring using C0. Therefore, we decided to compare the outcomes of patients transplanted using CsA levels monitored by C0 (1998 to 1999) who also had simultaneous C2 determinations (group A) with those of recipients transplanted after 2000 (group B). All received steroids, azathioprine, and CsA plus ketoconazole. Recipients were followed for at least a year after transplantation. Patients in group B showed higher CsA C2 levels, AUC concentrations, and drug doses during the immediate postsurgical period, and at 2 weeks as well as 4 and 6 months posttransplantation. Six group B patients (26%) but no group A recipients displayed, severe liver toxicity characterized by jaundice, elevated liver enzymes, with negative serological tests for CMV, HVC, and HVB. There was a correlation between the GOT and the C2 CsA levels; both normalized 15 to 55 days after CsA dose reduction. High C2 CsA levels, which have been recommended when the drug is used alone in renal transplantation, cannot be used in patients taking ketoconazole, because C2 neither represents nor correlates with AUC drug exposure. Thus high C2 levels may produce liver toxicity.

Population pharmacokinetics of cyclosporine in clinical renal transplant patients

Population pharmacokinetics of cyclosporine (CsA) in clinical renal transplant patients has been reported in the present study. A total of 2,548 retrospective drug monitoring data points were collected from 120 renal transplant patients receiving CsA. Population modeling was performed using the NONMEM (nonlinear mixed-effect modeling) program, using a one-compartment model with first-order absorption and elimination. The final regression model for CsA clearance (CL/F) with the influence of six significant covariates, comprising postoperative days (POD), total bilirubin level (TBIL, micromolar concentration), current body weight (CBW, kilograms), age (years), concurrent metabolic inhibitors of cyclosporine (INHI), and hematocrit (HCT, percentage), has been established and expressed as CL/F=28.5 -- 1.24 . POD -- 0.252 . (TBIL -- 11)+0.188 . (CBW -- 58) --0.191 . (Age -- 42) -- 2.45 . INHI -- 0.212 . (HCT-- 28) (liters per hour). The values in parentheses represent the median level for each of the corresponding covariates. The population estimates for CL/F (28.5 l/h), V/F (volume of distribution, 133 l), and interpatient variability (CV%=19.7%) for CL/F were achieved, respectively. The population model was further validated by internal and external approaches, and was demonstrated to be effective and stable. Moreover, simulation was conducted to facilitate the individualized treatment based on patient information and the final model.

CYP3A4 substrate selection and substitution in the prediction of potential drug-drug interactions

The complexity of in vitro kinetic phenomena observed for CYP3A4 substrates (homo- or heterotropic cooperativity) confounds the prediction of drug-drug interactions, and an evaluation of alternative and/or pragmatic approaches and substrates is needed. The current study focused on the utility of the three most commonly used CYP3A4 in vitro probes for the prediction of 26 reported in vivo interactions with azole inhibitors (increase in area under the curve ranged from 1.2 to 24, 50% in the range of potent inhibition). In addition to midazolam, testosterone, and nifedipine, quinidine was explored as a more "pragmatic" substrate due to its kinetic properties and specificity toward CYP3A4 in comparison with CYP3A5. Ki estimates obtained in human liver microsomes under standardized in vitro conditions for each of the four probes were used to determine the validity of substrate substitution in CYP3A4 drug-drug interaction prediction. Detailed inhibitor-related (microsomal binding, depletion over incubation time) and substrate-related factors (cooperativity, contribution of other metabolic pathways, or renal excretion) were incorporated in the assessment of the interaction potential. All four CYP3A4 probes predicted 69 to 81% of the interactions with azoles within 2-fold of the mean in vivo value. Comparison of simple and multisite mechanistic models and interaction prediction accuracy for each of the in vitro probes indicated that midazolam and quinidine in vitro data provided the best assessment of a potential interaction, with the lowest bias and the highest precision of the prediction. Further investigations with a wider range of inhibitors are required to substantiate these findings.

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.

Ketoconazole alters cyclosporine pharmacokinetic profile and may predispose to acute rejection

Ketoconazole (KET) is frequently used as a cyclosporine (CyA)-sparing agent. Adequate exposure to CyA is critical to avoid acute rejection (AR) or chronic rejection (CR). We compared the pharmacokinetic profiles of nine stable renal transplant patients on CyA before and after conversion to KET (200 mg/d of KET simultaneous with CyA). The CyA doses were adjusted to achieve similar drug exposures. The mean dose reduction of CyA while on KET was 83% from 3.2 +/- 0.9 to 0.6 +/- 0.2 (P <.000). Addition of KET produced a significant decrease in the absorption parameters of CyA and an increased elimination half-life. Coefficient of variation (CV) of AUC was significantly increased. Serum creatinine (SCr) at 3 months after switch remained the same. One patient experienced a grade I AR episode (11%). There were no adverse effects related to KET. Although addition of KET to CyA can significantly decrease the CyA dose, the observed pharmacokinetic changes involved decreased absorption and increased CV, which may respectively relate to AR and CR. The 11% AR rate observed is of concern. We do not recommend switching patients to KET as this may imply an increased risk of late AR or CR.

The use of a cyclosporin-ketoconazole combination: making renal transplantation affordable in developing countries

OBJECTIVES

The costs of immunosuppressive drugs in renal transplant recipients remains prohibitively high. Ketoconazole (KZ) has, in limited studies, been shown to significantly reduce the dose of cyclosporin (CyA) after renal transplantation. We report our long-term experience with the use of KZ in a large group of renal transplant recipients. Although this study was not a formal health economic assessment, we undertook a cost-saving analysis of the CyA-KZ combination usage.

METHODS

The 170 patients (174 transplants) undergoing renal transplantation between 1991 and 1997 included in the study received CyA (10 mg/kg/day), prednisone (30 mg/day) and azathioprine (100 mg/day) in the immediate perioperative period. At 1 month post-transplantation, KZ (100 mg twice daily) was added and the CyA dose reduced to 25% and the prednisone dose to 50%. The CyA dose was adjusted to maintain trough levels of 150-200 ng/ml.

RESULTS

There was an 85% reduction in the dose of CyA. The average costs were 10.61 pounds sterling for CyA alone compared with pound 2.26 (pounds sterling) for the CyA-KZ combination, which represents an average savings of 8.35 pounds sterling (79%) per patient per day. The estimated savings during the study period was 999,930 pounds sterling. The patient and graft survival for patients receiving KZ was similar to patients on the South African Dialysis and Transplant Registry. Graft survival was significantly worse in black patients.

CONCLUSION

The use of KZ with CyA in renal transplant recipients with stable allograft function results in a significant reduction in the dose of CyA and a significant cost savings, without compromising patient or graft survival. The regimen may be useful in countries with limited resources.

Effect of Atorvastatin on Cyclosporine Pharmacokinetics in Liver Transplant Recipients

BACKGROUND

The development of hyperlipidemia after liver transplant is frequently treated with hydroxymethylglutaryl coenzyme A reductase inhibitors (statins) such as atorvastatin. As atorvastatin and the primary immunosuppressant drug, cyclosporine, are metabolized by the same pathway, there is the potential for an interaction.

OBJECTIVE

To determine the effect of atorvastatin on cyclosporine pharmacokinetics in liver transplant recipients.

METHODS

Six stable, long-term adult liver transplant recipients from a single center who developed posttransplant dyslipidemia were recruited to participate in a 14-day, open-label study of atorvastatin 10 mg/d coadministered with standard posttransplant immunosuppression using constant oral doses of cyclosporine and corticosteroids. A 10-point pharmacokinetic profile was performed prior to and on day 14 after commencement of atorvastatin therapy. Cyclosporine concentrations were measured by HPLC-electrospray-tandem mass spectrometry. The AUC was calculated by the linear trapezoidal rule, with other parameters determined by visual inspection.

RESULTS

Atorvastatin coadministration increased the cyclosporine AUC by 9% (range 0–20.6%; 3018 vs 3290 ng•h/mL; p = 0.04). No significant change was evident for other cyclosporine pharmacokinetic parameters. Total cholesterol and low-density lipoprotein cholesterol levels were significantly lower on day 14 than at baseline (p < 0.02). One patient developed a twofold increase in transaminases after 2 weeks of atorvastatin therapy, but no other clinical or biochemical adverse events were recorded.

CONCLUSIONS

Atorvastatin coadministration increases the cyclosporine AUC by approximately 10% in stable liver transplant recipients. This change in systemic exposure to cyclosporine is of questionable clinical significance. Atorvastatin is effective in reducing cholesterol levels in liver transplant recipients.

Cyclosporin C(2) and C(0) concentration monitoring in stable, long-term heart transplant recipients receiving metabolic inhibitors

BACKGROUND

Cyclosporin (CsA) dose selection is complicated by significant pharmacokinetic variability between patients. Although therapeutic drug monitoring (TDM) has proven to be a useful tool for dose individualization, the search for an effective and practical measure of clinical effect has uncovered a number of options. Monitoring the CsA concentration in a blood sample taken 2 hours after the dose (C(2)) has been utilized but has not been rigorously evaluated in all clinical situations. The aim of this study was to evaluate C(2) and trough (C(0)) CsA concentrations as surrogate markers of area under the concentration-time curve (AUC) in stable, long-term heart transplant recipients receiving CsA alone or with diltiazem and/or ketoconazole.

METHODS

CsA blood concentration-time data were collected at steady state for 47 stable heart transplant recipients after the morning dose of Neoral. CsA concentration in whole blood was quantitated using the EMIT immunoassay. Patients were stratified into 4 groups, depending on the long-term concomitant administration of drugs known to inhibit CsA metabolism, as part of their routine therapy: Group A (n = 11), CsA alone; Group B (n = 10), CsA with slow-release diltiazem; Group C (n = 13), CsA with ketoconazole; and Group D (n = 12), CsA with a combination of diltiazem and ketoconazole.

RESULTS

In Group A, C(2) correlated poorly with AUC(0-5) (r(2) = 0.197; p = 0.17), whereas C(0) (trough blood sample) showed a stronger correlation (r(2) = 0.710; p = 0.001). Correlations of C(0) and C(2) with AUC(0-5) were the same, but weaker in patients receiving CsA and diltiazem (r(2) = 0.650; p = 0.005); however, C(2) correlated strongly with AUC(0-5) in patients receiving ketoconazole (r(2) = 0.870; p < 0.0001) or ketoconazole with diltiazem (r(2) = 0.898; p < 0.0001). C(0) was a poor predictor of AUC(0-5) in the latter 2 groups.

CONCLUSIONS

C(2) showed a strong correlation with AUC(0-5) in cardiothoracic transplant recipients receiving CsA with ketoconazole, but not with CsA alone or diltiazem. TDM using C(2) as an estimate of AUC requires further evaluation before being applied in long-term, stable cardiac transplant patients, as it may lead to inappropriate dose adjustment of CsA in patients receiving concomitant metabolic inhibitors.

Cyclosporin monitoring in Australasia: 2002 update of consensus guidelines

Therapeutic drug monitoring of cyclosporin (CsA) has been established as part of the routine clinical treatment of patients following organ transplantation for more than 20 years, and based on contemporary knowledge, many consensus guidelines have been published to assist clinics and laboratories attain optimal strategies for patient care. This article addresses the newer directions in CsA monitoring, with particular reference to the Australasian situation that has evolved since the 1993 Australasian guideline. These changes have included the introduction of alternative assay methodologies, changed CsA formulation from Sandimmun to Neoral throughout Australasia, and alternatives to trough concentration (C0) monitoring, especially 2-hour concentration (C2) monitoring and associated validated dilution protocols to accurately quantitate the higher whole blood CsA concentrations. The revision was prepared following a recent survey of all Australasian CsA-monitoring laboratories where discordant practices were evident.

Protein drug oral delivery: the recent progress

Rapid development in molecular biology and recent advancement in recombinant technology increase identification and commercialization of potential protein drugs. Traditional forms of administrations for the peptide and protein drugs often rely on their parenteral injection, since the bioavailability of these therapeutic agents is poor when administered nonparenterally. Tremendous efforts by numerous investigators in the world have been put to improve protein formulations and as a result, a few successful formulations have been developed including sustained-release human growth hormone. For a promising protein delivery technology, efficacy and safety are the first requirement to meet. However, these systems still require periodic injection and increase the incidence of patient compliance. The development of an oral dosage form that improves the absorption of peptide and especially protein drugs is the most desirable formulation but one of the greatest challenges in the pharmaceutical field. The major barriers to developing oral formulations for peptides and proteins are metabolic enzymes and impermeable mucosal tissues in the intestine. Furthermore, chemical and conformational instability of protein drugs is not a small issue in protein pharmaceuticals. Conventional pharmaceutical approaches to address these barriers, which have been successful with traditional organic drug molecules, have not been effective for peptide and protein formulations. It is likely that effective oral formulations for peptides and proteins will remain highly compound specific. A number of innovative oral drug delivery approaches have been recently developed, including the drug entrapment within small vesicles or their passage through the intestinal paracellular pathway. This review provides a summary of the novel approaches currently in progress in the protein oral delivery followed by factors affecting protein oral absorption.

Pharmacokinetic aspects of treating infections in the intensive care unit: focus on drug interactions

Pharmacokinetic interactions involving anti-infective drugs may be important in the intensive care unit (ICU). Although some interactions involve absorption or distribution, the most clinically relevant interactions during anti-infective treatment involve the elimination phase. Cytochrome P450 (CYP) 1A2, 2C9, 2C19, 2D6 and 3A4 are the major isoforms responsible for oxidative metabolism of drugs. Macrolides (especially troleandomycin and erythromycin versus CYP3A4), fluoroquinolones (especially enoxacin, ciprofloxacin and norfloxacin versus CYP1A2) and azole antifungals (especially fluconazole versus CYP2C9 and CYP2C19, and ketoconazole and itraconazole versus CYP3A4) are all inhibitors of CYP-mediated metabolism and may therefore be responsible for toxicity of other coadministered drugs by decreasing their clearance. On the other hand, rifampicin is a nonspecific inducer of CYP-mediated metabolism (especially of CYP2C9, CYP2C19 and CYP3A4) and may therefore cause therapeutic failure of other coadministered drugs by increasing their clearance. Drugs frequently used in the ICU that are at risk of clinically relevant pharrmacokinetic interactions with anti-infective agents include some benzodiazepines (especially midazolam and triazolam), immunosuppressive agents (cyclosporin, tacrolimus), antiasthmatic agents (theophylline), opioid analgesics (alfentanil), anticonvulsants (phenytoin, carbamazepine), calcium antagonists (verapamil, nifedipine, felodipine) and anticoagulants (warfarin). Some lipophilic anti-infective agents inhibit (clarithromycin, itraconazole) or induce (rifampicin) the transmembrane transporter P-glycoprotein, which promotes excretion from renal tubular and intestinal cells. This results in a decrease or increase, respectively, in the clearance of P-glycoprotein substrates at the renal level and an increase or decrease, respectively, of their oral bioavailability at the intestinal level. Hydrophilic anti-infective agents are often eliminated unchanged by renal glomerular filtration and tubular secretion, and are therefore involved in competition for excretion. Beta-lactams are known to compete with other drugs for renal tubular secretion mediated by the organic anion transport system, but this is frequently not of major concern, given their wide therapeutic index. However, there is a risk of nephrotoxicity and neurotoxicity with some cephalosporins and carbapenems. Therapeutic failure with these hydrophilic compounds may be due to haemodynamically active coadministered drugs, such as dopamine, dobutamine and furosemide, which increase their renal clearance by means of enhanced cardiac output and/or renal blood flow. Therefore, coadministration of some drugs should be avoided, or at least careful therapeutic drug monitoring should be performed when available. Monitoring may be especially helpful when there is some coexisting pathophysiological condition affecting drug disposition, for example malabsorption or marked instability of the systemic circulation or of renal or hepatic function.

Minimal effect of ketoconazole on cyclosporine (SangCyA) oral absorption and first-pass metabolism in rats: evidence of a significant vehicle effect on SangCyA absorption

The current work evaluated the effect of the CYP3A inhibitor ketoconazole on the oral absorption and first-pass metabolism of cyclosporine administered as the SangCyA formulation. Groups of 6 male Sprague-Dawley rats were administered SangCyA (5 and 15 mg/kg) by oral gavage alone and with ketoconazole (30 mg/kg). Blood cyclosporine levels were measured over 6 h, encompassing the cyclosporine absorption window. A significant vehicle effect on SangCyA absorption was observed. Comparing a 15 mg/kg dose, cyclosporine C(max) (mean+/-SD 1.12+/-0.16 microg/ml) and AUC(0-6) (5.34+/-0.71 microg h/ml) were 50% lower when propylene glycol was used as gavage vehicle instead of saline (2.19+/-0.94 microg/ml and 9.52+/-2.52 microg h/ml, respectively). Coefficients-of-variation for these parameters were halved in the propylene glycol vehicle however T(max) was unaffected. Ketoconazole increased cyclosporine C(max) and AUC(0-6) by 50-60%, regardless of the vehicle or the cyclosporine dose, without altering T(max) (2-3 h). The small effect of ketoconazole suggests that CYP3A-mediated intestinal and first-pass hepatic metabolism are minor determinants of cyclosporine oral bioavailability in rats.

Peppermint oil enhances cyclosporine oral bioavailability in rats: comparison with D-alpha-tocopheryl poly(ethylene glycol 1000) succinate (TPGS) and ketoconazole

Peppermint oil inhibits cyclosporine metabolism in vitro. The current work compared the effects of peppermint oil, ketoconazole, and D-alpha-tocopheryl poly(ethylene glycol 1000) succinate (TPGS) on cyclosporine oral bioavailability. Male Sprague-Dawley rats were administered cyclosporine (25 mg/kg) as the Sandimmune formulation. Peppermint oil (100 mg/kg) tripled the mean cyclosporine maximum concentration (C(max)) from 0.60 to 1.6 microg/mL and increased the area under the concentration versus time curve (AUC(0-infinity)) from 8.3 to 24.3 microg x h/mL. The median time to reach C(max) (t(max)) was increased from 2 to 6 h. Terminal half-life (10 h) and mean residence time (MRT; 15 h) were unaffected. Coadministration of TPGS (50 mg/kg) with cyclosporine in a saline vehicle doubled cyclosporine C(max) from 1.3 to 2.9 microg/mL and increased AUC(0-infinity) from 28.5 to 59.7 microg x h/mL. The t(max) was unchanged (3 h). Terminal half-life and MRT were increased by 44% (15.4 versus 10.7 h) and 24% (19.9 versus 16.0 h), respectively. Cyclosporine pharmacokinetics were not altered when corn oil was used instead of saline as a gavage vehicle, however the TPGS effect was abolished. Ketoconazole (10 and 20 mg/kg) had no effect on cyclosporine absorption. The lack of a significant ketoconazole effect may reflect poor metabolism of cyclosporine in rat intestinal tissue and suggests that inhibition of cytochrome P450 3A is not the only means by which peppermint oil enhances cyclosporine oral bioavailability.

Can oral midazolam predict oral cyclosporine disposition?

Effective cyclosporine therapy is confounded by large interindividual differences in oral bioavailability and a narrow therapeutic window. Because cytochrome P450 (CYP) 3A-mediated first-pass metabolism contributes to this unpredictable bioavailability, an in vivo oral CYP3A phenotyping probe could be a valuable tool in optimizing cyclosporine therapy. Based on similarities in the metabolic kinetics of cyclosporine and midazolam by the liver and intestinal mucosa, we evaluated whether midazolam oral clearance would predict cyclosporine oral clearance when the two drugs were administered to 20 medically stable kidney transplant recipients. Despite earlier findings in liver transplant recipients who displayed a strong correlation between the systemic clearances of midazolam and cyclosporine, there was a weak correlation between their oral clearances in the current group of subjects (r(s)=0.50, P=0.03). Differing extents of intestinal first-pass metabolic extraction between the two drugs, inhibition of midazolam metabolism by cyclosporine at the level of the intestine, and/or P-glycoprotein-mediated intestinal efflux of cyclosporine (but not midazolam) may account for this poor correlation. We conclude that although oral midazolam is unlikely to be clinically useful as a probe for cyclosporine disposition, its utility in the prediction of other orally administered CYP3A substrates cannot be out ruled.

Effects of the antifungal agents on oxidative drug metabolism: clinical relevance

This article reviews the metabolic pharmacokinetic drug-drug interactions with the systemic antifungal agents: the azoles ketoconazole, miconazole, itraconazole and fluconazole, the allylamine terbinafine and the sulfonamide sulfamethoxazole. The majority of these interactions are metabolic and are caused by inhibition of cytochrome P450 (CYP)-mediated hepatic and/or small intestinal metabolism of coadministered drugs. Human liver microsomal studies in vitro, clinical case reports and controlled pharmacokinetic interaction studies in patients or healthy volunteers are reviewed. A brief overview of the CYP system and the contrasting effects of the antifungal agents on the different human drug-metabolising CYP isoforms is followed by discussion of the role of P-glycoprotein in presystemic extraction and the modulation of its function by the antifungal agents. Methods used for in vitro drug interaction studies and in vitro-in vivo scaling are then discussed, with specific emphasis on the azole antifungals. Ketoconazole and itraconazole are potent inhibitors of the major drug-metabolising CYP isoform in humans, CYP3A4. Coadministration of these drugs with CYP3A substrates such as cyclosporin, tacrolimus, alprazolam, triazolam, midazolam, nifedipine, felodipine, simvastatin, lovastatin, vincristine, terfenadine or astemizole can result in clinically significant drug interactions, some of which can be life-threatening. The interactions of ketoconazole with cyclosporin and tacrolimus have been applied for therapeutic purposes to allow a lower dosage and cost of the immunosuppressant and a reduced risk of fungal infections. The potency of fluconazole as a CYP3A4 inhibitor is much lower. Thus, clinical interactions of CYP3A substrates with this azole derivative are of lesser magnitude, and are generally observed only with fluconazole dosages of > or =200 mg/day. Fluconazole, miconazole and sulfamethoxazole are potent inhibitors of CYP2C9. Coadministration of phenytoin, warfarin, sulfamethoxazole and losartan with fluconazole results in clinically significant drug interactions. Fluconazole is a potent inhibitor of CYP2C19 in vitro, although the clinical significance of this has not been investigated. No clinically significant drug interactions have been predicted or documented between the azoles and drugs that are primarily metabolised by CYP1A2, 2D6 or 2E1. Terbinafine is a potent inhibitor of CYP2D6 and may cause clinically significant interactions with coadministered substrates of this isoform, such as nortriptyline, desipramine, perphenazine, metoprolol, encainide and propafenone. On the basis of the existing in vitro and in vivo data, drug interactions of terbinafine with substrates of other CYP isoforms are unlikely.