Prediction ofin vivodrug interactions with eplerenone in man fromin vitrometabolic inhibition data

Hyperkalemia by eplerenone or esaxerenone in the presence or absence of clarithromycin in hypertensive patients: a retrospective observational cohort study

OBJECTIVE

Mineralocorticoid receptor antagonists (MRAs), eplerenone and esaxerenone, cause hyperkalemia dose-dependently. We investigated the cytochrome P450 3A4-mediated drug-drug interaction between the MRAs and clarithromycin.

METHODS

This retrospective observational study included adult hypertensive patients with MRA plus clarithromycin or MRA alone with a propensity score matching (1:1). The difference in serum potassium level (ΔK, maximum level - baseline level) between groups was compared using the Mann-Whitney U -test. Linear regression analysis was used to detect variables that correlated with ΔK in patients with MRA plus clarithromycin.

RESULTS

After propensity score matching (each nine patients), serum potassium level was elevated after treatment with MRA plus clarithromycin [4.3 (3.5 to 5.1) meq/l to 4.9 (4.0 to 5.5) meq/l, P  = 0.0234] and MRA alone [4.3 (4.0 to 4.7) meq/l to 4.6 (4.4 to 5.2) meq/l, P  = 0.0469]. Although there was no significant difference in ΔK between groups [MRA plus clarithromycin: 0.5 (0.1 to 1.1) meq/l vs. MRA alone: 0.3 (0.1 to 1.2) meq/l, P  = 0.7231], ΔK was significantly higher in esaxerenone plus clarithromycin than in esaxerenone alone [0.6 (0.5 to 1.1) meq/l vs. 0.1 (0.1 to 0.2) meq/l, P  = 0.0495]. Conversely, clarithromycin did not show a significant effect on ΔK in patients with eplerenone [0.4 (-0.2 to 1.2) meq/l vs. 0.8 (0.1 to 1.3) meq/l, P  = 0.5745]. A positive correlation was found between ΔK and age in patients with MRA plus clarithromycin ( y  = 0.03 ×  x - 1.38, r  = 0.71, P  = 0.0336).

CONCLUSION

The drug-drug interaction between MRAs and clarithromycin was evident, particularly in esaxerenone. Serum potassium levels should be closely monitored in older patients.

Cytochrome P450 3A Time-Dependent Inhibition Assays Are Too Sensitive for Identification of Drugs Causing Clinically Significant Drug-Drug Interactions: A Comparison of Human Liver Microsomes and Hepatocytes and Definition of Boundaries for Inactivation Rate Constants

Time-dependent inhibition (TDI) of CYP3A is an important mechanism underlying numerous drug-drug interactions (DDIs), and assays to measure this are done to support early drug research efforts. However, measuring TDI of CYP3A in human liver microsomes (HLMs) frequently yields overestimations of clinical DDIs and thus can lead to the erroneous elimination of many viable drug candidates from further development. In this investigation, 50 drugs were evaluated for TDI in HLMs and suspended human hepatocytes (HHEPs) to define appropriate boundary lines for the TDI parameter rate constant for inhibition (k_obs) at a concentration of 30 µM. In HLMs, a k_obs value of 0.002 minute^-1 was statistically distinguishable from control; however, many drugs show k_obs greater than this but do not cause DDI. A boundary line defined by the drug with the lowest k_obs that causes a DDI (diltiazem) was established at 0.01 minute^-1 Even with this boundary, of the 33 drugs above this value, only 61% cause a DDI (true positive rate). A corresponding analysis was done using HHEPs; k_obs of 0.0015 minute^-1 was statistically distinguishable from control, and the boundary was established at 0.006 minute^-1 Values of k_obs in HHEPs were almost always lower than those in HLMs. These findings offer a practical guide to the use of TDI data for CYP3A in early drug-discovery research. SIGNIFICANCE STATEMENT: Time-dependent inhibition of CYP3A is responsible for many drug interactions. In vitro assays are employed in early drug research to identify and remove CYP3A time-dependent inhibitors from further consideration. This analysis demonstrates suitable boundaries for inactivation rates to better delineate drug candidates for their potential to cause clinically significant drug interactions.

Severe hyperkalaemia due to a potential drug-drug interaction between eplerenone and antiretrovirals in a HIV-positive patient after a myocardial infarction

We present a case of a 48-year-old white HIV-1 positive man who presented an acute myocardial infarction. The patient was on ART for the last ten years with emtricitabine/tenofovir and ritonavir-boosted fosamprenavir. Eplerenone 25 mg/day was also initiated due to a left ventricular dysfunction. A week after discharge a routine laboratory examination revealed severe hyperkalaemia. Due to suspicion of a potential drug-drug interaction, both eplerenone and ARVs were interrupted. Despite daily treatment for hyperkalaemia, serum potassium levels normalized after two weeks. Eplerenone is metabolized by the hepatic P450 cytochrome isoenzyme CYP3A4; therefore, concomitant administration with CYP3A4 inhibitors, like ritonavir, may increase plasma levels of eplerenone and, therefore, the risk of side effects, mainly hyperkalaemia. Based on this case, it is important to alert the medical community of this possible life-threatening drug-drug interaction between eplerenone and ritonavir-boosted protease inhibitor.

Effects of itraconazole and rifampicin on the single-dose pharmacokinetics of the nonsteroidal mineralocorticoid receptor blocker esaxerenone in healthy Japanese subjects

AIMS

To investigate the effects of the strong cytochrome P450 (CYP) 3A inhibitor itraconazole and the strong CYP3A inducer rifampicin on the pharmacokinetics of single-dose esaxerenone, a nonsteroidal mineralocorticoid receptor blocker, in healthy Japanese subjects.

METHODS

Two open-label, single-sequence, crossover studies were conducted in healthy Japanese males aged 20-45 years. In Study 1 (n = 20), subjects received a single oral 2.5 mg dose of esaxerenone (Days 1, 13), with itraconazole 200 mg twice daily (Day 8) and once daily (Days 9-16). In Study 2 (n = 12), subjects received a single oral 5 mg dose of esaxerenone (Days 1, 13), with rifampicin 600 mg once daily (Days 8-16). The plasma concentration of esaxerenone and esaxerenone metabolites were measured using liquid chromatography-tandem mass spectrometry. Pharmacokinetic parameters were calculated using noncompartmental analysis, and safety was assessed.

RESULTS

Esaxerenone exposure increased when coadministered with itraconazole. Geometric least-square mean ratios (90% confidence interval) of peak plasma esaxerenone concentration (C_max ), area under the plasma concentration-time curve (AUC) from zero until the last measurable concentration (AUC_last ) and AUC from zero until infinity (AUC_inf ) were 1.13 (1.05, 1.20) ng mL^-1 , 1.47 (1.40, 1.54) ng h mL^-1 and 1.53 (1.45, 1.62) ng h mL^-1 , respectively. Esaxerenone exposure decreased when coadministered with rifampicin. Geometric least-squares mean ratios (90% confidence interval) of esaxerenone C_max , AUC_last and AUC_inf were 0.659 (0.599, 0.724), 0.315 (0.300, 0.332) and 0.312 (0.297, 0.328), respectively.

CONCLUSION

Itraconazole increased esaxerenone AUC_inf by 53.1%, and rifampicin decreased esaxerenone AUC_inf by 68.8%. These results suggest that caution is recommended when coadministering esaxerenone with strong inhibitors and inducers of CYP3A.

Pharmacokinetics, Metabolism, and Excretion of [14C]Esaxerenone, a Novel Mineralocorticoid Receptor Blocker in Humans

Esaxerenone (CS-3150) is a novel, nonsteroidal, selective mineralocorticoid receptor blocker. The absorption, metabolism, distribution, and excretion of esaxerenone were assessed in in vitro studies and in a clinical study, where [¹⁴C]esaxerenone (150 μCi/20 mg) was administered orally to six healthy male subjects. The plasma concentrations of esaxerenone and its metabolites (M4, M11, and M1) were measured using liquid chromatography-tandem mass spectrometry. The recovery of radioactivity was 92.5%, with 38.5% and 54.0% excreted in the urine and feces, respectively. The half-life of radioactivity in blood and plasma was approximately 30 hours, similar to that of the unchanged form in plasma. The blood-to-plasma ratio was 0.628, demonstrating low partitioning to blood components. In plasma, esaxerenone was the most abundant moiety (40.8%), followed by O-glucuronide (21.4%; M4), acyl-glucuronide of amide-bond hydrolysate (8.0%; M11), and the deshydroxyethyl form (1.7%; M1). In vitro studies showed that esaxerenone was a substrate of CYP3A and multiple UDP-glucuronosyltransferase isoforms. Oxidation contributed approximately 30% to its clearance, as indicated by the excretion ratio of oxidized metabolites into urine and feces. Caco-2 studies showed that esaxerenone was a substrate of P-glycoprotein and breast cancer resistance protein; however, the excretion ratios of the unchanged form in the feces and urine were 18.7% and 1.6%, respectively, indicating that these transporters were not important for the absorption and elimination of esaxerenone. Low urinary excretion of esaxerenone suggested that the plasma exposure of esaxerenone was not affected by renal dysfunction. Multiple elimination pathways including oxidation, glucuronidation, and hydrolysis, and the low contribution of transporters, indicated limited drug-drug interaction potential.

Onset timing of statin-induced musculoskeletal adverse events and concomitant drug-associated shift in onset timing of MAEs

To evaluate the onset timing of musculoskeletal adverse events (MAEs) that develop during statin monotherapy and to determine whether concomitant drugs used concurrently with statin therapy shifts the onset timing of MAEs. Cases in which statins (atorvastatin, rosuvastatin, simvastatin, lovastatin, fluvastatin, pitavastatin, and pravastatin) were prescribed were extracted from the US Food and Drug Administration (FDA) Adverse Event Reporting System (FAERS) Data Files. The onset timing of MAEs during statin monotherapy was evaluated by determining the difference between statin start date and MAE onset date. The use of concomitant drugs with statin therapy was included in the analysis. Statins used in combination with concomitant drugs were compared with statin monotherapy to determine if the use of concomitant drugs shifted the onset timing of MAEs. The onset of MAEs was significantly faster with atorvastatin and rosuvastatin than with simvastatin. A difference in onset timing was not detected with other statins because the number of cases was too small for analysis. When evaluating concomitant drug use, the concomitant drugs that shifted the onset timing of MAEs could not be detected. Statins with strong low-density lipoprotein cholesterol-lowering effects (atorvastatin and rosuvastatin) contributed not only to a high risk of MAE onset, but also to a shorter time-to-onset. No concomitant drug significantly shifted the onset timing of MAEs when used concurrently with statins.

Do Diuretics have Antinociceptive Actions: Studies of Spironolactone, Eplerenone, Furosemide and Chlorothiazide, Individually and with Oxycodone and Morphine

Spironolactone, eplerenone, chlorothiazide and furosemide are diuretics that have been suggested to have antinociceptive properties, for example via mineralocorticoid receptor antagonism. In co-administration, diuretics might enhance the antinociceptive effect of opioids via pharmacodynamic and pharmacokinetic mechanisms. Effects of spironolactone (100 mg/kg, i.p.), eplerenone (100 mg/kg, i.p.), chlorothiazide (50 mg/kg, i.p.) and furosemide (100 mg/kg, i.p.) were studied on acute oxycodone (0.75 mg/kg, s.c.)- and morphine (3 mg/kg, s.c.)-induced antinociception using tail-flick and hot plate tests in male Sprague Dawley rats. The diuretics were administered 30 min. before the opioids, and behavioural tests were performed 30 and 90 min. after the opioids. Concentrations of oxycodone, morphine and their major metabolites in plasma and brain were quantified by mass spectrometry. In the hot plate test at 30 and 90 min., spironolactone significantly enhanced the antinociceptive effect (% of maximum possible effect) of oxycodone from 10% to 78% and from 0% to 50%, respectively, and that of morphine from 12% to 73% and from 4% to 83%, respectively. The brain oxycodone and morphine concentrations were significantly increased at 30 min. (oxycodone, 46%) and at 90 min. (morphine, 190%). We did not detect any independent antinociceptive effects with the diuretics. Eplerenone and chlorothiazide did not enhance the antinociceptive effect of either opioid. The results suggest that spironolactone enhances the antinociceptive effect of both oxycodone and morphine by increasing their concentrations in the central nervous system.

Use of a Physiologically Based Pharmacokinetic Model for Quantitative Prediction of Drug-Drug Interactions via CYP3A4 and Estimation of the Intestinal Availability of CYP3A4 Substrates

The purpose of this study was to predict the drug-drug interactions (DDIs) via CYP3A4 by estimating the extent of hepatic CYP3A4 inhibition based on a physiologically based pharmacokinetic (PBPK) model of both substrate and inhibitor and the increase in the intestinal availability (Fg ) due to the enzyme inhibition. For the DDIs resulting from reversible inhibition of CYP3A4, the prediction using in vivo Ki values estimated from other clinical DDI studies and predicted in vivo Ki values calculated using the correlation between the log P and the in vivo Ki /in vitro Ki ratio was more accurate than that using in vitro Ki values. Incorporating inhibition of both intestinal and hepatic metabolism resulted in better prediction than that obtained considering inhibition in the liver alone, and all the DDIs (AUC increase by the inhibitor) were predicted within 2-fold accuracy when in vivo Ki values were used. In addition, Fg values were successfully back-calculated from the clinical DDI data based on the present model. In conclusion, the present PBPK model incorporating the in vivo Ki values was found to be useful for quantitative prediction of clinical DDIs and for estimation of the Fg values for CYP3A4 substrates for which intravenous data were not available.

Risk assessment of mechanism-based inactivation in drug-drug interactions

Drug-drug interactions (DDIs) that occur via mechanism-based inactivation of cytochrome P450 are of serious concern. Although several predictive models have been published, early risk assessment of MBIs is still challenging. For reversible inhibitors, the DDI risk categorization using [I]/K(i) ([I], the inhibitor concentration; K(i), the inhibition constant) is widely used in drug discovery and development. Although a simple and reliable methodology such as [I]/K(i) categorization for reversible inhibitors would be useful for mechanism-based inhibitors (MBIs), comprehensive analysis of an analogous measure reflecting in vitro potency for inactivation has not been reported. The aim of this study was to evaluate whether the term λ/k(deg) (λ, first-order inactivation rate at a given MBI concentration; k(deg), enzyme degradation rate constant) would be useful in the prediction of the in vivo DDI risk of MBIs. Twenty-one MBIs with both in vivo area under the curve (AUC) change of marker substrates and in vitro inactivation parameters were identified in the literature and analyzed. The results of this analysis show that in vivo DDIs with >2-fold change of object drug AUC can be identified with the cutoff value of λ/k(deg) = 1, where unbound steady-state C(max) is used for inhibitor concentration. However, the use of total C(max) led to great overprediction of DDI risk. The risk assessment using λ/k(deg) coupled with unbound C(max) can be useful for the DDI risk evaluation of MBIs in drug discovery and development.

Aldosterone Antagonists in Heart Failure

Aldosterone antagonists represented by nonselective spironolactone and mineralocorticoid-selective eplerenone are approved for treatment of symptomatic heart failure with reduced systolic function. Their cardioprotective, antifibrotic, and antiarrhythmic effects have been proven in animal experiments, and their effects on morbidity and mortality have been demonstrated in randomized clinical trials. Yet, they remain the most underutilized of all classes of medications for heart failure, primarily because of fear of hyperkalemia. Thorough patient screening and selection is the key for minimizing risks and optimizing benefits from these drugs. Ongoing trials will demonstrate whether the indication for aldosterone antagonists can be expanded to less severe heart failure or patients with preserved systolic function.

Review article: eplerenone: an underused medication?

In this article, we review the evidence supporting the use of eplerenone for improving cardiovascular prognosis. Activation of the renin-angiotensin-aldosterone system plays a major role in the pathogenesis of heart disease, and blockage of this system has been shown to improve prognosis in several cardiovascular conditions. The 2 marketed aldosterone antagonists, spironolactone and eplerenone, improve prognosis in patients with left ventricular (LV) dysfunction and are effective antihypertensive medications. In addition, a potential role for aldosterone antagonists in the treatment of patients with heart failure and preserved LV function has been suggested and is currently being evaluated in clinical trials. In patients with myocardial infarction having LV dysfunction and evidence of heart failure, eplerenone improves cardiovascular outcomes and attenuates myocardial remodeling. In addition, eplerenone is effective for the treatment of hypertension, where it regresses both LV hypertrophy and proteinuria (2 powerful markers of increased cardiovascular risk). In contrast to spironolactone, eplerenone essentially lacks the sexual side effects that sometimes limit the use of spironolactone. Hyperkalemia is the main potential side effect of eplerenone, especially when used in combination with other medications that can cause hyperkalemia. Adequate patient selection and monitoring are therefore of utmost importance when using this medication. In conclusion, eplerenone is a medication that offers the cardiovascular therapeutic and prognostic benefits of aldosterone antagonism but with fewer side effects compared to spironolactone.

Updates on cytochrome p450-mediated cardiovascular drug interactions

Cytochrome P (CYP) 450 is a superfamily of hemoproteins that play an important role in the metabolism of steroid hormones, fatty acids, and many medications. Many agents used for management of cardiovascular diseases are substrates, inhibitors, or inducers of CYP450 enzymes. When two agents that are substrates, inhibitors, or inducers of CYP450 are administered together, drug interactions with significant clinical consequences may occur. This review discusses CYP450-mediated cardiovascular drug interactions as well as noncardiovascular drug interactions that produced significant cardiovascular side effects. The principles in predicting drug interactions are also discussed.

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.

Updates on cytochrome P450-mediated cardiovascular drug interactions

Cytochrome P (CYP) 450 is a superfamily of hemoproteins that play an important role in the metabolism of steroid hormones, fatty acids, and many medications. Many agents used for management of cardiovascular diseases are substrates, inhibitors, or inducers of CYP450 enzymes.When two agents that are substrates, inhibitors, or inducers of CYP450 are administered together, drug interactions with significant clinical consequences may occur. This review discusses CYP450-mediated cardiovascular drug interactions as well as noncardiovascular drug interactions that produced significant cardiovascular side effects. The principles in predicting drug interactions are also discussed.

Inhibition and induction of human cytochrome P450 enzymes: current status

Variability of drug metabolism, especially that of the most important phase I enzymes or cytochrome P450 (CYP) enzymes, is an important complicating factor in many areas of pharmacology and toxicology, in drug development, preclinical toxicity studies, clinical trials, drug therapy, environmental exposures and risk assessment. These frequently enormous consequences in mind, predictive and pre-emptying measures have been a top priority in both pharmacology and toxicology. This means the development of predictive in vitro approaches. The sound prediction is always based on the firm background of basic research on the phenomena of inhibition and induction and their underlying mechanisms; consequently the description of these aspects is the purpose of this review. We cover both inhibition and induction of CYP enzymes, always keeping in mind the basic mechanisms on which to build predictive and preventive in vitro approaches. Just because validation is an essential part of any in vitro-in vivo extrapolation scenario, we cover also necessary in vivo research and findings in order to provide a proper view to justify in vitro approaches and observations.

Prediction of pharmacokinetic drug-drug interactions using human hepatocyte suspension in plasma and cytochrome P450 phenotypic data. II. In vitro-in vivo correlation with ketoconazole

Traditional cytochrome P450 (P450) based drug-drug interaction (DDI) predictions are based on the ratio of an inhibitor's physiological concentration [I] and its inhibition constant K(i). Determining [I] at the enzymatic site, although critical for predicting clinical DDIs, remains a technical challenge. In our previous study, a novel approach using cryopreserved human hepatocytes suspended in human plasma was investigated to mimic the in vivo concentration of ketoconazole at the enzymatic site (Lu et al., 2007), effectively eliminating the estimation of the elusive [I] value. P450 inhibition in this system appears to model that in vivo. Using the ketoconazole inhibition information in a human hepatocyte-plasma suspension together with quantitative P450 phenotypic information, we successfully predicted the pharmacokinetic DDIs for a small set of drugs, such as theophylline, tolbutamide, omeprazole, desipramine, midazolam, loratadine, cyclosporine, and alprazolam, as well as an investigational compound. For the applicability of this model on a wider scale the in vitro-in vivo correlation data set needed to be expanded. However, for most drugs in the literature there is not enough quantitative information on the involvement of individual P450s to predict DDIs retrospectively. To facilitate that, in this study we determined quantitative P450 phenotyping for seven marketed drugs: budesonide, buprenorphine, loratadine, sirolimus, tacrolimus, docetaxel, and methylprednisolone. Augmentation of the new data set with the one generated previously produced broader a database that provided further support for the wider applicability of this approach using ketoconazole as a potent CYP3A inhibitor. This application is predicted to be equally effective with other P450 inhibitors that are not substrates of efflux pumps.

Simultaneous determination of docetaxel and ketoconazole in rat plasma by liquid chromatography/electrospray ionization tandem mass spectrometry

A rapid and sensitive liquid chromatography/tandem mass spectrometry (LC/MS/MS) method has been developed and validated for simultaneous quantification of docetaxel and ketoconazole in rat plasma with paclitaxel as internal standard (IS). The analytes were extracted from rat plasma by using a liquid-liquid extraction technique with ethyl acetate and the LC separation was performed on a Cosmosil-C(18) analytical column (150 mm x 2.0 mm i.d., Nacalai Tesque Inc., Japan). The extracted samples were analyzed with LC/MS/MS, operating in selected reaction monitoring (SRM) mode. The SRM transitions of precursor ions to product ions were 830.3-->549.1 (m/z) for docetaxel, 531.2-->489.3 (m/z) for ketoconazole, and 876.7-->307.9 (m/z) for the IS. The calibration curves were linear over the range of 2-500 ng/mL for docetaxel and 50-20 000 ng/mL for ketoconazole, with coefficients of correlation above 0.999. The limits of quantification for docetaxel and ketoconazole were both 2 ng/mL. The limit of detection for each analyte was 1 ng/mL. The intra- and inter-day precision (CV) of analysis were within 7%, and the accuracy ranged from 95 to 110%. The overall recoveries for docetaxel and ketoconazole were about 89.0% and 91.1%, respectively. The total analysis time was only 9.0 min. This quantitation method was successfully applied to the simultaneous determination of docetaxel and ketoconazole in rat plasma and some potential interaction was found in the current coadministration pharmacokinetic study. This established method was also utilized in the in vitro and in vivo drug-drug interaction study of docetaxel and ketoconazole (to be published).

A Novel Model for the Prediction of Drug-Drug Interactions in Humans Based on in Vitro Cytochrome P450 Phenotypic Data

Ketoconazole has generally been used as a standard inhibitor for studying clinical pharmacokinetic drug-drug interactions (DDIs) of drugs that are primarily metabolized by CYP3A4/5. However, ketoconazole at therapeutic, high concentrations also inhibits cytochromes P450 (P450) other than CYP3A4/5, which has made the predictions of DDIs less accurate. Determining the in vivo inhibitor concentration at the enzymatic site is critical for predicting the clinical DDI, but it remains a technical challenge. Various approaches have been used in the literature to estimate the human hepatic free concentrations of this inhibitor, and application of those to predict DDIs has shown some success. In the present study, a novel approach using cryopreserved human hepatocytes suspended in human plasma was applied to mimic the in vivo concentration of ketoconazole at the enzymatic site. The involvement of various P450s in the metabolism of compounds of interest was quantitatively determined (reactive phenotyping). Likewise, the effect of ketoconazole on various P450s was quantitated. Using this information, P450-mediated change in the area under the curve has been predicted without the need of estimating the inhibitor concentrations at the enzyme active site or the K(i). This approach successfully estimated the magnitude of the clinical DDI of an investigational compound, MLX, which is cleared by multiple P450-mediated metabolism. It also successfully predicted the pharmacokinetic DDIs for several marketed drugs (theophylline, tolbutamide, omeprazole, desipramine, midazolam, alprazolam, cyclosporine, and loratadine) with a correlation coefficient (r(2)) of 0.992. Thus, this approach provides a simple method to more precisely predict the DDIs for P450 substrates when coadministered with ketoconazole or any other competitive P450 inhibitors in humans.

The risks and benefits of aldosterone antagonists

Spironolactone and eplerenone are mineralocorticoid- blocking agents used for their ability to block a host of epithelial and nonepithelial actions of aldosterone. These compounds are of proven benefit in reducing blood pressure and urine protein excretion, and in conferring cardiovascular gain in diverse circumstances of heart failure. However, as enthusiasm grows for use of mineralocorticoid-blocking agents, the risks inherent to use of such drugs become more pertinent. Whereas the endocrine side effects of spironolactone are in reality little more than a cosmetic disfigurement, the potassium-sparing properties of spironolactone and eplerenone can prove life-threatening if hyperkalemia develops. However, for most patients the risk of developing hyperkalemia should not dissuade the prudent clinician from use of these compounds. Hyperkalemia should be considered as a possibility in any patient receiving these medications and as such is best addressed preemptively.

Eplerenone blocks nongenomic effects of aldosterone on the Na+/H+ exchanger, intracellular Ca2+ levels, and vasoconstriction in mesenteric resistance vessels

There is increasing evidence for rapid nongenomic effects of aldosterone. Aldosterone has been demonstrated to alter intracellular pH and calcium in isolated cells. However, few studies have correlated these effects with aldosterone-mediated physiological responses. Therefore, we studied rapid effects of aldosterone on vascular reactivity, intracellular Ca2+, and pH in resistance vessels. Furthermore, we explored whether the new antimineralocorticoid drug eplerenone could effectively block nongenomic aldosterone-mediated effects. The vasoconstrictor action of aldosterone was examined directly by determining the diameter of small resistance mesenteric vessels (160-200 microm resting diameter), simultaneously with intracellular pH or Ca2+. Aldosterone (10 nm) caused a rapid constriction of resistance vessels (8.1% +/- 1.0% reduction in the diameter below control conditions, P < 0.05). Aldosterone potentiated phenylephrine-mediated constriction in small and large mesenteric vessels. Aldosterone induced a rapid increase of intracellular Ca2+ and cellular alkalinization. Vasoconstrictor action of aldosterone and nongenomic effects on the sodium-proton exchanger (NHE1) activity or intracellular Ca2+ responses was abolished by eplerenone. The vasoconstrictor response of aldosterone was related to phosphatidylinositol 3-kinase (PI3-K): the hormone decreased protein kinase B phosphorylation; pharmacological inhibition of PI3-K (10 microm LY294002 or 1 microm wortmannin) increased arterial contractility. Inhibitors of ERK 1/2 phosphorylation (15 microm PD98059) had no effect on aldosterone-mediated vasoconstriction. Inhibition of protein kinase C with 1 microm bi-sindolylmaleimide I and/or inhibition of NHE1 with 100 microm amiloride abolished aldosterone vasoconstrictor action of resistance mesenteric arteries. We conclude that aldosterone-mediated increase in vascular tone is related to a nongenomic mechanism that involves protein kinase C, PI3-K, and NHE1 activity. Eplerenone is an effective blocker of nongenomic effects of aldosterone in vascular tissue.

Pharmacokinetics and Pharmacodynamics of Mineralocorticoid Blocking Agents and their Effects on Potassium Homeostasis

Spironolacotone and eplerenone are mineralocorticoid-blocking agents used for their ability to block both the epithelial and non-epithelial actions of aldosterone. Spironolactone is a non-selective mineralocorticoid receptor antagonist with moderate affinity for both progesterone and androgen receptors. The latter property increases the likelihood of endocrine side effects with spironolactone including loss of libido, menstrual irregularities, gynecomastia and impotence. Eplerenone is a next generation aldosterone receptor antagonist selective for aldosterone receptors alone. This lesser affinity for progesterone and androgen receptors was arrived at by replacing the 17-alpha -thioacetyl group of spironolactone with a carbomethoxy group. Eplerenone is further distinguished from spironolactone by its shorter half-life and the fact that it does not have any active metabolites. Both eplerenone and spironolactone are effective antihypertensive agents and each has been shown to improve the morbidity and mortality of heart failure. Eplerenone or spironolactone use can increase serum potassium values and occasionally results in clinically relevant hyperkalemia. This is more apt to occur with spironolactone due to the very long half-life of several of its active metabolites.

In Vivo Effects of Cyclosporin A and Ketoconazole on the Pharmacokinetics of Representative Substrates for P-Glycoprotein and Cytochrome P450 (CYP) 3A in Rats

In this study, the in vivo effects of cyclosporin A (CsA) and ketoconazole (KCZ), which are used as inhibitors of P-glycoprotein (Pgp) and cytochrome P450 (CYP) 3A, respectively, on the pharmacokinetics of rhodamine 123 (Rho123), nelfinavir (NFV) and erythromycin (EM) were evaluated in rats. The biliary excretion clearance (Clbile) of a known Pgp substrate, Rho123, after intravenous pretreatment with CsA or KCZ (0-20 mg/kg after i.v. administration) showed maximum reduction by 85.6 or 54.1%, respectively, suggesting that the inhibitory potency of KCZ is about half that of Pgp in the liver. Without pretreatment with CsA or KCZ, the clearance ratio of Clbile relative to the total body clearances of Rho123, NFV and EM was 10.5, 0.07 and 31.1%, respectively. After CsA pretreatment, these ratios decreased markedly in a manner dependent on the dose of CsA, while after CZ pretreatment the clearance ratios of NFV and EM increased significantly in a manner dependent on the dose of KCZ. However, in the liver, the contribution of Pgp to the changes in the pharmacokinetic parameters of Rho123, NFV and EM after intravenous administration was much less than that of CYP3A. The portal levels of Rho123 and EM but not NFV after intra-loop administration in the presence of 10 microM CsA in the jejunal loop increased significantly, while in the presence of 25 microM KCZ in the jejunal loop, the portal levels of those substrates showed no notable change as compared to the control levels. In conclusion, KCZ had dual potency to inhibit CYP3A and Pgp, and its inhibitory potency for Pgp was half that of CsA in the rat liver. In addition, metabolism via CYP3A contributed more significantly to the clearance of these substrates that did excretion via Pgp in the liver. In the small intestine, the contribution of Pgp is a more important factor in determining the oral bioavailability of EM than metabolism via CYP enzymes. The elimination of NFV is mainly dependent on liver metabolism via CYP3A, and the Pgp efflux mechanism in the liver and intestine did not contribute as importantly to the oral bioavailability of NFV under in vivo conditions, although NFV has been demonstrated to be a substrate of Pgp under in vitro conditions.