Stereoselective disposition of carvedilol is determined by CYP2D6

Pharmacogenetics, drug-metabolizing enzymes, and clinical practice

The application of pharmacogenetics holds great promise for individualized therapy. However, it has little clinical reality at present, despite many claims. The main problem is that the evidence base supporting genetic testing before therapy is weak. The pharmacology of the drugs subject to inherited variability in metabolism is often complex. Few have simple or single pathways of elimination. Some have active metabolites or enantiomers with different activities and pathways of elimination. Drug dosing is likely to be influenced only if the aggregate molar activity of all active moieties at the site of action is predictably affected by genotype or phenotype. Variation in drug concentration must be significant enough to provide "signal" over and above normal variation, and there must be a genuine concentration-effect relationship. The therapeutic index of the drug will also influence test utility. After considering all of these factors, the benefits of prospective testing need to be weighed against the costs and against other endpoints of effect. It is not surprising that few drugs satisfy these requirements. Drugs (and enzymes) for which there is a reasonable evidence base supporting genotyping or phenotyping include suxamethonium/mivacurium (butyrylcholinesterase), and azathioprine/6-mercaptopurine (thiopurine methyltransferase). Drugs for which there is a potential case for prospective testing include warfarin (CYP2C9), perhexiline (CYP2D6), and perhaps the proton pump inhibitors (CYP2C19). No other drugs have an evidence base that is sufficient to justify prospective testing at present, although some warrant further evaluation. In this review we summarize the current evidence base for pharmacogenetics in relation to drug-metabolizing enzymes.

Pharmacokinetic properties of a new controlled-release formulation of carvedilol

This review summarizes the pharmacokinetics (PK) of carvedilol after administration of a new once-daily controlled-release (CR) formulation. The plasma concentration-time profiles for both R(+)- and S(-)-carvedilol indicate that carvedilol CR will provide coverage over a 24-hour period similar to the current immediate-release (IR) twice-daily formulation. Exposures for both enantiomers, based on area under the curve (AUC), maximum plasma concentrations (C(max)), and trough concentrations, are equivalent for carvedilol CR compared with carvedilol IR. C(max) and AUC of the enantiomers of carvedilol increase in an approximate dose-proportional manner after administration of carvedilol CR over the dose range of 10-80 mg, indicating that the formulation provides consistent PK performance across the dose strengths proposed for marketing. The intrasubject and intersubject variability of carvedilol CR was comparable to carvedilol IR. For carvedilol CR, mean AUC and C(max) were increased <20% after a high-fat meal compared with a standard meal. The CR and IR formulations of carvedilol exhibited equivalent steady-state PK characteristics in the target hypertension and heart failure populations. The availability of once-daily dosing is expected to improve treatment adherence and thereby enhance the effectiveness of carvedilol in routine clinical use.

Multiple regression analysis of pharmacogenetic variability of carvedilol disposition in 54 healthy Japanese volunteers

The aim of this study was to evaluate the pharmacogenetic variability in the disposition of carvedilol in the Japanese population. Five or 10 mg of carvedilol was orally administered to 54 healthy Japanese subjects (22-44 years old), and blood samples were taken at 2 and 6 h after dosing. We determined the polymorphic alleles of CYP2D6, CYP2C9, CYP2C19, CYP3A5, UGT2B7, and MDR1 in each subject. The whole blood concentration of R- and S-carvedilol was measured by an HPLC method. The pharmacokinetic parameters in individual subjects were estimated by the Bayesian method using the nonlinear mixed effects model (NONMEM) program. We then examined the effect of the genetic polymorphisms on the variability in the pharmacokinetics of carvedilol using a multiple regression analysis. The oral clearance (CL/F) and also apparent volume of distribution (V/F) of both enantiomers were significantly lower in the subjects with the CYP2D6*10 allele than those with the CYP2D6*1/*1, *1/*2, or *2/*2 genotype, confirming our previous finding that the bioavailability (F) and systemic clearance (CL) of R- and S-carvedilol in the liver is significantly altered in Japanese with the CYP2D6*10 allele. On the other hand, CYP2C9*3, CYP2C19*2, CYP2C19*3, CYP3A5*3, UGT2B7*2, and MDR1 C3435T did not significantly affect the pharmacokinetics of carvedilol in Japanese subjects.

Drug disposition of chiral and achiral drug substrates metabolized by cytochrome P450 2D6 isozyme: case studies, analytical perspectives and developmental implications

The concepts of drug development have evolved over the last few decades. Although number of novel chemical entitities belonging to varied classes have made it to the market, the process of drug development is challenging, intertwined as it is with complexities and uncertainities. The intention of this article is to provide a comprehensive review of novel chemical entities (NCEs) that are substrates to cytochrome P450 (CYP) 2D6 isozyme. Topics covered in this review aim: (1) to provide a framework of the importance of CYP2D6 isozyme in the biotransformation of NCEs as stand-alones and/or in conjunction with other CYP isozymes; (2) to provide several case studies of drug disposition of important drug substrates, (3) to cover key analytical perspectives and key assay considerations to assess the role and involvement of CYP2D6, and (4) to elaborate some important considerations from the development point of view. Additionally, wherever applicable, special emphasis is provided on chiral drug substrates in the various subsections of the review.

Carvedilol in chronic heart failure: past, present and future

Large randomized clinical trials of bisoprolol, carvedilol and metoprolol have conclusively demonstrated the efficacy and confirmed safety of β-blockers in patients with chronic heart failure. Recently, the beneficial effects of carvedilol in patients with heart failure soon after an acute myocardial infarction have also been shown. Despite this, β-blockers remain under-prescribed in this condition. This is of particular importance as heart failure is common and increasing in prevalence. In this article, when to start β-blockade and which β-blocker to use is considered. Since carvedilol is the most studied β-blocker in heart failure and has a broad range of activities that extend beyond β-blockade, whether it has possible advantages over other β-blockers is discussed. Also, how the use of β-blockade might evolve with the introduction of device-related therapy in heart failure is considered.

Long-term oral carvedilol in chronic heart failure

The long-term beta-blockade strategy with carvedilol, metoprolol succinate or bisoprolol is now strongly recommended to reduce the rates of mortality and morbidity in patients with chronic heart failure (CHF). Although the benefits observed with such drugs are viewed as a class effect, theoretically, carvedilol might be superior to the other two agents, considering its unique pharmacological profile, which includes a more comprehensive antiadrenergic activity and potentially relevant ancillary properties. So far, carvedilol has been proven to be effective and safe in a broader range of CHF patients than metoprolol and bisoprolol. Moreover, a recent large clinical trial has shown a significantly greater survival benefit with carvedilol as directly compared with metoprolol tartrate. Therefore, carvedilol may be the preferred beta-blocking agent to treat patients with CHF.

Effect of CYP2D6*10 on the Pharmacokinetics of R- and S-Carvedilol in Healthy Japanese Volunteers

This study was performed to investigate the effect of CYP2D6*10 on the pharmacokinetics of R- and S-carvedilol in healthy Japanese volunteers. Five or 10 mg of carvedilol was orally administered to 23 subjects (22-44 years old), and blood samples were taken at 2 and 6 h after dosing. We determined the polymorphic alleles of CYP2D6 in each subject. The whole blood concentration of R- and S-carvedilol was measured by an HPLC method. The pharmacokinetic parameters in individual subjects were estimated by the Bayesian method using the nonlinear mixed effects model (NONMEM) program. The mean values of oral clearance for R- and S-carvedilol were estimated to be 1.01 and 2.15 l/h/kg, respectively. The oral clearance was highly correlated with the apparent volume of distribution among the subjects, suggesting that the interindividual difference in bioavailability was largely responsible for the pharmacokinetic variability of carvedilol. The oral clearance and also volume of distribution of both enantiomers were significantly lower in the subjects with the CYP2D6*10 allele than with the CYP2D6*1/*1 or *1/*2 genotype. These results suggested that the systemic and/or pre-systemic metabolism of R- and S-carvedilol in the liver is significantly decreased in Japanese with the CYP2D6*10 allele.

Stereoselective Effect of Amiodarone on the Pharmacokinetics of Racemic Carvedilol

We investigated whether there was a stereoselective effect of amiodarone on the pharmacokinetics of carvedilol. Among a series of 106 inpatients with heart failure, 52 received carvedilol monotherapy (carvedilol group) and 54 received carvedilol plus amiodarone (carvedilol+amiodarone group). The serum carvedilol concentration administered/dose ratio was compared between the two groups based on HPLC measurement of the serum levels of carvedilol, amiodarone, and desethylamiodarone. In 6 patients from the carvedilol group, serum carvedilol levels were compared before and after coadministration of amiodarone. There was no significant between-group difference of the serum concentration to dose (C/D ratio) for the R-enantiomer carvedilol, however, the C/D ratio for the S-enantiomer and the serum S-carvedilol to R-carvedilol (S/R) ratio were both significantly lower in the carvedilol group than in the carvedilol+amiodarone group(47.8+/-56.7 versus 95.3+/-105 ng/mg/kg, P=0.0048 and 0.460+/-0.207 versus 0.879+/-0.377 ng/mg/kg, P<0.001), respectively. Furthermore, the mean S-carvedilol concentration over 14 days of coadministration with amiodarone was higher than that before coadministration (6.54+/-1.73 ng/mL versus 3.03+/-0.670 ng/mL, P<0.001). These results suggest that metabolism of S-carvedilol was markedly inhibited by coadministration of amiodarone.

INVOLVEMENT OF HUMAN HEPATIC UGT1A1, UGT2B4, AND UGT2B7 IN THE GLUCURONIDATION OF CARVEDILOL

Carvedilol ((+/-)-1-carbazol-4-yloxy)-3-[[2-(o-methoxyphenoxy)ethyl]amino]-2-propanol) is metabolized primarily into glucuronide conjugates. In the present study, we identified the human UDP-glucuronosyltransferase (UGT) isoforms involved in the glucuronidation of carvedilol by thin-layer chromatography using microsomes from human liver or insect cells expressing recombinant UGT isoforms. We observed two forms of carvedilol glucuronides, namely G1 and G2, in hepatic microsomes. The glucuronidation of carvedilol was catalyzed by at least three recombinant UGT isoforms: UGT1A1, UGT2B4, and UGT2B7. UGT2B4 formed both G1 and G2, whereas UGT1A1 and UGT2B7 were responsible for the formation of glucuronide G2 and G1, respectively. The enzyme kinetics for carvedilol glucuronidation by UGT1A1, UGT2B4, and UGT2B7 in addition to human liver microsomes were examined by Lineweaver-Burk analysis. The values of Km and Vmax for human liver microsomes were 26.6 microM and 106 pmol/min/mg protein for G1, and 46.0 microM and 44.5 pmol/min/mg protein for G2, respectively. The Km values for UGT1A1, UGT2B4, and UGT2B7 for G1 and G2 (22.1-55.1 microM) were comparable to those of the liver microsomes, whereas the Vmax values were in the range of 3.33 to 7.88 pmol/min/mg protein. The Km and Vmax/Km values for UGT2B4 and UGT2B7 for G1 were similar, whereas UGT2B4 had lower Km and higher Vmax/Km values for G2 compared with those of UGT1A1. These results suggest that G1 formation is catalyzed by UGT2B4 and UGT2B7, whereas G2 is formed by UGT2B4 and UGT1A1. These three hepatic UGT isoforms may have important roles in carvedilol metabolism.

Carvedilol: a review of its use in chronic heart failure

Carvedilol (Dilatrend) blocks beta(1)-, beta(2)- and alpha(1)-adrenoceptors, and has antioxidant and antiproliferative effects. Carvedilol improved left ventricular ejection fraction (LVEF) in patients with chronic heart failure (CHF) in numerous studies. Moreover, significantly greater increases from baseline in LVEF were seen with carvedilol than with metoprolol in a double-blind, randomised study and in a meta-analysis. Carvedilol also reversed or attenuated left ventricular remodelling in patients with CHF and in those with left ventricular dysfunction after acute myocardial infarction (MI). Combined analysis of studies in the US Carvedilol Heart Failure Trials Program (patients had varying severities of CHF; n = 1094) revealed that mortality was significantly lower in carvedilol than in placebo recipients. In addition, the risk of hospitalisation for any cardiovascular cause was significantly lower with carvedilol than with placebo. Mortality was significantly lower with carvedilol than with metoprolol in patients with mild to severe CHF in the Carvedilol Or Metoprolol European Trial (COMET) [n = 3029]. The Carvedilol Prospective Randomised Cumulative Survival (COPERNICUS) trial (n = 2289) demonstrated that compared with placebo, carvedilol was associated with significant reductions in all-cause mortality and the combined endpoint of death or hospitalisation for any reason in severe CHF. All-cause mortality was reduced in patients who received carvedilol in addition to conventional therapy compared with those who received placebo plus conventional therapy in the Carvedilol Post-Infarct Survival Control in LV Dysfunction (CAPRICORN) trial (enrolling 1959 patients with left ventricular dysfunction following acute MI). Carvedilol was generally well tolerated in patients with CHF. Adverse events associated with the alpha- and beta-blocking effects of the drug occurred more commonly with carvedilol than with placebo, whereas placebo recipients were more likely to experience worsening heart failure. In conclusion, carvedilol blocks beta(1)-, beta(2)- and alpha(1)-adrenoceptors and has a unique pharmacological profile. It is thought that additional properties of carvedilol (e.g. antioxidant and antiproliferative effects) contribute to its beneficial effects in CHF. Carvedilol improves ventricular function and reduces mortality and morbidity in patients with mild to severe CHF, and should be considered a standard treatment option in this setting. Administering carvedilol in addition to conventional therapy reduces mortality and attenuates myocardial remodelling in patients with left ventricular dysfunction following acute MI. Moreover, mortality was significantly lower with carvedilol than with metoprolol in patients with mild to severe CHF, suggesting that carvedilol may be the preferred beta-blocker.

Comparison between capillary electrophoresis and high-performance liquid chromatography for the stereoselective analysis of carvedilol in serum

A high-performance liquid chromatographic (HPLC) assay using a chiral stationary phase was developed and validated for the determination of carvedilol enantiomers in human serum and was compared with a previously developed capillary electrophoresis (CE) method. The CE and the HPLC assay were compared by analyzing a series of serum samples containing racemic carvedilol in different concentrations using the two methods. The concentrations obtained by the two assays were not found to be significantly different indicating that CE and HPLC are comparable in terms of reproducibility and precision for the stereoselective analysis of carvedilol in human serum.

Pharmacogenomics of congestive heart failure

Information on the probable efficacy and safety of drugs for individualized patients should change both the economics and the management of congestive heart failure. Rapidly profiling patients should enhance the ability to develop "designer" drugs for those with similar disease phenotypes. Pharmacogenomics also may resurrect promising drugs that did not show benefit when previously added to standard therapy. The promise of pharmacogenomics has to be tempered, however, by a sensitivity to concerns regarding the potential misuse of genomic information, including loss of confidentiality for the patient, as well as possible stigmatization of groups. Such difficult genoethical issues must be addressed in order to reap the full benefits of this evolving and exciting field.

Pharmacogenetics, pharmacogenomics, and cardiovascular therapeutics: the way forward

The completion of sequencing of the human genome will be the vanguard for numerous advances in medicine. The first discernible application is likely to occur in pharmacogenomics, a field focused on the influence of genetic differences on the variability in patients' response to medications. While an inherited basis for drug response has been recognized for some time, it is the confluence of molecular biology, high-throughput genotyping, and bioinformatics that has made it practical to study the genetic basis of variability to medications on a large scale. Pharmacogenomics may enable clinicians to prospectively identify patients most likely to derive benefit from a drug, with minimal likelihood of adverse events. This DNA-based approach to predicting clinical drug efficacy and toxicity would shift the current prescribing paradigm from its empirical nature to a more patient-specific model, ushering in a new era of personalized medicine. Polymorphisms in drug metabolizing enzymes, drug targets, and disease pathogenesis genes are associated with therapeutic effect to cardiovascular pharmacotherapy. Moreover, pharmacogenomics and functional genomics are expected to have a profound impact on the process of drug discovery and development. Finally, pharmacogenomics is likely to transform the way clinical trials are conducted by allowing for the selection of a more homogeneous study population, thereby reducing the size and cost of clinical investigation.

The genetic basis of variability in drug responses

It is almost axiomatic that patients vary widely in their beneficial responses to drug therapy, and serious and apparently unpredictable adverse drug reactions continue to be a major public health problem. Here, we discuss the concept that genetic variants might determine much of this variability in drug response, and propose an algorithm to enable further evaluation of the benefits and pitfalls of this enticing possibility.

ABCs of molecular cardiology and the impact of the Human Genome Project on clinical cardiology

The last decade was marked by a revolution in molecular biology, culminating with the Human Genome Project. This revolution has changed the classic practice of clinical cardiology in many ways, increasing our awareness of inheritance of defective genes and their impact on health and disease, and providing new diagnostic and therapeutic tools. On the other hand, identification of new diseases in the clinical setting has triggered research into previously unexplored areas of molecular biology. As a result of this interaction, both fields underwent major paradigm shifts. This article presents a primer of molecular biology for the cardiologist, followed by a discussion of the impact the Human Genome Project will have on the clinical practice of cardiology.

Separation of carvedilol enantiomers in very small volumes of human plasma by capillary electrophoresis with laser-induced fluorescence

A sensitive capillary electrophoretic method for the determination of carvedilol enantiomers in 100 microl of human plasma has been developed and validated. Carvedilol and the internal standard carazolol are isolated from plasma samples by liquid-liquid extraction using diethylether. A sensitive and selective detection is provided by helium-cadmium laser-induced fluorescence. The total analysis time is 17.5 min, about 30 min are needed for the sample preparation. The linearity of the assay ranges from 1.56 to 50 ng/ml per carvedilol enantiomer. The limits of quantification (LOQ) for the carvedilol enantiomers in 100 microl of human plasma are 1.56 ng/ml. The inter-day accuracy for R-carvedilol is between 95.8 and 103% (104% at LOQ) and for S-carvedilol between 97.1 and 103% (107% at LOQ); the inter-day precision values are between 3.81 and 8.64% (10.9% at LOQ) and between 5.47 and 7.86% (7.91% at LOQ) for R- and S-carvedilol, respectively. The small sample volume needed is especially advantageous for the application in clinical studies in pediatric patients. As an application of the assay concentration/time profiles of the carvedilol enantiomers in a 5-year-old patient receiving a test dose of 0.09 mg/kg carvedilol are reported.

Development of a capillary electrophoresis assay for the determination of carvedilol enantiomers in serum using cyclodextrins

A capillary electrophoresis method using cyclodextrins as the chiral selectors was developed for the determination of carvedilol enantiomers in serum. Several types of cyclodextrins were evaluated. The effect of cyclodextrin concentration on enantiomer resolution was investigated. Best results were obtained using 10 mM hydroxypropyl-beta-cyclodextrin in the run buffer. The effect of voltage on efficiency was assessed. Other electrophoretic conditions were optimized. The method was validated for carvedilol enantiomers in serum. Linearity of detection was assessed over the concentration range of 50-4000 ng/ml of each enantiomer in serum. Intra- and inter-assay variability obtained were under 8% for both enantiomers.

Effect of fluoxetine on carvedilol pharmacokinetics, CYP2D6 activity, and autonomic balance in heart failure patients

The objective of this study was to examine the pharmacokinetic and pharmacodynamic consequences of concomitant administration of fluoxetine and carvedilol in heart failure patients. Fluoxetine (20 mg) or matching placebo was administered in a randomized, double-blind, two-period crossover study to 10 patients previously identified as extensive metabolizers of CYP2D6 substrates. Patients were maintained on a carvedilol dose of 25 or 50 mg bid and given fluoxetine/placebo for a minimum of 28 days. Plasma was collected over the 12-hour carvedilol dosing interval, and the concentrations of the R(+) and S(-) enantiomers of carvedilol were measured. CYP2D6 phenotype was assessed during each study period using dextromethorphan (30 mg). Changes in autonomic modulation between study periods were measured by heart rate variability in the time and frequency domains using ambulatory electrocardiographic monitoring. Compared to placebo, fluoxetine coadministration resulted in a 77% increase in mean (+/- SD) R(+) enantiomer AUC0-12 (522 +/- 413 vs. 927 +/- 506 ng.h/mL, p = 0.01) and a nonsignificant increase in S(-) enantiomer AUC (244 +/- 185 vs. 330 +/- 179 ng.h/mL, p = 0.17). Mean apparent oral clearance for both enantiomers decreased significantly with fluoxetine administration (R(+): 10.3 +/- 7.2 vs. 4.5 +/- 2.2 mL/min/kg; S(-): 22.5 +/- 12.3 vs. 12.6 +/- 7.4 mL/min/kg; p = 0.004 and 0.03, respectively). No differences in adverse effects, blood pressure, or heart rate were noted between treatment groups, and there were no consistent changes in heart rate variability parameters. In conclusion, fluoxetine administration resulted in a stereospecific inhibition of carvedilol metabolism, with the R(+) enantiomer increasing to a greater extent than the S(-) enantiomer. However, this interaction was of little clinical significance in our sample population.

Differential effects of carvedilol on norepinephrine release in normoxic and ischemic heart

Carvedilol is a beta-adrenoceptor antagonist with multiple actions, which may contribute to superior cardioprotection in heart failure and myocardial infarction. We hypothesized that carvedilol may modulate presynaptic norepinephrine release in the heart. Therefore, we compared the effects of carvedilol (racemate and both enantiomers) and beta1-selective as well as nonselective beta-adrenoceptor blockers on norepinephrine release in isolated perfused rat hearts under normoxic and brief ischemic conditions. Exocytotic release of endogenous norepinephrine was induced by paired electric field stimulations to compare the release before (S1) and after (S2) beta-adrenoceptor blocker application. Metoprolol, bisoprolol, and pindolol (0.1-10 microM) had essentially no effect on exocytotic norepinephrine release under normoxic and ischemic conditions. In contrast, carvedilol exerted a biphasic concentration-response curve (increase followed by suppression) on norepinephrine release. The increase in norepinephrine release was more pronounced with R-carvedilol than with S-carvedilol, indicating an effect independent from beta-receptor antagonism. During ischemia, the facilitatory effect of carvedilol on norepinephrine release was lost, resulting in a concentration-dependent suppression of the release. These results indicate that carvedilol in contrast to classic beta1-selective and -nonselective beta-adrenoceptor blockers has pronounced effects on cardiac norepinephrine release with a remarkable difference between normoxic and ischemic conditions. Whereas a facilitation of norepinephrine release prevailed in normoxia, we observed a suppression of the release in ischemia. It remains to be established whether this unique action of carvedilol on cardiac sympathetic neurotransmission is of clinical relevance.

Pharmacogenetic diagnostics of cytochrome P450 polymorphisms in clinical drug development and in drug treatment

The current use and future perspectives of molecular genetic characterisation of cytochrome P450 enzymes (CYP) for drug development and drug treatment are summarised. CYP genes are highly polymorphic and the enzymes play a key role in the elimination of the majority of drugs from the human body. Frequent variants of some enzymes, CYP2A6, 2C9, 2C19 and 2D6, should be analysed in participants of clinical trials whenever these enzymes may play a role. It is suggested that a CYP genotype certificate is handed out to the volunteers or patients to avoid replicate analyses, and to allow that this information is available for future research and also for treatment with eventually needed drugs. Guidelines on what CYP alleles have to be analysed in drug development, as well as on analytical validation and CYP genotype data handling will be required. Treatment with several drugs may be improved by prior genotyping. The concepts and problems of CYP genotype-based clinical dose recommendations are presented and illustrated for selected drugs. The requirement for prospective trials on the medical and economic benefits of routine CYP genotyping is emphasised. Specific operationally defined recommendations dependent on genotype are a prerequisite for such studies and this review presents tentative CYP genotype-based dose recommendations systematically calculated from published data. Because of the multiplicity of factors involved, these doses will not be the optimal doses for each given individual, but should be more adequate than doses generally recommended for an average total population. Those CYP alleles and polymorphically metabolised drugs which are currently most interesting in drug development and drug treatment are reviewed, and more complete information is available from websites cited in this article.

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.

Hypotension with dobutamine: beta-adrenergic antagonist selectivity at low doses of carvedilol

OBJECTIVE

To report a case of marked hypotension resulting from the concomitant use of low-dose carvedilol and intravenous dobutamine.

CASE SUMMARY

A 54-year-old white man with severe heart failure was placed on carvedilol 3.125 mg orally twice a day; three days later the dosage was increased to 6.25 mg orally twice a day. His symptoms of heart failure worsened with increasing fluid retention, orthopnea, paroxysmal nocturnal dyspnea, and elevated blood urea nitrogen and creatinine. He was admitted for treatment of decompensated heart failure with intravenous dobutamine. With each increase in intravenous dobutamine, systolic blood pressure fell. Dobutamine was discontinued when systolic blood pressure reached 56 mm Hg. In a subsequent admission for decompensated heart failure, when the patient was not taking carvedilol, he was treated with intravenous dobutamine and systolic blood pressure increased.

DISCUSSION

Although carvedilol is a nonselective beta-adrenergic antagonist, at low doses it is a selective beta1-adrenergic antagonist. Dobutamine is a beta1-, beta2-, and alpha1-adrenergic agonist. Typically, patients with heart failure treated with intravenous dobutamine have a small increase in systolic blood pressure. We propose that the drop in blood pressure with dobutamine in this patient was caused by a fall in systemic vascular resistance due to vascular beta2-adrenergic receptor activation. The normal increase in cardiac output was partially blocked by selective beta1-adrenergic blockade at low doses of carvedilol.

CONCLUSIONS

Beta-adrenergic blockade with carvedilol is now common therapy for patients with congestive heart failure. Intravenous dobutamine is often used when these patients have worsening heart failure. Recognition that treatment with dobutamine in patients taking low doses of carvedilol may result in hypotension is important for appropriate monitoring and therapy.

Clinical Pharmacology of Carvedilol

BACKGROUND: There is now a wealth of data supporting the use of beta-blockers in heart failure and the additional pharmacological properties of carvedilol are thought to play an important role in the therapeutic efficacy of carvedilol in this disease. METHODS AND RESULTS: Carvedilol is licensed for the treatment of essential hypertension, chronic stable angina, and mild to moderate chronic heart failure. This article provides an up-to-date review of the clinical pharmacology of carvedilol, with particular emphasis on its clinical effects in heart failure. CONCLUSION: Carvedilol is a multiple-action neurohormonal antagonist that offers nonselective beta-blockade, alpha-1 blockade, antioxidant, anti-ischemic mortality, and anti-proliferative properties. In addition to reductions in hospitalization and mortality rates, benefits of carvedilol in heart failure include dramatic improvements in left ventricular function and other parameters of cardiac remodeling.

Genomics and hypertension: concepts, potentials, and opportunities

We are at the beginning of a biological revolution, spurred on by the Human Genome Project and associated studies. Within the next few years, expressed sequence tags (ESTs) representing all sequences expressed in humans will be determined and their genomic positions will be defined (STSs). The discovery of all the variants in the human genome that contribute to the genetic diversity of the human population will result in the construction of dense polymorphic maps. The rapid growth of the EST, STS, and single-nucleotide polymorphism (SNP) databases, coupled with impressive technological advances, will surely have a dramatic effect on biomedical research. In this review, we will examine the recent advances in genetics and genomics and place these within the context of medical research and patient care, with an emphasis on studies in the cardiovascular system.

Pharmacological importance of stereochemical resolution of enantiomeric drugs

Drug enantiomers have identical properties in an achiral environment, but should be considered as different chemical compounds. This is because they often differ considerably in potency, pharmacological activity and pharmacokinetic profile, since the modules with which they interact in biological systems are also optically active. Within biological systems, the metabolism of one isomer may be via a different pathway or occur at a different rate from that of the other isomer. Preferential binding of one isomer to plasma proteins may cause differences in circulating free drug and hence alter concentrations at active sites. Interactions of both isomers may differ at the active sites through which pharmacological action is mediated. Actions and levels of activity of the stereoisomers in vivo may also differ. All the pharmacological activity may reside in a single enantiomer, whereas several possibilities exist for the other enantiomer-- it may be inactive, have a qualitatively different effect, an antagonistic effect or produce greater toxicity. Two isomers may have nearly identical qualitative pharmacological activity, qualitatively similar pharmacological activity but quantitatively different potency, or qualitatively different pharmacological activity. To avoid adverse effects and optimise the therapeutic value of enantiomeric drugs, it is necessary that methods for the resolution of racemates be evolved and devolved to determine isomeric purity, establish the effectiveness of isomers of the drug, and detect the presence of an enantiomer with lower therapeutic activity and undesirable adverse effects. Even if a drug is given as a pure enantiomer, methods to discriminate between enantiomers are required because racemisation can occur both in vitro and in vivo. Methods developed for resolution of drug enantiomers should facilitate routine testing of single isomers and their metabolites, studies of pharmacological, toxicological and clinical effectiveness, routine analysis of racemates, pure enantiomers or intermediates in manufacturing processes, and investigation of the potential for inversion of an enantiopure drug substance during the early stages of drug development and therapeutic drug monitoring.