Lack of effect of grapefruit juice on the pharmacokinetics and pharmacodynamics of amlodipine

The Challenge and Importance of Integrating Drug-Nutrient-Genome Interactions in Personalized Cardiovascular Healthcare

Despite the rich armamentarium of available drugs against different forms of cardiovascular disease (CVD), major challenges persist in their safe and effective use. These include high rates of adverse drug reactions, increased heterogeneity in patient responses, suboptimal drug efficacy, and in some cases limited compliance. Dietary elements (including food, beverages, and supplements) can modulate drug absorption, distribution, metabolism, excretion, and action, with significant implications for drug efficacy and safety. Genetic variation can further modulate the response to diet, to a drug, and to the interaction of the two. These interactions represent a largely unexplored territory that holds considerable promise in the field of personalized medicine in CVD. Herein, we highlight examples of clinically relevant drug–nutrient–genome interactions, map the challenges faced to date, and discuss their future perspectives in personalized cardiovascular healthcare in light of the rapid technological advances.

Association of Amlodipine with the Risk of In-Hospital Death in Patients with COVID-19 and Hypertension: A Reanalysis on 184 COVID-19 Patients with Hypertension

Association between calcium channel blockers (CCBs) or functional inhibitors of acid sphingomyelinase (FIASMAs) use and decreased mortality in people with COVID-19 has been reported in recent studies. Since amlodipine is both a CCB and a FIASMA, the aim of this study was to investigate the association between chronic amlodipine use and the survival of people with hypertension infected with COVID-19. This retrospective cohort study used data extracted from the medical records of adult inpatients with hypertension and laboratory-confirmed COVID-19 between 1 March 2020 and 31 August 2020 with definite outcomes (discharged from hospital or deceased) from Erasme Hospital (Brussels, Belgium). We re-analyzed the data of the retrospective cohort study using only the 184 patients (103 males, 81 females) with a mean age of 69.54 years (SD = 14.6) with hypertension. The fifty-five participants (29.9%) receiving a chronic prescription of amlodipine were compared with the 129 patients who did not receive a chronic prescription of amlodipine. Univariate and multivariate logistic regressions were used to explore the relationships between mortality and sex, age, comorbidities, smoking, and amlodipine status. Out of the 184 participants, 132 (71.7%) survived and 52 (28.3%) died. The mortality rates were, respectively, 12.73% (n = 7) and 34.88% (n = 45) for the amlodipine and non-amlodipine groups. Multivariate logistic regression was significant (Chi square (5) = 29.11; p < 0.0001). Chronic kidney disease and malignant neoplasm were significant predictors as well as amlodipine status. For chronic kidney disease and malignant neoplasm, the odds ratio with 95% confidence interval (95% CI) were, respectively, 2.16 (95% CI: 1.04−4.5; p = 0.039) and 2.46 (95% CI: 1.01−6.01; p = 0.047). For amlodipine status the odds ratio was 0.29 (95% CI: 0.11−0.74; p = 0.009). The result of the present study suggests that amlodipine may be associated with reduced mortality in people with hypertension infected with COVID-19. Further research and randomized clinical trials are needed to confirm the potential protective effect of amlodipine in people with hypertension infected with COVID-19.

Extracorporeal treatment for calcium channel blocker poisoning: systematic review and recommendations from the EXTRIP workgroup

BACKGROUND

Calcium channel blockers (CCBs) are commonly used to treat conditions such as arterial hypertension and supraventricular dysrhythmias. Poisoning from these drugs can lead to severe morbidity and mortality. We aimed to determine the utility of extracorporeal treatments (ECTRs) in the management of CCB poisoning.

METHODS

We conducted systematic reviews of the literature, screened studies, extracted data, summarized findings, and formulated recommendations following published EXTRIP methods.

RESULTS

A total of 83 publications (6 in vitro and 1 animal experiments, 55 case reports or case series, 19 pharmacokinetic studies, 1 cohort study and 1 systematic review) met inclusion criteria regarding the effect of ECTR. Toxicokinetic or pharmacokinetic data were available on 210 patients (including 32 for amlodipine, 20 for diltiazem, and 52 for verapamil). Regardless of the ECTR used, amlodipine, bepridil, diltiazem, felodipine, isradipine, mibefradil, nifedipine, nisoldipine, and verapamil were considered not dialyzable, with variable levels of evidence, while no dialyzability grading was possible for nicardipine and nitrendipine. Data were available for clinical analysis on 78 CCB poisoned patients (including 32 patients for amlodipine, 16 for diltiazem, and 23 for verapamil). Standard care (including high dose insulin euglycemic therapy) was not systematically administered. Clinical data did not suggest an improvement in outcomes with ECTR. Consequently, the EXTRIP workgroup recommends against using ECTR in addition to standard care for patients severely poisoned with either amlodipine, diltiazem or verapamil (strong recommendations, very low quality of the evidence (1D)). There were insufficient clinical data to draft recommendation for other CCBs, although the workgroup acknowledged the low dialyzability from, and lack of biological plausibility for, ECTR.

CONCLUSIONS

Both dialyzability and clinical data do not support a clinical benefit from ECTRs for CCB poisoning. The EXTRIP workgroup recommends against using extracorporeal methods to enhance the elimination of amlodipine, diltiazem, and verapamil in patients with severe poisoning.

Effect of Korean Red Ginseng extracts on drug-drug interactions

Background

Ginseng has been the subject of many experimental and clinical studies to uncover the diverse biological activities of its constituent compounds. It is a traditional medicine that has been used for its immunostimulatory, antithrombotic, antioxidative, anti-inflammatory, and anticancer effects. Ginseng may interact with concomitant medications and alter metabolism and/or drug transport, which may alter the known efficacy and safety of a drug; thus, the role of ginseng may be controversial when taken with other medications.

Methods

We extensively assessed the effects of Korean Red Ginseng (KRG) in rats on the expression of enzymes responsible for drug metabolism [cytochrome p450 (CYP)] and transporters [multiple drug resistance (MDR) and organic anion transporter (OAT)] in vitro and on the pharmacokinetics of two probe drugs, midazolam and fexofenadine, after a 2-wk repeated administration of KRG at different doses.

Results

The results showed that 30 mg/kg KRG significantly increased the expression level of CYP3A11 protein in the liver and 100 mg/kg KRG increased both the mRNA and protein expression of OAT1 in the kidney. Additionally, KRG significantly increased the mRNA and protein expression of OAT1, OAT3, and MDR1 in the liver. Although there were no significant changes in the metabolism of midazolam to its major metabolite, 1′-hydroxymidazolam, KRG significantly decreased the systemic exposure of fexofenadine in a dose-dependent manner.

Conclusion

Because KRG is used as a health supplement, there is a risk of KRG overdose; thus, a clinical trial of high doses would be useful. The use of KRG in combination with P-glycoprotein substrate drugs should also be carefully monitored.

Irbesartan/amlodipine: a review of its use in adult patients with essential hypertension not adequately controlled with monotherapy

Combination therapy is often required in patients with hypertension, and fixed-dose single-pill combinations have been shown to provide an easier regimen for patients, improving adherence. Irbesartan/amlodipine (Aprovasc®) is an angiotensin-receptor blocker/calcium-channel blocker fixed-dose single-pill combination, whose constituent drugs exert additive effects when coadministered. In two randomized, open-label, multicentre, phase III trials, fixed-dose combination therapy with irbesartan/amlodipine was more effective than continuation of irbesartan or amlodipine monotherapy in patients with hypertension not adequately controlled with initial irbesartan or amlodipine monotherapy; there was a significantly greater decrease from baseline in mean seated home systolic blood pressure (primary endpoint) with the fixed-dose combination. The fixed-dose combination was also associated with a greater decrease in mean seated home diastolic blood pressure and mean seated office systolic and diastolic blood pressure than monotherapy. The fixed-dose combination of irbesartan/amlodipine was well tolerated in these patients; most treatment-emergent adverse events were of mild or moderate severity. The most frequent adverse event was peripheral oedema, generally associated with amlodipine treatment.

Grapefruit-Drug Interactions: Can Interactions With Drugs Be Avoided?

Grapefruit is rich in flavonoids, which have been demonstrated to have a preventive influence on many chronic diseases, such as cancer and cardiovascular disease. However, since the early 1990s, the potential health benefits of grapefruit have been overshadowed by the possible risk of interactions between drugs and grapefruit and grapefruit juice. Several drugs interacting with grapefruit are known in different drug classes, such as HMG-CoA reductase inhibitors, calcium antagonists, and immunosuppressives. Currently known mechanisms of interaction include the inhibition of cytochrome P450 as a major mechanism, but potential interactions with P-glycoprotein and organic anion transporters have also been reported. This review is designed to provide a comprehensive summary of underlying mechanisms of interaction and human clinical trials performed in the area of grapefruit drug interactions and to point out possible replacements for drugs with a high potential for interactions.

Antihypertensive Drugs in Patients Treated with Antiretroviral

Objective:

To review the literature for information regarding pharmacokinetic interactions between antiretrovirals and antihypertensive agents, evaluate the clinical significance of these interactions, and analyze the effect of antihypertensive drugs on the metabolic complications frequently observed in HIV-infected patients to emphasize the advantages and inconveniences of every class of antihypertensive drugs in association with antiretrovirals.

Data Sources:

A literature search was conducted via PubMed and MEDLINE (1950-November 2011) using the search terms drug interactions, cytochrome P450, names of antiretrovirals, names of commonly prescribed antihypertensive drugs, pharmacokinetics, and metabolic complications. Reference citations from relevant publications, manufacturers’ product information, and www.HIV-druginteracttons.org were also reviewed.

Study Selection And Data Extractions:

All articles with an English abstract identified through the data search were examined. Of these, pharmacologic reviews, studies, and case reports were evaluated.

Data Synthesis:

Antihypertensive drugs interact with several antiretroviral drugs, non nucleoside reverse transcriptase inhibitors (NNRTIs) and protease inhibitors (Pls) in particular. Pharmacokinetic interactions are less expected with diuretics, jî-blockers excreted by the kidney, angiotensin-converting enzyme (ACE) inhibitors, and angiotensin II receptor blockers (ARBs) other than losartan and irbesartan. Calcium channel blockers (CCBs) are metabolized by CYP3A4, with the potential for interaction with NNRTIs and Pls. Because CCBs do not adversely affect glucose or lipid metabolism or renal function, they may be preferred in patients with such complications. ACE inhibitors and ARBs may exert favorable effects on glucose homeostasis. In addition, they may significantly reduce protein excretion and further slow the progression of renal disease.

Conclusions:

The choice of antihypertensive drugs in HIV-infected patients is complex and must take into account the metabolic pathways of antiretroviral drugs and antihypertensive drugs with the potential of pharmacokinetic interactions, as well as the effect of antihypertensive drugs on some biological parameters.

Grapefruit-drug interactions

Grapefruit juice and grapefruit product consumption have potential health benefits; however, their intake is also associated with interactions with certain drugs, including calcium channel blockers, immunosuppressants and antihistamines. The primary mechanism through which interactions are mediated is mechanism-based intestinal cytochrome P450 3A4 inhibition by furanocoumarins resulting in increased bioavailability of administered medications that are substrates. Grapefruit products have also been associated with interactions with P-glycoprotein (P-gp) and uptake transporters (e.g. organic anion-transporting polypeptides [OATPs]). Polyphenolic compounds such as flavonoids have been proposed as the causative agents of the P-gp and OATP interactions. The mechanisms and magnitudes of the interactions can be influenced by the concentrations of furanocoumarins and flavonoids in the grapefruit product, the volume of juice consumed, and the inherent variability of specific enzymes and transporter components in humans. It is therefore challenging to predict the extent of grapefruit product-drug interactions and to compare available in vitro and in vivo data. The clinical significance of such interactions also depends on the disposition and toxicity profile of the drug being administered. The aim of this review is to outline the mechanisms of grapefruit-drug interactions and present a comprehensive summary of those agents affected and whether they are likely to be of clinical relevance.

The effect of grapefruit juice on drug disposition

INTRODUCTION

Since their initial discovery in 1989, grapefruit juice (GFJ)-drug interactions have received extensive interest from the scientific, medical, regulatory and lay communities. Although knowledge regarding the effects of GFJ on drug disposition continues to expand, the list of drugs studied in the clinical setting remains relatively limited.

AREAS COVERED

This article reviews the in vitro effects of GFJ and its constituents on the activity of CYP enzymes, organic anion-transporting polypeptides (OATPs), P-glycoprotein, esterases and sulfotransferases. The translational applicability of the in vitro findings to the clinical setting is discussed for each drug metabolizing enzyme and transporter. Reported AUC ratios for available GFJ-drug interaction studies are also provided. Relevant investigations were identified by searching the PubMed electronic database from 1989 to 2010.

EXPERT OPINION

GFJ increases the bioavailability of some orally administered drugs that are metabolized by CYP3A and normally undergo extensive presystemic extraction. In addition, GFJ can decrease the oral absorption of a few drugs that rely on OATPs in the gastrointestinal tract for their uptake. The number of drugs shown to interact with GFJ in vitro is far greater than the number of clinically relevant GFJ-drug interactions. For the majority of patients, complete avoidance of GFJ is unwarranted.

The grapefruit: an old wine in a new glass? Metabolic and cardiovascular perspectives

Grapefruit is a popular, tasty and nutritive fruit enjoyed globally. Biomedical evidence in the last 10 years has, however, shown that consumption of grapefruit or its juice is associated with drug interactions, which, in some cases, have been fatal. Grapefruit-induced drug interactions are unique in that the cytochrome P450 enzyme CYP3A4, which metabolises over 60% of commonly prescribed drugs as well as other drug transporter proteins such as P-glycoprotein and organic cation transporter proteins, which are all expressed in the intestines, are involved. However, the extent to which grapefruit-drug interactions impact on clinical settings has not been fully determined, probably because many cases are not reported. It has recently emerged that grapefruit, by virtue of its rich flavonoid content, is beneficial in the management of degenerative diseases such as diabetes and cardiovascular disorders. This potentially explosive subject is reviewed here.

Evaluation of the effect of lime fruit juice on the anticoagulant effect of warfarin

AIM

Citrus aurantifolia (Family Rutaceae) is commonly known as a familiar food and medicine, and s therapeutic effectiveness in a variety of diseases has been suggested in traditional medicine. Various complementary and alternative medicines (CAM) have been shown to interact with orthodox medicines. Hence, the aim of this study is to investigate such a phenomenon particularly the interaction of lime fruit juice with warfarin.

MATERIALS AND METHOD

Wistar strain albino rats of both sexes weighing between 190 and 230g were administered with oral doses of the respective drugs used depending on the groups of animals. Effects on the anticoagulant activity of warfarin were determined by standard laboratory methods.

RESULT

Lime fruit juice caused a reduction in the anticoagulant activity of warfarin.

CONCLUSION

This finding has shown that CAM can interact with orthodox medicines hence, warfarin prescribers need to be aware of the usage of CAM and monitor the international normalized ratio (INR) of their patients more frequently.

Effect of cytochrome P450 3A5*3 genotype on the stereoselective pharmacokinetics of amlodipine in healthy subjects

Amlodipine is a racemic mixture composed of S- and R-form and metabolized stereoselectively. Cytochrome P450 3A (CYP3A) including CYP3A5 are involved in the metabolism of amlodipine and it was reported that polymorphic CYP3A5 genotype modulates the plasma levels of amlodipine and thus affect its pharmacokinetics. This study was conducted to find whether stereoselective pharmacokinetics of amlodipine was affected by the polymorphic CYP3A5 genotype. Seventeen healthy subjects were genotyped for CYP3A5*3 variant. After a single dose of 10-mg amlodipine, enantiomers of amlodipine were analyzed using HPLC-MS/MS equipped with an AGP column. Amlodipine showed stereoselective pharmacokinetics. S-amlodipine exhibited higher plasma levels than R-amlodipine in both genotype groups. S-amlodipine showed 15% higher mean peak plasma concentrations (Cmax) in CYP3A5*1/*3 carriers (3.28 ng/ml) than CYP3A5*3/*3 carriers (2.85 ng/ml) (P = 0.194) and R-amlodipine also showed 21% higher Cmax in CYP3A5*1/*3 carriers (3.33 ng/ml) than CYP3A5*3/*3 carriers (2.75 ng/ml) (P = 0.114). CYP3A5*1/*3 carriers also have 23 and 12% higher mean area under the time versus concentration curve of R-amlodipine and S-amlodipine than CYP3A5*3/*3 carriers, respectively (for R-amlodipine, 147.1 ng*h/ml for CYP3A5*1/*3 carriers versus 121.8 ng*h/ml for CYP3A5*3/*3 carriers, P = 0.234; for S-amlodipine, 161.6 ng*h/ml for CYP3A5*1/*3 carriers vs. 144.2 ng*h/ml for CYP3A5*3/*3 carriers, P = 0.353). Other pharmacokinetic parameters also showed no significant difference between them. In conclusion, the present study showed that despite the evidence that amlodipine is stereoselectively metabolized, CYP3A5*3 genotype did not affect stereoselective disposition of amlodipine. It provides the evidence that CYP3A5*3genotype plays a minor role in the interindividual variability of stereoselective disposition of amlodipine in humans.

Pharmacokinetics and bioequivalence evaluation of two formulations of 10-mg amlodipine besylate: an open-label, single-dose, randomized, two-way crossover study in healthy Chinese male volunteers

BACKGROUND

Amlodipine is a third-generation dihydropyridine calcium antagonist for the treatment of angina and hypertension. The relative bioavailability of a newly developed dispersible tablet as compared with an established branded formulation has not been reported in a Chinese population.

OBJECTIVE

The aim of this study was to assess and compare the pharmacokinetic properties, bioavailability, and bioequivalence of a newly developed dispersible tablet formulation of amlodipine besylate with those of an established branded formulation in healthy Chinese adult male volunteers.

METHODS

An open-label, single-dose, randomized, 2-way crossover study was conducted in fasted healthy Chinese male volunteers. Eligible participants were randomly assigned in a 1:1 ratio to receive 2 tablets (5 mg each) of the test or reference formulation, followed by a 2-week washout period and administration of the alternate formulation. The study drugs were administered after a 10-hour overnight fast. Serum samples were collected over 120 hours. Amlodipine concentrations in the serum were analyzed by liquid chromatography tandem mass spectrometry with positive ion electrospray ionization using the multiple reaction monitoring mode. The visible detection of the method was in the range of 0.2 to 32.0 ng/mL, and the lower limit of quantification for amlodipine was 0.2 ng/mL. The amlodipine serum concentration-time curves were used to obtain pharmacokinetic parameters including AUC(0-t), AUC(0-infinity)), and C(max). The criteria for bioequivalence were 90% CIs of 80% to 125% for AUC and 70% to 143% for C(max), according to guidelines of the State Food and Drug Administration of the People's Republic of China. Tolerability was based on the recording of adverse events (AEs), monitoring vital signs, electrocardiograms, and clinical laboratory tests at baseline and completion of the study.

RESULTS

A total of 20 healthy Chinese male volunteers (mean [SD] age, 21.4 [2.6] years [range, 1926 years]; weight, 61.3 [5.4] kg [range, 54.0-75.0 kg]; and height, 171.2 [3.6] cm [range, 162.0-177.0 cm) were included in the study. The mean (SD) C(max), T(max), AUC(0-t), and AUC(0-infinity)) values after administration of the test and reference formulations, respectively, were as follows: 5.46 (1.13) versus 5.88 (1.24) ng/mL, 7.70 (2.08) versus 9.20 (4.18) hours, 284.56 (77.59) versus 311.34 (75.97) ng/mL/h, and 331.37 (111.03) versus 358.74 (101.10) ng/mL/h. The mean (SD) t(1/2) was 38.52 (10.51) hours for the test formulation and 38.75 (7.07) hours for the reference formulation. On analysis of variance, no period or sequence effects were observed for any pharmacokinetic property; however, a significant formulation effect was observed for C(max), AUC(0-t), and AUC(0-infinity)). The relative bioavailability of the test formulation was 90.9% by mean AUC(0-t) and 91.2% by mean AUC(0-infinity). The 90% CIs for the ratios of C(max), AUC(0-t), and AUC(0-infinity) were 88.4% to 97.5%, 86.4% to 95.7%, and 85.8% to 97.0%, respectively, meeting the predetermined criteria for bioequivalence. One subject (5%) reported 2 AEs. The AEs were mild, possibly associated with study drug, and resolved spontaneously by the next evaluation. No serious AEs were reported.

CONCLUSIONS

In this small study in healthy Chinese adult male volunteers, a single 10-mg dose of the dispersible tablet formulation (test) of amlodipine besylate met the regulatory criteria for bioequivalence to the established tablet formulation (reference) based on the rate and extent of absorption. Both formulations were well tolerated.

Medicinal importance of grapefruit juice and its interaction with various drugs

Grapefruit juice is consumed widely in today's health conscious world as a protector against cardiovascular diseases and cancers. It has however, been found to be an inhibitor of the intestinal cytochrome P - 450 3A4 system, which is responsible for the first pass metabolism of many drugs. The P - glycoprotein pump, found in the brush border of the intestinal wall which transports many of these cytochrome P - 450 3A4 substrates, has also been implicated to be inhibited by grapefruit juice. By inhibiting these enzyme systems, grapefruit juice alters the pharmacokinetics of a variety of medications, leading to elevation of their serum concentrations. Most notable are its effects on the calcium channel antagonist and the statin group of drugs. In the case of many drugs, the increased serum concentration has been found to be associated with increased frequency of dose dependent adverse effects. In this review, we have discussed the phytochemistry of grapefruit juice, the various drugs involved in the drug - grapefruit juice reaction with their mechanisms of action and have presented the clinical implications of these interactions.

Pharmacokinetic and pharmacodynamic characteristics of a new S-amlodipine formulation in healthy Korean male subjects: A randomized, open-label, two-period, comparative, crossover study

BACKGROUND

Amlodipine, a dihydropyridine calcium channel antagonist, is prescribed for the management of angina and hypertension. It is used therapeutically as a racemic mixture, composed of S- and R-enantiomers, but its calcium channel-blocking effect is confined to S-amlodipine; R-amlodipine has 1000-fold less activity than its S-enantiomer.

OBJECTIVE

The objective of this study was to compare the pharmacokinetic and pharmacodynamic properties and safety profiles of a newly developed amlodipine formulation, composed wholly of S-amlodipine, with those of the conventionally prescribed racemic formulation.

METHODS

This randomized, open-label, 2-period, comparative, crossover study was conducted with healthy volunteers at the Gil Medical Center and Gachon Medical School, Incheon, Korea. Male subjects, aged 20 to 50 years, were eligible to participate if their weight was within 20% of ideal body weight and if they were judged by physicians to be healthy. All subjects were randomly assigned in a 1:1 ratio to 1 of 2 treatment sequences: (1) a single dose of the test amlodipine formulation (S-enantiomer amlodipine 5 mg p.o.) (Lodien [Hanlim Pharmaceutical Co., Seoul, Korea]) in the first study period, followed by a single dose of the reference amlodipine formulation (racemate 10 mg p.o.) (Norvasc [Pfizer Pharmaceuticals Korea Ltd., Seoul, Korea]) in the second study period, or (2) a single dose of the reference formulation in the first study period, followed by a single dose of the test formulation in the second period. A 3-week washout occurred between study periods. Blood samples for pharmacokinetic analysis of S-amlodipine were collected at 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 24, 48, 72, 96, 120, 144, and 168 hours after drug administration. Pharmacodynamic variables (ie, systolic and diastolic blood pressure and heart rate) were measured at 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 24, 48, and 72 hours after administration. Safety profiles were also assessed. Hematology, biochemistry, electrocardiography, and urinalysis were performed at baseline and end of study. Adverse events were monitored throughout the study period. Pharmacokinetic characteristics were compared using noncompartmental analysis. Pharmacokinetic equivalence was concluded if the geometric mean ratios of the plasma Cmax and AUC were within the predetermined range of 80% to 125%.

RESULTS

Twenty-six healthy Korean male volunteers were screened and 18 subjects (mean [SD] age, 23.4 [1.5] years [range, 21-26 years]; mean [SD] weight, 69.3 [6.8] kg [range, 60-88 kg]) were enrolled and completed the study. The plasma concentration-time profiles of S-amlodipine were comparable after administration of both formulations. The mean (SD) values for Cmax AUC from time 0 to the last available measurement (AUC(last)), and AUC from 0 to infinity (AUC(0-infinity)) for the reference formulation (3.0 [0.6] ng/mL, 151.4 [35.7] ng x h/mL, and 175.3 [45.1] ng x h/mL, respectively) did not differ significantly from those for the test formulation (3.1 [0.6] ng/mL, 139.7 [40.3] ng x h/mL, and 161.7 [43.8] ng x h/mL, respectively). The calculated 90% Cls for the corresponding ratios of log-transformed Cmax, AUCO(0-infinity), and AUC(last) were 97.56% to 112.51%, 86.31% to 98.74%, and 83.46% to 100.04%, respectively, which met the predetermined criteria for pharmacokinetic equivalence. Despite the single administration, significant changes in maximal blood pressure and heart rate were observed after drug administration for both formulations, compared with baseline values (all, P < 0.001). However, no significant differences were observed between the 2 formulations in terms of pharmacodynamic profiles, and no clinically relevant changes were observed for either formulation with respect to physical examination, hematology, biochemistry, electrocardiography, or urinalysis. Neither formulation caused any serious adverse events.

CONCLUSIONS

Two amlodipine formulations were found to be equivalent in terms of the pharmacokinetics of S-amlodipine. The newly developed formulation, comprised of only S-amlodipine, had pharmacodynamic profiles comparable to those of the conventional racemic amlodipine formulation in these healthy Korean male subjects. Both formulations were well tolerated.

Intestinal permeability and excretion into bile control the arrival of amlodipine into the systemic circulation after oral administration

The objective of this study was to identify the factors controlling the arrival of amlodipine into the systemic circulation after oral administration in the fasting state. Dissolution data were collected with the rotating paddle and the flow-through apparatus. Caco-2 cell lines were used to assess the intestinal permeability characteristics. Actual in-vivo data were collected in 24 fasted healthy subjects after single-dose administration of the same amlodipine besylate tablet formulation used in the in-vitro dissolution studies. Regardless of the hydrodynamics, dissolution of amlodipine besylate tablets was rapid and complete in media simulating the contents of the upper gastrointestinal tract in the fasting state. Permeability of amlodipine through Caco-2 cell lines was lower than propranolol's and higher than ranitidine's, indicating that transport through the intestinal mucosa may be one process that limits the arrival into the systemic circulation. Indeed, the de-convoluted profile indicated that arrival into portal blood occurs at rates much slower than gastric emptying or dissolution rates. However, prediction of amlodipine's mean plasma profile after oral administration became possible only after additionally assuming excretion of amlodipine into the bile and a reasonable gastrointestinal residence time. Interestingly, in-vitro permeability data collected in this or in previous studies were inappropriate for simulating the mean actual plasma profile.

Major determinant factors of the extent of interaction between grapefruit juice and calcium channel antagonists

AIM

To evaluate quantitatively the influence of bioavailability and plasma unbound fraction (f(u)) on the extent of interaction between grapefruit juice (GFJ) and calcium antagonists.

METHODS

The extent of drug-GFJ interaction was evaluated from reported clinical interaction studies of 11 calcium antagonists. The relationship between extent of interaction and bioavailability or f(u) was analysed.

RESULTS

The following significant hyperbolic relationships were observed: [Extent] = 12.2/[bioavailability] + 1 and [Extent] = 0.718/[f(u)] + 1.

CONCLUSIONS

Bioavailability and f(u) are major determinant factors of calcium antagonist-GFJ interaction: a drug with smaller bioavailability or lower f(u) is likely to exhibit a more potent interaction, and vice versa.

Undesirable effects of citrus juice on the pharmacokinetics of drugs: focus on recent studies

It is well known that intake of grapefruit juice affects the pharmacokinetics of various kinds of drugs. It has been reported that other citrus juices also interact with certain drugs. To re-evaluate citrus juice-drug interactions based on currently available evidence, a literature search was conducted for new and updated information since the grapefruit juice-drug interaction was last reviewed in 1998. MEDLINE (1998-October 2004) was accessed and more than 200 reports were found. The effects of grapefruit juice ingestion on the pharmacokinetics of orally administered drugs have been reported for 40 drugs since the reviews published in 1998. Increases in either area under the concentration-time curve (AUC) or maximum plasma concentration (C(max)) were found with 34 of these, the major mechanism being considered to be inactivation of intestinal cytochrome P450 3A4, a so-called mechanism-based inhibition. Although recent reports point to the inhibitory effects of grapefruit juice on the function of P-glycoprotein, which transports substrates from enterocytes back into the lumen, the contribution to the bioavailability of drugs that are substrates of P-glycoprotein has not been established yet. Dramatic decreases in AUC and C(max) for two drugs in association with grapefruit juice ingestion has been reported and, in these cases, inhibitory effects on organic anion transporting polypeptide, which mediates absorption from the intestinal lumen to enterocytes, might be involved. Other citrus juices such as Seville (sour) orange juice and commonly ingested varieties of orange juice also showed significant effects on the AUC and C(max) of some drugs. Although the situation is complex and uncertainties remain, we recommend that patients avoid citrus juice intake while taking medications and that healthcare providers advise against citrus juice intake in this setting until any interactions with subject drugs can be clarified in clinical studies.

Resveratrol as an anti-inflammatory and anti-aging agent: mechanisms and clinical implications

Resveratrol is a phytoalexin polyphenolic compound found in various plants, including grapes, berries, and peanuts. Multiple lines of compelling evidence indicate its beneficial effects on neurological, hepatic, and cardiovascular systems. Also one of the most striking biological activities of resveratrol soundly investigated during the late years has been its cancer-chemopreventive potential. In fact, recently it has been demonstrated that this stilbene blocks the multistep process of carcinogenesis at various stages: tumor initiation, promotion, and progression. One of the possible mechanisms for its biological activities involves downregulation of the inflammatory response through inhibition of synthesis and release of pro-inflammatory mediators, modification of eicosanoid synthesis, inhibition of activated immune cells, or inhibiting such as inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) via its inhibitory effects on nuclear factor (kappa)B (NF-(kappa)B) or the activator protein-1 (AP-1). More recent data provide interesting insights into the effect of this compound on the lifespan of yeast and flies, implicating the potential of resveratrol as an anti-aging agent in treating age-related human diseases. It is worthy to note that the phenolic compound possesses a low bioavailability and rapid clearance from the plasma. As the positive effects of resveratrol on inflammatory response regulation may comprise relevant clinical implications, the purpose of this article is to review its strong anti-inflammatory activity and the plausible mechanisms of these effects. Also, this review is intended to provide the reader an up-date of the bioavailability and pharmacokinetics of resveratrol and its impact on lifespan.

Drug–drug interactions in oncology: Why are they important and can they be minimized?

Adverse drug-drug interactions are a major cause of morbidity and mortality. Cancer patients are at particularly high risk of such interactions because they commonly receive multiple medications, including cytotoxic chemotherapy, hormonal agents and supportive care drugs. In addition, the majority of cancer patients are elderly, and so require medications for co-morbid conditions such as cardiovascular, gastrointestinal, and rheumatological diseases. Furthermore, the age-related decline in hepatic and renal function reduces their ability to metabolize and clear drugs and so increases the potential for toxicity. Not all drug-drug interactions can be predicted, and those that are predictable are not always avoidable. However, increased awareness of the potential for these interactions will allow healthcare providers to minimize the risk by choosing appropriate drugs and also by monitoring for signs of interaction. This review considers the basic principles of drug-drug interactions, and presents specific examples that are relevant to oncology.

Assessment of sex differences in pharmacokinetics and pharmacodynamics of amlodipine in a bioequivalence study

AIMS

This study was conducted to assess the bioequivalence between two 10-mg amlodipine tablet formulations. As secondary objectives, sex-related differences and tolerability profile were evaluated.

METHODS

Thirty-six healthy volunteers (18 males and 18 females; age 20-32 years, weight 49.5-98.0 kg) were included in a randomised crossover study. Subjects were administered a single 10-mg oral dose of each formulation separated by a 14-day washout period. Plasma amlodipine levels were determined by a high performance liquid chromatographic method with tandem mass spectrometry detection.

RESULTS

All subjects completed the study and 90% confidence intervals for relevant pharmacokinetic parameters were within the ranges defined by European and US Regulatory Authorities: the geometric mean and the 90% confidence interval test/reference ratios calculated from log-transformed values were 104.54 (101.46-107.72%) for AUC(0-infinity) and 100.32 (97.41-103.33%) for Cmax. There were no serious or severe adverse events. The tolerability profile appeared to be comparable for the two products. On average, bioavailability of amlodipine was slightly higher in females than in males, but these differences could be explained by the lower body weight of women. There were no sex-related differences in drug clearance. Bioequivalence was also demonstrated within each gender group. Amlodipine treatment produced a slight decrease of systolic blood pressure and an increased in heart rate, which were more pronounced in women. The incidence of adverse events was similar in men and women.

CONCLUSIONS

The two formulations were considered bioequivalent. Although there were no relevant gender-related differences in the pharmacokinetics of amlodipine, women reached higher amlodipine concentrations most likely because of their lower body weight, and therefore, the reported pharmacodynamic effects were higher within this gender group.

Herbal modulation of P-glycoprotein

P-glycoprotein (Pgp) is a 170 kDa phosphorylated glycoprotein encoded by human MDR1 gene. It is responsible for the systemic disposition of numerous structurally and pharmacologically unrelated lipophilic and amphipathic drugs, carcinogens, toxins, and other xenobiotics in many organs, such as the intestine, liver, kidney, and brain. Like cytochrome P450s (CYP3A4), Pgp is vulnerable to inhibition, activation, or induction by herbal constituents. This was demonstrated by using an ATPase assay, purified Pgp protein or intact Pgp-expressing cells, and proper probe substrates and inhibitors. Curcumin, ginsenosides, piperine, some catechins from green tea, and silymarin from milk thistle were found to be inhibitors of Pgp, while some catechins from green tea increased Pgp-mediated drug transport by heterotropic allosteric mechanism, and St. John's wort induced the intestinal expression of Pgp in vitro and in vivo. Some components (e.g., bergamottin and quercetin) from grapefruit juice were reported to modulate Pgp activity. Many of these herbal constituents, in particular flavonoids, were reported to modulate Pgp by directly interacting with the vicinal ATP-binding site, the steroid-binding site, or the substrate-binding site. Some herbal constituents (e.g., hyperforin and kava) were shown to activate pregnane X receptor, an orphan nuclear receptor acting as a key regulator of MDR1 and many other genes. The inhibition of Pgp by herbal constituents may provide a novel approach for reversing multidrug resistance in tumor cells, whereas the stimulation of Pgp expression or activity has implication for chemoprotective enhancement by herbal medicines. Certain natural flavonols (e.g., kaempferol, quercetin, and galangin) are potent stimulators of the Pgp-mediated efflux of 7,12-dimethylbenz(a)-anthracene (a carcinogen). The modulation of Pgp activity and expression by these herb constituents may result in altered absorption and bioavailability of drugs that are Pgp substrates. This is exemplified by increased oral bioavailability of phenytoin and rifampin by piperine and decreased bioavailability of indinavir, tacrolimus, cyclosporine, digoxin, and fexofenadine by coadministered St. John's wort. However, many of these drugs are also substrates of CYP3A4. Thus, the modulation of intestinal Pgp and CYP3A4 represents an important mechanism for many clinically important herb-drug interactions. Further studies are needed to explore the relative role of Pgp and CYP3A4 modulation by herbs and the mechanism for the interplay of these two important proteins in herb-drug interactions.

Drug interactions in the hematopoietic stem cell transplant (HSCT) recipient: what every transplanter needs to know

Pharmacokinetic drug interactions among hematopoietic stem cell transplant recipients can result in either increases in serum concentrations of medications, which may lead to enhanced toxicity; or reduced serum concentrations, which can lead to treatment failure and the emergence of post transplant complications. The use of drugs that have a narrow therapeutic index, such as cyclosporine/tacrolimus (calcineurin inhibitors), increases the significance of these interactions when they occur. This report will review the clinical data evaluating the drug interactions of relevance to HSCT clinical practice, focusing on the pharmacokinetic interactions, and provides recommendations for managing these interactions to avoid both toxicity and treatment failure.

Randomized, open-label, two-period crossover comparison of the pharmacokinetic and pharmacodynamic properties of two amlodipine formulations in healthy adult male Korean subjects

BACKGROUND

Amlodipine, a third-generation dihydropyridine calcium antagonist, is prescribed in the management of angina and hypertension. A newly developed amlodipine formulation (amlodipine camsylate) is associated with similar physical properties, melting point, and solubility-and improved stability against long-term stability test and accelerated temperature test-compared with the conventional formulation (amlodipine besylate).

OBJECTIVE

This study was performed to compare the pharmacokinetic (PK) and pharmacodynamic (PD) properties and safety profiles of a newly developed amlodipine formulation with a conventional formulation in healthy male subjects.

METHODS

This randomized, open-label, 2-period crossover comparative study was conducted at the Clinical Trial Center, Gil Medical Center, Gachon Medical School (Incheon, Korea). Eighteen healthy male Korean subjects aged 20 to 40 years were enrolled. All subjects received a single oral dose (5-mg tablet) of a conventional (reference) or newly developed (test) amlodipine formulation. Blood samples for PK analysis of amlodipine were obtained during the 144-hour period after dosing. Systolic and diastolic blood pressure (BP) (SBP and DBP, respectively) and pulse rate (PR) were measured just before each blood sampling. Assessment of safety profiles, including hematology and biochemistry, electrocardiography, urinalysis, and monitoring of adverse events (AEs), was performed.

RESULTS

All participants completed both treatment periods. Their mean (SD) age was 22.3 (1.5) years (range, 20-25 years) and their mean (SD) body weight was 67.9 (5.6) kg (range, 57-77 kg). The plasma concentration-time profiles of amlodipine were similar after administration of the 2 formulations. The reference and test formulations were pharmacokinetically equivalent. The 90% CIs for the mean treatment ratios of the log-transformed peak plasma concentration and the area under the plasma concentration-time curve were within the predetermined equivalence range of 80% to 125%. Despite administration of a single dose, significant maximal changes in SBP, DBP, and PR were achieved after drug administration for both formulations compared with baseline values (all, P < 0.001). No significant differences in PD profiles were found between the 2 formulations. No clinically relevant changes were observed in physical, biochemical, hematologic, electrocardiographic, or urinalysis findings during the study. Neither formulation caused any AEs during the study.

CONCLUSIONS

The 2 amlodipine formulations were pharmacokinetically equivalent and showed similar PD characteristics in these healthy male subjects.

Food-drug interaction: grapefruit juice augments drug bioavailability--mechanism, extent and relevance

More than a decade has passed since it was unintentionally discovered that grapefruit juice interacts with certain drugs. The coadministration of these drugs with grapefruit juice can markedly elevate drug bioavailability, and can alter pharmacokinetic and pharmacodynamic parameters of the drug. The predominant mechanism for this interaction is the inhibition of cytochrome P-450 3A4 in the small intestine, resulting in a significant reduction of drug presystemic metabolism. An additional mechanism is, presumably, the inhibition of P-glycoprotein, a transporter that carries drug from the enterocyte back to the gut lumen, resulting in a further increase in the fraction of drug absorbed. Some calcium channel antagonists, benzodiazepines, HMG-CoA reductase inhibitors and cyclosporine are the most affected drugs. A single exposure to one glass of the juice can usually produce the maximal magnitude of the interaction. The data available so far, concerning this interaction and its clinical implications, are reviewed in this article. It is likely that more information regarding this interaction will accumulate in the future, and awareness of such is necessary for achieving optimal drug therapy.

Antihypertensive therapy: special focus on drug interactions

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

Herbal bioactivation: the good, the bad and the ugly

It has been well established that the formation of reactive metabolites of drugs is associated with drug toxicity. Similarly, there are accumulating data suggesting the role of the formation of reactive metabolites/intermediates through bioactivation in herbal toxicity and carcinogenicity. It has been hypothesized that the resultant reactive metabolites following herbal bioactivation covalently bind to cellular proteins and DNA, leading to toxicity via multiple mechanisms such as direct cytotoxicity, oncogene activation, and hypersensitivity reactions. This is exemplified by aristolochic acids present in Aristolochia spp, undergoing reduction of the nitro group by hepatic cytochrome P450 (CYP1A1/2) or peroxidases in extrahepatic tissues to reactive cyclic nitrenium ion. The latter was capable of reacting with DNA and proteins, resulting in activation of H-ras oncogene, gene mutation and finally carcinogenesis. Other examples are pulegone present in essential oils from many mint species; and teucrin A, a diterpenoid found in germander (Teuchrium chamaedrys) used as an adjuvant to slimming diets. Extensive pulegone metabolism generated p-cresol that was a glutathione depletory, and the furan ring of the diterpenoids in germander was oxidized by CYP3A4 to reactive epoxide which reacts with proteins such as CYP3A and epoxide hydrolase. On the other hand, some herbal/dietary constituents were shown to form reactive intermediates capable of irreversibly inhibiting various CYPs. The resultant metabolites lead to CYP inactivation by chemical modification of the heme, the apoprotein, or both as a result of covalent binding of modified heme to the apoprotein. Some examples include bergamottin, a furanocoumarin of grapefruit juice; capsaicin from chili peppers; glabridin, an isoflavan from licorice root; isothiocyanates found in all cruciferous vegetables; oleuropein rich in olive oil; dially sulfone found in garlic; and resveratrol, a constituent of red wine. CYPs have been known to metabolize more than 95% therapeutic drugs and activate a number of procarcinogens as well. Therefore, mechanism-based inhibition of CYPs may provide an explanation for some reported herb-drug interactions and chemopreventive activity of herbs. Due to the wide use and easy availability of herbal medicines, there is increasing concern about herbal toxicity. The safety and quality of herbal medicine should be ensured through greater research, pharmacovigilance, greater regulatory control and better communication between patients and health professionals.

Interactions between grapefruit juice and cardiovascular drugs

Grapefruit juice can alter oral drug pharmacokinetics by different mechanisms. Irreversible inactivation of intestinal cytochrome P450 (CYP) 3A4 is produced by commercial grapefruit juice given as a single normal amount (e.g. 200-300 mL) or by whole fresh fruit segments. As a result, presystemic metabolism is reduced and oral drug bioavailability increased. Enhanced oral drug bioavailability can occur 24 hours after juice consumption. Inhibition of P-glycoprotein (P-gp) is a possible mechanism that increases oral drug bioavailability by reducing intestinal and/or hepatic efflux transport. Recently, inhibition of organic anion transporting polypeptides by grapefruit juice was observed in vitro; intestinal uptake transport appeared decreased as oral drug bioavailability was reduced. Numerous medications used in the prevention or treatment of coronary artery disease and its complications have been observed or are predicted to interact with grapefruit juice. Such interactions may increase the risk of rhabdomyolysis when dyslipidemia is treated with the HMG-CoA reductase inhibitors atorvastatin, lovastatin, or simvastatin. Potential alternative agents are pravastatin, fluvastatin, or rosuvastatin. Such interactions might also cause excessive vasodilatation when hypertension is managed with the dihydropyridines felodipine, nicardipine, nifedipine, nisoldipine, or nitrendipine. An alternative agent could be amlodipine. In contrast, the therapeutic effect of the angiotensin II type 1 receptor antagonist losartan may be reduced by grapefruit juice. Grapefruit juice interacting with the antidiabetic agent repaglinide may cause hypoglycemia, and interaction with the appetite suppressant sibutramine may cause elevated BP and HR. In angina pectoris, administration of grapefruit juice could result in atrioventricular conduction disorders with verapamil or attenuated antiplatelet activity with clopidrogel. Grapefruit juice may enhance drug toxicity for antiarrhythmic agents such as amiodarone, quinidine, disopyramide, or propafenone, and for the congestive heart failure drug, carvediol. Some drugs for the treatment of peripheral or central vascular disease also have the potential to interact with grapefruit juice. Interaction with sildenafil, tadalafil, or vardenafil for erectile dysfunction, may cause serious systemic vasodilatation especially when combined with a nitrate. Interaction between ergotamine for migraine and grapefruit juice may cause gangrene or stroke. In stroke, interaction with nimodipine may cause systemic hypotension. If a drug has low inherent oral bioavailability from presystemic metabolism by CYP3A4 or efflux transport by P-gp and the potential to produce serious overdose toxicity, avoidance of grapefruit juice entirely during pharmacotherapy appears mandatory. Although altered drug response is variable among individuals, the outcome is difficult to predict and avoiding the combination will guarantee toxicity is prevented. The elderly are at particular risk, as they are often prescribed medications and frequently consume grapefruit juice.

Pharmacokinetics of amlodipine in hypertensive patients undergoing haemodialysis

OBJECTIVE

Amlodipine is a calcium antagonist of the dihydropyridine class. This study was performed to examine the single and multiple dose pharmacokinetics of amlodipine in hypertensive patients undergoing haemodialysis and to measure the index of accumulation during multiple dosage. An additional objective was to examine the dialysability of amlodipine by measuring its appearance in dialysate.

METHODS

Seventeen hypertensive out-patients on haemodialysis were enrolled in this prospective, open, non-comparative phase-IV study. Fifteen patients completed the study. In the first part of the study, the patients were given a single oral 5-mg dose of amlodipine and blood samples taken 0, 0.5, 1, 2, 3, 4, 6, 8, 10, 12, 24, 72 and 144 h later. After a 4-week wash-out period, the patients were given a daily 5-mg oral dose of amlodipine for 30 days. The trough levels of amlodipine were measured on days 8, 15, 22 and 29. On day 30, the total time course of amlodipine concentrations in plasma was measured, as in the first part of the study. In addition, on day 15, the concentrations of amlodipine in dialysate were measured at the same time as in blood. Blood pressure and heart rate were measured and adverse events monitored. Pharmacokinetic parameters [Cmax, tmax, kel and AUC(0-24)] were calculated for the single dose and for the dose on day 30 and the accumulation index calculated on the basis of AUC(0-24) and Cmax.

RESULTS

The mean values of the single-dose pharmacokinetic parameters were as follows: Cmax 3.83 microg l(-1), tmax 5.01, AUC(0-24) 59.90 microg l(-1) h, kel 0.0177 h(-1). The mean accumulation index on the basis of AUC(0-24) was calculated as 3.70. Very low levels of amlodipine were detectable in dialysate fluid. The most frequently reported adverse events were hypertension, hypotension and muscle cramps.

CONCLUSION

There are only minor differences in the pharmacokinetics of amlodipine between healthy subjects and hypertensive patients on haemodialysis. Comparison with literature values for healthy volunteers suggests that amlodipine is rapidly and extensively absorbed in the patient group. Amlodipine is essentially not dialysable. These findings do not indicate a need for dose adjustment in renal failure patients on haemodialysis.

Interactions between antiretroviral drugs and drugs used for the therapy of the metabolic complications encountered during HIV infection

Treatment of HIV infection with potent combination antiretroviral therapy has resulted in major improvement in overall survival, immune function and the incidence of opportunistic infections. However, HIV infection and treatment has been associated with the development of metabolic complications, including hyperlipidaemia, diabetes mellitus, hypertension, lipodystrophy and osteopenia. Safe pharmacological treatment of these complications requires an understanding of the drug-drug interactions between antiretroviral drugs and the drugs used in the treatment of metabolic complications. Since formal studies of most of these interactions have not been performed, predictions must be based on our understanding of the metabolism of these agents. All HIV protease inhibitors are metabolised by and inhibit cytochrome P450 (CYP) 3A4. Ritonavir is the most potent inhibitor of CYP3A4. Ritonavir and nelfinavir also induce a host of CYP isoforms as well as some conjugating enzymes. The non-nucleoside reverse transcriptase inhibitor delavirdine potently inhibits CYP3A4, whereas nevirapine and efavirenz are inducers of CYP3A4. Drug interaction studies have been performed with HIV protease inhibitors and HMG-CoA reductase inhibitors. Coadministration of ritonavir plus saquinavir to HIV-seronegative volunteers resulted in increased exposure to simvastatin acid by 3059%. Atorvastatin exposure increased by 347%, but exposure to active atorvastatin increased by only 79%. Conversely, pravastatin exposure decreased by 50%. Similar results have been obtained with combinations of simvastatin and atorvastatin with other HIV protease inhibitors. Thus, the lactone prodrugs simvastatin and lovastatin should not be used with HIV protease inhibitors. Atorvastatin may be used with caution. Although there are no formal studies available, calcium channel antagonists and repaglinide may have significant interactions and toxicity when used with HIV protease inhibitors because of their metabolism by CYP3A4. Sulfonylurea drugs utilise mainly CYP2C9 for metabolism, and this isoenzyme may be induced by ritonavir and nelfinavir with a resulting decrease in efficacy of the sulfonylurea. Losartan may have increased effect when coadministered with ritonavir and nelfinavir because of the induction of CYP2C9 and the expected increase in formation of the active metabolite, E-3174. Overall, well-designed drug-drug interaction studies at steady state are needed to determine whether antiretroviral drugs may be safely coadministered with many of the drugs used in the treatment of the metabolic complications of HIV infection.

Interactions of herbs with cytochrome P450

A resurgence in the use of medical herbs in the Western world, and the co-use of modern and traditional therapies is becoming more common. Thus there is the potential for both pharmacokinetic and pharmacodynamic herb-drug interactions. For example, systems such as the cytochrome P450 (CYP) may be particularly vulnerable to modulation by the multiple active constituents of herbs, as it is well known that the CYPs are subject to induction and inhibition by exposure to a wide variety of xenobiotics. Using in vitro, in silico, and in vivo approaches, many herbs and natural compounds isolated from herbs have been identified as substrates, inhibitors, and/or inducers of various CYP enzymes. For example, St. John's wort is a potent inducer of CYP3A4, which is mediated by activating the orphan pregnane X receptor. It also contains ingredients that inhibit CYP1A2, CYP2C9, CYP2C19, CYP2D6, and CYP3A4. Many other common medicinal herbs also exhibited inducing or inhibiting effects on the CYP system, with the latter being competitive, noncompetitive, or mechanism-based. It appears that the regulation of CYPs by herbal products complex, depending on the herb type, their administration dose and route, the target organ and species. Due to the difficulties in identifying the active constituents responsible for the modulation of CYP enzymes, prediction of herb-drug metabolic interactions is difficult. However, herb-CYP interactions may have important clinical and toxicological consequences. For example, induction of CYP3A4 by St. John's wort may partly provide an explanation for the enhanced plasma clearance of a number of drugs, such as cyclosporine and innadivir, which are known substrates of CYP3A4, although other mechanisms including modulation of gastric absorption and drug transporters cannot be ruled out. In contrast, many organosulfur compounds, such as diallyl sulfide from garlic, are potent inhibitors of CYP2E1; this may provide an explanation for garlic's chemoproventive effects, as many mutagens require activation by CYP2E1. Therefore, known or potential herb-CYP interactions exist, and further studies on their clinical and toxicological roles are warranted. Given that increasing numbers of people are exposed to a number of herbal preparations that contain many constituents with potential of CYP modulation, high-throughput screening assays should be developed to explore herb-CYP interactions.

Interactions of grapefruit juice and cardiovascular medications: A potential risk of toxicity

Recently, drug interactions with grapefruit juice (GFJ) have received considerable attention from basic scientists, physicians, industry and drug regulatory agencies. GFJ has been shown to inhibit cytochrome P-450 3A4 isoenzyme and P-glycoprotein transporters in the intestine and liver. The GFJ-induced inhibitory effects are considered to be responsible for alterations in drug bioavailability, and pharmacokinetic and pharmacodynamic changes when drugs are ingested concurrently with GFJ. However, little or no interaction is observed when GFJ is taken concomitantly with parentally administered drugs. It is well known that risk factors for cardiovascular disease increase with advancing age, while hepatic metabolic activity decreases in elderly individuals. It is, therefore, possible that the combination of GFJ and cardiovascular medications may pose a health risk, especially in elderly patients. A number of studies have shown interactions of GFJ with cardiovascular drugs such as calcium-channel blockers, angiotensin II receptor antagonists, beta-blockers, and statins. Such interactions are likely to change the pharmacokinetics and pharmacodynamics of these drugs, consequently causing undesirable health effects. Therefore, health care professionals and the public need to be advised of the potential risks associated with the concomitant use of GFJ and interacting medications, especially cardiovascular drugs and agents with a narrow therapeutic index. This review focuses on the adverse interactions of GFJ and cardiovascular medications, and the proposed underlying mechanisms of such interactions.

Stereoselective determination and pharmacokinetics of dihydropyridines: an updated review

All dihydropyridines, except nifedipine, have at least one chiral center, and their pharmacokinetics and clinical effects differ from one enantiomer to another. Chiral separation methods for dihydropyridines using chromatographic techniques are discussed. The stereoselective pharmacokinetics of dihydropyridine calcium antagonists were reviewed in detail in 1995. The present review article updates the methods for the stereoselective determination of dihydropyridines using chromatographic techniques and summarizes the pharmacokinetics of the dihydropyridines, including the newest drugs under development.