Further insight into the stereoselective interaction between warfarin and cimetidine in man

Effects of Cimetidine, Ranitidine and Omeprazole on Tolbutamide Pharmacokinetics

This randomized, four-way crossover study in 16 healthy subjects compared the effects of cimetidine 800 mg, ranitidine 300 mg and omeprazole 40 mg against placebo given daily after breakfast for seven days on the pharmacokinetics of a single oral dose of tolbutamide (500 mg) given on day 4.

Plasma tolbutamide and urinary hydroxytolbutamide and carboxytolbutamide concentrations were determined by HPLC. Ranitidine had no significant effects on tolbutamide metabolism. Cimetidine produced a 20% increase in AUC (P < 0·001) and a 14% increase in t 1/2 (P < 0·01), while omeprazole produced a 10% increase in AUC (P < 001). The effect of these agents on urinary concentrations of the tolbutamide metabolites was small.

These results do not indicate that interactions of major clinical significance occur in healthy subjects.

Inhibitory effects of tricyclic antidepressants (TCAs) on human cytochrome P450 enzymes in vitro: mechanism of drug interaction between TCAs and phenytoin

The ability of tricyclic antidepressants (TCAs) to inhibit phenytoin p-hydroxylation was evaluated in vitro by incubation studies of human liver microsomes and cDNA-expressed cytochrome p450s (p450s). The TCAs tested were amitriptyline, imipramine, nortriptyline, and desipramine. Amitriptyline and imipramine strongly and competitively inhibited phenytoin p-hydroxylation in microsomal incubations (estimated K(i) values of 5.2 and 15.5 micro M, respectively). In contrast, nortriptyline and desipramine produced only weak inhibition. In the incubation study using cDNA-expressed P450s, both CYP2C9 and CYP2C19 catalyzed phenytoin p-hydroxylation, whereas TCAs inhibited only the CYP2C19 pathway. All of the TCAs tested inhibited CYP2D6-catalyzed dextromethorphan-O-demethylation competitively, with estimated K(i) values of 31.0, 28.6, 7.9, and 12.5 micro M, respectively. The tertiary amine TCAs, amitriptyline and imipramine, also inhibited CYP2C19-catalyzed S-mephenytoin 4'-hydroxylation (estimated K(i) of 37.7 and 56.8 micro M, respectively). The secondary amine TCAs, nortriptyline and desipramine, however, showed minimal inhibition of CYP2C19 (estimated IC(50) of 600 and 685 micro M, respectively). None of the TCAs tested produced remarkable inhibition of any other p450 isoforms. These results suggest that TCAs inhibit both CYP2D6 and CYP2C19 and that the interaction between TCAs and phenytoin involves inhibition of CYP2C19-catalyzed phenytoin p-hydroxylation.

Beraprost sodium-fluindione combination in healthy subjects: pharmacokinetic and pharmacodynamic aspects

Beraprost sodium (BPS), an orally active PGI2 (prostaglandine 12) analogue possesses vasodilatating and platelet aggregation inhibiting properties. It is being developed in peripheral arterial occlusive disease. As in future clinical practice BPS might be co-prescribed with oral anticoagulants, we investigated its interaction with fluindione, a vitamin K antagonist in healthy subjects in a randomised, double-blind, placebo-controlled, crossover study. Twelve healthy Caucasian male subjects randomly received BPS 40 microg t.i.d. or placebo for 3 days. There was a 7 day wash out between the two treatment periods. On day 3 of each treatment, the subjects ingested concomitantly a single oral dose of 20 mg of fluindione. The main assessment criterion was fluindione's pharmacokinetics. Secondarily, pharmacodynamic measurements of coagulation (prothrombin time, and International Normalised Ratio, INR) and platelet function (in vitro closure time assessed by PFA-100) were performed. Fluindione was assayed by HPLC with UV detection up to 96 h post-drug. No statistical difference could be evidenced on any fluindione pharmacokinetic parameters between BPS and placebo phases: t 1/2 (h): 35.9 (8.2) vs. 34.0 (4.2) [90% CI 105.8 (95.5-116.2)]; T(max) (h): 2.0 (0.5-6.0) vs. 4.0 (0.5-6.0) [90% CI 136.4 (70.7-208.9)]; Cmax (mg/L): 3.1 (0.6) vs. 2.9 (0.5) [90% CI 94.1 (85.8-103.2)]; AUC 0-inf (mg/h/L): 117.0 (31.5) vs. 113.9 (33.8) [90% CI 97.6 (87.5-108.8)]. The studied doses of BPS did not affect platelet function, at least as assessed by the in vitro platelet function testing. Twenty milligrams of fluindione marginally modified the PT ratio and INR, however, no statistically significant difference was found between BPS and placebo phases. In conclusion, a 3 day regimen of BPS 40 microg t.i.d. by oral route does not seem to affect pharmacokinetic parameters of a fluindione 20 mg single dose.

Inhibition and induction of cytochrome P450 and the clinical implications

The cytochrome P450s (CYPs) constitute a superfamily of isoforms that play an important role in the oxidative metabolism of drugs. Each CYP isoform possesses a characteristic broad spectrum of catalytic activities of substrates. Whenever 2 or more drugs are administered concurrently, the possibility of drug interactions exists. The ability of a single CYP to metabolise multiple substrates is responsible for a large number of documented drug interactions associated with CYP inhibition. In addition, drug interactions can also occur as a result of the induction of several human CYPs following long term drug treatment. The mechanisms of CYP inhibition can be divided into 3 categories: (a) reversible inhibition; (b) quasi-irreversible inhibition; and (c) irreversible inhibition. In mechanistic terms, reversible interactions arise as a result of competition at the CYP active site and probably involve only the first step of the CYP catalytic cycle. On the other hand, drugs that act during and subsequent to the oxygen transfer step are generally irreversible or quasi-irreversible inhibitors. Irreversible and quasi-irreversible inhibition require at least one cycle of the CYP catalytic process. Because human liver samples and recombinant human CYPs are now readily available, in vitro systems have been used as screening tools to predict the potential for in vivo drug interaction. Although it is easy to determine in vitro metabolic drug interactions, the proper interpretation and extrapolation of in vitro interaction data to in vivo situations require a good understanding of pharmacokinetic principles. From the viewpoint of drug therapy, to avoid potential drug-drug interactions, it is desirable to develop a new drug candidate that is not a potent CYP inhibitor or inducer and the metabolism of which is not readily inhibited by other drugs. In reality, drug interaction by mutual inhibition between drugs is almost inevitable, because CYP-mediated metabolism represents a major route of elimination of many drugs, which can compete for the same CYP enzyme. The clinical significance of a metabolic drug interaction depends on the magnitude of the change in the concentration of active species (parent drug and/or active metabolites) at the site of pharmacological action and the therapeutic index of the drug. The smaller the difference between toxic and effective concentration, the greater the likelihood that a drug interaction will have serious clinical consequences. Thus, careful evaluation of potential drug interactions of a new drug candidate during the early stage of drug development is essential.

Cytochrome P4502C9: an enzyme of major importance in human drug metabolism

Accumulating evidence indicates that CYP2C9 ranks amongst the most important drug metabolizing enzymes in humans. Substrates for CYP2C9 include fluoxetine, losartan, phenytoin, tolbutamide, torsemide, S-warfarin, and numerous NSAIDs. CYP2C9 activity in vivo is inducible by rifampicin. Evidence suggests that CYP2C9 substrates may also be induced variably by carbamazepine, ethanol and phenobarbitone. Apart from the mutual competitive inhibition which may occur between alternate substrates, numerous other drugs have been shown to inhibit CYP2C9 activity in vivo and/or in vitro. Clinically significant inhibition may occur with coadministration of amiodarone, fluconazole, phenylbutazone, sulphinpyrazone, sulphaphenazole and certain other sulphonamides. Polymorphisms in the coding region of the CYP2C9 gene produce variants at amino acid residues 144 (Arg144Cys) and 359 (Ile359Leu) of the CYP2C9 protein. Individuals homozygous for Leu359 have markedly diminished metabolic capacities for most CYP2C9 substrates, although the frequency of this allele is relatively low. Consistent with the modulation of enzyme activity by genetic and other factors, wide interindividual variability occurs in the elimination and/or dosage requirements of prototypic CYP2C9 substrates. Individualisation of dose is essential for those CYP2C9 substrates with a narrow therapeutic index.

The effect of cimetidine on the formation of sulfamethoxazole hydroxylamine in patients with human immunodeficiency virus

Hypersensitivity reactions from trimethoprim/sulfamethoxazole are likely caused by a reactive nitroso intermediate formed from sulfamethoxazole hydroxylamine. This pilot study tested whether cimetidine inhibits the urinary excretion of sulfamethoxazole hydroxylamine. Ten outpatients infected with human immunodeficiency virus (HIV) and currently receiving trimethoprim/sulfamethoxazole prophylaxis were randomly selected from 59 eligible patients. Five received cimetidine 800 mg twice daily for 1 week and five served as controls. Two spot urine samples one week apart were obtained after a trimethoprim/sulfamethoxazole dose for all patients. Patients taking cimetidine had a significant decrease in excretion of sulfamethoxazole hydroxylamine relative to total excreted drug in the two urine samples compared with control patients. Cimetidine likely caused this decrease in sulfamethoxazole hydroxylamine excretion through inhibition of CYP3A4. Because of potential differences between HIV-infected patients and healthy subjects in oxidative metabolism, future studies of inhibitors of sulfamethoxazole hydroxylamine formation should be conducted in the HIV population.

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.

Stereochemistry in pharmacotherapy: when mirror images are not identical

OBJECTIVE

To describe how drug stereoisomers may differ in pharmacokinetic and pharmacodynamic properties and how these differences may affect therapeutic outcomes.

STUDY SELECTION

Representative studies were chosen from the drug literature demonstrating stereoisomeric differences in drug absorption, protein binding and distribution, metabolism, and elimination. Furthermore, examples of pharmacodynamic differences between drug stereoisomers are presented to demonstrate that these stereoisomers not only may differ in pharmacologic potency, but may possess entirely different pharmacologic actions.

DATA SYNTHESIS

Examples are presented demonstrating that when stereoiosomeric differences in pharmacokinetics are linked to pharmacodynamic differences, alterations in therapeutic effect can result. Additionally, drug interactions are discussed in which 1 isomer is affected to a greater extent than the other, potentially causing not only an increase or decrease in effect, but also a change in pharmacologic action. Examples also are presented of the marketing of single isomer entities, with a discussion of the use of these products. Finally, preliminary policies of the Food and Drug Administration are discussed, as well as the potential implications of these policies.

CONCLUSIONS

Drugs that are administered as stereoisomers can differ with respect to both pharmacokinetics and pharmacodynamics, and these differences may have profound implications in pharmacotherapy. All future investigations of drugs that exist as stereoisomers must take into account the pharmacokinetics and pharmacodynamics of both isomers to understand fully the observed phenomena.

Clinically significant drug interactions with the oral anticoagulants

Oral anticoagulants were introduced in the late 1940s and remain widely used today. Indications include prevention of thrombosis associated with atrial fibrillation, structural cardiac diseases and following prosthetic valvular replacement. They have been used for both treatment and prophylaxis of deep venous thrombosis and in efforts to decrease the frequency and rate of second myocardial infarction. These compounds include the coumarin derivatives [dicoumarol (bishydroxycoumarin), phenprocoumon, nicoumalone (acenocoumarol)] and the indanedione derivatives (diphenadione, phenindione, anisindione) which, because of adverse reactions, are largely unavailable. The oral anticoagulants, and warfarin in particular, are highly interactive with other drugs. Mechanisms of those interactions include both pharmacokinetic and pharmacodynamic mechanisms and may result in either hyper- or hypoprothrombinaemia. Because their principal adverse reaction is haemorrhage, and interactions are widespread across many therapeutic specialties, it becomes imperative for the practising physician to be aware of the possibility of interaction whenever these agents are coadministered with other drugs.

Human liver microsomal metabolism of the enantiomers of warfarin and acenocoumarol: P450 isozyme diversity determines the differences in their pharmacokinetics

1. To explain the large differences in (the stereoselectivity of) the clearances of the enantiomers of warfarin and acenocoumarol (4'-nitrowarfarin) their human liver microsomal metabolism has been studied and enzyme kinetic parameters determined. The effects of cimetidine, propafenone, sulphaphenazole, and omeprazole on their metabolism has been investigated. 2. The 4-hydroxycoumarins follow similar metabolic routes and are mainly hydroxylated at the 6- and 7-position (accounting for 63 to 99% of the metabolic clearances). 3. Due to the lower Km values of R- and S-acenocoumarol and higher Vmax values of S-acenocoumarol, the overall metabolic clearances of R/S acenocoumarol exceed those of R/S warfarin 6 and 66 times respectively. 4. The metabolism of both compounds is stereoselective for the S-enantiomers, which is 10 times more pronounced in the case of acenocoumarol. 5. Except for the 7-hydroxylation of the R-enantiomers (r = 0.90; P < 0.025), the 6- and 7-hydroxylation rates of R/S warfarin do not correlate with those of R/S acenocoumarol. 6. Sulphaphenazole competitively inhibits the 7- and in some samples partly (up to 50%) the 6-hydroxylation of S-warfarin as well as the 7-hydroxylation of R- and S-acenocoumarol and the 6-hydroxylation of S-acenocoumarol (Kis ranging from 0.5-1.3 microM). 7. Omeprazole partly (40-80%) inhibits the 6- and 7-hydroxylation of R-warfarin (Ki = 99 and 117 microM) and of R- (Ki = 219 and 7.2 microM) and S-acenocoumarol (Ki = 6.1 and 7.7 microM) but not S-warfarin in a competitive manner. 8. Differences in the partial (up to 40%) inhibition of the metabolism of the enantiomers of the 4-hydroxycoumarins were also observed for the relatively weak inhibitors, propafenone and cimetidine.9. The results suggest that the coumarin ring hydroxylations of both compounds are catalysed by different combinations of P450 isozymes. The 7-hydroxylation of R/S acenocoumarol and the 6-hydroxylation of S-acenocoumarol are at least partly conducted by (a) P450 isozyme(s) of the 2C subfamily different from P450 2C9 (the main S-warfarin 7- and 6-hydroxylase).

Stereoselective and isozyme-selective drug interactions

A surprisingly large number of marketed drugs are racemic mixtures. The pharmacokinetic literature on racemic drugs contains a vast amount of information on drug-drug interactions derived from the measurement of total drug concentrations in plasma and urine. The appreciation of the role of stereochemistry in drug interactions with racemic warfarin resulted in a long-overdue scientific rigor being applied to the study of drug interactions. It also compelled us to recognize that much of the literature was uninterpretable. A better understanding of oxidative metabolism, particularly the complexity of the cytochrome P-450 family of enzymes, has also strengthened the scientific basis of drug interactions. We now recognize that investigators and clinicians must consider both stereoselectivity and isozyme selectivity in the study of drug interactions to understand the nature of the interaction so as to more effectively use new and potent drugs.