Phenytoin intoxication during treatment with parenteral miconazole

Risk prediction of drug-drug interaction potential of phenytoin and miconazole topical formulations

The drug-drug interaction (DDI) risk of phenytoin with several topical formulations of miconazole is still unclear. The present investigation conducted in vitro-in vivo extrapolation to predict the potential risks. Our data indicated that miconazole potently inhibited phenytoin hydroxylation in both pooled human liver microsomes (HLMs) and recombinant cytochrome P450 2C9 (CYP2C9) with the K_i values of 125 ± 7 nM and 30 ± 2 nM, respectively. Quantitative prediction of DDI risk suggests that, beside intravenous administration or swallowed tablet, combination of phenytoin and miconazole high dose oral gel or buccal tablet may also result in a clinically significant increase of phenytoin AUC (>53%) by the inhibition of miconazole against phenytoin hydroxylation, consequently a higher frequency of adverse events, while the coadministration of miconazole vaginal formulation and phenytoin will be safe.

Miconazole oral gel and drug interactions

Miconazole oral gel is frequently prescribed for the treatment of oral Candidal infections. Its ability to be systemically absorbed and interact with other drugs has previously been recorded but is not universally known. As a reminder, a further case of derangement of anticoagulation following concomitant use of warfarin and miconazole is reported. Other potential drug interactions of miconazole and fluconazole are highlighted.

Clinically important drug interactions in epilepsy: interactions between antiepileptic drugs and other drugs

Antiepileptic drugs (AEDs) are commonly prescribed for long periods, up to a lifetime, and many patients will require treatment with other agents for the management of concomitant or intercurrent conditions. When two or more drugs are prescribed together, clinically important interactions can occur. Among old-generation AEDs, carbamazepine, phenytoin, phenobarbital, and primidone are potent inducers of hepatic enzymes, and decrease the plasma concentration of many psychotropic, immunosuppressant, antineoplastic, antimicrobial, and cardiovascular drugs, as well as oral contraceptive steroids. Most new generation AEDs do not have clinically important enzyme inducing effects. Other drugs can affect the pharmacokinetics of AEDs; examples include the stimulation of lamotrigine metabolism by oral contraceptive steroids and the inhibition of carbamazepine metabolism by certain macrolide antibiotics, antifungals, verapamil, diltiazem, and isoniazid. Careful monitoring of clinical response is recommended whenever a drug is added or removed from a patient's AED regimen.

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.

Down-regulation of mammalian 3-hydroxy-3-methylglutaryl coenzyme A reductase activity with highly purified liposomal cholesterol

Chinese hamster ovary-215 cells (CHO-215) cannot synthesize C27 and C28 sterols because of a defect in the reaction that decarboxylates 4-carboxysterols [Plemenitas, A., Havel, C.M. & Watson, J.A. (1990) J. Biol. Chem. 265, 17012-17017]. Thus, CHO-215 cell growth is dependent on an exogenous metabolically functional source of cholesterol. We used CHO-215 cells to (a) determine whether highly purified (> 99.5%) cholesterol, in egg lecithin liposomes, could down-regulate derepressed 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity and if so (b) determine whether the loss in reductase catalytic activity correlated kinetically with the synthesis and accumulation of detectable oxycholesterol derivatives. Liposomal cholesterol (26-39 microM) supported maximum CHO-215 growth and initiated suppression of HMG-CoA reductase activity at concentrations greater than 50 microM. Maximum suppression (50-60%) of reductase activity was achieved with 181.3 microM liposomal cholesterol in 6 h. Also, regulatory concentrations of highly purified liposomal [3H]cholesterol were not converted (biologically or chemically) to detectable levels of oxy[3H]cholesterol derivatives during 3-6 h incubations. Lastly, a broad-spectrum cytochrome P450 inhibitor (miconazole) had no effect on liposomal cholesterol-mediated suppression of HMG-CoA reductase activity. These observations established that (a) highly purified cholesterol, incorporated into egg lecithin liposomes, can signal the down-regulation of derepressed mammalian cell HMG-CoA reductase activity and (b) if oxycholesterol synthesis was required for liposomal cholesterol-mediated down-regulation, the products had to be more potent than 24-, 25-, or 26-/27-hydroxycholesterol.

Treatment of concomitant illnesses in patients receiving anticonvulsants: drug interactions of clinical significance

As epilepsy often is a chronic condition requiring prolonged therapy with anticonvulsants, patients being treated for epilepsy can be at risk when they are prescribed other drugs for concomitant diseases. Pharmacokinetic interactions can occur at each step of drug disposition (absorption, distribution, metabolism and elimination). Although such interactions may occur frequently with some drugs, only some will be clinically relevant. Alterations in the hepatic biotransformation of metabolised drugs due to hepatic isoenzyme induction or inhibition is of particular concern. The consequences of pharmacokinetic interactions are either accumulation of the drug leading to toxicity, or lowering of plasma concentrations resulting in reduced efficacy. Clinically relevant interactions depend on the structure, dosage and duration of administration of interacting agents, and on the individual's genetic make-up. In the past, drug interactions have been analysed empirically. At present, at least for interactions between drugs that are biotransformed in the liver, the risk should be predicted by considering the individual cytochrome P450 isoforms involved in the metabolism of coadministered drugs. Although drug-drug interactions can be predicted, their extent cannot be due to large interindividual variability. Even if nearly all drug combinations could be used with close clinical surveillance and blood concentration determinations, drugs that are not metabolised and are not highly protein bound, as are several of the new anticonvulsants, such as gabapentin, lamotrigine and vigabatrin, have a clear advantage in terms of a lower interaction potential.

Systemic antifungal agents. Drug interactions of clinical significance

There are 3 main classes of systemic antifungals: the polyene macrolides (e.g. amphotericin B), the azoles (e.g. the imidazoles ketoconazole and miconazole and the triazoles itraconazole and fluconazole) and the allylamines (e.g. terbinafine). Other systemic antifungals include griseofulvin and flucytosine. Most drug-drug interactions involving systemic antifungals have negative consequences. The interactions of amphotericin B, flucytosine, griseofulvin, terbinafine and azole antifungals can be divided into the following categories: (i) additive dangerous interactions; (ii) modifications of antifungal kinetics by other drugs; and (iii) modifications of the kinetics of other drugs by antifungals. Amphotericin B and flucytosine mainly interact with other agents pharmacodynamically. Clinically important drug interactions with amphotericin B cause nephrotoxicity, hypokalaemia and blood dyscrasias. The most important drug interaction of flucytosine occurs with myelotoxic agents. Hypokalaemia can precipitate the long QT syndrome, as well as potentially lethal ventricular arrhythmias like torsade de pointes. Synergism is likely to occur when either QT interval-modifying drugs (e.g. terfenadine and astemizole) and drugs that induce hypokalaemia (e.g. amphotericin B) are coadministered. Induction and inhibition of cytochrome P450 enzymes at hepatic and extrahepatic sites are the mechanisms that underlie the most serious pharmacokinetic drug interactions of the azole antifungals. These agents have been shown to notably decrease the catabolism of numerous drugs: histamine H1 receptor antagonists, warfarin, cyclosporin, tacrolimus, digoxin, felodipine, lovastatin, midazolam, triazolam, methylprednisolone, glibenclamide (glyburide), phenytoin, rifabutin, ritonavir, saquinavir, nevirapine and nortriptyline. Non-antifungal drugs like carbamazepine, phenobarbital (phenobarbitone), phenytoin and rifampicin (rifampin) can induce the metabolism of azole antifungals. The bioavailability of ketoconazole and itraconazole is also reduced by drugs that increase gastric pH, such as H2 receptor antagonists, proton pump inhibitors, sucralfate and didanosine. Griseofulvin is an enzymatic inducer of coumarin-like drugs and estrogens, whereas terbinafine seems to have a low potential for drug interactions. Despite important advances in our understanding of the mechanisms underlying pharmacokinetic drug interactions during the 1990s, at this time they still remain difficult to predict in terms of magnitude in individual patients. This is because of the large interindividual and intraindividual variations in the catalytic activity of those metabolising enzymes that can either be induced or inhibited by various drugs. Notwithstanding these variations, increasing clinical experience is allowing pharmacokinetic interactions to be used to advantage in order to improve the tolerability of some drugs, as recently exemplified by the use of a fixed combination of ketoconazole and cyclosporin.

In vivo pharmacokinetics and pharmacodynamics of topical ketoconazole and miconazole in human stratum corneum

A direct study evaluating whether differential drug uptake of topical 2% miconazole and 2% ketoconazole from cream formulations into human stratum corneum correlated with differential pharmacological activity against Candida albicans was investigated in healthy human subjects. A single 24-h topical dose of 2% ketoconazole cream or 2% miconazole cream was applied unoccluded, at the same dose (2.6 mg of formulation per cm2 of surface area), at four skin sites on both ventral forearms of six human subjects. At the end of the treatment, residual drug was removed with a tissue from all sites and the treated site was tape stripped 11 times, either 1, 4, 8, or 24 h later. The first tape disc was discarded. The remaining tape discs, 2 through 11, were combined and extracted for drug quantification by high-performance liquid chromatography and bioactivity against C. albicans growth in vitro. Topical 2% ketoconazole produced 14-, 10-, and 7-fold greater drug concentrations in stratum corneum than 2% miconazole at 1, 4, and 8 h after a single topical dose. Ketoconazole and miconazole concentrations in the stratum corneum were similar 24 h after drug removal. Tape disc extracts from 2% ketoconazole-treated skin sites demonstrated significantly greater bioactivity in the bioassay than 2% miconazole. The increased efficacy of 2% ketoconazole compared with that of 2% miconazole in vitro reflects their differential uptake into the stratum corneum and inherent pharmacological activity. Tape stripping the drug-treated site in conjunction with a bioassay is therefore a useful approach in the determination of bioavailability of topical antifungal agents.

Antiepileptic drugs. A review of clinically significant drug interactions

Approximately 20 to 30% of patients with active intractable epilepsy are commonly treated with polytherapy antiepileptic drug regimens, and these patients may experience complicated drug interactions. Furthermore, because of the long term nature of treatment, the possibility of drug interactions with drugs used for the treatment of concomitant disease is high. Classically, clinically significant drug interactions, both pharmacokinetic and pharmacodynamic, have been considered to be detrimental to the patient, necessitating dosage adjustment. However, this need not always be the case. With the introduction of new drugs (e.g. vigabatrin and lamotrigine) with known mechanisms of action, the possibility exists that these can be used synergistically. The most commonly observed clinically significant pharmacokinetic interactions can be attributed to interactions at the metabolic and serum protein binding levels. The best known examples relate to induction (e.g. phenobarbital, phenytoin, carbamazepine and primidone) or inhibition [e.g. valproic acid (sodium valproate)] of hepatic monoxygenase enzymes. The extent and direction of interactions between the different antiepileptic drugs are varied and unpredictable. Interactions in which the metabolism of phenobarbital, phenytoin or carbamazepine is inhibited are particularly important since these are commonly associated with toxicity. Some inhibitory drugs include macrolide antibiotics, chloramphenicol, cimetidine, isoniazid and numerous sulphonamides. A reduction in efficacy of antibiotic, cardiovascular, corticosteroid, oral anticoagulant and oral contraceptive drugs occurs during combination therapy with enzyme-inducing antiepileptic drugs. Discontinuation of the enzyme inducer or inhibitor will influence the concentrations of the remaining drug(s) and may necessitate dosage readjustment.

Adverse drug reactions to systemic antifungals. Prevention and management

Systemic administration of antifungal agents for invasive mycoses has dramatically increased over the past 10 years in many fields of medicine. The increase has been due both to an increasing immune compromised population and to potent antibacterial agents which allow these fungi to invade tissue. It is apparent that our understanding of the use of both the old and new antifungal agents has significantly increased in the last few years. In this review, we attempt to document our extensive knowledge of the adverse effects of the polyenes, flucytosine, griseofulvin and azoles when given systemically for treatment. Interwoven in this documentation of the adverse reactions to these agents is the attempt to help clinicians potentially avoid some of these adverse effects and if they do occur, to be able to identify and successfully manage them.

Antifungal agents used for deep-seated mycotic infections

The increased use of immunosuppressive regimens in organ transplantation and in the treatment of malignant lesions and the epidemic of acquired immunodeficiency syndrome (AIDS) are major reasons for the greater prevalence of fungal infections seen in clinical practice during the past decade. The traditional cornerstone of antifungal treatment, amphotericin B, continues to play a major role in deep-seated mycotic infections. The indications for intravenously administered miconazole have become limited. Orally administered flucytosine remains useful in certain infections, particularly cryptococcal meningitis. The new orally administered antifungal agents ketoconazole and fluconazole have been approved for clinical use and have supplanted amphotericin B in certain situations. Investigational antifungal agents, including liposomal amphotericin B, itraconazole, and saperconazole, hold promise for the future. Active investigation in the development of new antifungal agents is expected to continue.

Imidazoles and triazoles in antifungal therapy

Fungal infections range from superficial mycoses involving skin or mucous membranes to severe opportunistic infections that may be fatal. The selection of chemotherapeutic agents useful for the treatment of fungal infections includes many topical or systemic imidazoles and triazoles. This overview compares and contrasts the pharmacology and therapeutic use of these agents. This review will focus primarily on these agents' mechanism of action, pharmacokinetics, clinical studies, and adverse reactions.

Clinically relevant anti-epileptic drug interactions

Anti-epileptic drugs frequently interact due to pharmacokinetic features (induction or inhibition of metabolism, production of active metabolites, low therapeutic indices) and the need for prolonged treatment with possible addition of other drugs to treat concomitant diseases. The most important pharmacokinetic interactions are those that inhibit phenytoin, carbamazepine and phenobarbitone metabolism and thus increase their toxicity. Drugs inhibiting metabolism include antibiotic macrolides, chloramphenicol, isoniazide, some sulphonamides, propoxyphene, cimetidine, valproic acid and sulthiame. Anti-epileptic drugs can induce hepatic microsomal enzymes and, therefore, may increase metabolism of corticosteroids, oral contraceptives, oral anticoagulants, cardiovascular agents, antibiotics, chemotherapeutic agents, psychotropic drugs and non-opiate analgesics, thereby reducing their efficacy. Advantageous pharmacodynamic interactions include synergism of ethosuximide plus valproic acid and of carbamazepine plus valproic acid. A pharmacodynamic mechanism may be responsible for the reduced sensitivity of chronically treated epileptics to some neuromuscular blockers.

Cutaneous and immunologic reactions to phenytoin

Phenytoin (diphenylhydantoin; Dilantin) is a highly effective and widely prescribed anticonvulsant and antiarrhythmic agent. Since 1938 it has been invaluable in the treatment of grand mal and psychomotor epilepsy. Hydantoin derivatives have been used medicinally for more than a half-century. In recent years dermatologists have broadened the indications for phenytoin use to include recessive dystrophic epidermolysis bullosa, linear scleroderma, and pachyonychia congenita. In spite of widespread use and popularity, it is interesting that the frequency of complications relating to drug therapy remains low, relatively speaking. Nevertheless, a broad spectrum of cutaneous and immunologic reactions to phenytoin have been reported. These range from tissue proliferative syndromes (side effects), drug hypersensitivity syndromes (allergic effects), and a possible linkage with lymphoma (idiosyncratic effects). Therapeutic and toxic reactions to this commonly prescribed drug are comprehensively reviewed, analyzed, and summarized in this monograph.

Antifungal agents used for deep-seated mycotic infections

The main antifungal agents used for deep-seated mycotic infections are the broad-spectrum antifungal drug amphotericin B, the narrow-spectrum agent flucytosine, and the newer broad-spectrum agents ketoconazole, miconazole, and itraconazole. Amphotericin B remains the cornerstone of antifungal therapy. For the treatment of cryptococcal meningitis, the current recommendation is for the combined use of amphotericin B and flucytosine. Published clinical experience with the newer agents is limited. Not all patients from whom fungal agents have been isolated require treatment; the extent of the fungal infection should be determined, when possible, for evaluation of the need for treatment.

Omeprazole: effects on oxidative drug metabolism in the rat

Omeprazole, a substituted benzimidazole and a potent gastric antisecretory drug has been tested for inhibition of microsomal drug oxidative function in the rat. A single dose of 40 mg/kg prolonged pentobarbitone sleeping times from 118 (range 73-168) min to 195 (159-222) min (P less than 0.01), pentobarbitone half-lives from 89 (63-114) to 112 (54-146) min (P less than 0.05) and aminopyrine breath 14CO2 half-lives from 43 (37-51) to 56 (49-79) min (P less than 0.05). Omeprazole in doses of 20 mg/kg or less had no significant effect. In prolonging pentobarbitone sleeping times omeprazole 40 mg/kg and an equimolar (30 mg/kg) dose of cimetidine were approximately equipotent. These results contrast with studies in man in which much smaller doses of omeprazole have been shown to produce clinically significant inhibition of drug metabolism.

Pharmacokinetics of phenytoin in acute adult and child intoxication

The pharmacokinetics of phenytoin was studied in 4 acute intoxications. Two patients were identical twins aged 2 years 5 months being concomitantly poisoned, whereas one adult male was admitted twice. Their maximal phenytoin plasma concentrations were 246, 200, 168 and 164 mumol/l; the lowest values were in the twins. Despite this, consciousness in the children was more depressed whereas severe ataxia and involuntary movements dominated the course in the adult. All patients survived without any sequelae upon symptomatic treatment. Saturation kinetics of phenytoin could be demonstrated in all cases. For the twins, a Km in the range of 10.3-48.6 mumol/l was calculated, indicating saturation kinetics even within the therapeutic range of 40-80 mumol/l. In the twins, the Vmax was in the range of 47.3-79.4 mumol/l/day with a maximal elimination rate of 37.7-63.6 mumol/l/kg/day. We suggest that these kinetic parameters for phenytoin probably are independent of age.