Comparative bioavailability of a microsize and ultramicrosize griseofulvin formulation in man

The Biopharmaceutics Classification System: subclasses for in vivo predictive dissolution (IPD) methodology and IVIVC

The Biopharmaceutics Classification System (BCS) has found widespread utility in drug discovery, product development and drug product regulatory sciences. The classification scheme captures the two most significant factors influencing oral drug absorption; solubility and intestinal permeability and it has proven to be a very useful and a widely accepted starting point for drug product development and drug product regulation. The mechanistic base of the BCS approach has, no doubt, contributed to its wide spread acceptance and utility. Nevertheless, underneath the simplicity of BCS are many detailed complexities, both in vitro and in vivo which must be evaluated and investigated for any given drug and drug product. In this manuscript we propose a simple extension of the BCS classes to include sub-specification of acid (a), base (b) and neutral (c) for classes II and IV. Sub-classification for Classes I and III (high solubility drugs as currently defined) is generally not needed except perhaps in border line solubility cases. It is well known that the , pKa physical property of a drug (API) has a significant impact on the aqueous solubility dissolution of drug from the drug product both in vitro and in vivo for BCS Class II and IV acids and bases, and is the basis, we propose for a sub-classification extension of the original BCS classification. This BCS sub-classification is particularly important for in vivo predictive dissolution methodology development due to the complex and variable in vivo environment in the gastrointestinal tract, with its changing pH, buffer capacity, luminal volume, surfactant luminal conditions, permeability profile along the gastrointestinal tract and variable transit and fasted and fed states. We believe this sub-classification is a step toward developing a more science-based mechanistic in vivo predictive dissolution (IPD) methodology. Such a dissolution methodology can be used by development scientists to assess the likelihood of a formulation and dosage form functioning as desired in humans, can be optimized along with parallel human pharmacokinetic studies to set a dissolution methodology for Quality by Design (QbD) and in vitro-in vivo correlations (IVIVC) and ultimately can be used as a basis for a dissolution standard that will ensure continued in vivo product performance.

Pharmacokinetics of antifungal agents in onychomycoses

Onychomycosis is caused by infection by fungi, mainly dermatophytes and nondermatophyte yeasts or moulds; it affects the fingernails and, more frequently, the toenails. Dermatophytes are responsible for about 90 to 95% of fungal infections. Trichophyton rubrum is the most common dermatophyte; Candida albicans is the major nondermatophyte yeast. Although topical therapy of onchomycosis does not lead to systemic adverse effects or interactions with concomitantly taken drugs, it does not provide high cure rates and requires complete compliance from the patient. At present there are 3 oral antifungal medications that are generally used for the short term treatment of onychomycosis: itraconazole, terbinafine and fluconazole. The persistence of these active drugs in nails allows weekly administration, reduced treatment or a pulse regimen. Good clinical and mycological efficacies are obtained with itraconazole 100 to 200 mg daily, terbinafine 250mg daily for 3 months, or fluconazole 150 mg weekly for at least 6 months. Itraconazole is a synthetic triazole with a broad spectrum of action. It is well absorbed when administered orally and can be detected in nails 1 to 2 weeks after the start of therapy. The nail : plasma ratio stabilises at around 1 by week 18 of treatment. Itraconazole is still detectable in nails 27 weeks after stopping administration. Nail concentrations are higher than the minimum inhibitory concentration (MIC) for most dermatophytes and Candida species from the first month of treatment. The elimination half-life of itraconazole from nails is long, ranging from 32 to 147 days. Terbinafine is a synthetic allylamine that is effective against dermatophytes. Terbinafine is well absorbed from the gastrointestinal tract, and the time to reach effective concentrations in nail is 1 to 2 weeks. The half-life is from 24 to 156 days, explaining the observed persistence of terbinafine in nails for longer than 252 days. Fluconazole is a bis-triazole broad spectrum antifungal with high oral bioavailability. The uptake of fluconazole by nail increases with the length of treatment, and nail : plasma ratios are generally 1.5 to 2 at steady state. Fluconazole concentrations exceed the MIC for Candida species soon after the start of treatment. Fluconazole concentrations fall slowly after the drug is stopped, with a half-life of 50 to 87 days, and fluconazole is still detectable in nails 5 months after the end of treatment. All these drugs are potent inhibitors of cytochrome P450 (CYP) enzymes and may increase the plasma concentrations of concomitantly used drugs. Itraconazole inhibits CYP3A4. Fluconazole inhibits CYP3A4, but to a lesser degree than itraconazole, CYP2C9 and CYP2C19. Terbinafine inhibits CYP2D6.

Pharmacokinetic optimisation of oral antifungal therapy

The range of oral antifungal therapy has been expanded recently by the introduction of itraconazole, and terbinafine. These agents have a broader spectrum of activity than griseofulvin and flucytosine, and induce less liver toxicity than ketoconazole. Treatment with these agents may be optimised by application of pharmacokinetic principles. Griseofulvin, ketoconazole and itraconazole should be administered with food to ensure adequate absorption. Maximal absorption of griseofulvin is achieved by administration of the drug as a solid solution in polyethylene glycol. Absorption of azole antifungal agents is impaired by high gastric pH, which is observed in some patients with acquired immunodeficiency syndrome. It is also impaired by frequent vomiting, which commonly occurs in patients with neutropenia. Furthermore, antacids, H2-antagonists and sucralfate interfere with absorption of ketoconazole. The newer oral antifungals are more slowly eliminated and associated with less pronounced drug interactions than ketoconazole. As with ketoconazole, itraconazole and fluconazole influence cyclosporin metabolism. These effects are of clinical relevance and necessitate cyclosporin dosage reduction. However, the cyclosporin dosage reduction required during coadministration of itraconazole and fluconazole (50 to 55%) is less than that required when ketoconazole is concomitantly administered (85%). Monitoring of cyclosporin concentrations during coadministration with these agents is necessary to avoid nephrotoxicity. Drug monitoring is also advisable when phenytoin, carbamazepine or rifampicin (rifampin) are administered concomitantly with azoles, due to a mutual influence on drug metabolism. The antifungal activity of itraconazole is not related exclusively to free drug concentrations. Therefore, the low protein binding of fluconazole does not place this agent at an advantage over itraconazole in the treatment of fungal meningitis. However, terbinafine may be superior to itraconazole for the treatment of tinea unguium, another recalcitrant fungal disease, because terbinafine more rapidly penetrates the nail plate. During repeated use, itraconazole concentrations increase slowly in the nail plate. Steady-state concentrations are reached in the stratum corneum only after several weeks' administration. Following cessation of treatment, terbinafine, itraconazole and ketoconazole concentrations in keratinised tissues decline slowly. This allows a short duration of drug treatment. Some clinical trials suggest that low concentrations of flucytosine, griseofulvin and itraconazole are associated with treatment failure. Flucytosine-induced myelotoxicity also appears to be concentration dependent. This adverse reaction may be caused by fluorouracil (which is produced by metabolism of flucytosine by enterobacillary flora in the gut) rather than by the parent compound.

Treatment of tinea unguium with medium and high doses of ultramicrosize griseofulvin compared with that with itraconazole

Toenail tinea is a very recalcitrant dermatosis. Griseofulvin at > or = 500 mg/day is the current medication of choice, but it is minimally successful. In a controlled open trial ultramicrosize griseofulvin (UMSG) at doses of 660 and 990 mg/day was compared with itraconazole at 100 mg/day in 109 patients. At 4-week intervals, the patients were evaluated for their clinical and mycological statuses and adverse reactions. Treatment was given for up to 18 months. Compliance was checked by tablet counting. Response (cure, partial cure, marked improvement) was analyzed by the intent-to-treat method. Cured and partially cured patients were followed up. Except for one early dropout, the toenails (mean, 6 to 7) were involved. Cure or partial cure was found in 6% (UMSG at 660 mg), 14% (UMSG at 990 mg), and 19% (itraconazole at 100 mg) of patients (P = 0.2097); marked improvement was found in 36, 44, and 39% of patients in the three treatment groups, respectively. Most patients had to be treated for 18 months. Failure was related to short medication periods (adverse drug reactions, dropout). While stable cure was not obtained with UMSG at 660 mg, the higher dose of UMSG and itraconazole gave stable cures in the other patients. Side effects of nausea, diarrhea, and headache were found in 20, 26, and 11 patients, respectively (P = 0.0028), and the numbers in whom medication had to be discontinued differed, too (P = 0.0137). While there was no major difference with glutamic-pyruvic transaminase and gamma-GT, total and low-density lipoprotein cholesterol levels declined slightly in the itraconazole group (P = 0.0357 and P = 0.0639, respectively, at 3 months). More than 70% of the patients had an average compliance of > or = 90%; four patients (two dropouts) were poor compliers. In conclusion, it appears questionable whether griseofulvin can continue to be considered the "gold standard" in the treatment of toenail tinea. At present, itraconazole at 100 mg shows better efficacy and is better tolerated.

Human plasma and skin blister fluid levels of griseofulvin following a single oral dose

Griseofulvin and 6-demethylgriseofulvin (6-DMG) in plasma, suction blister fluid (SBF) and cantharides blister fluid (CBF) and urinary excretion of 6-DMG, were evaluated following administration of single oral doses of an ultramicrosize and a microsize formulation of griseofulvin to 6 healthy volunteers. The bioavailability of griseofulvin was higher following the ultramicrosize formulation when 64% of the dose was recovered (via metabolites) versus 52% after the microsize preparation. Penetration into skin blister fluid was delayed as compared to plasma levels; the peak concentration in plasma was observed at 3-4 h, whereas griseofulvin in CBF increased up to 6 h. The terminal half-live was calculated from plasma levels to 9.3 h. The half-lives calculated from SBF and CBF concentrations were 9.2 and 9.8 h, respectively, (n = 5). In plasma 84% of griseofulvin was bound to proteins, predominantly to albumin; binding in SBF and CBF was 72 and 82%, respectively. 3 h after drug administration the free concentration in plasma significantly exceeded the free concentrations in SBF and CBF. Distribution equilibrium between plasma and skin blister fluid was observed after 27 h. Thus, during chronic administration, the plasma griseofulvin level should reflect its concentration in the target organ.