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Aims Carbamazepine can cause hypersensitivity reactions in ~10% of patients. An immunogenic effect can be produced by the electrophilic 10,11-epoxide metabolite but not by carbamazepine. Hypothetically, certain single nucleotide polymorphisms might increase the formation of immunogenic metabolites, leading ultimately to hypersensitivity reactions. This study explores the role of clinical and genetic factors in the pharmacokinetics (PK) of carbamazepine and 3 metabolites known to be chemically reactive or formed through reactive intermediates. Methods A combination of rich and sparse PK samples were collected from healthy volunteers and epilepsy patients. All subjects were genotyped for 20 single nucleotide polymorphisms in 11 genes known to be involved in the metabolism or transport of carbamazepine and carbamazepine 10,11-epoxide. Nonlinear mixed effects modelling was used to build a population-PK model. Results In total, 248 observations were collected from 80 subjects. A 1-compartment PK model with first-order absorption and elimination best described the parent carbamazepine data, with a total clearance of 1.96 L/h, central distribution volume of 164 L and absorption rate constant of 0.45 h-1 . Total daily dose and coadministration of phenytoin were significant covariates for total clearance of carbamazepine. EPHX1-416G/G genotype was a significant covariate for the clearance of carbamazepine 10,11-epoxide. Conclusion Our data indicate that carbamazepine clearance was affected by total dose and phenytoin coadministration, but not by genetic factors, while carbamazepine 10,11-epoxide clearance was affected by a variant in the microsomal epoxide hydrolase gene. A much larger sample size would be required to fully evaluate the role of genetic variation in carbamazepine pharmacokinetics, and thereby predisposition to carbamazepine hypersensitivity.

Peroxides represent a new chemical class of antimalarial drugs (Figure 1). The first generation compounds are all derivatives of artemisinin, the active principle of a Chinese herbal remedy [1]. Artemisinin, or qinghaosu (green plant extract), is extracted from Artemesia annua, a plant which grows widely in South China. The first generation compounds currently in use are derivatives of the lactol, dihydroartemisinin, which is a major metabolite for each of the derivatives, and may be the most important chemical species for antimalarial activity in vivo. The promise of these compounds lies in their effectiveness against chloroquine-resistant parasites and their rapid action against malaria. Moreover, they have been used for the treatment of malaria in three million patients with few reports of clinically significant toxicity [2]. Figure 1 Chemical structures of ‘First’ and ‘Second’ generation peroxides. The fenozan, tetroxane and tricyclic trioxanes are totally synthetic compounds prepared from readily available starting materials. The first generation DHA ... The drawback with artemisinin and related compounds is that they suffer from poor bioavailability: the relative bioavailability of oral artemisinin (vs intramuscular) and intramuscular artemether (vs oral) and the bioavailability of artesunate are all within a range of 15–30% [3–5]. This may be one of the reasons for the relatively high rate of recrudescence associated with peroxide monotherapy. Their complex chemical structures are not essential for pharmacological activity and there has, therefore, been a drive to design and synthesize simpler endoperoxides which have greater in vivo potency. To date, more than 1000 new endoperoxides belonging to several chemical classes have been prepared. A number of ‘second generation’ peroxides have been developed, including arteflene (Figure 1). This is a synthetic peroxide developed from yingzhaosu A, the complex active constituent of the Chinese plant Artabotrys uncinatus, after it was shown to have antimalarial activity [6]. It has been tested in clinical trials in Africa. The ‘first generation’ artemisinin derivatives, as described above, are chemically simple esters and ethers of the parent lactol, dihydroartemisinin (Figure 1) [7, 8]. These compounds are generally more potent than artemisinin both in vitro and in vivo, and provide a wider choice of formulations which are more easily administered to patients in the field. Artemether and arteether are more oil soluble than artemisinin and are currently undergoing clinical trials under WHO sponsorship [8]. Other first generation derivatives include sodium artesunate, the DHA β-O-succinate half ester sodium salt, and sodium artelinate, the corresponding DHA β-O-p-carboxybenzyl ether sodium salt. Both of these compounds, which are water soluble, are currently under investigation as intravenous treatments for severe malarial infections [8]. Given the obvious pharmacokinetic limitations of the ‘first generation’ analogues and the realization that the peroxide bridge is the essential pharmacophore for biological activity, medicinal chemists have developed a range of second generation derivatives. Representative compounds include the bicyclic fenozan type derivatives, synthesized by Jefford et al. [9], the tetroxanes synthesized by Vennerstrom [10] and tricyclic simplified derivatives prepared by Posner and coworkers [11]. The discovery that several of the first generation artemisinin derivatives are metabolised extensively to DHA has prompted other workers to prepare C-10-carba deoxy derivatives (for a review see Meshnick et al. [1]). These compounds cannot be metabolised to DHA, which undergoes fairly rapid clearance [8], and should therefore have longer half-lives in vivo. The initial focus of the medicinal chemist is clearly directed towards the discovery of simpler (hence often less expensive) and more potent compounds, which provides not only lead compounds for therapeutic use, but also pharmacological tools to define the mechanism of action of this class of agent and the essential pharmacophore. A QSAR study of antimalarial activity indicates that docking between an active trioxane and the receptor, haem, could be the crucial step preliminary to drug action [12]. Drug safety is an important consideration which has to be evaluated in concert with efficacy at the earliest possible stage in drug development. It is therefore essential to define the potential hazards associated with second or third generation peroxides that present greater and more prolonged systemic exposure and design appropriate test systems for screening for drug toxicity. The specific questions which need to be addressed during further chemical development of peroxide antimalarials are as follows. Will such chemical modifications lead to loss of the high degree of selective therapeutic potency noted for first generation peroxide antimalarials? Will chemical modification result in greater exposure of mammalian systems to hazards posed by the peroxide group such as the neurotoxicity observed in rodents given first generation peroxides? The purpose of this review is to address each of these questions from chemical and clinical perspectives.

1. Cytochrome P450-mediated bioactivation of sulphamethoxazole to a hydroxylamine has been implicated in the hypersensitivity reactions associated with co-trimoxazole administration. Inhibiting the formation of the hydroxylamine may be one method of preventing the high frequency of toxicity which is observed in HIV-infected patients. Therefore, in this study, we have investigated the ability of fluconazole and ketoconazole, known cytochrome P450 inhibitors, to inhibit the formation of sulphamethoxazole hydroxylamine. 2. Ten healthy male volunteers were given co-trimoxazole (800 mg sulphamethoxazole and 160 mg trimethoprim) alone or 1 h after either fluconazole (150 mg) or ketoconazole (200 mg) in a randomized fashion with a washout period of at least 1 week between each phase. Urine was collected for 24 h, and sulphamethoxazole and its metabolites were quantified by electrospray LC-MS. 3. Ketoconazole had no effect on the urinary recovery of sulphamethoxazole or any of its metabolites. In contrast, fluconazole significantly (P < 0.001) inhibited the formation of sulphamethoxazole hydroxylamine by 50.0 +/- 15.1%. Fluconazole also inhibited the oxidation of sulphamethoxazole to the 5-methylhydroxy and 5-methylhydroxy acetate metabolites by 69.9 +/- 15.8% and 64.0 +/- 12.0%, respectively, but had no effect on the amount of sulphamethoxazole, N4-acetyl sulphamethoxazole, or sulphamethoxazole N1-glucuronide excreted in urine. 4. The potential clinical benefit of using fluconazole to prevent hypersensitivity to co-trimoxazole in patients with AIDS needs to be assessed in a prospective study using both metabolite formation and the clinical occurrence of adverse reactions as end-points.

1. The metabolism of amodiaquine (A), ethinyloestradiol (E), mianserin (M), phenytoin (Ph), sulphanilamide (S) and paracetamol (Pa) to both stable and chemically reactive, i.e. irreversibly protein bound, metabolites was investigated using microsomes prepared from histologically normal human liver obtained from eight kidney donors. 2. All drugs, except amodiaquine, were metabolized by NADPH-dependent microsomal enzymes to chemically reactive metabolites. The degree of NADPH-dependent binding varied between drugs (E, 11.5 +/- 5.8% incubated drug; M, 3.0 +/- 1.9%; Ph, 0.10 +/- 0.09%; S, 0.57 +/- 0.38%; Pa, 1.2 +/- 1.2%; mean of eight livers +/- s.d.). 3. Inclusion of glutathione (1 mM) or ascorbic acid (1 mM) in the incubation reduced the NADPH-dependent binding for all substrates, indicating the involvement of electrophilic oxidation products. 4. Binding of M and Pa correlated with each other (Spearman's r = 0.86) and with total cytochrome P-450 content (r = 0.76 and 0.78 respectively). E binding also correlated with the binding of M (r = 0.79) and Pa (r = 0.81) but not with cytochrome P-450. Binding of Ph and S did not correlate with any of the other measured metabolic parameters.

The metabolism of [3H]ethinyloestradiol (EE2) was investigated in six male subjects who had been phenotyped with respect to sparteine metabolism (three metabolizers and three non-metabolizers). Urinary metabolite profiles of EE2 were virtually identical. Following enzyme hydrolysis of sulphate and glucuronide conjugates the major urinary metabolite was 2-methoxyEE2. The ratio EE2:2-methoxyEE2 was taken as a measure of EE2 2-hydroxylation (metabolizers, 2.4 +/- 0.3; non-metabolizers, 2.5 +/- 0.4). Primaquine (45 mg), previously shown to inhibit antipyrine metabolism, had no effect on EE2 2-hydroxylation. Supporting studies in rats showed that acute administration of primaquine (50 mg/kg) and 1-methylimidazole (50 mg/kg) inhibited antipyrine but not EE2 metabolism. It is concluded that the cytochrome P-450 enzyme responsible for 2-hydroxylation of EE2 is distinct from the enzymes involved in the oxidation of sparteine and antipyrine.