These results suggest that plasma clozapine levels are correlated with clinical effects, although there is considerable variability in the response achieved at any given drug concentration. Because many patients respond well at plasma clozapine concentrations in a low range, aiming initially at plasma clozapine concentrations of 350 ng/ml or greater would require in some patients use of unrealistically high dosages and imply an excessive risk of side effects. Increasing dosage to achieve plasma levels above 350-400 ng/ml may be especially indicated in patients without side effects who failed to exhibit amelioration of psychopathology at standard dosages or at lower drug concentrations.

The role of the polymorphic cytochrome P450 2D6 (CYP2D6) in the metabolism of risperidone to its major active metabolite, 9-hydroxyrisperidone (9-OH-risperidone), has been documented after single oral doses of the drug. In this study, the influence of the CYP2D6 polymorphism on the steady-state plasma concentrations of risperidone and 9-OH-risperidone was investigated. Thirty-seven schizophrenic patients on monotherapy with risperidone, 4-8 mg/day, were genotyped by RFLP and PCR for the major functional variants of the CYP2D6 gene. Steady state plasma levels of risperidone and 9-OH-risperidone were analysed by HPLC. Based on the genotype analysis, three patients were classified as ultrarapid metabolizers (UM) with an extra functional CYP2D6 gene, 16 were homozygous extensive metabolizers (EM), 15 heterozygous EM and three poor metabolizers (PM). The median steady-state plasma concentration-to-dose (C/D) ratios of risperidone were 0.6, 1.1, 9.7 and 17.4 nmol/l per mg in UM, homozygous EM, heterozygous EM and PM, respectively, with statistically significant differences between PM and the other genotypes (P < 0.02). The C/D of 9-OH-risperidone also varied widely but was not related to the genotype. The risperidone/9-OH-risperidone ratio was strongly associated with the CYP2D6 genotype, with the highest ratios in PM (median 0.79). Heterozygous EM also had significantly higher ratios than homozygous EM (median value 0.23 versus 0.04; P < 0.01) or UM (median 0.03; P < 0.02). No significant differences were found in the C/D of the sum of the plasma concentrations of risperidone and 9-OH-risperidone between the genotype groups. In conclusion, the steady-state plasma concentrations of risperidone and the risperidone/9-OH-risperidone ratio are highly dependent on the CYP2D6 genotype. However, as risperidone and 9-OH-risperidone are considered to have similar pharmacological activity, the lack of relationship between the genotype and the sum of risperidone and 9-OH-risperidone indicates that the CYP2D6 polymorphism may be of limited importance for the clinical outcome of the treatment.

Our results strongly suggest that 6-hydroxylation, the main metabolic pathway of melatonin, is mediated mainly, but not exclusively, by CYP1A2, the high-affinity enzyme involved in melatonin metabolism, confirming the observation that a single oral dose of fluvoxamine increases nocturnal serum melatonin levels in healthy subjects. Furthermore, the results indicate that there is a potential for interaction with drugs metabolised by CYP1A2 both at physiological levels and after oral administration of melatonin, while CYP2C19 and CYP2C9 are assumed to be less important.

In chronic schizophrenic patients experiencing an acute exacerbation of the disorder, plasma levels of risperidone and its active metabolite correlate with the occurrence of parkinsonian side effects, whereas no significant correlation appears to exist with the degree of clinical improvement.

To evaluate the pharmacokinetic interaction between risperidone and the mood-stabilizing agents carbamazepine and valproic acid, steady state plasma concentrations of risperidone and 9-hydroxyrisperidone (9-OH-risperidone) were compared in patients treated with risperidone alone (controls, n = 23) and in patients comedicated with carbamazepine (n = 11) or sodium valproate (n = 10). The three groups were matched for sex, age, body weight, and antipsychotic dosage. Plasma concentrations of risperidone and 9-OH-risperidone did not differ between valproate-comedicated patients and controls. By contrast, the concentrations of both compounds were lower in patients taking carbamazepine, although the difference reached statistical significance only for the metabolite (p < 0.001). The sum of the concentrations of risperidone and 9-OH-risperidone in patients receiving carbamazepine (median 44 nmol/L) was also significantly lower than in patients receiving valproate (168 nmol/L) and in controls (150 nmol/L). In five patients assessed with and without carbamazepine comedication, dose-normalized plasma risperidone and 9-OH-risperidone concentrations were significantly lower when the patients received combination therapy than when they received risperidone alone. In three patients assessed with and without valproate, no major changes in the levels of risperidone and its metabolite were observed. These findings demonstrate that carbamazepine markedly decreases the plasma concentrations of risperidone and its active 9-OH-metabolite, probably by inducing CYP3A4-mediated metabolism. This interaction is likely to be clinically significant. Conversely, valproic acid does not cause any major change in plasma antipsychotic levels.