Concentrations of total (R) + (S) and of the enantiomers (R) and (S) of thioridazine and metabolites were measured in 21 patients who were receiving 100 mg thioridazine for 14 days and who were comedicated with moclobemide (450 mg/day). Two patients were poor metabolizers of dextromethorphan and one was a poor metabolizer of mephenytoin. Cytochrome P450IID6 (CYP2D6) is involved in the formation of thioridazine 2-sulfoxide (2-SO) from thioridazine and also probably partially in the formation of thioridazine 5-sulfoxide (5-SO), but not in the formation of thioridazine 2-sulfone (2-SO2) from thioridazine 2-SO. Significant correlations between the mephenytoin enantiomeric ratio and concentrations of thioridazine and metabolites suggest that cytochrome P450IIC19 could contribute to the biotransformation of thioridazine into yet-unknown metabolites, other than thioridazine 2-SO, thioridazine 2-SO2, or thioridazine 5-SO. An enantioselectivity and a large interindividual variability in the metabolism of thioridazine have been shown: measured (R)/(S) ratios of thioridazine, thioridazine 2-SO fast eluting (FE), thioridazine 2-SO slow eluting (SE), thioridazine 2-SO (FE+SE), thioridazine 2-SO2, thioridazine 5-SO(FE), and thioridazine 5-SO(SE) were (mean +/- SD) 3.48 +/- 0 .93 (range, 2.30 to 5.80), 0.45 +/- 0.22 (range, 0.21 to 1.20), 2.27 +/- 8.1 (range, 6.1 to 40.1), 4.64 +/- 0.68 (range, 2.85 to 5.70), 3.26 +/- 0.58 (range, 2.30 to 4.30), 0.049 +/- 0.019 (range, (0.021 to 0.087), and 67.2 +/- 66.2 (range, 16.8 to 248), respectively. CYP2D6 is apparently involved in the formation of (S)-thioridazine 2-SO(FE), (R)-thioridazine 2-SO(SE), and also probably (S)-thioridazine 5-SO(FE) and (R)-thioridazine 5-SO(SE).

In a multicenter study of 78 severely depressed inpatients (44 women and 34 men; age range, 23 to 70 years), the efficacy, onset of efficacy, and tolerability of the reversible monoamine oxidase-A inhibitor moclobemide (450 mg/day) in combination with thioridazine (100 mg/day) were compared with those of moclobemide (450 mg/day) plus placebo. Patients enrolled met the DSM-III-R criteria for severe depression and had a severity score of at least 20 on the first 17 items of the Hamilton Rating Scale for Depression (HAM-D). Additionally, these patients had not responded to at least two standard antidepressants during the 2 years preceding screening and the mean duration of the current episode was 6 months. After a washout period of 3 to 5 days, patients were randomized to one of the two treatment groups, which at the outset had similar characteristics. Efficacy was assessed by the HAM-D, a depression observation rating for nurses, and a Clinical Global Impression (CGI) scale. Tolerability assessments included an overall rating, a description of adverse events, vital signs, electrocardiogram, and laboratory tests. After 4 weeks of therapy, both groups of patients showed significant improvements in HAM-D and CGI scores. The response rates (based on HAM-D > or = 50% decrease) were 74% for moclobemide/thioridazine and 77% for moclobemide/placebo, and according to CGI scores, 76 and 72% were "very much improved" or "much improved," respectively. Onset of effect was noted after 9.2 and 9.8 days, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)