BRITISH MEDICAL JOURNAL 19 FEBRUARY 1972 463 PAPERS AND ORIGINALS Drug Interaction: Inhibitory Effect of Neuroleptics on Metabolism of Tricyclic Antidepressants in Man LARS F. GRAM, KERSTIN FREDRICSON OVERO British Medical Journal, 1972, 1, 463-465 excretion (Anders, 1971). There are relatively few reports on interaction concerning neuroleptics or tricyclic antidepressants. Summary Chlorpromazine has both stimulatory and inhibitory effects on the metabolism of hexobarbitone in Total urinary excretion of radioactivity after oral or experimental animals of a test of (Riimke and Bout, 1960-1). Furthermore, chlorpromazine intravenous administration dose '4C-imi- accelerates the metabolism pramine was measured in were tested ofmeprobamate (Kato and Vassanelli, eight patients. They 1962). Tricyclic antidepressants have been shown to inhibit the before, during, and after treatment with neuroleptics. metabolism Excretion diminished while the were of tremorine and oxotremorine in rats in vitro and patients being in vivo (Hammer and Sjoqvist, 1967). It has also been found treated with perphenazine, haloperidol, or chlorproma- that zine, not treatment. desipramine hydrochloride inhibits the metabolism of though during flupenthixol amphetamine in isolated, perfused rat liver (Dingell and Bass, Total urinary excretion of radioactivity and plasma 1969). Pretreatment levels of metabolites and were measured with phenobarbitone decreased steady-state unchanged drug plasma levels of desipramine and nortriptyline in man (Sjoqvist in five patients after a test dose of 14C-nortriptyline. Each et was before and al. 1968). These findings were later confirmed in a twin study patient tested again during perphenazine (Alexanderson et al., 1969). Forrest et treatment. In all patients perphenazine treatment caused: al. (1970) found increased (1) decrease of total urinary excretion, (2) decreased urinary excretion of chlorpromazine metabolites when patients plasma level of metabolites, and (3) increased plasma were given additional treatment with phenobarbitone. Vesell level of unchanged nortriptyline. et al. (1970) found that nortriptyline prolonged the plasma half- that inhibit lives of phenazone (antipyrine) and dicoumarol (bishydroxy- These results indicate neuroleptics the in man. P. G. metabolism of in man. coumarin) Dayton has reported a rise in steady- tricyclic antidepressants state plasma level of imipramine and desipramine when methyl- phenidate was administered concurrently with the imipramine treatment (Cosmides, 1970). Introduction Interaction between neuroleptics and antidepressants has, treatment with and despite its clinical relevance, not been investigated thoroughly. Combined neuroleptics tricyclic antidepres- Moody et al. (1967) reported on one sants has been recommended in several clinical reports (Davis patient, who had a rise in et al., 1970). It has been used for schizophrenic patients with plasma levels of imipramine and desipramine when chlorproma- and for with schizo- zine was given in addition to the imipramine treatment. They depressive symptoms depressive patients did not, however, interpret these findings as being a result of in- affective symptoms. For this reason possible pharmacokinetic interaction between and hibited metabolism of imipramine. neuroleptics tricyclic antidepressants We have previously studied the pharmacokinetic properties was investigated. of imipramine and in Pharmacokinetic interaction has been to occur particular the metabolism and urinary drug reported excretion of metabolites (Christiansen et al., 1967). Urinary in many drug combinations, particularly in respect of the meta- excretion of bolism rather than their or radioactivity after oral administration of 14C- of drugs absorption, distribution, imipramine was relatively rapid. Over 95% of the excretion takes place as polar metabolites: hydroxymetabolites, glucuroni- des, and several non-extractable unidentified metabolites. These Psychochemistry Institute, University of Copenhagen School of data were the basis for using total urinary excretion of radio- Medicine, Copenhagen, Denmark activity after administration of I4C-imipramine as a measure of LARS F. GRAM, M.D., Research Assistant metabolism of the drug. The pharmacokinetic interaction be- Research Laboratories, H. Lundbeck & Co. A/S, 2500 Copenhagen tween neuroleptics and antidepressants was investigated in this Valby, Denmark in an FREDRICSON Research Assistant way initial study, where, however, the plasma level of imi- KERSTIN OVER0, M.PHARM., pramine or metabolites could not be determined. Later the drug 464 BRITISH MEDICAL JOURNAL 19 FEBRUARY 1972 interaction between neuroleptics and antidepressants was studied radioactivity in plasma and concentration of unchanged drug by measuring plasma levels of unchanged drug and metabolites could be used as an estimate of the total concentration of as well as urinary excretion of radioactivity. 14C-nortriptyline was nortriptyline metabolites in plasma. usedas a test substance in these studies since the unchanged form of this compound could be determined in plasma. Results The results ofthe studies on urinary excretion of 14C-imipramine Methods and Patients are summarized in Fig. 1. More detailed accounts of these results have been published elsewhere (Gram et al., 1971b). 14C-Imipramine, Initial Studies on Urinary Excretion.-Eight In all patients there was a 25-500o decrease in total urinary women aged 49-60 took part in the investigation. All were diag- excretion of radioactivity after oral administration of "4C-imi- nosed as suffering from schizophrenia according to the criteria pramine when perphenazine (20-48 mg/day), haloperidol (12- of Bleuler (1911). A test dose of 3 ieCi of "4C-imipramine in a 20 mg/day), or chlorpromazine (300 mg/day) was administered. total of 10 mg of imipramine was given by mouth or intra- In two patients on flupenthixol treatment (3-6 mg/day) the venously. Urinary excretion of radioactivity was measured when urinary excretion was not influenced. The two patients on halo- the patients were on treatment with neuroleptics and 10-30 peridol were on concomitant treatment with biperiden. Repeated days after discontinuation of the drug. Most patients were testing during a period of separate discontinuation of biperiden tested both before and after periods of discontinuation as well showed that haloperidol, but not biperiden, inhibited the urinary as during the period of discontinuation. The influence of per- excretion of radioactivity. phenazine treatment was tested in five patients, chlorpromazine The results of the studies with 14C-nortriptyline are shown in treatment in one patient, haloperidol treatment in two patients, Fig. 2. The inhibition of urinary excretion of radioactivity during and flupenthixol treatment in two. Urine was collected at inter- perphenazine treatment was the same as that seen in the initial vals of two to eight hours for 48 hours after administration of studies. During perphenazine treatment the plasma level of the test dose. 14C-imipramine was labelled at the 10-11 position metabolities after administration of a single test dose of 14CG in the central ring system. nortriptyline was reduced by 25-600'. The plasma level of un- 1 4C-Nortriptyline, Studies on Plasma Levels and Urinary changed nortriptyline at the same time rose to a level 10-30% Excretion.-Five patients (three men aged 22, 23, and 44 and higher than that found before the start of perphenazine treat- two women aged 32 and 56) were tested before and during ment. There was a pronounced inter-individual variation of all treatment with perphenazine 20-48 mg/day. All patients were three pharmacokinetic measurements. The change in these diagnosed as suffering from schizophrenia according to the measurements in relation to perphenazine administration was, criteria ofBleuler (1911). A test dose of 3 ,uCi of 14C-nortriptyline however, identical in all five patients. in a total dose of 50 mg of nortriptyline was given by mouth. Urine was collected at intervals of four to eight hours for the first 24 hours. Blood samples were drawn at intervals of two to Discussion four hours during the first 10-15 hours and then 24 hours after administration of the test dose. '4C-nortriptyline was labelled Theoretically, the observed inhibition of urinary excretion of in position a to the N-atom in the side chain. 14C-imipramine could be a result of drug interaction at various levels: absorption, metabolism, or excretion. The changes that might be expected in the three pharmacokinetic measurements in these cases are shown in the Table. The results of the study ANALYSIS Radioactivity in urine was measured according to the principles Theoretical Consequences of Different Mechanisms of Neuroleptic-induced previously described (Gram et al., 1971a). For counting radio- Decrease of Urinary Excretion of 14C-Nortriptyline active in plasma 1-ml samples were taken. Soluene (Packard) 1 ml and dioxan 1 ml were added and the plasma was incubated Change at 50-60°C for 12-16 hours for total solution. Then 450 1d of Mechanism of Drug Plasma Levels of 1 M HC1 and finally 15 ml of Instagel (Packard) were added. Interaction Nortriptyline Urinary The plasma specimens were counted for 100 minutes in a liquid Unchanged Metabolites Excretion scintillation spectrometer. Drug After the administration of 14C-nortriptyline counts 50-100% Inhibited absorption . Decrease Decrease Decrease Inhibited metabolism .. Increase Decrease Decrease above the background level were obtained. Plasma