Interactions of CNS Drugs: Psychotherapeutic Agents—The Antipsychotic Drugs

Aspects of the pharmacology of the phenothiazine antipsychotic agents pertinent to possible interactions are reviewed. Pharmacokinetic interactions of possible clinical importance are the interference with the absorption of the phenothiazines by antacids, reversal of the hypotensive effect of guanethidine by psychotropic drugs, blockade of the effects of levodopa in Parkinson's disease, and stimulation of phenothiazine-metabolizing enzymes by phenobarbital. Pharmacologically, phenothiazines may potentiate certain depressant effects of other CNS depressants or add to the anticholinergic side effects of a number of other drugs. The phenothiazines appear to antagonize both the CNS stimulant and the anorexic effects of amphetamines and related substances. Other reactions of possible concern are the phenothiazine-induced hypoglycemia, the relationship of lithium toxicity to sodium intake, and the number of false positive pregnancy tests in patients on phenothiazine therapy.

A priori lithium dosage regimen using population characteristics of pharmacokinetic parameters

The important problem of initiation of long-term lithium treatment is tackled by means of the selection of an a priori dosage regimen based on the presumed efficacy of lithium and absence of toxicity. The pharmacokinetics of Li+ ion is represented by a four-compartment open model including the supposed first-order processes for the release of the active compound from the dosage form and its absorption. Experimental protocols for measurements of serum concentrations and of urinary amounts after single and multiple dosing to healthy volunteers were derived with several oral dosage forms. Estimation of the pharmacokinetic parameters for each subject made it possible to validate the model for the various dosage forms. The interindividual variability of these parameters is taken into account by estimating the characteristics of the statistical distribution for the whole population. A dosage regimen is considered optimum when serum concentration profiles at steady state range from the threshold of efficacy (0.8 mmol/liter) to the threshold of toxicity (2.0 mmol/liter). When the number of daily intakes is fixed, the search for the optimum dose for the whole population is effected by minimizing the expected value of the random variable which characterizes the risks of excursion out of the therapeutic range. By this means universal dosages are shown to be unsatisfactory. However, certain dosage regimens individualized with respect to the renal clearance value of lithium and based on two or three daily intakes can give excellent results even when conventional dosage forms are used.