Clinical Pharmacokinetics of Atypical Antipsychotics: An Update

Therapeutic drug monitoring (TDM) is the quantification and interpretation of drug concentrations in blood to optimize pharmacotherapy. It considers the interindividual variability of pharmacokinetics and thus enables personalized pharmacotherapy. In psychiatry and neurology, patient populations that may particularly benefit from TDM are children and adolescents, pregnant women, elderly patients, individuals with intellectual disabilities, patients with substance abuse disorders, forensic psychiatric patients or patients with known or suspected pharmacokinetic abnormalities. Non-response at therapeutic doses, uncertain drug adherence, suboptimal tolerability, or pharmacokinetic drug-drug interactions are typical indications for TDM. However, the potential benefits of TDM to optimize pharmacotherapy can only be obtained if the method is adequately integrated in the clinical treatment process. To supply treating physicians and laboratories with valid information on TDM, the TDM task force of the Arbeitsgemeinschaft für Neuropsychopharmakologie und Pharmakopsychiatrie (AGNP) issued their first guidelines for TDM in psychiatry in 2004. After an update in 2011, it was time for the next update. Following the new guidelines holds the potential to improve neuropsychopharmacotherapy, accelerate the recovery of many patients, and reduce health care costs.

The purpose of this study was to investigate the safety and efficacy of risperidone in the treatment of schizophrenic patients and determine its optimal dose. This double-blind study included 388 schizophrenic patients drawn from 20 sites in the United States. Patients were randomly assigned to 8 weeks' treatment with placebo, one of four doses of risperidone (2, 6, 10, or 16 mg), or 20 mg of haloperidol daily. Clinical improvement (20% reduction in total scores on the Positive and Negative Syndrome Scale for Schizophrenia) at the study end point was shown by 35% of the patients receiving 2 mg of risperidone, 57% receiving 6 mg, 40% receiving 10 mg, and 51% receiving 16 mg; and by 30% receiving haloperidol and 22% receiving placebo. Statistically significant differences in clinical improvement were found between 6 and 16 mg of risperidone versus placebo and versus haloperidol. Positive symptom scores were significantly lower after 6, 10, and 16 mg of risperidone and 20 mg of haloperidol than placebo; negative symptom scores, however, were reduced significantly, compared with placebo, only after 6 and 16 mg of risperidone. The incidence of extra-pyramidal side effects (measured by the Extrapyramidal Symptom Rating Scale) was significantly higher in patients treated with 16 mg of risperidone or 20 mg of haloperidol than placebo. The results indicate that the optimal daily dose of risperidone for most schizophrenic patients in this study was 6 mg; this dose was as effective as 16 mg, and the incidence of extrapyramidal symptoms in patients receiving 6 mg of risperidone was no higher than that in patients receiving placebo. Risperidone is a safe antipsychotic that is effective against both the positive and negative symptoms of schizophrenia.

Brexpiprazole (OPC-34712, 7-{4-[4-(1-benzothiophen-4-yl)piperazin-1-yl]butoxy}quinolin-2(1H)-one) is a novel drug candidate in clinical development for psychiatric disorders with high affinity for serotonin, dopamine, and noradrenaline receptors. In particular, it bound with high affinity (Ki 1000 nM). Brexpiprazole potently bound to rat 5-HT2A and D2 receptors in vivo, and ex vivo binding studies further confirmed high 5-HT1A receptor binding potency. Brexpiprazole inhibited DOI (2,5-dimethoxy-4-iodoamphetamine)-induced head twitches in rats, suggestive of 5-HT2A antagonism. Furthermore, in vivo D2 partial agonist activity of brexpiprazole was confirmed by its inhibitory effect on reserpine-induced DOPA accumulation in rats. In rat microdialysis studies, brexpiprazole slightly reduced extracellular dopamine in nucleus accumbens but not in prefrontal cortex, whereas moderate increases of the dopamine metabolites, homovanillic acid and DOPAC (3,4-dihydroxy-phenyl-acetic acid), in these areas also suggested in vivo D2 partial agonist activity. In particular, based on a lower intrinsic activity at D2 receptors and higher binding affinities for 5-HT1A/2A receptors than aripiprazole, brexpiprazole would have a favorable antipsychotic potential without D2 receptor agonist- and antagonist-related adverse effects. In conclusion, brexpiprazole is a serotonin-dopamine activity modulator with a unique pharmacology, which may offer novel treatment options across a broad spectrum of central nervous system disorders.