Phenotyping and genotyping of drug metabolism to guide pharmacotherapy in psychiatry.

In psychiatry, around 130 drugs are available which have been detected and developed during the last 60 years. These drugs are effective and essential for the treatment of many psychiatric disorders and symptoms. Therapeutic outcomes, however, are still far from satisfactory for many patients. One major reason underlying insufficient response or problems of tolerability or toxicity is considerable interindividual variability in the pharmacokinetic properties of the patients. At the exact same dose, a more than 20-fold interindividual variation in steady state concentrations of the drugs in the body may result. Patients differ in their ability to absorb, distribute, metabolize and excrete drugs due to concurrent disease, age, concomitant medication or genetic peculiarities. A valuable tool for tailoring the dosage of the prescribed medication(s) to the individual characteristics of a patient is therapeutic drug monitoring (TDM). For a considerable number of psychopharmacologic compounds, TDM has become a clinical routine for dose adjustment. The benefits of TDM regarding optimization of psychopharmacotherapy can be further enhanced by genotyping of cytochrome P450 enzymes (CYP). These enzymes are highly polymorphic and involved in the metabolism of most psychotherapeutic drugs. Genotyping related information may be supportive especially for drugs that are predominantly metabolized by CYP2C19 or CYP2D6. The in vivo activity of CYP isoenzymes can be determined using probe drug assays. When used appropriately, phenotyping and genotyping methods are most helpful for problem solving and improvement of the outcomes of psychopharmacotherapy for many patients.