481. THERAPEUTIC DRUG MONITORING FOR ANTIPSYCHOTIC MEDICATION TO ESTIMATE DRUG ADHERENCE IN JAPANESE PATIENTS WITH PSYCHIATRIC DISORDERS

Abstract Background It has been reported that the adherence rate of antipsychotics among patients with psychiatric disorders (approximately 50%) is relatively lower than other diseases. Although there are numerous factors influencing adherence such as polypharmacy, younger age, lack of insight into the illness and stigma associated with the disorder, the reliability of the evaluation methods also would be a matter since the assessments were mostly done by subjective patient self-reports. Aims & Objectives In the present study, we evaluated drug adherence by using therapeutic drug monitoring (TDM) in Japanese patients with psychiatric disorders. Method Patients who diagnosed with psychiatric disorders, including schizophrenia (SCZ), bipolar disorder, or major depressive disorder, at the Department of Psychiatry, Gifu University Hospital and prescribed antipsychotics (haloperidol, levomepromazine, aripiprazole, paliperidone, risperidone, olanzapine, and/or quetiapine) were enrolled between August 2021 and October 2023. Plasma concentrations of these drugs were measured using LC-MS/MS. Based on the Consensus Guidelines for Therapeutic Drug Monitoring in Neuropsychopharmacology (2018), plasma concentrations of antipsychotics within dose-related reference range defined as better adherence. Factor analysis was done by using factors possibly influencing adherence such as sex, age, diagnosis, number of drugs, antipsychotic dosage [calculated as chlorpromazine (CP) equivalents] and the use of concomitant drugs including antiparkinsonian drugs, antidepressants, benzodiazepines, and mood stabilizers. Additionally, we compared the objective adherence to subjective adherence using the Drug Attitude Inventory-10 (DAI-10). Results Forty-seven patients who met the inclusion criteria were enrolled, including 35 patients with SCZ. TDM defined that the rate of better adherence was 59.6 % in all patients and was 51.4 % in SCZ patients. Nine patients among them (19.1 %) were determined to have no medication intake. Patients in better adherence group were characterized by higher proportion of antipsychotics with concomitant drugs, higher CP equivalent doses, and higher proportion of inpatients compared to poor adherence group. Notably, the beneficial effects of concomitant drug use were more pronounced in patients receiving antipsychotic monotherapy compared to those on polytherapy. In contrast, no significant differences in DAI-10 scores were observed between better and poor adherence groups. Discussion & Conclusions Consistent with previous reports, the better adherence rate determined by TDM for antipsychotics in Japanese patients was mostly same. Our results revealed that the concomitant medications contributed to improved adherence of antipsychotic medications with relatively high intensity treatment of antipsychotics, as indicated by CP-equivalent doses. Furthermore, the lack of significant differences in DAI-10 scores between better and poor adherence groups suggest the adherence evaluation by TDM is more objective and useful. Taken together, these results highlight the relative usefulness of TDM for adherence assessment of antipsychotics.