Patients
The study was conducted at the University of the Ryukyus between April 2011 and February 2020. The present study originally included 60 Japanese depressed patients. The inclusion criteria were (a) diagnoses of major depressive disorder, bipolar I disorder, or bipolar II disorder according to the DSM-IV-TR criteria11 (b) insufficient response to at least three psychotropics including antidepressants, mood stabilizers or atypical antipsychotics despite sufficient therapeutic doses and at least 4 weeks of treatment (c) a Montgomery Åsberg Depression Rating Scale (MADRS) score of 21 or more3, 4, 7–9 (d) no previous treatment with lamotrigine (e) no lifetime history of substance abuse, organic brain syndrome, delirium or dementia (f) physically healthy without any clinically significant findings in clinical laboratory examinations, electrocardiography, and electroencephalography (g) no oral contraceptives in female patients. The study protocol was approved by the Ethics Committee of the University of the Ryukyus (No. 30). All patients had given a written informed consent to participate in this study.
Procedure
The medication of all subjects was unchanged for at least 4 weeks prior to the study. Antiepileptics inducing enzymes were excluded. After that, lamotrigine was co-given at 6 pm for 8 weeks as an augmentation therapy. The dose timing was chosen to avoid possible sedative effects. The mean ± SD of the number of psychotropic drugs coadministered with lamotrigine was 1.7 ± 0.6. These drugs were maintained during the study period. In case of 36 patients not given valproate, lamotrigine 25 mg/d was prescribed in the first 2 weeks, and the dose was incremented at the rate of 25 mg/2 weeks. As for the remaining 24 subjects given valproate, lamotrigine 25 mg/every other day was initiated during the first 2 weeks, the dose was incremented to 25 mg/d for the next 2 weeks, and thereafter at the rate of 25 mg/2 weeks. The final doses were 100 mg/d for the former and 75 mg/d for the latter, respectively. These doses were decided by a finding from Kagawa et al.4 indicating that a mean effective dose of lamotrigine was 88 mg/d in lamotrigine augmentation therapy. No other medications were allowed, except for the occasional use of flunitrazepam (1-4 mg/d) as a hypnotic and sennoside (12-36 mg/d) as a laxative. Nursing staff or their families confirmed patients’ adherence. Depressive symptoms were evaluated by the MADRS before and after the 8-week treatment. A responder was defined as a case with a final MADRS score of 10 or less. Spontaneously reported or observed adverse effects were reported during the study. Blood samplings were performed taken at 8 am after the 8th week of treatment.
Measurement Tools
Genotyping Data: DNA was isolated from peripheral leucocytes using a QIA-amp DNA Blood Maxi (Qiagen, Tokyo, Japan). The genotypes of UGT1A4 142T>G, UGT2B7 -161C>T, and UGT2B7 372A>G were detected by real-time polymerase chain reaction (PCR) methods according to Singkham et al.12 The PCR conditions were 95℃ for 10 minutes followed by 92℃ for 15 seconds and 60℃ for 90 seconds.12
Lamotrigine Concentration Data: Plasma lamotrigine concentrations were measured using the high-performance liquid chromatography method reported by Brzaković et al.13 The detection limit was 0.04 µmol/L using 0.5 mL of plasma, and the coefficients of variation were 6.57 ± 2.01% for between-days and 4.95 ± 1.87% for within-day imprecision.
Statistical Analyses
Analysis of variance was used to compare the percentage improvements at week 8 among the 3 diagnoses. The Wilcoxon rank-sum test was used to assess differences in the percentage improvements and plasma lamotrigine concentrations between two genotype groups of each polymorphism. The chi-square test was used to compare the number of responders at week 8 between two genotype groups of each polymorphism. A stepwise multiple regression analysis was applied to analyze the correlation between the percentage improvements and several factors including the three genetic polymorphisms, age, gender, duration of illness, number of previous mood episodes, duration of the present episode, entry MADRS score, valproate coadministration, and plasma lamotrigine concentrations, in which dummy variables were used for these polymorphisms (T/T=0; T/G + G/G=1 for UGT1A4 142T>G polymorphism, C/C=0; C/T + T/T =1 for UGT2B7 -161C>T polymorphism, and A/A=0; A/G + G/G=1 for UGT2B7 372A>G polymorphism), gender (male=0, female=1), and valproate coadministration (without=0, with=1). A two-tailed P value of less than 0.05 was regarded as statistically significant. SPSS 22.0 for Windows (SPSS, Japan Inc, Tokyo, Japan) was used to perform statistical analyses. Power analyses were performed with G*Power (Franz Faul, University of Kiel, Kiel, Germany) version 3.1.9.7.