Optimal sampling time and clinical implication of the SCN5A promoter haplotype in propafenone therapeutic drug monitoring

Purpose The clinical usefulness of therapeutic drug monitoring (TDM) of propafenone, a sodium channel blocker, has been unclear due to the lack of information regarding optimal blood sampling time and therapeutic concentration range. Antiarrhythmic effects of sodium channel blockers are affected by the activity of the cardiac sodium channel (SCN5A). We investigated the optimal sampling time and the clinical implication of the SCN5A promoter haplotype in propafenone TDM. Methods We evaluated serum concentrations of propafenone, the SCN5A promoter haplotype, and antiarrhythmic efficacy in 55 patients with supraventricular tachy-arrhythmias. Blood samples obtained 1.5–6 and 10–24 h after the last dose were categorized as peak and trough samples, respectively. Results The peak propafenone concentration was significantly higher in effectively treated patients than that in patients showing insufficient response (337 ± 213 vs. 177 ± 93 ng/mL, P = 0.005), but the trough propafenone concentration was not significantly different between the two groups (68 ± 48 vs. 42 ± 36 ng/mL). Clinically relevant propafenone efficacy was achieved significantly more often in SCN5A haplotype B carriers than in wild-type haplotype A homozygotes (90 vs. 60%, P < 0.05). Among the haplotype A homozygotes, peak propafenone concentration was higher in effectively treated patients than that in patients showing insufficient response (299 ± 177 vs. 177 ± 93 ng/mL, P = 0.061). Conclusion The present study found that antiarrhythmic efficacy of propafenone was associated with peak propafenone concentration rather than trough concentration and was affected by the SCN5A promoter haplotype.