Drug Interactions between Phenergan with Dextromethorphan and siponimod

GENERALLY AVOID: Due to its significant bradycardic effects, the risk of QT prolongation and torsade de pointes arrhythmia may be increased during initiation of siponimod treatment in patients receiving drugs that prolong the QT interval. Siponimod can cause a decrease in heart rate during initiation of therapy that is apparent within an hour of the first dose, and the day 1 decline is maximal at approximately 3 to 4 hours. The maximal decrease in heart rate from baseline was seen between day 5 and 6. After day 6, heart rate starts increasing and reaches placebo levels within 10 days after treatment initiation. The highest daily postdose-dose decrease in absolute hourly mean heart rate is observed on day 1, with a decrease of 5 to 6 bpm. Following day 1, decreases in heart rate are less pronounced. Heart rates below 40 bpm were rarely observed. In controlled clinical trials, bradycardia (including sinus bradycardia and decreased heart rate) occurred in 6% of siponimod-treated patients compared to 3% of patients receiving placebo. Initiation of siponimod treatment has also resulted in transient AV conduction delays. First-degree AV block (prolonged PR interval on ECG) occurred in 5.1% of siponimod-treated patients and 1.9% of patients receiving placebo. Second-degree AV blocks, usually Mobitz type I (Wenckebach), have been observed at the time of siponimod initiation in less than 1.7% of patients. Bradycardia and conduction abnormalities were usually transient and asymptomatic, and resolved within the first 24 hours, but they occasionally required treatment with atropine. In a study evaluating the effect on QT interval of siponimod 2 or 10 mg at steady-state, siponimod treatment resulted in maximum prolongations of the QTc of 7.8 and 7.2 msec, respectively, with upper bounds of the 90% confidence interval of 9.93 and 9.72 msec, respectively. In general, the risk of an individual agent or a combination of agents causing ventricular arrhythmia in association with QT prolongation is largely unpredictable but may be increased by certain underlying risk factors such as congenital long QT syndrome, cardiac disease, and electrolyte disturbances (e.g., hypokalemia, hypomagnesemia). In addition, the extent of drug-induced QT prolongation is dependent on the particular drug(s) involved and dosage(s) of the drug(s).

MANAGEMENT: Siponimod has not been studied in patients receiving drugs that can prolong the QT interval. Because bradycardia and AV block are recognized risk factors for QT prolongation and torsade de pointes arrhythmia, treatment with siponimod should generally not be initiated in patients who are concurrently treated with QT prolonging drugs with known arrhythmogenic properties. Advice from a cardiologist should be sought if treatment with siponimod is considered in patients on concurrent therapy with QT prolonging drugs with a known risk of torsades de pointes or drugs that slow heart rate or AV conduction.

MONITOR: Central nervous system- and/or respiratory-depressant effects may be additively or synergistically increased in patients taking multiple drugs that cause these effects, especially in elderly or debilitated patients. Sedation and impairment of attention, judgment, thinking, and psychomotor skills may increase.

MANAGEMENT: During concomitant use of these drugs, patients should be monitored for potentially excessive or prolonged CNS and respiratory depression. Cautious dosage titration may be required, particularly at treatment initiation. Ambulatory patients should be counseled to avoid hazardous activities requiring mental alertness and motor coordination until they know how these agents affect them, and to notify their physician if they experience excessive or prolonged CNS effects that interfere with their normal activities.