ENCAINIDE
Drugs in Pregnancy and Lactation.
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Name: ENCAINIDE
Class: Antiarrhythmic
Risk Factor: BM
Fetal Risk Summary
Encainide is a cardiac agent used for the treatment of ventricular arrhythmias. Neither animal nor human teratogenicity has been observed, but human experience is very limited.
The parent compound and its two active metabolites (more potent than encainide on a per milligram basis), o-demethylencainide and 3-methoxy-o-demethylencainide, cross the placenta to the fetus (B.D. Quart, personal communication, Bristol-Myers Company, 1988). Concentrations of encainide and its metabolites in fetal plasma have ranged from 30%–300% of simultaneously collected maternal serum levels (B.D. Quart, personal communication, 1988). Excessive accumulation of the drug or its metabolites in the fetal plasma apparently does not occur. Amniotic fluid levels of the three compounds in one case were 2–3 times greater than in fetal plasma (B.D. Quart, personal communication, 1988).
Encainide has been successfully used to treat a fetal cardiac arrhythmia in utero, but a high maternal dose, 50 mg four times a day, was required to control the abnormality (B.D. Quart, personal communication, 1988). The gestational age of the fetus was not given nor was the length of therapy. In this case, encainide could not be found in fetal plasma, and concentrations of the two metabolites were less than 100 ng/mL. Very high concentrations of the metabolites were measured in the newborn's first several urine samples.
Breast Feeding Summary
Encainide and its active metabolites are excreted in breast milk. In one patient taking 50 mg four times a day, milk levels of encainide and one metabolite, o-demethylencainide, were 200–400 ng/mL and 100–200 ng/mL, respectively (B.D. Quart, personal communication, 1988). These levels were comparable to maternal peak plasma levels. A third metabolite, 3-methoxy-o-demethylencainide, evidently was not produced by the mother because it was not found in her plasma or milk. However, based on animal experiments, it is also expected to cross into the milk if present in the maternal plasma (B.D. Quart, personal communication, 1988).
