In two Segment II Teratology studies, timed-pregnant Crl:CD[BR] (Sprague-Dawley) rats were treated orally (gastric intubation) on days 6-15 of gestation with ibutilide fumarate (ibutilide), a class III antiarrhythmic that has been shown to increase the refractory period and action potential duration of myocardial cells. In the first study, ibutilide does of 20, 40, and 80 mg/kg/ day were tested. Although maternal toxicity was equivocal in the 80 mg/kg/day group, all 23 rats that conceived had entirely resorbed liters when the animals were killed on day 20 of gestation. Similarly, 12 of 24 litters were completely resorbed in the 40 mg/kg/day group, with an 87.7% postimplantational loss. Of the surviving fetuses in this group, 48.6% had at least one malformation. The incidences of malformed pharynx and malformed palate, along with adactyly, were statistically significantly higher in this group than in the control group. In addition, a significant (P < 0.05) increase in total malformations (5.7% of the fetuses), relative to the controls (0.8%), was found for the 20 mg/kg/day group. Since a no observed adverse effect level (NOAEL) was not found, a second teratology study was performed. In this study, the ibutilide doses were 5, 10, and 20 mg/kg/day. The 20 mg/kg/day dose was again teratogenic with 9.2% of the fetuses malformed, as compared to a control value of 1.0%. Also, the incidences of scoliosis and interventricular septal defect were statistically significantly higher in this group. Although statistically significant differences were not detected, scoliosis was also found in the 10 mg/kg/day group (3 fetuses in 2 litters), along with a significant dose-response trend for this malformation. As the result, the NOAEL for ibutilide teratogenicity in rats was set at 5 mg/kg/day. This dose is 4 times the proposed maximum clinical dose (two 1 mg doses, each infused over 10 minutes, or 0.033 mg/kg for a 60 kg person), when corrected for 2.6% oral bioavailability in the rat at a dose of 10 mg/kg, as determined in separate studies.