Abstract
Mated Crl:CD VAF/Plus female rats, in a range-finding study (n = 5–6 per dose) and a subsequent definitive study (n = 30 per dose) were used to determine the developmental toxicity, including the teratogenic potential of levo-alpha-acetyhnethadol (LAAM) hydrochloride, in tolerant rats. Tolerance was induced by initially administering the drug by gavage (10 ml/kg) at 2 mg/kg/day and increasing the dose every 2 weeks for 12 weeks until the doses of 2, 6, 9, 12, and 15, or 2, 6, and 12 mg/kg/day were achieved in the range-finding or definitive study, respectively. Females were then mated to stock males and treated throughout mating and gestation. Controls received distilled water on a similar regimen. The range-finding experiment was used for initial clinical evaluations and to determine tissue concentrations of LAAM and metabolites. In plasma, liver, and brain collected from dams and fetuses pooled by litter on gestation day 20, LAAM and its two N-demethylated metabolites, norLAAM and dinorLAAM, showed dose-dependent increases in concentration and in tissue to plasma ratios. Tissue to dam plasma ratios were highest in dam liver (17–60), intermediate in fetal liver (3–16), and fetal brain (3–14), and lowest in dam brain (0.8–5.6) and fetal plasma (0.3–2.1). In the definitive study, caesarean section examinations were performed following euthanization on gestation day 20 on all surviving females followed by teratologic examination of the fetuses. Drug-related outcomes, including increased activity, secondary hair loss, scabbing, focal swelling, and material around the nose, were exhibited by all groups receiving LAAM. Maternal toxicity was evident as decreased body weights, with maximum reduction at the 6-mg/kg/day dose, and reduction in feed consumption. There was also evidence of developmental toxicity in the form of postimplantaüon losses at all doses of LAAM. There were no deaths attributable to LAAM. No grossly observable visceral or skeletal anomalies related to LAAM were observed in the fetuses. In conclusion, the no-observable-effect level when administered to tolerant rats was less than 2 mg/kg/day with regard to clinical signs, body weight, body weight gain, and feed consumption, and with regard to developmental toxicity as reflected by postimplantaüon losses. Despite maternal and developmental toxicity, there was no evidence of selective fetal toxicity or teratogenic activity attributable to LAAM.