The following abstract presents results of a study conducted by a contract laboratory for the National Toxicology Program. The findings may not have been peer reviewed and were not evaluated in accordance with the levels of evidence criteria established by NTP in March 2009. For more information, see the Explanation of Levels of Evidence for Developmental Toxicity. The findings and conclusions for this study should not be construed to represent the views of NTP or the U.S. Government.
Isoproterenol hydrochloride, a widely used bronchodilator, was evaluated for teratogenicity following maternal exposure. Timed-pregnant CD rats were dosed by intraperitoneal injection on gestational days 6 through 15 with isoproterenol hydrochloride (0, 20, 40 or 80 mg/kg/day) in sterile 0.9% saline. These dose groups are referred to as IP-0, IP-20, IP-40 and IP-80, respectively. Dams were weighed on gestational days 0, 6-15 (prior to daily dosing) and 20 (immediately following sacrifice), and were also observed for clinical signs of toxicity. At sacrifice on gestational day 20, dams were evaluated for body weight, liver weight, gravid uterine weight and status of uterine implantation sites (i.e., resorptions, dead fetuses, live fetuses). Live fetuses were dissected from the uterus and evaluated for live litter size, body weights, sex ratios and gross morphological abnormalities. All live fetuses were examined for visceral malformations employing the Staples' fresh tissue dissection method. Half of the fetuses were decapitated prior to dissection and the heads were fixed in Bouin's solution for free hand sectioning and examination (Wilson's Technique). All fetal carcasses were prepared with Alizarin Red S stain and examined for skeletal malformations.
The maternal mortality rate in the present study was 25.0% (8/32 dams) for IP-20 dams, 12.1% (4/33 dams) for IP-40 dams and 8.6% (3/35 dams) for IP-80 dams. All dams from the control group survived to terminal sacrifice on gestational day 20. No significant dose-response trends or differences between dose groups were observed on measures of maternal body weight (gestational day 11, 15, or 20), maternal weight gain (gestation period, treatment period or absolute weight gain),gravid uterine weight, absolute liver weight or relative liver weight. Weight loss of more than 5 grams/day, considered to be a clinical sign of toxicity in individual dams, was observed in 18.8% (6/32) of IP-20 dams, 45.5% (15/33) of IP-40 dams and 57.1% (20/35) of IP-80 dams as early as the second day of treatment. Other clinical signs seen across dose groups included piloerection, prone positioning, lethargy, ataxia, polydipsia, hyperventilation, and excessive consumption of wood-chip bedding (pica).
Data concerning the status of uterine implantation sites showed no statistically significant dose-response related differences on the following measures: number of implantation sites per dam; number or percent of resorptions, fetal deaths, non-live fetuses (dead plus resorbed) or affected fetuses (non-live plus malformed) per litter; proportion of litters with one or more resorptions, fetal deaths, nonlive or affected. In litters with one or more live fetuses (i.e. live litters) there was no difference in the number of live fetuses per litter, or in the proportion of males to females per litter across treatment groups. Body weight of live fetuses in live litters exhibited a significant dose-response trend and was significantly lower than controls in all three experimental groups. When separated by sex, average male and female body weights per litter were also significantly depressed in all experimental groups relative to controls with a significant dose-response trend. There was no significant difference between treated and control litters in the number or percentage of males, females or live fetuses malformed, nor was there any statistical difference in the number of litters with malformed fetuses. Examination of malformation incidence by category produced no evidence of dose-response trends nor evidence of any malformation unique to, or with higher incidence in, any of the experimental groups relative to controls.
In conclusion, no evidence for teratogenicity of isoproterenol hydrochloride administered by intraperitoneal injection during the time of organogenesis was seen in pregnant CD rats at doses which produced evidence of fetal and maternal toxicity (20-80 mg/kg/day).