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.
L-5-Hydroxytryptophan, a metabolite of tryptophan and direct precursor of the biogenic amine, 5-hydroxytryptamine, was evaluated for teratogenicity following maternal exposure. Timed-pregnant CD rats were dosed by gavage (po) on gestational days (gd) 6 through 15 with HTP (0.0, 50.0, 100.0, 150.0 or 300.0 mg/kg/day) in corn oil. These dose groups were referred to as HTP-0, HTP-50, HTP-100, HTP-150, or HTP-300 respectively.
Dams were weighed on gd 0, 6 through 15 (prior to daily dosing) and 20 (immediately after sacrifice) and were also observed for clinical signs of toxicity. Food and water intake was monitored throughout the study for replicates II and III. At sacrifice on gd 20, dams were evaluated for body weight, liver weight, kidney weight (replicate II and III), gravid uterine weight and status of uterine implantation sites (i.e., sites, resorptions, dead fetuses, live fetuses). Live fetuses were dissected from the uterus and evaluated for live litter size, body weight, sex 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 cleared and stained with Alizarin Red S and examined for skeletal malformations.
There was one maternal death at HTP-100 (2.7% mortality); all other dams survived to scheduled sacrifice on gd 20. A significant dose-response trend for reduction in maternal body weight was observed on gd 11, 15 and 20 but not on gd 0 or 6 (prior to onset of dosing), with no significant pairwise comparisons. There was also a significant downward trend for maternal weight gain during gestation period and treatment period with the values for both periods significantly lower at HTP-300 versus HTP-0. Absolute weight gain exhibited a significant downward trend but no significant pairwise comparisons. Gravid uterine weight also exhibited a significant downward trend with the value at HTP-300 significantly lower than at HTP-0. Relative maternal liver weight, absolute and relative maternal kidney weight all exhibited a significant upward trend with the values at HTP-300 significantly higher than in the HTP-0. Maternal kidney weight was also significantly higher at the HTP-150 versus HTP-0. The following clinical signs were exhibited by dams in a clear dose-dependent manner: piloerection, rough coat, reddened feet and ears, lethargy and excessive urination. There was a significant downward trend for food intake during the dosing period (gd 6-15) with HTP-300 values significantly lower than those of HTP-0. During the post-dosing period (gd 15-20) the trend was significantly upward with the value for HTP-300 significantly higher than for HTP-0. There were no dose-related effects on food intake for the gestation period (gd 0-20). Water intake exhibited a significant dose-related upward trend for treatment period, post-dosing period and gestation period. In pairwise comparisons, HTP-300 values were significantly higher than those of HTP-0 for all three periods. In addition the value for water intake in the post-dosing period at HTP-150 was significantly higher than for HTP-0. Histopathology of maternal kidneys indicated a clear dose-response in incidence and severity of renal lesions, most severe and originating in the cortical tubules. There were no significant differences among dose groups in the number of implantation sites per litter, number of resorptions, or of fetal deaths or of non-live (dead plus resorbed) fetuses per litter, or for the number or proportion of litters with one or more resorptions, fetal deaths or non-live fetuses. The percent resorptions and affected (non-live plus malformed) fetuses per litter exhibited a significant upward trend with the value at HTP-300 significantly higher than HTP-0. Number of litters with affected fetuses also exhibited a significant upward trend. In live litters, there were no significant differences among dose groups in the number of live fetuses, male or female per litter. Percent males per litter was elevated in the HTP-50 versus HTP-0 with no significant trend and no other significant pairwise comparisons. Average fetal body weight per litter (all fetuses or separated by sex) exhibited a significant downward trend with the values at HTP-300 significantly different from HTP-0. Percent fetuses malformed per litter and number of litters with malformations exhibited a significant upward trend with the values at HTP-300 significantly higher than that at HTP-0. Percent males malformed per litter exhibited a significant (p<0.05) upward trend but no significant pairwise comparisons. Percent female fetuses malformed per litter exhibited a significant (p<0.001) upward trend, with the value at HTP-300 significantly higher than that at HTP-0. Examination of malformation incidence by category indicated that the number of litters with visceral malformations exhibited a significant upward trend across dose groups. The visceral defects observed were hydroureter, hydronephrosis, renal agenesis (one fetus), and hydrocephaly (one fetus). Gross malformations included anophthalmia and microphthalmia. Skeletal defects included short rib and missing rib.
In conclusion, administration of L-5-hydroxytryptophan in corn oil by gavage to pregnant CD rats during organogenesis produced teratogenicity at a dose (HTP-300) which also produced clear indications of maternal and other fetal toxicity.