National Toxicology Program

National Toxicology Program

Abstract for TR-71 - L-Tryptophan (CASRN 73-22-3)

Bioassay of L-Tryptophan for Possible Carcinogenicity (CAS No. 73-22-3)

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Chemical Formula: C11H12N2O2

L-Tryptophan is an essential amino acid for humans, and a precursor of the neurohormones serotonin (5-hydroxytryptamine) and melatonin (N-acetyl-5-methoxytryptamine), and the B vitamin nicotinic acid. It is found in small concentrations in casein, and in many foods.

A bioassay of the amino acid L-tryptophan for possible carcinogenicity was conducted by administering the test chemical in feed to Fischer 344 rats and B6C3F1 mice.

Groups of 35 rats and 35 mice of each sex were administered L-tryptophan at one of two doses, either 25,000 or 50,000 ppm, 5 days per week for 78 weeks, and then observed for 26 or 27 weeks. Matched controls consisted of groups of 15 rats or 15 mice of each sex. All surviving rats and mice were killed at 104 or 105 weeks.

L-Tryptophan had little toxic effect on the rats; mean body weight loss was minimal and survival of dosed groups of both sexes was high. In the mice, mean body weights of dosed animals were lower than those of controls throughout most of the bioassay, particularly in the females. Sufficient numbers of rats were at risk to termination of the study for development of late-appearing tumors, and sufficient numbers of mice were at risk beyond 52 weeks of the study for development of tumors.

No neoplasms occurred in a statistically significant incidence among dosed rats when compared with controls.

In both male and female mice, neoplasms of the hematopoietic system occurred at higher incidences in the low-dose groups than in the matched-control groups (males: controls 0/12, low-dose 9/34, high-dose 2/33; females: controls 2/13, low-dose 6/33, high-dose 1/35). These incidences, however, are not statistically significant, using the Bonferroni correction, and therefore, no tumors are considered to be related to the administration of the test chemical.

It is concluded that under the conditions of this bioassay, L-tryptophan was not carcinogenic for Fischer 344 rats or B6C3F1 mice.


Levels of Evidence of Carcinogenicity:
Sex Species Results
Male Rats: Negative
Female Rats: Negative
Male Mice: Negative
Female Mice: Negative

Synonym: L-a-amino-b-indolepropionic acid

Report Date: 1978

Target Organs from 2-year Studies

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