Thymidine

Endpoint summary

Description of key information

Repeated dose toxicity was tested using L-Thymidine in a six month toxicity study with three-month interim sacrifice and one month recovery groups in rats according to OECD Guideline 408 (Repeated Dose 90-Day Oral Toxicity in Rodents). The read-across approach to the L-isomer of the nucleoside is justified. No treatment related adverse effects were noted in any of the dose groups. The NOAEL was determined to be 1,000 mg/kg/day.
According to REACH Annex VIII column 1 and 2, repeated dose toxicity testing via the inhalation and dermal routes was waived. Instead, data on repeated dose toxicity testing via the oral route is presented. The oral route is the most appropriate route of administration, having regard to the likely route of human exposure. 

Key value for chemical safety assessment

Repeated dose toxicity: via oral route - systemic effects

Link to relevant study records

Reference

Endpoint:

sub-chronic toxicity: oral

Type of information:

migrated information: read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: See read-across justification attached.

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 408 (Repeated Dose 90-Day Oral Toxicity Study in Rodents)

Deviations:

no

GLP compliance:

yes

Limit test:

no

Species:

rat

Strain:

other: Crl:CD(SD) BR

Sex:

male/female

Route of administration:

oral: gavage

Vehicle:

CMC (carboxymethyl cellulose)

Remarks:

0.5 % (wt/vol)

Duration of treatment / exposure:

Six month treatment with three month interim sacrifice and one month recovery groups (control and high dose group)

Frequency of treatment:

Daily

Remarks:

Doses / Concentrations:
0, 250, 500, 1000 mg/kg bw/day
Basis:
actual ingested

No. of animals per sex per dose:

10/sex/group for control and treatment groups and 5/sex/recovery group

Control animals:

yes, concurrent vehicle

Observations and examinations performed and frequency:

Repeat-dose toxicity was evaluated based on mortality; clinical observations; body weight; food consumption; ophthalmology; clinical pathology, including hematology, coagulation, and chemistry; and/or macroscopic and microscopic pathology.

Sacrifice and pathology:

GROSS PATHOLOGY: Yes
HISTOPATHOLOGY: Yes

Clinical signs:

no effects observed

Mortality:

no mortality observed

Body weight and weight changes:

no effects observed

Food efficiency:

no effects observed

Ophthalmological findings:

no effects observed

Haematological findings:

no effects observed

Clinical biochemistry findings:

no effects observed

Organ weight findings including organ / body weight ratios:

no effects observed

Gross pathological findings:

no effects observed

Histopathological findings: non-neoplastic:

no effects observed

Details on results:

In the 6-month repeat-dose toxicity study with 3-month interim sacrifice in rats, six rats in total were found dead (one control male, two males dosed at 500 mg/kg/day, and two females dosed at 1,000 mg/kg/day) or sacrificed for humane reasons (one male dosed at 1,000 mg/kg/day). Except for one male dosed at 500 mg/kg/day that was found dead on day 161 and whose cause of death was undetermined, the cause of death for the remaining four rats was considered to be gavage related. The male dosed at 1,000 mg/kg/day that was sacrificed for humane reasons on study day 133 was diagnosed with widespread lymphoma, a common spontaneous tumor. None of these deaths was attributed to the test item. Treatment had no adverse effects on food consumption, but some dosed rats had intervals of increased food intake. This transient increased appetite was not considered evidence for toxicity from the test item. Ophthalmic examinations of rats in all dose groups did not identify any treatment-related changes; there were some age-related changes. Hematology data did not indicate toxicity from the test item after 92 days (3 months), 176 days (6 months), or 204 days (1-month recovery). There were no conclusive findings from the clinical chemistry data to indicate evidence for toxicity at any interval. Mean absolute and relative organ weights were somewhat variable but failed to demonstrate a clear pattern for toxicity from the test item. No macroscopic or microscopic morphological changes were associated with test item exposure. The NOAEL was reported to be 1,000 mg/kg/day.

Dose descriptor:

NOAEL

Effect level:

1 000 mg/kg bw/day (nominal)

Based on:

test mat.

Sex:

male/female

Basis for effect level:

other: No treatment related adverse effects observed up to the highest dose (limit dose) tested.

Critical effects observed:

not specified

Conclusions:

No treatment related adverse effects were noted in this 6-month repeat-dose toxicity study with 3-month interim sacrifice in rats, including recovery groups. The NOAEL for males and females was determined to be 1000 mg/kg bw/day (limit dose).

Executive summary:

A study was carried out similar or equivalent to OECD Guideline 408 (Repeated Dose 90-Day Oral Toxicity in Rodents) using L-Thymidine. The read-across approach to the L-isomer of the nucleoside is justified.

A total of 10 males and 10 females was dosed at 0, 250, 500 and 1000 mg/kg bw/day (up to the limit dose) for six month with three month interim sacrifice and one month recovery groups (control and high dose group). Repeat-dose toxicity was evaluated based on mortality; clinical observations; body weight; food consumption; ophthalmology; clinical pathology, including hematology, coagulation, and chemistry; and/or macroscopic and microscopic pathology.

In the 6-month repeat-dose toxicity study with 3-month interim sacrifice in rats, none of the observed deaths was attributed to the test item. Treatment had no adverse effects on food consumption, but some dosed rats had intervals of increased food intake. Ophthalmic examinations of rats in all dose groups did not identify any treatment-related changes. Hematology data did not indicate toxicity from the test item after 92 days (3 months), 176 days (6 months), or 204 days (1-month recovery). There were no conclusive findings from the clinical chemistry data to indicate evidence for toxicity at any interval. Mean absolute and relative organ weights were somewhat variable but failed to demonstrate a clear pattern for toxicity from the test item. No macroscopic or microscopic morphological changes were associated with test item exposure. The NOAEL was determined to be 1000 mg/kg/day.

Endpoint conclusion

Endpoint conclusion:

no adverse effect observed

Dose descriptor:

NOAEL

1 000 mg/kg bw/day

Study duration:

subchronic

Species:

rat

Quality of whole database:

Good quality data base.

Repeated dose toxicity: inhalation - systemic effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: inhalation - local effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: dermal - systemic effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: dermal - local effects

Endpoint conclusion

Endpoint conclusion:

no study available

Justification for classification or non-classification

Based on the data available the substance is not classified and labeled according to Regulation 1272/2008/EEC (CLP) and Directive 67/548/EEC (DSD).