Cystine

Carcinogenicity

Description of key information

The effects found in the study are not relevant as different depletion diets are examined. A 14 to 17 months cystine depletion diet in mice results      even in high protein diets in pathologic calcification, primarily in the lungs but also in the aorta and arteries of other organs sectioned. 
The effect of a L-Cystine overdose is not examined and not not discussed. 

Key value for chemical safety assessment

Carcinogenicity: via oral route

Link to relevant study records

Reference

Endpoint:

carcinogenicity: oral

Type of information:

experimental study

Adequacy of study:

disregarded due to major methodological deficiencies

Reliability:

3 (not reliable)

Rationale for reliability incl. deficiencies:

other: Unsuitable test system

Principles of method if other than guideline:

Mice were fed different deficiency diets over 14 and 17 months and several organs were subject to histologic studies.
Pathologic calcification as well as a decrease in nephritis and amyloidosis were observed. The incidence of spontaneous pulmonary
tumors was determined, tumors were sectioned.

GLP compliance:

not specified

Species:

mouse

Strain:

other: strain A

Sex:

male/female

Details on test animals or test system and environmental conditions:

Strain A mice, 4-6 weeks old until 14 and 17 months of age.
Four groups: one control and three dose groups.

Route of administration:

oral: feed

Details on exposure:

1. Control group: control Purina dog chow diet (129 mice)
2. high cystine - low protein diet ad libitum (60 mice)
3. high cystine - low protein diet, restricted to the average food consumption of group 4 (95 mice)
4. low cystine - low protein diet (61 mice)

Analytical verification of doses or concentrations:

no

Details on analytical verification of doses or concentrations:

no data

Duration of treatment / exposure:

14 and 17 months

Frequency of treatment:

Depletion diet ad libitum or restricted diet (group 3).

Post exposure period:

The mice were killed at 14 to 17 months.
Animals that were found dead before were eliminated from the results.

No. of animals per sex per dose:

345 animals in one control and four dose groups

Control animals:

yes

Details on study design:

At necropsy: observations on the gross organs, histologic sections of lung, heart, aorta, kidneys, testes, ovaries and uterus.
Pulmonary tumors were sectioned, if found.

Positive control:

None

Observations and examinations performed and frequency:

At necropsy: observations on the gross organs, histologic sections of lung, heart, aorta, kidneys, testes, ovaries and uterus.
Pulmonary tumors were sectioned, if found.

Sacrifice and pathology:

All surviving mice were killed after 14 or 17 months.

Statistics:

no data

Clinical signs:

effects observed, treatment-related

Description (incidence and severity):

mortality was monitored

Mortality:

mortality observed, treatment-related

Description (incidence):

mortality was monitored

Body weight and weight changes:

not specified

Food consumption and compound intake (if feeding study):

not specified

Food efficiency:

not specified

Clinical biochemistry findings:

not specified

Organ weight findings including organ / body weight ratios:

not specified

Gross pathological findings:

effects observed, treatment-related

Description (incidence and severity):

see study design

Histopathological findings: non-neoplastic:

effects observed, treatment-related

Description (incidence and severity):

see study design

Histopathological findings: neoplastic:

effects observed, treatment-related

Description (incidence and severity):

lung tumors, if found

Relevance of carcinogenic effects / potential:

The effects found in the study are not relevant as different depletion diets are examined. A 14 to 17 months Cystine depletion diet in mice results - even in high protein diets - in pathologic calcification, primarily in the lungs but also in the aorta and arteries of other organs sectioned. Strain A mice develop nephritis in practically 100% of the animals after 12 months of age or older. The deficiency diet seems to inhibit nephritis.
The effect of a L-Cystine overdose is not examined and not not discussed.
The study therefore cannot be used to discuss the carcinogenic potential of high doses of L-Cystine.

Conclusions:

Extensive pathologic calcification was observed in strain A mice 14 and 17 months of age that had been fed a low cystine-low protein diet.
The deficient diets exerted an inhibitory effect on the development of the type of nephritis that commonly occurs in strain A mice.
Throughout the carious groups there was a correlation between the appearance of nephritis and of amyloidosis.

Executive summary:

Extensive pathologic calcification was observed in strain A mice14 and 17 months of age that had been fed a low cystine-low protein diet. The highest degree of calcification occurred in the lungs of certain animals, but it was also observed in the aorta and arteries of the other organs sectioned, including the heart, kidneys, and gonad. With the exception of the testes, the lesion was infrequent and irregular in the organs of strain A mice of the same age, fed the high cystine-low protein diet ad libitum, high cystine-low protein diet restricted, and Purina dog chow. In the arteries of the testes it occurred in males fed all these diets but was more frequent in those fed the high-cystine diets than in those fed dog-chow. The deficient diets exerted an inhibitory effect on the development of the type of nephritis that commonly occurs in strain A mice. Practically all the mice fed Purina dog chow had nephritis, but the incidence was less in those fed the high cystine-low protein diet ad libitum, still less in those fed the high cystine-low protein diet restricted, and least in the animals fed the low cystine-low protein diet. Throughout the carious groups there was a correlation between the appearance of nephritis and of amyloidosis.

Endpoint conclusion

Endpoint conclusion:

no adverse effect observed

Study duration:

chronic

Species:

mouse

Carcinogenicity: via inhalation route

Endpoint conclusion

Endpoint conclusion:

no study available

Carcinogenicity: via dermal route

Endpoint conclusion

Endpoint conclusion:

no study available

Justification for classification or non-classification

For L-Cystine no experimental study on cancerogenocity is available. There is no indication from the existing studies on CMR- toxicity that L-Cystine has toxicity referring to CMR endpoints. L-Cystine is a natural occurring amino acid present in all living cells. L-Cystine (and L-Cysteine, respectively) is an amino acid that is required for normal functioning of humans. There is sufficient weight of evidence for the absence of reprotoxic effects. There is no indication that the substance is able to induce hyperplasia or pre-neoplastic lesions in subchronic studies. Extensive genotoxicity testing does not raise any concerns for further studies. Additionally L-Cystine (and L-Cysteine respectively) are natural amino acids. It can be stated that amino acids are generally quickly absorbed and utilised in the mammal metabolism and that the amounts used in studies are generally far below the daily intake of an adult European individual (EFSA): as discussed in section 5.1.3. the intake of 2.2 g L-Cysteine which corresponds to approximately 1.1 g L-Cystine is a realistic value for daily intake. The same is for L-Cysteine. Both substances are linked by metabolic pathways and via chemical oxidation. A significant amount of L-Cystine (and L-Cysteine, respectively) is usually taken up via the food.

In usual diet, most amino acids are supplied as constituents of protein and not as free amino acid. Protein intake clearly modifies plasma amino acid levels. However, amino acid concentrations are subject to homeostasis and the plasma concentrations vary within fixed limits and are tightly regulated. Exposure with L-Cystine (and L-Cysteine, respectively) from uses which are covered by this registration would only marginally increase the total daily L-cystine dose (and L-Cysteine dose, respectively) which is taken up via the food. Even if the plasma amino acid concentration would increase/vary by any use such fluctuations are physiological and subject to homeostasis. Therefore it is highly unlikely that L-Cystine (and L-Cysteine, respectively) taken up via any use covered by this registration would result in systemic effects. A repeated dose toxicity studies consistently indicated the very low toxicity of L-Cystine (and L-Cysteine, respectively). Even in very high doses no toxicity is observed and no adverse effects were reported for the reproductive organs. Moreover, L-Cystine (and L-Cysteine, respectively) is an amino acid that is required for normal functioning of humans. The substance is of low toxicological activity. Therefore, reprotoxic effects are not expected for L-Cystine (and L-Cysteine, respectively) and no test is required for this substance. There is sufficient weight of evidence for the absence of reprotoxic effects. Therefore, in accordance with Annex X 8.9.1 column 2 and because of reasons of animal welfare, a study on carcinogenicity is not deemed necessary.

Additional information

Extensive pathologic calcification was observed in strain A mice 14 and 17 months of age that had been fed a low cystine-low protein diet for 14 and 17 months. The deficient diets exerted an inhibitory effect on the development of the type of nephritis that commonly occurs in strain A mice. Throughout the carious groups there was a correlation between the appearance of nephritis and of amyloidosis.

Justification for selection of carcinogenicity via oral route endpoint:
After an extended literature search the only study available is from 1945 and has to be rated as Klimisch 3 due to the unsuitable test system.
As cystine-depletion was examined instead of a cystine overdose, the adverse effects observed in the study don`t need to be considered.