1,3,5-triazine-2,4,6-triamine phosphate

Carcinogenicity

Description of key information

There is no study available with melamine phosphate but with melamine.

 

Melamine was administered to rats and mice in a daily diet over a period of 103 weeks. Due to melamine induced formation of bladder stones in male rats only, leading to bladder tumors at 4500 ppm (ca. 263 mg/kg bw/day), the NOAEL is considered to be 2250 ppm for males (126 mg/kg bw/day) and 4500 ppm for females (269 mg/kg bw/day).

Hard et al. (2009), see section 7.5.1, re-analysed histological specimen of the NTP-studies in rat and detected additional lesions, turning the NOAEL of 2250 ppm into a LOAEL.

No neoplastic findings related to treatment were observed in male or female mice.

 

A further 2-years carcinogenicity study (Reno 1983) was performed at lower doses and after the NTP study. It did not reveal carcinogenicity in male rats (and also not in female rats) up to the highest used doses of 1000 ppm in male and 2000 ppm in female rats.

Key value for chemical safety assessment

Carcinogenicity: via oral route

Link to relevant study records

Reference

Endpoint:

carcinogenicity: oral

Type of information:

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

Adequacy of study:

key study

Justification for type of information:

For Read Across Justification please refer to section 13.

Reason / purpose for cross-reference:

read-across: supporting information

Reason / purpose for cross-reference:

read-across source

Key result

Dose descriptor:

NOAEL

Effect level:

2 250 ppm

Sex:

male

Basis for effect level:

other: other: transitional cell carcinoma in the urinary bladder

Remarks on result:

other: CAS 108-78-1

Key result

Dose descriptor:

NOAEL

Effect level:

4 500 ppm

Sex:

female

Basis for effect level:

other: other: Chronic inflammation, kidney

Remarks on result:

other: CAS 108-78-1

Critical effects observed:

yes

System:

urinary

Organ:

bladder

Treatment related:

yes

Critical effects observed:

yes

System:

immune system

Organ:

thyroid gland

Treatment related:

yes

Endpoint conclusion

Endpoint conclusion:

adverse effect observed

Dose descriptor:

LOAEL

126 mg/kg bw/day

Study duration:

chronic

Species:

rat

Quality of whole database:

similar to OECD TG 451 (Read-Across)

System:

urinary

Organ:

bladder

Carcinogenicity: via inhalation route

Endpoint conclusion

Endpoint conclusion:

no study available

Carcinogenicity: via dermal route

Endpoint conclusion

Endpoint conclusion:

no study available

Mode of Action Analysis / Human Relevance Framework

Regarding melamine, several studies to evaluate the mechanism of bladder neoplasms were performed. Experimental data show that melamine is not genotoxic, confirmed in many studies using different methods.

The relevant trigger for the toxicity of melamine are the urinary tract calculi at sufficiently high doses, because of the precipitation of melamine in the urine above the solubility limit. The calculi in the urinary bladder are leading to chronic irritation, hyperplasia and finally to tumours. This mechanism is also known for implanted foreign inert bodies but also for low toxic substances such as thymine, uracil, silicates, sodium ascorbate, etc. (Okumura 1992, Ogasawara 1995, IARC 1999, McGregor 2010). For IARC 1999 and McGregor 2010 see Section 13. No tumour formation is to be expected when the urinary levels of melamine are maintained below the threshold level of precipitation.

Additional dosing of NaCl, which causes a higher water intake, and a dilution in the urine, suppresses the calculus formation and hyperplasia of the papilla in the kidney (Ogasawara 1995).

The mode of action is not relevant for humans, as no high enough doses has ever been observed to occur in humans to produce bladder calculi, except for the past cases of fraudulent tainting infant formulas in China. No such high exposures are also anticipated in the future. Recently, Cohen SM stated: "The risk assessment for such chemicals as melamine is not related to its carcinogenicity in the animal model, but to the actual toxicity of the formation of calculi, which can occur in humans. Thus, if urinary solids formation is the mode of action concluded for a particular chemical, the risk assessment is related to the toxicity of urinary solid formation, not to carcinogenicity." [Cohen SM. Screening for human urinary bladder carcinogens: two-year bioassay is unnecessary. Toxicol. Res., 2018, 7, 565].

In the next publication, SM Cohen stated: "The mode of action analysis shows that the key events are ingestion of high doses of the test material leading to formation of the urinary tract crystals or calculi producing urothelial toxicity and consequent urothelial proliferation. Tumors occur in rodents if this persists for a long period of time.

The question is whether these tumorigenic effects occur in all species, whether rodents, pets, or humans. It turns out that the urothelial tumors secondary to urinary crystals or calculi do not predict to tumors in other species such as cats, dogs, nonhuman primates, and humans" [Cohen SM.Crystalluria and Chronic Kidney Disease. Toxicol Pathol. 2018 Dec;46(8):949-955]. The main point is that in humans,calculi are present for short periods of time and tumors do not develop. ...There are a few unusual circumstances where calculi can be present in the urinary bladder of humans for long periods oftime, such as bladder diverticuli or neurogenic bladder secondary to paraplegia. ... Consequently, most regulatory authorities consider urinary tract crystals and calculi not to be relevant to human carcinogenesis.

Justification for classification or non-classification

Classification, Labelling, and Packaging Regulation (EC) No. 1272/2008

The available experimental test data are reliable and suitable for classification purposes under Regulation 1272/2008. As a result, the substance is not classified for carcinogenicity under Regulation (EC) No. 1272/2008.

Additional information

Melamine phosphate was not tested for carcinogenicity. It is acceptable to derive the systemic toxicity from experimental data of melamine since both melamine and its phosphate salt have a high solubility in water and a log POW of less than 1 and phosphate is an abundant in cells and body fluids. Melamine and melamine phosphate have a similar solubility in water (3.5 – 3.9 g/L at 20°C). In addition, the higher molecular weight of melamine phosphate compared to melamine gives a safety margin.

 

Effects of melamine to urinary bladder

A carcinogenesis study of melamine was conducted by feeding diets containing 2250 or 4500 ppm melamine to groups of 50 male F344 rats and 50 B6C3F1, mice of each sex for 103 weeks. Groups of 50 female rats were fed diets containing 4500 or 9000 ppm melamine. Groups of 49 male rats, 50 female rats, 49 male mice, and 50 female mice served as controls. The predominant effect of melamine in rats was formation of urinary bladder stones in association with bladder tumors in male rats. Moreover, chronic inflammation was significantly increased in the kidney of dosed female rats and is attributed to the administration of melamine. Acute and chronic inflammation and epithelial hyperplasia of the urinary bladder were found in increased incidence in dosed male mice. However, there was no evidence of bladder tumor development in this species (NTP rat/mouse, 1983).

 

In the publication of Melnick R.L. et al. (1984) the study results for rats mentioned above were summarized. Melamine was administered in the diet to F344 rats or B6C3F1 mice for 103 weeks to determine carcinogenic potential. The dose levels of melamine were 2250 or 4500 ppm for male rats and mice of each sex, and 4500 or 9000 ppm for female rats. Transitional-cell carcinomas in the urinary bladder of male rats occurred at a significantly higher incidence in the high dose group than in the controls. Seven of the eight male rats with transitional-cell carcinomas of the urinary bladder also had bladder stones. There was a significant association between bladder stones and bladder tumors in male rats fed with high doses of melamine. Urinary bladder tumors were not observed in the low-dose male rat group, while bladder stones were observed in one rat in this group. In female rats, chronic inflammation of the kidney was observed at an increased incidence in both the low and high dose groups.

 

Hard et al. 2009 re-evaluated the histological specimen of the kidneys from NTP-studies and detected retrograde nephropathy in 7 out of 50 rats in the low dose group (2250 ppm), turning the NOAEL of 2250 ppm into a LOAEL. 

 

Discussion

The primary toxicity of melamine is based on the formation of bladder stones. This is also the primary toxic effect for other substances, e.g. dimethyl terephthalate and terephthalic acid. In several studies (especially related to melamine and also for the other compounds) it was shown that the irritative stimulation by calculi leads to proliferative lesions in the urinary tract which can result in the formation of tumors in the urinary bladder. As no genotoxic properties were shown for melamine, it can be assumed that the formation of bladder calculi is the only effect which leads to the formation of bladder calculi.

The production of calculi is a high-dose-only phenomenon. Therefore, the extrapolation to humans from the rodent bioassay should be dependent on dose requirements for formation of calculi rather than any type of statistical extrapolation to lower doses. It must be presumed that calculus formation also poses some carcinogenic risk to humans. However, the doses frequently required to form calculi are several orders of magnitude greater than the doses expected for human exposure.

 

Conclusion

Melamine was administered to rats and mice in a daily diet over a period of 103 weeks. Due to melamine induced formation of bladder stones in male rats only leading to bladder tumors at 4500 ppm, the LOAEL is considered to be 2250 ppm for males (126 mg/kg bw/day) and 4500 ppm for females (269 mg/kg bw/day).

 

A chronic study with rats and with doses at the lower end of the NTP-study was performed by Reno 1985. The NOAEL was 1000 ppm for male and 2000 ppm for female rats.