Methenamine

Endpoint summary

Description of key information

Oral (Animal data): In the key study (Lijinsky, 1977), it was concluded that >= 0.1%methenamine(equivalent to >= 80 mg/kgbw/d in males and >= 100 mg/kgbw/d in females, highest test dose applied) was established as the no-toxic-effect-level (NOAELsys) for Sprague-Dawleyrats. Human Data: Dose level causing no toxic effects in man: 2 to 4 g/d, equivalent to ca. 28 to 57 mg/kgbw/d (Goodman and Gilman, 1975; Martindale, 2005). The NOAEL of 4 g/d (57 mg/kgbw/d based on a body weight of 70 kg person) in man (Goodman and Gilman, 1975; Martindale, 2005) is derived from decades of long experience withmethenamineas a therapeutic substance (Human:NOAELsys: 57 mg/kgbw/d) and used as a reference figure in the risk assessment.

Key value for chemical safety assessment

Repeated dose toxicity: via oral route - systemic effects

Link to relevant study records

Reference

Reference 1

Endpoint:

sub-chronic toxicity: oral

Remarks:

combined repeated dose and carcinogenicity

Type of information:

experimental study

Adequacy of study:

key study

Study period:

1968

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: public available literature (non GLP, non guideline) but good documentation

Reason / purpose for cross-reference:

reference to same study

Qualifier:

equivalent or similar to guideline

Guideline:

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

Deviations:

yes

Remarks:

only one single dose, limited number of parameters investigated

Principles of method if other than guideline:

Public avaialble literature. No guideline indicated. For details on method see materials and methods section.

GLP compliance:

not specified

Limit test:

yes

Species:

rat

Strain:

Wistar

Sex:

male/female

Details on test animals or test system and environmental conditions:

10 weeks old, outbread Wistar rats

Route of administration:

oral: drinking water

Vehicle:

water

Details on oral exposure:

Groups of 48 male and 48 female outbred Wistar rats (10 weeks old) received 0, or 1.0% methenamine in the drinking water for 104 weeks (calculated intake 2.0-1.5 g/kg bw/d in males and 2.5-2.0 g/kg bw/d in females).

Analytical verification of doses or concentrations:

no

Details on analytical verification of doses or concentrations:

no analytical measurement

Duration of treatment / exposure:

104 weeks

Frequency of treatment:

continuous (drinking water ad libitum)

Remarks:

Doses / Concentrations:
0, or 1.0% methenamine (calculated intake 2.0-1.5 g/kg bw/d in males and 2.5-2.0 g/kg bw/d in females)
Basis:
nominal in water

No. of animals per sex per dose:

48

Control animals:

yes, plain diet

Details on study design:

Groups of 48 male and 48 female outbred Wistar rats (10 weeks old) received 0, or 1.0% methenamine in the drinking water for 104 weeks (calculated intake 2.0-1.5 g/kg bw/d in males and 2.5-2.0 g/kg bw/d in females). After the termination of treatment rats were observed for a subsequent treatment-free period of up to 3 years of age.

Positive control:

no positive control.

Observations and examinations performed and frequency:

Animals were inspected daily and weighted every two weeks. Water intake was determined periodically (no further information). There were no data on hematology and clinical biochemistry.

Sacrifice and pathology:

Necropsy and microscopic examination of organ samples were carried on animals dying during the study or were killed at the end of the study.

Other examinations:

no other examinations.

Statistics:

not indicated.

Clinical signs:

no effects observed

Mortality:

no mortality observed

Body weight and weight changes:

no effects observed

Food consumption and compound intake (if feeding study):

not specified

Food efficiency:

not specified

Water consumption and compound intake (if drinking water study):

no effects observed

Ophthalmological findings:

not examined

Haematological findings:

not examined

Clinical biochemistry findings:

not examined

Urinalysis findings:

not examined

Behaviour (functional findings):

not examined

Organ weight findings including organ / body weight ratios:

no effects observed

Gross pathological findings:

no effects observed

Histopathological findings: non-neoplastic:

no effects observed

Histopathological findings: neoplastic:

no effects observed

Details on results:

Water intake was comparable in both control and methenamine treated test groups throughout the study. Body weights showed no significant differences between controls and methenamine treated groups. At the end of the second year 84% of survivors were noted in methenamine-treated and untreated animals. In all methenamine treated rats a yellow coloration of the coat was observed. At necropsy and microscopic examination no specific pathological lesions related to methenamine treatment were observed in rats which died during the study or were sacrificed at the end of the test.

Key result

Dose descriptor:

NOAEL

Effect level:

>= 1 500 - 2 000 mg/kg bw/day (nominal)

Sex:

male

Basis for effect level:

other: no treatment related effects observed.

Key result

Dose descriptor:

NOAEL

Effect level:

>= 2 000 - 2 500 mg/kg bw/day (nominal)

Sex:

female

Basis for effect level:

other: no treatment related effects observed.

Key result

Critical effects observed:

no

Conclusions:

Based on the results of the study, >= 1.0% (calculated intake >= 2.0-1.5 g/kg bw/d in males and >= 2.5- 2.0 g/kg bw/d in females) methenamine was established as no-toxic-effect-level (NOAELsys) for outbred Wistar rats.

Executive summary:

Groups of 48 male and 48 female outbred Wistar rats (10 weeks old) received 0, or 1.0% methenamine in the drinking water for 104 weeks (calculated intake 2.0-1.5 g/kg bw/d in males and 2.5-2.0 g/kg bw/d in females). After the termination of treatment rats were observed for a subsequent treatment-free period of up to 3 years of age. Animals were inspected daily and weighted every two weeks. Water intake was determined periodically (no further information). There were no data on hematology and clinical biochemistry. Necropsy and microscopic examination of organ samples were carried on animals dying during the study or were killed at the end of the study. Water intake was comparable in both control and methenamine treated test groups throughout the study. Body weights showed no significant differences between controls and methenamine treated groups. At the end of the second year 84% of survivors were noted in methenamine-treated and untreated animals. In all methenamine treated rats a yellow coloration of the coat was observed. At necropsy and microscopic examination no specific pathological lesions related to methenamine treatment were observed in rats which died during the study or were sacrificed at the end of the test. Based on the results of the study, >=1.0% (calculated intake >=2.0-1.5 g/kg bw/d in males and >= 2.5- 2.0 g/kg bw/d in females) methenamine was established as no-toxic-effect-level (NOAELsys) for outbred Wistar rats.

Reference 2

Endpoint:

chronic toxicity: oral

Type of information:

experimental study

Adequacy of study:

key study

Study period:

1977

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: public available literature (non GLP, non guideline)

Reason / purpose for cross-reference:

reference to same study

Qualifier:

no guideline followed

Principles of method if other than guideline:

Public avaialble literature. No guideline indicated. For details on method see IUCLID5 materials and methods section.

GLP compliance:

not specified

Limit test:

no

Species:

rat

Strain:

Sprague-Dawley

Sex:

male/female

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Age at study initiation: 8-10 weeks
- Housing: 3 rats in one cage
- Diet: ad libitum
- Water: ad libitum

Route of administration:

oral: drinking water

Vehicle:

water

Details on oral exposure:

Groups of 15 male and 15 female Sprague-Dawley rats (8-10 weeks old) received on 5 days/week 0.1% methenamine in drinking water or 0.1% methenamine with 0.2% sodium nitrite in drinking water (no data of negative controls).

Analytical verification of doses or concentrations:

not specified

Details on analytical verification of doses or concentrations:

not indicated.

Duration of treatment / exposure:

50 weeks

Frequency of treatment:

5 days/per week

Remarks:

Doses / Concentrations:
5 g/day methenamine (equivalent to 80 mg/kg bw/d in males and 100 mg/kg bw/d in females, calculated on an assumed body weight of 250 g in males and 200 g in females)
Basis:
nominal in water

No. of animals per sex per dose:

15

Control animals:

not specified

Details on study design:

Groups of 15 male and 15 female Sprague-Dawley rats (8-10 weeks old) received on 5 days/week 0.1% methenamine in drinking water or 0.1% methenamine with 0.2% sodium nitrite in drinking water (no data of negative controls). Animals were treated for 50 weeks (a total of 250 days) and kept until they died or were killed due to moribund condition. Each rat received a total of 5 g methenamine over 50 weeks (equivalent to 80 mg/kg bw/d in males and 100 mg/kg bw/d in females, calculated on an assumed body weight of 250 g in males and 200 g in females).

Positive control:

no positive control, but other subsatance with positive effects tested in parallel.

Observations and examinations performed and frequency:

There were no data on hematology and clinical biochemistry.
Survival rate, body weight gain, behavior, macroscopic and microscopic findings were observed.

Sacrifice and pathology:

A complete necropsy and histopathologic examination was performed from all experimental animals.

Other examinations:

none

Statistics:

not indicated

Clinical signs:

no effects observed

Mortality:

no mortality observed

Body weight and weight changes:

no effects observed

Food consumption and compound intake (if feeding study):

not specified

Food efficiency:

not specified

Water consumption and compound intake (if drinking water study):

not specified

Ophthalmological findings:

not examined

Haematological findings:

not examined

Clinical biochemistry findings:

not examined

Urinalysis findings:

not examined

Behaviour (functional findings):

not examined

Organ weight findings including organ / body weight ratios:

no effects observed

Gross pathological findings:

no effects observed

Histopathological findings: non-neoplastic:

no effects observed

Histopathological findings: neoplastic:

no effects observed

Details on results:

There was no significant difference in the survival rate. The chronic oral administration of methenamine to Sprague-Dawley rats at dose of 80 mg/kg bw/d for males and 100 mg/kg bw/d for females resulted in no effects on body weight gain, behavior, macroscopic or microscopic findings.

Key result

Dose descriptor:

NOAEL

Effect level:

>= 80 mg/kg bw/day (nominal)

Sex:

male

Basis for effect level:

other: no adverse effects observed.

Key result

Dose descriptor:

NOAEL

Effect level:

>= 100 mg/kg bw/day (nominal)

Sex:

female

Basis for effect level:

other: no adverse effects observed.

Key result

Critical effects observed:

no

Conclusions:

It was concluded that >=0.1% methenamine (equivalent to >=80 mg/kg bw/d in males and >=100 mg/kg bw/d in females) was established as the no-toxic-effect-level (NOAELsys) for Sprague-Dawley rats.

Executive summary:

Groups of 15 male and 15 female Sprague-Dawley rats (8-10 weeks old) received on 5 days/week 0.1% methenamine in drinking water or 0.1% methenamine with 0.2% sodium nitrite in drinking water (no data of negative controls). Animals were treated for 50 weeks (a total of 250 days) and kept until they died or were killed due to moribund condition. Each rat received a total of 5 g methenamine over 50 weeks (equivalent to 80 mg/kg bw/d in males and 100 mg/kg bw/d in females, calculated on an assumed body weight of 250 g in males and 200 g in females). There were no data on hematology and clinical biochemistry. A complete necropsy and histopathologic examination was performed from all experimental animals. There was no significant difference in the survival rate. The chronic oral administration of methenamine to Sprague-Dawley rats at dose of 80 mg/kg bw/d for males and 100 mg/kg bw/d for females resulted in no effects on body weight gain, behavior, macroscopic or microscopic findings. Therefore, it was concluded that 0.1% methenamine (equivalent to 80 mg/kg bw/d in males and 100 mg/kg bw/d in females) was established as the no-toxic-effect level (NOAELsys) for Sprague-Dawley rats.

Endpoint conclusion

Endpoint conclusion:

no adverse effect observed

Dose descriptor:

NOAEL

80 mg/kg bw/day

Study duration:

subacute

Species:

rat

Quality of whole database:

Public available literature (non GLP, non guideline)

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

Additional information

Animal data:

Oral:

The lowest dose level causing no adverse toxic effects in rats could be derived from a chronic oral administration of methenamine in drinking water to Sprague-Dawley rats. At the only tested dose level of 0.1% methenamine (equivalent to 80 mg/kg bw/d in males and 100 mg/kg bw/d in females) no effects on body weight gain, behaviour, macroscopic or microscopic findings were observed (Lijinsky and Taylor, 1977).

50-week oral (drinking water)/Sprague-Dawley rat

NOAELsys >= 80 mg/kg bw/d in males and >= 100 mg/kg bw/d in females.

In a further long-term drinking water study in Wistar rats that was equal in test design and quality to the above-mentioned study the administration of methenamine at 2 g/kg bw/d in males and 2.5 g/kg bw/d in females for 104 weeks was tolerated without adverse effects. There was no evidence of toxic effects on the general behavior and the main organs (Della Porta et al., 1968(B)).

104-week oral (drinking water)/Wistar rat

NOAELsys >= 2 g/kg bw/d in males and >= 2.5 g/kg bw/d in females.

In a lifetime diet study in Wistar rats, in which only one dose (0.16%, equivalent to approximately to about 100 mg/kg bw/d in both sexes) was tested, no significant differences between control and test group with respect to body weight, organ weight, histopathological findings, life-span and causes of death were seen (Natvig et al., 1971).

Lifetime study oral (diet)/Wistar rat

NOAELsys >= 100 mg/kg bw/d in males and females

In a comparative investigation oral methenamine administrations of ca. 1130 mg/kg bw/d in males and 1570 mg/kg bw/d in female BD (cPah) rats for 333 days showed no differences in macroscopic lesions in the main organs of test and control groups (Brendel, 1964).

333-day study oral (gavage)/ BD (cPah) rats

NOAELsys >= 1130 mg/kg bw/d in males and >= 1570 mg/kg bw/d in females

In an oral toxicity study given methenamine in the drinking water for 60 weeks and following a subsequent treatment-free period male and female mice of the CTM, SWR/Dp and C3Hf strains showed no methenamine-related effects at 2.5 g/kg bw/d (Della Porta et al., 1968(C)).

In the SWR/Dp mice (7 weeks old) the control group consisted of 45 males and 30 females.29 male and 27 female mice received 1.0% methenamine in drinking water for 60 weeks (calculated daily intake of 2.5 g/kg bw/d in either sex). In the C3Hf mice (5 weeks old) 30 males and 63 females received water only, and 49 males and 44 females received 1.0% methenamine in the drinking water over a period of 60 weeks (calculated daily intake of 2.5 g/kg bw/d in either sex). After termination of treatment, mice were observed up to 100 weeks of age. There were no data on hematology and clinical biochemistry. Necropsy was performed on all animals that died on study or were killed at termination of the study. Organ samples (no data for the list) and all gross lesions taken at necropsy were evaluated microscopically.

Water intake was similar in both control and methenamine treated groups throughout the study. Body weight gain showed no significant differences between control and methenamine treated SWR and C3Hf strain groups. Treatment of CTM mice with 5.0% (12.5 g/kg bw/d) methenamine for 30 weeks resulted in a significant reduction in survival rates and slight reduction of growth in the surviving animals. Slight retardation of growth was also seen in SWR mice treated with 1.0% (2.5 g/kg bw/d) methenamine. The effect on growth in SWR mice was very small, not statistically significant, and no corroborating findings were noted at necropsy and microscopy. In addition there were no methenamine-related gross and microscopic findings in mice of all tested strains which died during or were killed at the end of the experiment. Therefore, for mice a NOAELsys of 2.5 g/kg bw/d methenamine in either sex was obtained.

No treatment-related effects were found in food consumption, body weight gain, or behavior in male and female cats (strain unspecified) receiving methenamine in feed at a dose of 60.65 mg/kg bw/ up to two years (Kewitz, 1966, unpublished report).

106 week oral (feed)/cats (strain unspecified) NOAELsys >= 60.65 mg/kg bw/d

Dermal:

In a supporting study (Zondlo, 1992), two groups of rabbits (strain unspecified), 6 males per group, were used in this repeated dose dermal toxicity test. Two ml of 0.20% methenamine (equivalent to 1.3 mg/kg bw/d assuming a mean body weight of 3 kg) were applied in distilled water to one group of rabbits; the other group served as a control receiving the vehicle (water) only. The methenamine treated animals received applications of test solution during 5 days a week for a period of 6 weeks; the applications were given without occlusive patches. General behavior, hair growth, and body weight gain were comparable in both the controls and methenamine treated groups. No erythema, edema, scratching, or variation of the cutaneous fold were observed in the methenamine treated animals treated with a 0.2 % solution of methenamine when compared to the controls.

Inhalation:

No studies are available to assess toxicity after repeated inhalation exposure.

Summary of animal toxicity data after repeated exposure to methenamine

There are no oral repeated dose toxicity studies with a full range of parameters to be examined according to the current regulatory requirements (EEC methods, B.7, B.9, B.26, B.30). Nevertheless, there are a number of older diet, gavage and drinking water studies in several animal species. None of these studies provided data on hematology and clinical chemistry; data on histopathology were limited. However a number of these repeated dose toxicity studies by oral administration (gavage, feed, drinking water) showed that methenamine did not cause any toxic effects in experimental animals up to and including 2.5 g/kg bw/d. All in-life parameters, which included body weight gain, food consumption, and survival, were unaffected by exposure to methenamine. Similarly, postmortem analyses, which included organ weights, gross pathology and histopathology, were unchanged following exposure to methenamine. The only clinical observation in studies with rats was a yellow staining of the perineal hair in some cases which is of no toxicological relevance. The yellow coloration of the fur observed in treated rats was reported by Brendel (1964), both after repeated feeding and after intramuscular injection of methenamine. Such yellow discoloration of the fur was only noted in rats and not in other experimental animals studied. This fur discoloration in methenamine treated rats may be a consequence of a reaction between formaldehyde in the urine and kynurenine, a normal constituent in the rat hair (Kewitz, 1966).

In a subchronic dermal toxicity study in rabbits using an aqueous methenamine solution at a concentration of 0.20% (equivalent to 1.3 mg/kg bw/d) no systemic or local effects were noted in animals of both sexes (COLIPA, 1989, cited by Zondlo, 1992).

There were no animal studies on repeated dose toxicity of methenamine after inhalation.

Human data:

From the use of methenamine for long-term therapy or the prevention of recurrent urinary infections in man it was known that dose levels of 2 to 4 g/d produced no harmful reactions or complications (Goodman and Gilman, 1975; Martindale, 2005). However, therapeutic doses of 8 g/d for 3 to 4 weeks produced side effects such as bladder irritation, painful and frequent micturition, albuminuria and hematuria (Goodman and Gilman, 1975; Mon. 144, 1988).

Dose level causing no toxic effects in man: 2 to 4 g/d, equivalent to ca. 28 to 57 mg/kg bw/d (Goodman and Gilman, 1975; Martindale, 2005). The NOAEL of 4 g/d (57 mg/kg bw/d based on a body weight of 70 kg person) in man (Goodman and Gilman, 1975; Martindale, 2005) is derived from decades of long experience with methenamine as a therapeutic substance.

Human NOAELsys: 57 mg/kg bw/d

This value is in line with the animal findings revealing effects at concentrations of about 2.5 kg/kg bw/day and applieing a safety factor of 100 (intra- and interspecies extrapolation).

Summary of human toxicity data after repeated exposure to methenamine

Methenamine is widely used as an accelerator and a hardener in the rubber and plastics industries. However, the number of available studies on the effects of methenamine on man following occupational exposure is limited. Toxic effects in humans at the workplace have only been reported after repeated exposure to mixtures of several compounds, including methenamine. Workers in production plants, in the lacquer and plastics industries, in tire manufacturing plants and in foundries can be exposed to methenamine by inhalation or skin contact. In all these workplaces, the workers are also exposed to other chemicals (e.g. formaldehyde, ammonia, resorcinol, phenol, furfuryl alcohol, cyanides, epoxy resins, curing agents). Therefore, the available occupational exposure studies were not adequately designed to specifically address the nature and origin of symptoms occurring in rubber and foundry workers, or to establish a plausible dose-response relationship relating to a single substance. Considering the lack of information on the exact exposure situation, especially the actual levels of methenamine exposure, it is not possible to make qualitative assessments of the observed effects in relation to methenamine exposure alone. Lung function measurements in one of the studies revealed significant reductions in expiratory flow rates at low lung volumes. In another study, an intracutaneous skin test with methenamine gave positive reactions in all workers, and a provocative inhalation test with an aerosol of a lacquer product revealed allergic reactions from the lungs, the nose or the skin. Since the early use of methenamine in the rubber and resins industries, however, increased incidences of wheeze and further respiratory tract symptoms like cough, and nasal and eye irritation were reported in workers who were simultaneously exposed to methenamine and other chemicals such as resorcinol.

Reliable information about systemic toxicity of Methenamine can however be derived from the therapeutical use of this chemical. No adverse effects were observed in patients receiving methenamine for long-term prophylaxis or therapy especially as urinary antibacterial-antiseptic substance at dose levels of 2 to 4 g/d (corresponding to ca. 28 to 57 mg/kg bw/d) for several weeks or months. However, with a higher dose of 8 g/d (corresponding to ca. 114 mg/kg bw/d) over 3 to 4 weeks clinical symptoms such as bladder irritation, painful and frequent micturition, albuminuria and haematuria were reported. Albuminuria or hematuria were not observed in experimental animals. Therefore human data are considered as a more reliable data base of the NOAEL deduction and the hazard assessment of methenamine. In conclusion a NOAEL of 57 mg/kg bw/day is used for the risk assessment.

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 (EC) No 1272/2008. As a result the substance is not considered to be classified under Regulation (EC) No 1272/2008, as amended for the sixth time in Regulation No 605/2014.