First draft prepared by
Dr D.J. Benford
Food Standards Agency, Aviation House, London, United Kingdom
Cross-linked sodium carboxymethyl cellulose is manufactured by acidifying an aqueous suspension of sodium carboxymethyl cellulose and heating the suspension to achieve cross-linking. The product is then washed and dried. It is also produced during the manufacture of sodium carboxymethyl cellulose by lowering the pH and heating to cause cross-linking. Cross-linked sodium carboxymethyl cellulose is used in tablets of table-top sweeteners and dietary food supplements, as it facilitates disintegration in aqueous solutions, with a maximum level of use of 30 g/kg. It is also widely used as an excipient in pharmaceutical applications.
Cross-linked sodium carboxymethyl cellulose has not been evaluated previously by the Committee. The parent compound, sodium carboxymethyl cellulose, was considered by the Committee in its evaluations of modified celluloses at its fifth, seventh, tenth, thirteenth, seventeenth, twenty-sixth, twenty-seventh, thirtieth and thirty-fifth meetings (Annex 1, references 5, 7, 13, 19, 32, 59, 62, 73 and 88) and of ethyl cellulose and ethylhydroxyethyl cellulose at the twenty-sixth and twenty-seventh meetings (Annex 1, references 59 and 62). At its thirty-fifth meeting, the Committee reviewed new data on seven modified celluloses (Annex 1, reference 89), including sodium carboxymethyl cellulose. The data consisted of the results of studies in rats on caecal enlargement, effects on caecal flora and developmental toxicity and studies of mutagenicity in vitro. New data on humans were also available, indicating that some individuals may experience laxative effects at a dose as low as 5 g per person per day. The Committee concluded that modified celluloses are of low toxicity and allocated a group ADI ‘not specified’1 to seven modified celluloses: methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, hydroxypropylmethyl cellulose, sodium carboxymethyl cellulose, ethyl cellulose and ethylhydroxyethyl cellulose. The Committee pointed out that their laxative properties should be taken into account when they are used as food additives.
Enzymatically hydrolysed sodium carboxymethyl cellulose was evaluated by the Committee at its fifty-first meeting (Annex 1, reference 137). The Committee compared the toxicity and absorption, distribution and metabolism of enzymatically hydrolysed carboxymethyl cellulose with that of the parent compound and concluded that the similarities in the results were consistent with the absence of any toxicologically significant difference between the two. Therefore, the enzymatically hydrolysed form was included in the group ADI ‘not specified’ with the other seven modified celluloses.
No specific information was available. Studies reviewed previously by the Committee relating to sodium carboxymethyl cellulose and its enzymatically hydrolysed form indicated that 90–99% of both forms was excreted in faeces. The carboxymethyl cellulose was partially degraded during passage through the gastrointestinal tract (Annex 1, reference 137).
Cross-linking of carboxymethyl cellulose results in it becoming insoluble in water, and it is therefore less likely to be absorbed and degraded than the parent compound.
Studies of acute toxicity with cross-linked sodium carboxymethyl cellulose have been conducted in rats and mice, apparently demonstrating low toxicity, but the reports were not available to the Committee.
In a study that complied with good laboratory practice (GLP) and was conducted to OECD guideline 408, groups of 20 male and female Sprague-Dawley CD rats were given diets containing 10 000 or 50 000 ppm of Ac-Di-Sol SD 711 or 50 000 ppm of ‘modified cellulose gum’ for about 90 days. Both compounds were defined in the report as ‘a mixture of polymers of internally cross-linked sodium carboxymethyl cellulose’, and Ac-Di-Sol was reported to be the brand name of ‘modified cellulose gum’. However, this terminology is inappropriate, since ‘modified cellulose gum’ should more correctly be used as a synonym for the parent sodium carboxymethyl cellulose rather than the cross-linked form. As control animals received a diet consisting of unblended basal diet, the test diets had a lower caloric value. Clinical signs were recorded daily, and body weights and food consumption were recorded weekly. Ophthalmic examinations were performed before the beginning of the study and before terminal sacrifice. Blood samples were taken before termination for haematological and clinical chemical determinations. At necropsy, brain, liver, adrenals, testes and kidneys were weighed, and a wide range of organs were taken for histopathological examination. Urine was not analysed.
Three males (one receiving 50 000 ppm Ac-Di-Sol and two receiving ‘modified cellulose gum’) were killed because of mechanical injury to the snout. There were no treatment-related deaths or clinical signs. The average doses received by the animals during the study were 760, 3900 and 4000 mg/kg bw per day for males and 890, 4700 and 4600 mg/kg bw per day for females given 10 000 ppm Ac-Di-Sol, 50 000 ppm Ac-Di-Sol and 50 000 ppm ‘modified cellulose gum’, respectively. Males receiving 50 000 ppm Ac-Di-Sol showed no change in food consumption but had a statistically significant decrease in body-weight gain during the final 3 weeks of the study, resulting in a mean terminal body weight that was 93% that of control animals. Males receiving ‘modified cellulose gum’ had significantly increased food consumption throughout the study, but their body-weight gain was unaffected. Females in all treated groups showed increased food consumption and no effect on body-weight gain. The authors considered that these results were not toxicologically significant.
No significant effects were reported in ophthalmicl examinations, at necropsy or in haematological parameters. The only significant changes in clinical chemistry were small decreases in aspartate aminotransferase activity in males and females receiving 50 000 ppm ‘modified cellulose gum’. The authors considered that these alterations were not toxicologically significant. The absolute weights of the kidney and the weight of the kidney relative to that of brain were significantly increased by 7–8% in males receiving 50 000 ppm of ‘modified cellulose gum’. The relative adrenal weights were significantly decreased by 10–12% in rats of each sex receiving 50 000 ppm of ‘modified cellulose gum’. These changes were not accompanied by histopathological changes and were considered not to be treatment-related.
Renal mineralization at the corticomedullary junction was observed in females in all groups but not in males. This lesion was equated to nephrocalcinosis and was observed in 4/20 control females (three minimal, one mild), 7/20 at 10 000 ppm Ac-Di-Sol (six minimal, one mild), 7/20 at 50 000 ppm Ac-Di-Sol (five minimal, one mild, one moderate) and 9/20 at 50 000 ppm ‘modified cellulose gum’ (five minimal, three mild, one moderate). In addition, one female receiving 50 000 ppm ‘modified cellulose gum’ had mineralization in the urinary space of the kidney. No statistical analysis was conducted. The study pathologist considered that, although renal mineralization at the corticomedullary junction is a common spontaneous change in control female rats, it was a microscopic lesion related to treatment in female rats fed 50 000 ppm Ac-Di-Sol or ‘modified cellulose gum’. No other treatment-related lesions and no caecal enlargement were observed. (Freeman, 1997a).
The Committee recalled the results of a 90-day toxicity study in rats in which the effects of enzymatically hydrolysed sodium carboxymethyl cellulose were compared with those of the parent compound, sodium carboxymethyl cellulose, which was discussed by the Committee at its fifty-first meeting (Annex 1, reference 137). In this study, the test substances were added to the diet at a concentration of 2.5, 5 or 10%, and 10% wheat starch was added to the diet of control animals. The monograph and report of the fifty-first meeting focused on the histopathological observations that were statistically significantly different from those in control animals, i.e. pelvic urothelial hyperplasia and pelvic nephrocalcinosis in male animals. However, corticomedullary nephrocalcinosis was noted more frequently in females than in males in all groups of animals on test diets and was not found in control animals. The increased frequency was statistically significant in females receiving a diet containing 10% sodium carboxymethyl cellulose (Bär et al., 1995).
In a study of teratogenicity that complied with GLP and was conducted according to OECD guideline 414, groups of 25 female Sprague-Dawley CD rats were fed diets containing 10 000 or 50 000 ppm of Ac-Di-Sol SD 711 or ‘modified cellulose gum’ on days 6–15 of gestation. The animals were then fed untreated basal diet until the end of the study on day 20 of gestation. The report defined both compounds as ‘a mixture of polymers of internally cross-linked sodium carboxymethyl cellulose’; however, this is ian nappropriate use of the term ‘modified cellulose gum’, which is more correctly used as a synonym for the parent sodium carboxymethyl cellulose, rather than the cross-linked form. Control animals received a diet consisting of unblended basal diet. Clinical signs were recorded daily, and maternal body weights and food consumption were recorded on days 0, 6, 15 and 20. At necropsy, the dams were examined for gross lesions, the number and distribution of implantation sites, early and late resorptions, live and dead fetuses and corpora lutea. Body weight, sex and external signs were recorded for all fetuses, and half were subjected to visceral examination and half to skeletal examination.
There were no deaths, although one animal receiving 10 000 ppm ‘modified cellulose gum’ was killed early after premature delivery on day 20. A small number of clinical signs were ascribed to mechanical injury and were considered not to be treatment-related. The average doses received by the animals during the study were 910, 4600, 910 and 4600 mg/kg bw per day for animals receiving 10 000 ppm Ac-Di-Sol, 50 000 ppm Ac-Di-Sol, 10 000 ppm ‘modified cellulose gum’ and 50 000 ppm ‘modified cellulose gum’, respectively. Food consumption, body-weight gain and gravid uterine weight did not differ between groups. All dams appeared normal at necropsy, and there were no differences between treated groups in the numbers of implants, live and dead fetuses or corpora lutea, litter size or resorptions. Dams receiving 50 000 ppm ‘modified cellulose gum’ had a statistically significant increase in preimplantation loss, which was considered not to be treatment-related because implantation occurred before administration of the test material.
The fetal body weights and sex ratios were comparable between test groups. External malformations were observed in one fetus of each of three dams receiving 10 000 ppm ‘modified cellulose gum’ but in no other group. These were considered not to be treatment-related because the finding was not statistically significant and they occurred only in fetuses of dams at the lowest dietary concentration. Visceral malformations were observed in one fetus of one dam receiving 10 000 ppm of Ac-Di-Sol and one fetus of one dam receiving 50 000 ppm of Ac-Di-Sol. This finding was considered not to be treatment-related as it was not statistically significant. There were no visceral malformations in fetuses of control dams or those receiving ‘modified cellulose gum’. Sporadic skeletal variations, but not malformations, were reported in fetuses of control dams. One fetus of one dam receiving 50 000 ppm of Ac-Di-Sol had a non-ossified lumbar vertebra. This finding was not statistically significant and therefore considered not to be treatment-related. Skeletal malformations were reported in one fetus of each of two dams receiving 10 000 ppm of ‘modified cellulose gum’. These findings were considered not to be treatment-related because they were isolated occurrences in the group at the lowest dose. A statistically significant increase in the incidence of a skeletal variation (incompletely ossified thoracic vertebra centra) was noted among fetuses of dams receiving 50 000 ppm. The fetal incidence was 27% and the litter incidence was 72%. This was considered not to be treatment-related because it occurred in the absence of any statistically or biologically significant effects on other parameters in the study, and because the incidence was within the range of that in other control groups in the laboratory (16–28% and 72–84% for fetal and litter incidences, respectively). There was no evidence of maternal or developmental toxicity at a limit dose of 50 000 ppm in the diet (Freeman, 1997b).
At its present meeting, the Committee reviewed two studies on cross-linked sodium carboxymethyl cellulose and considered its properties in the light of previous evaluations of modified celluloses. Like other modified celluloses, cross-linked sodium carboxymethyl cellulose is considered to be of low toxicity. Because cross-linking renders it insoluble in water, it is less likely to be absorbed than the parent compound. In a 90-day dietary study in rats, the only treatment-related observations were increased food consumption, decreased body-weight gain and effects on the kidneys of animals receiving cross-linked sodium carboxymethyl cellulose at 50 000 mg/kg of diet, equivalent to 3900–4700 mg/kg bw per day averaged over the study period. The changes in food consumption or body-weight gain were consistent with a 5% reduction in the nutritional content of the diet, as compared with the basal diet fed to control animals, due to addition of the test substance. In contrast to the effects of some other non-digestible substances evaluated previously by the Committee, caecal enlargement was not observed. Mineralization at the corticomedullary junction was observed in the kidneys of some female rats, with a higher incidence in treated groups than in controls. The report equated the renal mineralization to nephrocalcinosis and noted that, although this is a spontaneous change in control female rats, it was related to treatment in female rats fed cross-linked sodium carboxymethyl cellulose at 50 000 mg/kg of diet, but not at 10 000 mg/kg of diet. Pelvic nephrocalcinosis and corticomedullary nephrocalcinosis were observed in rats given sodium carboxymethyl cellulose or enzymatically hydrolysed carboxymethyl cellulose in a study evaluated by the Committee at its fifty-first meeting (Annex 1, reference 137). In that study, it was also reported that corticomedullary nephrocalcinosis was observed more frequently in females than in males. Because of the inconsistency of the dose–response relationship, however, the Committee at its fifty-first meeting had not been convinced that these changes were of toxicological concern. The changes observed in the study evaluated at the current meeting appeared to represent a minimal increase in the incidence and severity of a spontaneous lesion, which may be within normal variation. As discussed in the report of the twenty-sixth meeting (Annex 1, reference 59), nephrocalcinosis is a common finding in laboratory rodents. No implications for human hazard have been established. Therefore, the NOEL for cross-linked sodium carboxymethyl cellulose in the above study was 3900 mg/kg bw per day.
The study of developmental toxicity in rats showed no evidence of maternal toxicity or teratogenicity at the highest possible dose of cross-linked sodium carboxymethyl cellulose of 50 000 mg/kg of diet. The NOEL was 4500 mg/kg bw per day.
The Committee noted that evaluation of the results of the 90-day toxicity study was not straightforward, because of nutritional differences between the basal diet given to control animals and the diets supplemented with cross-linked sodium carboxymethyl cellulose. However, taking into account evaluations of other modified celluloses, the Committee concluded that the effects of cross-linked sodium carboxymethyl cellulose were consistent with dietary overloading with a poorly absorbed substance of low toxicity. The Committee included cross-linked sodium carboxymethyl cellulose in the group ADI ‘not specified’2 with the other modified celluloses.
Bär, A., Til, H.P. & Timonen, M. (1995) Subchronic oral toxicity study with regular and enzymatically depolymerized sodium carboxymethylcellulose in rats. Food Chem. Toxicol., 33, 909–917.
Freeman, C. (1997a) Ac-Di-Sol SD-711 and modified cellulose gum ninety-day feeding study in rats. Unpublished report No. I95-2054 from FMC Corporation Toxicology Laboratory, Princeton, New Jersey, USA. Submitted to WHO by the Organisation des Fabricants de Produits Cellulosiques Alimentaires, Leidschendam, Netherlands.
Freeman, C. (1997b) Ac-Di-Sol SD-711 and modified cellulose gum dietary teratology study in rats. Unpublished report No. I96-2090 from FMC Corporation Toxicology Laboratory, Princeton, New Jersey, USA. Submitted to WHO by the Organisation des Fabricants de produits Cellulosiques Alimentaires, Leidschendam, Netherlands.
ENDNOTES
See Also:
Toxicological Abbreviations
CROSS-LINKED SODIUM CARBOXYMETHYL CELLULOSE (JECFA Evaluation)