Potassium triboron pentaoxide

Endpoint summary

Key value for chemical safety assessment

Effects on fertility

Effect on fertility: via oral route

Dose descriptor:

NOAEL

100 mg/kg bw/day

Additional information

Effects on male fertility have been investigated in detail. A dose related effect on the testis was observed in rats, mice and deer mice, with confirmation from limited studies in dogs. Effects in rats start with reversible inhibition of spermiation after 14 days (at 39 mg B/kg bw/day) and 28 days (at 26 mg B/kg bw/day). At doses equal to and above 26 mg B/kg bw/day testicular atrophy, degeneration of seminiferous tubules and reduced sperm counts were observed. Male fertility was further investigated in two serial mating studies of treated male rats with untreated female rats. Infertility of treated males correlated well with germinal aplasia. Similar effects on male fertility were described in deer mice (Peromyscus maniculatus) after treatment with boric acid. Fertility studies in rats (two three-generation study with for boric acid and disodium tetraborate decahydrate) and mice (a continuous breeding study with boric acid) further support effects on testes as the underlying cause for reduced male fertility.

Diminished sperm production may be due to testicular effects on germ cell, Sertoli cell, or Leydig cell function or act via an alteration of the pituitary-hypothalamic axis. There is an indication that LH and FSH are elevated under boric acid treatment (Lee et al., 1978) and that serum testosterone may be decreased in CD-1 mice and F344 rats (Grizzle et al., 1989; reviewed in Fail et al., 1991; Treinen & Chapin, 1991). The decrease in prostate weight at 111.3 mg B/kg bw/day observed by Fail et al. (1991) might be caused by reduced testosterone levels.

 

A NOAEL of 17.5 mg B/kg bw/day for effects on female fertility was derived in the Transitional Annex XV dossier (TD 2008) based on Weir (1966c-d) and Fail et al,1991.   However, the TD failed to adequately distinguish between effects on female fertility and effects on development. Fertility is generally defined in males as the ability to produce sperm which are capable of producing fertilisation of an ovum leading to conception.  In females, it is defined as the ability to produce and release ova which can be fertilised leading to conception.  To test fertility in animals males and females are pretreated to cover the period of development of the sperm and eggs, then mate and treat until the time of implantation, around Day 6 following mating, and then stop treatment in the females.   To test for effects on development pregnant females are treated from Day 6 till the end of pregnancy. Neither the Weir and Fisher multigeneration study nor the Fail RACB studies were performed with this division of treatments.  They both treated animals continuously before and during pregnancy and also after delivery. In a three generation study in rats groups of 8 males and 16 females were treated with boric acid or disodium tetraborate decahydrate equivalent to 0, 5.9, 17.5 and 58.8 mg B/kg bw/day (Weir 1966c,d). An attempt was made to study the fertility of the P1 females at the top dose level by mating them with untreated males but only one litter of 16 pairs was produced. This highest dose level was clearly clinically toxic to the females after 2-3 weeks of dosing, with rough fur, scaly tails, inflamed eyelids and staining of the fur on the face and abdomen. The mating procedure to test the fertility of the females was not a satisfactory one. To avoid treatment of the males used for pairing, food was withdrawn from the cages of the females for 8 hours per day during the pairing process, and this is known to be very stressful to laboratory rats. There was no evidence on whether mating actually occurred for any of the rats, and no vaginal examinations for the presence of sperm were carried out. The females of the top dose P1 generation were sacrificed after 45 weeks of treatment and histopathological examination of the ovaries and uterus carried out. In the ovaries the presence of corpora lutea was regarded as a major indication of cyclic function, and these were found in 7 of 15 females, with reduced or absent function in the remaining 8 animals. The changes in the ovaries were not clearly different from those of controls.  No treatment related changes were found in the uterus. No changes were found that could account for the reduced litter production, and no conclusions could be drawn about fertility in the top dose females.  Comparable results were found in the Weir and Fisher multigeneration study on borax, with clear testicular atrophy at the top dose levels in males, and no clear explanation of the reduced number of litters in the top dose females, using the same unsatisfactory mating technique.  The authors of the study concluded that testis atrophy was clearly produced in males at the top dose level, but that the evidence of the decreased ovulation in females did not account for the reduced number of litters in the cross mating study in females.  Thus the Weir and Fisher studies produced clear evidence of adverse effects on male fertility, but did not produce clear evidence for an adverse effect on female fertility.

 

In a continuous breeding study of boric acid in Swiss mice (NTP, 1990; Fail et al., 1991), the three administered doses were 1000 ppm (26,6 mg B/kg bw/day), 4500 ppm (111,3 mg B/kg bw/day) and 9000 ppm (220,9 mg B/kg bw/day). A dose-related effect on the testis (testicular atrophy and effects on sperm motility, morphology and concentration) was noted; fertility was partially reduced at 111 mg B/kg bw/day, and absent at 221 mg B/kg bw/day.

 

For cross over mating only the mid dose group (111,3 mg B/kg bw/day) could be mated with control animals, since the high dose produced no litter. Indices of fertility for mid dose males with control females, control males with mid dose females and control males with control females were 5%, 65% and 74%, respectively. The according indices of mating (incidence of copulatory plugs) were 30%, 70% and 79%. This indicates that the primary effect was seen in males, however, slight effects were also noted in females. Live pup weight (adjusted for litter size) was significantly reduced compared to control litters, the average dam weight was significantly lower on postnatal day 0 compared to control dams and the average gestational period of the mid dose females was 1 day longer than in control females. The latter finding has also been observed in the developmental toxicity study by Price et al. (1996).

 

In task 4 of this continuous breeding study control animals and low-dose F1 animals were mated because in the 9000 ppm groups no litters and in the 4500 ppm group only 3 litters were produced. While mating, fertility and reproductive competence were un-altered compared to control, the adjusted pup-weight (F2) was slightly but significantly decreased. F1 females had significantly increased kidney/adrenal and uterus weights and the oestrus cycle was significantly shorter compared to control females. A crossover mating study of controls and 4500 ppm groups confirmed the males as the affected sex.Necropsy at 27 weeks confirmed reduced testes weight, seminiferous tubule degeneration, decreased sperm count and motility and increase in abnormal sperm.In females at 27 weeks, 4500 ppm boric acid was toxic with decreased liver, kidney and adrenal weights, but no effect on oestrous cycles, mating, number of litters and number of pups. In F1 males a reduction in sperm concentration was observed, but no other sperm parameters were influenced.

 

While in this study the NOAEL for females of the F0-generation is 1000 ppm this is a LOAEL for males of the F0-generation (motility of epididymal sperms was significantly reduced: 78% ± 3 in controls vs. 69% ± 5 at 1000 ppm). For the F1-generation 1000ppm can be identified as a LOAEL, based on the 25% reduction of sperm concentration in males at this dose. Further, though normal in number, the F2-pups had reduced adjusted bodyweights at 1000 ppm, which is therefore also a LOAEL for F2-generation.

The authors concluded that the male is the most sensitive sex and that the testis is the primary target organ for boron. The NOAEL for testicular pathology in the present mouse study is probably 1000 ppm (26mg B/kg bodyweight). While males are more sensitive to boron induced toxicity, data also suggest an effect of boron on the female reproductive system. A reduced number of pups per litter and number of pups born alive at high dose levels are in agreement with earlier reports and could result from an effect of boron to alter implantation or to disrupt cell division in the embryo. This is supported by results of developmental toxicity studies in rats and mice in which higher dose levels can reduce the number of implants. Although F1 females had significantly increased kidney/adrenal and uterus weights and the oestrus cycle was significantly shorter compared to control female, similar effects were not observed in the 4500 ppm dose group, therefore the NOAEL for fertility in females was the dose level in diet of 4500 ppm, 846 mg/kg bw of boric acid or equivalent to 148 mg B/kg bodyweight.

In conclusion, the effects described in the Fail study on fertility show that 4500 ppm (111.3 mgB/kg bw) is a NOAEL for the females, and that other small effects in females are the result of developmental toxicity for which a NOAEL of <1000ppm (26.6mg B/kg bw) may be valid.

No further studies on the effects of boron on female fertility were reported by the National Toxicology Program team who published several other studies on the mechanism of action of boron on male fertility and on spermatogenesis. No effects on steroidogenic function were found in Leydig cells, and no clear mechanism of action to cause testis atrophy was identified by Ku and Chapin (1994).

 

Although boron has been shown to adversely affect male reproduction in laboratory animals, male reproductive effects attributable to boron have not been demonstrated in studies of highly exposed workers. For further information on epidemiologic studies with workers exposed to high concentrations of boron, please refer to chapter 7.10.2 of this dossier and the respective endpoint summary.

Short description of key information:
A multigeneration study in the rat (Weir, 1966) gave a NOAEL for fertility in males of 17.5 mg B/kg/day.

Effects on developmental toxicity

Description of key information

A benchmark dose of 59 mg/kg bw/day (10.3 mg B/kg bw/day) for developmental toxicity developed by Allen et al. (1996) was based on the studies of  Heindel et al. (1992), Price, Marr & Myers (1994) and Price et al. (1996).

Effect on developmental toxicity: via oral route

Dose descriptor:

BMDL05

59 mg/kg bw/day

Additional information

Developmental effects have been observed in three species, rats, mice and rabbits. The most sensitive species being the rat with a NOAEL of 9.6 mg B/kg bw/day. This is based on a reduction in mean foetal body weight/litter, increase in wavy ribs and an increased incidence in short rib XIII at 13.3 mg B/kg bw/day. The reduction in foetal body weight and skeletal malformations had reversed, with the exception of short rib XIII, by 21 days postnatal. At maternally toxic doses, visceral malformations observed included enlarged lateral ventricles and cardiovascular effects.

The NOAEL for this endpoint is 9.6 mg B/kg bw/day corresponding to 55 mg boric acid/kg bw/day; 85 mg disodium tetraborate decahydrate/kg, 65 mg disodium tetraborate pentahydrate/kg and 44.7 mg disodium tetraborate anhydrous/kg.

The critical effect is considered to be decreased fetal body weight in rats, for which the NOAEL was 9.6 mg/kg body weight per day. A benchmark dose developed by Allen et al. (1996) was based on the studies of Heindel et al. (1992), Price, Marr & Myers (1994) and Price et al. (1996). The benchmark dose is defined as the 95% lower bound on the dose corresponding to a 5% decrease in the mean fetal weight (BMDL₀₅). The BMDL₀₅of 10.3 mg/kg body weight per day as boron is close to the Price et al. (1996) NOAEL of 9.6 mg/kg body weight per day.

There is no evidence of developmental effects in humans attributable to boron in studies of populations with high exposures to boron (Tuccar et al 1998; Col et al. 2000; Chang et al. 2006).

Justification for classification or non-classification

There is no existing reproductive toxicity study for orthoboric acid, potassium salt. As such, read across from boric acid in combination with intended use conditions is used to evaluate the potential of the substance to cause reproductive toxicity. The substance to be registered is expected to have predictable adsorption, distribution, metabolism and excretion based on boric acid, meeting the criteria for inclusion.

The registered substance is manufactured in highly refined mineral oil and further diluted with lubricant additives and base oil in finished fluid. The oil and additives are expected to stabilize the substance to hydrolysis and hydrolysis to boric acid is not expected to occur under normal conditions of handling and use. However, under undesired hydrolytic conditions, including aqueous environments, the borated salt will quickly convert to boric acid. Exposure to boric acid is not expected under normal use conditions to the worker or consumer. The registered substance does not contain detectable levels of residual boric acid. The hazard communications requirement for classification is based on the constituents known to be present under normal conditions of use. Assuming worst case scenarios, if orthoboric acid, potassium salt were to hydrolyze completely, the only relevant route of exposure to workers is dermal and the dermal absorption of boric acid in humans is relatively low at <0.3% (Draize and Kelley, 1959). Animal ingestion studies in several species, at high doses, indicate that boric acid causes reproductive and developmental effects. The doses are many times in excess of those which humans would be exposed to. A human study of occupational exposure to borates showed no adverse effect on reproduction (Whorton et al., 1994). Therefore, orthoboric acid, potassium salt is not classified for reproductive toxicity based on exposure considerations.