Reaction products of sodium glucoheptonate with zinc sulfate and sodium hydroxide

Endpoint summary

Link to relevant study record(s)

Reference

Endpoint:

basic toxicokinetics, other

Remarks:

expert satement

Type of information:

other: expert satement

Adequacy of study:

key study

Study period:

2017

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: An extended assessment of the toxicokinetic behaviour of zinc glucoheptonate was performed, taking into account the chemical structure, the available physico-chemical-data and the available toxicity data of the structural analogues.

Objective of study:

absorption

distribution

excretion

metabolism

Qualifier:

according to guideline

Guideline:

other: TGD, Part I, Annex IV, 2003; ECHA guidance R7c., 2012

Principles of method if other than guideline:

Physical chemical properties of zinc glucoheptonate were integrated with the published toxicological data and data on ADME parameters of the structurally related substance zinc gluconate to create a prediction of its toxicokinetic behaviour. Additionally, well investigated ADME data on zinc from different sources (food, medications and other inorganic and organic compounds) have been taken into account, because the systemic toxicity of zinc glucoheptonate is considered to be driven by released zinc from the zinc glucoheptonate complex.

Radiolabelling:

no

Type:

absorption

Results:

Due to the MW of 619.2 g/mol and logPow -16.9, it is readily absorbed via the GI tract. Low absorption potential via dermal route and inhalation is expected due to its high water solubility (500-550 g/L) and low vapour pressure (1.55 x 10E-4 Pa).

Type:

distribution

Results:

Zinc, if released from glucoheptonate, is distributed to all organs and tissues and will be bound with organic ligands rather than existing free in solution as a cation

Type:

metabolism

Results:

The enzyme 6-phosphogluconolactonase (catalysing the second step of pentose phosphate pathway (PPP)) was shown to possess a broad substrate specificity hydrolysing gluconolactone moieties including glucoheptonate.

Type:

excretion

Results:

Glucoheptonate is mainly excreted via the kidneys. About 70-80% of the ingested amount of zinc is excreted via faeces (5 to 10 mg/day depending upon the dietary zinc concentration).

Details on absorption:

Zinc glucoheptonate is expected to be moderately absorbed after oral exposure, based on its high water solubility and molecular weight suggestive for favoured absorption through gastrointestinal tract. As worst-case, 100 % oral absorption is considered appropriate. Concerning absorption after exposure via inhalation, as the chemical has a low vapour pressure, is highly hydrophilic, has a negative LogPow, and has 13.9 % of particles less than 100 µm, it is clear, that the substance is poorly available for inhalation and will not be absorbed significantly via lungs. However, an absorption by aspiration cannot be fully ruled out. Therefore, 100% inhalation absorption is considered. Zinc glucoheptonate is not expected to be absorbed following dermal exposure into the stratum corneum and into the epidermis, due to its very high water solubility and considering low absorption potential of zinc and glucoheptonate moieties. 10 % absorption is therefore considered for dermal route of exposure.

Details on distribution in tissues:

Glucoheptonate moieties, are expected to be distributed predominantly to kidneys and organs with higher expression of glucose transporters. The substance does not indicate a significant potential for accumulation. Zinc, if released from glucoheptonate, is distributed to all organs and tissues and will be bound with organic ligands rather than existing free in solution as a cation

Details on excretion:

Glucoheptonate is involved into intermediary carbohydrate metabolism and eliminated unchanged primarily via the urine and to a lesser extent via the bile. About 70-80% of the ingested amount of zinc is excreted via faeces (5 to 10 mg/day depending upon the dietary zinc concentration).

Details on metabolites:

Metabolism of glucoheptonate in mammalian tissues is described in several publications dealing with investigations of substrate specificity of a various number of aldonic acids and its isomeric analogues lactones. The enzyme 6-phosphogluconolactonase (catalysing the second step of pentose phosphate pathway (PPP)) was shown to possess a broad substrate specificity hydrolysing gluconolactone moieties including glucoheptonate. The enzyme is present in almost all mammalian tissues including humans. Further investigations revealed that glucoheptonate moiety undergoes a series of biochemical transformations similar to those of PPP.

Executive summary:

Zinc glucoheptonate is expected to be moderately absorbed after oral exposure, based on its high water solubility and molecular weight suggestive for favoured absorption through gastrointestinal tract. As worst-case, 100 % oral absorption is considered appropriate. Concerning absorption after exposure via inhalation, as the chemical has a low vapour pressure, is highly hydrophilic, has a negative LogPow, and has 13.9 % of particles less than 100 µm, it is clear, that the substance is poorly available for inhalation and will not be absorbed significantly via lungs. However, an absorption by aspiration cannot be fully ruled out. Therefore, 100% inhalation absorption is considered. Zinc glucoheptonate is not expected to be absorbed following dermal exposure into the stratum corneum and into the epidermis, due to its very high water solubility and considering low absorption potential of zinc and glucoheptonate moieties. 10 % absorption is therefore considered for dermal route of exposure. Glucoheptonate moieties, are expected to be distributed predominantly to kidneys and organs with higher expression of glucose transporters. The substance does not indicate a significant potential for accumulation. Zinc homeostasis is regulated in mammals by gastrointestinal absorption, excretion via faeces and via the urine as well as by the release from tissues. The total body zinc is maintained constant at the physiologically required levels of zinc in the various tissues at low and high dietary zinc intakes. Zinc, if released from glucoheptonate, is distributed to all organs and tissues and will be bound with organic ligands rather than existing free in solution as a cation. Glucoheptonate is involved into intermediary carbohydrate metabolism and eliminated unchanged primarily via the urine and to a lesser extent via the bile.

Description of key information

Toxicokinetic assessment of ZnGHA (for IUCLID’s endpoint summary)

General

There are no ADME studies available for zinc glucoheptonate. The toxicokinetic profile of the registered substance was not determined by actual absorption, distribution, metabolism or excretion measurements. Rather, the physical chemical properties of zinc glucoheptonate were integrated with the published toxicological data and data on ADME parameters of the structurally related substance zinc gluconate (here named as source substance) to create a prediction of its toxicokinetic behaviour. Additionally, well investigated ADME data on zinc from different sources (food, medications and other inorganic and organic compounds) have been taken into account because the systemic toxicity of zinc glucoheptonate is considered to be driven by released zinc from the zinc glucoheptonate complex (please refer also to read-across statement).

Toxicological profile of Zinc Glucoheptonate

There are a limited number of studies available for toxicological endpoints of zinc glucoheptonate. Its structurally similar substance zinc gluconate is of low toxicological activity in oral acute toxicity studies in rats and mice with LD50 values around 2000 mg/kg bw (Salgueiro et al., 2000) and > 5000 mg/kg bw (Ash and Ash, 2004). In a study with rats, a LD50 of 1290 mg/kg bw is reported for zinc gluconate (RTECS, ...). In acute toxicity studies with inorganic zinc compounds, dietary zinc intakes up to 1 g/kg body weight have been well tolerated, but dietary zinc intakes above 2 g/kg body weight have usually led to death (EFSA, 2006). The acute toxicity of zinc varied, depending on the zinc salt tested, and ranged from 237 to 623 mg zinc (elemental) /kg in rats and from 86 to 605 mg zinc (elemental)/kg in mice after oral administration (Domingo et al. 1988, cited in ATSDR, 2005 and in WHO, 2001). The principal findings included gastrointestinal and haematological effects resulting in alterations of the blood lipid profile in humans and animals. In addition, lesions have been observed in the liver, pancreas, and kidneys of animals (ATSDR, 2005).

No acute dermal or inhalation studies are identified for the source substance zinc gluconate.“The target substance zinc glucoheptonate was also not tested in acute dermal or inhalation studies. The dermal toxicity of zinc compounds can vary widely with the chemical form of zinc. For example, zinc chloride is caustic and can cause severe irritation while zinc sulfate is irritating but not as caustic as zinc chloride, and zinc oxide does not appear to be a dermal irritant”(ATSDR, 2005). Zinc glucoheptonate was not irritating to skin and eyes inin vivoirritation studies in rabbits (Patel, 2016a,b).

Zinc deficiency is known to be associated with adverse effects of immune function in animals and humans. Ingestion of 150 mg of elemental zinc led also to adversed immune function in humans while no delayed dermal hypersensitivity was observed in patch tested patients (WHO, 2001). Zinc sulfate was negative in LLNA (Basketter et al., 1999; Ikarashi et al., 1992). Zinc gluconate is used to treat inflammatory skin diseases such as acne vulgaris (Sirikudta et al., 2013) pointing to its antiallergenic action. Gluconic acid and its derivatives are not skin sensitisers and used in a variety of food, cosmetic and consumer products (SIDS, 2004, CIR, 2014; Regulation (EC) No 1925/2006). Based on these pieces of evidence, one may conclude absence of skin sensitisation of zinc compounds. Thus, no skin sensitisation can be associated with zinc glucoheptonate too.

Administration of a diverse zinc compounds to animal and humans under long-term conditions (4 week and longer) resulted predominantly in gastrointestinal effects including vomiting, nausea, abdominal cramps and diarrhoea. Another main haematological effect observed was anaemia in animals and humans. Excess of zinc led also to some cardiovascular and neurological disturbances in animals and humans. Kidney was a target organ in the repeated dose toxicity studies (ATSDR, 2005; WHO, 2001). In a 3-month drinking water study in rats with zinc acetate dehydrate, NOEL of 160 mg/kg bw was established based on affected renal function observed at higher dose levels (320 and 640 mg/kg bw) (Llobet et al., 1988). Zinc sulfate caused an increase in the absolute and relative kidney weights and regressive kidney lesions (not specified) in female mice that consumed 1,110 mg zinc/kg/dayin the diet for 13 weeks, but no effects occurred in rats that consumed 565 mg zinc/kg/day or in mice that consumed 104 mg zinc/kg/day under similar conditions (Maita et al., 1981; ATSDR, 2005). The maximum no-effect level of ZnSO4 was determined to be 3, 000 ppm, which is approximately equivalent to the following milligram doses: mice; male 458 mg/kg/day, female 479 mg/kg/day, rats; male 234 mg/kg/day, female 243 mg/kg/day. (Maita et al. 1981). Zinc supplementation at dose level of 50 mg zinc (elemental)/ kg bw did not result in adverse systemic effects in postmenopausal women during 90-day period (Davis et al., 2000), in adult men during a 2-month study (Fisher et al., 1984) or in a 10-week supplementation study in females (Yadrick et al., 1989). With regard to the toxicity of glucoheptonate moiety, no clinical signs or other abnormalities at necropsy were detected in the repeated studies with gluconates. No toxicological effects have been observed up to 1000 mg/kg bw in males and up to 2000 mg/kg bw in females (SIDS, 2004). “Potential side effects were attributed to high doses of cation intake, evidenced by results from assays designed for the gluconate anion effect specifically”(SIDS, 2004). This statement clearly indicates that no toxicity can be attributed to sugar-like organic moiety while metal is responsible for toxicity effects.

Genotoxicity studies conducted in a variety of test systems have failed to provide evidence for mutagenicity of zinc. However, there are indications of weak clastogenic effects following zinc exposure in in vivo studies (ATSDR, 2005). Exposure to zinc as zinc sulfate or zinc chloride does not increase mutation frequencies in bacterial or mammalian cell culture test systems (Amacher and Paillet 1980; Gocke et al., 1981; Marzin and Vo Phi, 1985; Nishioka, 1975; Thompson et al., 1989; Venitt and Levy, 1974; Wong, 1988 (ATSDR, 2005)). Similarly, there was no convincing evidence of a clastogenic effect in human lymphocytes exposed to 0.0003–0.00003 M zinc chloride (Deknudt and Deminatti, 1978).

There are a lot of data on a great number of zinc compounds available for reproductive and developmental toxicity in the scientific literature. Zinc deficiency is reported to be associated with adverse reproductive and developmental effects (Heimbach et al., 2000); ATSDR, 2005, WHO, 2001; RAR, 2004). High zinc doses (200 mg elemental zinc/kg bw/day) resulted in significantly lower numbers of implantation sites and live birth (Samantha and Pal, 1986; Pal and Pal, 1987), while 1,100 mg or 565 mg Zn2+/kg bw/day for mice and rats can be considered as NOAEL for effects on reproductive organs (Maita et al., 1981). The lowest NOAEL of 30 mg/kg bw and 42.5 mg/kg bw were reported for mice and rat, respectievely, for zinc sulfate heptahydrate (RAR, 2004). Additionally, a NOAEL of 25 mg/kg bw was reported for rats for zinc carbonate (Uriu-Hare et al., 1989).

Toxicokinetic analysis of Zinc Glucoheptonate

Zinc glucoheptonate complex is an odourless, brown solid in microgranulated form (molecular weight of 325.6 g/mol (monomer as Zn:GHA as 1:1) or 619.2 g/mol (dimer as Zn:GHA as 2:2) at 20°C. The substance is completely soluble in water (500 g/L at 20°C) and has a negative partition coefficient (logPow = -16.9, KOWWIN v1.68 estimate). The substance has a very low vapour pressure (1.55 x 10E-4 Pa) and has a melting point of 216.4 °C under atmospheric conditions. The boiling point could not be determined because the substance decomposed at 150-200 °C. Hydrolysis as a function of pH does not apply, as the substance forms stable complexes that are not hydrolyse

The stability of zinc glucoheptonate complex is higher at alkaline conditions while the complex is expected to be not stable enough at acidic conditions as determined in several studies with zinc gluconate and other metal –glucoheptonate complexes (please refer to read-across statement). This is because gluconate or glucoheptonate anions are fully protonated at low pH values and are not able to participate in complexation of metal cations (Alekseev et al., 1998). Moreover, lactonisation occurs at low pHs that would hinder complexation (Pallagi et al., 2010). These findings provide evidence that metals dissociate from the complexes at low pH that prevails in the stomach. It would mean that the metal cation originated from the glucoheptonate complexes is subjected to more or less an independent from the organic moiety fate of absorption into the systemic circulation. In small intestines, where pH raises, new complexes with other organic natural chelating agent i.e. from food can be formed, impacting the absorption. Therefore, the existing ADME data on several organic and inorganic zinc compounds have been accounted to assess absorption behaviour and a further fate of zinc cations released from glucoheptonate moiety.

 

Absorption

Oral absorption

Absorption of zinc glucoheptonate via gastrointestinal tract can be carried out by the intact zinc-glucoheptonate complex and/or its dissociated products: zinc and glucoheptonate moieties. In case of absorption of intact complex, physicochemical properties define the absorption behaviour. Oral absorption is favoured for molecules with MW below 500 g/mol. Since the molecular weight of zinc glucoheptonate is 325.6 g/mol (monomer as Zn:GHA as 1:1) or 619.2 g/mol (dimer as Zn:GHA as 2:2), and it has high water solubility (500 g/L) and the very low logP_owvalue (-6.58), it is expected to be readily absorbed via the gastrointestinal (GI) tract. The complex may be taken up also by passive diffusion through aqueous pores of the gastrointestinal epithelial by the bulk passage of water. However, absorption of very hydrophilic substances by passive diffusion may be limited by the rate at which the substance partitions out of the gastrointestinal fluid.

Since zinc is expected to dissociate from the complex at acidic conditions of stomach, it will follow an independent way of absorption which is regulated by the body needs. Zinc is an essential trace element which is homeostatically regulated and maintained in the various tissues mainly by the gastrointestinal absorption and secretion during high and low dietary zinc intake and because of the limited exchange of zinc between tissues, a constant supply of zinc is required to sustain the physiological requirements. Zinc absorption in the gastrointestinal tract occurs throughout the entire small intestine with the highest rate in the jejunum and the rate of total absorption appears to be concentration-dependent (Cousins, 1985; Lee et al., 1989; RAR, 2004). The zinc absorption process in the intestines includes both passive diffusion and a carrier-mediated process. The absorption can be influenced by several factors such as ligands in the diet and the zinc status. Persons with adequate nutritional levels absorb 20-30% and animals absorb 40-50% (Salgueiro et al., 2000; ATSDR, 2005). Persons that are zinc deficient absorb more, while persons with excessive zinc intake absorb less. “Knowledge of the form used and its molecular weight is necessary to calculate the amount of elemental zinc administered under a given set of circumstances, and is similarly important in that different chemical forms of zinc may be absorbed to differing degrees depending on their in vivo solubility, resulting in differing levels of toxicity” (ATSDR, 2005). For the soluble zinc compounds, the available information suggests an oral absorption value of 20%. This value can be considered as the lower bound range at adequate nutritional levels.

Referring to absorption of glucoheptonate moiety via gastrointestinal tract, it is assumed to be similar to other well-investigated structural carbohydrates. Glucoheptonic acid is a carbohydrate and is one of the natural occurring metabolites in plants found in the potato tuber (Roessner et al., 2000), in orange trees (Liu et al., 2015), in avocado (Septon and Richmyer, 1963) and in other plants (Fraser-Reid et al., 2012). Gluconate and its isomerised product glucono-delta-lactone as the most structurally similar analogues are known to be readily absorbed in the small intestines (OECD SIDS, 2004; WHO, 1999). Absorption of glucoheptonate moiety via gastrointestinal tract is considered to be similar to gluconate moiety. Ca gluconante was extensively absorbed in animals and in humans (WHO, 1999). On the other hand, other carbohydrates i.e. isomalt, lactitol, lactulose and sucralose are absorbed either only to a limited extent or not absorbed (CIR, 2014).

Based on this information, zinc from zinc glucoheptonate is expected to be absorbed following an independent pattern typical for soluble zinc compounds. Absorption of glucoheptonate moiety is assumed to be similar to that of gluconate. Since toxicity effects are assumed to be driven zinc in case if zinc glucoheptonate is ingested, a prediction of its absorption rate is essential for purposes of the hazard assessment of zinc glucoheptonate (please refer to read-across statement). 12 % oral absorption would be theoretically appropriate for elemental zinc from zinc glucoheptonate, while 100 % absorption is appropriate for glucoheptonate moiety. Regarding the intact complex zinc glucoheptonate, its physico-chemical characteristics are in the range suggestive of moderate absorption from the gastro-intestinal tract according to ECHA guidance. Thus, taken together, a worst-case value of 100 % will be used for the calculation of hazard values (DNELs), if required, by route-to route extrapolation, because no substance-specific data is available on oral absorption in mammalian species for zinc glucoheptonate.

Dermal absorption

Based on physico-chemical properties of zinc glucoheptonate, the substance is not likely to penetrate the skin to a large extent as the substance is not sufficiently lipophilic to cross thestratum corneum(negative logP_owof -16.9 and water solubility of 500-550 g/L). The water solubility above 10,000 mg/L together with the log P value below 0 further indicates that the substance is too hydrophilic to cross the lipid rich environment of thestratum corneum. Dermal uptake of such substances will be low. In case, if certain amounts of zinc glucoheptonate dissociate in the moisture of skin, only negligible amounts of zinc will be available for systemic absorption. Studies regarding the absorption of zinc through the skin are very limited. Insoluble or less soluble zinc compounds (i.e. zinc oxide) have been reported to promote wound healing while soluble zinc compounds (i.e. zinc sulfate) is rapidly transferred into the blood and, therefore, caused decreased wound-tissue zinc levels (Agren et al. 1991, cited in ATSDR, 2005). According to RAR (2004), the available information from in vivo as well as the in vitro studies suggests the dermal absorption of soluble zinc compounds through intact skin to be less than 2% (solutions or suspensions).

There is no experimental data available on dermal absorption of the glucoheptonate ions as well as on structurally similar gluconates. The molecular weight of 325.6 g/mol (monomer) 619.2 g/mol (dimer of zinc glucoheptonate indicates a certain potential to penetrate the skin. However, from the molecular structure (dissociating chemical to polar ions), it is suggested that it is unlikely that significant amounts of zinc glucoheptonate can be resorbed through intact skin. Zinc glucoheptonate is not irritating to skin and is considered to be not sensitising to skin (Patel, 2016; Basketter et al., 1999; Ikarashi et al., 1992). Thus, an enhancement of penetration due to damage of the skin can be ruled out. According to ECHA guidance R.7C (2014), 10% of dermal absorption is considered for zinc glucoheptonate as a worst case value, due to negative logP_owand the very high water solubility, although realistically any dermal absorption is very unlikely.

 

Inhalation absorption

Based on the low vapour pressure (1.55 x 10E-4 Pa) of zinc glucoheptonate, inhalation exposure via vapour is not likely. Moreover, the final product has a granulated form. The majority of the particles (87.1 %) have a size between 100 and 800 µm. 13.9 % of the particles are smaller than 100 µm but bigger than 40 µm. There are no particles smaller than 40 µm (Dabeer, 2014). Thus, it is very unlikely, that considerable amounts of the substance reach thoracic and alveolar regions of the lung. In cases, if the substance reaches the lung, it is taken up rapidly because of molecular weight of 325.6 (monomer) 619.2 g/mol (dimer). The substance could be absorbed extensively through aqueous pores. Since the substance is a water-soluble dust, it is also expected to diffuse/dissolve into the mucus lining the respiratory tract. The negative LogP_owpoints also to a low absorption potential across the respiratory epithelium.

With regard to absorption by inhalation of zinc, animal data with various zinc compounds suggest that there is pulmonary absorption following inhalation exposure (RAR, 2004). The absorption of inhaled zinc depends on the particle size and the deposition of these particles. The deposition of soluble zinc compounds occurred mainly in the head region, which is rapidly translocated to the gastrointestinal tract, and much less in the tracheobronchial and pulmonary region that is absorbed locally. Local absorption of the soluble zinc compounds is considered to be approximately 20% of the material deposited in the head region, 50 % of the material deposited in the tracheobronchial region and 100% of the material deposited in the pulmonary region. As maximum, the inhalation absorption for the soluble zinc compounds is considered 40 %. These values can be assumed as a reasonable worst case, because they are considered to cover existing differences between the different zinc industry sectors with respect to the type of exercise activities (and thus breathing rate) and particle size distribution.

Based on this data, a low to moderate systemic availability after inhalation can be predicted. Since 13.9 % of particles are under 100 µm and physico-chemical characteristics of zinc glucoheptonate are not in a range suggestive of significant absorption via the respiratory tract, it is very likely that the substance can be absorbed rather by aspiration. As worst-case, 100 % inhalation absorption as default guidance value is appropriate for the dust and other fractions of the microgranulated product.

Distribution and accumulative potential

Distribution and accumulation of glucoheptonate

Since zinc can dissociate from the glucoheptonate moiety before absorption, their distribution and accumulative potential can follow more or less independent ways.

The distribution of glucoheptonate moiety can be assessed using data on absorption of other glucoheptonate compounds, especially those used as radiotracer for imaging tumors. Tc-99m Glucoheptonate (GHA) is used as a renal imaging agent (Arnold et al., 1975; Lee and Blaufox, 1985; Wenzel et al., 1977). In a study investigating distribution of different renal imaging agents, distribution of Tc-99m glucoheptonate was into renal cortex and was similar to that of Tc-labelled gluconate (Arnold et al., 1975; Adler et al., 1976). After ip injection of [99]Tc-Sn-glucoheptonate to mice, the radiopharmaceutical was also distributed predominantly to kidney. The other organs were liver, lungs, blood, spleen and muscle (Wenzel et al., 1977). Kiewiet (1981) measured also glucoheptonate in stomach, intestines, thyroid glands and bone marrow. The renal uptake was between 14-18 % in rats (Kiewiet, 1981), 13 % in rabbits and 21 % in dogs after 1 hour injection (Arnold et al., 1975). In rats with experimental myocardial infarction, Tc-99m glucoheptonate showed significant uptake in myocardial lesions (Adler et al., 1976). Blood and urinary clearance were very fast (Arnold et al., 1975; Adler et al., 1976).Like gluconate, about 50% of the plasma activity of the GHA complex is loosely bound to plasma proteins initially, increasing to about 75% after 6 hr (Arnold et al., 1975).

Tc-99m glucoheptonate has been also used for imaging brain and lung tumors, hypoxia and ischemia (Waxman et al., 1976; Vorne et al., 1982; Vorne et al., 1987; Barai et al., 2004; Ramchandra, 2011).

According to the label on Tc99-glucoheptonate of Anazao Health corporation (2012): “When injected intravenously, Technetium Tc 99m Glucoheptonate is rapidly cleared from the blood. In patients with normal renal function, less than 15% of the initial activity remains in the blood after one hour. About 40% of the injected dose is excreted in the urine in one hour, while about 70% is excreted in 24 hours. In patients with renal disease, the blood clearance and urinary excretion of the radiopharmaceutical are delayed. Up to 15% of the injected dose is retained in the kidneys. The renal retention is greater in the cortex than in the medulla. The radiopharmaceutical may be bound to the proximal convoluted tubules, which are located primarily in the renal cortex. Technetium Tc 99m Glucoheptonate tends to accumulate in intracranial lesions that are associated with excessive neovascularity or an altered blood-brain barrier. The drug does not accumulate in the choroid plexus or salivary glands”.

According to Jaiswal et al. (2009), glucoheptonate has high degree of specificity for neoplastic tissues allowing to differentiate neoplastic lesions from non-neoplastic ones. The uptake mechanism by the cells may be linked to GLUT-1 (Glucose transporter) and GLUT-4 expression that are overexpressed in malignant tissues. Ramchandra (2011) concluded that glucoheptonates behaves as a glucose analogue, actively transported as a source of energy.

There are no published studies in the scientific literature on ADME behaviour of glucoheptonate after oral intake. Since Ca glucoheptonate (Ca gluceptate) is routinely used for treatment of hypocalcaemia, the substance is well investigated and therefore no classical toxicity studies were carried out with calcium glucoheptonate in laboratory animals (EMEA, 1998). Therefore, no significant bioaccumulation is expected.

 

Distribution and accumulation of zinc

 

With regard to distribution of zinc, a number of experimental studies are available. The highest levels of radioactivity were found in the small intestine followed by the kidney, liver and large intestine six hours after a single oral administration of 0.1 µCi of [65]Zn²⁺as ZnCl₂to Wistar rats. Smaller amounts were found in the lungs and spleen. Fourteen days after administration, the highest levels of radioactivity were found in the hair, testicles, liver and large intestines (Kossakowski and Grosicki, 1983). 

Organs with high zinc concentrations (ranging from 20 to 60 mg/kg fresh weight) are liver, gut, kidney, skin, lung, brain, heart and pancreas as cited in EU RARs (Bentley and Grubb, 1991; He et al., 1991; Llobet et al., 1988). High concentrations of zinc were also detected in the retina and in sperm as cited in EU RARs (EU 2004, a, b, c, d, e, f; Bentley and Grubb, 1991).

The tissue uptake of [65]Zn²⁺(as zinc chloride) was determined in adult male Wistar rats after intraperitonealinjection of 15 µCi [65]Zn²⁺. The liver displayed the greatest uptake for zinc ions, followed by the kidney, pancreas, spleen, ileum, lung, heart, bone, testis, blood cells, muscle and brain. Additional data on Zn uptake by the brain indicates that the blood-brain barrier is minimally permeable to zinc cations (Pullen et al., 1990).

Eight hours following intravenous administration of 65[Zn]-chloride to rabbits, tissue levels were highest in the liver, intestine and kidney with levels being ≥ 10 %/g in tissue (Lorber et al., 1970; Llobet et al., 1988).

 

After absorption from the gastrointestinal tract, the zinc is bound in plasma primarily to albumin and then transported to the liver and subsequently throughout the body. The normal plasma zinc concentration is ca. 1 mg/L, the total zinc content of the human body (70 kg) is in the range of 1.5-2 g (ATSDR, 2005).

Zinc is found in all tissues and tissue fluids and it is a co-factor in over 200 enzyme systems. In animal studies, the highest zinc concentrations were found in bone, liver and kidneys as well as in blood (Llobet et al., 1988). In humans, the major part of total body zinc is found in muscle and bone, approximately 60% and 30%, respectively (Wastney et al., 1986). Under normal conditions, the highest zinc concentration per tissue weight is found in bone, hair and in the prostate (Cleven et al., 1993).

The distribution of zinc in humans appears to be influenced by age. The zinc concentration levels increase in the liver, pancreas and prostate and decreases in the uterus and aorta with age. Levels in kidneys and heart peak at approximately 40-50 years of age and then declines. Levels in the aorta decline after 30 years of age (Schroeder et al., 1967).

Metabolism and excretion

Metabolism and excretion of zinc and glucoheptonate are considered to follow their independent pathways.

As described in EU RARs, zinc is primarily bound to organic ligands rather than existing free in solution as a cation (Gordon et al., 1981). It is found in diffusible and non-diffusible forms in the blood. About 66% of the diffusible form of zinc in the plasma is freely exchangeable and loosely bound to albumin (Cousins et al., 1985). A small amount of the non-diffusible form of zinc is tightly bound to α2-macroglobulin in the plasma and is not freely exchangeable with other zinc ligands. Zinc is incorporated into and dissociated from α2-macroglobulin only in the liver (Henkin et al., 1974).

After a single oral dose of 86 – 130 µg of 65[Zn] as ZnCl₂, ZnCO₃or Zn₅(OH)₈Cl₂x H₂O, male rats eliminated 65[Zn] from the body with a rate of about 1.7 % of the absorbed dose during day 5 to 14 post-dosing as determined from stool, urinary and in vivo whole-body gamma counting results. Male rats who received 25 mg ZnCO₃/kg feed or 100 mg Zn₅(OH)₈Cl₂x H₂O / kg feed for 14 days, the radioactivity from a subcutaneous dose of 37 kBq of 65[Zn]Cl₂disappeared from the body with a rate of approximately 1% during the period 5 to 14 days post dosing (Galvez-Morros et al., 1992).

 

As described in EU RARs (2004 a-f) within certain limits, mammals can maintain the total body zinc and thephysiologically required levels of zinc in the various tissues, constant, both at low and high dietary zinc intakes. The sites of regulation of zinc metabolism are: absorption of zinc from the gastrointestinal tract, excretion of zinc in urine, exchange of zinc with erythrocytes, release of zinc from tissue, and secretion of zinc into the gastrointestinal tract. Regulation of gastrointestinal absorption and gastrointestinal secretion most likely contributes the most to zinc homeostasis. In spite of the mechanism for whole-body zinc homeostasis, a regular exogenous supply of zinc is necessary to sustain the physiological requirements because of the limited exchange of zinc between tissues (EU RAR 2004 a-f). It has been hypothesized by Hempe and Cousins (1992) that zinc entering the luminal cells is associated with cysteine-rich intestinal protein (CRIP), a diffusible intracellular zinc carrier, and that a small amount is bound to metallothionein; however, as the luminal zinc concentration increases, the proportion of cytosolic zinc associated with CRIP is decreased and zinc binding to metallothionein is increased. CRIP binds 40% of radiolabelled zinc entering the intestinal cells of rats when zinc concentration is low; but only 14% when the concentration is high.

Zinc is initially concentrated in the liver after ingestion, and is subsequently distributed throughout the body. When plasma zinc levels are high, liver metallothionein synthesis is stimulated, which facilitates the retention of zinc by hepatocytes (EU RAR 2004 a-f).

 

In humans, the faecal zinc consists of un-absorbed dietary zinc and endogenous zinc from bile, pancreatic juice and other secretions. About 70-80% of the ingested amount of zinc is excreted via faeces (5 to 10 mg/day depending upon the dietary zinc concentration) (Spencer et al., 1966; Venugopal and Lucky, 1978; Reinhold et al., 1991; Wastney et al., 1986). In humans, of the amount of zinc consumed, about 10% is lost through urine (approximately 200 to 600 µg zinc/day). The urinary zinc excretion appears to be sensitive to alterations in the zinc status (Babcock et al., 1982; Aamodt et al., 1982). Saliva, hair loss, mother’s milk and sweat appear to be minor routes for zinc excretion. In tropical climates about 2-3 mg Zn/day may be lost in sweat (Venugopal and Lucky, 1978; Rivlin, 1983; Prasad et al., 1963; Rossowka and Nakamoto, 1992; Henkin et al., 1975). In humans with no excessive intake of zinc, the half-life of absorbed radio-labelled zinc ranges from 162 to 500 days. After parenteral administration of 65[Zn], half-lives ranged from 100 to 500 days (Elinder, 1986).

Metabolism of glucoheptonate in mammalian tissues is described in several publications dealing with investigations of substrate specificity of a various number of aldonic acids and its isomeric analogues lactones. The enzyme 6-phosphogluconolactonase (catalysing the second step of pentose phosphate pathway (PPP)) was shown to possess a broad substrate specificity hydrolysing gluconolactone moieties including glucoheptonate. The enzyme is present in almost all mammalian tissues including humans. Further investigations revealed that glucoheptonate moiety undergoes a series of biochemical transformations similar to those of PPP. Since glucoheptonic acid is a naturally occurring substance in plants (potato, orange trees, avocado etc.) and a derivative of glucoheptonic acid participates in the biosynthesis of aromatics compounds in plants as part of the shikimic acid pathway, it is involved into intermediary carbohydrate metabolism in mammals (please refer to read-across statement).

Glucoheptonate is mainly excreted via the kidneys (Kiewiet, 1981). About 12% activity remains in the renal cortex for up to 6 h, while most of the injected activity appears in the urine (Ramchandra, 2011). In a subacute toxicity study with Tc-99m glucoheptonate in rats, dogs and rabbits by iv injection, a large proportion of the dose was cleared from the plasma by glomerular filtration and was rapidly excreted (Belbeck et al., 1981). Some of glucoheptonate actively secreted in the bile and intestines (Ramchandra, 2011). Intense biliary excretion of glucoheptonate has been described in patients in the fasting state and in patients with renal insufficiency and with obstruction of the abdominal aorta. Several medications are also known to increase the biliary excretion of glucoheptonate (Siegel et al., 1992).

Summary

Zinc glucoheptonate is expected to be moderately absorbed after oral exposure, based on its high water solubility and molecular weight suggestive for favoured absorption through gastrointestinal tract. As worst-case, 100 % oral absorption is considered appropriate. Concerning absorption after exposure via inhalation, as the chemical has a low vapour pressure, is highly hydrophilic, has a negative LogPow, and has 13.9 % of particles less than 100 µm, it is clear, that the substance is poorly available for inhalation and will not be absorbed significantly via lungs. However, an absorption by aspiration cannot be fully ruled out. Therefore, 100% inhalation absorption is considered. Zinc glucoheptonate is not expected to be absorbed following dermal exposure into the stratum corneum and into the epidermis, due to its very high water solubility and considering low absorption potential of zinc and glucoheptonate moieties. 10 % absorption is therefore considered for dermal route of exposure. Glucoheptonate moieties, are expected to be distributed predominantly to kidneys and organs with higher expression of glucose transporters. The substance does not indicate a significant potential for accumulation. Zinc homeostasis is regulated in mammals by gastrointestinal absorption, excretion via faeces and via the urine as well as by the release from tissues. The total body zinc is maintained constant at the physiologically required levels of zinc in the various tissues at low and high dietary zinc intakes. Zinc, if released from glucoheptonate, is distributed to all organs and tissues and will be bound with organic ligands rather than existing free in solution as a cation. Glucoheptonate is involved into intermediary carbohydrate metabolism and eliminated unchanged primarily via the urine and to a lesser extent via the bile.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Absorption rate - oral (%):

100

Absorption rate - dermal (%):

10

Absorption rate - inhalation (%):

100