Propyl 3,4,5-trihydroxybenzoate

Endpoint summary

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

basic toxicokinetics, other

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

4 (not assignable)

Rationale for reliability incl. deficiencies:

secondary literature

Conclusions:

Several studies on toxicokinetics of Propyl gallate are available for different species including mice, rats, rabbits, dogs, pigs and humans.

Executive summary:

Several studies on toxicokinetics of Propyl gallate are available for different species.

Mice

A mouse study by Vora et al. (1999) compared the toxicokinetics of Propyl gallate when administered in 2 different vehicles: an ethanol:saline solution (2:3, 0.9 % w/v) and a solution of the inclusion complex hydroxypropyl-beta-cyclodextrin (concentration not specified) in saline (0.9 % w/v). No dosing volumes were specified. Harlan Sprague-Dawley mice (4-6 animals/group; 20-30 g in bw) were administered 100 mg Propyl gallate/kg bw p.o. and sacrificed in groups at various post-dosing time-points (0-180 minutes) for blood collection and plasma separation. The results suggested that the rate of absorption of Propyl gallate was slower in the ethanol:saline solution. This might be explained by the effects of ethanol on blood flow to the gastrointestinal tract and liver. However, the overall absorption of approximately 5 % was not different between the 2 vehicles. No data on the distribution of Propyl gallate to organs has been identified.

Rats

Adult albino rats were administered 100 mg Propyl gallate per animal by gavage (vehicle not specified) in a study by Booth et al. (1959). The major metabolite detected in urine was 4-O-methyl-gallic acid. Gallic acid, 2-Methoxypyrogallol and glucuronides of the methoxylated products were minor metabolites. This indicates hydrolysis of the ester followed by 4-O-methylation of Gallic acid.

In a study in which rats were given 100 mg Gallic acid per animal either by gavage or by intraperitoneal (i.p.) injection (vehicles not specified) or fed a diet containing 0.5 % (equivalent to 600 mg/kg bw) Gallic acid, urinary excretion of 4-O-methyl gallic acid as well as Gallic acid was reported. In rats given the i.p. injection of Gallic acid, an additional metabolite, suspected to be Pyrogallol, was also detected along with trace levels of 2-O-methyl-pyrogallol.

Metabolism of Propyl gallate by intestinal bacteria in the rat was investigated in a study by Niimura et al. (1986). Several strains were isolated from the faeces of rats, which converted Propyl gallate to Gallic acid and then further decarboxylated to produce Pyrogallol.

Rabbits

4-O-methyl-gallic acid, and Pyrogallol were found in the urine of New Zealand White rabbits (weighing approximately 2 kg) fed 0.5% (equivalent to approximately 150 mg/kg bw/day) Gallic acid in the diet (duration not specified) in a study by Booth et al. (1959).

In rabbits fed 1 g Propyl gallate, the major urinary metabolite was a glucuronide conjugate hypothesised to be 4-O-methyl galloyl-β:D-glucosiduronic acid (72.0 % of the dose administered) and unconjugated phenols: 4-O-methyl gallate, Gallic acid and Pyrogallol (6.7 % of the dose administered) (Dacre, 1960). In rabbits fed Gallic acid for 10 days, most of the dose was excreted in the urine unchanged although some 4-O-methyl-gallic acid and Pyrogallol were also detected (Conning et al., 1986).

Dogs

No Propyl gallate was detected in the urine of dogs fed 0.0117 % Propyl gallate in the diet for 14 months (Orten et al., 1948).

Pigs

Madhavi (1996) reported that the metabolism of gallic esters in the pig is similar to that in rats.

Rat and human comparison

In a project for the UK Food Standards Agency, Tullberg and colleagues (2004) compared the kinetics of four food additives (BHT, Curcumin, Propyl gallate and Thiabendazole) in vivo in rats and humans and in hepatocytes from human and rat to examine the adequacy of the kinetic uncertainty factors.

The plasma concentration-time curves for Propyl gallate in rats showed rapid absorption and elimination with significantly higher concentrations in males than in females. The plasma concentrations in humans dosed at the current group ADI (0.5 mg/kg bw) were close to the limit of quantification, whereas more reliable data were obtained at a dose of 10 times the ADI. The kinetics were essentially linear in both rats and humans at the doses studied. The data indicated that the default uncertainty factors for interspecies differences and human variability would be adequate for Propyl gallate. Studies with Octyl gallate and Dodecyl gallate showed the presence of extremely low plasma concentrations, which, combined with the low ADI values for these food additives, meant that further in vivo studies were impracticable.

In vitro studies on Propyl gallate using rat and human liver preparations showed that there were minor species differences in the rates of metabolism and in the estimated Vmax and Km values for the BHT, Propyl gallate and TBZ metabolising enzyme systems. The estimated interspecies adjustment factors based on Vmax/Km were 2.4 for Propyl gallate compared with in vivo values of 2.0 for Propyl gallate (at dose equivalence). The in vitro values do not take into account differences in organ blood flow, and such analyses would require the development of a full PBPK model. Interestingly the Km values for Propyl gallate were considerably higher than the in vivo plasma levels, indicating little likelihood of saturation of metabolism (Tullberg et al, 2004).

Reference 2

Endpoint:

basic toxicokinetics in vitro / ex vivo

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

4 (not assignable)

Rationale for reliability incl. deficiencies:

abstract

The transformation, transport and distribution of n-Propyl gallate and two analogues were investigated in the rat liver in an in vitro study. Isolated perfused rat liver was used. n-Propyl gallate, Methyl gallate, n-Octyl gallate and transformation products were quantified by high pressure-liquid chromatography coupled to fluorescence detection. The interactions of n-Propyl gallate and analogues with the liver presented three main characteristics: (1) the hydrolytic release of Gallic acid from n-Propyl gallate and Methyl gallate was very fast compared with the subsequent transformations of the Gallic acid moiety; (2) transport of the esters was very fast and flow-limited in contrast to the slow and barrier-limited transport of Gallic acid; (3) the apparent distribution volume of n-Propyl gallate, but probably also of Methyl gallate and n-Octyl gallate, greatly exceeded the water space in the liver, contrary to the Gallic acid space, which is smaller than the water space. It can be concluded in this study that at low portal concentrations (<50 μM) the Gallic acid esters are 100% extracted during a single passage through the liver, releasing mainly Gallic acid into the systemic circulation. For the latter a considerable time is required until complete biotransformation. The exposure of the liver to the esters, however, was shown to be quite prolonged due to extensive intracellular binding.

Conclusions:

The interactions of n-Propyl gallate and analogues with the liver presented three main characteristics: (1) the hydrolytic release of Gallic acid from n-Propyl gallate and Methyl gallate was very fast compared with the subsequent transformations of the Gallic acid moiety; (2) transport of the esters was very fast and flow-limited in contrast to the slow and barrier-limited transport of Gallic acid; (3) the apparent distribution volume of n-Propyl gallate, but probably also of Methyl gallate and n-Octyl gallate, greatly exceeded the water space in the liver, contrary to the Gallic acid space, which is smaller than the water space. At low portal concentrations (<50 μM) the Gallic acid esters (including n-Propyl gallate) are 100% extracted during a single passage through the liver, releasing mainly Gallic acid into the systemic circulation.

Executive summary:

The transformation, transport and distribution of n-Propyl gallate and two analogues were investigated in the rat liver in an in vitro study. Isolated perfused rat liver was used. n-Propyl gallate, Methyl gallate, n-Octyl gallate and transformation products were quantified by high pressure-liquid chromatography coupled to fluorescence detection. The interactions of n-Propyl gallate and analogues with the liver presented three main characteristics: (1) the hydrolytic release of Gallic acid from n-Propyl gallate and Methyl gallate was very fast compared with the subsequent transformations of the Gallic acid moiety; (2) transport of the esters was very fast and flow-limited in contrast to the slow and barrier-limited transport of Gallic acid; (3) the apparent distribution volume of n-Propyl gallate, but probably also of Methyl gallate and n-Octyl gallate, greatly exceeded the water space in the liver, contrary to the Gallic acid space, which is smaller than the water space. It can be concluded in this study that at low portal concentrations (<50 μM) the Gallic acid esters are 100% extracted during a single passage through the liver, releasing mainly Gallic acid into the systemic circulation. For the latter a considerable time is required until complete biotransformation. The exposure of the liver to the esters, however, was shown to be quite prolonged due to extensive intracellular binding.

Reference 3

Endpoint:

basic toxicokinetics, other

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

4 (not assignable)

Rationale for reliability incl. deficiencies:

abstract

Propyl gallate was quickly metabolised and excreted when administered orally to rats and rabbits. CIR (1985) reported that "When fed to rats, most of the Propyl Gallate was passed in the feces as the original ester. The urinary components detected were the original ester and gallic acid, and these were excreted completely within 24 hours". When administered orally to rabbits, 79 percent of the administered dose of Propyl gallate was excreted in the urine, "72 percent as 4-methoxygallic acid glucuronide" and "6.7 percent as unconjugated phenolic compounds. Minor metabolites included Pyrogallol (free and conjugated) and free 4-methoxy gallic acid" (CIR 1985).

Conclusions:

Propyl gallate is quickly metabolised to gallic acid and other metabolites and excreted when administered orally to rats and rabbits.

Executive summary:

It is mentioned in the US EPA (2005) scientific assessment report that Propyl gallate was quickly metabolised and excreted when administered orally to rats and rabbits. CIR (1985) reported that "When fed to rats, most of the Propyl Gallate was passed in the feces as the original ester. The urinary components detected were the original ester and gallic acid, and these were excreted completely within 24 hours". When administered orally to rabbits, 79 percent of the administered dose of Propyl gallate was excreted in the urine, "72 percent as 4-methoxygallic acid glucuronide" and "6.7 percent as unconjugated phenolic compounds. Minor metabolites included Pyrogallol (free and conjugated) and free 4-methoxy gallic acid" (CIR 1985).

Reference 4

Endpoint:

basic toxicokinetics in vitro / ex vivo

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

4 (not assignable)

Rationale for reliability incl. deficiencies:

abstract

Detailed metabolic pathways for Propyl gallate have been described by Dacre (1960) as mentioned in the WHO 1993 evaluation of certain food additives and contaminants including Propyl gallate.

In vitro incubations with Propyl, Octyl and Dodecyl gallate were performed using homogenates of liver, mucosa of the small intestine, and contents of caecum/colon as a source of intestinal microflora. The various homogenates were incubated at 37 °C with the individual gallate esters. At various time points up to 24 hours, samples were taken and analysed by HPLC in order to determine the concentration of Gallic acid and residual ester. From the time-course of Gallic acid formation, as well as the disappearance of the specific esters, the rate of hydrolysis of the three esters was calculated.

All test substances were extensively metabolised by the homogenate of the intestinal mucosa, which was demonstrated by the appearance of peaks in the chromatograms. Furthermore, the caecum and colon contents also showed a high metabolic capacity, especially towards Propyl gallate. The amount of Gallic acid detected in the incubations was always much smaller than the total decrease of the amount of ester. It seems likely that apart from hydrolysis of the ester bond, other biotransformation routes (oxidation and/ or conjugation) are of major importance for all three gallate esters.

The three homogenates show quantitatively different structure-activity relationships for the three esters. Homogenates of liver and of contents of caecum and colon metabolise Propyl gallate most extensively followed by Octyl or Dodecyl gallate. Homogenate of the mucosa of the small intestine shows the highest rates with Octyl gallate, lower rates with Dodecyl gallate and Propyl gallate. For this homogenate, the rate of formation of Gallic acid is inversely related to the chain length of the ester (de Bie & van Ommen, 1992).

Conclusions:

Propyl gallate is extensively metabolised by homogenates of liver, mucosa of the small intestine and contents of caecum and colon. Apart from hydrolysis of the ester bond, other biotransformation routes (e.g. oxidation and/or conjugation) are of major importance for Propyl gallate.

Executive summary:

Detailed metabolic pathways for Propyl gallate have been described by Dacre (1960) as mentioned in the WHO 1993 evaluation of certain food additives and contaminants including Propyl gallate.

In vitro incubations with Propyl, Octyl and Dodecyl gallate were performed using homogenates of liver, mucosa of the small intestine, and contents of caecum/colon as a source of intestinal microflora. The various homogenates were incubated at 37 °C with the individual gallate esters. At various time points up to 24 hours, samples were taken and analysed by HPLC in order to determine the concentration of Gallic acid and residual ester. From the time-course of Gallic acid formation, as well as the disappearance of the specific esters, the rate of hydrolysis of the three esters was calculated.

All test substances were extensively metabolised by the homogenate of the intestinal mucosa, which was demonstrated by the appearance of peaks in the chromatograms. Furthermore, the caecum and colon contents also showed a high metabolic capacity, especially towards Propyl gallate. The amount of Gallic acid detected in the incubations was always much smaller than the total decrease of the amount of ester. It seems likely that apart from hydrolysis of the ester bond, other biotransformation routes (oxidation and/ or conjugation) are of major importance for all three gallate esters.

The three homogenates show quantitatively different structure-activity relationships for the three esters. Homogenates of liver and of contents of caecum and colon metabolise Propyl gallate most extensively followed by Octyl or Dodecyl gallate. Homogenate of the mucosa of the small intestine shows the highest rates with Octyl gallate, lower rates with Dodecyl gallate and Propyl gallate. For this homogenate, the rate of formation of Gallic acid is inversely related to the chain length of the ester (de Bie & van Ommen, 1992).

Description of key information

Several studies on toxicokinetics on Propyl gallate are available.

Absorption:

In a study from Vora et al. (1999), Propyl gallate (administered p.o. in 2 different vehicles: ethanol:saline solution and a solution of the inclusion complex hydroxypropyl-beta-cyclodextrin) was shown to have a low absorption rate (approximately 5%) in mice. On the other hand, as described in the EFSA (2014) report, the plasma concentration-time curves for Propyl gallate in rats showed rapid absorption in one kinetic study (Tullberg et al., 2004).

Distribution:

Eler et al. (2013) reported that the apparent distribution volume of Propyl gallate in rat greatly exceeded the water space in the liver. This study concluded that at low portal concentrations (< 50 µM) gallic acid esters (including n-Propyl gallate) will be extracted 100% during a single passage through the liver by releasing mainly Gallic acid into the systemic circulation. In human, Propyl gallate was not detected in plasma samples but in omentum samples (Bianchi et al., 1997).

Metabolism & Excretion:

Metabolism studies identified several metabolites such as 4-O-methyl-gallic acid, Gallic acid, 2 -Methoxypyrogallol and glucuronides of the methoxylated products in urine from rats. This supports the hypothesis that Propyl gallate will undergo hydrolysis of the esters to yield Gallic acid (Booth et al. 1959). It has been shown that the hydrolytic release of Gallic acid from Propyl gallate is very fast compared to the transformations of the Gallic acid moiety (Eler et al., 2013).

In rabbits, metabolites as glucuronide conjugate hypothesised to be 4-O-methyl galloyl-β: D-glucosiduronic acid (72.0 % of the dose administered) and unconjugated phenols: 4-O-methyl gallate, Gallic acid and Pyrogallol was found in the urine after oral administration of Propyl gallate (Dacre, 1960).

The metabolism of Propyl gallate as well as the excretion was found to be fast in rats and rabbits after oral administration. A complete excretion of the original ester and Gallic acid was identified within 24 hours. Propyl gallate is metabolised quickly in the liver, intestinal mucosa, caecum and colon (WHO 1960).

CIR (1985) described that 79% of Propyl gallate was excreted in the urine, which 72% of the excreted dose was identified as 4-methoxygallic acid glucuronide and 6.7% as unconjugated phenolic compounds along with minor metabolites such as Pyrogallol and 4-Methoxy gallic acid. Similar findings on the metabolism of gallic esters in rats were also seen in pigs (Madhavi 1996).

 

Minor species differences between rats and humans were identified with regards to kinetics and the metabolising enzyme systems of Propyl gallate (EFSA, 2014).

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Additional information

Several studies on toxicokinetics on Propyl gallate are available.

 

Absorption:

In a study from Vora et al. (1999), the toxicokinetics of Propyl gallate was determined in Harlan Sprague-Dawley mice. A comparison of the rate of oral absorption of Propyl gallate, dissolved in two different vehicles (ethanol: saline solution and a solution of the inclusion complex hydroxypropyl-beta-cyclodextrin (concentration not specified) in saline) was carried out. It has been shown that the rate of oral absorption of Propyl gallate was slower in the ethanol:saline solution compared to the other solution, but the overall oral absorption of approximately 5% was equal between the two vehicle solutions. On the other hand, as described in the EFSA (2014) report, the plasma concentration-time curves for Propyl gallate in rats showed rapid absorption in one kinetic study (Tullberg et al., 2004).

 

Distribution:

In a study from Eler et al. (2013), transport and distribution of Propyl gallate was investigated using isolated perfused rat liver and high pressure-liquid chromatography coupled to fluorescence detection. They reported that the apparent distribution volume of Propyl gallate greatly exceeded the water space in the liver. This study concluded that at low portal concentration (< 50 µM) gallic acid esters will be extracted 100% during a single passage through the liver by releasing mainly Gallic acid into the systemic circulation. A study from Bianchi et al. (1997) investigated Propyl gallate in human tissues. Tissues of omentum were collected in 50 patients and blood was collected. In 30% of the omentum samples, Propyl gallate was detected but not in the plasma.

 

Metabolism & Excretion:

Metabolism studies identified several metabolites such as 4-O-methyl-gallic acid, Gallic acid, 2-Methoxypyrogallol and glucuronides of the methoxylated products in urine from rats after oral administration of 100 mg Propyl gallate, and 4 -O-methyl-gallic acid was found to be the major metabolite. This supports the hypothesis that Propyl gallate will undergo hydrolysis of the esters to yield Gallic acid (Booth et al. 1959). It has been shown that the hydrolytic release of Gallic acid from Propyl gallate is very fast compared to the transformations of the Gallic acid moiety (Eler, 2013).

In rabbits, metabolites as glucuronide conjugate hypothesised to be 4-O-methyl galloyl-β: D-glucosiduronic acid (72.0 % of the dose administered) and unconjugated phenols: 4-O-methyl gallate, Gallic acid and Pyrogallol was found in the urine after oral administration of Propyl gallate (Dacre, 1960).

The metabolism of Propyl gallate as well as the excretion was found to be fast in rats and rabbits after oral administration. A complete excretion of the original ester and Gallic acid was identified within 24 hours. Propyl gallate is metabolised quickly in the liver, intestinal mucosa, caecum and colon (WHO 1960).

CIR (1985) described that 79% of Propyl gallate was excreted in the urine, which 72% of the excreted dose was identified as 4-methoxygallic acid glucuronide and 6.7% as unconjugated phenolic compounds along with minor metabolites such as Pyrogallol and 4-Methoxy gallic acid.Similar findings on the metabolism of gallic esters in rats were also seen in pigs (Madhavi 1996).

A comparison of rats and humans regarding the kinetics of four food additives (one of which includes Propyl gallate) was described in the EFSA (2014) report. In vivo and in vitro studies in rats and humans were carried out in a study from Tullberg et al. (2004) in order to examine kinetic uncertainty factors. In both rats and humans, the kinetics were essentially linear. In vitro studies revealed that minor differences in species were observed in the rates of metabolism and in the Vmax and Km values for the metabolising enzymes. Interspecies adjustment factors of Km and Vmax were determined to be 2.4, whereas in vivo values were found to be 2.0. It should be mentioned that in vitro values do not take into account differences in organ blood flow. Higher Km values found in vitro were higher than in the in vivo plasma levels, leading to the hypothesis that the metabolism shows saturation. Based on these results, species differences between rats and humans were identified to be minor with regards to kinetics.