Guidelines for Canadian Drinking Water Quality: Guideline Technical Document - Haloacetic Acids

8.0 Kinetics and metabolism

In the following studies, and in those in subsequent sections, HAAs were administered as the free acid, as the sodium salt or as a neutralized solution, depending on the study methodology. The sodium salt and neutralized solution of the HAA both result in the salt being formed and are designated as, for example, "MCA (sodium salt)." The free acid is designated as, for example, "MCA (acid)." The form of HAA used in each study is noted, because the form can influence the effects seen in the test systems. When it is described in the following studies that the HAA was neutralized, it signifies that sodium hydroxide was the base used to adjust the pH. In the event that another base was used, this is noted in the description of the study.

8.1 Absorption

8.1.1 Monochloroacetic acid

Experiments with buffered solutions of MCA at pH 7 across human skin using in vitro diffusion chambers failed to show evidence of significant dermal absorption (Xu et al., 2002). The authors stated that ionization may be the most significant factor limiting the permeability of HAAs.

ECETOC (1999) reported that MCA given orally to rats or mice was rapidly and extensively absorbed.

8.1.2 Dichloroacetic acid

DCA is also readily absorbed into the bloodstream from the gastrointestinal tract following oral exposure in both rats and humans (Stacpoole et al., 1987, 1998a; James et al., 1998; Schultz et al., 1999). Dermal absorption in humans is minor both in vivo (Kim and Weisel, 1998) and in vitro (using diffusion chambers with a buffered solution of DCA) (Xu et al., 2002;). DCA exists primarily as an ionic species in drinking and swimming pool waters that are kept within a neutral pH range (Kim and Weisel, 1998), which limits its dermal absorption (Xu et al., 2002;).

8.1.3 Trichloroacetic acid

TCA is readily absorbed from the gastrointestinal tract following oral exposure in both rats and humans (Kim and Weisel, 1998; Schultz et al., 1999). The concentration of TCA in blood in rats following oral ingestion peaked at approximately 2 hours post-dosing (Schultz et al., 1999). No evidence of significant dermal absorption was seen with TCA in humans in vivo (Kim and Weisel, 1998) or using diffusion chambers in vitro (Xu et al., 2002;).

8.1.4 Monobromoacetic acid

No specific studies measuring the absorption of MBA following different routes of exposure were undertaken; however, acute oral studies have shown that MBA is absorbed and causes adverse effects (see Section 10.1).

8.1.5 Dibromoacetic acid

DBA is rapidly absorbed into the bloodstream from the gastrointestinal tract following oral exposure in rats; the blood concentration peaked at approximately 1 hour post-dosing (Schultz et al., 1999). Schultz et al. (1999) estimated the oral bioavailability of DBA (using only a single high dose) at only 30%. They postulated that it was due to first-pass metabolism. No other doses were used to confirm this value.

Other short-term studies (Linder et al., 1994a, b, 1995, 1997b; Parrish et al., 1996; Cummings and Hedge, 1998; Vetter et al., 1998; NTP, 1999b) report effects on the liver, kidney, spleen and male reproductive system, demonstrating that DBA is sufficiently absorbed to have caused adverse effects.

No evidence of significant dermal absorption was seen with DBA at pH 7 across human skin using in vitro diffusion chambers (Xu et al., 2002;). The authors stated that ionization may be the most significant factor limiting the permeability of HAAs, including DBA.

8.2 Metabolism

8.2.1 Monochloroacetic acid

Two different pathways have been proposed for the breakdown of MCA in biological systems (ECETOC 1999):

• formation of S-carboxymethyl glutathione and subsequently S-carboxymethyl cysteine, which is then metabolized to thiodiacetic acid (main route); and
• formation of glycolic acid following hydrolysis of the C-Cl bond; subsequent oxidation leads to the formation of oxalic acid and carbon dioxide. Other metabolic pathways suggested are via dehalogenation to form oxalate and glycine

and/or dehalogenation and reduction to thiodiacetic acid via glutathione conjugation (Bhat et al., 1990). MCA (acid) has also been reported to bind to lipids (Yllner, 1971; Bhat and Ansari, 1989; Kaphalia et al., 1992).

8.2.2 Dichloroacetic acid

DCA's principal metabolic pathway occurs via oxidative dechlorination to form glyoxylate (Keys et al., 2004). Glyoxylate can be further biotransformed to oxalate (by oxidation), to glycine (by transamination) and subsequently to glycine conjugates such as serine and/or 5,10 methylene tetrahydrofolate, or to glycolate (by reduction); all of these metabolites are excreted in variable quantities in the urine (Stacpoole, 1989; James et al., 1998; Stacpoole et al., 1998a; U.S. EPA, 2003c). Some DCA is also converted to carbon dioxide and eliminated via exhaled air (James et al., 1998). DCA can also be metabolized through reductive dechlorination to form MCA and subsequently thiodiacetate (James et al., 1998).

The enzyme that initially catalyses the glutathione-dependent oxygenation of DCA has been identified as glutathione-S-transferase-zeta (GST-zeta) and is found primarily in the cytosol (Tong et al., 1998a, b). Appreciable differences in the metabolism of DCA exist between species.

The half-lives of DCA in mice and rats following oral dosing were 1.5 hours and 0.9 hour, respectively (Larson and Bull, 1992). Repeat dosing with DCA has also shown an increased plasma elimination half-life in both rats and humans (Anderson et al., 1999).

Toxicokinetics studies indicate that DCA is able to inhibit its own metabolism (also known as suicide inhibition) by irreversibly inactivating the GST-zeta enzyme (U.S. EPA, 2003c; Keys et al., (2004). Prior treatment with DCA has been shown to inhibit the metabolic clearance of subsequent doses of DCA in rats (James et al., 1998), mice (Schultz et al., 2002) and humans (Curry et al., 1985; Stacpoole et al., 1998a).

Species- and age-related differences in GST-zeta activity were observed. The relative rate of DCA transformation was greater in mice and rat cytosol than in human hepatic cytosol (Tong et al., 1998a). Reduced liver metabolism was seen in young mice, accompanied by a decrease in immunoreactive GST-zeta, whereas the levels of that protein remained unchanged in aged mice (Schultz et al., 2002).

Pharmacokinetic models, created to help estimate concentrations of DCA in the liver, may be useful to refine the tissue dose-response for liver tumours. However, these models are limited, as they can provide estimates of liver concentrations only where the metabolism is not inhibited or is at its maximum inhibition. Partial inhibition is difficult to model, since concentrations may vary depending on GST-zeta activity (U.S. EPA, 2003c).

Carcinogenic and genotoxic effects have been associated with high doses of DCA, where its metabolism is inhibited (U.S. EPA, 2003c).

However, as reported in the U.S. EPA, (2003c) Toxicological Review of DCA, there are still many unanswered questions regarding DCA's metabolism, including whether there is more than one metabolic pathway, and its relevance to toxicity in laboratory animals and humans.

8.2.3 Trichloroacetic acid

A relatively small proportion of TCA is metabolized in the liver. The formation of carbon dioxide, glyoxylic acid, oxalic acid, glycolic acid and DCA was observed in rats and mice following oral administration of radiolabelled TCA (neutralized). It was suggested that TCA was metabolized by reductive dehalogenation to DCA (Larson and Bull, 1992). Further reductive dehalogenation of DCA to MCA and ultimately to thiodiglycolate has been proposed as a metabolic pathway (Bull, 2000). However, other investigators have suggested that metabolism to DCA may have been over-reported in earlier studies due to analytical methodologies that convert TCA to DCA due to the presence of a reagent (Ketcha et al., 1996; Lash et al., 2000).

8.2.4 Monobromoacetic acid

As part of a larger metabolism study (Jones and Wells, 1981), a group of three male Sprague-Dawley rats was orally administered MBA (sodium salt), equivalent to 50 mg MBA/kg bw, and the urine was collected for 24 hours. Unchanged MBA was excreted in the urine within 24 hours, along with N-acetyl-S-(carboxymethyl)cysteine. No other details were provided in the study.

8.2.5 Dibromoacetic acid

An in vitro metabolism study conducted by Tong et al. (1998a) demonstrated that GST-zeta enzyme catalysed the oxygenation of DBA to glyoxylic acid, a pathway shared by DCA. WHO (2004c) reported that glyoxylic acid can be metabolized to glycine, glycolate, carbon dioxide or oxalic acid, based on a study by Stacpoole et al. (1998b).

8.3 Distribution

8.3.1 Monochloroacetic acid

After oral administration of a single toxic dose (225 mg/kg bw) of MCA (acid) to rats, levels initially remained below those seen following administration of a subchronic dose (10 mg/kg bw), because most of the toxic dose was retained in the stomach for up to 8 hours (a spasm of the pyloric sphincter prohibited further flux for several hours) (Saghir and Rozman, 2003).

Kaphalia et al. (1992) found that the liver, kidney, intestine and spleen were the organs containing the highest MCA levels when it was orally administered (as an acid) to rats as a single dose.

8.3.2 Dichloroacetic acid

DCA, when administered via gavage, is distributed initially to the liver and muscle and subsequently to other target organs (Evans, 1982; James et al. (1998). James et al. (1998) administered a single radiolabelled oral dose of DCA (sodium salt) to young adult rats, and the dose distributed mostly to the muscles (11.9%), liver (6.19%), gastrointestinal tract (3.74%), fat (3.87%) and kidney (0.53%). Other tissues, including plasma, spleen, heart, skin, bone, brain, lung and testes, accounted for 9.5% of the administered dose. DCA also exhibited low binding to plasma when given intravenously (Schultz et al., 1999). Schultz et al. (1999) noted that the lipophilicity of DCA was low when measured at a pH value close to that of blood (pH 7.4), which indicates that DCA would not tend to accumulate in fat.

8.3.3 Trichloroacetic acid

Following oral and intravenous administration in rats, TCA appears to bind significantly to plasma proteins and is also distributed to the liver (Templin et al., 1993; Schultz et al., 1999; Yu et al., 2000). Because of the significant binding to plasma, only the free TCA is available to tissues for uptake and elimination (Yu et al., 2000). Plasma protein binding has been found to vary across species and is highest in humans (Lumpkin et al., 2003).

8.3.4 Monobromoacetic acid

No specific studies measuring the distribution of MBA in various tissues following different routes of exposure were undertaken; however, acute oral studies have shown that MBA is absorbed and causes adverse effects (see Section 10.1).

8.3.5 Dibromoacetic acid

DBA (acid) was detected in the plasma of male and female Sprague-Dawley rats following exposure via deionized drinking water in a range-finding reproductive/developmental study (Christian et al., 2001). DBA was not detected in the plasma of female B6C3F1 mice administered DBA in drinking water for 28 days (NTP, 1999b). This may be due to extensive metabolism and excretion and not to limited absorption (U.S. EPA, 2005a). Detectable and quantifiable levels of DBA were also found in the placenta, amniotic fluid and milk (Christian et al., 2001). According to Christian et al. (2001), no apparent accumulation of DBA was observed. DBA was also detected in testicular interstitial fluid when male Sprague-Dawley rats (number not given) were gavaged for 5 days with DBA (neutralized) at 250 mg/kg bw (Holmes et al., 2001). The concentration of DBA peaked at 30 minutes and exhibited a half-life of 1.5 hours.

The lipophilicity of DBA was low when measured at a pH value close to that of blood (pH 7.4), indicating that DBA would not tend to accumulate in fat. DBA also exhibited low binding to plasma when given intravenously (Schultz et al., 1999).

8.4 Excretion

8.4.1 Monochloroacetic acid

Urination is reported as the major route of MCA elimination in rats when dosed orally or dermally with MCA (acid) (Saghir and Rozman, 2003). Approximately 90% of a single oral dose of MCA (acid) given to rats was excreted in the urine within 24 hours (Kaphalia et al., 1992).

8.4.2 Dichloroacetic acid

DCA is mostly eliminated either unchanged or by metabolic transformation primarily in expired air or in the urine. In the urine of rodents, the amount eliminated as unmetabolized DCA or as metabolites varies according to the dose. At low doses, DCA is almost completely eliminated in the urine as metabolites, while a higher percentage of unmetabolized DCA was seen using higher or repeat doses of DCA (Lukas et al., 1980; Lin et al., 1993; Gonzalez-Leon et al., 1997; Cornett et al., 1999), possibly due to the inhibition of its metabolism. In rodents and humans, variable levels of metabolites are found in the urine.

DCA is also eliminated from the lungs as carbon dioxide, but the levels may differ between species. Studies with rats and mice showed that carbon dioxide represented 24-30% and 2-45% of the total dose, respectively (Larson and Bull, 1992; Lin et al., 1993; Xu et al., 1995). Less than 2% of DCA was recovered in the faeces in animal studies (Larson and Bull, 1992; Lin et al., 1993).

DCA is a metabolite of TCE in humans and has been detected in the seminal fluid of some workers exposed to TCE (Forkert et al., 2003).

8.4.3 Trichloroacetic acid

The primary route of excretion for TCA given orally or intravenously is the urine (Templin et al., 1993; Schultz et al., 1999; Yu et al., 2000).

In a limited metabolism study, three volunteers ingested TCA (sodium salt) at a dose of 3 mg/kg bw; the elimination half-life from the blood was 50.6 hours (Muller et al., 1974). In a longitudinal exposure pilot study, Bader et al. (2005) measured the elimination half-life of TCA in the urine of five volunteers. These volunteers were provided tap water (concentrations of TCA ranged from 50 to 180 µg/L) for the first 2 weeks and then TCA-free bottled water for the last 2 weeks. Individual TCA urinary elimination rates ranged from 2.1 to 6.3 days. TCA appears to persist for several days when a steady state is almost reached within the plasma, therefore reflecting average exposure over several days. The authors inferred that TCA in plasma may be a viable biomarker for drinking water exposure.

Since TCA is one of the major metabolites of TCE and PCE in humans (Monster et al., 1979; IARC, 1995; ACGIH, 2001; Forkert et al., 2003), several metabolism studies looked at the half-life of the parent compounds as well as their metabolites, such as TCA, and were included in this review.

After volunteers inhaled 50-100 ppm TCE (6 hours/day for 5-10 days), the elimination half-life of TCA from the blood ranged from 85 to 99 hours, a higher value than that obtained following ingestion of TCA (sodium salt) (Muller et al., 1974).

Allen and Fisher (1993) developed a physiologically based pharmacokinetic model for humans exposed to TCE with emphasis on the metabolite, TCA, and compared it with findings in mice and rats (Allen and Fisher 1993). The volume of TCA distribution in humans was found to be lower than in rats or mice. The model estimated that, in humans, 93% of total TCA eliminated was excreted unchanged in the urine, while the remainder may be metabolized or eliminated by other routes (Allen and Fisher 1993).

Another inhalation study looked at the comparative excretion rates of inhaled PCE and its metabolites in rats and humans (Volkel et al., 1998). The mean elimination half-life of TCA in the urine was 45.6 hours in humans, compared with 11.0 hours in rats, suggesting that elimination of TCA in rats is more rapid and may be due to differences in PCE metabolism (Volkel et al., 1998).

TCA, being one of the major metabolites of TCE and PCE in humans, has been used as a biomarker of occupational exposure to these chemicals (Monster et al., 1979; IARC, 1995; ACGIH, 2001; Forkert et al., 2003). TCA is also a metabolite of 1,1,1-trichloroethane, 1,1,2,2 tetrachloroethane and chloral hydrate (IARC, 1995).

8.4.4 Monobromoacetic acid

No studies on MBA excretion were identified.

8.4.5 Dibromoacetic acid

Only one study on DBA excretion was located. Schultz et al. (1999) exposed rats intravenously to a single dose of 109 mg DBA/kg bw, and the elimination half-life was calculated at 0.72 hour. The major route of elimination was believed to be via biotransformation. The urine and faeces were very minor contributors to overall blood clearance when DBA was given intravenously to rats, with the urine representing less than 1% of total clearance and the amount in the faeces being negligible (Schultz et al., 1999).