Acetonitrile

Endpoint summary

Description of key information

A number of published studies on the toxicokinetics of acetonitrile are available.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Absorption rate - oral (%):

50

Absorption rate - dermal (%):

50

Absorption rate - inhalation (%):

100

Additional information

Absorption

Quantitative data are not available for absorption of acetonitrile from the gastrointestinal or respiratory tract, although sufficient absorption can occur to induce systemic toxicity as evidenced by animal and human toxicity data.

The dermal absorption of acetonitrile has been studied quantitatively. In a guideline (OECD 428 equivalent) and GLP study conducted by Haskell Laboratories (2005), the in vitro absorption of acetonitrile through human cadaver skin was shown to be low. The permeability coefficient was calculated to be 1.82 x 10^-4cm/h. Short-term penetration rates were calculated to be 375.6 μg equiv/cm²/h (10 minute exposure) and 66.0 μg equiv/cm²/h (60 minute exposure). The majority of an infinite dose of acetonitrile was removed from the skin surface at the end of the 8-hour exposure, and only a small portion penetrated the skin and was recovered in the receptor fluid (0.24%). Following a 10- and 60-minute exposure, a major portion of a finite dose was recovered in the organic volatile trap (~96%), with only a minor portion accounted for in the receptor fluid (≤0.74%). These findings are consistent with reliable animal study results showing low acute dermal toxicity of acetonitrile (MPI Research, 1998).

Distribution

Given its high water solubility, absorbed acetonitrile would be expected to have a high volume of distribution. Ahmed et al (1992) reported that radioactivity was widely distributed throughout the body following intravenous injection of 14C-acetonitrile in mice. Free and conjugated hydrogen cyanide has been detected in a wide range of tissues following administration of acetonitrile (Haguenoer et al, 1975).

There is no evidence that repeated exposure to acetonitrile results in its accumulation in animal tissues.

Metabolism

The toxic effects of acetonitrile have been ascribed to the metabolic formation of cyanide. The symptoms produced during acute acetonitrile intoxication are similar to those of cyanide poisoning, except the onset is delayed. A number of human case reports of accidental or intentional ingestion of acetonitrile have reported delays in the onset of severe cyanide poisoning of 11 – 16 hours following intake of acetonitrile (Turchen et al, 1991; Kurt et al, 1991; Geller et al, 1991; Mueller et al, 1996). These cases responded well to cyanide antidotal therapy. Studies in mice showed that the cyanide antidote sodium thiosulphate protected against acetonitrile toxicity (Willhite & Smith, 1981). Similar results were obtained with hamsters (Willhite, 1983). Pretreatment of mice with carbon tetrachloride, a suicide inactivator of cytochromes P450, decreased tissue cyanide concentrations and mortality due to acetonitrile administration (Willhite & Smith, 1981; Willhite, 1981; Tanii & Hashimoto, 1984).  The role of cytochromes P450 and other enzymes in acetonitrile bioactivation has been elucidated throughin vivoandin vitrostudies with enzyme inducers and inhibitors. Treatment of rats with acetone along with acetonitrile potentiated acetonitrile toxicity (Pozzani et al., 1959; Smyth et al., 1969; Freeman & Hayes, 1985). The onset of toxicity and appearance of blood cyanide were delayed by acetone co-administration. Peak cyanide concentrations were significantly greater in rats dosed with both solvents than in rats given only acetonitrile (Freeman & Hayes, 1985). These data indicate that acetone produced an initial inhibition of acetonitrile metabolism to cyanide followed by a stimulation of metabolism upon acetone elimination. In vitro studies further characterized acetonitrile metabolism (Freeman & Hayes, 1988). Liver microsomes from female Sprague-Dawley rats catalysed the NADPH- and oxygen-dependent metabolism of acetonitrile to cyanide. The reaction was inhibited by carbon monoxide, metyrapone and SKF 525-A, consistent with cytochromes P450 as the catalyst. Pre-treatment of rats with acetone increased the Vmax for cyanide production from acetonitrile by approximately 5-fold, indicating induction of cytochrome P450. Further studies demonstrated competitive inhibition by added acetone (KIof 0.41 mM), dimethyl sulphoxide (KI of 0.51 mM), or ethanol (KI of 0.11 mM) (Freeman & Hayes, 1988). These data are consistent with competitive inhibition of acetonitrile bioactivation by co-administered acetone and induction of cytochromes P450 by acetone leading to a higher capacity for acetonitrile bioactivation.  A similar situation has been described for the effect of ethanol on acetonitrile bioactivation in mice. Treatment of mice with ethanol increased the (in vitro) liver microsomal metabolism of acetonitrile to cyanide. Addition of ethanol to the microsomal incubation resulted in inhibition of cyanide formation (Tanii & Hashimoto, 1984). These results are typical for many cytochrome P450 substrates that are also inducing agents; it is not known whether acetonitrile can induce its own metabolism.

The metabolic picture that emerged from the previous studies involves the monooxygenation of acetonitrile to form a cyanohydrin, with subsequent release of cyanide and an aldehyde (Ohkawa et al., 1972). However, formaldehyde was not a metabolite of acetonitrile or its cyanohydrin glycolonitrile (Freeman & Hayes, 1987b). Therefore other potential mechanisms of cyanide formation have been investigated. One potential mechanism involves nucleophilic attack of glutathione, either on acetonitrile or glycolonitrile, resulting in the SN2 displacement of cyanide (Freeman & Hayes, 1987a). Freeman & Hayes (1987a) investigated the metabolism of acetonitrile in freshly isolated rat hepatocytes. Depletion of glutathione content by >80% by buthionine sulfoximine treatment of the rats did not affect acetonitrile metabolism to cyanide. Cyanide formation correlated with cytochrome P450 content. Depletion of cytochrome P450 by pretreatment of the rats with cobalt heme decreased cyanide production while pretreatment of the rats with acetone to induce cytochrome P450 2E1 increased cyanide production 2-fold (Freeman & Hayes, 1987a). Moreover, addition of glutathione to microsomal incubations with acetonitrile had no effect on cyanide formation and a glutathione S-transferase preparation did not catalyze cyanide release from acetonitrile (Freeman & Hayes, 1987a). These data indicate that glutathione and glutathione S-transferase do not play a significant role in the bioactivation of acetonitrile.

Further studies demonstrated a key role for cytochrome P450 2E1 in the metabolism of acetonitrile to cyanide. Treatment of rats with cytochrome P450 2E1 inducers such as pyrazole, 4-methylpyrazole, and ethanol increased cyanide production from acetonitrile 4- to 5-fold by isolated microsomes (Freierman & Cederbaum, 1989). Carbon monoxide inhibited cyanide production as did added substrates for cytochrome P450 2E1. Antibodies to cytochrome P450 2E1 strongly inhibited the microsomal oxidation of acetonitrile to cyanide, indicating that cytochrome P450 2E1 was the major catalyst (Freierman & Cederbaum, 1989). Hydroxyl radical scavengers or the iron chelator desferrioxamine had no effect on microsomal cyanide formation from acetonitrile, indicating no role for hydroxyl radicals in the reaction (Freierman & Cederbaum, 1989). Azide, an inhibitor of catalase, inhibited production of cyanide from acetonitrile and added catalase overcame the inhibition. Formate, a substrate for the peroxidatic activity of catalase, also inhibited microsomal cyanide formation. These results implicate a role for microsomal catalase in the production of cyanide from acetonitrile (Freierman & Cederbaum 1989). However, catalase-H2O2 did not oxidize acetonitrile to cyanide under conditions where other peroxidatic substrates were oxidized. These results suggest the coupled activity of cytochrome P450 2E1 and catalase in the microsomal oxidation of acetaldehyde to cyanide. A reconstituted system containing purified cytochrome P450 2E1, NADPH-cytochrome P450 reductase, and catalase efficiently oxidized acetonitrile to cyanide (Freierman & Cederbaum, 1989). Cytochrome P450 2E1 was absolutely required for the reaction. Omission of catalase decreased cyanide formation by 82%. The reaction was inhibited by antibodies to cytochrome P450 2E1, ethanol, and 4-methylpyrazole (Freierman & Cederbaum, 1989). These data provide strong evidence for a duel role of cytochrome P450 2E1 and catalase in the microsomal metabolism of acetonitrile to cyanide. However, the nature of the reaction products other than cyanide is not known. It is not known whether the cyanohydrin of acetonitrile, glycolonitrile, is a substrate for catalase oxidation. The microsomal oxidation of glycolonitrile to cyanide required a NADPH-generating system, was inhibited by cytochrome P450 inhibitors and cobalt heme pretreatment, and induced by acetone pretreatment (Freeman & Hayes, 1987b), consistent with catalysis by cytochrome P450 2E1.

Microsomes from rat nasal ethmoturbinates contain cytochrome P450 enzymes that catalyzed the formation of cyanide from acetonitrile at rates an order of magnitude greater than rat liver microsomes on a mg microsomal protein basis (Dahl & Waruszewski, 1989). Rat nasal tissues also contain a high amount of rhodanese, which detoxifies cyanide by transferring a sulfur atom from thiosulfate to cyanide to form thiocyanate (Dahl, 1989). It is not known if the high enzyme activities observed in rat nasal tissues also occur in human nasal tissues.

Kinetic parameters

Freeman & Hayes (1988) reported that the oxygen-dependent metabolism of acetonitrile to cyanide by rat liver microsomes followed Michaelis-Menten saturation kinetics with a Vmax of 395 pmol cyanide/min/mg microsomal protein and a KM of 21.3 mM acetonitrile. Feierman & Cederbaum, 1989 reported similar Km and Vmax values in non-induced rat liver microsomes of 28 ± 7 mM and 0.33 ± 0.06 nMol cyanide produced per min/ mg protein, respectively. Tanii & Hashimoto (1984) reported Km and Vmax values of 4.19 mM and 14.3 ng CN formed / 15 min / mg of protein in mouse hepatic microsomes incubated in contact with acetonitrile.

Excretion

Michaelis et al (1991) reported blood acetonitrile and cyanide levels over a 120-hour period in the case of intentional ingestion by a 30 year old man. The maximum cyanide blood level was 17.3 µg/mL (0.66 mmol/L) after an estimated acetonitrile dose of 5 mL, which is equivalent to 65 mg/kg. The calculated elimination half-lives were 32 h for acetonitrile and 15 h for cyanide.

Mueller et al (1996) reported the case of intentional ingestion of acetonitrile by a 39 year old woman. Harmful levels of blood cyanide persisted for over 24 hours following ingestion, and the half-life of acetonitrile in blood was calculated to be 36 hours.

Ahmed et al (1992) reported that pharmacokinetics analysis of acetonitrile in tissues indicated that the distribution and excretion kinetics of the acetonitrile parent molecule in various tissues follows one compartment, first order kinetics. The half-life of elimination of 14C-acetonitrile from blood and most tissues ranged from 5.52 hr in the liver to 8.45 hr in the blood. Haguenoer et al (1975) reported that pulmonary clearance of unchanged acetonitrile is also an important pathway of elimination, especially at high exposure levels.

Eschbach (1991) reported recoveries and primary routes of excretion following a single oral administration of radiolabelled acetonitrile to rats. Acetonitrile is slowly absorbed from the stomach and eliminated through the lungs. In the first 24 hours following oral administration of 14C-labelled acetonitrile, the majority of the radioactivity was eliminated by exhalation or urinary excretion. The total 14C-recovery in urine at 24 hours ranged from 2.45-3.76% and most probably represents a parent metabolite, thiocyanate. Based on the data obtained, it appears that 71.2% of the dose is exhaled as either parent material, CO2 or free cyanide. As radioactivity was recovered at later intervals, metabolism of acetonitrile to CO2 and free cyanide is indicated. The majority of 14C-activity is eliminated by 24 hours either in expired air or via urinary excretion. The majority 14C-activity in the urine is associated with thiocyanate.