Clinical pharmacology and toxicology of dichloroacetate.

Dichloroacetate (DCA) is a xenobiotic of interest to both environmental toxicologists and clinicians. The chemical is a product of water chlorination and of the metabolism of various drugs and industrial chemicals. Its accumulation in groundwater and at certain Superfund sites is considered a potential health hazard. However, concern about DCA toxicity is predicated mainly on data obtained in inbred rodent strains administered DCA at doses thousands of times higher than those to which humans are usually exposed. In these animals, chronic administration of DCA induces hepatotoxicity and neoplasia. Ironically, the DCA doses used in animal toxicology experiments are very similar to those used clinically for the chronic or acute treatment of several acquired or hereditary metabolic or cardiovascular diseases. As a medicinal, DCA is generally well tolerated and stimulates the activity of the mitochondrial pyruvate dehydrogenase enzyme complex, resulting in increased oxidation of glucose and lactate and an amelioration of lactic acidosis. By this mechanism, the drug may also enhance cellular energy metabolism. DCA is dehalogenated in vivo to monochloroacetate and glyoxylate, from which it can be further catabolized to glycolate, glycine, oxalate, and carbon dioxide. It remains to be determined whether important differences in its metabolism and toxicology exist in humans between environmentally and clinically relevant doses. ImagesFigure 1Figure 2


Introduction
Dichloroacetate (DCA) is one of a host of organohalides to which humans have long been chronically exposed. Environmental sources of DCA include chlorinated drinking water (1)(2)(3) and groundwater contamination by certain industrial solvents and other chlorinated precursors (4). DCA is also a metabolite in the biotransformation of several pharmaceuticals (5,6) and has been administered orally and parenterally for decades as an investigational drug for the treatment of numerous cardiovascular and metabolic disorders. Collectively, these data indicate that daily human exposure to DCA encompasses a 10,000-fold concentration range, from approximately 4 pg/kg consumed in finished drinking water to approximately 50 mg/kg administered for therapeutic purposes.
A comparative assessment of the kinetics, metabolism, and toxicology of DCA in animals and humans has recently been summarized (7). Here we focus on the clinical pharmacology and safety of this unusual chemical, based in large part on investigations conducted in healthy volunteers From a medicinal standpoint, a principal site of action of DCA is the pyruvate dehydrogenase (PDH) enzyme complex (PDC) located in the mitochondrial inner membrane. PDC catalyzes the reversible decarboxylation of pyruvate to acetyl coenzyme A (CoA), which is the rate-limiting step in the aerobic oxidation of glucose, pyruvate, and lactate in animal cells (Figure 1).
PDC undergoes rapid posttranslational modulation in activity, due in part to reversible phosphorylation. PDC kinase phosphorylates and inactivates PDC, whereas PDC phosphatase dephosphorylates the complex and restores catalytic activity. DCA inhibits the kinase, thereby locking PDC in its unphosphorylated active form. This effect occurs within minutes of oral or parenteral administration of DCA, which is rapidly transported across cell membranes via the monocarboxylate carrier (8) and is concentrated in mitochondria (5).
The ability of mitochondria to oxidize substrates and produce adenosine triphosphate (ATP) by oxidative phosphorylation is integral to normal homeostasis and to the ability of cells to survive in the face of impending energy failure. Experimental and clinical investigations with DCA suggest that the drug primes the tricarboxylic acid (TCA) cycle with acetyl groups and the respiratory chain with electrons donated by the reducing equivalents generated by the PDCand TCA-cycle-catalyzed reactions, thereby promoting ATP synthesis ( Figure 2) (5). The ability of DCA to stimulate the efficient conversion of substrate fuel (glucose or lactate) into energy provides a biochemical rationale for its utility in ameliorating acquired or congenital causes of lactic acidosis and other pathologic conditions associated with mitochondrial energy failure.
Indeed, studies by many investigators have firmly established DCA as the most potent lactate-lowering agent ever used clinically and a potential treatment for myocardial ischemia or failure (9).

Kinetics and Metabolism
Dichloroacetate is rapidly and virtually completely absorbed following oral dosing and about 20% is bound to human plasma proteins (10). In   date, the first dose is cleared from plasma more rapidly than subsequent doses, although the mechanism for this effect is unknown. Also unclear is the quantitative importance of its varied metabolic routes and the influence of repeat administration thereon. Glyoxylate is an intermediate in DCA metabolism, and oxalate and CO2 are terminal end products. Neither glyoxylate nor oxalate stimulate PDC activity (5). However, because the actions of DCA in humans often persist several days after its clearance from plasma (5), it is possible PDH-P, inactive that other reactive intermediates of DCA accumulate intracellularly at active sites and bind covalently to target proteins, or that DCA (or a metabolite) induces enzymes responsible for its pharmacodynamic effects. DCA is extensively metabolized in rodents and humans, with little of the dose excreted unchanged (7). Although most drug metabolism studies have been ereby maintaining prompted by concern about DCA as a ation of glucose. potential environmental pollutant, nearly all experiments to date have administered doses to animals and humans in the therapeutic (i.e., milligram per kilogram) rather than the environmental (microgram per kilogram) range. Thus, it remains unknown whether the kinetics and metabolism of the compound differ at these extremes of human exposure.

Adults
To investigate the in vivo metabolism of DCA in humans, we recently developed and validated a gas chromatography/mass spectrometry technique that simultaneously measures DCA and its metabolites monochloroacetate (MCA), glyoxylate, glycolate, and oxalate in human plasma (11 3-month washout period occurs between studies, as our previous investigations (10,12) demonstrated that an initial DCA dose in healthy subjects increases the plasma t112 of a subsequent dose, even after a washout period of several weeks. These ongoing studies were initiated using the 25 mg/kg dose for two practical reasons: first, to optimize our chances of identifying metabolites of ['3C]DCA in plasma and urine, and second, to generate comparative data with our pediatric investigations early on.  4. In general, the kinetic profile of a 25-mg/kg dose appears to be independent of gender and administration. Peak plasma concentration (Cmax) did not change appreciably between genders or between the first and fifth intravenous doses, but the Cm,, after the final oral dose (158±99 pg/ml) was 68% greater than after the initial dose (94 ± 45 pg/l). We speculate this difference may reflect faster gastrointestinal absorption of DCA upon repeated administration, perhaps by induction of some transport mechanism. Chronic dosing in all subject categories led to a striking increase in tl12, and this was associated with a fall in plasma clearance and an increase in the area under the plasma concentration curve (AUC). Urinary DCA accounts for approximately 1 to 2% of the administered initial dose and approximately 3 to 5% of the fifth dose, based on 24-hr urine collections. and its metabolites ( Figure 3B)     ['3C]DCA kinetics are performed at entry with a 2.5-pg/g dose of compound and 3 to 4 times with 12.5 mg/kg DCA during the initial 24-month period. The dose of 2.5 pg/kg is chosen because it is too small to exert any important pharmacodynamic effect and because it reflects the amount expected to be consumed by environmental exposure. Thus, the study affords the opportunity to examine repeatedly the kinetics and metabolism of DCA during both environmental and dinical dose phases. Table 2 and Figure 4 show representative data on DCA kinetics and metabolism in CLA patients. The patient in Table 2 received 12.5 mg/kg every 12 hr by mouth.
Cmax levels were usually achieved 1 hr after the dose and minimum plasma levels were obtained at the dosing interval. In a second patient, Figure 4 illustrates the expected change in plasma drug clearance. In sharp contrast to our prior studies in adults with acquired causes of lactic acidosis (14,15) or in healthy volunteers, both Table 2 and Figure 4 demonstrate that repeated dosing in children appears to lead to progressive increases in Cmax. Figure 5 illustrates the plasma metabolism of an oral 12.5 mg/kg dose of DCA (1:1 isotope ratio) in a child with CLA. As with healthy adults, MCA is an immediate and short-lived product, whereas glyoxylate and oxalate have more protracted time courses.
In the CLA study, children are administered 50 mg/kg chloral hydrate (CH) by  mouth to induce short-term conscious sedation for various neurologic tests. During the course of our DCA kinetic experiments, we noted the presence of a major interfering substance in some plasma samples. We determined this substance was CH and, in the course of these investigations, discovered that DCA is a major metabolite of CH in humans (6). We also found that CH and DCA may compete for similar routes of biotransformation. This conclusion is based in part on the observation that prior administration of CH delays the plasma clearance of DCA. The discovery that humans catabolize CH to DCA has important implications for the kinetics, dynamics, and toxicology of both compounds. Both are found in microgram per liter quantities in municipal drinking water and are considered by the U.S. Environmental Protection Agency to be environmental hazards. The interesting possibilities are raised that the mental status changes induced by CH and (in some individuals) DCA may have a common mechanism and that CH may alter intermediary metabolism in the host by virtue of its conversion to DCA. Toxicology A valid assessment of the human risk of DCA is confounded by the fact that most animal toxicology has focused on its chronic administration to subprimates, usually at doses nearer to those used clinically than to levels anticipated from environmental exposure of the chemical, its precursors, or its catabolites. The animal toxicology of DCA has recently been reviewed (7). In brief, the liver, kidney, nervous system, testes, and eye are potential target organs of chronic DCA toxicity. Studies in rats and mice have shown that DCA can induce hepatocellular injury, hypertrophy, hyperplasia, adenomas, and carcinomas after chronic oral exposure to levels similar to those administered clinically (7).
To date, evidence of DCA toxicity in humans is limited and is primarily restricted to the nervous system and the liver. Approximately 50% of healthy adults receiving single or repeated oral or intravenous doses of 25 or 50 mg/kg DCA exhibit anxiolytic or sedative effects. These symptoms usually occur within 60 min of drug administration and may last several hours (16,17). Mild drowsiness or diminished anxiety have also been observed in a few patients with diabetes mellitus (18) or lactic acidosis (14) treated briefly with DCA. These central nervous system changes occur unpredictably and without obvious predilection for any particular gender, race, age group, or route of drug administration.
A reversible peripheral neuropathy occurred after several months of daily oral 50 to 100 mg/kg DCA treatment in two children with CIA due to PDC deficiency or a respiratory chain defect (7). Return to pretreatment neurologic states was achieved, both clinically and by nerve conduction velocity testing, within 6 months of discontinuing DCA. In the patient with PDC deficiency, resolution of the neuropathy led to retreatment with DCA at doses of 10 to 25 mg/kg/day, which has been continued for over 2 years without evidence of peripheral neuropathy.
Mild (-2-fold) asymptomatic elevation of serum transaminases has been noted in at least two children with CIA (17)   to the lowest dose in one subject led to normalization of transaminase levels. There is no clinical evidence that DCA is toxic to the eyes, kidneys, or gonads or induces neoplasia in any human tissue.

Summary and Conclusions
Together, the results of these and other (5,7) investigations indicate that the kinetics and biotransformation of DCA are qualitatively similar when the chemical is administered to children or adults at therapeutically relevant doses, as depicted in Figure 6. DCA, and/or an intermediate, strongly inhibits its own metabolism, but the mechanism for this is unknown. Likewise, the quantitative significance of the putative pathways of its metabolism in any species remains to be determined, as do the early molecular events in the dehalogenation of DCA. The chemical appears to be relatively nontoxic when administered acutely and parenterally to healthy adults or to critically ill patients with acquired causes of lactic acidosis, but its long-term safety can be more adequately addressed by chronic treatment of children with CLA.
DCA is unique among chlorinated hydrocarbons in that research on its kinetics, metabolism, and toxicology has been driven by both environmental and therapeutic concerns. The interesting consequence is that much of the animal experimentation has been conducted using doses much closer to those used clinically than to levels to which human populations may be exposed, either by chlorination of municipal drinking water or by groundwater contamination as occurs, for example, at certain Superfund sites. Thus, very little animal and almost no human studies have examined the pharmacology or toxicology of DCA at environmentally relevant concentrations. Therefore, the appropriateness of extrapolating animal toxicology studies to humans in determining the health risk from DCA exposure is problematic thus far. In fact, this caveat may apply to the extrapolation of nonhuman toxicologic data generated for a number of chlorinated hydrocarbons, induding precursors of DCA. In the case of DCA, however, its status as an investigational drug for both pediatric and adult diseases affords an invaluable opportunity to prospectively assess its chronic toxicity in humans.