Chronic toxicity of dichloroacetate: possible relation to thiamine deficiency in rats.
The chronic use of dichloroacetate (DCA) for diabetes mellitus or hyperlipoproteinemias has been compromised by neurologic and other forms of toxicity. DCA is metabolized to glyoxylate, which is converted to oxalate and, in the presence of adequate thiamine levels, to other metabolites. DCA stimulates the thiamine-dependent enzymes pyruvate dehydrogenase and alpha-ketoacid dehydrogenase. We postulated that the neurotoxicity from chronic DCA administration could result from depletion of body thiamine stores and abnormal metabolism of oxalate, a known neurotoxin. For 7 weeks, rats were fed ad lib. Purina chow and water or chow plus sodium DCA (50 mg/kg or 1.1 g/kg) in water. A portion of the DCA-treated animals also received intraperitoneal injections of 600 micrograms thiamine three times weekly or 600 micrograms thiamine daily by mouth. Thiamine status was assessed by determining red cell transketolase activity and, in a blinded manner, by recording the development of clinical signs known to be associated with thiamine deficiency. At the 50 mg/kg dose, chronic administration of DCA showed no clinical toxicity or effect on transketolase activity. At the 1.1 g/kg dose, however, DCA markedly increased the frequency and severity of toxicity and decreased transketolase activity 25%, compared to controls. Coadministration of thiamine substantially reduced evidence of thiamine deficiency and normalized transketolase activity. Inhibition of transketolase by DCA in vivo was not due to a direct action on the enzyme, however, since DCA, glyoxylate, or oxalate had no appreciable effects on transketolase activity in vitro. After 7 weeks, plasma DCA concentrations were similar in rats receiving DCA alone or DCA plus thiamine, while urinary oxalate was 86% above control in DCA-treated rats but only 28% above control in DCA plus thiamine-treated animals. No light microscopic changes were seen in peripheral nerve, lens, testis, or kidney morphology in either DCA-treated group, nor was there disruption of normal sperm production in the DCA-treated group. We conclude that stimulation by DCA of thiamine-requiring enzymes may lead to depletion of total body thiamine stores and to both a fall in transketolase activity and an increase in oxalate accumulation in vivo. DCA neurotoxicity may thus be due, at least in part, to thiamine deficiency and may be preventable with thiamine treatment.