Age-dependent kinetics and metabolism of dichloroacetate: possible relevance to toxicity

Abstract

Dichloroacetate (DCA) is an investigational drug for certain metabolic diseases. It is biotransformed principally by the zeta-1 family isoform of glutathione transferase (GSTz1), also known as maleylacetoacetate isomerase (MAAI), which catalyzes the penultimate step in tyrosine catabolism. DCA causes a reversible peripheral neuropathy in several species, including humans. However, recent clinical trials indicate that adults are considerably more susceptible to this adverse effect than children. We evaluated the kinetics and biotransformation of DCA and its effects on tyrosine metabolism in nine patients treated for 6 months with 25 mg/kg/day and in rats treated for 5 days with 50 mg/kg/day. We also measured the activity and expression of hepatic GSTz1/MAAI. Chronic administration of DCA causes a striking age-dependent decrease in its plasma clearance and an increase in its plasma half-life in patients and rats. Urinary excretion of unchanged DCA in rats increases with age, whereas oxalate, an end product of DCA metabolism, shows the opposite trend. Low concentrations of monochloroacetate (MCA), which is known to be neurotoxic, increase as a function of age in the urine of dosed rats. MCA was detectable in plasma only of older animals. Hepatic GSTz1/MAAI-specific activity was inhibited equally by DCA treatment among all age groups, whereas plasma and urinary levels of maleylacetone, a natural substrate for this enzyme, increased with age. We conclude that age is an important variable in the in vivo metabolism and elimination of DCA and that it may account, in part, for the neurotoxicity of this compound in humans and other species.

Figures

Fig. 1

Reactions catalyzed by the glutathione-dependent bifunctional enzyme GSTz/MAAI. GSTz dehalogenates DCA to glyoxylate, and it is the major route of DCA biotransformation. MAAI is the penultimate enzyme in the catabolism of phenylalanine and tyrosine that isomerizes maleylacetoacetate to fumarylacetoacetate and maleylacetone to fumarylacetone.

Fig. 2

Concentration-time profile of [13C]DCA in plasma of samples obtained in five previously drug-naive young children (ages 2.2–7.1 years) after receiving an initial dose of 12.5 mg/kg (♦) and after 6 months of 12.5 mg/kg administered twice daily (▲).

Fig. 3

Concentration-time profile of [12C]DCA in plasma of samples obtained in four previously drug-naive older subjects (ages 14.0–33.9 years) after receiving an initial dose of 12.5 mg/kg (♦) and after 6 months of 12.5 mg/kg administered twice daily (▲).

Fig. 4

Concentration-time profile of [13C]DCA in plasma of a representative young (A) and old (B) rat on day 5, after four daily doses of 50 mg/kg [12C]DCA.

Fig. 5

Kinetics and biotransformation of [12C]DCA in rats as a function of age. Blood was collected on the fifth day of DCA administration at a dose of 50 mg/kg/day. Results are expressed as data from individual animals (n = 5/group) and group means (bars). AUC of [12C]DCA (A), [12C]DCA concentrations (B), and [12C]oxalate concentrations in urine (C). No urinary DCA was detected in two young rats, and no urinary oxalate was detected in one young rat, and this value was set as LOD/√2 for statistical analyses.

Fig. 6

Urinary (A and B) and plasma (C) concentrations of MCA as a function of age. Data are from individual animals (n = 5/group) and group means (bars). No urinary [12C]MCA was detected in three young rats, and no urinary [13C]MCA was detected in one young rat, and these values were set at LOD/√2 for statistical analysis.

Fig. 7

[12C]MCA concentrations (nanomoles per milliliter) in human whole blood incubated with [12C]DCA (millimolar)-fortified whole blood. Data are the mean ± S.D. of the measured MCA concentrations of four separate replicates. Studies were conducted in whole blood incubated for 1 h with 1, 2, or 5 mM DCA and from blood from an adult female Sprague-Dawley rat incubated for 1 h with 2 mM DCA.

Fig. 8

GSTz/MAAI specific activity with DCA as substrate in liver cytosol of control and DCA-treated rats, expressed as nanomoles of glyoxylate formed per minute per milligram of protein. Data are mean ± S.D., with five animals per group. p < 0.04 in old control rats compared with young and adult rats; p < 0.001 between age-appropriate control and DCA-treated groups.

Fig. 9

Plasma (A) and urinary (B) MA concentrations after 5 days of DCA (50 mg/kg/day) as a function of age. Data are values of individual animals (n = 5/group) and group means (bars). There was no detectable urinary MA in one young rat and this value was set at LOD/√2 for statistical analysis.