Therapeutic applications of dichloroacetate and the role of glutathione transferase zeta-1

Abstract

Dichloroacetate (DCA) has several therapeutic applications based on its pharmacological property of inhibiting pyruvate dehydrogenase kinase. DCA has been used to treat inherited mitochondrial disorders that result in lactic acidosis, as well as pulmonary hypertension and several different solid tumors, the latter through its ability to reverse the Warburg effect in cancer cells and restore aerobic glycolysis. The main clinically limiting toxicity is reversible peripheral neuropathy. Although administration of high doses to rodents can result in liver cancer, there is no evidence that DCA is a human carcinogen. In all studied species, including humans, DCA has the interesting property of inhibiting its own metabolism upon repeat dosing, resulting in alteration of its pharmacokinetics. The first step in DCA metabolism is conversion to glyoxylate catalyzed by glutathione transferase zeta 1 (GSTZ1), for which DCA is a mechanism-based inactivator. The rate of GSTZ1 inactivation by DCA is influenced by age, GSTZ1 haplotype and cellular concentrations of chloride. The effect of DCA on its own metabolism complicates the selection of an effective dose with minimal side effects.

Keywords: Dichloroacetate; GSTZ1; Inhibition of metabolism; Pharmacogenetics; Pharmacokinetics; Pyruvate dehydrogenase kinase.

Figure 1

Role of the pyruvate dehydrogenase complex in intermediary metabolism and site of action of DCA. PDC-PO₄ is the inactive phosphorylated form of pyruvate dehydrogenase complex; PDC is the active unphosphorylated form. PDK is pyruvate dehydrogenase kinase; PDP is pyruvate dehydrogenase phosphatase; e⁻ represents transfer of an electron.

Figure 2

Structures of representative DCA derivatives and co-drugs, with pyruvate and sodium DCA shown for comparison. Bet-CA is an ester of betulinic acid with DCA.

Figure 3

The known metabolites of DCA; the enzymes responsible are shown in italics and the subcellular location is given, where this is known. Abbreviations are: DCA, dichloroacetate; MCA, monochloroacetate; GSTZ1, glutathione transferase zeta 1.

Figure 4

Tyrosine catabolism showing the role of GSTZ1 in isomerization of maleylacetoacetate and maleylacetone. Compounds shown with an asterisk are electrophilic, chemically reactive substances.

Figure 5

Mechanism of DCA dechlorination (adapted from Tzeng et al., 2000). Abbreviations are: GSH, glutathione; GSTZ1, glutathione transferase zeta 1.

Figure 6

Mechanism of nucleophilic attack of a cellular nucleophile (Nuc) to the electrophilic carbon of maleylacetone. The cellular nucleophile could be a free thiol (−SH) group from cysteine present in a protein.

Figure 7

Development of GSTZ1 activity adjusted to account for the relatively larger liver size in children. The box and whiskers plot shows the median with the minimum and maximum values. A single outlier in the birth to 2 months of age group is shown separately. The birth to 2 months group had significantly lower activity than the 2 months to 7 years or the 7 to 18 years age groups, which did not differ from each other. Asterisks (****) indicate significant differences, p<0.0001. Adapted from Li et al. (2011).

Figure 8

Effect of a physiological concentration of chloride on the time course of GSTZ1 inactivation by DCA, 0.5 mM, in human liver cytosol of EGT/EGT (GSTZ1C/1C, black) and EGT/KRT (GSTZ1C/1A, red). Solid lines show the linear regression for each individual in the absence of chloride; dashed lines show the linear regression in the presence of 38 mM chloride. Each data point is the mean of duplicate determinations with individual cytosol samples. The X axis shows the incubation time and the Y axis shows the natural log of activity remaining at the time point shown (A) divided by control activity (A₀, activity at time 0). In the absence of chloride, half-lives of inactivation (t½_inact) values were 0.53 and 0.49 h for EGT/EGT (1C/1C) samples, and were 0.38 h for both EGT/KRT (1C/1A) samples. In the presence of 38 mM chloride, t½_inact values for EGT/EGT (1C/1C) samples were 5.73 and 5.02 h and for EGT/KRT (1C/1A) samples were 2.66 and 2.37 h (Zhong et al., 2014a).

Figure 9

Proposed effect of chloride concentration and GSTZ1 expression on the efficacy of DCA in penetrating the tumor (adapted from Jahn et al., 2016).