Novel S-adenosylmethionine decarboxylase inhibitors for the treatment of human African trypanosomiasis

Abstract

Trypanosomiasis remains a significant disease across the sub-Saharan African continent, with 50,000 to 70,000 individuals infected. The utility of current therapies is limited by issues of toxicity and the need to administer compounds intravenously. We have begun a program to pursue lead optimization around MDL 73811, an irreversible inhibitor of S-adenosylmethionine decarboxylase (AdoMetDC). This compound is potent but in previous studies cleared rapidly from the blood of rats (T. L. Byers, T. L. Bush, P. P. McCann, and A. J. Bitonti, Biochem. J. 274:527-533). One of the analogs synthesized (Genz-644131) was shown to be highly active against Trypanosoma brucei rhodesiense in vitro (50% inhibitory concentration, 400 pg/ml). Enzyme kinetic studies showed Genz-644131 to be approximately fivefold more potent than MDL 73811 against the T. brucei brucei AdoMetDC-prozyme complex. This compound was stable in vitro in rat and human liver microsomal and hepatocyte assays, was stable in rat whole-blood assays, did not significantly inhibit human cytochrome P450 enzymes, had no measurable efflux in CaCo-2 cells, and was only 41% bound by serum proteins. Pharmacokinetic studies of mice following intraperitoneal dosing showed that the half-life of Genz-644131 was threefold greater than that of MDL 73811 (7.4 h versus 2.5 h). Furthermore, brain penetration of Genz-644131 was 4.3-fold higher than that of MDL 73811. Finally, in vivo efficacy studies of T. b. brucei strain STIB 795-infected mice showed that Genz-644131 significantly extended survival (from 6.75 days for controls to >30 days for treated animals) and cured animals infected with T. b. brucei strain LAB 110 EATRO. Taken together, the data strengthen validation of AdoMetDC as an important parasite target, and these studies have shown that analogs of MDL 73811 can be synthesized with improved potency and brain penetration.

FIG. 1.

Measured and predicted levels of MDL 73811 and Genz-644131 in the blood of mice. (A) Mice were injected i.p. with MDL 73811 either at 50 mg/kg (open circles) or 20 mg/kg (open triangles); samples were collected and analyzed over an 8-h period. Based upon those values, the predicted exposure level was modeled. Filled circles indicated predicted levels following a single injection of 50 mg/kg, plotted from 8 to 24 h. Filled triangles indicated predicted blood levels modeled on thrice daily injections over a 24-h period. The dashed line indicates the calculated IC₅₀ level, corrected for protein binding. (B) Mice were injected once with 50 mg/kg Genz-644131, and blood levels were sampled over 8 h (open circles). Predicted values extending clearance of the 50-mg/kg dose through 24 h are modeled (open triangles). Filled triangles indicate predicted values for a hypothetical dose of 5 mg/kg, based upon results with 50 mg/kg. The dashed line indicates the calculated IC₅₀ level, corrected for protein binding.