Thiol exchange: An in vitro assay that predicts the efficacy of novel homocysteine lowering therapies

Elevated plasma total homocysteine (tHcy) is a risk factor for atherosclerosis. Hcy is 70-80% bound to albumin as a disulfide. Recent trials have evaluated ability of thiol-containing drugs to exchange with protein bound Hcy and consequently increase its renal clearance. The objective of this study was to develop an in vitro assay to predict the efficacy of thiol-containing drugs to lower tHcy in the clinical setting. The assay was used to test the effects of N-acetylcysteine (NAC), mesna, captopril, dimercaptosuccinic acid (DMSA), and penicillamine. Hcy was added in vitro to plasma of healthy subjects (n = 6) and equilibrated. Concentrations of thiol exchange agent were added and incubated at 37°C. Aliquots were removed at selected intervals and free Hcy determined. Mesna, captopril, and NAC caused a concentration-dependent increase in free Hcy. Three-hundred micromolar mesna and captopril had a greater effect than equimolar NAC, increasing free Hcy by 33.9 ± 5.0% and 32.0 ± 2.6%, respectively compared to 22.3 ± 2.4% for NAC, p < 0.001. Our in vitro results indicate that mesna, captopril, and NAC effectively exchange with covalently bound Hcy. This assay can act as screening tool for novel tHcy lowering therapies and should spare the expense of negative trials.