Probucol. A reappraisal of its pharmacological properties and therapeutic use in hypercholesterolaemia.

Probucol is a lipid-regulating agent structurally dissimilar to other known agents, with a unique pharmacodynamic and clinical profile. It is effective in the treatment of primary Type IIa and IIb hyperlipoproteinaemias, including polygenic (non-familial) hypercholesterolaemia and both heterozygous and homozygous forms of familial hypercholesterolaemia, with reductions in plasma total cholesterol and low density lipoprotein (LDL)-cholesterol levels of about 10 to 20% being attained. Marked effects on cutaneous and tendinous xanthomas have been observed, with significant regression often apparent after 2 or 3 months' therapy. Preliminary trials also indicate efficacy in hyperlipoproteinaemia secondary to nephrotic syndrome and diabetes mellitus. The mechanism of the reduction in LDL-cholesterol levels is yet to be fully elucidated, but it is thought that the decrease results from enhanced catabolism, and there is preliminary evidence of an independent antioxidant effect. In contrast with all other known lipid-lowering agents, probucol also effects a consistent reduction in serum high density lipoprotein (HDL)-cholesterol levels, of around 20 to 30%; the clinical significance of this observation is unclear, although some preliminary investigations suggest a beneficial effect in enhancing reverse cholesterol transport. The influence of probucol treatment on cardiovascular morbidity and mortality remains to be fully investigated; a large trial quantifying the potential effect of probucol against the development of atherosclerotic lesions is currently in progress. Adverse effects of probucol are generally mild, seldom requiring treatment withdrawal, with gastrointestinal effects such as diarrhoea predominating. However, indications of an increased frequency of ventricular arrhythmias and sudden death in association with QT interval prolongation in some animals have prompted some concern. Although there is evidence of a degree of QT prolongation in a number of trials in humans, the nature and clinical significance of this effect requires clarification, as no increased incidence of cardiac arrhythmias is apparent. Thus, probucol appears to be of benefit in primary and secondary hyperlipoproteinaemia of Types IIa and IIb, and particularly in homozygous familial hypercholesterolaemia, with marked effects on xanthomas, and a generally favourable adverse effect profile. There is no evidence to date causally relating occasional QT interval prolongation in patients to any incidence of arrhythmias or sudden death. Pharmacodynamic investigations are likely to clarify further the place of probucol in therapy, particularly with respect to its distinctive lowering of plasma HDL-cholesterol levels.