Niacin/Laropiprant and Endothelial Function

Endothelial dysfunction can be regarded as a syndrome which exhibits systemic manifestations and is detectable prior to obvious intimal lesions which are considered to be important factors in the pathogenesis of atherosclerosis and its complications. Endothelial function is determined by the integrated index of all atherogenic and atheroprotective factors present in an individual, including known, as well as yet unknown variables and genetic predispositions. Should the hypothesis of endothelial dysfunction reflecting a vascular phenotype prone to atherogenesis be true, endothelial function could then serve as a marker of an inherent atherosclerotic risk. Dysfunction of either the coronary or peripheral vascular endothelium can be considered an independent predictor of cardiovascular events, offering valuable prognostic information additional to that derived from conventional risk factor assessment. Interventions, such as risk factor modification and treatment with various drugs, including statins and angiotensin-converting enzyme inhibitors, have shown to improve endothelial function.

Endothelial dependant vasomotion has been used as a clinical endpoint for assessment of endothelial function. Testing involves pharmacological and/or physiological stimulation of endothelial release of NO and other vasoactive compounds, and often a comparison of vascular responses to endothelium-independent dilators such as nitroglycerine. Determination of local NO bioavailability not only reflects its influence on vascular tone, but also the other important functions of this molecule, which include thromboregulation, cell adhesion, and proliferation

A non-invasive technique has been developed to evaluate endothelium-dependent, brachial artery FMD. Endothelium of the brachial artery is provoked to release nitric oxide (NO) by the pressure created by inflated sphygmomanometer cuff placed on the forearm distal to the brachial artery and its subsequent release 4-5 minutes later.FMD occurs predominantly as a result of local endothelial release of NO and it can be imaged and quantitated as an index of vasomotor function. The advantages of this high-frequency ultrasonographic imaging of the brachial artery are two-fold: it is non-invasive and it allows repeated measurements.

Large prospective epidemiological studies have demonstrated an inverse correlation of high density lipoprotein cholesterol (HDL-C) and the risk of cardiovascular events. Although the atheroprotective effects of HDL have mainly been attributed to its function in reverse cholesterol transport, lately numerous beneficial effects of HDL such as the improvement of endothelial function, anti-inflammatory, anti-thrombotic, antioxidative effects and the stimulation of endothelial regeneration have been identified. Consequently, therapeutic elevation of HDL become more and more important goal of the treatment of patients with coronary artery disease (CAD). Warnholtz et al recently demonstrated that extended-release (ER) niacin 1000 mg daily significantly improved endothelial function in CAD patients with low HDL-C, but not in normal HDL-C. The biggest obstacle for niacin therapy is flushing which makes many patients non compliant to the prescribed medication.

Tredaptive TM [(ERN/LRPT); Merck \& Co., Inc, Whitehouse Station, NJ, USA)] is a combination tablet containing 1 g of extended release niacin (ERN) and 20 mg of laropiprant, highly selective PGD2-receptor (DP1) antagonist, which offers improved tolerability, supporting a simplified 1-2 g dosing paradigm and improved compliance.

Statins and niacin have been shown to improve endothelial function in CAD patients, however, the question whether niacin improves endothelial function in statin treated patients still remains open. What would be the time needed to show an effect is also not known, but based on the recent publication by Warholtz it seems that 12 weeks should be sufficient.

This study would prospectively test influence of the higher dose of ER-niacin (2000mg) than used in the latest publications (1000 mg) in addition to laropiprant (40mg) in the CHD patients with low HDL-C, the subgroup which in the recent publication by Warhnoltz seem to have benefited from the treatment with niacin.