Nicotinic acid (NA) is commonly used to treat dyslipidemia, but it elicits an adverse effect, termed flushing, which consists of cutaneous vasodilation with associated discomfort. An animal model of NA-induced flushing has been established in mice. As in humans, NA stimulated vasodilation in a dose-dependent manner, was associated with an increase of the vasodilatory prostaglandin (PG) D2 in plasma and could be blocked by pretreatment with aspirin. Two PGD2 receptors have been identified: PGD2 receptor 1 (DP1, also called DP) and PGD2 receptor 2 (DP2, sometimes termed CRTH2). DP2 does not mediate NA-induced vasodilation; the DP2-specific agonist DK-PGD2 (13,14-dihydro-15-keto-PGD2) did not induce cutaneous vasodilation, and DP2 ؊/؊ mice had a normal vasodilatory response to NA. By contrast, BW245C, a DP1-selective agonist, induced vasodilation in mice, and MK-0524, a DP1-selective antagonist, blocked both PGD2-and NA-induced vasodilation. NA-induced vasodilation was also studied in DP1 ؉/؉ , DP1 ؉/؊ , and DP1 ؊͞؊ mice; although NA-induced vasodilation depended almost completely on DP1 in female mice, it depended only partially on DP1 in male mice. The residual NA-induced vasodilation in male DP ؊͞؊ mice was aspirin-sensitive. Thus, in the mouse, DP1 appears to be an important component involved in NA-induced vasodilation, but other cyclooxygenase-dependent mechanisms also may be involved. A clinical study in healthy men and women demonstrated that treatment with MK-0524 reduced the symptoms of flushing and the increase in skin perfusion after the administration of NA. These studies suggest that DP1 receptor antagonism may be an effective means to suppress NA-induced flushing in humans.aspirin ͉ prostaglandin D2 receptor 1 antagonist ͉ MK-0524 ͉ niacin ͉ flushing N icotinic acid (NA), sometimes called niacin, is a watersoluble B vitamin that has been used to treat dyslipidemia for almost 50 years (1, 2). Used in high doses, it reduces plasma low-density lipoprotein cholesterol, apolipoprotein B, triglycerides, and lipoprotein(a) and increases high-density lipoprotein cholesterol and apolipoprotein A-I (3, 4). NA has been shown to have cardiovascular benefit when used alone or in combination with statins (hydroxymethylglutaryl-CoA reductase inhibitors) in several clinical trials (5-7).Despite these demonstrated beneficial effects, widespread use of NA for dyslipidemia has been limited by symptoms of flushing, which is associated with cutaneous vasodilation of the face, neck, and torso that occurs in nearly all patients (5, 8). Administration of cyclooxygenase (COX) inhibitors, such as aspirin and indomethacin, before ingestion of NA can attenuate the NA-induced cutaneous reactions in most patients (9-13) without affecting its plasma free fatty acid-lowering effect (14). However, doses of aspirin of 325 mg or higher are generally required to significantly block flushing (15), which compromises the utility of this approach for the chronic suppression of flushing. Although the mechanism of action of NA-induced flushi...