Sickle cell disease is an autosomal recessive disorder and the most common genetic disease affecting African-Americans. Approximately 0.15% of African-Americans are homozygous for sickle cell disease, and 8% have sickle cell trait. Hemoglobin S polymerization leads to red cell rigidity, microvascular obstruction, inflammation, and end-organ ischemic injury. Our published data indicate that up to 50% of sickle cell patients have vascular dysfunction due to impaired bioavailability of endogenous nitric oxide, due in large part to scavenging of nitric oxide by cell-free hemoglobin. We recently have completed studies that directly demonstrate endothelial dysfunction in patients with sickle cell disease, characterized by decreased ACh dependent vasorelaxation in forearm blood flow studies, distinct from the nitric oxide resistance above. Further, we have found in sickle cell patients a new association between low levels of apoA-I, pulmonary hypertension and endothelial dysfunction. Raising levels of HDL and therefore apoA-1, could have the effect of ameliorating the endothelial dysfunction characteristic of sickle cell disease by affecting endothelium dependent vasorelaxation. Therapies directed at restoring HDL in these patients may be beneficial. HDL is thought to promote vascular health in a variety of ways, some of which are unrelated to lipid transport. One of the best-known mechanisms relates to efflux of cholesterol from atherosclerotic plaque, yet HDL is thought to have several antithrombotic and anti-inflammatory effects. In vitro HDL attenuates formation of oxidized LDL and inhibits endothelial cell expression of inflammatory cell adhesion molecules. It is also thought to mediate NO production via stimulation of eNOS5, thereby modulating endothelial function. In a study of subjects with atherosclerosis, low HDL levels correlated with impaired vasomotor relaxation via brachial artery FMD. Another study utilizing recombinant HDL cholesterol infused into brachial arteries of hypercholesterolemic men resulted in increased acetylcholine mediated blood flow that was inhibited by the infusion of L-NAME, an eNOS inhibitor, suggesting that HDL increased blood flow via an eNOS dependent mechanism. This may have implications not only for subjects with atherosclerosis, but also for those with sickle cell disease and endothelial dysfunction. We propose that niacin therapy could improve vascular reactivity in response to acetylcholine. Several options for increasing HDL levels have been previously utilized in forearm flow studies using venous occlusion plethysmography or flow-mediated dilation. Reconstituted HDL (rHDL), apoA-1 mimetics and niacin therapy were all shown to improve endothelial dysfunction, and proved safe and effective. This trial will aim to 1) establish the effects of niacin treatment on raising HDL levels in subjects with sickle cell disease, 2) investigate whether niacin treatment would result in improvement of endothelial-dependent relaxation via venous occlusion plethysmography, and 3) compare the efficacy of peripheral arterial tonometry measurements to venous occlusion plethysmography and flow-mediated dilation as indicators of vascular dysfunction. The study was approved on July 24, 2007 and the first volunteer was enrolled on September 17, 2007. To date we have enrolled 30 subjects. Below is the status of the enrolled subjects: 24 subjects have completed the study 1 subject completed the screening process and elected not to participate 2 subject were withdrawn due to Serious Adverse Events 2 subject were found ineligible during the screening process. 1 subject withdrew prior to study completion due to personnel reasons Twenty-four subjects with HbSS sickle cell disease participated in this study. Based on an expected increase in HDL-C of 10.4 mg/dL, enrolling 40 subjects would yield 98% power with alpha = 0.05, with a planned interim analysis. Patients were recruited if baseline apoA-I <99 mg/dL or HDL-C <39 mg/dL. 27 adults with sickle cell anemia were randomized to either placebo or niacin, increased in 500mg steps every 4 weeks as tolerated, to a maximum dose of 1500mg daily. Vascular function was assessed at baseline and after 12 weeks of treatment with FBF, with monthly laboratory monitoring. Our primary outcome was change in HDL-C, and secondary outcomes included changes in apoA-I or FBF. At baseline, subjects were similar on all variables (except for a mild discrepancy in baseline HDL-C: placebo 30.7 4.9 vs. niacin 33.7 7.5 mg/dL, p=0.11). After 12 weeks, the niacin group showed a small non-significant increase in HDL-C from baseline (placebo 0.9 3.8 vs. niacin 5.1 7.7 mg/dL, p=0.07), but no change in apoA-I (placebo 5.3 15.4 vs. niacin 5.5 20.1 mg/dL, p=0.49). Likewise, there was no significant evidence of improved vascular function. Post-hoc subgroup analyses failed to identify consistent predictors of those subjects who would respond to niacin treatment. Patients with sickle cell anemia and low apoA-I/HDL-C appear to be more resistant to niacin effect than that reported for other dyslipidemia subjects. A manuscript has been submitted. Analysis of blood flow parameters with infrared imaging has been published, and the summary below is adapted from the abstract: Vascular dysfunction is an important pathophysiologic manifestation of sickle cell disease (SCD), a condition that increases risk of pulmonary hypertension and stroke. We hypothesized that infrared (IR) imaging would detect changes in cutaneous bloodflow reflective of vascular function. We performed IR imaging and conventional strain gauge plethysmography in twenty-five adults with SCD at baseline and during intra-arterial infusions of an endothelium-dependent vasodilator acetylcholine (ACh), an endothelium-independent vasodilator sodium nitroprusside (SNP), and a NOS inhibitor L-NMMA. Skin temperature measured by IR imaging increased in a dose-dependent manner to graded infusions of ACh (+1.1C, p<0.0001) and SNP (+0.9C, p<0.0001), and correlated with dose-dependent increases in forearm blood flow (ACh: +19.9mL/min/100mL, p<0.0001;r(s)=0.57, p=0.003;SNP: +8.6mL/min/100mL, p<0.0001;r=0.70, p=0.0002). Although IR measurement of skin temperature accurately reflected agonist-induced increases in blood flow, it was less sensitive to decreases in blood flow caused by NOS inhibition. Baseline forearm skin temperature measured by IR imaging correlated significantly with baseline forearm blood flow (31.80.2C, 6.00.4mL/min/100mL;r=0.58, p=0.003), and appeared to represent a novel biomarker of vascular function. It predicted a blunted blood flow response to SNP (r=-0.61, p=0.002), and was independently associated with a marker of pulmonary artery pressure, as well as hemoglobin level, diastolic blood pressure, homocysteine, and cholesterol (R(2)=0.84, p<0.0001 for the model). IR imaging of agonist-stimulated cutaneous blood flow represents a less cumbersome alternative to plethysmography methodology. Measurement of baseline skin temperature by IR imaging may be a useful new marker of vascular risk in adults with SCD.
| Scoffone, Heather M; Krajewski, Megan; Zorca, Suzana et al. (2013) Effect of extended-release niacin on serum lipids and on endothelial function in adults with sickle cell anemia and low high-density lipoprotein cholesterol levels. Am J Cardiol 112:1499-504 |
| Gorbach, Alexander M; Ackerman, Hans C; Liu, Wei-Min et al. (2012) Infrared imaging of nitric oxide-mediated blood flow in human sickle cell disease. Microvasc Res 84:262-9 |
| Kato, Gregory J (2008) Novel small molecule therapeutics for sickle cell disease: nitric oxide, carbon monoxide, nitrite, and apolipoprotein A-I. Hematology Am Soc Hematol Educ Program :186-92 |
