E-Mail karger@karger.com In-Depth Topic Review Am J Nephrol 2014;39:348–356 DOI: 10.1159/000360609 Effects of Allopurinol on Endothelial Dysfunction: A Meta-Analysis Mehmet Kanbay a Dimitrie Siriopol b Ionut Nistor b Omer C. Elcioglu a Ozge Telci a Mumtaz Takir a Richard J. Johnson c Adrian Covic b a Department of Medicine, Division of Nephrology, Istanbul Medeniyet University School of Medicine, Istanbul , Turkey; b Department of Nephrology, University of Medicine ‘Grigore T. Popa’, Iasi , Romania; c Division of Renal Diseases and Hypertension, University of Colorado, Denver, Colo. , USA using the Cochran Q test and the χ 2 statistic (with substantial heterogeneity defined as values >50%). Results: The final analysis consisted of 11 studies (2 observational and 9 ran- domized). For the endothelial-dependent vasodilatation there were 6 studies, including 257 patients, that evaluated flow-mediated dilatation and 5 studies with 87 patients that reported data on forearm blood flow response to acetylcho- line or flow-dependent vasodilatation. Overall, there was a significant increase in the endothelium-dependent vasodila- tation with allopurinol treatment (MD 2.69%, 95% CI 2.49, 2.89%, p < 0.001; heterogeneity χ 2 = 319.1, I 2 = 96%, p < 0.001). There was only 1 study (100 patients) assessing ni- trate-mediated dilatation and 4 studies (73 patients) evaluat- ing forearm blood flow response to sodium nitroprusside as measures of endothelial-independent vasodilatation. The overall analysis (MD –0.08, 95% CI –0.50, 0.34, p = 0.70; het- erogeneity χ 2 = 9.0, I 2 = 44%, p = 0.11) showed no effect of allopurinol treatment on endothelium-independent vasodi- latation. Conclusions: We found that treatment of hyperuri- cemia with allopurinol is associated with an improvement in the endothelial-dependent, but not with the endothelial-in- dependent vasodilatation. © 2014 S. Karger AG, Basel Key Words Uric acid · Allopurinol · Endothelial dysfunction · Cardiovascular disease Abstract Objective: Several studies have assessed the effect of allopu- rinol on endothelial function, but these studies were relative- ly small in size and used different methods of evaluating en- dothelial function. We conducted a meta-analysis to investi- gate the effect of allopurinol on both endothelial-dependent and -independent vasodilatation. Methods: Electronic data- bases, Medline, PubMed, EMBASE, SCOPUS, EBSCO and the Cochrane Library Central Register of Clinical Trials were searched from January 1985 to July 2013 on clinical trials (randomized and non-randomized) which assessed the effect of allopurinol on endothelial function. We conducted a sen- sitivity analysis to assess the contribution of each study to the pooled treatment effect by excluding each study one at a time and recalculating the pooled treatment effect for the remaining studies. Treatment effect was significant if p < 0.05. We assessed for heterogeneity in treatment estimates Received: January 20, 2014 Accepted: February 12, 2014 Published online: April 17, 2014 NephrologyAmerican Journal of Mehmet Kanbay, MD Sağlık Bakanlığı Istanbul Medeniyet Universitesi Goztepe Egitim ve Arastirma Hastanesi Nefroloji Kliniği TR–34710 Kadıkoy, Istanbul (Turkey) E-Mail drkanbay   @   yahoo.com © 2014 S. Karger AG, Basel 0250–8095/14/0394–0348$39.50/0 www.karger.com/ajn D ow nloaded from http://karger.com /ajn/article-pdf/39/4/348/2207895/000360609.pdf by Bezm ialem Vakif Ü niversitesi user on 01 O ctober 2025 http://dx.doi.org/10.1159%2F000360609 Allopurinol Improves Endothelial Function Am J Nephrol 2014;39:348–356 DOI: 10.1159/000360609 349 Introduction Uric acid, the circulating end product of purine me- tabolism, is associated with a wide variety of cardiovascu- lar conditions including hypertension [1] , coronary ar- tery disease [2] , cerebrovascular disease [3] , vascular de- mentia [4] , preeclampsia [5] and chronic kidney disease [6] . While it was originally posited that uric acid was el- evated secondary to these conditions, there is now strong evidence that elevated uric acid levels independently pre- dict the development of hypertension [7–9] , kidney dis- ease [10] and diabetes [11, 12] . Endothelial dysfunction (ED) represents a preliminary phase in the atherosclerotic process. It has also been im- plicated in the pathophysiology of chronic heart failure, diabetes, hypertension, coronary heart disease and chron- ic kidney disease [13, 14] . The etiology of ED is complex and involves dysregulation of multiple pathways [14] , and the association between high serum uric acid concentra- tions and ED has been repeatedly shown [15–17] . Experi- mental studies also show that uric acid can directly alter nitric oxide (NO) bioavailability [18–23] and mediate ED in the animal models [18] . Most importantly, allopurinol, a xanthine oxidase inhibitor, has been shown to improve endothelial function in different studies that included a relative low number of individuals with a variety of pa- thologies [24–35] . Therefore, the aim of this meta-analysis was to establish more clearly the benefits of uric acid-low- ering therapy with allopurinol on endothelial function. Materials and Methods Study Protocol We have conducted a systematic review and meta-analysis ac- cording to a previously published protocol (CRD42014006978 at http://www.crd.york.ac.uk/PROSPERO). Search STRATEGY Electronic databases, Medline, PubMed, EMBASE, SCOPUS, EBSCO and the Cochrane Library Central Register of Clinical Trials were searched using the MESH terms ‘allopurinol’, ‘endothelial dys- function’, ‘endothelial function’ with the key words ‘xanthine oxidase inhibitor’, ‘uric acid’, ‘hyperuricemia’, and ‘oxypurinol’. Our search was limited to studies in humans and in peer-reviewed journals from January 1985 to July 2013 without language restriction. In addition, we searched other potentially relevant studies by using a manual search of references from all eligible studies, review articles and Sci- ence Citation Index Expanded on the Web of Science, and searched the top 25 citations for each paper through the ‘related articles’ fea- ture of PubMed. We also reviewed congress proceedings of the American Society of Nephrology, International Society of Nephrol- ogy, European Renal Association, American College of Cardiology, American Heart Association, and European Society of Cardiology. Study Selection Eligible studies included (1) prospective (randomized or non- randomized) or retrospective study designs assessing the effect of allopurinol on endothelial function, (2) parallel or cross-over study design, (3) and documentation of change in ED. There was no re- striction criteria imposed on the type of patients studied. Exclusion criteria included (1) no description of primary or secondary out- comes, (2) absence of adequate and reproducible results, and (3) articles that are not designed as clinical trials (review, etc.) and (4) studies with duplicated data, including same group of patients. Data Extraction, Validity Assessment and Quality Assessment Data extraction was done independently by two authors (D.S. and O.C.E.) using standard data extraction forms. Where more than one publication of one study exists, reports were grouped to- gether and only the publication with the most complete data was included. Any unclear or missing information was requested from the original author by written correspondence and any relevant information obtained in this manner was included in the review. Disagreements were resolved by consultation with all authors. Two reviewers (O.T. and O.C.E.) evaluated the quality of the selected studies independently. The scale used three categories to evaluate: selection, comparability and outcome. The study quality was determined with a Jadad composite score [36] , which is a 5-point quality scale with 1 lowest and 5 highest. A study has been defined a high-quality study if it has a Jadad score ≥ 3. Publication bias was assessed using the funnel plot technique [37] . Outcome Assessment Primary outcome of this analysis was change in endothelial- dependent and -independent vasodilatation with allopurinol treat- ment from the beginning of the treatment to follow-up. Secondary outcomes were change in different subgroups of endothelial func- tion measurements (for endothelial-dependent vasodilatation: flow-mediated dilatation (FMD) and forearm blood flow (FBF) re- sponse to acetylcholine or flow-dependent flow assessment and for endothelial-independent vasodilatation: nitrate-mediated dilata- tion (NMD) and FBF response to sodium nitroprusside). Data ex- tracted included identifying information, focus of the study, details of the study protocol and demographic data. We extracted charac- teristics of each study including baseline and follow-up endothelial function, baseline clinical characteristics of the study population, known diagnosis of hypertension, hyperuricemia, or chronic kid- ney disease, type of study design, and use of agents that might affect endothelial function and total duration of follow-up. Statistical Analysis In this meta-analysis to investigate the impact of allopurinol use on endothelial function, two-tailed variance analysis was performed in the paired samples with known arithmetic means and standard deviations Effect size method and adjusted arithmetic mean differ- ences were used in the studies detected with sampling method. Analyses were performed with the packet program Comprehensive Meta-Analysis (Biostat, Englewood, N.J., USA) and with Review Manager Version 5.2 (The Cochrane Collaboration 2012) [38] . Parallel-group and cross-over design trials were combined in the same meta-analysis based on recommendations by Elbourne et al. [39] . Generic inverse variance based on calculating absolute differ- ences of mean changes in endothelial function (assessed by FMD, NMD, FBF response to acetylcholine or sodium nitroprusside or D ow nloaded from http://karger.com /ajn/article-pdf/39/4/348/2207895/000360609.pdf by Bezm ialem Vakif Ü niversitesi user on 01 O ctober 2025 Kanbay   /Siriopol   /Nistor   /Elcioglu   /Telci   / Takir   /Johnson   /Covic   Am J Nephrol 2014;39:348–356 DOI: 10.1159/000360609 350 flow-dependent flow) between the experimental and control groups and standard errors for each comparison within each study was used. We converted standard deviation and 95% confidence interval (CI) to standard error by using a standard formula [37] . When necessary, we estimated mean and standard deviation from median, range and sample size using the method proposed by Hozo et al. [40] . We con- ducted a sensitivity analysis to assess the contribution of each study to the pooled treatment effect by excluding each study one at a time and recalculating the pooled treatment effect for the remaining stud- ies. Treatment effect was significant if p < 0.05. We assessed for het- erogeneity in treatment estimates using the Cochran Q test and the χ 2 statistic (with substantial heterogeneity defined as values >50%). Results Selection and Description of Studies We included in our final analysis 11 studies (2 obser- vational and 9 randomized) involving 344 patients (min- imum 9 and maximum 100 patients) ( fig. 1 ). The follow- up was between 1 week and 4 months. We analyzed en- dothelium-dependent vasodilatation (6 studies evaluated FMD and 5 studies FBF response to acetylcholine or flow- dependent flow assessment) and endothelium-indepen- Potentially relevant studies after initial search (n = 378) Studies selected for detailed review (n = 74) 11 full-text studies met the inclusion criteria and were included in the analysis Studies excluded based on title and abstract screening (n = 304) 63 studies excluded due to: - Design of the study - Review - Did not report endothelial function outcome - Study with incomplete data Fig. 1. Selection and description of studies. Table 1. Demographic and characteristics of studies included in the meta-analysis Reference (first author) Country Age Gender BMI Study population Jadad scoreallopurinol control allopurinol control all opurinol control M F M F Yelken 2012 [24] Turkey 34 ± 11 – 13 6 27.38 ± 3.77 – non-diabetic CKD patients 0 Meléndez-Ramirez 2012 [27] Mexico 49.1 ± 6.7 – 9 0 pre-treatment 27.3 ± 1.1 – normotensive male patients without DM, CKD and HL 0 Kanbay 2011 [25] Turkey 54.4 ± 8.0 50.4 ± 11.2 16 14 18 19 28.4 ± 2.9 29.7 ± 3.4 subjects with asymptomatic hyperuricemia 1 Dogan 2011 [28] Turkey 50.5 ± 5.0 50.0 ± 6.0 25 25 26 24 25.5 ± 4.5 25.8 ± 5.0 normotensive subjects with DM 1 Yiginer 2008 [29] Turkey 63.1 ± 11.7 59.7 ± 9.7 7 21 7 15 25.0 ± 5.3 26.2 ± 4.2 subjects with metabolic syndrome 2 George 2006 [30] UK 69.7 ± 8 – 25 5 – – 29.7 ± 4.7 – CHF (NYHA class II–III) with documented LV systolic dysfunction 4 El Solh 2006 [31] USA 47.3 ± 8.7 (29 – 60) – 7 5 – – 39.1 ± 13.6 (23 – 67) – moderate-to-severe OSA patients 4 Farquharson 2002 [32] UK 67.5 ± 5.5 – 10 1 – – – – compensated mild-to-moderate CHF 4 Doehner 2002 [33] UK 68 ± 2 69 ± 3 – – – – 26.7 ± 2.1 27.2 ± 1.6 <70 years with CHF caused by dilated CMP or IHD 1 Butler 2000 [34] UK 65 ± 6.7 58 ± 8 10 1 12 0 26.2 ± 3.5 27.4 ± 3 type 2 DM and coexisting mild HT 4 O’Driscoll 1999 [35] Australia 48 ± 2 – 7 2 – – – – hypercholesterolemic volunteers 4 BMI = Body mass index; CHF = congestive heart failure; CKD = chronic kidney disease; CMP = cardiomyopathy; DM = diabetes mellitus; HL = hyper- lipidemia; HT = hypertension; IHD = ischemic heart diesease; LV = left ventricular; OSA = obstructive sleep apnea. D ow nloaded from http://karger.com /ajn/article-pdf/39/4/348/2207895/000360609.pdf by Bezm ialem Vakif Ü niversitesi user on 01 O ctober 2025 Allopurinol Improves Endothelial Function Am J Nephrol 2014;39:348–356 DOI: 10.1159/000360609 351 dent vasodilatation (1 study evaluated NDM and 4 stud- ies FBF response to sodium nitroprusside). Allopurinol was the intervention agent in all trials. The dose of allo- purinol varied between 150 and 900 mg daily. Quality Assessment We included both randomized and observational studies in our analyses, therefore the included studies had of variable quality. There were 6 studies with good qual- ity (Jadad score ≥ 3) with low risk of bias and 5 studies with low quality (Jadad score <3) and high risk of bias. Endothelium-Dependent and -Independent Measurements Baseline characteristics of the included studies are stated in tables 1 and 2 . In all studies the end-diastolic brachial diameter was used for evaluation. The technique used for assessing FDM and NMD was in agreement with previously described methods [41–43] in all studies, with the exception of that of El Solh et al. [31] . Transducers with frequencies between 5 and 12 MHz were used, but in one study this detail was not presented [25, 29, 31] . In the majority of the studies [24, 25, 27, 31] , measurements were made by a single observer. Reproducibility was only reported in two trials [28, 31] . Dogan et al. [28] reported the intra- and inter-observer coefficients of variation for overall FMD and NMD measurements (3.2, 2.8, 4.1 and 3.3%, respectively). El Solh et al. [31] reported the vari- ability for baseline diameter measurements expressed as intra-session variability of 0.6%. Forearm venous occlusion plethysmography was used in all studies to evaluate FBF. Three studies used a Meda- sonics VascuLab SPG-16 plethysmograph [32, 34, 35] , one study used an EC4, Hokanson plethysmograph [33] , while one study did not report what type of device was used [30] . For the assessment of endothelial-dependent vasodilatation, four studies used acetylcholine infusions (maximum dose 100 nmol/min [30, 32, 34] and 40 μg/ Table 2. Baseline characteristics of the studies included in the meta-analysis Reference (first author) Design of study Duration of follow-up Basal uric acid, mg/dl Allopurinol dose, mg Inclusion criteria Exclusion criteria allop urinol control Yelken 2012 [24] prospective cohort 16 weeks 8.6 ± 1.2 – 150 UA >7 mg/dl in men, UA >6 mg/dl in women in CKD ACS, CHF, DM, IHD, TIA, VHD, taking ULM Meléndez-Ramírez 2012 [27] prospective cohort 30 days 7.8 ± 0.36 – 300 normotensive, >18 age <65 years history of smoking, DM, HT, CKD, HL Kanbay 2011 [25] RCT 4 months 8.3 ± 1.1 7.9 ± 0.7 300 asymptomatic hyperuricemia without DM, HT, CHF smoking, history of CAD, and patients on RAS blockers, statins Dogan 2011 [28] RCT 12 weeks 5.0 ± 0.8 4.8 ± 1.1 900 patients with DM HT, smoking, CHF, history of CAD Yiginer 2008 [29] RCT 1 month 6.6 ± 0.6 6.5 ± 0.6 300 metabolic syndrome history of gout, CAD, uncontrolled HT, smoking, CHF George 2006 [30] RCT 1 month 7.12 ± 1.53 – 300 mild-to-moderate CHF serum creatinine >2.03 mg/dl, uncontrolled HT, on allopurinol El Solh 2006 [31] RCT 4 weeks 4.6 ± 0.9 4.6 ± 0.9 moderate-to-severe OSA smoking, history of CHF, HT, DM, gout Farquharson 2002 [32] RCT, cross-over 1 month 4.6 ± 0.9 – 300 mild-to-moderate CHF history of DM, HL, uncontrolled HT Doehner 2002 [33] RCT, cross-over, 1 week + 1 week (cross-over) 9 ± 0.37 9.9 ± 0.62 300 CHF patients with hyperuricemia history of malignancy, heart and liver disease, stroke Butler 2000 [34] RCT, cross-over 1 month – – 300 type I DM with mild HT and without end-organ damage evidence of target organ damage, history of CAD, CKD O’Driscoll 1999 [35] RCT, cross-over 1 month 0.25 ± 0.03 mmol/l 0.35 ± 0.02 mmol/l 300 patients with HL not receiving cholesterol-lowering therapy history of CKD, DM and using RAS blockers ACS = Acute coronary syndrome; CAD = coronary artery disease; CHF = congestive heart failure; CKD = chronic kidney disease; DM = diabetes mel- litus; FDF = flow-dependent flow; HL = hyperlipidemia; HT = hypertension; IHD = ischemic heart disease; OSA = obstructive sleep apnea; RAS = renin angiotensin system; RCT = randomized controlled trial; TIA = transient ischemic attack; UA = uric acid; ULM = urate-lowering medication (allopurinol and probenecid); VHD = valvular heart disease. D ow nloaded from http://karger.com /ajn/article-pdf/39/4/348/2207895/000360609.pdf by Bezm ialem Vakif Ü niversitesi user on 01 O ctober 2025 Kanbay   /Siriopol   /Nistor   /Elcioglu   /Telci   / Takir   /Johnson   /Covic   Am J Nephrol 2014;39:348–356 DOI: 10.1159/000360609 352 min [35] ) and one study flow-dependent flow evaluation [33] . For the determination of endothelial-independent vasodilatation the same four studies used sodium nitro- prusside infusions (maximum dose 37.8 nmol/min [30, 32, 34] and 8 μg/min [35] ). Only one study reported re- producibility data. Butler et al. [34] reported the baseline variability of the data was <10%, when blood flow was analyzed repeatedly in a steady state and a quiet environ- ment. The variability of repeated analysis of the same raw plethysmographic data was <5%. Outcome Measures Reporting Allopurinol Effect on Endothelium-Dependent Vasodilatation All studies assessed the effect of allopurinol on endo- thelium-dependent vasodilatation. Six studies, including 257 patients, evaluated FMD and the remaining 5 studies with 87 patients reported data on FBF response to acetyl- choline or flow-dependent vasodilatation. Overall, there was a significant increase in the endothelium-dependent vasodilatation with allopurinol treatment (MD 2.69%, 95% CI 2.49, 2.89%, p < 0.001; heterogeneity χ 2 = 319.1, I 2 = 96%, p < 0.001) ( fig. 2 ). Analyzed separately, there was also a significant in- crease in both FMD (MD 2.75%, 95% CI 2.49, 3.01%, p < 0.001; heterogeneity χ 2 = 73.6, I 2 = 93%, p < 0.001) and FBF (MD 2.62, 95% CI 2.32, 2.91, p < 0.001; heterogene- ity χ 2 = 245.1, I 2 = 98%, p < 0.001). The results of our analysis were not affected if the 2 non-randomized stud- ies from the FMD subgroup were excluded (MD 2.69, 95% CI 2.43, 2.96, p < 0.001; heterogeneity χ 2 = 68.5, I 2 = 96%, p < 0.001). Similarly, the conclusion was also the Study or subgroup (first author) Allopurinol Control Weight Mean difference IV, fixed [95% CI] Mean difference IV, fixed, 95% CImean SD total mean SD total 1.1.1 Flow-mediated dilatation Dogan, 2011 [28] 9.5 1.2 50 6.1 0.8 50 24.0% 3.40 [3.00, 3.80] El Solh, 2006 [31] 10.4 3.2 6 7.4 2.8 6 0.3% 3.00 [–0.40, 6.40] Kanbay, 2011 [27] 8.12 1.56 30 7.77 0.85 37 9.9% 0.35 [–0.27, 0.97] Meléndez-Ramirez, 2012 [27] 13.28 1.09 9 9.07 2.02 9 1.7% 4.21 [2.71, 5.71] Yelken, 2012 [24] 11.37 9 19 5.42 8.3 19 0.1% 5.95 [0.44, 11.46] Yiginer, 2008 [29] 11.8 0.6 28 8.8 0.9 22 20.1% 3.00 [2.56, 3.44] Subtotal (95% CI) 142 143 56.1% 2.75 [2.49, 3.01] Heterogeneity: χ2 = 73.60, d.f. = 5 (p < 0.00001), I2 = 93% Test for overall effect: Z = 20.60 (p < 0.00001) 1.1.2 Forearm blood flow – response to acetylcholine and flow-dependent flow Butler, 2000 [34] 3.11 1.08 12 3.04 1.23 12 4.5% 0.07 [–0.86, 1.00] Butler, 2000 [34] 3.16 1.21 11 2.54 0.76 11 5.4% 0.62 [–0.22, 1.46] Doehner, 2002 [33] 10.6 2 8 6.7 1 6 1.5% 3.90 [2.30, 5.50] Farquharson, 2002 [32] 7.24 0.64 11 5.88 0.9 11 9.0% 1.36 [0.71, 2.01] George, 2006 [30] 11.19 1.17 10 3.88 0.36 5 6.1% 7.31 [6.52, 8.10] George, 2006 [30] 7.33 0.65 10 3.66 0.36 5 14.6% 3.45 [2.94, 3.96] O’Driscoll, 1999 [35] 6.7 1 9 7.5 1.5 9 2.8% –0.80 [–1.95, 0.38] Subtotal (95% CI) 71 59 43.9% 2.62 [2.32, 2.91] Heterogeneity: χ2 = 245.05, d.f. = 6 (p < 0.00001), I2 = 98% Test for overall effect: Z = 17.35 (p < 0.00001) Total (95% CI) 213 202    100% 2.69 [2.49, 2.89] Heterogeneity: χ2 = 319.06, d.f. = 12 (p < 0.00001), I2 = 96% Test for overall effect: Z = 26.93 (p < 0.00001) Test for subgroup differences: χ2 = 0.41, d.f. = 1 (p = 0.52), I2 = 0% –10 –5 0 5 Favors control Favors allopurinol 10 Fig. 2. Forest plot of endothelial-dependent dilatation. D ow nloaded from http://karger.com /ajn/article-pdf/39/4/348/2207895/000360609.pdf by Bezm ialem Vakif Ü niversitesi user on 01 O ctober 2025 Allopurinol Improves Endothelial Function Am J Nephrol 2014;39:348–356 DOI: 10.1159/000360609 353 same if we excluded from the FBF subgroup the study that evaluated endothelium-dependent vasodilatation using flow-dependent flow [33] (MD 2.57, 95% CI 2.27, 2.87, p < 0.001; heterogeneity χ 2 = 242.5, I 2 = 98%, p < 0.001). Allopurinol Effect on Endothelium-Independent Vasodilatation Only 1 study assessed NMD (100 patients) and 4 stud- ies (73 patients) evaluated FBF response to sodium nitro- prusside. Although allopurinol treatment improved NMD (MD 4.00, 95% CI 2.47, 5.53, p < 0.001), the FBF response to sodium nitroprusside (MD 0.20, 95% CI –0.20, 0.61, p = 0.33; heterogeneity χ 2 = 34.5, I 2 = 83%, p < 0.001) and the overall analysis (MD –0.08, 95% CI –0.50, 0.34, p = 0.70; heterogeneity χ 2 = 9.0, I 2 = 44%, p = 0.11) showed no effect of allopurinol treatment on endo- thelium-independent vasodilatation ( fig. 3 ). Yiginer et al. [29] also evaluated NMD, but they only reported that it did not change from baseline in both groups (allopurinol and placebo). Study or subgroup (first author) Allopurinol Control Weight Mean difference IV, fixed [95% CI] Mean difference IV, fixed, 95% CImean SD total me an SD total 2.1.1 Nitrate-induced dilatation Dogan, 2011 [28] 14 4 50 10 3.8 50 7.0% 4.00 [2.47, 5.53] Subtotal (95% CI) 50 50 7.0% 4.00 [2.47, 5.53] Heterogeneity: not applicable Test for overall effect: Z = 5.13 (p < 0.00001) 2.1.2 Response to sodium nitroprusside Butler, 2000 [34] 3.57 1.42 12 3.86 2.47 12 6.3% –0.29 [–1.90, 1.32] Butler, 2000 [34] 3.49 1.56 11 3.36 0.76 11 15.5% 0.13 [–0.90, 1.16] Farquharson, 2002 [32] 9.37 0.65 11 9.42 1.32 11 21.6% –0.05 [–0.92, 0.82] George, 2006 [30] 10.11 0.66 10 9.72 0.9 5 20.7% 0.39 [–0.50, 1.28] George, 2006 [30] 9.84 0.7 10 9.72 0.9 5 20.2% 0.12 [–0.78, 1.02] O’Driscoll, 1999 [35] 8.5 0.9 9 10.6 1.9 9 8.7% –2.00 [–3.37, –0.63] Subtotal (95% CI) 63 53 93.0% –0.08 [–0.50, 0.34] Heterogeneity: χ2 = 9.00, d.f. = 5 (p = 0.11), I2 = 44% Test for overall effect: Z = 0.39 (p = 0.70) Total (95% CI)      113    103  100% 0.20 [–0.20, 0.61] Heterogeneity: χ2 = 34.47, d.f. = 6 (p < 0.00001), I2 = 83% Test for overall effect: Z = 0.98 (p = 0.33) Test for subgroup differences: χ2 = 25.47, d.f. = 1 (p < 0.00001), I2 = 96.1% Fig. 3. Forest plot of endothelial-independent dilatation. –10–20 0 10 Favors control Favors allopurinol 20 5 –10 4 3 2 1 –5 0 5 10 SE (M D ) MD 0 Fig. 4. Funnel plot of the mean differences in endothelial-depen- dent dilatation versus standard errors of the mean differences of all 11 studies which assessed allopurinol effect on endothelial-de- pendent dilatation. The x-axis is in m/s. D ow nloaded from http://karger.com /ajn/article-pdf/39/4/348/2207895/000360609.pdf by Bezm ialem Vakif Ü niversitesi user on 01 O ctober 2025 Kanbay   /Siriopol   /Nistor   /Elcioglu   /Telci   / Takir   /Johnson   /Covic   Am J Nephrol 2014;39:348–356 DOI: 10.1159/000360609 354 Evaluation of Heterogeneity and Publication Bias The funnel plot ( fig. 4 ) showed an asymmetrical plot in the presence of reporting bias indicating that small studies with negative results have not been published. Homogeneity test for Q statistics was calculated, accord- ing to a fixed-effect model, Q statistics was found to be negative in terms of homogeneity test. According to the fixed-effect model, effect sizes were found to be heteroge- neous. Because the Q statistics for the homogeneity test was significant, the variability of the effect sizes were con- sidered to be higher than expected from a bias caused by a sampling error. Because the homogeneity test originat- ed from sampling error was found to be higher than ex- pected, the model was switched to a random-effect mod- el by calculating the variance of random-effect compo- nents. Discussion Endothelial-dependent vasodilatation is primarily a function of endothelial NO. ED is usually reported as an impaired NO release in response to acetylcholine or FMD, which results in impaired endothelium-dependent vasodilatation. Our primary finding was that allopurinol use is associated with an improvement in the endothelial- dependent, but not endothelial-independent vasodilata- tion. While there was significant heterogeneity between trials included in this meta-analysis, most of it could be explained by differences in the methodological quality of the trials. Allopurinol is a xanthine oxidase inhibitor. The me- tabolism of xanthine by xanthine oxidase generates uric acid and oxidants. Thus, by blocking xanthine oxidase, allopurinol may reduce both uric acid and xanthine oxi- dase-associated oxidants. There is evidence that uric acid per se may inhibit NO bioavailability in endothelial cells. Several mechanisms have been reported, including uric acid-induced oxidative stress with scavenging of NO [20, 44–46] , alteration in L -arginine transport or metabolism [23, 47] , or by direct scavenging of NO [19] . It is also possible that the beneficial effect of allopu- rinol on endothelial function involves the blockade of xanthine oxidase-associated oxidants rather than uric acid per se. In line with this assertion, George et al. [30] reported that high-dose allopurinol, but not probene- cid (a uricosuric), could improve endothelial function in subjects with congestive heart failure. However, al- lopurinol will also reduce intracellular uric acid levels more effectively by blocking intracellular uric acid gen- eration, and the mechanism by which uric acid induces ED is via it intracellular effects [46] . The use of uricase, which degrades uric acid, has also been reported to not improve endothelial function [48] , but uricase treat- ment also generates oxidants when uric acid is degrad- ed. Hence, the exact protective mechanism by which allopurinol protects the endothelial NO levels remains unclear. In our analysis, there was only one study that found a significant increase in endothelial-independent dilata- tion [28] following allopurinol treatment. Although it was the only included study that evaluated NMD, these positive findings could be related to initial higher NMD values in the placebo group and also to a longer period of administration. This meta-analysis has several limitations. Although the authors were contacted to identify the missing data and to clarify areas of concern, we received no respons- es on our queries. Therefore, this meta-analysis includ- ed published data only. This meta-analysis was also lim- ited by the methodological quality of studies included, the large number of studies with small cohorts, non- randomized or cross-over design, variable duration of follow-up and dosage and the lack of multicenter stud- ies. Taken together, the results of this meta-analysis show- ing a beneficial effect of lowering serum uric acid level with allopurinol only on endothelial-dependent dilata- tion imply that this effect is not related to correcting a generally abnormal responsiveness to vasodilator stimuli. Because this review is based on observational studies, and because the number of included studies and the sample size are limited, the application of allopurinol for im- provement of endothelial function in patients with hy- peruricemia is not yet routinely recommended. Never- theless, given that there is abundant evidence of an asso- ciation between hyperuricemia and ED from clinical and animal studies, we recommend adequately powered, high-quality, randomized placebo-controlled trials to de- finitively evaluate the benefits and risks of treatment of hyperuricemia with allopurinol on endothelial function and cardiovascular disease. Acknowledgements Part of this study was funded by the University of Medicine and Pharmacy, Iasi (grants Nos. 1640/01.02.2013, 1641/01.02.2013 and IDEI-PCE 2011, PN-II-ID-PCE-2011-3-0637). D ow nloaded from http://karger.com /ajn/article-pdf/39/4/348/2207895/000360609.pdf by Bezm ialem Vakif Ü niversitesi user on 01 O ctober 2025 Allopurinol Improves Endothelial Function Am J Nephrol 2014;39:348–356 DOI: 10.1159/000360609 355 Disclosure Statement Dr. R.J. Johnson is listed as an inventor on patents for lowering uric acid as a means for reducing blood pressure and insulin resis- tance. The patents have been licensed by XORT Therapeutics, Inc., for which he has received shares. He has also written lay books on uric acid and sugar (The Fat Switch, Mercola.com and the Sugar Fix, Rodale), and he is also on the Scientific Board of Nutrilite. He has received research grants from Danone, Amway, Questcor and the National Institutes of Health. 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