Such outstanding results have raised suspicions of a possible spectrum bias, which originates on a diagnostic test not being used under clinically significant conditions.Further studies of the relationship between CCSVI and MS have had variable results, with many failing to reproduce the association between MS and CCSVI. Moreover, the greatest predictor of positive results is researchers involvement in the administration of the "liberation procedure". This effect goes to the extent that, when only fully independent studies are considered, no association at all is found. The poor reproducibility across studies and diagnostic modalities has led some authors to conclude that CCVSI might be nothing more than a clinically irrelevant sonographic construct.Already by 2010, there were "a growing number of papers that raise serious questions about its (CCSVI) validity", although evidence had been "both for and against the controversial hypothesis". It was agreed that it was urgent to perform appropriate epidemiological studies to define the possible relationship between CCSVI and MS, although existing data did not support CCSVI as the cause of MS.

Venous malformations
Most of the venous problems in MS patients have been reported to be truncular venous malformations, including azygous stenosis, defective jugular valves and jugular vein aneurysms. Problems with the innominate vein and superior vena cava have also been reported to contribute to CCSVI. A vascular component in MS had been cited previously.Several characteristics of venous diseases make it difficult to include MS in this group. In its current form, CCSVI cannot explain some of the epidemiological findings in MS.
Drug interactions
Febuxostat is contraindicated with concomitant use of theophylline and chemotherapeutic agents, namely azathioprine and 6-mercaptopurine, because it could increase blood plasma concentrations of these drugs and thereby their toxicity. Pharmacology
Mechanism of action
Febuxostat is a non-purine-selective inhibitor of xanthine oxidase. It works by non-competitively blocking the molybdenum pterin center, which is the active site of xanthine oxidase. Xanthine oxidase is needed to oxidize successively hypoxanthine and xanthine to uric acid. Thus, febuxostat inhibits xanthine oxidase, thereby reducing production of uric acid. Febuxostat inhibits both the oxidized and the reduced forms of xanthine oxidase by virtue of its tight binding to the molybdenum pterin site. Pharmacokinetics
After oral intake, at least 84% of the febuxostat dose is absorbed in the gut, and highest blood plasma concentrations are reached after 60 to 90 minutes. When taken together with a fatty meal, febuxostat reaches lower concentrations in the body; but this is not considered clinically relevant. When in the bloodstream, 99.2% of the substance is bound to the plasma protein albumin, and 82–91% of the active metabolites are bound to plasma proteins. Febuxostat has three active metabolites in humans, which are formed mainly by a number of cytochrome P450 liver enzymes (CYP1A1, 1A2, 2C8, 2C9). One of them is a dicarboxylic acid, the other two are hydroxylated derivatives. These, as well as the original drug, are further glucuronidated, mainly by the enzymes UGT1A1, 1A8, and 1A9.