CHAPTER-IV an Improved Process for Synthesis of Febuxostat Which Is an Inhibitor of Xanthine Oxidase

CHAPTER-IV An improved process for synthesis of Febuxostat which is an inhibitor of xanthine oxidase. INTRODUCTION : The clinical manifestations of gout, a spectrum of monoarthritic disorders characterized by crystallization of monosodium urate from supersaturated body fluids into tissues, have been well described for centuries. Although historically associated with royalty and affluent societies, increased longevity and shifts in patterns of diet and lifestyle have led to an increasing prevalence of gout worldwide, including in less-developed countries. 1,2 Attacks of acute gouty arthritis are usually treated with NSAIDs, colchicine, or corticosteroids; however, because hyperuricemia is the primary antecedent biochemical abnormality observed in patients with acute gouty arthritis, urate-lowering agents are the foundation for prevention of further attacks. Colchicine was used originally to treat rheumatic complaints, especially gout. It has toxic side effects which include gastrointestinal upset and neutropenia. 3 Colchicine poisoning has been compared to arsenic poisoning; symptoms start 2 to 5 hours after the toxic dose has been ingested and include burning in the mouth and throat, fever, vomiting, diarrhea, abdominal pain and kidney failure. 4 Hyperuricemia in humans is best defined as a serum uric acid of >6.8 mg/dL, which approaches the limit of solubility for monosodium urate in extracellular fluids. 5 Uric acid is the terminal product of a cascade of metabolic steps produced by xanthine oxidase from xanthine and hypoxanthine, which in turn are produced from purine. Uric acid is more toxic to tissues than either xanthine or hypoxanthine. Uric acid is released in hypoxic conditions. 6 In humans and higher primates, uric acid is the final oxidation (breakdown) product of purine metabolism and is excreted in urine. Excess serum accumulation of uric acid can lead to a type of arthritis known as gout. 7 This painful condition is the result of needle-like crystals of uric acid precipitating in joints and capillaries. Various factors, such as age, body weight, diet, temperature, and pH, are known to influence both the concentration and solubility of monosodium urate; however, normal 128 physiologic homeostasis is able to maintain serum uric acid levels below the point of supersaturation and subsequent crystal formation. As the total pool of serum uric acid in the body rises, either because of overproduction or underexcretion, the risk of an acute gout attack increases in a continuous manner. The estimated 5-year cumulative risk of gout is <1% in patients with serum uric acid <7 mg/dL, but >25% of those with urate levels >10 mg/dL will likely experience an attack. 8 In chronic gout, polyarticular involvement may be noted. If left untreated, acute gout attacks generally resolve within 2 weeks. Approximately 80% of patients experiencing their first gout attack will have a recurrence within 2 years. 9 If the underlying hyperuricemia is left untreated, intercritical periods become shorter and attacks become more common. Therefore, reducing the total body pool of urate with lifestyle and pharmacologic interventions is an important step in preventing recurrent attacks. The main reasons for high uric acid level are: • Diet may be a factor in Metabolic Syndrome, fructose and sucrose can cause increased levels of uric acid. • Eating large amounts of sea salt can cause increased levels of uric acid • Serum uric acid can be elevated due to reduced excretion by the kidneys. • Serum uric acid can be elevated due to high intake of dietary purine. • Fe activates xanthine oxidase (XO) and Cu deactivates it, so that as men accumulate Fe with age and Cu levels decline as testosterone levels drop with age (testosterone increases Cu half life), eventually the high Fe/Cu results in more active XO and higher urate levels. A xanthine oxidase inhibitor is any substance that inhibits the activity of xanthine oxidase, an enzyme involved in purine metabolism. In humans, inhibition of xanthine oxidase reduces the production of uric acid, and several medications that inhibit xanthine oxidase are indicated for treatment of hyperuricemia and related medical conditions including gout. Xanthine oxidase inhibitors are being investigated for management of reperfusion injury. 129 Xanthine oxidase inhibitors are of two kinds: purine analogues and others. Purine analogues include allopurinol, oxypurinol,10 and tisopurine. Other group of xanthine oxidase inhibitors include febuxostat 11 and inositols. In some cases, for allopurinol, severe life- threatening side effects have been reported. These include a toxicity syndrome dramatized by eosinophilia, vasculitis, rash hepatitis, and progressive renal failure. Therefore, novel non-purine alternatives to allopurinol with potent XO inhibitory activity, but possessing fewer side effects are in great demand. Under efforts to find novel XO inhibitors without purine backbone, 2-phenylthiazoles and 1-phenylpyrazoles had been designed and tested as xanthine oxidase inhibitors. Among them, febuxostat is shown to be promising xanthine oxidase inhibitor. Febuxostat received marketing approval by the European Medicines Agency on April 21, 2008 12 and was approved by the U.S. Food and Drug Administration on February 16, 2009. 13 OMe OMe O OMe O N H3C H N NH O OMe N N H Cholchicine Allopurinol CH3 CH3 O S O NC N OH CH3 Febuxostat 130 PRESENT WORK Chemically febuxostat is 2-(3-cyano-4-isobutoxyphenyl)-4-methyl-1,3-thiazole-5- carboxylic acid which is an inhibitor of xanthine oxidase that is indicated for the use in treatment of hyperuricemia and gout. It is a non-purine selective inhibitor of xanthine oxidase. It works by non-competitively blocking the channel leading to the active site on xanthine oxidase. Xanthine oxidase is needed to successively oxidate both hypoxanthine and xanthine to uric acid. Febuxostat inhibits xanthine oxidase activity, therefore reducing production of uric acid. CH3 CH3 O S O NC N OH CH3 Febuxostat (1) Process development work was undertaken to make drug available in country. The reported methods for synthesis of febuxostat involve construction of thiazole ring from properly substituted benzene derivative. One of the method to prepare febuxostat ( 1) as per Scheme-114 involves reaction of 4- nitrobezonitrile with KCN in hot DMSO followed by treatment with isobutyl bromide and potassium carbonate to give intermediate 4. Reaction of 4 with thioacetamide in hot DMF gives intermediate 5 which on cyclization using Ethyl-2-chloroacetoacetate followed by alkaline hydrolysis gives febuxostat ( 1). 131 CH 3 CH3 CH 3 CH3 KCN/DMSO, O MeCSNH 2 O2N O Isobutyl bromide/K 2CO 3 DMF 70°C/6.0 h 45°C/ 40 h S CN NC CN NC 4 5 NH2 CH CH3 3 CH CH3 3 Ethyl-2-chloroacetoacetate THF/Ethanol O O Ethanol NaOH O 100°C/ 2 h S 60°C S NC NC COOEt N N OH CH CH3 3 3 Febuxostat (1) Scheme 1 In another synthesis 15 of febuxostat ( 1), 4-hydroxy-3-nitobenzaldehyde is reacted with hydroxylamine and sodium formate in refluxing formic acid to give 4-hydroxy-3- nitobenzbenzonitrile ( 6) which is further treated with thioacetamide in hot DMF to yield corresponding thiobenzamide 7. Cyclization of 7 with ethyl-2-chloroacetoacetate in refluxing ethanol gives intermediate 8, which on o-alkylation with isobutyl bromide in presence of K2CO 3 in hot DMF providing the isobutyl ether 9. The reduction of nitro group of 9 with H 2/Pd-C gives amino derivative 10 , which on diazotization with NaNO 2/HCl followed by treatment with CuCN and KCN gives 3. The alkaline hydrolysis of 3 gives febuxostat ( 1) (Scheme-2) . 132 MeCSNH Ethyl-2-chloroacetoacetate NH OH 2 2 HO - HO Ethanol HO HCOONa DMF HCl Formic acid 80°C/ 1 h S reflux/ 5 h O N CN O2N O2N CHO reflux/ 5.0 h 2 NH 6 7 2 CH3 CH3 CH3 Isobutyl bromide CH HO 3 H2,Pd-C DMF/K CO O 2 3 O EtOH, EtOAc S 70°C/18 h RT/24 h S O2N H N COOEt S 2 COOEt O N N 2 COOEt N N CH CH 3 10 3 8 9 CH3 CH3 CH3 CH CH 3 3 THF/Ethanol NaNO 2, HCl O CuCN, KCN O NaOH 60°C S O S NC NC COOEt N N OH CH3 CH3 Febuxostat (1) 3 Scheme 2 16 In one more synthesis of febuxostat ( 1) (scheme 3), condensation and cyclization of 4-hydroxythiobenzamide with 2-bromoacetoacetic acid ethyl ester in refluxing ethanol provides 11 which is formylated by reaction with hexamethylenetetramine and polyphosphoric acid in hot acetic acid/water to afford 12 . Alkylation of 12 with isobutyl bromide in presence of potassium carbonate and potassium iodide in dimethylformamide gives 13 , which on treatment with formic acid, sodium formate and hydroxylamine hydrochloride yields 3. Finally alkaline hydrolysis of 3 using sodium hydroxide in THF/ethanol gives febuxostat ( 1). 133 Ethyl-2-bromo- OH HO HO acetoacetate HMTA Ethanol S S OHC reflux COOEt PPA/100°C COOEt N N CH 3 CH3 S NH2 4-Hydroxythiobenzamide 11 12 CH3 CH3 CH3 CH3 Isobutyl bromide O O NH OH.HCl DMF/K 2CO 3 2 S S HCOONa/HCOOH NC OHC COOEt COOEt N N CH3 CH3 13 3 CH3 CH3 O NaOH S O THF/Ethanol NC N OH CH3 Febuxostat (1) Scheme 3 The first two methods (scheme 1 & 2) have following drawbacks in comparison to scheme 3. These drawbacks include: • Use of hazardous reagents like KCN, CuCN which are industrially unsafe. • Low yields and use of column chromatography for purification process, which makes it industrially unviable. A key step in the synthesis of febuxostat as per scheme 3 was introduction of formyl group selectively at ortho position to the hydroxyl group in compound 11 . Classical Duff reaction 17 of phenol derivative using HMTA and acetic acid results in ortho formylation.

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