ALLOPURINOL

Allopurinol is known chemically as 1,5-Dihydro-4 H -pyrazolo[3,4- d ]pyrimidin-4-one. It is a xanthine oxidase inhibitor which is administered orally. Its solubility in water at 37°C is 80 mg/dL and is greater in an alkaline solution. The structural formula is represented below: C 5 H 4 N 4 O M.W. 136.11 Allopurinol Tablets USP, 100 mg and 300 mg contain the following inactive ingredients: croscarmellose sodium, lactose monohydrate, magnesium stearate, microcrystalline cellulose, pregelatinized starch and sodium lauryl sulfate. Allopurinol Tablets USP, 300 mg also contain FD&C Yellow No. 6. Allopurinol structural formula

THIS IS NOT AN INNOCUOUS DRUG. IT IS NOT RECOMMENDED FOR THE TREATMENT OF ASYMPTOMATIC HYPERURICEMIA. Allopurinol reduces serum and urinary uric acid concentrations. Its use should be individualized for each patient and requires an understanding of its mode of action and pharmacokinetics (see CLINICAL PHARMACOLOGY , CONTRAINDICATIONS , WARNINGS and PRECAUTIONS ). Allopurinol is indicated in: the management of patients with signs and symptoms of primary or secondary gout (acute attacks, tophi, joint destruction, uric acid lithiasis, and/or nephropathy). the management of patients with leukemia, lymphoma and malignancies who are receiving cancer therapy which causes elevations of serum and urinary uric acid levels. Allopurinol treatment should be discontinued when the potential for overproduction of uric acid is no longer present. the management of patients with recurrent calcium oxalate calculi whose daily uric acid excretion exceeds 800 mg/day in male patients and 750 mg/day in female patients. Therapy in such patients should be carefully assessed initially and reassessed periodically to determine in each case that treatment is beneficial and that the benefits outweigh the risks.

The dosage of allopurinol to accomplish full control of gout and to lower serum uric acid to normal or near-normal levels varies with the severity of the disease. The average is 200 to 300 mg per day for patients with mild gout and 400 to 600 mg per day for those with moderately severe tophaceous gout. The appropriate dosage may be administered in divided doses or as a single equivalent dose with the 300 mg tablet. Dosage requirements in excess of 300 mg should be administered in divided doses. The minimal effective dosage is 100 to 200 mg daily and the maximal recommended dosage is 800 mg daily. To reduce the possibility of flare-up of acute gouty attacks, it is recommended that the patient start with a low dose of allopurinol (100 mg daily) and increase at weekly intervals by 100 mg until a serum uric acid level of 6 mg/dL or less is attained but without exceeding the maximal recommended dosage. Normal serum urate levels are usually achieved in one to three weeks. The upper limit of normal is about 7 mg/dL for men and postmenopausal women and 6 mg/dL for premenopausal women. Too much reliance should not be placed on a single serum uric acid determination since, for technical reasons, estimation of uric acid may be difficult. By selecting the appropriate dosage and, in certain patients, using uricosuric agents concurrently, it is possible to reduce serum uric acid to normal or, if desired, to as low as 2 to 3 mg/dL and keep it there indefinitely. While adjusting the dosage of allopurinol in patients who are being treated with colchicine and/or anti-inflammatory agents, it is wise to continue the latter therapy until serum uric acid has been normalized and there has been freedom from acute gouty attacks for several months. In transferring a patient from a uricosuric agent to allopurinol, the dose of the uricosuric agent should be gradually reduced over a period of several weeks and the dose of allopurinol gradually increased to the required dose needed to maintain a normal serum uric acid level. It should also be noted that allopurinol is generally better tolerated if taken following meals. A fluid intake sufficient to yield a daily urinary output of at least 2 liters and the maintenance of a neutral or, preferably, slightly alkaline urine are desirable. Since allopurinol and its metabolites are primarily eliminated only by the kidney, accumulation of the drug can occur in renal failure, and the dose of allopurinol should consequently be reduced. With a creatinine clearance of 10 to 20 mL/min, a daily dosage of 200 mg of allopurinol is suitable. When the creatinine clearance is less than 10 mL/min the daily dosage should not exceed 100 mg. With extreme renal impairment (creatinine clearance less than 3 mL/min) the interval between doses may also need to be lengthened. The correct size and frequency of dosage for maintaining the serum uric acid just within the normal range is best determined by using the serum uric acid level as an index. For the prevention of uric acid nephropathy during the vigorous therapy of neoplastic disease, treatment with 600 to 800 mg daily for two or three days is advisable together with a high fluid intake. Otherwise similar considerations to the above recommendations for treating patients with gout govern the regulation of dosage for maintenance purposes in secondary hyperuricemia. The dose of allopurinol recommended for management of recurrent calcium oxalate stones in hyperuricosuric patients is 200 to 300 mg/day in divided doses or as the single equivalent. This dose may be adjusted up or down depending upon the resultant control of the hyperuricosuria based upon subsequent 24 hour urinary urate determinations. Clinical experience suggests that patients with recurrent calcium oxalate stones may also benefit from dietary changes such as the reduction of animal protein, sodium, refined sugars, oxalate-rich foods, and excessive calcium intake as well as an increase in oral fluids and dietary fiber. Children, 6 to 10 years of age, with secondary hyperuricemia associated with malignancies may be given 300 mg allopurinol daily while those under 6 years are generally given 150 mg daily. The response is evaluated after approximately 48 hours of therapy and a dosage adjustment is made if necessary.

Patients who have developed a severe reaction to allopurinol should not be restarted on the drug.

Data upon which the following estimates of incidence of adverse reactions are made are derived from experiences reported in the literature, unpublished clinical trials and voluntary reports since marketing of allopurinol began. Past experience suggested that the most frequent event following the initiation of allopurinol treatment was an increase in acute attacks of gout (average 6% in early studies). An analysis of current usage suggests that the incidence of acute gouty attacks has diminished to less than 1%. The explanation for this decrease has not been determined but may be due in part to initiating therapy more gradually (see PRECAUTIONS and DOSAGE AND ADMINISTRATION ). The most frequent adverse reaction to allopurinol is skin rash. Skin reactions can be severe and sometimes fatal. Therefore, treatment with allopurinol should be discontinued immediately if a rash develops (see WARNINGS ). Some patients with the most severe reaction also had fever, chills, arthralgias, cholestatic jaundice, eosinophilia and mild leukocytosis or leukopenia. Among 55 patients with gout treated with allopurinol for 3 to 34 months (average greater than 1 year) and followed prospectively, Rundles observed that 3% of patients developed a type of drug reaction which was predominantly a pruritic maculopapular skin eruption, sometimes scaly or exfoliative. However, with current usage, skin reactions have been observed less frequently than 1%. The explanation for this decrease is not obvious. The incidence of skin rash may be increased in the presence of renal insufficiency. The frequency of skin rash among patients receiving ampicillin or amoxicillin concurrently with allopurinol has been reported to be increased (see PRECAUTIONS ). Most Common Reactions* Probably Causally Related Gastrointestinal: diarrhea, nausea, alkaline phosphatase increase, SGOT/ SGPT increase Metabolic and Nutritional: acute attacks of gout Skin and Appendages: rash, maculopapular rash *Early clinical studies and incidence rates from early clinical experience with allopurinol suggested that these adverse reactions were found to occur at a rate of greater than 1%. The most frequent event observed was acute attacks of gout following the initiation of therapy. Analyses of current usage suggest that the incidence of these adverse reactions is now less than 1%. The explanation for this decrease has not been determined, but it may be due to following recommended usage (see ADVERSE REACTIONS introduction, INDICATIONS AND USAGE , PRECAUTIONS and DOSAGE AND ADMINISTRATION ). Incidence Less Than 1% Probably Causally Related Body as a whole: ecchymosis, fever, headache Cardiovascular: necrotizing angiitis, vasculitis Gastrointestinal: hepatic necrosis, granulomatous hepatitis, hepatomegaly, hyperbilirubinemia, cholestatic jaundice, vomiting, intermittent abdominal pain, gastritis, dyspepsia Hemic and Lymphatic: thrombocytopenia, eosinophilia, leukocytosis, leukopenia Musculoskeletal: myopathy, arthralgias Nervous: peripheral neuropathy, neuritis, paresthesia, somnolence Respiratory: epistaxis Skin and Appendages: erythema multiforme exudativum (Stevens-Johnson syndrome), toxic epidermal necrolysis (Lyell’s syndrome), hypersensitivity vasculitis, purpura, vesicular bullous dermatitis, exfoliative dermatitis, eczematoid dermatitis, pruritus, urticaria, alopecia, onycholysis, lichen planus Special Senses: taste loss/perversion Urogenital: renal failure, uremia (see PRECAUTIONS ) Incidence Less Than 1% Causal Relationship Unknown Body as a whole: malaise Cardiovascular: pericarditis, peripheral vascular disease, thrombophlebitis, bradycardia, vasodilation Endocrine: infertility (male), hypercalcemia, gynecomastia (male) Gastrointestinal: hemorrhagic pancreatitis, gastrointestinal bleeding, stomatitis, salivary gland swelling, hyperlipidemia, tongue edema, anorexia Hemic and Lymphatic: aplastic anemia, agranulocytosis, eosinophilic fibrohistiocytic lesion of bone marrow, pancytopenia, prothrombin decrease, anemia, hemolytic anemia, reticulocytosis, lymphadenopathy, lymphocytosis Musculoskeletal: myalgia Nervous: optic neuritis, confusion, dizziness, vertigo, foot drop, decrease in libido, depression, amnesia, tinnitus, asthenia, insomnia Respiratory: bronchospasm, asthma, pharyngitis, rhinitis Skin and appendages: furunculosis, facial edema, sweating, skin edema Special Senses: cataracts, macular retinitis, iritis, conjunctivitis, amblyopia Urogenital: nephritis, impotence, primary hematuria, albuminuria

In patients receiving mercaptopurine or azathioprine, the concomitant administration of 300-600 mg of allopurinol per day will require a reduction in dose to approximately one-third to one-fourth of the usual dose of mercaptopurine or azathioprine. Subsequent adjustment of doses of mercaptopurine or azathioprine should be made on the basis of therapeutic response and the appearance of toxic effects (see CLINICAL PHARMACOLOGY ). It has been reported that allopurinol prolongs the half-life of the anticoagulant, dicumarol. The clinical basis of this drug interaction has not been established but should be noted when allopurinol is given to patients already on dicumarol therapy. Since the excretion of oxipurinol is similar to that of urate, uricosuric agents, which increase the excretion of urate, are also likely to increase the excretion of oxipurinol and thus lower the degree of inhibition of xanthine oxidase. The concomitant administration of uricosuric agents and allopurinol has been associated with a decrease in the excretion of oxypurines (hypoxanthine and xanthine) and an increase in urinary uric acid excretion compared with that observed with allopurinol alone. Although clinical evidence to date has not demonstrated renal precipitation of oxypurines in patients either on allopurinol alone or in combination with uricosuric agents, the possibility should be kept in mind. The reports that the concomitant use of allopurinol and thiazide diuretics may contribute to the enhancement of allopurinol toxicity in some patients have been reviewed in an attempt to establish a cause-and-effect relationship and a mechanism of causation. Review of these case reports indicates that the patients were mainly receiving thiazide diuretics for hypertension and that tests to rule out decreased renal function secondary to hypertensive nephropathy were not often performed. In those patients in whom renal insufficiency was documented, however, the recommendation to lower the dose of allopurinol was not followed. Although a causal mechanism and a cause-and-effect relationship have not been established, current evidence suggests that renal function should be monitored in patients on thiazide diuretics and allopurinol even in the absence of renal failure, and dosage levels should be even more conservatively adjusted in those patients on such combined therapy if diminished renal function is detected. An increase in the frequency of skin rash has been reported among patients receiving ampicillin or amoxicillin concurrently with allopurinol compared to patients who are not receiving both drugs. The cause of the reported association has not been established. Enhanced bone marrow suppression by cyclophosphamide and other cytotoxic agents has been reported among patients with neoplastic disease, except leukemia, in the presence of allopurinol. However, in a well-controlled study of patients with lymphoma on combination therapy, allopurinol did not increase the marrow toxicity of patients treated with cyclophosphamide, doxorubicin, bleomycin, procarbazine and/or mechlorethamine. Tolbutamide’s conversion to inactive metabolites has been shown to be catalyzed by xanthine oxidase from rat liver. The clinical significance, if any, of this observation is unknown. Chlorpropamide’s plasma half-life may be prolonged by allopurinol, since allopurinol and chlorpropamide may compete for excretion in the renal tubule. The risk of hypoglycemia secondary to this mechanism may be increased if allopurinol and chlorpropamide are given concomitantly in the presence of renal insufficiency. Rare reports indicate that cyclosporine levels may be increased during concomitant treatment with allopurinol. Monitoring of cyclosporine levels and possible adjustment of cyclosporine dosage should be considered when these drugs are co-administered.