Gout is a metabolic disorder which is characterized by hyperuricemia and resultant deposition of monosodium urate in the tissues, particularly the joints and kidneys. The etiology of this hyperuricemia is the overproduction of uric acid in relation to the patient's ability to excrete it. If progressive deposition of urates is to be arrested or reversed, it is necessary to reduce the serum uric acid level below the saturation point to suppress urate precipitation.

Administration of ZYLOPRIM generally results in a fall in both serum and urinary uric acid within 2 to 3 days. The degree of this decrease can be manipulated almost at will since it is dose-dependent. A week or more of treatment with ZYLOPRIM may be required before its full effects are manifested; likewise, uric acid may return to pretreatment levels slowly (usually after a period of 7 to 10 days following cessation of therapy). This reflects primarily the accumulation and slow clearance of oxipurinol. In some patients a dramatic fall in urinary uric acid excretion may not occur, particularly in those with severe tophaceous gout. It has been postulated that this may be due to the mobilization of urate from tissue deposits as the serum uric acid level begins to fall.

The action of ZYLOPRIM differs from that of uricosuric agents, which lower the serum uric acid level by increasing urinary excretion of uric acid. ZYLOPRIM reduces both the serum and urinary uric acid levels by inhibiting the formation of uric acid. The use of ZYLOPRIM to block the formation of urates avoids the hazard of increased renal excretion of uric acid posed by uricosuric drugs.

ZYLOPRIM can substantially reduce serum and urinary uric acid levels in previously refractory patients even in the presence of renal damage serious enough to render uricosuric drugs virtually ineffective. Salicylates may be given conjointly for their antirheumatic effect without compromising the action of ZYLO-PRIM. This is in contrast to the nullifying effect of salicylates on uricosuric drugs.

ZYLOPRIM also inhibits the enzymatic oxidation of mercapto-purine, the sulfur-containing analogue of hypoxanthine, to 6-thiouric acid. This oxidation, which is catalyzed by xanthine oxi-dase, inactivates mercaptopurine. Hence, the inhibition of such oxidation by ZYLOPRIM may result in as much as a 75% reduction in the therapeutic dose requirement of mercaptopurine when the two compounds are given together.

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