uricases

Purine catabolism in most mammals proceeds: adenine/guanine → xanthine → uric acid (via xanthine oxidase) → allantoin (via uricase). Allantoin is far more soluble than uric acid and is readily excreted in urine. Humans and higher primates lost uricase function through evolutionary mutations, resulting in uric acid being the terminal excretory product of purine metabolism. This makes humans uniquely susceptible to hyperuricemia, gout, and urate nephropathy/tumor lysis syndrome, conditions rare in other mammals.

Clinically, two recombinant uricase formulations are important: 1. Rasburicase - a recombinant non-pegylated uricase used for prevention/treatment of tumor lysis syndrome (TLS) in patients undergoing chemotherapy for hematologic malignancies. It rapidly converts uric acid to allantoin, preventing urate nephropathy. Contraindicated in G6PD deficiency because the reaction produces hydrogen peroxide, which can trigger severe hemolysis in these patients. 2. Pegloticase - a PEGylated uricase used for refractory chronic gout in patients who fail conventional urate-lowering therapy (allopurinol, febuxostat). It is given as an IV infusion; anti-drug antibodies can develop, reducing efficacy and increasing infusion reaction risk.

Mechanistically, uricase therapy works upstream of xanthine oxidase inhibitors (allopurinol, febuxostat) by degrading uric acid itself rather than preventing its formation. This distinction is tested on boards: xanthine oxidase inhibitors reduce uric acid production, while uricases enhance its clearance/degradation.

Key testable points: uricase deficiency in humans (pseudogene) underlies susceptibility to gout and hyperuricemia-related disease; rasburicase is contraindicated in G6PD deficiency due to hemolysis risk from H2O2 byproduct; pegloticase is reserved for severe refractory gout; uricases are especially useful in oncologic emergencies like TLS where rapid uric acid reduction is needed to prevent acute kidney injury.