Pegloticase (Krystexxa)

Pegloticase is a PEGylated recombinant enzyme that breaks down uric acid directly — an enzyme humans lost through evolution but most other mammals still make. Approved in 2010 for chronic gout that has failed conventional drugs, it works differently from any pill-based gout treatment, but roughly a third to half of patients develop antibodies against it that both cause the drug to stop working and raise the risk of a serious infusion reaction. A 2022 label expansion, based on a real randomized trial adding a second, unrelated drug alongside it, addressed that problem directly rather than papering over it.

An enzyme humans don't have

Most mammals produce an enzyme called uricase (urate oxidase) that breaks down uric acid into allantoin, a compound the kidneys excrete easily. Humans and other great apes lost a functional copy of that gene somewhere in primate evolution, which is a large part of why gout — caused by uric acid building up and crystallizing in joints — is a distinctly human problem. Standard gout treatment lowers uric acid production or increases its excretion using drugs like allopurinol or febuxostat, but a minority of patients with severe, longstanding gout do not respond adequately to those drugs, cannot tolerate them, or have progressed to large deposits of urate crystals (tophi) that oral therapy alone cannot resolve. Pegloticase restores the missing function directly: it is a recombinant, PEGylated form of mammalian uricase — engineered by researchers at Duke University starting in the mid-1990s, then licensed for commercial development to Bio-Technology General, later renamed Savient Pharmaceuticals — that metabolizes uric acid the way the human body no longer can. The polyethylene glycol (PEG) coating is there to reduce the immune system's reaction to a foreign enzyme and extend how long it stays active in the blood — a strategy this site's pegcetacoplan and pegfilgrastim pages also describe for different drugs, though as this page explains, that same PEG coating turned out to be pegloticase's central problem.

The pivotal trials, in a genuinely narrow population

The FDA approved pegloticase on 14 September 2010, specifically for chronic gout in adults refractory to conventional therapy — not as a general-purpose gout drug, but for patients who had already failed the standard options. The approval rested on two replicate, randomized, placebo-controlled Phase 3 trials (often referred to as GOUT1 and GOUT2) in this refractory population, comparing biweekly or monthly pegloticase infusions against placebo over six months. The pooled responder rate — the proportion of patients who kept plasma uric acid below 6 mg/dL for at least 80% of the time during months 3 and 6 combined — was 42% on the approved biweekly dosing schedule, versus 0% on placebo (Sundy JS, Baraf HSB, Yood RA, et al., "Efficacy and Tolerability of Pegloticase for the Treatment of Chronic Gout in Patients Refractory to Conventional Treatment: Two Randomized Controlled Trials," JAMA 2011;306(7):711-720, PMID 21846852, DOI: 10.1001/jama.2011.1169).¹ A 42% response rate against a 0% placebo response is a real, clinically meaningful effect in a population with no other approved option left — but it also means the majority of even this narrowly-selected, refractory population did not reach a durable response on pegloticase alone, a limitation this page's later sections explain.

The immunogenicity problem: antibodies against the PEG, not the enzyme

MIRROR: adding methotrexate to fix what PEG breaks

Rather than reformulating the drug itself, Horizon Therapeutics (by then pegloticase's owner) tested whether co-administering a low, short-course dose of methotrexate — an old, unrelated immunosuppressant drug used for decades in rheumatoid arthritis — alongside pegloticase could blunt the antibody response driving both the loss of efficacy and the infusion reactions. The randomized, double-blind, placebo-controlled MIRROR trial enrolled 152 adults with uncontrolled gout, all receiving pegloticase, randomized to also receive methotrexate or placebo. The primary endpoint — maintaining serum urate below 6 mg/dL for at least 80% of visits during weeks 20-24 — was met by 71% of the methotrexate group versus 39% of the placebo group, and infusion reactions during the first six months fell from 30.6% on placebo to 4.2% with methotrexate co-therapy (Botson JK, Saag K, Peterson J, et al., "A Randomized, Placebo-Controlled Study of Methotrexate to Increase Response Rates in Patients with Uncontrolled Gout Receiving Pegloticase: Primary Efficacy and Safety Findings," Arthritis Rheumatol 2023;75(2):293-304, PMID 36099211, DOI: 10.1002/art.42335).³ The FDA approved this combination use on 8 July 2022 — a genuine case of a real randomized trial identifying the mechanism behind a drug's biggest limitation and testing a specific, unrelated fix for it, rather than simply narrowing the label or adding a caution.

A drug that outlived three owners

Pegloticase's corporate history is unusually turbulent for an approved biologic that never lost its approval. Savient Pharmaceuticals, which brought pegloticase through approval in 2010, spent far more on sales and marketing than the drug earned in its first years on the market and filed for Chapter 11 bankruptcy on 14 October 2013. Crealta Pharmaceuticals acquired pegloticase and Savient's other assets out of bankruptcy court later that year for roughly $120 million. Horizon Pharma then acquired Crealta in 2016 for roughly $510 million, folding pegloticase into a larger rare-disease portfolio; Horizon later reincorporated as Horizon Therapeutics. Finally, Amgen completed a $27.8 billion acquisition of Horizon Therapeutics in October 2023, after the US Federal Trade Commission briefly challenged the deal on antitrust grounds and reached a settlement permitting it to close. None of these changes of ownership were driven by a safety or efficacy problem with the drug itself — this is a business-history footnote, not a regulatory one — but four different owners across thirteen years is a genuinely unusual path for one continuously-approved biologic.

Current status

Pegloticase remains FDA-approved and currently marketed by Amgen as Krystexxa, both alone and, since 2022, with co-administered methotrexate for patients who can tolerate it. It remains a narrow, later-line option — reserved for chronic gout that has already failed standard oral urate-lowering therapy — administered by infusion with real, well-characterized immunogenicity risk that clinicians are now expected to actively manage rather than simply monitor. There is no wellness or grey-market interest in pegloticase: its cost, infusion-only administration, and prescription-only distribution through specialty pharmacies put it well outside that market.