Pegloticase (Krystexxa): The Last Resort That Actually Works

Somewhere back in your evolutionary history, your ancestors carried an enzyme called uricase. Its job was simple. It broke uric acid down into allantoin, a far more water-soluble waste product that the kidneys flush out without complaint¹. Then the gene broke. Most other mammals still carry a working copy. We do not, which is a large part of why gout is such a stubbornly human problem.

Pegloticase hands the tool back. It is a lab-grown uricase, wrapped in a protective coating that helps it survive in the bloodstream, and once it is circulating it does the work your body stopped doing generations ago: it converts uric acid into allantoin, which the kidneys clear.

Every other urate-lowering medicine works around the edges of your chemistry, turning down production or coaxing your kidneys into excreting more. Pegloticase destroys the uric acid itself, fast enough that deposits which took years to build finally have room to come apart. That directness is why it works when tablets have failed, and why it is handled with so much care.

Pegloticase is not where treatment starts. The American College of Rheumatology strongly recommends against it as first-line therapy². For most people, an oral medicine like allopurinol, started low and raised until serum urate sits below 6 mg/dL, does the job without anyone ever putting a line in your arm³.

Where pegloticase earns its place is at the end of that road. When xanthine oxidase inhibitors, uricosurics and everything else have genuinely failed to reach target, and a person is still having frequent flares or carrying tophi that will not dissolve, the ACR strongly recommends switching to pegloticase⁴. That is the specific job it was built for: uncontrolled, tophaceous gout that has resisted every oral medicine.

It is just as clearly not for milder disease. If your uric acid is above target but your flares are infrequent, fewer than two a year, and you have no tophi, the guideline strongly recommends against switching to pegloticase⁵. The reasoning is plain arithmetic: the risks below only make sense weighed against a genuinely heavy disease burden.

Pegloticase is an intravenous infusion, given in a clinic or infusion suite by people trained to watch you while it goes in. The trials that established the drug used a dose given every two weeks, and that fortnightly rhythm is still the familiar pattern⁶. Your rheumatologist sets the schedule that fits you.

Plan for the day rather than the hour. There is the drip itself, which runs slowly on purpose, and there is a stretch of monitoring afterwards, because reactions tend to announce themselves during or shortly after the infusion. Many teams give an antihistamine and a steroid beforehand to take the edge off that risk. Most will draw blood to check your uric acid before they start anything.

That pre-infusion blood draw carries real weight, and the next section explains why.

Pegloticase is a foreign protein. Your immune system, which is very good at its job, sometimes notices and starts building antibodies against it. When that happens the drug gets swept out of your blood before it can finish its work, uric acid climbs back up, and the odds of a bad infusion reaction climb with it. A uric acid level that has started rising again, measured before the next infusion, is the sign your team watches for that the drug is losing its grip.

For years that was pegloticase's ceiling. Then a randomized trial called MIRROR tested a straightforward idea: give methotrexate alongside it and quiet the immune response before it gets started. The share of people who responded rose from 38.5% on pegloticase alone to 71.0% with methotrexate added, and infusion reactions in that trial fell from 30.6% in the comparison group to 4.2%⁷.

Two caveats. Those figures come from a single trial, and they arrived after the 2020 ACR guideline was written, so the guideline does not account for them. Even so, they changed practice: pairing pegloticase with a medicine that calms the immune response is now a common approach, and worth raising with your specialist before the first infusion rather than after the third.

When pegloticase breaks uric acid down, the reaction leaves hydrogen peroxide behind. Most red blood cells handle that without any trouble. Red cells in people with G6PD deficiency, an inherited shortage of a protective enzyme, cannot. The result can be hemolysis, where red cells rupture, and methemoglobinemia, where blood loses its ability to carry oxygen properly. Pegloticase is contraindicated in G6PD deficiency, and anyone at higher risk should be screened before starting it⁸.

G6PD deficiency is not rare, and it is not evenly spread. It affects roughly 12% of African Americans⁸. A screening step treated as optional gets skipped most often for exactly the people it was designed to protect.

The screen itself is one blood test, done once, before you start. Ask whether it has been done. Good teams will be glad you did.

The trials that brought pegloticase to market enrolled people whose gout had refused everything else. Over six months, about 42% of those given the fortnightly infusion and 35% of those given it monthly reached the uric acid endpoint. In the placebo group the figure was zero⁶.

So roughly four in ten responded, which means most did not, in a group with tophi and years of failed tablets behind them. When the drug did work, uric acid fell below the level where crystals stay solid, and holding it there is what lets old deposits dissolve. That threshold is the same logic behind the guideline's treatment target³. The methotrexate pairing described above was developed to improve those odds.

If this drug is on the table, you have likely spent years on tablets that never got you to target. Take the questions below to your rheumatologist before the first infusion, not after the third.