Pegvaliase (Palynziq)

Pegvaliase is an enzyme-substitution drug for phenylketonuria (PKU) that works nothing like the diet or the one other approved PKU drug most patients already know: instead of restoring the enzyme PKU patients are missing, it substitutes a different, PEGylated bacterial enzyme that breaks down phenylalanine through an entirely separate chemical route — described at the time as the first bacterially-derived therapeutic protein ever approved for a chronic human disease. Approved for adults in 2018 on real placebo-controlled trial evidence, it carries a boxed warning and a mandatory safety program for a real, frequent anaphylaxis risk — and only reached adolescent patients in February 2026, nearly eight years later.

PKU and the diet that isn't always enough

Phenylketonuria (PKU) is an inherited metabolic disorder caused by deficient activity of phenylalanine hydroxylase (PAH), the liver enzyme that normally converts the amino acid phenylalanine (Phe) into tyrosine. Without adequate PAH activity, phenylalanine builds up in the blood and, if untreated from infancy, causes severe and irreversible intellectual disability — one of the original successes of newborn screening, introduced in the US starting in the 1960s specifically to catch PKU before that damage occurs. The mainstay treatment, then and now, is a severely phenylalanine-restricted diet relying on specialized medical formula — a demanding, lifelong regimen that many patients don't sustain into adulthood, when a real minority develop or redevelop poorly controlled blood Phe levels with associated cognitive, psychiatric and neurological symptoms, short of the profound intellectual disability untreated infantile PKU causes. A separate oral drug, sapropterin, approved in 2007, helps a genetic subset of patients whose remaining PAH enzyme responds to a cofactor boost — a real option, but not one that works for patients, a majority of those with more severe disease, whose PAH mutations leave no functional enzyme for a cofactor to boost.

Pegvaliase is a fundamentally different approach: rather than restoring or boosting the patient's own PAH enzyme, it substitutes an entirely different enzyme that metabolizes phenylalanine through a separate chemical pathway altogether.

A bacterial enzyme, not a human one

Pegvaliase is a PEGylated, recombinant form of phenylalanine ammonia lyase (PAL), an enzyme found in the cyanobacterium Anabaena variabilis that breaks phenylalanine down into ammonia and trans-cinnamic acid — a completely different metabolic route than the one PAH normally uses, and one no human enzyme performs. Because PAL is a bacterial protein rather than a human one, injecting it provokes a real, expected immune response; researchers addressed that by attaching multiple strands of polyethylene glycol (PEG) to the enzyme's surface — the same general strategy this site's pegloticase and pegfilgrastim pages describe for other drugs — to partially shield it from antibody recognition and extend how long it circulates. BioMarin Pharmaceutical carried the molecule from its original academic characterization through PEGylation, dose-optimization and its Phase 3 program; researchers describing the drug afterward called it the first bacterially-derived therapeutic protein ever approved for a chronic human disease (Qi Y, et al., “Pharmacokinetic, pharmacodynamic, and immunogenic rationale for optimal dosing of pegvaliase, a PEGylated bacterial enzyme, in adult patients with phenylketonuria,” Clin Transl Sci 2021;14(5):1894-1905, PMID 34057292) — a genuine manufacturing and regulatory first, distinct from any specific claim about how well it works.¹

PRISM: from open-label dose-finding to a randomized discontinuation trial

Pegvaliase's approval rested on the PRISM Phase 3 program, run in two linked parts. PRISM-1 was an open-label, dose-finding study that enrolled 261 pegvaliase-naive adults with PKU, starting everyone on a slow induction/titration schedule (beginning at 2.5 mg once weekly) before randomizing them 1:1 to a 20 mg/day or 40 mg/day maintenance dose; by 24 months, mean blood Phe had fallen from a baseline of 1232.7 µmol/L to 311.4 µmol/L, a 68.7% reduction, with 51.2% of participants reaching a blood Phe of 120 µmol/L or below (Thomas J, Levy H, Amato S, Vockley J, Zori R, Dimmock D, Harding CO, Bilder DA, Weng HH, Olbertz J, Merilainen M, Jiang J, Larimore K, Gupta S, Gu Z, Northrup H, “Pegvaliase for the treatment of phenylketonuria: Results of a long-term phase 3 clinical trial program (PRISM),” Mol Genet Metab 2018;124(1):27-38, PMID 29653686, DOI: 10.1016/j.ymgme.2018.03.006).²

Because PRISM-1 had no placebo arm, a second study embedded within the ongoing PRISM-2 extension added a genuinely controlled comparison: a randomized discontinuation trial (RDT) that took participants who had already responded to pegvaliase and randomized them, 2:1, to continue their existing dose or switch to blinded placebo for 8 weeks. Blood Phe rose sharply in both placebo groups (least-squares mean increases of roughly 950 µmol/L and 665 µmol/L in the 20 mg/day and 40 mg/day placebo arms respectively) while remaining essentially flat in patients who continued on pegvaliase (a roughly 27 µmol/L mean change), a difference significant at P<0.0001 against each placebo arm (Harding CO, Amato RS, Stuy M, Longo N, Burton BK, Posner J, Weng HH, Merilainen M, Gu Z, Jiang J, Vockley J, “Pegvaliase for the treatment of phenylketonuria: A pivotal, double-blind randomized discontinuation Phase 3 clinical trial,” Mol Genet Metab 2018;124(1):20-26, PMID 29628378, DOI: 10.1016/j.ymgme.2018.03.003). Together, the two studies gave the FDA both real-world, long-term dose-response data and a genuine, if short, placebo-controlled demonstration that the effect was attributable to the drug itself rather than to diet changes or regression to the mean.³

The anaphylaxis problem, and the REMS program built around it

2026: the label finally reaches adolescents

For nearly eight years after its original 2018 approval, pegvaliase remained restricted to adults, even though PKU is a lifelong condition often diagnosed and managed from infancy. That changed on 27 February 2026, when the FDA approved a supplemental Biologics License Application extending Palynziq's indication to adolescents aged 12 through 17, based on PEGASUS, an open-label, randomized, controlled Phase 3 trial comparing pegvaliase against continued diet-only management in adolescents with uncontrolled blood Phe above 600 µmol/L. The FDA had accepted BioMarin's adolescent application for Priority Review on 29 October 2025, with a target decision date of 28 February 2026 — meaning the actual approval landed a day ahead of its own regulatory deadline. The same boxed warning and REMS enrollment requirements that apply to adult patients now extend to adolescent patients as well.

Current status

Pegvaliase remains FDA-approved and marketed by BioMarin as Palynziq, with parallel approval in the European Union (EMA, 2019, for patients 16 and older) and other markets. It remains a later-line option specifically for patients whose blood Phe stays uncontrolled despite diet and, where applicable, sapropterin — not a first-line PKU treatment — reflecting both its genuinely demanding administration schedule and its real, boxed-warning-level anaphylaxis risk. Within that narrower population, it remains the only approved drug that substitutes for the missing PAH enzyme's function altogether, rather than working around it.