Abbreviations: BH₄: Tetrahydrobiopterin, PAH: Phenylalanine hydroxylase, Pal: Phe ammonia lyase, Phe: Phenylalanine, PKU: Phenylketonuria, Tyr: Tyrosine.

Phenylketonuria (PKU) is an autosomal recessive disorder caused by a deficiency of the enzyme phenylalanine hydroxylase (PAH), most commonly due to mutations in the PAH gene¹. PAH catalyzes the oxidation of phenylalanine (Phe) to tyrosine (Tyr) as part of the phenylalanine metabolism, a process that requires the reduced pterin cofactor tetrahydrobiopterin (BH₄). BH₄ can be synthesized de novo and recycled through a salvage pathway to sustain Phe hydroxylation² (Figure 1).

Although rare, PKU is the most detected disorder through newborn screening, with a global prevalence of approximately 1 in 12,000 live births (based on Italian data) and an estimated half a million individuals affected worldwide³.

PAH deficiency impairs the conversion of the essential amino acid L-Phe to L-Tyr, leading to the accumulation of Phe in the blood and, more critically, in the brain⁴.

Although the mechanisms are not fully understood, elevated Phe levels in the brain of individuals with untreated PAH deficiency are associated with acquired microcephaly, growth failure, seizures, and severe global developmental delays⁵.

The implementation of neonatal screening programs and early initiation of treatment has enabled countries with advanced health systems to effectively control PKU^6,7.

However, elevated and persistent blood Phe levels in adults are associated with executive dysfunction and significant behavioral and psychiatric disturbances, negatively impacting patients’ quality of life^5,8.

Chronic elevation of blood Phe levels is associated with inattention, impaired memory, executive dysfunction, and psychiatric disorders⁹.

The cornerstone of PKU therapy is the strict restriction of dietary Phe intake, with low-Phe or Phe-free amino acid–fortified medical foods and specially-formulated low-protein foods¹⁰.

Initiating dietary Phe restriction within the first days of life prevents the clinical manifestations of PKU. Early dietary intervention should begin as soon as possible, with strict control of Phe levels, due to the heightened vulnerability of the developing brain during the first years of life. Fluctuations of the Phe levels, often triggered by intercurrent febrile episodes, should be carefully managed^5,11.

According to the current European guidelines^4,6, lifelong treatment is recommended, with blood Phe levels maintained within the target range of 120-360 μmol/L until 12 years of age and within 120-600 μmol/L thereafter. Continuous monitoring and follow-up are essential to prevent neurocognitive decline or deterioration^9,12.

In pediatric patients, severe restriction of dietary intact protein to reduce blood and brain Phe concentrations, combined with supplementation using synthetic medical foods to ensure adequate nutrition, has been the cornerstone of therapy for individuals with PAH deficiency since the 1950s⁸. However, lifelong adherence to such a highly restrictive diet therapy can have a profound psychological impact on both individuals and their families.

Long-term neurocognitive outcomes remain suboptimal, and adverse health effects associated with chronic dietary treatment are increasingly recognized^9,13.

There is a growing need for novel treatments that maintain low blood Phe levels while permitting a greater intake of intact protein. However, non-adherence to the complex and unpalatable diet regimen is common during adolescence and adulthood¹¹.

A lifelong phenylalanine-restricted diet remains the primary therapy for PKU; however, adherence to this stringent regimen typically declines as patients transition into adulthood. Therefore, effective therapeutic alternatives are urgently needed to support long-term treatment compliance¹⁴.

Sapropterin dihydrochloride, a synthetic form of BH₄ – the cofactor of PAH – was the first pharmacological treatment introduced as standard care for PKU. In approximately one-third of patients with PKU, oral administration of sapropterin dihydrochloride has been shown to induce PAH enzyme stabilization and increase residual enzymatic activity¹⁴.

Still, controversies remain regarding the criteria for BH₄ responsiveness. A reduction of 30% or more in blood Phe levels during a BH₄ loading test is often used as the threshold for classifying patients as “BH₄ responsive” and the role of mutations. Moreover, the BH₄ loading test lacks standardization, and the 30% threshold is considered arbitrary, with no clear consensus despite BH₄ being now available as a generic drug and included in clinical guidelines^5,6. Pegvaliase, a PEGylated recombinant Phe ammonia lyase (PAL) enzyme, converts Phe into trans-cinnamic acid and ammonia, which are excreted in urine and further metabolized in the liver⁴. Pegvaliase is an innovative enzyme substitution therapy that received approval from the Italian Medicines Agency (AIFA) in 2023⁶.

Pegvaliase is indicated for patients with PKU aged 16 years and older who have uncontrolled disease, defined as Phe levels exceeding 600 μmol/L, despite prior treatment efforts^4,7.

The most common adverse events associated with pegvaliase treatment include injection-site reactions, dizziness and immune reactions¹⁴.

Although pegvaliase is highly effective in reducing Phe concentrations, cases of severe drug-induced hypersensitivity adverse events have been reported. In clinical trials⁸, all subjects treated with pegvaliase developed anti-drug antibodies against both the PAL protein and the PEG component, which can lead to hypersensitivity adverse events.

However, drug-specific IgE antibodies have not been detected, and cases of anaphylaxis are rare. In clinical trials⁸, all patients exhibited elevated levels of anti-PEG antibodies and anti-PAL IgM at the initiation of pegvaliase treatment, contributing to type III immune complex-mediated reactions.

This special issue highlights the Italian experience in managing patients with PKU, including outcomes from the use of PAL-based therapies.

Acknowledgments:

Editorial assistance was provided by Aashni Shah and Massimiliano Pianta (Polistudium srl).

Artificial Intelligence-Assisted Technologies:

No artificial intelligence-assisted technologies were used in the production of this article.

Availability of Data and Materials:

Data sharing is not applicable to this article, as no datasets were generated or analyzed during the current study.

Conflicts of Interest:

The author declares no conflict of interest related to the content of the manuscript. Although the author did not receive any financial benefit from BioMarin Pharmaceutical Italia S.R.L. and the integrity of the article was not influenced or affected by BioMarin Pharmaceutical Italia S.R.L., the company did provide support for the editorial assistance of the article through an unconditional grant, as stated in the Funding section.

Consent to Participate:

Not applicable.

Ethics Approval:

Not applicable.

Funding:

Editorial assistance was supported by BioMarin Pharmaceutical Italia S.R.L, with no influence on the content or the preparation of the manuscript.

ORCID ID

A. Burlina: 0000-0001-7724-137X

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