Effectiveness of Pyridoxal-5'-Phosphate in PNPO Deficiency: A Systematic Review.

Pyridox(am)ine 5'-phosphate oxidase (PNPO) deficiency is an ultrarare inherited neurometabolic disease, characterized by primarily neonatal-onset B6-responsive epileptic encephalopathies. Treatment often requires sustainable access to high-quality pyridoxal-5'-phosphate (PLP, i.e., active vitamin B6), although some patients (also) respond to pyridoxine (PN). While PN is authorized as a medicinal product, PLP is not, and this forces reliance on lesser-regulated food supplements, which risks dosing inaccuracies. This systematic review evaluates the effectiveness and safety of PLP in PNPO deficiency (PROSPERO, CRD42024542199). A systematic search was conducted in PubMed, Embase, and ClinicalTrials.gov, with risk of bias assessed and observational evidence summarized using a narrative synthesis approach. A total of 30 studies were included reporting on 49 patients treated with PLP. Clinical seizure responsiveness following PLP therapy was observed in the majority of patients (n = 38, 77.6%) and PLP treatment significantly improved survival (p < 0.001) compared with untreated siblings with a similar phenotype. The majority of PLP-responsive patients responded exclusively to PLP, with PN being attempted but ineffective in most of them (n = 30/33, 90.9%) Liver toxicity was the most frequently observed adverse event (n = 10, 20.4%) and although the underlying pathophysiological mechanism remains unclear, it may be associated with high-dose PLP. Therefore, regular liver disease screening is recommended during PLP therapy. This means that PLP remains the only effective therapy for achieving and maintaining seizure control in the majority of PNPO deficient patients, but the therapeutic window for optimal management is narrow. Thus, it is essential to ensure patient access to high-quality and appropriate forms of PLP.