Pyridoxal-5'-phosphate (PLP) is in most patients the effective treatment for pyridox(am)ine-5'-phosphate oxidase (PNPO) deficiency, a rare autosomal recessive cause of neonatal-onset developmental and epileptic encephalopathy. Although generally considered safe, long-term high-dose PLP exposure may have hepatotoxic effects, particularly in the absence of pharmaceutical-grade formulations.We report a series of four pediatric patients with vitamin B6-dependent epilepsy who received long-term PLP therapy. Two had genetically confirmed PNPO deficiency, and two were later diagnosed with ALDH7A1 deficiency. All received high-dose oral PLP, with frequent changes in formulation due to availability issues.Three of the four patients developed hepatocellular carcinoma after several years of PLP treatment; one developed fully reversible severe hepatotoxicity. The shared exposure to prolonged high-dose PLP across all affected patients, despite differing metabolic conditions, suggests a possible role for PLP toxicity independent of the underlying metabolic disorder. Known toxic mechanisms include mitochondrial dysfunction, Schiff base-mediated protein modification, and accumulation of reactive PLP degradation products. In two patients, the total PLP dose was successfully reduced by over 30% through increasing administration frequency, without loss of seizure control.These findings raise significant concerns about the long-term hepatic safety of oral PLP in patients with vitamin B6-dependent epilepsies. As intravenous PLP is unfeasable for lifelong therapy, there is an urgent need for standardized, high-quality PLP preparations and exploration of alternative delivery routes such as intranasal administration. Regular hepatic monitoring should be implemented in all patients receiving chronic PLP therapy.

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.

Pyridoxamine 5'-phosphate oxidase deficiency is a rare inborn error of vitamin B6 metabolism that presents with drug-resistant epileptic seizures. However, the condition is responsive to supplementation with the active vitamin B6 metabolite pyridoxal 5'-phosphate and, in some cases, pyridoxine. In this case report, a 10-year-old Iranian male of Fars ethnicity came to a regional hospital in Tehran, Iran with a chief complaint of tic-like movement. He had a history of unintentional, repetitive, and stereotypic movements of both arms since the age of 4 years. The physical examination depicted facial dimorphism. During admission, the patient experienced habitual hypermotor seizures and generalized tonic-clonic seizures. Ictal electroencephalography demonstrated a generalized background attenuation and bursts of generalized, predominantly left-sided, biphasic spike-wave complexes. Whole-genome sequencing revealed a pyridoxamine 5'-phosphate oxidase deficiency as the underlying cause of the drug-resistant seizures, resulting in a low serum level of pyridoxal 5'-phosphate. The patient underwent pyridoxine supplementation therapy, which ultimately resolved his seizures. At 6 months, he was seizure free. Physicians ought to be aware of manifestations of vitamin B6 deficiency such as mimicking tic and consider it in the differential for drug-resistant epilepsy.

Several variants of the enzyme pyridox(am)ine 5'-phosphate oxidase (PNPO), responsible for a rare form of vitamin B6-dependent neonatal epileptic encephalopathy known as PNPO deficiency (PNPOD), have been reported. However, only a few of them have been characterised with respect to their structural and functional properties, despite the fact that the knowledge of how variants affect the enzyme may clarify the disease mechanism and improve treatment. Here, we report the characterisation of the catalytic, allosteric and structural properties of recombinantly expressed D33V, R161C, P213S, and E50K variants, among which D33V (present in approximately 10% of affected patients) is one of the more common variants responsible for PNPOD. The D33V and E50K variants have only mildly altered catalytic properties. In particular, the E50K variant, given that it has been found on the same chromosome with other known pathogenic variants, may be considered non-pathogenic. The P213S variant has lower thermal stability and reduced capability to bind the FMN cofactor. The variant involving Arg161 (R161C) largely decreases the affinity for the pyridoxine 5'-phosphate substrate and completely abolishes the allosteric feedback inhibition exerted by the pyridoxal 5'-phosphate product.

Pyridoxamine-5'-phosphate oxidase (PNPO) deficiency is an autosomal recessive pyridoxal 5'-phosphate (PLP)-vitamin-responsive epileptic encephalopathy. The emerging feature of PNPO deficiency is the occurrence of refractory seizures in the first year of life. Pre-maturity and fetal distress, combined with neonatal seizures, are other associated key characteristics. The phenotype results from a dependency of PLP which regulates several enzymes in the body. We present the phenotypic and genotypic spectrum of (PNPO) deficiency based on a literature review (2002-2020) of reports (n = 33) of patients with confirmed PNPO deficiency (n = 87). All patients who received PLP (n = 36) showed a clinical response, with a complete dramatic PLP response with seizure cessation observed in 61% of patients. In spite of effective seizure control with PLP, approximately 56% of patients affected with PLP-dependent epilepsy suffer developmental delay/intellectual disability. There is no diagnostic biomarker, and molecular testing required for diagnosis. However, we noted that cerebrospinal fluid (CSF) PLP was low in 81%, CSF glycine was high in 80% and urinary vanillactic acid was high in 91% of the cases. We observed only a weak correlation between the severity of PNPO protein disruption and disease outcomes, indicating the importance of other factors, including seizure onset and time of therapy initiation. We found that pre-maturity, the delay in initiation of PLP therapy and early onset of seizures correlate with a poor neurocognitive outcome. Given the amenability of PNPO to PLP therapy for seizure control, early diagnosis is essential.