SAT-729 - Pyridoxic Acid Measurement Improves Interpretation of Pyridoxal-5′-Phosphate in the Evaluation of Suspected Hypophosphatasia

Background
Elevated serum pyridoxal-5′-phosphate (PLP) is a key biochemical criterion for hypophosphatasia (HPP) in patients with low alkaline phosphatase (ALP). PLP must be hydrolyzed by tissue-nonspecific alkaline phosphatase to pyridoxal to enter cells, where it is metabolized to pyridoxic acid (PA). Elevated PLP may reflect impaired hydrolysis, as in HPP, or excess vitamin B6 intake. We hypothesized that PA measurement would improve interpretation of PLP in patients evaluated for HPP.

Methods
Serum PLP and PA (mcg/L; reference ranges PLP 5–50, PA 3–30) were measured in patients evaluated for suspected HPP. The HPP cohort included 20 patients with pathogenic ALPL variants. The non-HPP cohort included nine patients negative for ALPL variants, several with repeated paired measurements. PLP:PA ratios were calculated. Group comparisons used nonparametric testing. Exploratory measurements of PLP and PA were also performed in a small number of asfotase alfa–treated HPP patients to assess biomarker interpretability on therapy.

Results
Mean ALP was 25.0 ± 9.8 U/L in HPP patients and 33.1 ± 4.8 U/L in non-HPP patients. Elevated PLP (>50 mcg/L) was present in 11/20 HPP patients (58–132) and 4/9 non-HPP patients (51–108). Among those with elevated PLP, PA was significantly lower in HPP than non-HPP patients (9.0 ± 7.2 vs 30.7 ± 19.0 mcg/L; p = 0.0047), with higher PLP:PA ratios (15.8 ± 12.0 vs 3.25 ± 2.29; p = 0.00065). Normal PLP values were observed in 9/20 HPP patients and 5/9 non-HPP patients; in this subgroup, PA and PLP:PA ratios did not differ significantly. However, PA identified six genetically confirmed HPP patients with borderline or low PA consistent with vitamin B6 deficiency. In one patient, supplementation increased PLP from 39 to 304 mcg/L and PA from 3 to 88 mcg/L with transient symptomatic improvement. In patients receiving asfotase alfa with markedly elevated ALP levels, PLP values were frequently low or uninterpretable, whereas PA levels remained measurable and clinically informative, identifying both vitamin B6 deficiency and elevations related to supplementation.

Conclusion
PA measurement improves biochemical interpretation of PLP in suspected HPP. PA distinguishes impaired PLP hydrolysis from excess vitamin B6 intake when PLP is elevated and identifies vitamin B6 deficiency in genetically confirmed HPP patients, a relevant finding given potential symptom reversibility. Importantly, PA remains interpretable in patients receiving asfotase alfa, supporting its utility for assessing vitamin B6 status during enzyme replacement therapy. Normal PLP and PA in some HPP patients may reflect variant-specific ALPL effects, supporting future studies incorporating paired PLP, pyridoxal, and PA measurements.

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