Circulating and tissue-derived isoforms of bone alkaline phosphatase in Paget's disease of bone

Alkaline phosphatase (ALP) is routinely used in the assessment of Paget's disease of bone (PDB); however, the individual bone ALP isoforms (B/I, B1, and B2) have not been investigated in this disorder.

Subjects comprised 37 patients (mean age 74 years) with symptomatic PDB confirmed by radiograph and stratified into high and low total ALP activity groups and 66 healthy individuals (mean age 64 years). Extracts of human cancellous and cortical bone tissues were also investigated. The bone ALP isoforms, measured by HPLC, were compared with two bone ALP immunoassays (Metra and Ostase), and the bone formation marker intact amino-terminal procollagen type I propeptide (iPINP).

All bone ALP isoforms were increased in high ALP activity PDB compared with the low ALP activity and control groups (p < 0.0001). The B2 isoform had the greater relative activity representing 36%, 50%, and 71%, of the total ALP activity in the control, low and high ALP activity groups, respectively. Compared with controls, B2 was increased in the low ALP activity PDB group (p < 0.05). ROC analysis showed a validity of approximately 80% for B2 to discriminate patients with PDB.

All bone ALP isoforms were increased in patients with high ALP activity PDB and the B2 isoform was even elevated in the low ALP activity PDB group. The bone ALP isoform B2 may be of use in the management of PDB but that has to be further elucidated in subsequent studies.