Parathormone and bone-specific alkaline phosphatase for the follow-up of bone turnover in hemodialysis patients: Is it so simple?

Alkaline Phosphatases in the Complex Chronic Kidney Disease-Mineral and Bone Disorders

Alkaline phosphatases (APs) remove the phosphate (dephosphorylation) needed in multiple metabolic processes (from many molecules such as proteins, nucleotides, or pyrophosphate). Therefore, APs are important for bone mineralization but paradoxically they can also be deleterious for other processes, such as vascular calcification and the increasingly known cross-talk between bone and vessels. A proper balance between beneficial and harmful activities is further complicated in the context of chronic kidney disease (CKD). In this narrative review, we will briefly update the complexity of the enzyme, including its different isoforms such as the bone-specific alkaline phosphatase or the most recently discovered B1x. We will also analyze the correlations and potential discrepancies with parathyroid hormone and bone turnover and, most importantly, the valuable recent associations of AP's with cardiovascular disease and/or vascular calcification, and survival. Finally, a basic knowledge of the synthetic and degradation pathways of APs promises to open new therapeutic strategies for the treatment of the CKD-Mineral and Bone Disorder (CKD-MBD) in the near future, as well as for other processes such as sepsis, acute kidney injury, inflammation, endothelial dysfunction, metabolic syndrome or, in diabetes, cardiovascular complications. However, no studies have been done using APs as a primary therapeutic target for clinical outcomes, and therefore, AP's levels cannot yet be used alone as an isolated primary target in the treatment of CKD-MBD. Nonetheless, its diagnostic and prognostic potential should be underlined.

Variations of parathyroid hormone and bone biomarkers are concordant only after a long term follow-up in hemodialyzed patients

End-stage renal disease is associated with mineral and bone disorders. Guidelines recommending therapies should be based on serial assessments of biomarkers, and thus on variations (Δ), rather than scattered values. We analyzed the correlations between ΔPTH and Δbone biomarkers such as bone-specific alkaline phosphatase (b-ALP), Beta-CrossLaps (CTX), osteocalcin, intact serum procollagen type-1 N-propeptide (P1NP), and tartrate-resistant acid phosphatase 5B (TRAP-5B) at different time-points. In this prospective observational analysis, variations of biomarkers were followed after 6-week (n = 129), 6-month (n = 108) and one-year (n = 93) period. Associations between variations were studied by univariate linear regression. Patients followed for one-year period were classified (increaser or decliner) according to variations reaching the critical difference. Over the 6-week period, only ΔCTX was correlated with ΔPTH (r = 0.38, p < 0.0001). Over the one-year period, correlations between ΔPTH and Δbone biomarkers became significant (r from 0.23 to 0.47, p < 0.01), except with ΔTRAP-5b. Correlations between Δbone biomarkers were all significant after one-year period (r from 0.31 to 0.68, p < 0.01), except between Δb-ALP and ΔTRAP-5b. In the head-to-head classifications (decliners/increasers), the percentage of concordant patients was significantly higher over the one-year than the 6-week period. A concordance between ΔPTH and Δbone biomarkers is observed in dialysis patients, but only after a long follow-up.

Circulating markers of bone turnover

Renal osteodystrophy is a feature of chronic kidney disease (CKD), with increasing prevalence as CKD progresses. This bone disease is responsible for major morbidity, including fractures, and a deterioration in the quality of life and its sequelae. Circulating biomarkers of renal osteodystrophy typically indicate bone turnover, but not other features of bone, like bone volume, mineralization, quality or strength. Bone turnover can be considered to be primarily a reflection of bone cell activity, in particular that of osteoblasts and osteoclasts. Since current treatments for bone disease usually target cellular activity, biomarkers are considered to be able to contribute to the decision-making for treatment and its follow-up. In CKD, one has to consider the impact of a diminished clearance of biomarkers or their altered metabolism, both potentially limiting its clinical use. Here, several aspects of the most frequently used biomarkers of bone turnover are reviewed, with an emphasis on the specific situation represented by CKD. This review is based on the overview lecture at the symposium held in Amsterdam, September 23, 2016: “The Bone In CKD”, organized by the CKD-MBD working group of ERA-EDTA.