A Study to Inform the Design of a National Multicentre Randomised Controlled Trial to Evaluate If Reducing Serum Phosphate to Normal Levels Improves Clinical Outcomes including Mortality, Cardiovascular Events, Bone Pain, or Fracture in Patients on Dialysis

Serum phosphate is associated with increased risk of bone fragility fractures in haemodialysis patients

BACKGROUND

Bone fragility fractures are associated with high morbidity and mortality. This study analysed the association between the current biochemical parameters of chronic kidney disease-mineral and bone disorders (CKD-MBD) and bone fragility fractures in the COSMOS (Current management Of Secondary hyperparathyroidism: a Multicentre Observational Study) project.

METHODS

COSMOS is a 3-year, multicentre, open cohort, prospective, observational study carried out in 6797 haemodialysis patients (227 centres from 20 European countries). The association of bone fragility fractures (outcome) with serum calcium, phosphate and parathyroid hormone (PTH) (exposure), was assessed using standard Cox proportional hazards regression and Cox proportional hazards regression for recurrent events. Additional analyses were performed considering all-cause mortality as a competitive event for bone fragility fracture occurrence. Multivariable models were used in all strategies, with the fully adjusted model including a total of 24 variables.

RESULTS

During a median follow-up of 24 months, 252 (4%) patients experienced at least one bone fragility fracture (incident bone fragility fracture rate 28.5 per 1000 patient-years). In the fractured and non-fractured patients, the percentage of men was 43.7% and 61.4%, mean age 68.1 and 63.8 years and a haemodialysis vintage of 55.9 and 38.3 months, respectively. Baseline serum phosphate >6.1 mg/dL (reference value 4.3-6.1 mg/dL) was significantly associated with a higher bone fragility fracture risk in both regression models {hazard ratio (HR) 1.53 [95% confidence interval (CI) 1.10-2.13] and HR 1.44 (95% CI 1.02-2.05)}. The significant association persisted after competitive risk analysis [subHR 1.42 (95% CI 1.02-1.98)] but the finding was not confirmed when serum phosphate was considered as a continuous variable. Baseline serum calcium showed no association with bone fragility fracture risk in any regression model. Baseline serum PTH >800 pg/mL was significantly associated with a higher bone fragility fracture risk in both regression models, but the association disappeared after a competitive risk analysis.

CONCLUSIONS

Hyperphosphatemia was independently and consistently associated with an increased bone fracture risk, suggesting serum phosphate could be a novel risk factor for bone fractures in haemodialysis patients.

Phosphate and bone fracture risk in chronic kidney disease patients

In chronic kidney disease (CKD), phosphate homoeostasis plays a central role in the development of mineral and bone disorder (MBD) together with decreased serum calcium and elevated serum parathyroid hormone, fibroblast growth factor 23 and sclerostin levels. Today there are only a few data exploring the direct role of abnormal phosphate homoeostasis and hyperphosphataemia in the development of CKD-MBD. On the other hand, several studies have looked at the link between hyperphosphataemia and cardiovascular morbidity and mortality in CKD, but there is a lack of evidence to indicate that lowering phosphate levels improves cardiovascular outcomes in this population. Furthermore, the impact of liberalizing phosphate targets on CKD-MBD progression and bone fracture is currently not known. In this review we discuss the central role of phosphate in the pathogenesis of CKD-MBD and how it may be associated with fracture risk, both in hyper- and hypophosphataemia.

A randomized controlled trial of different serum phosphate ranges in subjects on hemodialysis

Background

Hyperphosphataemia in dialysis subjects is associated with increased mortality. However cause and effect has not been proven, and the ideal phosphate target range is unknown despite KDOQI’s call for studies over 12 years ago. The design and conduct of a randomized controlled trial is challenging because maintaining two groups within differing target ranges of serum phosphate has not been achieved over a long follow-up of 1 year, in a trial setting, before. The SPIRiT study examines the subject acceptance, recruitment and retention rates for such a study in which subjects were randomised to two distinct serum phosphate concentrations, then titrated and maintained over 12 months.

Methods

A two center trial of 104 hemodialysis subjects randomized to lower range LRG 0.8–1.4 mmol/L or 2.5–4.3 mg/dL) and higher range (HRG 1.8–2.4 mmol/L or 5.6–7.4 mg/dL) serum phosphate groups. Two months’ titration and ten months’ maintenance phase. Interventions were non-calcium phosphate binders, self-help questionnaires, with blood tests at specified time intervals.

Results

Thirteen percent of the eligible dialysis population were successfully recruited. A mean separation by serum phosphate of 1.1 mg/dL was achieved and maintained between the groups over 10 months. Drop-out rate was 27% with mortality 10%. Nine subjects in the HRG (17.6%) and two subjects in the LRG (3.8%) died during the study, however the study was not powered to detect significant differences in outcomes.

Conclusion

Randomizing dialysis subjects to separate treatment targets for serum phosphate can achieve a clinically significant sustained separation over 12 months. A large scale longer term study is required to examine outcomes including mortality.

Trial registration

The trial registration number is ISRCTN24741445 – Date of registration 16th January, retrospectively registered.

Serum Phosphate Is Associated With Fracture Risk: The Rotterdam Study and MrOS

Extreme phosphate levels (P) have been associated with mineralization defects and increased fracture risk. Whether P within normal range is related to bone health in the general population is not well understood. To investigate the association of P with bone mineral density (BMD) and fracture risk, we assessed two population-based cohorts: the Dutch Rotterdam Study (RS-I, RS-II, RS-III; n = 6791) and the US Osteoporotic Fractures in Men (MrOS; n = 5425) study. The relationship of P with lumbar spine (LS) and femoral neck (FN) BMD was tested in all cohorts via linear models; fracture risk was tested in RS-I, RS-II, and MrOS through Cox models, after follow-up of 8.6, 6.6, and 10.9 years, respectively. Adjustments were made for age, body mass index, smoking, serum levels of calcium, potassium, 25-hydroxyvitamin D, estimated glomerular filtration rate (eGFR), FN-BMD, prevalent diabetes, and cardiovascular disease. Additional adjustments were made for phosphate intake, parathyroid hormone, and fibroblast growth factor 23 levels in MrOS. We further stratified by eGFR. Results were pooled through study-level meta-analyses. Hazard ratios (HR) and betas (β) (from meta-analyses) are expressed per 1 mg/dL P increase. P was positively associated with fracture risk in men and women from RS, and findings were replicated in MrOS (pooled HR all [95% CI]: 1.47 [1.31-1.65]). P was associated with fracture risk in subjects without chronic kidney disease (CKD): all (1.44 [1.26-1.63]) and in men with CKD (1.93 [1.42-2.62]). P was inversely related to LS-BMD in men (β: -0.06 [-0.11 to -0.02]) and not to FN-BMD in either sex. In summary, serum P was positively related to fracture risk independently from BMD and phosphate intake after adjustments for potential confounders. P and LS-BMD were negatively related in men. Our findings suggest that increased P levels even within normal range might be deleterious for bone health in the normal population. © 2017 American Society for Bone and Mineral Research.