Hyperphosphatemia in end-stage renal disease

Medical and Surgical Treatment for Secondary and Tertiary Hyperparathyroidism

Prevention and treatment of secondary hyperparathyroidism (SHPT) in patients on chronic maintenance hemodialysis and of tertiary hyperparathyroidism (THPT) in patients after kidney transplantation is a challenge for the nephrologist and for the surgeon. Indication and results of medical and surgical therapy for SHPT and THPT have remained under discussion during the last decades. This review resumes the current medical and surgical strategies for patients with SHPT and THPT.

The role of the gastrointestinal tract in phosphate homeostasis in health and chronic kidney disease

Purpose of review

For a number of years, there has been increasing interest in the concept of directly targeting intestinal phosphate transport to control hyperphosphatemia in chronic kidney disease. However, progress has been slow due to the paucity of information on the mechanisms involved in intestinal phosphate absorption. This editorial highlights the most recent developments in our understanding of this process and the role of the intestine in the maintenance of phosphate balance.

Recent findings

Recent studies in NaPi-IIb knockout mice have confirmed that this transport protein plays a significant role in intestinal phosphate absorption and is critical in the proposed feed-forward mechanism between the small intestine and kidney, which helps to maintain normal phosphate balance and steady-state plasma phosphate concentrations. In addition, renal failure-induced hyperphosphatemia is attenuated in NaPi-IIb knockout mice, confirming that NaPi-IIb is a suitable target in the prevention and treatment of hyperphosphatemia.

Summary

Recent findings suggest that consumption of processed foods containing phosphate preservatives may lead to excessive phosphate exposure (if not overload), toxicity, and cardiovascular disease in the general population, as well as in patients with declining renal function. Therefore, establishing more effective ways of targeting the intestine to limit dietary phosphate absorption could have wide-reaching health benefits.

Phosphate homeostasis and the renal-gastrointestinal axis

Transport of phosphate across intestinal and renal epithelia is essential for normal phosphate balance, yet we know less about the mechanisms and regulation of intestinal phosphate absorption than we do about phosphate handling by the kidney. Recent studies have provided strong evidence that the sodium-phosphate cotransporter NaPi-IIb is responsible for sodium-dependent phosphate absorption by the small intestine, and it might be that this protein can link changes in dietary phosphate to altered renal phosphate excretion to maintain phosphate balance. Evidence is also emerging that specific regions of the small intestine adapt differently to acute or chronic changes in dietary phosphate load and that phosphatonins inhibit both renal and intestinal phosphate transport. This review summarizes our current understanding of the mechanisms and control of intestinal phosphate absorption and how it may be related to renal phosphate reabsorption; it also considers the ways in which the gut could be targeted to prevent, or limit, hyperphosphatemia in chronic and end-stage renal failure.

Intestinal phosphate absorption in a model of chronic renal failure

Hyperphosphatemia is an important consequence of chronic renal failure (CRF). Lowering of the plasma phosphate concentration is believed to be critical in the management of patients with CRF, especially those on dialysis. Reports of the effect of CRF on the intestinal handling of phosphate in vitro have been conflicting; but what happens in vivo has not been studied. What effect a reduction in the dietary phosphate intake has on intestinal phosphate absorption in CRF in vivo is unclear. In this study, we have used the in situ intestine loop technique to determine intestinal phosphate absorption in the 5/6-nephrectomy rat model of CRF under conditions of normal and restricted dietary phosphate intake. In this model of renal disease, we found that there is no significant change in the phosphate absorption in either the duodenum or jejunum regardless of the dietary phosphate intake. There was also no change in the expression of the messenger RNA of the major intestinal phosphate carrier the sodium-dependent-IIb transporter. Furthermore, we found no change in the intestinal villus length or in the location of phosphate uptake along the villus. Our results indicate that in CRF, unlike the kidney, there is no reduction in phosphate transport across the small intestine. This makes intestinal phosphate absorption a potential target in the prevention and treatment of hyperphosphatemia.

Newer Paradigms in Renal Replacement Therapy: Will They Alter Cardiovascular Outcomes?

Cardiovascular disease remains the leading cause of morbidity and mortality for patients with end-stage renal disease. Conventional hemodialysis has had limited impact on cardiovascular risk factors and mortality. Increasing evidence suggests that nocturnal home hemodialysis has beneficial effects on cardiovascular parameter outcomes. This article reviews the documented effects of nocturnal home hemodialysis on blood pressure control, cardiac geometry and left ventricular systolic function, lipid profiles, calcium-phosphate metabolism, parathyroid hormone levels, homocysteine levels, sleep apnea, and autonomic modulation of heart rate. It discusses possible mechanisms to explain these observed changes.

Efficacy, Tolerability, and Safety of Lanthanum Carbonate in Hyperphosphatemia: A 6-Month, Randomized, Comparative Trial versus Calcium Carbonate

BACKGROUND/AIMS

Hyperphosphatemia is an important clinical consequence of renal failure, and its multiple adverse systemic effects are associated with significantly increased risks of morbidity and mortality in dialysis patients. Existing oral phosphate binders have not permitted control of serum phosphate within currently accepted guidelines. This study compares lanthanum carbonate with calcium carbonate for control of serum phosphate in hemodialysis patients.

METHODS

In this European multicentre study, 800 patients were randomised to receive either lanthanum or calcium carbonate and the dose titrated over 5 weeks to achieve control of serum phosphate. Serum levels of phosphate, calcium and parathryoid hormone were followed over the following 20 weeks.

RESULTS

Around 65% of patients in each group achieved phosphate control, but in the calcium carbonate group this was at the expense of significant hypercalcemia (20.2% of patients vs. 0.4%). Consequently, calcium x phosphate product tended to be better controlled in the lanthanum group.

CONCLUSION

This 6-month study demonstrates that serum phosphate control with lanthanum carbonate (750-3,000 mg/day) is similar to that seen with calcium carbonate (1,500-9,000 mg/day), but with a significantly reduced incidence of hypercalcemia. Lanthanum carbonate is well tolerated and may be more effective in reducing calcium x phosphate product than calcium carbonate.

DAILY HEMODIALYSIS-SELECTED TOPICS: Cardiovascular Effects of Frequent Intensive Hemodialysis

Cardiovascular disease remains the leading cause of morbidity and mortality for patients with end-stage renal disease (ESRD). Frequent intensive hemodialysis (short daily hemodialysis [2 hours per session, six sessions per week] and nocturnal home hemodialysis [6 hours per session, five to six sessions per week]) has recently gained increasing popularity as an alternative to conventional hemodialysis (4 hours per session, three sessions per week). There is an emerging body of evidence that frequent intensive hemodialysis offers superior uremic toxin clearance, blood pressure control, and other cardiovascular outcomes. The goals of the present review are to systematically evaluate the available evidence in blood pressure control and cardiovascular outcomes in ESRD and the achievable changes after converting from conventional dialysis to frequent intensive hemodialysis, and to provide possible physiological explanations to account for these important changes of potent markers of adverse events in this patient population.

Lanthanum Carbonate

Lanthanum carbonate is a novel, non-aluminium, non-calcium phosphate binding agent that forms a water-insoluble compound, lanthanum phosphate, in the gut. Lanthanum carbonate (elemental lanthanum 375-3000 mg/day) reduced serum phosphorus levels compared with placebo in two randomised, double-blind, multicentre 4-week trials in patients with chronic renal failure receiving regular haemodialysis. In two large, randomised trials in patients with chronic renal failure requiring haemodialysis, lanthanum carbonate (elemental lanthanum 375-3000 mg/day) was as effective as calcium carbonate and/or other conventional phosphate binders in reducing and maintaining serum phosphorus levels (< or =5.6 mg/dL over 6 months and < or =5.9 mg/dL over 2 years). Lanthanum carbonate was generally well tolerated. Most adverse events were mild-to-moderate in severity, with gastrointestinal events being the most common. The tolerability profile of lanthanum carbonate was similar to those of conventional phosphate binders; however, hypercalcaemic episodes occurred significantly less frequently over 6 months with lanthanum carbonate than with calcium carbonate. In a randomised 1-year trial, numerically fewer lanthanum carbonate (elemental lanthanum < or =3750 mg/day) recipients had renal bone disease at study end than at baseline; however, in the calcium carbonate < or =9000 mg/day group, numerically more patients had renal bone disease at study end compared with baseline.

The effects of sevelamer and calcium acetate on proxies of atherosclerotic and arteriosclerotic vascular disease in hemodialysis patients

BACKGROUND

We recently determined that in hemodialysis patients, the use of calcium salts to correct hyperphosphatemia led to progressive coronary artery and aortic calcification as determined by sequential electron beam tomography (EBT) while the use of the non-calcium-containing binder sevelamer did not. Whether the specific calcium preparation (acetate vs. carbonate) might influence the likelihood of progressive calcification was debated.

METHODS

To determine whether treatment with calcium acetate was specifically associated with hypercalcemia and progressive vascular calcification, we conducted an analysis restricted to 108 hemodialysis patients randomized to calcium acetate or sevelamer and followed for one year.

RESULTS

The reduction in serum phosphorus was roughly equivalent with both agents (calcium acetate -2.5 +/- 1.8 mg/dl vs. sevelamer -2.8 +/- 2.0 mg/dl, p = 0.53). Subjects given calcium acetate were more likely to develop hypercalcemia (defined as an albumin-corrected serum calcium > or =10.5 mg/dl) (36 vs. 13%, p = 0.015). Treatment with calcium acetate (mean 4.6 +/- 2.1 g/day - equivalent to 1.2 +/- 0.5 g of elemental calcium) led to a significant increase in EBT-determined calcification of the coronary arteries (mean change 182 +/- 350, median change +20, p = 0.002) and aorta (mean change 181 +/- 855, median change +73, p < 0.0001). These changes were similar in magnitude to those seen with calcium carbonate. There were no significant changes in calcification among sevelamer-treated subjects.

CONCLUSION

Despite purported differences in safety and efficacy relative to calcium carbonate, calcium acetate led to hypercalcemia and progressive vascular calcification in hemodialysis patients.