Control of predialytic hyperphosphatemia by oral calcium acetate and calcium carbonate. Comparable efficacy for half the dose of elemental calcium given as acetate without lower incidence of hypercalcemia

Design and baseline characteristics of the LANDMARK study

Background

Calcium (Ca)-based phosphate (P) binders, compared to non-Ca-based P binders, contribute to vascular calcification, which is associated with cardiovascular events.

Methods

The LANDMARK study is a multicenter, randomized, open-label, parallel comparative study of lanthanum carbonate (LC) and calcium carbonate (CC) in hemodialysis patients. Stable hemodialysis patients with intact parathyroid hormone ≤240 pg/mL meeting ≥1 of the following criteria (age >65 years, postmenopause, diabetes mellitus) were randomized into the LC and CC groups. LC group patients initially received LC 750 mg/day or the previously used dose and were titrated up to a maximum 2250 mg/day to achieve serum P levels of 3.5–6.0 mg/dL. CC group patients received CC 3 g/day or the previously used dose and were titrated to achieve the same P range. If the target serum P level was not achieved, non-Ca-based P binders (other than LC) could also be added. The primary endpoint is survival time free of cardiovascular events, including cardiovascular death, non-fatal myocardial infarction or stroke, and unstable angina.

Results

Overall, 2309 patients were allocated to the LC (N = 1154) or CC group (N = 1155). At baseline, the mean age was 68.4 years, 40.4 % were women, 55.9 % had diabetes, 18.3 % had a history of ischemic heart disease, and 13.9 % had cerebrovascular disease. A total of 184 patients (8.4 %) had undergone coronary intervention procedures. Baseline characteristics were well balanced between groups.

Conclusions

The LANDMARK study will determine whether LC, a non-Ca-based P binder, reduces cardiovascular mortality and morbidity in chronic hemodialysis patients.

PHOSPHORUS METABOLISM AND MANAGEMENT IN CHRONIC KIDNEY DISEASE: Phosphate Metabolism in the Setting of Chronic Kidney Disease: Significance and Recommendations for Treatment

Phosphorus is an essential mineral that plays a crucial role in cell structure and metabolism. In living organisms, phosphorus exists surrounded by four oxygen atoms to form phosphate (PO(4)). Within cells, PO(4) regulates enzymatic activity and serves as an essential component of nucleic acids, adenosine triphosphate, and phospholipid membranes. Outside cells, PO(4) primarily resides in bone and teeth as hydroxyapatite. A small amount of inorganic PO(4) circulates in serum, with levels balanced by gastrointestinal intake, renal excretion, and a set of specific hormones. Under normal conditions, PO(4) is excreted through the kidneys. Among patients with end stage renal disease (ESRD) receiving chronic dialysis, circulating PO(4) levels typically rise to levels well above the normal laboratory range. Higher serum PO(4) levels are strongly associated with arterial calcification and mortality in this setting. Among predialysis patients with chronic kidney disease (CKD), phosphaturic hormones enhance renal PO(4) excretion to maintain serum PO(4) levels within the high-normal laboratory range. Recently, high-normal serum PO(4) levels have been associated with cardiovascular (CV) events and mortality among individuals who have CKD and among those who have normal kidney function. This review discusses PO(4) metabolism in the context of CKD, examines associations of PO(4) levels with adverse outcomes in the CKD setting, and suggests treatment strategies for moderating serum PO(4) levels.

Emerging strategies for lowering serum phosphorous in patients with end-stage renal disease

Hyperphosphataemia is a major problem in patients with chronic kidney disease as it has been associated with increased morbidity and mortality. Over the last four decades, different modalities have been applied to treat hyperphosphataemia with varying degrees of success. Unfortunately, treatment strategies have led to unforeseen complications. These have prompted the development of new classes of medications with potentially better efficacy and few short-term and long-term side effects. This article reviews the causes, mechanism and management of hyperphosphataemia.

The safety of phosphate binders

Disturbances of mineral metabolism occur during the early stages of chronic kidney disease. As renal function worsens, excess dietary phosphorus accumulates and blood levels increase, that can be clearly seen when the glomerular filtration rate has fallen below 30 ml/min/1.73 m2. In patients with end stage renal disease, standard dialysis (three times/week) falls far short of removing adequate amounts of absorbed phosphorus; therefore, hyperphosphataemia is found in the majority of these patients. Hyperphosphataemia has long been associated with progression of secondary hyperparathyroidism and renal osteodystrophy, it can also lead to soft-tissue and vascular calcification. Recent observational data have associated hyperphosphataemia with increased cardiovascular mortality among dialysis patients. Adequate control of serum phosphorus remains a cornerstone in the clinical management and, despite the growing amount of available therapeutic options, achievement of NFK/KDOQI targets for mineral metabolism remain poor. Several reasons may explain the failure to adequately treat hyperphosphataemia: poor compliance with diet and phosphate binder prescriptions are common causes. Also, factors related with cost, tolerance, palatability, safety and efficacy are important. In this article, the authors review the advantages and drawbacks of conventional and emerging therapies in phosphorous binding.

Hyperphosphatemia and phosphate binders

PURPOSE

The pathophysiology of hyperphosphatemia associated with end-stage renal disease and treatment with phosphate binders are discussed.

SUMMARY

Phosphorus is an essential element necessary for the normal function of the human body, required for skeletal construction and synthesis of DNA, proteins, and adenosine triphosphate. In healthy individuals, serum phosphorus concentrations are maintained between 2.5 and 4.5 mg/dL through diet and renal excretion. In renal insufficiency, phosphorus excretion declines and hyperphosphatemia develops. The body's compensation mechanisms cause secondary hyperparathyroidism and renal osteodystrophy. Phosphate binders provide an effective means for managing serum phosphate. Commercially available phosphate binders include calcium carbonate, calcium acetate, sevelamer, lanthanum, and, rarely, aluminum hydroxide. Because of aluminum's known toxicities, aluminum-based phosphate binders have a limited place in therapy. Calcium carbonate's benefits are seen over a narrow gastric pH range, thereby limiting the drug's utility. Calcium acetate is effective over a wide pH range. Other phosphate binders, including sevelamer hydrochloride and lanthanum carbonate, have recently entered the market, but their use remains controversial.

CONCLUSION

If left untreated, hyperphosphatemia can result in secondary hyperparathyroidism, renal osteodystrophy, and metastatic calcification of blood vessels and soft tissue. The treatment of hyperphosphatemia in patients with chronic renal failure includes dialysis, dietary phosphorus restrictions, phosphate-binding medications, and vitamin D analogs. Selection of phosphate binders should be based on patient characteristics, including serum phosphate, serum calcium, and intact parathyroid hormone concentrations, and patient tolerability.

A comparison of clinically useful phosphorus binders for patients with chronic kidney failure

A comparison of clinically useful phosphorus binders for patients with chronic kidney failure. Over the past 30 years it has become apparent that hyperphosphatemia plays a major causative role across the entire spectrum of morbidity associated with advancing kidney dysfunction and failure. A large fraction (60% to 70%) of dietary phosphorus is absorbed and normally excreted by the kidneys. Ideally, as kidney function deteriorates, the net quantity of phosphorus absorbed from the GI tract should be proportionally reduced to match the decrease in kidney function. After initiation of chronic dialysis therapy, the absorbed phosphorus load should match the amount of phosphorus removed via dialysis plus any excreted by residual kidney function. Because it is very difficult to reduce dietary phosphorus to these levels, a variety of oral phosphorus binders have been employed. Currently available binders include alkaline aluminum, magnesium, and calcium salts (primarily calcium carbonate and calcium acetate), various iron salts, and the binding resin sevelamer hydrochloride. Lanthanum carbonate is the newest agent and will probably be released shortly. This review compares the theoretic and in vitro chemistry of these drugs with in vivo data obtained in both normal patients, and in patients with kidney failure. The clinical potency and potential toxicity of the binding agents are compared, and optimal drug administration strategies are also reviewed.

Safety of New Phosphate Binders for Chronic Renal Failure

Phosphate (Pi) retention is a common problem in patients with chronic kidney disease, particularly in those who have reached end-stage renal disease (ESRD). In addition to causing secondary hyperparathyroidism and renal osteodystrophy, recent evidence suggests that, in ESRD patients, high serum phosphorus concentration and increased calcium and phosphorous (Ca x P) product are associated with vascular and cardiac calcifications and increased mortality. Dietary phosphorus restriction and Pi removal by dialysis are not sufficient to restore Pi homeostasis. Reduction of intestinal Pi absorption with the use of Pi binders is currently the primary treatment for Pi retention in patients with ESRD. The use of large doses of calcium-containing Pi binders along with calcitriol administration may contribute to over-suppression of parathyroid hormone secretion and adynamic bone disease as well as to a high incidence of vascular calcifications. When used in patients with impaired renal function, aluminium salts were found to accumulate in bone and other tissues, resulting in osteomalacia and encephalopathy.Sevelamer, an aluminium- and calcium-free Pi binder can reduce serum phosphorus concentration and is associated with a significantly lower incidence of hypercalcaemia, while maintaining the ability to suppress parathyroid hormone production. An additional benefit of sevelamer is its ability to lower low density lipoprotein-cholesterol and total cholesterol levels. Sevelamer attenuates the progression of vascular calcifications in haemodialysis patients, which may lead to lower mortality. The use of sevelamer in non-dialysed patients might aggravate metabolic acidosis, common in these patients. Several other calcium-free Pi binders are in development. Lanthanum carbonate has shown significant promise in clinical trials in ESRD patients. Magnesium salts do not offer a significant advantage over currently available Pi binders. Their use is restricted to patients receiving dialysis since excess magnesium must be removed by dialysis. Iron-based compounds have shown variable efficacy in short-term clinical trials in small numbers of haemodialysis patients. Mixed metal hydroxyl carbonate compounds have shown efficacy in animals but have not been studied in humans. Major safety issues include absorption of the metal component with possible tissue accumulation and toxicity.

Hyperphosphatemia in end-stage renal disease

Hyperphosphatemia occurs universally in end-stage renal disease (ESRD) unless efforts are made to prevent positive phosphate balance. Positive phosphate balance results from the loss of renal elimination of phosphate and continued obligatory intestinal absorption of dietary phosphate. Increased efflux of phosphate from bone because of excess parathyroid hormone-mediated bone resorption can also contribute to increased serum phosphate concentrations in the setting of severe hyperparathyroidism. It is important to treat hyperphosphatemia because it contributes to the pathogenesis of hyperparathyroidism, vascular calcifications, and increased cardiovascular mortality in ESRD patients. Attaining a neutral phosphate balance, which is the key to the management of hyperphosphatemia in ESRD, is a challenge. Control of phosphorus depends on its removal during dialysis and the limitation of gastrointestinal absorption by dietary phosphate restriction and chelation of phosphate. Knowledge of the quantitative aspects of phosphate balance is useful in optimizing our use of phosphate binders, dialysis frequency, and vitamin D sterols. The development of new phosphate binders and efforts to find new ways to inhibit gastrointestinal absorption of phosphate will lead to improvements in the control of serum phosphate levels in ESRD.

Calcium acetate versus calcium carbonate in the control of hyperphosphatemia in hemodialysis patients

CONTEXT

Hyperphosphatemia has an important role in the development of bone and mineral abnormalities in end-stage renal disease (ESRD).

OBJECTIVE

To compare the phosphorus binding power and the hypercalcemic effect of calcium acetate and calcium carbonate in hemodialysis patients.

TYPE OF STUDY

Crossover, randomized, double-blind study.

PLACE

A private hospital dialysis center.

PARTICIPANTS

Fifty-two patients who were undergoing regular hemodialysis three times a week ([Ca++] dialysate = 3.5 mEq/L).

PROCEDURES

Half of the patients were started on 5.6 g/day of calcium acetate and, after a 2 week washout period, received 6.2 g/day of calcium carbonate. The other half followed an inverse protocol.

MAIN MEASUREMENTS

Clinical interviews were conducted 3 times a week to monitor for side effects. Determinations of serum urea, calcium, phosphorus, hematocrit, Kt/V and blood gas analysis were obtained before and after each treatment.

RESULTS

Twenty-three patients completed the study. A significant increase in calcium plasma levels was only observed after treatment with calcium carbonate [9.34 mg/dl (SD 0.91) vs. 9.91 mg/dl (SD 0.79), P < 0.01]. The drop in phosphorus levels was substantial and significant for both salts [5.64 mg/dl (SD 1.54) vs. 4.60 mg/dl (SD 1.32), P < 0.01 and 5.89 mg/dl (SD 1.71) vs. 4.56 mg/dl (SD 1.57), P < 0.01, for calcium acetate and calcium carbonate respectively]. The percentage reduction in serum phosphorus (at the end of the study) per milliequivalent of salt administered per day tended to be higher with calcium acetate but statistical significance was not found.

CONCLUSION

Calcium acetate can be a good alternative to calcium carbonate in the handling of hyperphosphatemia in ESRD patients. When calcium acetate is used, control of hyperphosphatemia can be achieved with a lower administration of calcium, perhaps with a lower risk of hypercalcemia.

Phosphate-binding capacity of ferrihydrite versus calcium acetate in rats

Calcium salts, such as calcium carbonate and calcium acetate, are the principal compounds used as phosphate binders in patients with chronic renal failure. The dose required is three to six times the normal requirement for calcium. Use of these large doses of calcium salts in the diet can result in hypercalcemia. Other compounds have been investigated as phosphate binders with varying degrees of success. Synthetic ferrihydrite (5Fe(2)O(3).9H(2)O) has a high adsorptive capacity for phosphate and may be an effective phosphate binder. The phosphate-binding capacity of ferrihydrite was compared with that of calcium acetate in 250-g male Sprague Dawley rats. After an overnight fast, rats (n = 5 per group) were gavaged with an American Institute of Nutrition (AIN) 76 formula containing one third the daily phosphorus intake labeled with phosphorus-32 ((32)P). Either two levels of calcium acetate, representing three (1/2X) or six (1X) times the usual calcium intake for one third of the day, or equivalent amounts of ferrihydrite were added to the diet. An additional group received two times (2X) the larger dose, and a sixth control group received no binder in the diet. Phosphorus absorption curves were determined from (32)P appearance in the serum. The 1/2X dose of ferrihydrite reduced (32)P by approximately one half, and the 2X dose nearly completely suppressed (32)P absorption, similar to the 1X dose of calcium acetate. The advantage of using a ferrihydrite binder would be to avoid the hypercalcemia resulting from the use of high-dose calcium salts. An added advantage may result from the small amounts of iron absorbed in these chronically iron-deficient patients.

Role of calcium carbonate administration timing in relation to food intake on its efficiency in controlling hyperphosphatemia in patients on maintenance dialysis

A study has claimed that at an equal elemental calcium dose, CaCO3 was not less but equally as efficient in controlling predialysis hyperphosphatemia as calcium acetate, provided both calcium salts were ingested 5 min before meals instead of during meals because the higher acidity of the fasting gastric juice would allow for better dissociation of CaCO3. However, this study did not directly demonstrate that the efficiency of CaCO3 in controlling hyperphosphatemia was actually greater when it was administered before a meal than during a meal. To examine this point, we performed a 3 month randomized crossover trial in 12 reliable and stable patients maintained on chronic hemodialysis. Their plasma concentrations of calcium, protein, phosphate, bicarbonate, urea, and creatinine were measured before the first dialysis of each week and the amount of intact parathyroid hormone (PTH) at the beginning and at the end of each of the 3 months. Comparison of the plasma concentrations measured during the 2 modes of administration showed no significant differences in creatinine, urea, bicarbonate, or intact PTH. The mean (+/-SD) plasma concentration of PO4 was not significantly lower (1.88+/-0.50 vs. 1.74+/-0.41 mM) whereas the corrected level of plasma Ca was significantly lower (2.30+/-0.17 vs. 2.38+/-0.16 mM; p < 0.04) when CaCO3 was given before meals than during meals. In conclusion, the administration of CaCO3 before a meal does not increase its efficiency in controlling hyperphosphatemia because the level of plasma PO4 was actually slightly higher with this timing of administration whereas the comparison of the creatinine and urea levels suggested a stability of phosphate intake and the comparison of the PTH and bicarbonate levels suggested the stability of osteolysis and of the transcellular membrane shift of phosphate. Also, administration of CaCO3 before a meal is associated with significantly lower plasma corrected calcium, suggesting less absorption of calcium, which may be an advantage but only in hypercalcemic patients. There is no reason other than the prevention of its hypercalcemic effect to recommend the administration of CaCO3 just before meals rather than during meals.

Aluminum exposure and metabolism

Aluminum (Al) is a nonessential, toxic metal to which humans are frequently exposed. Oral exposure to aluminum occurs through ingestion of aluminum-containing pharmaceuticals and to a lesser extent foods and water. Parenteral exposure to aluminum can occur via contaminated total parenteral nutrition (TPN), intravenous (i.v.) solutions, or contaminated dialysates. Inhalation exposure may be important in some occupational settings. The gut is the most effective organ in preventing tissue aluminum accumulation after oral exposure. Typically gastrointestinal absorption of aluminum from diets is < 1%. Although the mechanisms of aluminum absorption have not been elucidated, both passive and active transcellular processes and paracellular transport are believed to occur. Aluminum and calcium may share some absorptive pathways. Aluminum absorption is also affected by the speciation of aluminum and a variety of other substances, including citrate, in the gut milieu. Not all absorbed or parenterally delivered aluminum is excreted in urine. Low glomerular filtration of aluminum reflects that most aluminum in plasma is nonfiltrable because of complexation to proteins, predominantly transferrin. The importance of biliary secretion of aluminum is debatable and the mechanism(s) is poorly understood and appears to be saturable by fairly low oral doses of aluminum.

Calcium acetate versus calcium carbonate as oral phosphate binder in pediatric and adolescent hemodialysis patients

Calcium carbonate is widely used as an oral phosphorus binder to control hyperphosphatemia in children on maintenance hemodialysis. Intestinal calcium absorption may induce hypercalcemia, particularly if calcitriol is given simultaneously. In adults, calcium acetate binds phosphorus more effectively than calcium carbonate, while reducing the frequency of hypercalcemic events. We therefore compared calcium acetate with calcium carbonate in nine pediatric patients on long-term maintenance hemodialysis. Following a 1-week withdrawal of phosphorus binders, calcium carbonate was administered for 7 weeks; after a second withdrawal, calcium acetate was given for another 7 weeks. All patients received calcitriol regularly. Both agents lowered the serum phosphorus concentration significantly (calcium carbonate 5.7 +/- 1.4 vs. 7.7 +/- 2.1 mg/ dl, P < 0.005; calcium acetate 5.8 +/- 1.4 vs. 7.8 +/- 2.0 mg/dl, P < 0.005). Significantly less elementary calcium was ingested with calcium acetate than with calcium carbonate: 750 (375-1,500) vs. 1,200 (0-3,000) mg calcium/day, P < 0.0001. Wit calcium carbonate serum calcium increased significantly. The number of episodes of hyperphosphatemia or hypercalcemia did not differ between treatments. Intact plasma parathyroid hormone (PTH) decreased significantly with both phosphate binders, and serum 25-hydroxyvitamin D3 increased. There was a close relationship between serum phosphorus and PTH in prepubertal but not in pubertal patients. We conclude that hyperphosphatemia can be controlled effectively by both calcium acetate and calcium carbonate in pediatric hemodialysis patients. The oral load of elementary calcium is reduced significantly by binding phosphorus with calcium acetate instead of calcium carbonate; nevertheless, hypercalcemic episodes remain equally frequent with both phosphate binders.

Magnesium carbonate as a phosphorus binder: a prospective, controlled, crossover study

The use of calcium carbonate (CaCO3) to bind phosphorus (P) in chronic hemodialysis patients has been a popular tactic in the past decade. Nonetheless, problems with hypercalcemia decrease its usefulness, particularly in patients treated with calcitriol. A P binder not containing calcium (Ca) would be of value in these circumstances. In short-term studies, we showed that magnesium carbonate (MgCO3) was well-tolerated and controlled P and Mg levels when given in conjunction with a dialysate Mg of 0.6 mg/dl. We, therefore, performed a prospective, randomized, crossover study to evaluate if the chronic use of MgCO3 would allow a reduction in the dose of CaCO3 and yet achieve acceptable levels of Ca, P, and Mg. We also assessed whether the lower dose of CaCO3 would facilitate the use of larger doses of calcitriol. The two phases were MgCO3 plus half the usual dose of CaCO3 and CaCO3 alone given in the usual dose. It was found that MgCO3 (dose, 465 +/- 52 mg/day elemental Mg) allowed a decrease in the amount of elemental Ca ingested from 2.9 +/- 0.4 to 1.2 +/- 0.2 g/day (P < 0.0001). The Ca, P, Mg levels were the same in the two phases. The maximum dose of i.v. calcitriol without causing hypercalcemia was 1.5 +/- 0.3 micrograms/treatment during the MgCO3 phase and 0.8 +/- micrograms/treatment during the Ca phase (P < 0.02). If these studies are confirmed, the use of MgCO3 and a dialysate Mg of 0.6 mg/dl may be considered in selected patients who develop hypercalcemia during treatment with i.v. calcitriol and CaCO3.