Safety of New Phosphate Binders for Chronic Renal Failure

Evaluation of the In Vitro Efficacy of Sevelamer Hydrochloride and Sevelamer Carbonate

The objective of this project is to develop an in vitro approach that can be used to determine the phosphate binding capacity of sevelamer hydrochloride and carbonate for both drug products and active pharmaceutical ingredients (APIs). A simple and efficient inductively coupled plasma spectrometer method for analysis of phosphate at physiologically relevant pH conditions has been developed and validated. The method addresses each of the analytical validation characteristics such as linearity, accuracy, precision, stability, and selectivity, and meets the acceptance criteria defined in the United States Food and Drug Administration guidance (Food and Drug Administration, Center for Drug Evaluation and Research. 2001. Guidance for industry-Bioanalytical method validation, May). The in vitro phosphate binding efficacies were systematically evaluated and compared for two drug products and two APIs. The phosphate binding profiles appeared similar between the drug products. Under all conditions, the sevelamer-phosphate binding reached equilibrium at 6 h. The 90% confidence interval for the k2 ratio (sevelamer carbonate vs. sevelamer hydrochloride) was well within 80%-125% under all pH conditions. However, the k1 ratio varied, indicating that there exists difference in the binding affinity. Our findings will be useful in assisting with "in vivo" biowaiver for the approval of generic sevelamer drug products.

Inorganic phosphate homeostasis in sodium-dependent phosphate cotransporter Npt2b⁺/⁻ mice

An inorganic phosphate (P(i))-restricted diet is important for patients with chronic kidney disease and patients on hemodialysis. Phosphate binders are essential for preventing hyperphosphatemia and ectopic calcification. The sodium-dependent P(i) (Na/P(i)) transport system is involved in intestinal P(i) absorption and is regulated by several factors. The type II sodium-dependent P(i) transporter Npt2b is expressed in the brush-border membrane in intestinal epithelial cells and transports P(i). In the present study, we analyzed the phenotype of Npt2b(-/-) and hetero(+/-) mice. Npt2b(-/-) mice died in utero soon after implantation, indicating that Npt2b is essential for early embryonic development. At 4 wk of age, Npt2b(+/-) mice showed hypophosphatemia and low urinary P(i) excretion. Plasma fibroblast growth factor 23 levels were significantly decreased and 1,25(OH)(2)D(3) levels were significantly increased in Npt2b(+/-) mice compared with Npt2b(+/+) mice. Npt2b mRNA levels were reduced to 50% that in Npt2b(+/+) mice. In contrast, renal Npt2a and Npt2c transporter protein levels were significantly increased in Npt2b(+/-) mice. At 20 wk of age, Npt2b(+/-) mice showed hypophosphaturia and reduced Na/P(i) cotransport activity in the distal intestine. Npt2b(+/+) mice with adenine-induced renal failure had hyperphosphatemia and high plasma creatinine levels. Npt2b(+/-) mice treated with adenine had significantly reduced plasma P(i) levels compared with Npt2b(+/+) mice. Intestinal Npt2b protein and Na(+)/P(i) transport activity levels were significantly lower in Npt2b(+/-) mice than in the Npt2b(+/+) mice. The findings of the present studies suggest that Npt2b is an important target for the prevention of hyperphosphatemia.

Sevelamer carbonate for the treatment of hyperphosphatemia in patients with kidney failure (CKD III - V)

IMPORTANCE OF THE FIELD

Altered mineral metabolism in chronic kidney disease (CKD) is associated with increased morbidity, mortality, hospitalization, cost of care and reduced quality of life. Phosphorus control, one component of CKD metabolic derangements, is potentially related to impaired outcomes and has significant room for improvement.

AREAS COVERED IN THIS REVIEW

Historical, present and future aspects of treatment of hyperphosphatemia focusing on sevelamer hydrochloride and sevelamer carbonate.

WHAT THE READER WILL GAIN

Comprehensive insight into the background and controversies regarding phosphate binders.

TAKE HOME MESSAGE

While calcium-free phosphate binders with a sevelamer backbone may offer therapeutic advantages for CKD patients at risk, more studies comprising significant patient numbers are warranted to answer compelling clinical questions.

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.

Treatment Options for Hyperphosphatemia in Feline CKD: What's Out there?

Practical relevance Phosphorus is retained in chronic kidney disease (CKD), promoting renal secondary hyperparathyroidism and eventually resulting in hyperphosphatemia. Most agree that phosphate retention is a major contributor to the progression of CKD in many species and it is well known that hyperphosphatemia is associated with a significant mortality risk in humans with end-stage renal disease.

Patient group Chronic kidney disease is a common ailment of geriatric cats.

Evidence base There is evidence in cats suggesting that the use of a phosphate-restricted diet in IRIS stage 2–3 disease has a beneficial effect on clinical outcome. However, despite the fact that intestinal phosphate binders are commonly used in veterinary practice for patients with CKD, there have been few published reports focusing on the safety and efficacy of these products in veterinary medicine. No phosphorus binders are licensed as medications for dogs or cats. This article draws on data from clinical trials in humans and studies in cats to discuss treatment goals and options for phosphate retention and hyperphosphatemia in feline CKD.

Clinical significance With careful monitoring of serum phosphate and parathyroid hormone, and implementation of phosphate-restricted dietary management and intestinal phosphate binders, progression of CKD and the degree of hyperparathyroidism in cats may be reduced.

Audience Companion animal and feline practitioners are at the forefront in the management of CKD in cats.

Comparison of sevelamer hydrochloride and sevelamer carbonate: risk of metabolic acidosis and clinical implications

Hyperphosphatemia is highly prevalent in patients with chronic kidney disease (CKD), particularly in those with advanced or end-stage renal disease. Sevelamer hydrochloride is an ion-exchange resin that reduces serum phosphorus concentrations. The agent also produces favorable lipid profile effects and does not cause hypercalcemia. However, reported drawbacks of this agent are metabolic acidosis, high pill burden, and a relatively low affinity and selectivity for phosphate anions. Sevelamer carbonate is a new buffered formulation that does not increase the risk of metabolic acidosis. To determine the roles of these two agents in the treatment of hyperphosphatemia in patients with CKD, we performed a MEDLINE search (June 1995-June 2008) focusing on the mechanism of action of resin binding with phosphate and the development of metabolic acidosis. We also reviewed studies that evaluated the effects of sevelamer hydrochloride or sevelamer carbonate on serum bicarbonate concentrations. Several studies in patients with CKD and hyperphosphatemia who received hemodialysis or peritoneal dialysis found decreases in serum bicarbonate concentrations with the use of sevelamer hydrochloride, whereas sevelamer carbonate did not have this negative effect on bicarbonate concentrations. Both drugs appear to be equivalent in their abilities to lower serum phosphorus concentrations. However, as sevelamer carbonate does not decrease serum bicarbonate levels, it may be more appropriate for patients at risk for metabolic acidosis who require phosphate binders that do not contain calcium or aluminum.

Sevelamer hydrochloride: a review of its use for hyperphosphataemia in patients with end-stage renal disease on haemodialysis

Sevelamer (Renagel), an orally administered metal-free cationic hydrogel polymer/resin that binds dietary phosphate in the gastrointestinal (GI) tract, is approved for use in the US, Europe and several other countries for the treatment of hyperphosphataemia in adult patients with end-stage renal disease (ESRD) on haemodialysis or peritoneal dialysis.Clinical evidence shows that sevelamer was at least as effective as calcium acetate and calcium carbonate at controlling serum phosphorus, calcium-phosphorus product (Ca x P) and intact parathyroid hormone (iPTH) levels, but generally reduced serum calcium levels to a greater extent and was associated with a lower risk of hypercalcaemic episodes than calcium-based phosphate binders. Sevelamer appeared to slow the progression of cardiovascular calcification in patients with ESRD and also had a beneficial effect on serum low-density lipoprotein-cholesterol (LDL-C) levels. In patients receiving chronic haemodialysis, there was no between-group difference in all-cause mortality between sevelamer and calcium-based phosphate binder therapy in the primary efficacy analysis in the large (n >2100), 3-year DCOR trial; in the smaller (n = 109) nonblind RIND trial in patients new to dialysis, data suggest there is an overall survival benefit with sevelamer versus calcium-based phosphate binder treatment. The relative survival benefits and cost effectiveness of these phosphate binder therapies remains to be fully determined. Sevelamer treatment was generally as well tolerated as calcium acetate or calcium carbonate treatment. Overall, sevelamer is a valuable option for the management of hyperphosphataemia in patients with ESRD on haemodialysis.

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.

Accumulation of Metals and Minerals from Phosphate Binders

Metals and minerals that depend on renal clearance may accumulate to toxic levels in patients with marginal kidney function. Toxicities of aluminum-based phosphate binders became apparent approximately 25 years ago. Nephrologists now recognize cardiovascular calcification may follow use of calcium-based phosphate binders. Five lessons can be learned: (1)safety must not be assumed in absence of data; (2) all evidence for causal linkage of toxicities from therapeutics must be considered, including animal data; (3) clinical trials are unlikely to reveal the spectrum of problems from long-term drug exposure; (4) complications can remain unrecognized until late in post-introduction surveillance; (5) minerals important for normal function can be toxic with excess accumulation. Introduction of new agents necessitates caution - it is difficult to change practice once a therapeutic is commonplace. Lessons learned about hazards of past phosphate binders must be applied judiciously when evaluating long-term risks/safety of novel metal-based binders such as lanthanum carbonate.

Veterinary hemodialysis: advances in management and technology

Hemodialysis (HD) is a renal replacement therapy that can enable recovery of patients in acute kidney failure and prolong survival for patients with end-stage kidney failure. HD is also uniquely suited for management of refractory volume overload and removal of certain toxins from the bloodstream. Over the last decade, veterinary experience with HD has deepened and refined and its geographic availability has increased. As awareness of the usefulness and availability of dialytic therapy increases among veterinarians and pet owners and the number of veterinary dialysis facilities increases, dialytic management will become the standard of advanced care for animals with severe intractable uremia.