Beneficial effects of calcimimetics on progression of renal failure and cardiovascular risk factors

Impact of parathyroidectomy on renal graft function, blood pressure and serum lipids in kidney transplant recipients: a single centre study

BACKGROUND

Successful kidney transplantation is believed to reverse secondary hyperparathyroidism, but persistent disease has emerged in a significant number of allograft recipients. Parathyroid hormone (PTH) is not only involved in the aetiology of calcium/phosphate abnormalities and osteitis fibrosa, but it is also a permissive factor in the occurrence of hypertension, cardiovascular damage and dyslipidaemia. In experimental renal failure, abrogation of hyperparathyroidism by administration of a calcimimetic or parathyroidectomy (PTX) attenuates progression of renal failure. To evaluate the impact of PTX on blood pressure (BP), renal graft function and serum lipids, we performed a retrospective case-controlled study in renal graft recipients.

METHODS

Charts of 1647 kidney allograft recipients, transplanted between 1989 and 2004, were reviewed. Thirty-two patients with a functioning graft and a history of a successful PTX performed at least 9 months after transplantation were identified. Biochemical and clinical data available 6 months pre- and post-PTX were registered. Changes in BP, renal function and serum lipids were assessed. The data were compared with those obtained in a similar time frame in a control group closely matched for date of transplantation.

RESULTS

Systolic BP (149.9 vs 141.7 mmHg), diastolic BP (85.6 vs 81.9 mmHg), pulse pressure (64.3 vs 58.8 mmHg), total cholesterol concentration (221.4 vs 211.1 mg/dl) and low-density lipoprotein cholesterol concentration (123.9 vs 106.7 mg/dl) improved significantly after successful PTX. Serum creatinine, conversely, significantly increased after PTX (1.75 vs 2.13 mg/dl, P<0.0001). No significant changes were observed in the control group in the same time period.

CONCLUSION

In patients with a functioning renal graft, BP and dyslipidaemia improve, whereas serum creatinine worsens following successful PTX. Our data are in agreement with a stimulatory effect of PTH on plasma renin activity and an inhibitory effect on lipase activity, as previously demonstrated by others. To what extent the increased serum creatinine following PTX reflects a true deterioration of the glomerular filtration rate and/or is the consequence of vitamin D-induced reduction of the renal tubular secretion of creatinine needs to be elucidated by further research.

Treatment of secondary hyperparathyroidism of dialysis patients with calcimimetics as a valuable addition to established therapeutic means

Currently available options for the treatment of hyperparathyroidism secondary to chronic renal failure do not allow the achievement of target values for plasma calcium, phosphorus, and parathyroid hormone in the majority of patients with chronic kidney disease (CKD) stage 5. This is particularly true for CKD patients who have been referred to nephrologists late in the course of their disease and in whom prevention has not been possible. The advent of a new class of therapeutic agents, the calcimimetics, will allow an easier control of already established parathyroid overfunction, as has been demonstrated in several phase II studies and one phase III study with cinacalcet. Future studies will show whether an earlier start of treatment in patients with CKD stage 2, 3, and 4 allows the prevention of secondary hyperparathyroidism. Since all available experience has been gathered in adult patients it is also necessary to test the efficacy and safety of the calcimimetics in children with CKD.

Vascular calcification: a stiff challenge for the nephrologist: does preventing bone disease cause arterial disease?

There has been an explosion of interest in vascular calcification in the last 5 years. Four key "germinal" findings have fallen onto very fertile soil. First, on the background of an increasing cardiovascular disease burden it has been found that at least cross-sectionally, and in a limited fashion prospectively, achieved dialysis plasma phosphate levels are linked to all-cause and cardiovascular mortality. Second, there are increasing reports of calcific uremic arteriolopathy in Australia and the United States. Third, we know know that the mechanical properties of the carotid artery, and the aorta, have a profound influence on survival for dialysis patients. Vascular calcification itself (as assessed by x-ray films and ultrasound) has been linked to aortic stiffness. Fourth, increasing numbers of studies are showing extremely extensive coronary artery calcification (CAC) in dialysis patients, even at a young age. From these apparently unlinked observations the following assertion has been posited-that in the widespread (over) use of calcium-containing oral phosphate binders (OPB) to prevent uremic osteodystrophy in our dialysis population we have unwittingly accelerated widespread uremic vasculopathy and thereby contributed to premature cardiovascular mortality. It is the purpose of this article to discuss vascular calcification (and particularly CAC) in dialysis patients as we understand it today. We will review the published series, with special reference to the Sevelamer Treat to Goal trial and also discuss the new Kidney Disease Outcome Quality Initiative (K-DOQI) guidelines on the use of phosphate binders in chronic kidney disease.

Calcimimetics--fooling the calcium receptor

The cloning and characterization of a calcium-sensing receptor from bovine parathyroid cells has opened up the possibility of modulating the activity of this receptor protein by organic small molecules, either increasing the sensitivity for calcium (calcimimetics) or decreasing the sensitivity (calciolytics), thus suppressing or stimulating parathyroid hormone (PTH) secretion and synthesis, respectively. In primary and secondary hyperparathyroidism, calcimimetics have proven to effectively lower the PTH concentration with minimal side effects. In secondary hyperparathyroidism, a specific advantage is the concomitant lowering of serum calcium and phosphate concentrations.

Calcimimetics and the Treatment of Primary and Secondary Hyperparathyroidism

OBJECTIVE

To review the mechanism of action, development, and clinical application of the calcimimetic compounds being investigated for the treatment of primary and secondary hyperparathyroidism (HPT).

DATA SOURCES

A MEDLINE search (1990–April 2004) was performed to identify all published articles related to calcimimetics. Published abstracts over the previous 5 years from various scientific meetings (American Society of Nephrology, American Society for Clinical Pharmacology and Therapeutics, American Society of Bone and Mineral Research) were also searched for reports regarding investigational calcimimetic agents. Data on cinacalcet HCI were provided by Amgen, Inc.

STUDY SELECTION AND DATA EXTRACTION

Studies were selected based on number of patients included, relevance to the approved indications, and inclusion of pharmacokinetic and drug interaction information.

DATA SYNTHESIS

The investigational calcimimetic compounds directly modulate the calcium-sensing receptor and can produce at least a 30% reduction in parathyroid hormone (PTH) secretion in secondary HPT. Cinacalcet appears to have more predictable pharmacokinetics and a lower risk of symptomatic hypocalcemia than the older agent, R-568. The safety profile and effective reduction in PTH and simultaneous reductions in calcium and phosphorus (and the calcium–phosphorus product) induced by cinacalcet make this agent advantageous over previously used therapies, such as vitamin D and phosphate-binding agents. More studies are required to determine additional uses of cinacalcet.

CONCLUSIONS

Cinacalcet has a unique mechanism for reducing PTH concentration and appears to be a safe and effective oral therapy for both primary and secondary HPT.

The calcium-sensing receptor: a key factor in the pathogenesis of secondary hyperparathyroidism

Serum calcium levels are regulated by the action of parathyroid hormone (PTH). Major drivers of PTH hypersecretion and parathyroid cell proliferation are the hypocalcemia and hyperphosphatemia that develop in chronic kidney disease patients with secondary hyperparathyroidism (SHPT) as a result of low calcitriol levels and decreased kidney function. Increased PTH production in response to systemic hypocalcemia is mediated by the calcium-sensing receptor (CaR). Furthermore, as SHPT progresses, reduced expression of CaRs and vitamin D receptors (VDRs) in hyperplastic parathyroid glands may limit the ability of calcium and calcitriol to regulate PTH secretion. Current treatment for SHPT includes the administration of vitamin D sterols and phosphate binders. Treatment with vitamin D is initially effective, but efficacy often wanes with further disease progression. The actions of vitamin D sterols are undermined by reduced expression of VDRs in the parathyroid gland. Furthermore, the calcemic and phosphatemic actions of vitamin D mean that it has the potential to exacerbate abnormal mineral metabolism, resulting in the formation of vascular calcifications. Effective new treatments for SHPT that have a positive impact on mineral metabolism are clearly needed. Recent research shows that drugs that selectively target the CaR, calcimimetics, have the potential to meet these requirements.

Calcium sensing by endocrine cells

The elucidation of the structure and function of the Ca2+(o)-sensing receptor (CaR) has provided important insights into the normal control of Ca2+(o) homeostasis, particularly the key role of the receptor in kidney and parathyroid. Further studies are needed to define more clearly the homeostatic role of the CaR in additional tissues, both those that are involved and those that are uninvolved in systemic Ca2+(o) homeostasis. The availability of the cloned CaR has also permitted documentation of the molecular basis of inherited disorders of Ca2+(o) sensing, including those in which the receptor is less and or more sensitive than normal to Ca2+(o). Antibodies to the CaR that either activate it or inactivate it produce syndromes resembling the corresponding genetic diseases. Expression of the receptor is abnormally low in 1 degree and 2 degrees hyperparathyroidism, which could contribute to the defective Ca2+(o) sensing in these conditions. The recent discovery of calcimimetics, which sensitize the CaR to Ca2+(o), has provided what will likely be an effective medical therapy for the secondary/tertiary hyperparathyroidism of end stage renal failure as well as for 1 degree hyperparathyroidism.

The impact of calcimimetics on mineral metabolism and secondary hyperparathyroidism in end-stage renal disease

The impact of calcimimetics on mineral metabolism and secondary hyperparathyroidism in end-stage renal disease. Secondary hyperparathyroidism is often complicated by elevations in calcium and phosphorus either as a result of the disease per se or due to toxicity from current therapeutic options. These disturbances in mineral metabolism limit the successfulness of therapy and have been implicated as contributing to the development and progression of vascular calcification, an important and often overlooked component of cardiovascular disease in patients on dialysis. Phosphorus, active vitamin D, and calcium all play important roles in the pathogenesis of secondary hyperparathyroidism; however, serum calcium is the primary regulator of minute-to-minute parathyroid hormone secretion. Small changes in serum calcium are detected by a cell surface calcium sensing receptor that has recently been cloned. Calcimimetic agents modulate the activity of the calcium-sensing receptor and result in profound reductions in levels of circulating parathyroid hormone. Additionally, these agents result in decreases in serum calcium, phosphorus, and calcium-phosphorus product. Recently completed phase 2 clinical trials with the second-generation calcimimetic agent cinacalcet HCl confirm that this agent represents a safe and effective novel therapeutic agent which has the potential to dramatically alter the treatment and complications associated with secondary hyperparathyroidism in patients on dialysis.

Management of secondary hyperparathyroidism of dialysis patients

Hyperphosphatemia, vitamin D deficiency, and resulted hypocalcemia have been regarded as classical pathogeneses of secondary hyperparathyroidism. These factors have been treated by the administration of phosphorus binder and vitamin D derivatives. However, these therapies have not brought about a successful result for the prevention and treatment of secondary hyperparathyroidism. The reason could be mainly attributed to the hypercalcemia that results from the administration of calcium salts as a phosphorus binder and the calcemic action of vitamin D. To prevent hypercalcemia, non-calcium containing phosphorus binder (sevelamer hydrochloride) and vitamin D analogues, which suppress PTH secretion with minimum calcemic action, have been developed. These new vitamin D analogues include 19-nor-1-alpha, 25-dihydroxyvitamin D2 (paricalcitol), 1-alpha-hydroxyvitamin D2 (doxercalciferol), 22oxa-calcitriol (maxacalcitol) and F6-calcitriol (falecalcitriol). Furthermore, calcimimetics that stimulate calcium-sensing receptor of parathyroid cells as calcium and suppress PTH secretion are now under clinical trial. Percutaneous direct injection therapy of vitamin D, vitamin D analogue or calcimimetics into parathyroid gland has also been reported. The combination of these new strategies is expected to effectively and safely suppresses secondary hyperparathyroidism that has been resistant to conventional medical treatments.