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Why are Calcimimetics Prescribed for Kidney Patients?
Calcimimetics are prescribed to treat a condition called secondary hyperparathyroidism (hyper-para-thyroid-ism). In this condition the parathyroid glands are overactive. Kidney failure often causes the parathyroid glands to make too much parathyroid hormone (PTH). Too much PTH can make your calcium and phosphorus blood levels increase, leading to hardening of organs and soft tissues, bone disease, and other serious complications.
Overview of PTH
Parathyroid glands are small glands located behind the thyroid gland in your neck. These glands make a hormone called parathyroid hormone (PTH). This hormone helps keep the calcium and phosphorus levels in your blood at a normal, balanced level.
PTH and Kidney Disease
As kidney function declines, the metabolism of phosphorus and vitamin D becomes disordered. Since diseased kidneys have difficulty excreting excess phosphorus from your body, phosphorus levels in your blood rise. Also, the diseased kidneys lose the ability to convert the vitamin D that you get from sunlight and food to active vitamin D. This is the kind of vitamin D you need for proper calcium absorption. Without this active vitamin D, less calcium is absorbed from your intestines into your blood. Blood levels of calcium decrease.
Calcium-sensing receptors (CaSR) are located on the cells of the parathyroid glands. The CaSR controls how much PTH is made by the parathyroid glands. If blood levels of calcium are low, the CaSR tells the parathyroid gland to make more PTH. PTH will pull calcium from your bones into your bloodstream to correct the low blood calcium level.
Over time, as the parathyroid glands are signaled to make more and more PTH, the glands become enlarged and the production of PTH becomes steadily higher. This leads to excessive PTH production, high blood calcium levels, and weak bones.
Treatment with Calcimimetics
Calcimimetics are a type of medication that lower PTH and calcium levels by making the CaSR more sensitive to the amount of calcium in the blood. Currently, there are two calcimimetics available on the market – Oral Cinacalcet (Sensipar ®) and IV Parsabiv ®.
While Sensipar and Parsabiv both target the CaSR, they bind to different sites on the CaSR. According to the pharmaceutical websites:
- Sensipar is a small molecule compound that binds to the transmembrane domain of the CaSR.
- Parsabiv is a synthetic peptide that binds directly to the extracellular domain, at a site separate from the calcium-binding site.
Both types of calcimimetics have shown to be effective at lowering PTH and calcium levels.
What are My Options to Discuss with My Doctor?
Since Sensipar has been on the market longer than Parsabiv, most patients are likely already taking Sensipar to control their secondary hyperparathyroidism. There are risks and benefits associated with both calcimemetics. It is best to discuss with your doctor which one is best for your needs.
Sensipar Key Points:
- Taken orally (by mouth)
- Taken every day (unless prescribed otherwise by your doctor)
- Most common side effects are nausea, vomiting, and diarrhea
- May cause low calcium
Parsabiv Key Points:
- Given intravenously at the end of the dialysis treatment
- Usually given three times per week at dialysis
- Most common side effects are nausea, vomiting and diarrhea
- May cause low calcium
Only one calcimimetic should be taken at a time. Your doctor will decide which calcimimetic is the best fit for you if you need this type of medication.
Your healthcare team will monitor calcium, phosphorus and PTH levels and make adjustments as needed. Your dietitian will discuss your intake of calcium and phosphorus and suggest diet changes if needed.
This article is for informational purposes only and is not a substitute for medical advice or treatment. Consult your physician and dietitian regarding your specific diagnosis, treatment, diet and health questions.