Pharmacotherapeutic group: Calcium homeostasis, parathyroid hormones and analogues, ATC code: H05AA03
Mechanism of action
Endogenous parathyroid hormone (PTH) is secreted by the parathyroid glands as a polypeptide of 84 amino acids. PTH exerts its action via cell-surface parathyroid hormone receptors, present in bone, kidney and nerve tissue. Parathyroid hormone receptors belong to the family of G-coupled protein receptors.
PTH has a variety of critical physiological functions that include its central role in modulating serum calcium and phosphate levels within tightly regulated levels, regulating renal calcium and phosphate excretion, activating vitamin D, and maintaining normal bone turnover.
Natpar is produced in E. coli using recombinant DNA technology, and is identical to the 84 amino acid sequence of endogenous human parathyroid hormone.
Pharmacodynamic effects
PTH (1-84) is the principal regulator of plasma calcium homeostasis. In the kidney, PTH (1-84) increases renal tubular reabsorption of calcium and promotes phosphate excretion.
The overall effect of PTH is to increase serum calcium concentration, to reduce urinary excretion of calcium and to lower serum phosphate concentration.
Natpar has the same primary amino acid sequence as endogenous parathyroid hormone and may be anticipated to have the same physiological actions.
Clinical efficacy and safety
The safety and clinical efficacy of Natpar in adults with hypoparathyroidism is derived from 1 randomised, placebo-controlled study and an open-label extension study. In these studies, Natpar was self-administered, with daily doses ranging from 25 to 100 micrograms per subcutaneous injection.
Study 1 – REPLACE
The objective of this trial was to maintain serum calcium with Natpar while reducing or replacing oral calcium and active vitamin D. The study was a 24-week, randomised, double-blind, placebo-controlled, multicentre trial. In this trial, patients with chronic hypoparathyroidism receiving calcium and active forms of vitamin D (vitamin D metabolite or analogues) were randomised to Natpar (n=84) or placebo (n=40). The mean age was 47.3 years (range 19 to 74 years); 79% were females. Patients had hypoparathyroidism for an average of 13.6 years.
At randomisation, active forms of vitamin D were reduced by 50% and patients were allocated to Natpar 50 micrograms daily or placebo. Randomisation was followed by a 12-week Natpar titration phase and a 12-week Natpar dose maintenance phase.
Ninety percent of patients who were randomised completed 24 weeks of treatment.
For the efficacy analysis, subjects that fulfilled three components of a three-part response criterion were considered responders. A responder was defined using a composite primary efficacy endpoint of at least a 50% reduction from the baseline active vitamin D dose AND at least a 50% reduction from the baseline oral calcium AND an albumin-corrected total serum calcium concentration maintained or normalised compared with the baseline value (≥1.875 mmol/L) and did not exceed the upper limit of the laboratory normal range.
At the end of treatment, 46/84 (54.8%) patients treated with Natpar achieved the primary endpoint versus 1/40 (2.5%) with placebo (p<0.001).
At Week 24, for patients who completed the study, 34/79 (43%) Natpar patients were independent of active vitamin D treatment and were receiving no more than 500 mg of calcium citrate, compared with 2/33 (6.1%) placebo patients (p<0.001).
Sixty-nine percent (58/84) of subjects randomised to Natpar showed a reduction in oral calcium of ≥50% compared to 7.5% (3/40) of subjects randomised to placebo. The mean percent change from baseline in oral calcium was -51.8% (SD 44.6) in subjects receiving Natpar compared to 6.5% (SD 38.5) in the placebo group (p<0.001). In addition, 87% (73/84) of patients treated with Natpar showed a ≥50% reduction in oral active vitamin D versus 45% (18/40) in the placebo group.
Study 2 - RACE
Study 2 is a six year long-term, open-label extension study of daily subcutaneous dosing of Natpar in hypoparathyroidism subjects who completed prior studies with Natpar.
A total of 49 subjects were enrolled in the study. Subjects received doses of 25 micrograms, 50 micrograms, 75 micrograms or 100 micrograms/day for up to approximately 72 months (mean 2038 days (~5.6 years). The minimum time of exposure to Natpar was 41 days, and the maximum was 2497 days (~6.8 years).
61.2% (30/49) of subjects met the primary efficacy endpoint at end of treatment, defined as albumin-corrected total serum calcium concentration that was normalized or maintained compared to the baseline value and not exceeding the upper limit of normal values; ≥50% reduction from baseline or ≤500 mg of daily calcium supplementation; and ≥50% reduction from baseline or ≤0.25 µg of daily calcitriol supplementation.
The results demonstrate durability of the physiological effects of Natpar over 72 months including maintenance of mean albumin-corrected serum calcium levels (n=49, 2.09±0.174 mmol/L at baseline; n=38, 2.08±0.167 mmol/L at 72 months), a decrease in serum phosphate (n=49, 1.56±0.188 mmol/L at baseline; n=36, 1.26±0.198 mmol/L at 72 months) and the maintenance of normal calcium phosphate product product (<4.4mmol2/L2) for all subjects (n=49 at baseline, n=36 at 72 months).
The long-term effects included a decrease in mean urinary calcium excretion to the normal range (n=48, 8.92±5.009 mmol/day at baseline; n=32, 5.63±3.207 mmol/day at 72 months), and stabilization of normal mean serum creatinine levels (n=49, 84.7±18.16 µmol/L at baseline; n=38, 78.2±18.52 µmol/L at 72 months). In addition, there was maintenance of normal bone mineral density.
Paediatric population
The European Medicines Agency has deferred the obligation to submit the results of studies with Natpar in one or more subsets of the paediatric population in hypoparathyroidism (see section 4.2 for information on paediatric use).
This medicinal product has been authorised under a so-called 'conditional approval' scheme. This means that further evidence on this medicinal product is awaited.
The European Medicines Agency will review new information on this medicinal product at least every year and this SmPC will be updated as necessary.