Comparison of hypotensive response following intravenous injection of parathyroid hormone 1-84 and 1-34 in conscious rats

Reduced secretion of parathyroid hormone and hypocalcemia in systemic heterozygous ATP2B1-null hypertensive mice

The ATP2B1 gene is associated with hypertension. We previously reported that systemic heterozygous ATP2B1-null (ATP2B1^+/−) mice exhibited hypertension due to impaired endothelial nitric oxide synthase (eNOS) activity and decreased nitric oxide (NO) production. The ATP2B1 gene encodes plasma membrane calcium ATPase 1 (PMCA1), which has been thought to regulate only intracellular Ca²⁺ concentration. However, recently, it has been suggested that ATP2B1 works not only at cellular levels, but also throughout the entire body, including in the calcium metabolism, using small intestine-specific ATP2B1 knockout mice. To clarify the roles of ATP2B1 in the entire body and the effects of ATP2B1 on blood pressure, we examined the alterations of calcium related factors in ATP2B1^+/− mice. ATP2B1^+/− mice exhibited hypocalcemia. The expression of ATP2B1 in the kidney and small intestine decreased, and hypercalciuria was confirmed in ATP2B1^+/− mice. The intact-PTH levels were lower, and bone mineral density was increased in these mice. These results suggest that hypocalcemia is mainly a result of inhibited bone resorption without compensation by PTH secretion in the case of ATP2B1 knockout. Moreover, NO production may be affected by reduced PTH secretion, which may cause the increase in vascular contractility in these mice. The ATP2B1 gene is important for not only intra-cellular calcium regulation but also for calcium homeostasis and blood pressure control.

Pharmacokinetics and pharmacodynamics of subcutaneous recombinant parathyroid hormone (1-84) in patients with hypoparathyroidism: an open-label, single-dose, phase I study

BACKGROUND

Impaired mineral homeostasis affecting calcium, phosphate, and magnesium is a result of parathyroid hormone (PTH) deficiency in hypoparathyroidism. The current standard of treatment with active vitamin D and oral calcium does not control levels of these major minerals. Recombinant full-length human PTH 1-84 (rhPTH[1-84]) is being developed for the treatment of hypoparathyroidism.

OBJECTIVE

The goal of this study was to investigate the pharmacokinetics and pharmacodynamics of a single subcutaneous injection of rhPTH(1-84) in patients with hypoparathyroidism.

METHODS

This was an open-label, dose-escalating study of single subcutaneous administration of 50 µg and then 100 µg of rhPTH(1-84). Enrolled patients (age range, 25-85 years) had ≥12 months of diagnosed hypoparathyroidism defined according to biochemical evidence of hypocalcemia with concomitant low-serum intact PTH and were taking doses ≥1000 mg/d of oral calcium and ≥0.25 µg/d of active vitamin D (oral calcitriol). The patient's prescribed dose of calcitriol was taken the day preceding but not on the day of or during the 24 hours after rhPTH(1-84) administration. Each patient received a single 50-µg rhPTH(1-84) dose, had at least a 7-day washout interval, and then received a single 100-µg rhPTH(1-84) dose. The following parameters were assessed: plasma PTH; serum and urine total calcium, magnesium, phosphate, and creatinine; and urine cyclic adenosine monophosphate.

RESULTS

After administration of rhPTH(1-84) 50 µg (n = 6) and 100 µg (n = 7), the approximate t½ was 2.5 to 3 hours. Plasma PTH levels increased rapidly, then declined gradually back to predose levels at ~12 hours. The median AUC was similar with calcitriol and rhPTH(1-84) for serum 1,25-dihydroxyvitamin D (calcitriol, 123-227 pg · h/mL; rhPTH[1-84], 101-276 pg · h/mL), calcium (calcitriol, 3.3-3.7 mg · h/dL; rhPTH[1-84], 3.3-7.6 mg · h/dL), and magnesium (calcitriol, 0.7-0.9 mg · h/dL; rhPTH[1-84], 1.3-2.8 mg · h/dL). In contrast, the median AUC for phosphate was strongly negative with rhPTH(1-84) (calcitriol, -1.0 to 0.8 mg · h/dL; rhPTH[1-84], -21.3 to -26.5 mg · h/dL). Compared with calcitriol, rhPTH(1-84) 50 µg reduced 24-hour calcium excretion and calcium-to-creatinine ratios by 12% and 23%, respectively, and rhPTH(1-84) 100 µg reduced them by 26% and 27%. There was little overall impact on urine magnesium levels. Compared with calcitriol, rhPTH(1-84) 50 µg increased urinary phosphate excretion and phosphate-to-creatinine ratios by 53% and 54%, respectively, and rhPTH(1-84) 100 µg increased them by 45% and 42%. Urine cyclic adenosine monophosphate-to-creatinine ratio increased with rhPTH(1-84) by 2.3-fold (50 µg) and 4.4-fold (100 µg) compared with calcitriol.

CONCLUSIONS

PTH replacement therapy with rhPTH(1-84) regulated mineral homeostasis of calcium, magnesium, phosphate, and vitamin D metabolism toward normal in these study patients with hypoparathyroidism.

Hypertensive activity of synthesized PTH(25-34) and Ac-PTH(25-30)-NH2 in rats

Parathyroid hormone (PTH) is secreted by parathyroid glands and is the main known factor that control plasma calcium concentration. There are many indications that PTH or products of PTH degradation influence the mean arterial blood pressure (MAP). These observations might be important in diseases accompanied with the overproduction of PTH such as primary hyperparathyroidism (PHPT). It was shown that the six amino acids PTH precursor-PRO-PTH with reversed sequence (PRO-rs), which contains a rare tripeptide -Arg-Lys-Lys- fragment, induces significant hypertensive response in rats. This strong alkali tripeptide is also present in the position 25-27 of the PTH molecule. The aim of the present study was to synthesize, by the solid phase peptide synthesis method, PTH fragments including the -Arg-Lys-Lys- sequence and test their influence on blood pressure and calcium plasma concentration in rats. Our study demonstrated that PTH(25-34) and the acetylated amide analogue of PTH(25-30), (Ac-PTH(25-30)-NH(2)) were hypertensive in the physiological doses. The presence of strong alkali sequence -Arg-Lys-Lys- in PTH(25-30) fragment is not sufficient to induce hypertension either in physiological or pharmacological doses in rats. Therefore, both the proximity of the -Arg-Lys-Lys- sequence and length of the peptide might also play roles as pressure factors.