Design and Preclinical Development of TransCon PTH,an Investigational Sustained-Release PTH Replacement Therapy for Hypoparathyroidism

Hypoparathyroidism (HP) is a condition of parathyroid hormone (PTH) deficiency leading to abnormal calcium and phosphate metabolism. The mainstay of therapy consists of vitamin D and calcium supplements, as well as adjunct Natpara (PTH(1-84)). However, neither therapy optimally controls urinary calcium (uCa) or significantly reduces the incidence of hypercalcemia and hypocalcemia. TransCon PTH, a sustained-release prodrug of PTH(1-34) in development for the treatment of HP, was designed to overcome these limitations. To determine the pharmacokinetics and pharmacodynamics of TransCon PTH, single and repeat s.c. dose studies were performed in rats and monkeys. TransCon PTH demonstrated a half-life of 28 and 34 hours in rats and monkeys, respectively. After repeated dosing, an infusion-like profile of the released PTH, characterized by low peak-to-trough levels, was obtained in both species. In intact rats and monkeys, daily subcutaneous administration of TransCon PTH was associated with increases in serum calcium (sCa) levels and decreases in serum phosphate levels (sP). In monkeys, at a single dose of TransCon PTH that increased sCa levels within the normal range, a concurrent decrease in uCa excretion was observed. In 4-week repeat-dose studies in intact rats and monkeys, uCa excretion was comparable to controls across all dose levels despite increases in sCa levels. Further, in a rat model of HP, TransCon PTH normalized sCa and sP levels 24 hours per day. This was in contrast to only transient trends toward normalization of sCa and sP levels with an up to 6-fold higher molar dose of PTH(1-84). After repeated dosing to HP rats, uCa excretion transiently increased, corresponding to increases in sCa above normal range, but at the end of the treatment period, uCa excretion was generally comparable to sham controls. TransCon PTH was well tolerated and the observed pharmacokinetics and pharmacodynamics were in line with the expected action of physiological replacement of PTH. © 2019 American Society for Bone and Mineral Research.     

The effect of adding PTH(1–84) to conventional treatment of hypoparathyroidism: A randomized,placebo‐controlled study

In hypoparathyroidism, plasma parathyroid hormone (PTH) levels are inadequate to maintain plasma calcium concentration within the reference range. On conventional treatment with calcium supplements and active vitamin D analogues, bone turnover is abnormally low, and BMD is markedly increased. We aimed to study the effects of PTH‐replacement therapy (PTH‐RT) on calcium‐phosphate homeostasis and BMD. In a double‐blind design, we randomized 62 patients with hypoparathyroidism to daily treatment with PTH(1–84) 100 µg or similar placebo for 24 weeks as add‐on therapy to conventional treatment. Compared with placebo, patients on PTH(1–84) reduced their daily dose of calcium and active vitamin D significantly by 75% and 73%, respectively, without developing hypocalcemia. However, hypercalcemia occurred frequently during the downtitration of calcium and active vitamin D. Plasma phosphate and renal calcium and phosphate excretion did not change. Compared with placebo, PTH(1–84) treatment significantly increased plasma levels of bone‐specific alkaline phosphatase (+226% ± 36%), osteocalcin (+807% ± 186%), N‐terminal propeptide of procollagen 1 (P1NP; +1315% ± 330%), cross‐linked C‐telopeptide of type 1 collagen (CTX; +1209% ± 459%), and urinary cross‐linked N‐telopeptide of type 1 collagen (NTX; (+830% ± 165%), whereas BMD decreased at the hip (?1.59% ± 0.57%), lumbar spine (?1.76% ± 1.03%), and whole body (?1.26% ± 0.49%) but not at the forearm. In conclusion, the need for calcium and active vitamin D is reduced significantly during PTH‐RT, whereas plasma calcium and phosphate levels are maintained within the physiologic range. In contrast to the effect of PTH(1–84) treatment in patients with osteoporosis, PTH‐RT in hypoparathyroidism causes a decrease in BMD. This is most likely due to the marked increased bone turnover. Accordingly, PTH‐RT counteracts the state of overmineralized bone and, during long‐term treatment, may cause a more physiologic bone metabolism. © 2011 American Society for Bone and Mineral Research     

PTH(1–84) administration reverses abnormal bone‐remodeling dynamics and structure in hypoparathyroidism

Hypoparathyroidism is associated with abnormal structural and dynamic skeletal properties. We hypothesized that parathyroid hormone(1–84) [PTH(1–84)] treatment would restore skeletal properties toward normal in hypoparathyroidism. Sixty‐four subjects with hypoparathyroidism were treated with PTH(1–84) for 2 years. All subjects underwent histomorphometric assessment with percutaneous iliac crest bone biopsies. Biopsies were performed at baseline and at 1 or 2 years. Another group of subjects had a single biopsy at 3 months, having received tetracycline before beginning PTH(1–84) and prior to the biopsy (quadruple‐label protocol). Measurement of biochemical bone turnover markers was performed. Structural changes after PTH(1–84) included reduced trabecular width (144 ± 34 µm to 128 ± 34 µm, p = 0.03) and increases in trabecular number (1.74 ± 0.34/mm to 2.07 ± 0.50/mm, p = 0.02) at 2 years. Cortical porosity increased at 2 years (7.4% ± 3.2% to 9.2% ± 2.4%, p = 0.03). Histomorphometrically measured dynamic parameters, including mineralizing surface, increased significantly at 3 months, peaking at 1 year (0.7% ± 0.6% to 7.1% ± 6.0%, p = 0.001) and persisting at 2 years. Biochemical measurements of bone turnover increased significantly, peaking at 5 to 9 months of therapy and persisting for 24 months. It is concluded that PTH(1–84) treatment of hypoparathyroidism is associated with increases in histomorphometric and biochemical indices of skeletal dynamics. Structural changes are consistent with an increased remodeling rate in both trabecular and cortical compartments with tunneling resorption in the former. These changes suggest that PTH(1–84) improves abnormal skeletal properties in hypoparathyroidism and restores bone metabolism toward normal euparathyroid levels. © 2011 American Society for Bone and Mineral Research     

A Homozygous [Cys25]PTH(1‐84) Mutation That Impairs PTH/PTHrP Receptor Activation Defines a Novel Form of Hypoparathyroidism

Hypocalcemia and hyperphosphatemia are encountered in idiopathic hypoparathyroidism (IHP) and pseudohypoparathyroidism type Ib (PHP1B). In contrast to PHP1B, which is caused by resistance toward parathyroid hormone (PTH), the genetic defects leading to IHP impair production of this important regulator of mineral ion homeostasis. So far, only five PTH mutations were shown to cause IHP, each of which is located in the hormone's pre‐pro leader segment and thus impair hormone secretion. In three siblings affected by IHP, we now identified a homozygous arginine‐to‐cysteine mutation at position 25 (R25C) of the mature PTH(1‐84) polypeptide; heterozygous family members are healthy. Depending on the assay used for evaluating these patients, plasma PTH levels were either low or profoundly elevated, thus leading to ambiguities regarding the underlying diagnosis, namely IHP or PHP1B. Consistent with increased PTH levels, recombinant [Cys25]PTH(1‐84) and wild‐type PTH(1‐84) were secreted equally well by transfected COS‐7 cells. However, synthetic [Cys25]PTH(1‐34) was found to have a lower binding affinity for the PTH receptor type‐1 (PTH1R) than PTH(1‐34) and consequently a lower efficiency for stimulating cAMP formation in cells expressing this receptor. Consistent with these in vitro findings, long‐term infusion of [Cys25]PTH(1‐34) resulted only in minimal calcemic and phosphaturic responses, despite readily detectable levels of [Cys25]PTH(1‐34) in plasma. The mineral ion abnormalities observed in the three IHP patients are thus most likely caused by the inherited homozygous missense PTH mutation, which reduces bioactivity of the secreted hormone. Based on these findings, screening for PTH(1‐84) mutations should be considered when clinical and laboratory findings are consistent with PHP1B, but GNAS methylation changes have been excluded. Differentiating between IHP and PHP1B has considerable implications for genetic counseling, therapy, and long‐term outcome because treatment of IHP patients with inappropriately high doses of active vitamin D and calcium can contribute to development of nephrocalcinosis and chronic kidney disease. © 2015 American Society for Bone and Mineral Research.     

Diphtheria Toxin‐ and GFP‐Based Mouse Models of Acquired Hypoparathyroidism and Treatment With a Long‐Acting Parathyroid Hormone Analog

Hypoparathyroidism (HP) arises most commonly from parathyroid (PT) gland damage associated with neck surgery, and is typically treated with oral calcium and active vitamin D. Such treatment effectively increases levels of serum calcium (sCa), but also brings risk of hypercalciuria and renal damage. There is thus considerable interest in using PTH or PTH analogs to treat HP. To facilitate study of this disease and the assessment of new treatment options, we developed two mouse models of acquired HP, and used them to assess efficacy of PTH(1–34) as well as a long‐acting PTH analog (LA‐PTH) in regulating blood calcium levels. In one model, we used PTHcre‐iDTR mice in which the diphtheria toxin (DT) receptor (DTR) is selectively expressed in PT glands, such that systemic DT administration selectively ablates parathyroid cells. For the second model, we generated GFP‐PT mice in which green fluorescent protein (GFP) is selectively expressed in PT cells, such that parathyroidectomy (PTX) is facilitated by green fluorescence of the PT glands. In the PTHcre‐iDTR mice, DT injection (2 × 5 μg/kg, i.p.) resulted in moderate yet consistent reductions in serum PTH and sCa levels. The more severe hypoparathyroid phenotype was observed in GFP‐PT mice following GFP‐guided PTX surgery. In each model, a single subcutaneous injection of LA‐PTH increased sCa levels more effectively and for a longer duration (>24 hours) than did a 10‐fold higher dose of PTH(1–34), without causing excessive urinary calcium excretion. These new mouse models thus faithfully replicate two degrees of acquired HP, moderate and severe, and may be useful for assessing potential new modes of therapy. © 2015 American Society for Bone and Mineral Research.     

A Randomized Double-Blind Placebo-Controlled First-In-Human Phase 1 Trial of TransCon PTH in Healthy Adults

TransCon PTH is a sustained-release, essentially inactive prodrug transiently bound to an inert carrier, designed to release PTH(1-34), and in development for hypoparathyroidism (HP). This phase 1, randomized, placebo-controlled, single and multiple ascending dose (SAD and MAD, respectively) trial evaluated safety, tolerability, pharmacodynamics (PD), and pharmacokinetics (PK) of TransCon PTH in healthy adults. SAD and MAD cohorts consisted of 10 subjects (eight active, two placebo) who received up to seven single or six multiple ascending doses of TransCon PTH, respectively. TransCon PTH doses ranged from 3.5 to 124 μg PTH(1-34) for the SAD cohorts and 3.5 to 24 μg PTH(1-34)/day for the MAD cohorts. The primary PK endpoint was Free PTH. The PD endpoints included albumin adjusted serum calcium (sCa), fractional excretion of calcium (FECa), intact endogenous PTH(1-84), bone turnover markers, renal tubular maximum reabsorption of phosphate/glomerular filtration rate (TMP/GFR), serum phosphate (sP) and magnesium, and 1,25 dihydroxyvitamin D. TransCon PTH was generally well tolerated; there were no drug-related serious adverse events (SAEs), and all AEs were transient in nature. Free PTH demonstrated an effective half-life of approximately 60 hours and a dose-dependent, sustained exposure with an infusion-like profile within the calculated physiologic range for active PTH at steady-state. Albumin-adjusted sCa demonstrated a dose-dependent, sustained response with complete control of FECa despite modest hypercalcemia at higher doses. Renal tubular maximum reabsorption of phosphate/glomerular filtration rate (TMP/GFR) showed a dose-dependent decrease, resulting in a dose-dependent decrease in sP. TransCon PTH administered daily for 10 days showed no increase in the osteoblastic bone formation markers, serum bone-specific alkaline phosphatase (BSAP) or P1NP, or the osteoclastic bone resorption marker, urine NTx, but modestly and transiently increased the osteoclast marker, serum CTx. These phase 1 data support TransCon PTH as a daily replacement therapy for HP providing physiological levels of PTH 24 hours per day and advancement into phase 2 clinical development. © 2020 The Authors. Journal of Bone and Mineral Research published by American Society for Bone and Mineral Research.     

ZP2307, a novel, cyclic PTH(1-17) analog that augments bone mass in ovariectomized rats

Daily injections of human parathyroid hormone (1-34), hPTH(1-34), provide a highly effective treatment option for severe osteoporosis. However, PTH analogs shorter than 28 amino acids do not retain any bone augmenting potential. Here, we present ZP2307 ([Ac₅c¹, Aib³, Leu⁸, Gln¹⁰, Har¹¹, Ala¹², Trp¹⁴, Asp¹⁷]PTH(1-17)-NH₂), a novel, chemically modified and cyclized hPTH(1-17) analog, that augments bone mass in ovariectomized, osteopenic rats. Subcutaneous administration of this structurally constrained, K¹³-D¹⁷ side-chain-to-side-chain cyclized peptide reversed bone loss and increased bone mineral density (BMD) up to or above baseline levels in rat long bones and vertebrae. Highly significant effects of ZP2307 were achieved at doses of 40–320 nmol/kg. Micro-CT and histomorphometric analyses showed that ZP2307 improved quantitative and qualitative parameters of bone structure. Biomechanical testing of rat femora confirmed that ZP2307 dramatically increased bone strength. Over a broad maximally effective dose range (40–160 nmol/kg) ZP2307 did not increase serum concentrations of ionized free calcium above normal levels. Only at the highest dose (320 nmol/kg) ZP2307 induced hypercalcemic calcium levels in the ovariectomized rats.To our knowledge ZP2307 is the smallest PTH peptide analog known to exert augmentation of bone. Our findings suggest that ZP2307 has the potential to effectively augment bone mass over a broad dose range without a concomitant increase in the serum concentration of ionized free calcium above the normal range.     

Calcilytic Ameliorates Abnormalities of Mutant Calcium‐Sensing Receptor (CaSR) Knock‐In Mice Mimicking Autosomal Dominant Hypocalcemia (ADH)

Activating mutations of calcium‐sensing receptor (CaSR) cause autosomal dominant hypocalcemia (ADH). ADH patients develop hypocalcemia, hyperphosphatemia, and hypercalciuria, similar to the clinical features of hypoparathyroidism. The current treatment of ADH is similar to the other forms of hypoparathyroidism, using active vitamin D₃ or parathyroid hormone (PTH). However, these treatments aggravate hypercalciuria and renal calcification. Thus, new therapeutic strategies for ADH are needed. Calcilytics are allosteric antagonists of CaSR, and may be effective for the treatment of ADH caused by activating mutations of CaSR. In order to examine the effect of calcilytic JTT‐305/MK‐5442 on CaSR harboring activating mutations in the extracellular and transmembrane domains in vitro, we first transfected a mutated CaSR gene into HEK cells. JTT‐305/MK‐5442 suppressed the hypersensitivity to extracellular Ca²⁺ of HEK cells transfected with the CaSR gene with activating mutations in the extracellular and transmembrane domains. We then selected two activating mutations locating in the extracellular (C129S) and transmembrane (A843E) domains, and generated two strains of CaSR knock‐in mice to build an ADH mouse model. Both mutant mice mimicked almost all the clinical features of human ADH. JTT‐305/MK‐5442 treatment in vivo increased urinary cAMP excretion, improved serum and urinary calcium and phosphate levels by stimulating endogenous PTH secretion, and prevented renal calcification. In contrast, PTH(1‐34) treatment normalized serum calcium and phosphate but could not reduce hypercalciuria or renal calcification. CaSR knock‐in mice exhibited low bone turnover due to the deficiency of PTH, and JTT‐305/MK‐5442 as well as PTH(1‐34) increased bone turnover and bone mineral density (BMD) in these mice. These results demonstrate that calcilytics can reverse almost all the phenotypes of ADH including hypercalciuria and renal calcification, and suggest that calcilytics can become a novel therapeutic agent for ADH. © 2015 American Society for Bone and Mineral Research.     

Intermittent PTH Administration Stimulates Pre‐Osteoblastic Proliferation Without Leading to Enhanced Bone Formation in Osteoclast‐Less c‐fos−/− Mice

This study aimed to investigate the behavior and ultrastructure of osteoblastic cells after intermittent PTH treatment and attempted to elucidate the role of osteoclasts on the mediation of PTH‐driven bone anabolism. After administering PTH intermittently to wildtype and c‐fos^?/? mice, immunohistochemical, histomorphometrical, ultrastructural, and statistical examinations were performed. Structural and kinetic parameters related to bone formation were increased in PTH‐treated wildtype mice, whereas in the osteoclast‐deficient c‐fos^?/? mice, there were no significant differences between groups. In wildtype and knockout mice, PTH administration led to significant increases in the number of cells double‐positive for alkaline phosphatase and BrdU, suggesting active pre‐osteoblastic proliferation. Ultrastructural examinations showed two major pre‐osteoblastic subtypes: one rich in endoplasmic reticulum (ER), the hypER cell, and other with fewer and dispersed ER, the misER cell. The latter constituted the most abundant preosteoblastic phenotype after PTH administration in the wildtype mice. In c‐fos^?/? mice, misER cells were present on the bone surfaces but did not seem to be actively producing bone matrix. Several misER cells were shown to be positive for EphB4 and were eventually seen rather close to osteoclasts in the PTH‐administered wildtype mice. We concluded that the absence of osteoclasts in c‐fos^?/? mice might hinder PTH‐driven bone anabolism and that osteoclastic presence may be necessary for full osteoblastic differentiation and enhanced bone formation seen after intermittent PTH administration.     

PTH(1‐84) Administration in Hypoparathyroidism Transiently Reduces Bone Matrix Mineralization

Patients with hypoparathyroidism have low circulating parathyroid (PTH) levels and higher cancellous bone volume and trabecular thickness. Treatment with PTH(1‐84) was shown to increase abnormally low bone remodeling dynamics. In this work, we studied the effect of 1‐year or 2‐year PTH(1‐84) treatment on cancellous and cortical bone mineralization density distribution (Cn.BMDD and Ct.BMDD) based on quantitative backscattered electron imaging (qBEI) in paired transiliac bone biopsy samples. The study cohort comprised 30 adult hypoparathyroid patients (14 treated for 1 year; 16 treated for 2 years). At baseline, Cn.BMDD was shifted to higher mineralization densities in both treatment groups (average degree of mineralization Cn.Ca_Mean +3.9% and +2.7%, p < 0.001) compared to reference BMDD. After 1‐year PTH(1‐84), Cn.Ca_Mean was significantly lower than that at baseline (–6.3%, p < 0.001), whereas in the 2‐year PTH(1‐84) group Cn.Ca_Mean did not differ from baseline. Significant changes of Ct.BMDD were observed in the 1‐year treatment group only. The change in histomorphometric bone formation (mineralizing surface) was predictive for Cn.BMDD outcomes in the 1‐year PTH(1‐84) group, but not in the 2‐year PTH(1‐84) group. Our findings suggest higher baseline bone matrix mineralization consistent with the decreased bone turnover in hypoparathyroidism. PTH(1‐84) treatment caused differential effects dependent on treatment duration that were consistent with the histomorphometric bone formation outcomes. The greater increase in bone formation during the first year of treatment was associated with a decrease in bone matrix mineralization, suggesting that PTH(1‐84) exposure to the hypoparathyroid skeleton has the greatest effects on BMDD early in treatment. © 2015 American Society for Bone and Mineral Research.     

Hyperlipidemia Impairs Osteoanabolic Effects of PTH

Epidemiological and in vitro studies have suggested that hyperlipidemia/oxidized phospholipids adversely affect bone. We recently found that oxidized phospholipids attenuate PTH‐induced cAMP and immediate‐early gene (IEG) expression in MC3T3‐E1 cells, raising concerns that clinical hyperlipidemia may attenuate osteoanabolic effects of PTH in vivo. Thus, we studied whether intermittent PTH treatment has differential osteoanabolic effects in wildtype (C57BL/6) and hyperlipidemic (LDLR^?/?) mice. Consistent with our previous in vitro studies, induction of IEGs in calvarial tissue, 45 min after a single dose of recombinant hPTH(1‐34), was attenuated in LDLR^?/? mice compared with C57BL/6 mice. Daily hPTH(1‐34) injections for 5 wk significantly increased total and cortical BMD and BMC, assessed by pQCT, in C57BL/6 mice. However, this induction was completely abrogated in LDLR^?/? mice. Similarly, PTH(1‐34) failed to increase BMD in another hyperlipidemic mouse model, ApoE^?/? mice. Histomorphometric analysis showed that trabecular bone of both mice responded similarly to PTH(1‐34). Structural parameters improved significantly in response to PTH(1‐34) in both mouse strains, although to a lesser degree in LDLR^?/? mice. With PTH(1‐34) treatment, osteoblast surface trended toward an increase in C57BL/6 mice and increased significantly in LDLR^?/? mice. PTH(1‐34) did not alter resorption parameters significantly, except for the eroded surface (ES/BS), which was reduced in the C57BL/6 but not in the LDLR^?/? mice. These results show that PTH(1‐34) has adverse effects on cortical bones of the hyperlipidemic mice, suggesting that the therapeutic effects of PTH may be compromised in the presence of hyperlipidemia.     

Exogenous PTH and Endogenous 1,25‐Dihydroxyvitamin D Are Complementary in Inducing an Anabolic Effect on Bone

PTH and 1,25(OH)₂D each exert dual anabolic and catabolic skeletal effects. We assessed the potential interaction of PTH and 1,25(OH)₂D in promoting skeletal anabolism by comparing the capacity of exogenous, intermittently injected PTH(1‐34) to produce bone accrual in mice homozygous for the 1α(OH)ase‐null allele [1α(OH)ase^?/? mice] and in wildtype mice. In initial studies, 3‐mo‐old wildtype mice were either injected once daily (40 μg/kg) or infused continuously (120 μg/kg/d) with PTH(1–34) for up to 1 mo. Infused PTH reduced BMD, increased the bone resorption marker TRACP‐5b, and raised serum calcium but did not increase serum 1,25(OH)₂D. Injected PTH increased serum 1,25(OH)₂D and BMD, raised the bone formation marker osteocalcin more than did infused PTH, and did not produce sustained hypercalcemia as did PTH infusion. In subsequent studies, 3‐mo‐old 1α(OH)ase^?/? mice, raised on a rescue diet, and wildtype littermates were injected with PTH(1–34) (40 μg/kg) either once daily or three times daily for 1 mo. In 1α(OH)ase^?/? mice, baseline bone volume (BV/TV) and bone formation (BFR/BS) were lower than in wildtype mice. PTH administered intermittently increased BV/TV and BFR/BS in a dose‐dependent manner, but the increases were always less than in wildtype mice. These studies show that exogenous PTH administered continuously resorbs bone without raising endogenous 1,25(OH)₂D. Intermittently administered PTH can increase bone accrual in the absence of 1,25(OH)₂D, but 1,25(OH)₂D complements this PTH action. An increase in endogenous 1,25(OH)₂D may therefore facilitate an optimal skeletal anabolic response to PTH and may be relevant to the development of improved therapeutics for enhancing skeletal anabolism.     

A comparison of parathyroid hormone‐related protein (1‐36) and parathyroid hormone (1‐34) on markers of bone turnover and bone density in postmenopausal women: The PrOP study

Parathyroid hormone‐related protein (PTHrP)(1‐36) increases lumbar spine (LS) bone mineral density (BMD), acting as an anabolic agent when injected intermittently, but it has not been directly compared with parathyroid hormone (PTH)(1‐34). We performed a 3‐month randomized, prospective study in 105 postmenopausal women with low bone density or osteoporosis, comparing daily subcutaneous injections of PTHrP(1‐36) to PTH(1‐34). Thirty‐five women were randomized to each of three groups: PTHrP(1‐36) 400 µg/day; PTHrP(1‐36) 600 µg/day; and PTH(1‐34) 20 µg/day. The primary outcome measures were changes in amino‐terminal telopeptides of procollagen 1 (PINP) and carboxy‐terminal telopeptides of collagen 1 (CTX). Secondary measures included safety parameters, 1,25(OH)₂ vitamin D, and BMD. The increase in bone resorption (CTX) by PTH(1‐34) (92%) (p < 0.005) was greater than for PTHrP(1‐36) (30%) (p < 0.05). PTH(1‐34) also increased bone formation (PINP) (171%) (p < 0.0005) more than either dose of PTHrP(1‐36) (46% and 87%). The increase in PINP was earlier (day 15) and greater than the increase in CTX for all three groups. LS BMD increased equivalently in each group (p < 0.05 for all). Total hip (TH) and femoral neck (FN) BMD increased equivalently in each group but were only significant for the two doses of PTHrP(1‐36) (p < 0.05) at the TH and for PTHrP(1‐36) 400 (p < 0.05) at the FN. PTHrP(1‐36) 400 induced mild, transient (day 15) hypercalcemia. PTHrP(1‐36) 600 required a dose reduction for hypercalcemia in three subjects. PTH(1‐34) was not associated with hypercalcemia. Each peptide induced a marked biphasic increase in 1,25(OH)₂D. Adverse events (AE) were similar among the three groups. This study demonstrates that PTHrP(1‐36) and PTH(1‐34) cause similar increases in LS BMD. PTHrP(1‐36) also increased hip BMD. PTH(1‐34) induced greater changes in bone turnover than PTHrP(1‐36). PTHrP(1‐36) was associated with mild transient hypercalcemia. Longer‐term studies using lower doses of PTHrP(1‐36) are needed to define both the optimal dose and full clinical benefits of PTHrP. © 2013 American Society for Bone and Mineral Research. © 2013 American Society for Bone and Mineral Research.     

Treatment with Parathyroid Hormone hPTH(1-34), hPTH(1-31), and Monocyclic hPTH(1-31) Enhances Fracture Strength and Callus Amount After Withdrawal Fracture Strength and Callus Mechanical Quality Continue to Increase

The influence of intermittent hPTH(l-34)NH₂, hPTH(1-31)NH₂, and monocyclic [Leu²⁷]cyclo(Glu²²-Lys²⁶)hPTH(1-31)NH₂ treatment on callus formation, mechanical strength, and callus tissue mechanical quality of tibial fractures in rats was investigated after 8 and 16 weeks of healing. In the 8 weeks of healing animals, the PTH-peptides were injected subcutaneously during the entire observation period (15 nmol/kg/day [hPTH(1-34)NH₂: 15 nmol = 60 µg]), and control animals with fractures were given vehicle. In the 16 weeks of healing animals, the PTH-peptides were injected only during the first 8 weeks of healing (15 nmol/kg/day), after which the animals were left untreated during the rest of the healing period. After the first 8 weeks of healing, increased fracture strength and callus volume were seen in the PTH-treated rats (ultimate load 66%, ultimate stiffness 58%, callus volume 28%), and the three peptides were equally effective. No difference in callus tissue mechanical quality was found between PTH and vehicle animals. After 16 weeks of healing, no differences in fracture strength, callus volume, or callus tissue mechanical quality were seen between PTH and vehicle. When comparing PTH-treated animals at 8 and 16 weeks, fracture strength and callus tissue mechanical quality continued to increase after the withdrawal of PTH (ultimate load 23%, ultimate stress 88%, elastic modulus 87%) and external callus volume declined during this period (27%).     

Parathyroid hormone (PTH)–induced bone gain is blunted in SOST overexpressing and deficient mice

Intermittent parathyroid hormone (PTH) treatment is a potent bone anabolic principle that suppresses expression of the bone formation inhibitor Sost. We addressed the relevance of Sost suppression for PTH‐induced bone anabolism in vivo using mice with altered Sost gene dosage. Six‐month‐old Sost overexpressing and 2‐month‐old Sost deficient male mice and their wild‐type littermates were subjected to daily injections of 100 µg/kg PTH(1–34) or vehicle for a 2‐month period. A follow‐up study was performed in Sost deficient mice using 40 and 80 µg/kg PTH(1–34). Animals were sacrificed 4 hours after the final PTH administration and Sost expression in long bone diaphyses was determined by qPCR. Bone changes were analyzed in vivo in the distal femur metaphysis by pQCT and ex vivo in the tibia and lumbar spine by DXA. Detailed ex vivo analyses of the femur were performed by pQCT, µCT, and histomorphometry. Overexpression of Sost resulted in osteopenia and Sost deletion in high bone mass. As shown before, PTH suppressed Sost in wild‐type mice. PTH treatment induced substantial increases in bone mineral density, content, and cortical thickness and in aging wild‐type mice also led to cancellous bone gain owing to amplified bone formation rates. PTH‐induced bone gain was blunted at all doses and skeletal sites in Sost overexpressing and deficient mice owing to attenuated bone formation rates, whereas bone resorption was not different from that in PTH‐treated wild‐type controls. These data suggest that suppression of the bone formation inhibitor Sost by intermittent PTH treatment contributes to PTH bone anabolism. © 2010 American Society for Bone and Mineral Research     

New PTH assays and renal osteodystrophy

Parathyroid hormone (PTH) levels have been used instead of bone histomorphometric analysis in renal failure, but the assessment of tetracycline-labeled bone biopsy remains the most reliable method to diagnose the different subtypes of renal osteodystrophy. The availability of the first-generation immunometric PTH assay (1^st PTH-IMA) allowed the distinction between the different types of renal bone diseases. However, 1^st PTH-IMA not only detects the intact hormone PTH(1–84), but also additional PTH truncated fragments. A second-generation immunometric PTH assay (2^nd PTH-IMA) recognizes only PTH(1–84) and possible PTH fragments that are truncated at the carboxyl-terminus, but not PTH(7–84). In addition, whether assessment of the ratio PTH(1–84) and amino-terminally truncated PTH(1–84) fragments is a better predictor of bone turnover remains controversial. An initial study using the 2^nd PTH-IMA suggested that the ratio between PTH(1–84) and amino-terminally truncated PTH(1–84) fragments more accurately predicts bone turnover in adult patients treated with hemodialysis. However, subsequent studies using the Scantibodies assay have failed to better predict the underlying bone disease in adults undergoing maintenance hemodialysis. Furthermore, a different 2^nd PTH-IMA (Immutopics) with similar, but not identical, in vitro characteristics did not show a superior predictive value of the ratio in pediatric patients treated with peritoneal dialysis. Although the 2^nd PTH-IMA may provide important new insights into the physiology of parathyroid gland function, at present, measurement of PTH using either 1^st or 2^nd PTH-IMAs provides similar accuracy for predicting bone turnover in patients treated with dialysis. Thus, the current data do not yet support the claim that 2^nd PTH-IMAs provide an advantage over 1^st PTH-IMAs for the diagnosis of the different subtypes of renal bone diseases.     

PTH(1‐34) Treatment Increases Bisphosphonate Turnover in Fracture Repair in Rats

Bisphosphonates (BP) are antiresorptive drugs with a high affinity for bone. Despite the therapeutic success in treating osteoporosis and metabolic bone diseases, chronic BP usage has been associated with reduced repair of microdamage and atypical femoral fracture (AFF). The latter has a poor prognosis, and although anabolic interventions such as teriparatide (PTH_(1–34)) have been suggested as treatment options, there is a limited evidence base in support of their efficacy. Because PTH_(1–34) acts to increase bone turnover, we hypothesized that it may be able to increase BP in turnover in the skeleton, which, in turn, may improve bone healing. To test this, we employed a mixture of fluorescent Alexa647‐labelled pamidronate (Pam) and radiolabeled ¹⁴C‐ZA (zoledronic acid). These traceable BPs were dosed to Wistar rats in models of normal growth and closed fracture repair. Rats were cotreated with saline or 25 μg/kg/d PTH_(1–34), and the effects on BP liberation and bone healing were examined by X‐ray, micro‐CT, autoradiography, and fluorescent confocal microscopy. Consistent with increased BP remobilization with PTH_(1–34), there was a significant decrease in fluorescence in both the long bones and in the fracture callus in treated animals compared with controls. This was further confirmed by autoradiography for ¹⁴C‐ZA. In this model of acute BP treatment, callus bone volume (BV) was significantly increased in fractured limbs, and although we noted significant decreases in callus‐bound BP with PTH_(1–34), these were not sufficient to alter this BV. However, increased intracellular BP was noted in resorbing osteoclasts, confirming that, in principle, PTH_(1–34) increases bone turnover as well as BP turnover. © 2015 American Society for Bone and Mineral Research.     

Role of the Parathyroid Hormone Type 1 Receptor (PTH1R) as a Mechanosensor in Osteocyte Survival

Osteocytes have a major role in the control of bone remodeling. Mechanical stimulation decreases osteocyte apoptosis and promotes bone accrual, whereas skeletal unloading is deleterious in both respects. PTH1R ablation or overexpression in osteocytes in mice produces trabecular bone loss or increases bone mass, respectively. The latter effect was related to a decreased osteocyte apoptosis. Here, the putative role of PTH1R activation in osteocyte protection conferred by mechanical stimulation was assessed. Osteocytic MLO‐Y4 cells were subjected to mechanical stimuli represented by hypotonic shock (216 mOsm/kg) or pulsatile fluid flow (8 Hz, 10 dynes/cm²) for a short pulse (10 min), with or without PTH1R antagonists or after transfection with specific PTHrP or PTH1R siRNA. These mechanical stimuli prevented cell death induced within 6 hours by etoposide (50 μM), related to PTHrP overexpression; and this effect was abolished by the calcium antagonist verapamil (1 μM), a phospholipase C (PLC) inhibitor (U73122; 10 μM), and a PKA activation inhibitor, Rp‐cAMPS (25 μM), in these cells. Each mechanical stimulus also rapidly induced β‐catenin stabilization and nuclear ERK translocation, which were inhibited by the PTH1R antagonist PTHrP(7–34) (1 μM), or PTH1R siRNA, and mimicked by PTHrP(1–36) (100 nM). Mechanical stretching by hypotonic shock did not affect cAMP production but rapidly (<1 min) stimulated Ca_i²⁺ transients in PTH1R‐overexpressing HEK‐293 cells and in MLO‐Y4 cells, in which calcium signaling was unaffected by the presence of a PTHrP antiserum or PTHrP siRNA but inhibited by knocking down PTH1R. These novel findings indicate that PTH1R is an important component of mechanical signal transduction in osteocytic MLO‐Y4 cells, and that PTH1R activation by PTHrP‐independent and dependent mechanisms has a relevant role in the prosurvival action of mechanical stimulus in these cells. © 2014 American Society for Bone and Mineral Research © 2014 American Society for Bone and Mineral Research     

PTH(1‐84) replacement therapy in hypoparathyroidism: A randomized controlled trial on pharmacokinetic and dynamic effects after 6 months of treatment

Untreated, hypoparathyroidism (hypoPT) is a state of hypocalcemia with inappropriately low plasma parathyroid hormone (PTH) levels and hyperphosphatemia. PTH administration normalizes plasma calcium and phosphate levels and reduces the doses of calcium and active vitamin D analogues needed. To develop an evidence‐based clinical algorithm to monitor hypoPT patients treated with recombinant human PTH (rhPTH[1‐84]) injected subcutaneously in the thigh, we performed a 24‐hour monitoring study of pharmacokinetic and pharmacodynamic effects in a group of 38 patients who had completed a 6‐month randomized study on effects of treatment with a fixed rhPTH(1‐84) dose of 100 µg/d or similar placebo as an add‐on to conventional treatment. PTH levels rose immediately, reaching a median peak level of 26.5 (interquartile range [IQR], 20.1–42.5) pmol/L 15 minutes following injection. Thereafter, levels gradually decreased until reaching predosing levels after 16 hours, with a plasma half‐life of 2.2 (1.7–2.5) hours. rhPTH(1‐84) changed the diurnal rhythms of ionized calcium levels and 1,25‐dihydroxyvitamin D (1,25[OH]₂D) levels, with rising levels following injection. Ionized calcium peaked after 7.0 (5.0–10.0) hours. Asymptomatic hypercalcemia was present in 71% of the rhPTH(1‐84)‐treated patients. Compared with placebo, 24‐hour urinary calcium, phosphate, and magnesium did not change, although the diurnal variation in renal excretion rates changed significantly in response to treatment. In conclusion, as a safety precaution, we recommend occasionally measuring calcium levels at approximately 7 hours after administration in order to reveal episodes of hypercalcemia. A 100‐µg daily dose of rhPTH(1‐84) appears to be too high in some patients, suggesting a need for a device allowing for individual dose adjustments. © 2013 American Society for Bone and Mineral Research.