Plasma levels of parathyroid hormone that induce anabolic effects in bone of ovariectomized rats can be achieved by stimulation of endogenous hormone secretion

Physiological and Pharmacological Roles of PTH and PTHrP in Bone Using Their Shared Receptor, PTH1R

Parathyroid hormone (PTH) and the paracrine factor, PTH-related protein (PTHrP), have preserved in evolution sufficient identities in their amino-terminal domains to share equivalent actions upon a common G protein-coupled receptor, PTH1R, that predominantly uses the cyclic adenosine monophosphate-protein kinase A signaling pathway. Such a relationship between a hormone and local factor poses questions about how their common receptor mediates pharmacological and physiological actions of the two. Mouse genetic studies show that PTHrP is essential for endochondral bone lengthening in the fetus and is essential for bone remodeling. In contrast, the main postnatal function of PTH is hormonal control of calcium homeostasis, with no evidence that PTHrP contributes. Pharmacologically, amino-terminal PTH and PTHrP peptides (teriparatide and abaloparatide) promote bone formation when administered by intermittent (daily) injection. This anabolic effect is remodeling-based with a lesser contribution from modeling. The apparent lesser potency of PTHrP than PTH peptides as skeletal anabolic agents could be explained by lesser bioavailability to PTH1R. By contrast, prolongation of PTH1R stimulation by excessive dosing or infusion, converts the response to a predominantly resorptive one by stimulating osteoclast formation. Physiologically, locally generated PTHrP is better equipped than the circulating hormone to regulate bone remodeling, which occurs asynchronously at widely distributed sites throughout the skeleton where it is needed to replace old or damaged bone. While it remains possible that PTH, circulating within a narrow concentration range, could contribute in some way to remodeling and modeling, its main physiological role is in regulating calcium homeostasis.

Impact of Nutrition-Based Interventions on Athletic Performance during Menstrual Cycle Phases: A Review

Despite the steady increase in female participation in sport over the last two decades, comprehensive research on interventions attenuating the influence of female menstrual physiology on performance remains scarce. Studies involving eumenorrheic women often only test in one menstrual phase to limit sex hormone variance, which may restrict the application of these findings to the rest of the menstrual cycle. The impacts of nutrition-based interventions on athletic performance throughout the menstrual cycle have not been fully elucidated. We addressed this gap by conducting a focused critical review of clinical studies that reported athletic outcomes as well as menstrual status for healthy eumenorrheic female participants. In total, 1443 articles were identified, and 23 articles were included. These articles were published between 2011 and 2021, and were retrieved from Google Scholar, Medline, and PubMed. Our literature search revealed that hydration-, micronutrient-, and phytochemical-based interventions can improve athletic performance (measured by aerobic capacity, anaerobic power, and strength performance) or attenuate exercise-induced damage (measured by dehydration biomarkers, muscle soreness, and bone resorption biomarkers). Most performance trials, however, only assessed these interventions in one menstrual phase, limiting the application throughout the entire menstrual cycle. Improvements in athletic performance through nutrition-based interventions may be contingent upon female sex hormone variation in eumenorrheic women.

Development of a Novel Human Parathyroid Hormone Receptor 1 (hPTHR1) Agonist (CH5447240), a Potent and Orally Available Small Molecule for Treatment of Hypoparathyroidism

During the course of derivatization of HTS hit 4a, we have identified a novel small-molecule hPTHR1 agonist, 1-(3,5-dimethyl-4-(2-((2-((1 R,4 R)-4-methylcyclohexyl)-4-oxo-1,3,8-triazaspiro[4.5]dec-1-en-8-yl)sulfonyl)ethyl)phenyl)-1-methylurea (CH5447240, 14l). Compound 14l exhibited a potent in vitro hPTHR1 agonist effect with EC₂₀ of 3.0 μM and EC₅₀ of 12 μM and showed excellent physicochemical properties, such as high solubility in fasted state simulated intestinal fluid and good metabolic stability in human liver microsomes. Importantly, 14l showed 55% oral bioavailability and a significantly elevated serum calcium level in hypocalcemic model rats.

Parathyroid Hormone-Related Protein, Its Regulation of Cartilage and Bone Development, and Role in Treating Bone Diseases

Although parathyroid hormone-related protein (PTHrP) was discovered as a cancer-derived hormone, it has been revealed as an important paracrine/autocrine regulator in many tissues, where its effects are context dependent. Thus its location and action in the vasculature explained decades-long observations that injection of PTH into animals rapidly lowered blood pressure by producing vasodilatation. Its roles have been specified in development and maturity in cartilage and bone as a crucial regulator of endochondral bone formation and bone remodeling, respectively. Although it shares actions with parathyroid hormone (PTH) through the use of their common receptor, PTHR1, PTHrP has other actions mediated by regions within the molecule beyond the amino-terminal sequence that resembles PTH, including the ability to promote placental transfer of calcium from mother to fetus. A striking feature of the physiology of PTHrP is that it possesses structural features that equip it to be transported in and out of the nucleus, and makes use of a specific nuclear import mechanism to do so. Evidence from mouse genetic experiments shows that PTHrP generated locally in bone is essential for normal bone remodeling. Whereas the main physiological function of PTH is the hormonal regulation of calcium metabolism, locally generated PTHrP is the important physiological mediator of bone remodeling postnatally. Thus the use of intermittent injection of PTH as an anabolic therapy for bone appears to be a pharmacological application of the physiological function of PTHrP. There is much current interest in the possibility of developing PTHrP analogs that might enhance the therapeutic anabolic effects.

The Optimal Duration of PTH(1-34) Infusion Is One Hour per Day to Increase Bone Mass in Rats

Parathyroid hormone (PTH) is a potential medicine for osteoporosis, and subcutaneous (s.c.) PTH treatment enhances bone mass; however, continuous infusion of PTH elicits bone resorption and induces bone loss. To clarify this contradictory phenomenon, we examined bone markers and bone mass in rats to assess the optimal duration of PTH(1-34) infusion. Continuous infusion of PTH at 1 µg/kg/h (Css, steady-state concentration ca. 300 pg/mL) for 1-4 h clearly stimulated the expression both of bone formation-related genes (c-fos, Wnt4, EphrinB2) and of bone resorption-related genes (tnfsf11, tnfsf11b, encoding receptor activator of nuclear factor-kappaB ligand (RANKL), osteoprotegerin (OPG)), but s.c. treatment stimulated these genes only 1-h after the injection. Rats were treated with 1-, 2-, or 4-h infusions of PTH daily using a totally implanted catheter system, and the femoral bone mineral density (BMD) was measured at 4 weeks. The 1-h infusion of PTH significantly stimulated serum bone formation markers (procollagen I N-terminal propeptide (PINP) and osteocalcin) on day 14 and femoral BMD at 2 and 4 weeks, but the 4-h infusion of PTH did not enhance BMD. Since the 4-h infusion increased the levels of both the bone formation markers and a bone resorption marker (urinary C-terminal telopeptide of type 1 collagen (CTx)), the increased bone resorption may predominate over bone formation. The intermittent elevation of plasma PTH to 300 pg/mL for 1-h each day is optimal for increasing bone mass in rats. In osteoporosis therapy in human, using the optimal duration for the clinical dose of PTH may selectively stimulate bone formation.

Treadmill running reduces parathyroid hormone concentrations during recovery compared with a nonexercising control group

CONTEXT

Lower PTH concentrations reported in the hours after acute, endurance exercise compared with preexercise levels might be influenced by factors such as circadian fluctuations.

OBJECTIVE

The objective of the study was to compare postexercise PTH concentrations with a nonexercising control group.

DESIGN AND SETTING

A laboratory-based study with a crossover design, comparing a 60-minute (at 10:30 am) bout of treadmill running at 65% of the maximal rate of oxygen uptake (exercise) with semirecumbent rest (CON). Blood samples were obtained immediately before (baseline 10:15 am) and after (11:30 am) exercise and during recovery (12:30 am, 1:30 pm, and 2:15 pm).

PARTICIPANTS

Ten physically active men (mean ± 1 SD, age 26 ± 5 y; body mass 78.3 ± 5.8 kg; maximal rate of oxygen uptake 57.3 ± 6.9 mL/kg(-1) · min(-1)) participated in the study.

MAIN OUTCOME MEASURES

PTH, albumin-adjusted calcium, and phosphate concentrations were measured.

RESULTS

PTH concentrations increased (+85%, P < .01) during exercise and were higher than in CON immediately at the end of exercise (4.5 ± 1.9 vs 2.6 ± 0.9 pmol/L(-1), P < .05). In the postexercise period (12:30-2:15 pm), PTH was not different compared with baseline but was lower compared with CON at 1:30 pm (-22%; P < .01) and tended to be lower at both 12:30 pm (-12%; P = .063) and 2:15 pm (-13%; P = .057). Exercise did not significantly affect the albumin-adjusted calcium concentrations, whereas phosphate was higher than CON immediately after exercise (1.47 ± 0.17 vs 1.03 ± 0.17 pmol/L(-1), P < .001) and was lower at 1:30 pm (-16%: P < .05).

CONCLUSIONS

Lower PTH concentrations after acute endurance running compared with a rested control condition suggest a true effect of exercise.

Bone biology and anabolic therapies for bone: current status and future prospects

Bone is continuously remodelled at many sites asynchronously throughout the skeleton, with bone formation and resorption balanced at these sites to retain bone structure. Negative balance resulting in bone loss and osteoporosis, with consequent fractures, has mainly been prevented or treated by anti-resorptive drugs that inhibit osteoclast formation and/or activity, with new prospects now of anabolic treatments that restore bone that has been lost. The anabolic effectiveness of parathyroid hormone has been established, and an exciting new prospect is presented of neutralising antibody against the osteocyte protein, sclerostin. The cellular actions of these two anabolic treatments differ, and the mechanisms will need to be kept in mind in devising their best use. On present evidence it seems likely that treatment with either of these anabolic agents will need to be followed by anti-resorptive treatment in order to maintain bone that has been restored. No matter how effective anabolic therapies for the skeleton become, it seems highly likely that there will be a continuing need for safe, effective anti-resorptive drugs.

Effect of recovery duration between two bouts of running on bone metabolism

PURPOSE

Strenuous endurance exercise increases biochemical markers of bone resorption but not formation, although the effect of recovery duration between consecutive bouts of exercise is unknown. We examined the effect of recovery duration on the bone metabolic response to two bouts of running.

METHODS

Ten physically active men completed two 9-d trials. On days 4 and 5 (D4 and D5), participants completed two 60-min bouts of running at 65% V˙O2max separated by either a 23-h (LONG) or a 3-h (SHORT) recovery period. Osteoprotegerin (OPG), parathyroid hormone (PTH), albumin-adjusted calcium (ACa), and phosphate (PO4) were measured from blood samples obtained before and for 3 h after exercise and on four follow-up days (D6-D9). Markers of bone resorption (C-terminal telopeptide region of collagen type 1) and bone formation (N-terminal propeptides of procollagen type 1 and bone alkaline phosphatase) were measured in early morning fasted samples on D4-D9.

RESULTS

There were no significant changes in C-terminal telopeptide region of collagen type 1, N-terminal propeptides of procollagen type 1, or bone alkaline phosphatase with either protocol. OPG, PTH, ACa, and PO4 concentrations increased with all exercise bouts, but the response to the second bout was not altered by recovery duration.

CONCLUSIONS

Two 60-min bouts of running at 65% V˙O2max separated by either 23 or 3 h had no effect on the markers of bone resorption or formation from 1 to 4 d after exercise. Reducing recovery duration from 23 to 3 h between two bouts of running did not alter the increase in OPG, PTH, ACa, and PO4 to the second bout.

Pharmacodynamic responses to combined treatment regimens with the calcium sensing receptor antagonist JTT-305/MK-5442 and alendronate in osteopenic ovariectomized rats

Parathyroid hormone (PTH) is the anabolic standard of care for patients with severe osteoporosis. The CaSR allosteric antagonist JTT-305/MK-5442, a PTH secretagogue, could offer an oral osteoanabolic treatment alternative for postmenopausal women with osteoporosis. Here we disclose the pharmacokinetic profile of JTT-305/MK-5442 and its activity on bone remodeling in ovariectomized (OVX) osteopenic rats. Daily treatments (0.3 to 2.4 mg/kg/d) for 12 weeks resulted in plateaued BMD increases (3.8 to 5.3%) at axial and appendicular skeletal sites. However, treatment effects were not statistically significant, in agreement with effects seen in animals treated with low dose PTH (1-84) (5 μg/kg/d). In a consecutive study we tested JTT-305/MK-5442 effects on bone formation in OVX-rats challenged with combined alendronate (ALN) treatment paradigms. At 7 month, JTT-305/MK-5442 treatment significantly increased BMD in lumbar vertebrae (LV), while no change in BMD was observed in femora or tibiae. ALN add-on co-treatment produced incremental increases in LV, distal femur (DF) and proximal tibia (PT) BMD over the respective ALN control. Histological analyses confirmed modest increases in mineralized surface (MS/BS) and bone formation rate (30.5±1.9%) on trabecular surfaces by JTT-305/MK-5442. As expected, ALN administration profoundly reduced bone formation, however, JTT-305/MK-5442 significantly stimulated MS/BS and BFR in ALN treated groups. In summary, JTT-305/MK-5442 acts as a PTH secretagogue in the osteopenic OVX-rat, eliciting consistent, though modest effects on remediation of BMD due to estrogen depletion. Induction of bone formation by JTT-305/MK-5442 at trabecular bone surfaces appears to be resilient to ALN-mediated suppression of bone formation. This study provides for the first time, a mechanistic evaluation of combination treatment of a PTH secretagogue with ALN.

ATF936, a novel oral calcilytic, increases bone mineral density in rats and transiently releases parathyroid hormone in humans

Parathyroid hormone (PTH), when injected daily as either the intact hormone PTH(1-84) or the active fragment PTH(1-34) (teriparatide), is an efficacious bone anabolic treatment option for osteoporosis patients. Injections lead to rapid and transient spikes in hormone exposure levels, a profile which is a prerequisite to effectively form bone. Oral antagonists of the calcium-sensing receptor (calcilytics) stimulate PTH secretion and represent thus an alternative approach to elevate hormone levels transiently. We report here on ATF936, a novel calcilytic, which triggered rapid, transient spikes in endogenous PTH levels when given orally in single doses of 10 and 30mg/kg to growing rats, and of 1mg/kg to dogs. Eight weeks daily oral application of 30mg/kg of ATF936 to aged female rats induced in the proximal tibia metaphysis increases in bone mineral density, cancellous bone volume and cortical and trabecular thickness as evaluated by computed tomography. In healthy humans, single oral doses of ATF936 produced peak PTH levels in plasma after a median time of 1h and levels returned to normal at 24-h post-dose. The average maximum PTH concentration increase from baseline was 1.9, 3.6, and 6.0-fold at doses of 40, 70, and 140mg. ATF936 was well tolerated. The sharp, transient increase in PTH levels produced by the oral calcilytic ATF936 was comparable to the PTH profile observed after subcutaneous administration of teriparatide. In conclusion, ATF936 might hold potential as an oral bone-forming osteoporosis therapy.

Novel and potent calcium-sensing receptor antagonists: discovery of (5R)-N-[1-ethyl-1-(4-ethylphenyl)propyl]-2,7,7-trimethyl-5-phenyl-4,5,6,7-tetrahydropyrazolo[1,5-a]pyrimidine-3-carboxamide monotosylate (TAK-075) as an orally active bone anabolic agent

The calcium-sensing receptor antagonist (CaSR) has been recognized as a promising target of anabolic agents for treating osteoporosis. In the course of developing a new drug candidate for osteoporosis, we found tetrahydropyrazolopyrimidine derivative 1 to be an orally active CaSR antagonist that stimulated transient PTH secretion in rats. However, compound 1 showed poor physical and chemical stability. In order to work out this compound's chemical stability and further understand its in vivo efficacy, we focused on modifying the 2-position of the tetrahydropyrazolopyrimidine. As a result of chemical modification, we discovered (5R)-N-[1-ethyl-1-(4-ethylphenyl)propyl]-2,7,7-trimethyl-5-phenyl-4,5,6,7-tetrahydropyrazolo[1,5-a]pyrimidine-3-carboxamide monotosylate 10m (TAK-075), which showed improved solubility, chemical stability, and in vivo efficacy. Furthermore, we describe that evaluating the active metabolite is important during repeated treatment with short-acting CaSR antagonists.

Allosteric modulation of family C G-protein-coupled receptors: from molecular insights to therapeutic perspectives

Allosteric receptor modulation is an attractive concept in drug targeting because it offers important potential advantages over conventional orthosteric agonism or antagonism. Allosteric ligands modulate receptor function by binding to a site distinct from the recognition site for the endogenous agonist. They often have no effect on their own and therefore act only in conjunction with physiological receptor activation. This article reviews the current status of allosteric modulation at family C G-protein coupled receptors in the light of their specific structural features on the one hand and current concepts in receptor theory on the other hand. Family C G-protein-coupled receptors are characterized by a large extracellular domain containing the orthosteric agonist binding site known as the "venus flytrap module" because of its bilobal structure and the dynamics of its activation mechanism. Mutational analysis and chimeric constructs have revealed that allosteric modulators of the calcium-sensing, metabotropic glutamate and GABA(B) receptors bind to the seven transmembrane domain, through which they modify signal transduction after receptor activation. This is in contrast to taste-enhancing molecules, which bind to different parts of sweet and umami receptors. The complexity of interactions between orthosteric and allosteric ligands is revealed by a number of adequate biochemical and electrophysiological assay systems. Many allosteric family C GPCR modulators show in vivo efficacy in behavioral models for a variety of clinical indications. The positive allosteric calcium sensing receptor modulator cinacalcet is the first drug of this type to enter the market and therefore provides proof of principle in humans.

Mechanism-based pharmacokinetic/pharmacodynamic model of parathyroid hormone-calcium homeostasis in rats and humans

The purpose of this study was to develop a mechanism-based pharmacokinetic/pharmacodynamic model that describes the regulation of the parathyroid hormone (PTH)-Ca(2+) system in rats and humans. Temporal concentration data for endogenous PTH and Ca(2+) were extracted from literature for rats (normal adult males) and humans. In addition, exogenous PTH was administered subcutaneously to male Sprague-Dawley rats with jugular vein catheters, and plasma concentrations were measured over time. A mathematical model was developed and fitted simultaneously to endogenous PTH, Ca(2+), and exogenous PTH concentrations in rats. Ca(2+) concentrations were described using a turnover model, with its depletion being induced by a chelating agent, and PTH concentrations were characterized using a precursor-dependent indirect response model. The same structural model was used for fitting data obtained in humans. PTH stimulation was driven by occupancy of the Ca(2+) sensing receptor, and lowering of physiological Ca(2+) concentrations increased PTH secretion, with PTH profiles being adequately described by the model. PTH stimulatory capacity was baseline-dependent in rats [S(max_rats) = 34.8 x PTH(0)] and humans [S(max_humans) = 392/PTH(0)]. Modeling results suggest that normal rats are twice as sensitive to Ca(2+)-induced PTH stimulation compared with humans. In conclusion, the developed model adequately characterizes the PTH-Ca(2+) regulation across species and may be useful in the development of therapeutic drugs targeting this system.

Design, new synthesis, and calcilytic activity of substituted 3H-pyrimidin-4-ones

Design, new synthesis, structure-activity relationship studies and calcium receptor antagonist (calcilytic) properties of novel 3H-pyrimidin-4-ones are described.

3H-Quinazolin-4-ones as a new calcilytic template for the potential treatment of osteoporosis

Structure-activity relationship studies, focused on identification of the active pharmacophore fragments in a single high-throughput screening calcilytic hit, resulted in the discovery of potent calcium receptor antagonists, substituted 3H-quinazolin-4-ones.

Discovery and structure–activity relationships of 2-benzylpyrrolidine-substituted aryloxypropanols as calcium-sensing receptor antagonists

A structure-activity relationship study of the amine portion of the calcilytic compound NPS-2143 resulted in the discovery of substituted 2-benzylpyrrolidines as replacements for the 1,1-dimethyl-2-naphthalen-2-yl-ethylamine. When compared to NPS-2143, a newly discovered compound, 3h, exhibited similar potency as a calcium-sensing receptor (CaR) antagonist and a superior human ether-a-go-go related gene (hERG) profile.

The physiology of the circadian rhythm of parathyroid hormone and its potential as a treatment for osteoporosis

PURPOSE OF REVIEW

A circadian rhythm exists for parathyroid hormone, with a biphasic pattern showing a late afternoon/early evening rise and fall and a broader, longer-lasting increase late evening/early morning reaching nadir mid-morning. This review explores the characteristics of the circadian rhythm, factors regulating the rhythm and its role in bone metabolism.

RECENT FINDINGS

Gender differences exist in the circadian rhythm for parathyroid hormone. Ageing in women alters the response to calcium infusion, increasing the suppression of parathyroid hormone secretion and decreasing bone resorption. There is no difference between young and elderly men in the parathyroid hormone response to calcium infusion. Loop diuretic ingestion alters the parathyroid hormone circadian rhythm reflecting loop diuretic effects on phosphate and calcium metabolism. Adult growth hormone deficiency alters parathyroid hormone secretion and end organ sensitivity, but the circadian rhythm is retained. Growth hormone replacement therapy enhances the parathyroid hormone circadian rhythm and increases end organ responses. Exogenous parathyroid hormone (1-34) and (1-84) administered by daily injection has an anabolic effect on bone, increasing bone mass and decreasing fracture. Calcilytic drugs stimulate and calcimimetic drugs suppress parathyroid hormone secretion and have been used to treat disorders of bone metabolism.

SUMMARY

The circadian nature of parathyroid hormone secretion is confirmed by many publications. The underlying rhythm is endogenous. Life style factors and nutritional intake modulate the pattern of secretion. Direct association with bone resorption and formation is tentative. It is suggested that acute changes in these rhythms have little effect on resorption, but longer-term manipulation of parathyroid hormone secretion alters bone cell function. Growth hormone therapy in adult deficiency increases parathyroid hormone activity, indicating growth hormone may have therapeutic potential for osteoporosis. Manipulation of the endogenous parathyroid hormone rhythm, using timed supplements of phosphate or calcium or by calcilytic and calcimimetic molecules, offers a novel approach to osteoporosis treatment.

Harnessing the parathyroids to create stronger bones

PURPOSE OF REVIEW

Parathyroid hormone is anabolic to bone but when secreted in excess it is catabolic. It is important, therefore, to understand the mechanisms that determine the normal circadian rhythm of parathyroid hormone secretion and whether the cellular response to it will be anabolic or catabolic. This may lead to new strategies for the treatment of osteoporosis and the low turnover bone disease of some dialysis patients.

RECENT FINDINGS

The parathyroid plays a central role in normal mineral and bone homeostasis by acting on its receptor, the PTH/PTHrP receptor (PTH1R). In fact there is more than one receptor - the PTH2 receptor and a putative carboxy-terminal PTH receptor. The latter, in particular, may be particularly relevant to our understanding of the role of parathyroid hormone and its use in pharmacology. Parathyroid hormone in excess destroys bone, as in most patients with chronic renal failure, and when it is lacking this may result in low turnover bone disease. At a more subtle level, patients with postmenopausal osteoporosis may have a blunting of the normal circadian rhythm of parathyroid hormone, with its peak at night and nadir in the morning. Insights as to what determines whether parathyroid hormone will be anabolic or catabolic to bone are reviewed.

SUMMARY

Attempting to correct the circadian rhythm in osteoporotic patients by calcilytic drugs or perhaps physiological equivalents may have a role in the future in treating osteoporosis. In the meantime, the administration of recombinant parathyroid hormone is an effective agent in the management of osteoporosis.

Anabolic and catabolic bone effects of human parathyroid hormone (1-34) are predicted by duration of hormone exposure

Parathyroid hormone (PTH)(1-34), given once daily, increases bone mass in a variety of animal models and humans with osteoporosis. However, continuous PTH infusion has been shown to cause bone loss. To determine the pharmacokinetic profile of PTH(1-34) associated with anabolic and catabolic bone responses, PTH(1-34) pharmacokinetic and serum biochemical profiles were evaluated in young male rats using dosing regimens that resulted in either gain or loss of bone mass. Once-daily PTH(1-34) or 6 PTH(1-34) injections within 1 h, for a total daily dose of 80 microg/kg, induced equivalent increases in proximal tibia bone mass. In contrast, 6 PTH(1-34) injections/day over 6 h for a total dose of 80 microg/kg/day or 3 injections/day over 8 h for a total of 240 microg/kg/day decreased tibia bone mass. The PTH(1-34) pharmacokinetics of the different treatment regimens were distinctive. The magnitude of the maximum serum concentrations (Cmax) of PTH(1-34) and area under the curve (AUC) did not predict the catabolic bone outcome. Compared to the anabolic pharmacokinetic profile of a transient increase in PTH(1-34) with rapid decreases in serum calcium and phosphate, the catabolic regimen was associated with PTH(1-34) concentrations remaining above baseline values during the entire 6-h dosing period with a trend toward an increase in serum calcium and a prolonged decrease in phosphate. The pharmacokinetic profiles suggest that the anabolic or catabolic response of bone to PTH(1-34) is determined primarily by the length of time each day that serum concentrations of PTH(1-34) remain above baseline levels of endogenous PTH and only secondarily by the Cmax or AUC of PTH(1-34) achieved.

Systemic delivery of parathyroid hormone (1-34) using inhalation dry powders in rats

The aim of this work was to prepare and characterize inhalation dry powders of human parathyroid hormone (PTH), as well as to assess their efficacy for systemic delivery of the peptide and safety in rats. The powders were prepared by spray-drying using PTH, sugars, dipalmitoylphosphatidylcholine, and/or albumin. They presented an average primary particle diameter of 4.5 microm and tap density of 0.06 g/cm(3), a mass median aerodynamic diameter between 3.9 and 5.9 microm, and reached up to 98% emitted dose and up to 61% fine particle fraction in the multi-stage liquid impinger using a Spinhaler inhaler device. Varying the airflow rate from 30 to 100 L/min had limited influence on the aerodynamic behavior of the aerosols. The absolute PTH bioavailability was 21% after intratracheal administration of the powder formed of PTH/albumin/lactose/dipalmitoylphosphatidylcholine and 18% after subcutaneous injection in rats. Equilibrium dialysis revealed a 78% binding of PTH to albumin and the withdrawal of albumin from the powder increased absolute bioavailability after inhalation from 21 to 34%. No acute inflammation appeared in the lung up to 48 h after a single inhalation. The increased bioavailability of the optimized powder aerosol of PTH makes it a promising alternative to subcutaneous injection.

Prevention of bone loss in ovariectomized rats by pulsatile transdermal iontophoretic administration of human PTH(1-34)

Serum human parathyroid hormone (1-34)[hPTH(1-34)] levels and the anabolic effect of hPTH(1-34) were compared after administration using multiple pulses of iontophoresis or subcutaneous (sc) intermittent injections to ovariectomized (OVX) Sprague Dawley rats. Triple-pulse iontophoretic administration of hPTH(1-34) (doses: 40-400 microg/patch), achieved by repeated 30-min applications of a 0.1 mA/cm(2) current separated by 45-min rest intervals, produced three sharp peaks in the serum hPTH(1-34) level in response to application of the current. Each peak appeared at the end of the 30-min current application period and was proportional to the hPTH(1-34) dose. Compared with once-daily sc injections (7 pulses/week), triple-pulses iontophoretic administered 3 times/week (9 pulses/week) for 4 weeks produced dose-related increases in the bone mineral density (BMD) of the distal 1/3 femur. For the sc administration, the relative BMD values using the vehicle injection as a reference standard for 1, 5, and 25 microg/kg/day were 104, 114, and 121%, respectively. For iontophoretic administration, the relative BMD values using the placebo patch as a reference standard for 40, 120, and 400 microg/patch were 104, 110, and 116%, respectively. The increase in the BMD plotted against the area under the hPTH(1-34) serum level-time curve (AUC) over 1 week resulted in similar straight lines in the 9 pulses/week iontophoretic administration and the 7 pulses/week sc administration groups. The estimated iontophoretic dose giving an equivalent BMD to once-daily sc administration at 5 microg/kg/day was 120 microg/patch. These findings strongly suggest that three iontophoretic pulses administered on alternate days will exert an anabolic effect equivalent to that of daily sc administration at doses giving the same weekly AUC. Furthermore, this method of administering hPTH(1-34) might enable self-medication, a useful advance in the treatment of osteoporosis.

Influence of a low calcium and phosphorus diet on the anabolic effect of human parathyroid hormone (1-38) in female rats

Parathyroid hormone (PTH) or synthetic N-terminal PTH fragments administered intermittently have been established as anabolic agents in animal and human bones. In the present study, the influence of a low calcium diet on the anabolic effect of human PTH(1-38) [hPTH(1-38)] was investigated. Forty-eight 10-week-old female Sprague-Dawley rats were randomly assigned to a diet with a low calcium content (LCa) or a diet with the recommended amount of calcium (RCa). After an adaptation period of 15 days, the rats were randomly assigned to hPTH(1-38) treatment (+LCa/+RCa) or vehicle only (-LCa/-RCa) for an additional 14 days. Total bone mineral density (BMD) values of several bones were determined using quantitative computed tomography and from ratios of ash weight to volume. Biomechanical competence of the fourth lumbar vertebrae and of the right femora was assessed. An anabolic effect could be detected in both PTH-treated groups. However, the bones of the +LCa group showed significantly lower BMD and also a diminished increase in maximal breaking force compared with those of the +RCa group. The study demonstrates that the anabolic effect of hPTH(1-38) is blunted by the LCa diet. This suggests that, during PTH treatment, dietary calcium intake is critical.

Human parathyroid hormone 1-34 reverses bone loss in ovariectomized mice

The experimental work characterizing the anabolic effect of parathyroid hormone (PTH) in bone has been performed in nonmurine ovariectomized (OVX) animals, mainly rats. A major drawback of these animal models is their inaccessibility to genetic manipulations such as gene knockout and overexpression. Therefore, this study on PTH anabolic activity was carried out in OVX mice that can be manipulated genetically in future studies. Adult Swiss-Webster mice were OVX, and after the fifth postoperative week were treated intermittently with human PTH(1-34) [hPTH(1-34)] or vehicle for 4 weeks. Femoral bones were evaluated by microcomputed tomography (microCT) followed by histomorphometry. A tight correlation was observed between trabecular density (BV/TV) determinations made by both methods. The BV/TV showed >60% loss in the distal metaphysis in 5-week and 9-week post-OVX, non-PTH-treated animals. PTH induced a approximately 35% recovery of this loss and a approximately 40% reversal of the associated decreases in trabecular number (Tb.N) and connectivity. PTH also caused a shift from single to double calcein-labeled trabecular surfaces, a significant enhancement in the mineralizing perimeter and a respective 2- and 3-fold stimulation of the mineral appositional rate (MAR) and bone formation rate (BFR). Diaphyseal endosteal cortical MAR and thickness also were increased with a high correlation between these parameters. These data show that OVX osteoporotic mice respond to PTH by increased osteoblast activity and the consequent restoration of trabecular network. The Swiss-Webster mouse model will be useful in future studies investigating molecular mechanisms involved in the pathogenesis and treatment of osteoporosis, including the mechanisms of action of known and future bone antiresorptive and anabolic agents.

Osteocyte and osteoblast apoptosis and excessive bone deposition accompany failure of collagenase cleavage of collagen

Mice carrying a targeted mutation (r) in Col1a1, encoding a collagenase-resistant form of type I collagen, have altered skeletal remodeling. In hematoxylin and eosin-stained paraffin sections, we detect empty lacunae in osteocytes in calvariae from Col1a1(r/r) mice at age 2 weeks, increasing through age 10-12 months. Empty lacunae appear to result from osteocyte apoptosis, since staining of osteocytes/periosteal osteoblasts with terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling is increased in Col1a1(r/r) relative to wild-type bones. Osteocyte perilacunar matrices stained with Ab that recognizes collagenase collagen alpha1(I) chain cleavage ends in wild-type but not Col1a1(r/r) calvariae. Increased calvarial periosteal and tibial/femoral endosteal bone deposition was found in Col1a1(r/r) mice from ages 3-12 months. Calcein labeling of calvarial surfaces was increased in Col1a1(r/r) relative to wild-type mice. Daily injections of synthetic parathyroid hormone for 30 days increased calcein-surface labeling in wild-type but caused no further increase in the already high calcein staining of Col1a1(r/r) bones. Thus, failure of collagenase cleavage of type I collagen in Col1a1(r/r) mice is associated with osteocyte/osteoblast death but increases bone deposition in a manner that mimics the parathyroid hormone-induced bone surface activation seen in wild-type mice.

Daily transient decreases in plasma parathyroid hormone levels induced by the calcimimetic NPS R-568 slows the rate of bone loss but does not increase bone mass in ovariectomized rats

Daily parathyroid hormone (PTH) injections that transiently increase plasma PTH levels within the physiological range increase bone mass in osteopenic, ovariectomized (ovx) rats. This study tested the hypothesis that repeated transient decreases in plasma PTH levels from normal, induced by the daily oral administration of the calcimimetic NPS R-568, would induce an anabolic effect in bone of ovx rats with established osteopenia and/or prevent the rapid bone loss that occurs following ovx. In the reversal study, NPS R-568 was administered orally (10 or 100 micromol/kg) for 30 days to 14-month-old retired breeder rats that were ovx 5 months earlier. NPS R-568 treatment did not increase bone formation rate (BFR) or cancellous bone area (B.Ar) in the proximal tibial metaphysis, or bone mineral density (BMD), at any femoral site. In the prevention study, 3-month-old virgin rats were ovx and given NPS R-568 for the following 28 days. The 10 micromol/kg dose prevented the increase in osteoclast number and 42% of the loss of B.Ar, without affecting the elevated osteoblast populations or BFR. Surprisingly, the 100 micromol/kg dose had fewer protective effects, despite preventing the increase in BFR in both cancellous and cortical bone. Detailed analysis of cancellous bone showed that tendency for a dose-related protection of true cancellous bone occurred, but, while the 10 micromol/kg dose prevented 88% of the loss of calcified cartilage seen in control ovx rats, the 100 micromol/kg dose increased that loss by a further 31%. The mechanism underlying these disparate effects of NPS R-568 on calcified cartilage accumulation in the tibial metaphysis is unclear, but may be related to the different effects that the two doses have on plasma Ca(2+) levels. In conclusion, transient increases in PTH levels above basal, and not simple oscillations in hormone levels below normal, appear necessary for the anabolic properties of endogenous PTH to be manifested in the bones of osteopenic ovx rats.

Antagonizing the parathyroid calcium receptor stimulates parathyroid hormone secretion and bone formation in osteopenic rats

Parathyroid hormone (PTH) is an effective bone anabolic agent, but it must be administered parenterally. An orally active anabolic agent would provide a valuable alternative for treating osteoporosis. NPS 2143 is a novel, selective antagonist (a "calcilytic") of the parathyroid cell Ca(2+) receptor. Daily oral administration of NPS 2143 to osteopenic ovariectomized (OVX) rats caused a sustained increase in plasma PTH levels, provoking a dramatic increase in bone turnover but no net change in bone mineral density. Concurrent oral administration of NPS 2143 and subcutaneous infusion of 17beta-estradiol also resulted in increased bone turnover. However, the antiresorptive action of estrogen decreased the extent of bone resorption stimulated by the elevated PTH levels, leading to an increase in bone mass compared with OVX controls or to either treatment alone. Despite the sustained stimulation to the parathyroid gland, parathyroid cells did not undergo hyperplasia. These data demonstrate that an increase in endogenous PTH secretion, induced by antagonism of the parathyroid cell Ca(2+) receptor with a small molecule, leads to a dramatic increase in bone turnover, and they suggest a novel approach to the treatment of osteoporosis.

Mandibular bone formation rates in aged ovariectomized rats treated with anti-resorptive agents alone and in combination with intermittent parathyroid hormone

Anti-resorptive agents--including estrogen (E), calcitonin (CT), and bisphosphonates--are established in the treatment of osteoporosis. Intermittent administration of parathyroid hormone (PTH) stimulates bone formation and is a possible therapeutic agent for the restoration of bone mass. The purpose was to determine the effects of the anti-resorptive agents alone and in combination with intermittent PTH on bone formation in the mandible and a long bone in the aged ovariectomized (Ovx) rat. Female rats were ovariectomized or sham-operated. One year later, groups of Ovx rats were treated with E, CT, or the bisphosphonate, Risedronate (NE). Additional groups of Ovx rats were treated with each of these agents in combination with human PTH for 10 weeks. Estrogen treatment suppressed most indices of bone formation in the humerus and mandible, while NE decreased some indices of formation at the endocortical and endosteal surfaces of the mandible and humerus. Increased double-labeled surface and mineral apposition rates were observed only on the mandibular endosteal surfaces following CT treatment. When the anti-resorptive agents were combined with intermittent PTH, most indices of bone formation at all skeletal sites were substantially greater than those of the untreated Ovx controls as well as the E-, CT-, and NE-treated groups, respectively. These results provide additional evidence that established and emerging therapies for osteoporosis affect osseous tissues in the oral cavity, and this may influence the progression of diseases and/or aging changes at this site.

Daily intermittent decreases in serum levels of parathyroid hormone have an anabolic-like action on the bones of uremic rats with low-turnover bone and osteomalacia

The calcium receptor agonist (calcimimetic) compound NPS R-568 causes rapid decreases in circulating levels of parathyroid hormone (PTH) in rats and humans. We hypothesized that daily intermittent decreases in serum PTH levels may have different effects on bone than do chronically sustained decreases. To test this hypothesis, we compared two NPS R-568 dosing regimens in rats with chronic renal insufficiency induced by two intravenous injections of adriamycin. Fourteen weeks after the second adriamycin injection, creatinine clearance was reduced by 52%, PTH levels were elevated approximately 2.5-fold, and serum 25(OH)D3 and 1,25(OH)2D3 levels were reduced substantially. Treatment by daily per os gavage, which decreased PTH levels intermittently, or continuous subcutaneous infusion, which resulted in a sustained suppression of serum PTH levels, then began for 8 weeks. Despite the hyperparathyroidism, the adriamycin-injected rats developed a low-turnover bone lesion with osteomalacia (fourfold increase in osteoid volume in the proximal tibial metaphysis) and osteopenia (67% decrease in cancellous bone volume and an 18% reduction in bone mineral density at the distal femur). Daily administered (but not infused) NPS R-568 significantly increased cancellous bone volume solely by normalizing trabecular thickness, and increased femoral bone mineral density by 14%. These results indicate that daily intermittent, but not sustained, decreases in PTH levels have an "anabolic-like" effect on bones with a low-turnover lesion in this animal model of chronic renal insufficiency.

Food fractionation is a powerful tool to increase bone mass in growing rats and to decrease bone loss in aged rats: modulation of the effect by dietary phosphate

The incidence of osteoporotic fractures has been associated with low bone mass. To reduce this incidence, it is therefore important to try to prevent the development of low bone mass by either increasing bone mass built up during adolescence and/or preventing bone loss in later life. It has been shown that food fractionation, a procedure that prevents the diurnal rhythm of bone resorption, increases bone mass in growing rats fed a high calcium (Ca), high phosphate (Pi) diet. In this paper, data are presented that show that providing growing rats with the same daily amount of a high Ca, low Pi diet (a Pi content similar to that of a human diet) in portions every 6 h instead of one meal increases total bone mineral content, trabecular bone mineral density, and cortical thickness, and markedly reduces the decrease in these parameters in aged rats. The effect is smaller when a high Ca, high Pi diet is fractionated. This could be the consequence of the transient postprandial increase in parathyroid hormone (PTH) following one large meal of a high Ca, high Pi diet, since the effect is similar to that reported after PTH injections which are anabolic for bone. Thus, a high dietary Pi must be considered as a confounding factor when treatments affecting bone mass are investigated in rats. The present data show that feeding habits have a profound effect on bone mass in the rat, independent of age. Whether bone mass in humans is also under the control of dietary habits is not yet clear. If so, frequent small meals of appropriate composition may help to prevent osteoporosis.

Development of a scintillation proximity assay for high-throughput measurement of intact parathyroid hormone

A simple, high-throughput scintillation proximity assay (SPA) for parathyroid hormone (1-84) (PTH) has been developed. Fifteen commercially available N-terminal and six C-terminal anti-PTH antibodies were evaluated for detection of human PTH(1-84). Two C-terminal antibodies (CR1073M and 10-P55) gave the most consistent results. Using one of these antibodies (10-P55), an assay was developed with a sensitivity of 4 pg/ml for human and rat PTH(1-84). Porcine PTH(1-84) was not detectable. The intra-assay and inter-assay coefficients of variation for a 467 pg/ml sample were 6. 1 and 6.5%, respectively, and for a 21 pg/ml sample, 6.2 and 4.4%. Human PTH(1-34), while not detected in the assay, interfered with the detection of PTH(1-84). Smaller fragments [for example, human PTH(3-34)] and a C-terminal PTH fragment [PTH(53-84)] did not interfere in the assay. The procedure gave 106-110% recovery of human PTH(1-84) spiked into samples. Immunoreactive PTH concentrations in serum of rats administered EGTA were determined by SPA and by a commercially available PTH immunoassay. There was a good correlation between the two assays with significant increases in serum immunoreactive PTH concentrations at 15 and 30 min after EGTA injection and a rapid decrease to baseline values by 60 min. The SPA gives a high-throughput method for simply and accurately determining PTH(1-84) concentrations in serum.