Calcilytics: antagonists of the calcium-sensing receptor for the treatment of osteoporosis

The only bone anabolic agents currently available on the market are based on parathyroid hormone (PTH). Secretion of endogenous PTH is controlled by a calcium sensing receptor (CaSR) at the surface of the parathyroid glands. Antagonists of this receptor (calcilytics) induce the release of the hormone. Provided the effect of the calcilytic is of short duration, a bone anabolic effect should result as well, based on ample experience with exogenous PTH. Although the first calcilytic series became known about ten years ago, the number of different structural types is still small today. This article will outline the quest from hits to potent development candidates of all relevant calcilytic series currrently known. Even after the front-runners unexpectedly failed in the clinic, the approach for an oral alternative to parenteral PTH remains highly attractive

The Chemistry of Bisphosphonates: From anti-scaling agents to potent therapeutics

In the early 1960s inorganic pyrophosphate (PPi) was found to be present in body fluids and to act as a natural inhibitor of calcification by its interaction with hydroxyapatite. In addition to inhibiting the formation of calcium phosphate, PPi also inhibited dissolution of hydroxyapatite crystals which made it interesting for pharmalogical applications in the treatment of diseases associated with excessive bone resorption. However, pyrophosphate is metabolically unstable because of rapid hydrolysis of the P-O-P backbone by hydrolytic enzymes in the gastrointestinal tract. In the search for more stable analogues of PPi, attention turned to the chemical class of bisphosphonates (BPs). The first BPs were synthesized in the 19th century and have been widely used for industrial applications. Bisphosphonates are formally derived from pyrophosphate by replacement of the bridging oxygen atom by a carbon atom resulting in a P-C-P moiety which is resistant to enzymatic hydrolysis. In addition to its decisive role in stability, the central carbon atom also provides an attachment point for two additional substituents R1 and R2. While R1 is preferentially a hydroxy group, allowing such derivatives to act as powerful tridentate ligands for calcium (bone hook), R2 is mainly responsible for anti-resorptive potency. The clinically available BPs can be divided into 2 subclasses based on their structure and molecular mechanism of action. The simple, non-nitrogen containing derivatives can be incorporated into non-hydrolysable cytotoxic ATP analogues. The more potent nitrogen-containing BPs inhibit FPPS, a key enzyme in the mevalonic pathway. Members of this class have a wide therapeutic window between undesired inhibition of bone formation and bone resorption and several of them have found widespread use for the treatment of benign and malignant bone disease

Symmetric Geminal Bisphosphinic Acids RR´C[P(CH3)(O)OH]2 - NMR and Analytical Studies

Four bisphosphinic acids RR´C[P(CH3)(O)OH]2 are characterized by 1H, 13C{1H}, and 31P{1H} NMR data. H3C-P-C-P-CH3 skeletons give rise to [A3X]2 spin systems. Some algebraic equations are derived for manual analysis of [A3X]2 spectra. HR NMR data for heteroaromatic substituents R in RC(H)[P(CH3)(O)OH]2 are reported. Dissociation constants and ion-specific chemical shifts dP of CH3C(OH)[P(CH3)(O)OH]2 are determined by 31{1H} NMR controlled titrations of 2a

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

AXT914 a novel, orally-active parathyroid hormone-releasing drug in two early studies of healthy volunteers and postmenopausal women

Antagonism of the calcium-sensing receptor in the parathyroid gland leads to parathyroid hormone (PTH) release. Calcilytics are a new class of molecules designed to exploit this mechanism. In order to mimic the known bone-anabolic pharmacokinetic (PK) profile of s.c. administered PTH, such molecules must trigger sharp, transient and robust release of PTH. The results of two early clinical studies with the orally-active calcilytic AXT914, a quinazolin-2ne derivative are reported. These were GCP-compliant, single and multiple dose studies of PK/PD and tolerability in healthy volunteers and postmenopausal women.The first study, examined single ascending doses (4 to 120. mg) and limited multiple doses (60 or 120. mg. q.d. for 12. days) of AXT914. The second study was a randomized, double-blind, active- and placebo-controlled, 4-week repeat-dose parallel group study of healthy postmenopausal women (45 and 60. mg AXT914, placebo, 20. μg Forteo/teriparatide/PTH(1-34) fragment).AXT914 was well tolerated at all doses and reproducibly induced the desired PTH-release profiles. Yet, 4. weeks of 45 or 60. mg AXT914 did not result in the expected changes in circulating bone biomarkers seen with teriparatide. However total serum calcium levels increased above baseline in the 45 and 60. mg AXT914 treatment groups (8.0 % and 10.7%, respectively), compared to that in the teriparatide and placebo groups (1.3% and 1.0%, respectively). Thus the trial was terminated after a planned interim analysis due to lack of effect on bone formation biomarkers and dose-limiting effects on serum calcium.In conclusion, AXT914 was well tolerated but the observed transient and reproducible PTH-release after repeat oral administration of AXT914 which showed an exposure profile close to that of s c. PTH, did not translate into a bone anabolic response and was associated with a persistent dose-related increase in serum calcium concentrations. © 2014

Penta-Substituted Benzimidazoles as Potent Antagonists of the Calcium Sensing Receptor (CaSR-Antagonists)

A series of novel benzimidazole derivatives has been designed via a scaffold morphing approach based on known calcilytics chemotypes. Subsequent lead optimisation led to the discovery of penta-substituted benzimidazoles that exhibit attractive in vitro and in vivo calcium sensing receptor (CaSR) inhibitory profiles. In addition, synthesis and structure activity relationship data are provided

HDAC4 Controls Muscle Homeostasis through Deacetylation of Myosin Heavy Chain, PGC-1α, and Hsc70

HDAC4, a class IIa histone deacetylase, is upregulated in skeletal muscle in response to denervation-induced atrophy. When HDAC4 is deleted postnatally, mice are partially protected from denervation. Despite the name “histone” deacetylase, HDAC4 demonstrably deacetylates cytosolic and non-histone nuclear proteins. We developed potent and selective class IIa HDAC inhibitors. Using these tools and genetic knockdown, we identified three previously unidentified substrates of HDAC4: myosin heavy chain, peroxisome proliferator-activated receptor gamma co-activator 1alpha (PGC-1α), and heat shock cognate 71 kDa protein (Hsc70). HDAC4 inhibition almost completely prevented denervation-induced loss of myosin heavy chain isoforms and blocked the action of their E3 ligase, MuRF1. PGC-1α directly interacts with class IIa HDACs; selective inhibitors increased PGC-1α protein in muscles. Hsc70 deacetylation by HDAC4 affects its chaperone activity. Through these endogenous HDAC4 substrates, we identified several muscle metabolic pathways that are regulated by class IIa HDACs, opening up new therapeutic options to treat skeletal muscle disorders and potentially other disease where these specific pathways are affected. Luo et al. use class IIa HDAC inhibitors, along with skeletal-muscle-specific and whole-body inducible HDAC4 knockout mice, to demonstrate HDAC4 deacetylates three previously undescribed substrates: myosin heavy chain, PGC-1α, and Hsc70. Through these substrates, HDAC4 inhibition leads to rescue of muscle atrophy and increased succinate dehydrogenase activity