Parathyroid hormone (PTH) is a major regulator of calcium homeostasis and its main target organs are considered to be the bones and kidneys. Native human parathyroid hormone is a polypeptide consisting of 84 amino acids. This hormone is secreted from the parathyroid gland in response to low blood calcium levels, and acts on osteoblasts (bone-building cells) in the bones and tubular epithelial cells in the kidneys. This hormone interacts with a cell surface receptor molecule called PTH-1 receptor or PTH/PTHrP receptor, which is expressed by both osteoblasts and renal tubular cells.
PTHrP (PTH-related protein), the major cause of humoral hypercalcemia of malignancy (HHM), also has normal functions including developmental roles. PTHrP has 141 amino acids, although mutants also occur that result from alternative gene splicing mechanisms. PTHrP plays a key role in the formation of the skeleton through a process that also involves binding to the PTH-1 receptor (Non Patent Literature 1, Non Patent Literature 2).
Regulation of calcium concentrations is necessary for normal functions of the gastrointestinal system, skeletal system, nervous system, neuromuscular system and cardiovascular system. Synthesis and release of PTH are primarily controlled by the serum calcium level. Synthesis and release of PTH are stimulated at low serum calcium levels, and synthesis and release of PTH are suppressed at high serum calcium levels. PTH, in turn, maintains the serum calcium level by directly or indirectly promoting calcium entry into the blood at three calcium exchange sites: intestine, bone and kidney. PTH contributes to net gastrointestinal absorption of calcium by assisting in the renal synthesis of active vitamin D. PTH promotes calcium mobilization from the bone to serum by stimulating differentiation of osteoclasts that are bone-resorbing cells. This also mediates at least three main effects in the kidney (stimulation of tubular calcium resorption; enhancement of phosphate clearance; and promotion of an increase in the enzyme that completes the synthesis of active vitamin D). PTH is considered to exert these effects primarily through receptor-mediated activation of adenylate cyclase and/or phospholipase C.
Disruption of calcium homeostasis may produce many clinical disorders (e.g., serious bone disease, anemia, renal dysfunction, ulcers, myopathy and neuropathy), and this usually results from conditions that produce an alteration in the level of parathyroid hormone. Hypercalcemia is a condition characterized by an elevated serum calcium level. This is often associated with primary hyperparathyroidism in which excessive PTH production occurs as a result of parathyroid gland lesions (e.g., adenoma, hyperplasia or carcinoma). Humoral hypercalcemia of malignancy (HHM), another type of hypercalcemia, is the most common paraneoplastic syndrome. This appears to result in most instances from the production of a certain protein hormone that shares amino acid homology with PTH by tumors (e.g., squamous cell carcinoma, renal carcinoma, ovarian carcinoma or bladder carcinoma). These PTHrPs appear to mimic the effects of PTH on the kidney and skeleton in some degree, and are considered to interact with the PTH receptor in these tissues. PTHrP is usually found at low levels in many tissues including keratinocytes, brain, pituitary gland, parathyroid gland, adrenal cortex, medulla, fetal liver, osteoblast-like cells and lactating mammary tissues. For many HHM malignant tumors, high levels of PTHrP are observed in the circulatory system, and this leads to elevated calcium levels associated with HHM.
The pharmacological profiles of PTH and PTHrP are nearly identical in most in vitro assay systems, and elevated blood levels of PTH (i.e., primary hyperparathyroidism) or PTHrP (i.e., HHM) have comparable effects on inorganic ion homeostasis (Non Patent Literature 3, Non Patent Literature 4). The similarities in the biological activities of the two ligands can be explained by their interaction with the PTH/PTHrP receptor, a common receptor expressed abundantly in the bones and kidneys (Non Patent Literature 5).
The PTH-1 receptor is homologous in primary structure to some other receptors binding to peptide hormones, such as secretin (Non Patent Literature 6), calcitonin (Non Patent Literature 7) and glucagon (Non Patent Literature 8); these receptors together form a distinct family called receptor family B (Non Patent Literature 9). Within this family, the PTH-1 receptor is unique in that it binds to two peptide ligands and thereby regulates two separate biological processes. A recently identified PTH receptor subtype called PTH-2 receptor binds to PTH but not to PTHrP (Non Patent Literature 10). This finding has implied that the structural differences in the PTH and PTHrP ligands determine the selectivity for interaction with the PTH-2 receptor. The PTH-2 receptor has been detected by RNA methods in the brain, pancreas and vasculature; however, its biological functions have not been determined (Non Patent Literature 10). The family B receptors are assumed to use a common molecular mechanism for engagement with their own cognate peptide hormone (Non Patent Literature 11).
The PTH-1 receptor binds to both PTH and PTHrP and causes not only intracellular cAMP accumulation and adenyl cyclase (AC) activation but also signal transduction to phospholipase C (PLC), thereby leading to the production of inositol trisphosphate (IP3), diacylglycerol (DAG) and intracellular calcium (iCa2+) (Non Patent Literature 12, Non Patent Literature 13).
Osteoporosis is a potentially crippling bone disease and is observed in a substantial portion of the elderly population, in pregnant women and even in juveniles. The term “osteoporosis” refers to a group of disorders consisting of different constituents. Osteoporosis is clinically classified into type I and type II. Type I osteoporosis occurs primarily in middle-aged women and is associated with menopausal estrogen loss, while type II osteoporosis is associated with the elderly. Patients with osteoporosis are considered to benefit from novel therapies designed to promote fracture repair, or therapies designed to prevent or reduce fractures associated with the disease.
This disease is characterized by reduced bone mass, decreased bone mineral density (BMD), decreased bone strength and an increased risk of fracture. Currently, there is no effective cure for osteoporosis, although estrogen, calcitonin, and etidronate and alendronate that are bisphosphonates are used to treat the disease with various levels of success. These agents act to decrease bone resorption.
PTH(1-34) (teriparatide) has a strong bone anabolic effect and induces significant increases in bone mineral density and bone strength. Subcutaneous administration of human PTH(1-34) increased the spine bone mineral density (BMD) by 8% in one year and decreased the risks of vertebral fracture and nonvertebral fracture by 65% and 55% in two years, respectively (Non Patent Literature 14). Subcutaneous administration of human PTH(1-84) also increased the spine bone mineral density (BMD) by 6.9% in 18 months and decreased the risk of vertebral fracture by 58% (Non Patent Literature 15). Parathyroid hormone is currently believed to be one of the most effective treatments for osteoporosis (Non Patent Literature 16). Importantly, hPTH(1-34) must be administered in a pulsed manner (e.g., subcutaneous injection once daily) to achieve its bone-forming effect. Longer administration of PTH(1-34) such as by use of a continuous infusion pump mechanism activates bone-resorptive responses mediated by osteoclasts much stronger than bone-forming responses mediated by osteoblasts, and thus PTH(1-34) exerts a net degradation effect on the bone.
Although parathyroid hormone is believed to be one of the most effective treatments for osteoporosis, only less than 1% of patients with osteoporosis use teriparatide and the average duration of teriparatide is 12 months (Non Patent Literature 16). Teriparatide must be administered by self-injection. The fact that it is difficult to use a pen-type device for self-administration is the principal cause of the low compliance of teriparatide-administered patients. It is obvious that noninvasively, preferably orally, available compounds having a PTH-like effect with clinical efficacy in osteoporosis similar to that of parathyroid hormone can considerably improve the compliance of patients with regard to the administration, and that the compounds can be the most useful therapeutic option for patients with osteoporosis.
There are many low molecular weight agonists for the GPCR family A; however, only a limited number of low molecular weight ligands for the GPCR family B have been reported (Non Patent Literature 17). Low molecular weight agonists have been reported for the GLP-1 receptor, calcitonin receptor and PTH1 receptor belonging to the GPCR family B; however, there is no compound used in clinical applications for the treatment of diseases.