The precise control of whole-body calcium is essential for the maintenance of normal physiological function. Disruptions in calcium homeostasis can lead to pathology including osteoporosis, kidney stone formation, and cardiac arrythmias. During the 1960s and early 1970s, a full understanding of calcium metabolism was still emerging. This commentary spotlights a seminal Clinical Science paper published in 1972 that significantly advanced the field and contributed to the eventual approval of bisphosphonate drugs commonly used to treat postmenopausal osteoporosis, cancer metastases, and other calcium disorders.
Calcium homeostasis is essential for normal physiological function and is tightly regulated by a variety of mechanisms, the most important of which are parathyroid hormone and the vitamin D hormones. During the 1960s, the mechanisms controlling calcium metabolism were not fully understood, but were actively under investigation. In 1961, Drs. William Neumann and Herbert Fleisch made a critical observation that significantly advanced the field. They noted that calcium phosphate was present in bodily fluids at supersaturated concentrations and that if collagen was added, the calcium phosphate would precipitate and form hydroxyapatite crystals. From this observation, given that collagen is a common protein in soft tissues, Neumann and Fleisch surmised that a mechanism must exist in the body that prevents the high concentrations of calcium from crystallizing in tissues .
Because polyphosphates were chemically used to ‘soften’ water and prevent calcium crystallization in industrial and home settings, Fleisch postulated that endogenously produced polyphosphates may exist to inhibit calcium phosphate precipitation in soft tissues of the body as well . In 1962, Fleisch and Bisaz proved this hypothesis correct when they isolated urinary pyrophosphate and demonstrated it as an effective inhibitor of calcium crystallization . Another major advance in understanding bone physiology came in 1970 when it was demonstrated that both pyrophosphates and diphosphonates inhibited the dissolution of hydroxyapatite crystals and bone resorption in response to parathyroid hormone [4,5]. These findings advanced the line of scientific inquiry related to how bisphosphonates might regulate whole-body calcium metabolism and the flux of calcium into and out of bone. The study also helped drive the field toward the eventual approval and clinical use of bisphosphonates that are now widely used in the treatment of primary osteoporosis (i.e. osteogenesis imperfecta) and secondary osteoporosis (postmenopausal, chronic glucocorticoid usage, and chronic inflammatory and immune disorders) in both children and adults. Bisphosphonates are also commonly used, and effective, to prevent cancer metastasis particularly in postmenopausal women , thus further illustrating the importance of these early preclinical studies to ultimately benefit patients.
Clinical Science has been at the forefront of scientific discovery since its establishment in 1909 as the journal Heart: A study of the Circulation (renamed Clinical Science in 1933). Given the rich history of the journal, it is perhaps not surprising that influential papers on the control of calcium metabolism are contained within the volumes of Clinical Science. In 1972, the journal published ‘The Influence of Two Diphosphonates on Calcium Metabolism in the Rat’ . The paper was authored by Drs. A.B. Gasser, D.B. Morgan, H.A. Fleisch, and L.J. Richelle and was an important piece of the puzzle that eventually led to the clinical approval of bisphosphonate use.
The major goal of the study was to test the impact of two different diphosphonates (now called bisphosphonates) on calcium metabolism in the rat. Gasser et al. utilized a rat model to determine whether two different diphosphonates, disodium dichloromethylene diphosphonate (Cl2MDP), and disodium ethane-1-hydroxy-1,1-diphosphonate (EHDP) exerted differential effects on total body calcium balance along with bone formation and resorption. Female Wistar rats at 54 days of age were switched to a low calcium (0.5%), low phosphate (0.35%) diet. This age was selected because it represents a time at which the rate of bone formation is maximal in this strain. For ten consecutive days, rats assigned to random groups, received subcutaneous injections of either vehicle, Cl2MDP, or EHDP at concentrations ranging from 0.01 to 10.0 mg/kg of bodyweight, while maintained on the low calcium (0.5%), low phosphate (0.35%) diet. On day 7, the rats were injected with 45Ca after which plasma and urine samples were collected over the next 72 h at which time he rats were euthanized and the right femur was collected. Using the calcium measurements in blood, urine, feces, and bone, along with calculating calcium intake, the authors calculated calcium absorption, urinary and fecal calcium output, bone formation and resorption rates, and calcium intake and excretion.
The main findings of the present study were that administration of either compound had little impact on plasma calcium across most of the doses. At the highest dose of Cl2MDP (10.0 mg/kg), plasma calcium decreased with an even larger decrease in plasma phosphate likely reflecting the activation and secretion of parathyroid hormone. However, these changes did not occur at the highest dose of EHDP (10.0 mg/kg). Despite some differences between the compounds, both caused a net decrease in bone resorption, thus providing more support for the potential therapeutic benefit for bisphosphonates. In 1977, EHDP would become the first bisphosphonate (Etidronate) clinically approved for the treatment of Paget’s disease , a calcium metabolism disorder characterized by the coupling of excessive bone resorption with disorganized bone formation. The development and use of Etidronate paved the way for new bisphosphonates that eventually surpassed their predecessor, although Etidronate remains in use in some countries for the treatment of osteoporosis and Paget’s disease.
At the time of this publication, overall calcium metabolism was thought to result in part from the growth and dissolution of hydroxyapatite crystals based on previous in-vitro studies. This paper by Glasser et al.  added improved understanding of how calcium is regulated in vivo, and studies like this were instrumental in the development of a new class of therapeutics. It also further supported the concept that multiple organs, like the kidneys, are important for whole body calcium metabolism . Perhaps, not surprisingly, the study has been cited over 200 times, most recently in 2021 , demonstrating its relevance even 50 years after its original publication. It is routinely cited in the literature reviews on bisphosphonates, bone physiology, and the history of bone calcium metabolism. The collective works of these authors also demonstrate the importance of preclinical laboratory science toward the translational development of clinical therapeutics. Based on this work, and the work of others, bisphosphonates are now widely used clinically in the treatment of a variety of diseases including osteoporosis, Paget’s disease, oncology, and nephrological disorders. Clinical Science is pleased to publish works that have such lasting impact in the biomedical field.
The data are not applicable to the present paper.
The authors declare that there are no competing interests associated with the manuscript.
This material was the result of work supported by the resources and the use of facilities at the Columbia VA Health Care System, Columbia, SC.
CRediT Author Contribution
Elena L. Dent: Conceptualization, Writing—original draft, Writing—review & editing. Michael J. Ryan: Conceptualization, Resources, Writing—original draft, Writing—review & editing.