Osteoporosis Resources

Osteoporosis Research @ MGH
Study Reveals Potent New Osteoporosis Therapy

Current and Future Treatments for Osteoporosis
All of the currently available drugs for the treatment of osteoporosis (bisphonates, SERMS, and estrogen) act as anti-resorptive agents; that is, they slow bone loss but do not stimulate bone deposition. Nonetheless, they limit the deterioration of the bones and, in altering the metabolic balance, may result in some gain of bone mass and fewer fractures. The study, led by Dr. Robert Neer of Massachusetts General Hospital, published in the New England Journal of Medicine (May 10, 2001) demonstrated the effectiveness of a new treatment, parathyroid hormone (PTH). Compared with other treatments, PTH works more rapidly and has a significantly greater effect on increasing bone mass and reducing the risk of osteoporotic fractures.
return to top

The Quest for a Cure
While the results of the PTH trial are truly exciting, the investigators at MGH have not yet attained their goal of curing osteoporosis, especially as PTH treatment did not increase the density of cortical bone. However, investigators at MGH and elsewhere have shown that PTH can cure osteoporosis in rats, restoring cortical and trabecular bone to normal in a few months, but with PTH doses higher than humans tolerate. Such results suggest that, with further refinement, it may be possible to develop a treatment that will come closer to the desired goal. Can osteoporosis be cured by combination therapy, with PTH to promote bone formation together with an agent to reduce bone resorption?

At this time, a 2¹/₂year study has been initiated at MGH, comparing the effect of osteoporosis treatment with PTH with or without alendronate, a bisphosphonate drug which blocks the action of the bone destroying osteoclast cells. This study, which includes both men and women, will find out whether combination treatment increases the bone building response beyond that expected for PTH alone.
return to top

Future Improvements in Treatment
Even if the combination treatment of alendronate and PTH is highly effective, it will still be less than the ideal treatment. As discussed above, bisphosphonates are poorly absorbed and may be associated with side effects. PTH, like insulin is broken down in the digestive system and must be given by daily sub-cutaneous injection. This presents a major obstacle, especially among the elderly, who may find it difficult to self-administer the daily injections. For this reason, many people are interested in designing a drug which has the beneficial effects of PTH but can be administered orally or by inhalation. Before this goal can be achieved, it is necessary to understand how PTH acts on its receptor to transmit a signal to the interior of the cell and to characterize the series of events that are triggered within the cell by the activated receptor.
return to top

Parathyroid Hormone
Research at MGH into the physiology of parathyroid hormone and the disorders associated with it has a long and distinguished history. Normally, PTH is secreted from the parathyroid gland in response to several stimuli, principally a decrease in blood calcium, and can induce either bone resorption or bone deposition, depending on the conditions. That PTH could result in new bone formation was first demonstrated in the 1920s by scientists at MGH, who observed in rat experiments that an extract from parathyroid glands increased bone density if injected once a day. The significance of this effect was not realized at that time perhaps because hyperparathyroidism, a disease in which continuous excessive amounts of PTH are secreted, was known to cause bone loss. Thus, the finding was ignored, then forgotten.

The extremely small amounts of PTH normally present made its isolation very difficult and it was not until the late 1960s that investigators isolated pure PTH. The structure of the hormone was determined by investigators who began their work at NIH and later moved to the MGH Endocrine Unit (the structure was also determined by an independent group at NIH). The MGH investigators subsequently discovered that only a part of the peptide chain was necessary for hormonal activity, a sequence of 34 amino acids, which is less than half the size of the native hormone. Scientists at MGH were the first to synthesize this peptide fragment, which they used to study the action of PTH on bone tissue.

The possibility of using PTH to treat bone loss was inspired by a British study, which in 1970 confirmed the finding of fifty years earlier that PTH administered once daily increased bone mass in rats. However, the significance of this effect was once again dismissed by most scientists and clinicians. Nevertheless, the persistence of a few clinical investigators, many of whom were at MGH, has resulted in growing recognition of the importance of the ability of PTH to stimulate bone growth in humans, confirming the vision of the MGH scientists who believe that it could play a major role in the future treatment of osteoporosis.
return to top

Measuring bone density in humans
Up until the 1970s, measuring bone mass in humans was neither pleasant nor easy. A biopsy was necessary to remove a piece of bone, which could then be evaluated under a microscope. The alternative method of estimating bone gain or loss was by calcium retention analyses, which depended on precise collection and measurement of all food and liquid ingested and all excretions, for several weeks. Despite this difficulty, a multinational cooperative group, which included the Endocrine Unit at MGH initiated a human study of daily injections of PTH at this time. In 1980, the results of this first study were published, demonstrating that PTH can induce bone deposition in humans when it is administered once a day, and that this effect is not a phenomenon seen only in rats. However, while it was possible to demonstrate increases in bone density in bone biopsies, they were unable to detect any changes in the retention of calcium. In addition, the study did not include a control group since this would have required volunteers for bone biopsy studies. The results were reported as encouraging but were regarded as preliminary and largely ignored.

Nevertheless, the investigators at MGH persisted in their conviction that the measured increases in bone density were real but they had great difficulty in finding funds to support their work. There was little government research support for clinical research and pharmaceutical companies were not interested in a product, PTH, which was not patented.

While further studies with PTH languished, technological developments such as quantitative computerized tomography (QCT) and dual energy X-ray absorptometry (DEXA) made it possible to measure bone mass in humans quickly, easily, and non-invasively. MGH investigators played a significant role in these developments and were the first to report the use of this new technology to make accurate and reproducible non-invasive measurements of bone in the spine.
return to top

Successful Clinical Trials of the Benefits of Parathyroid Hormone Therapy
In 1986 and in 1990, MGH investigators published the first clinical research studies using QCT and DEXA technology which showed that PTH, injected once daily, dramatically increased bone mass in the spine of osteoporotic men and osteoporotic postmenopausal women. These findings were subsequently confirmed by studies in two other institutions that demonstrated the bone building effect of PTH in both men and women.

In addition, MGH investigators were the first to show that once daily injections of PTH prevented bone loss resulting from lack of estrogen. The subjects were women who were being treated with nafarelin, a drug which induces temporary, reversible menopause. While nafarelin is extremely effective for the treatment of some estrogen dependent disorders, it can only be given for a few months because it causes significant bone loss (which can become permanent with long term treatment). The effect of the PTH therapy in these women was dramatic; it completely prevented nafaralin induced bone loss and actually caused a slight increase in bone density in the spine.

The promise of PTH treatment was finally recognized and a pharmaceutical company sponsored a large multicenter, multinational trial comparing the effect of placebo with PTH in 1637 women, all of whom had already experienced an osteoporosis-related fracture. In this trial, women self-administered daily injections of PTH or placebo to find out whether PTH would not only increase bone mass, but also prevent further fractures which, of course, is the goal of any treatment of osteoporosis.

In May 2001, the exciting and dramatic results of this study that was led by MGH investigators were published the New England Journal of Medicine. The study demonstrated that PTH is markedly better than any available treatment in reducing vertebral fractures (70% fewer than in the control) and in increasing bone density (up 13% in the spine and 6% in the femoral neck) over the course of the study (18-26 months). This increase in bone density and dramatic drop in the number of fractures is far superior to any other treatment available.
return to top

How does Parathyroid Hormone Work?
The effects of PTH on bone growth and resorption are both paradoxical and complex. Continuous high levels of PTH result in high levels of calcium in the blood and a decrease in bone density. However, injections of PTH administered once a day, as described above, result in an increase in bone density.

The effects of PTH are mediated by a specific receptor, found on osteoblast cells in bone and on tubule cells in the kidney. The receptor is embedded in the cell membrane and when the hormone binds to the receptor, its shape is altered (Figure 1). This initiates two cascades of biochemical events within the cell, one of which starts with increased synthesis of cAMP and the other, increased calcium uptake into the cell and enzyme activation. At present, it is not clear whether one or both of these cascades initiates bone growth, and until the roles of these signaling pathways are better understood, it will prove difficult to design drugs to cure osteoporosis in humans.

In addition to PTH, which is secreted in the parathyroid gland, there is a similar protein that is secreted and acts locally within the bone, known as parathyroid hormone-related peptide (PTHrP). This acts on the same receptor as PTH, now known as the PTH/PTHrP receptor. MGH investigators have also been studying a second PTH receptor (PTH-2 receptor) that does not respond to PTHrP in order to better understand how the PTH/PTHrP receptor functions.
return to top

The Combined Power of Chemistry and Genetics
The traditional ways of studying the effects of a hormone are to inject the hormone and observe the effects or to observe what happens in its absence – i.e. the gland producing it is removed. However, since PTHrP is produced in many tissues, close to its site of action, it is not possible to precisely mimic its effects by injecting the hormone or to surgically remove its source. Instead, scientists have to manipulate the genes of experimental animals to "knock out" or delete the PTHrP gene and/or the PTH/PTHrP receptor gene.

Figure 1

When MGH scientists deleted the PTH/PTHrP receptor in mice, they found that these mice died in utero, indicating that this receptor is essential to embryonic development. The mice had widespread abnormalities in bone formation and very low blood calcium levels. The development of trabecular and cortical bone was abnormal, as well. These studies showed that the PTH/PTHrP receptor regulates cells in the growth plates of bone as well as calcium homeostasis. However, the scientists were able to isolate and grow bone cells from these fetal mice and studied their response to growth factors essential for normal bone development. In this way, they demonstrated that the cells were unable to respond to one particular growth factor, as well as to PTHrP.

They have also inserted a mutant, constitutively active, form of human PTH/PTHrP receptor into mice and observed an increase in trabecular bone but a decrease in cortical bone during development.

The MGH investigators have also deleted the PTHrP gene and they have compared bone development in mice missing the PTH/PTHrP gene with those missing the PTHrP gene. Although there were similarities in the development of the mice, the investigators found that the bone mineralization patterns and the levels of some critical bone forming proteins differed in the two mutants.

By genetically manipulating mice in this way, it was possible to demonstrate that the role of PTHrP is different from that of PTH, even though they share a receptor. PTHrP is primarily a regulator in fetal development, controlling the pace and synchrony of bone differentiation, whereas PTH functions throughout life as the major regulator of blood calcium levels, an effect involving bone as the major store of calcium.

While the results described above are illustrative of work at MGH that is probing the physiological function of PTH, PTHrP and the PTH/PTHrP receptor, others at MGH are probing the molecular structures of these peptides and proteins and determining how they interact.
return to top

Identifying Critical Sites for Hormone-Receptor Binding and Activation
An inspired insight led one investigator at MGH to realize that a very rare inherited disease in humans, Jansen-type metaphyseal chrondodysplasia, could be due a fault in the PTH/PTHrP receptor. The symptoms of this disease, which results in dwarfism, abnormal bone formation, and altered calcium metabolism, are in some ways similar to hyperparathyroidism. However, their PTH levels are not elevated, which previous investigators had taken as evidence of another unknown calcium regulating factor.

Investigators at MGH sought out some of these patients, extracted and analyzed DNA from their blood samples, and found variations in the PTH/PTHrP receptor gene. They discovered three genetic mutations, all of which result in a receptor that is permanently activated, even in the absence of hormone.

They have probed the significance of the mutated sites in the PTH/PTHrP receptor by placing mutated copies of the receptor gene into cells grown in culture. In this way, they have systematically substituted all the natural types amino acids at the first two sites of mutation that were discovered to be associated with Jansen’s disease. At the first site, they found that only two possible substitutions, both of basic amino acids, resulted in permanent activation. At the other site, most substitutions resulted in permanent activation.

They have obtained fundamental information on how the hormone and the receptor interact and of the critical conformation changes necessary for receptor activation by synthesizing and testing many analogs of PTH and PTHrP. Some of these analogs act as an inverse agonist, inactivating the mutant receptor.

They have learned more about how the molecular structure determines specificity by genetically manipulating the PTH-2 receptor and the PTH/PTHrP receptor and have found two significant receptor sites which alter the specificity. They have made modifications to PTHrP and have found the critical site which prevents it binding to the PTH-2 receptor.

They have identified additional binding sites on the PTH/PTHrP receptor by developing synthetic photoreactive, fully functional analogs PTH and PTHrP and analyzing the sites of hormone/receptor interaction.

These newly identified sites, illustrated in Figure 2, help define the molecular interface between hormone and receptor, information that is key to successful drug discovery efforts. As mentioned above, the action of PTH can be simulated by a fragment of the hormone consisting of first 34 of the amino acids in the entire hormone. By systematically studying peptide interactions with the PTH receptor, it may be possible to design or a non-peptide with PTH activity or an active peptide molecule small enough to be absorbed as a nose spray.

To date, the MGH investigators have discovered a peptide as small as 11 amino acids which has some PTH activity. Further work on this lead peptide could result in the discovery of other peptide analogs with sufficient activity to be administered without injection and which will be clinically effective.

By combining the first 9 amino acids of PTH with a modified PTH/PTHrP receptor, lacking the terminal extracellular part of the molecule, MGH investigators have constructed an active hybrid molecule, which will provide further information of hormone/receptor interaction. This structure suggests that it will be possible to design new, smaller molecules which will act to stimulate the PTH/PTHrP receptor.

Figure 2

return to top

Some Specific Goals for the Next Decade
The goal of osteoporosis research at MGH is a cure; that is the restoration of bone strength to that of a thirty year old. Scientists have demonstrated that it is possible to cure osteoporosis in rats and, therefore, it is reasonable to believe that a cure is also possible in man. The recent success with PTH therapy has brought treatment closer to the goal with some restoration of bone mass, but many questions still need to be answered. The MGH nvestigators want to:

understand what limits the action of PTH. If a net gain of bone mass continued for a longer time then perhaps the full strength of the bone could be restored. The scientists want to find out how the two distinct effects of PTH, bone resorption and bone deposition, alter with time;

explain why PTH does not affect all bones equally, and why cortical bone is much less sensitive to the hormone than trabecular bone;

design a safe and effective drug that can be taken by mouth and not injected, as is necessary for PTH;

explain the organ physiology of bone and describe the molecular pathways that lead to bone growth. return to top

If you are interested in supporting osteoporosis research at Mass. General, please visit our Development Office web site. Thank you.

News@MassGeneral | Mass. General home page | Return to osteoporosis resources