Denosumab as a Pharmacological Countermeasure Against Osteopenia in Long Duration Spaceflight

Changes During Spaceflight

Cumulative data from across the Apollo, Space Shuttle, Mir, and ISS missions shows that a prolonged exposure to microgravity results in bone resorption and a total BMD loss rate of 1–2%/mo. ^{10 , 38 , 45} However, the greatest loss is seen in the high load-bearing bones, with up to 20% loss in the femur, pelvis, and lumbar spine. ^{45 , 47} The loss in BMD is persistent after spaceflight, with preflight BMD in the trochanter only 50% recovered by 9 mo and returned to baseline by 3 yr. ³⁸

In addition to the fivefold increased risk of fractures, the rapid resorption of bone during early spaceflight may result in hypercalcemia and hypercalciuria, contributing to the formation of renal calculi and atherosclerotic disease. ^{12 , 41 , 47} In comparison, terrestrial bone loss in older populations at the femoral neck is estimated at 0.82–0.96% per yr. ²⁴ It is postulated that in antigravity, a lack of activity in the extensor muscles of the lower limbs and trunk reduces tension on the surrounding bones, which, when combined with the lack of mechanical force normally induced by gravity, results in rapid reductions in bone density. ^{12 , 32 , 47}

Currently, astronauts on the ISS perform aerobic and resistive exercise for up to 2.5 h daily, inclusive of setup time. ¹² When combined with strict adherence to the assigned diet with sufficient calorie, calcium, and vitamin D intake, overall BMD is maintained in the upper body. ⁴⁰ However, significant loss is still seen in the lumbar spine, femur, and pelvis; this is associated with elevation of resorption markers C-terminal telopeptide (CTX) and N-terminal telopeptide (NTX). ^{39 , 40} Whether the BMD decrease observed was associated with clinically significant bone geometry changes, including cortical thinning at the femoral neck, is not known due to CT imaging not being performed in either study.

Regardless of physical activity in-flight, increased levels of CTX and NTX are observed compared to preflight measurements, ^{42 , 43} indicating increased osteoclast activity. Due to bone resorption, there is an increased risk of renal stone formations from an increase in urinary calcium and oxalate excretion. ^{40 , 42 , 43} While baseline differences in bone biochemistry may exist between sexes prior to spaceflight, there is no difference in response to microgravity in the maintenance of bone density. ⁴³

Therefore, even with current countermeasures, astronauts on prolonged spaceflights are at risk of osteoporotic fractures in load-bearing bones upon return to a normal gravitational environment, as well as at elevated risk of renal calculus formation. However, numerous biochemical mechanisms exist through which bone density can be altered or maintained in microgravity; this is achieved through altering calcium homeostasis, enhancing bone deposition, or selectively targeting the RANKL-OPG axis.

Alendronate

Bisphosphonates are structurally similar moieties to pyrophosphate, with a high affinity for calcium that causes them to accumulate within skeletal tissue. Nitrogenous bisphosphonates, including alendronate and zoledronate, are antagonists of farnesyl diphosphate synthase, which interrupts subsurface protein trafficking in osteoclasts. This alters the cytoskeletal structure required for bone contact and, hence, inhibits osteoclastogenesis and bone resorption. ²⁷

Alendronate is taken as an oral tablet either daily, weekly, or monthly. ³⁵ It can be stored for prolonged periods in a well-sealed container between 15–30 °C. ³⁵ As dietary calcium, magnesium, and aluminum interact with bisphosphonates, alendronate must be taken after fasting and at least 30 min prior to food. It must be taken sitting upright to minimize gastroesophageal reflux—a common side-effect that often contributes to reduced compliance and discontinuation. This may be troublesome in microgravity, as astronauts are unable to sit upright and gastric emptying is often delayed during early spaceflight due to space motion sickness. ²³ An alternative, zoledronate, is given as a yearly IV infusion to avoid most gastrointestinal side-effects, although post-infusion arthralgias are very common. ³⁴

Bisphosphonates are also associated with two rare phenomena. Osteonecrosis of the jaw can occur spontaneously; however, it is associated with dental trauma, poor oral health, and as a function of dose and number of years on therapy. ³⁶ Atypical femur fractures are atraumatic, occurring along the diaphysis (shaft or subtrochanteric) and, like stress fractures, are transverse. ^{16 , 27} Counterintuitively, the atypical fracture risk increases with long-term therapy, with annual incidence of 1.78 per 100,000 with <1.9 yr of use increasing to 113.1 per 100,000 for >8 yr of use. ¹⁶ Both conditions are considered unlikely to occur in the astronaut population, due to their high baseline of health and therapy duration being unlikely to extend much beyond the 3 yr of an expected return Mars mission.

To date, alendronate is the only pharmacological agent to be assessed as a countermeasure against microgravity-induced osteopenia. ²⁵ A 17-wk bed rest study demonstrated that, compared to controls, administration of alendronate not only maintained bone density in all bones examined (with exception of the calcaneus), but also suppressed markers of bone turnover and loss of calcium. ²⁶ A follow-up in-flight study examined 10 astronauts who were prescribed 70 mg of alendronate weekly, commencing 3 wk prior to a 5.5-mo ISS expedition and continuing throughout. ²⁵ Astronauts were required to undertake the normal 2.5-h daily exercise regime during the study.

BMD at the femoral neck, trochanter, total hip, pelvis, and lumbar spine was assessed pre- and postflight using dual-energy X-ray absorption (DXA) and quantitative computed tomography (QCT). Markers of bone turnover, including urinary and serum calcium, NTX, CTX, Vitamin D, and PTH, were recorded at specified intervals pre-, during, and post-flight. The data was compared to historical ISS data of 18 astronauts who undertook ISS missions prior to 2008 using the interim resistive exercise device, as well as to data from missions post-2008 with 11 astronauts who used the ARED. Of note, no QCT was available for the ARED group for comparison to the alendronate group. All astronauts continued to take standard vitamin D and calcium supplementation with a normal expedition diet. During the study, three astronauts withdrew from the alendronate group – one for personal reasons, one from gastrointestinal discomfort following a test dose, and one from developing dyspepsia in flight. ²⁵

Compared to the exercise-only groups, the alendronate group showed a clinically significant maintenance of preflight bone density scores on DXA, with relative suppression of markers of bone turnover. ²⁵ There was also a significant difference in BMD and bone mineral content (BMC) on QCT across all sites between pre-ARED and the alendronate group. ²⁵ As it was unclear whether the use of ARED in the alendronate group accounted for the significant difference in QCT BMD/BMC, a follow-up study recruiting an additional 10 astronauts using the same protocol and investigations was undertaken. ³⁹ Although this demonstrated that ARED did ameliorate overall bone loss, it did not significantly reduce trabecular BMD and BMC loss in the hip, nor did it suppress markers of bone turnover. ³⁹ Therefore, it can be concluded that the effects in BMD maintenance seen are due to the antiresorptive effect of alendronate. ^{25 , 39}

Denosumab

Denosumab is a novel human monoclonal antibody approved by the FDA in 2010 for treatment of osteoporosis and fracture prevention in bone metastasis, which acts as a specific inhibitor of RANKL. ^{1 , 27} RANK is expressed on the surface of preosteoclasts, and by binding to RANKL, it triggers maturation into osteoclasts. The maturation of osteoclasts is controlled by OPG, which is expressed by osteoblasts and regulated by exposure to estradiol. It is thought that in osteoporosis, decreased estradiol exposure leads to decreased OPG expression and, hence, unchecked osteoclast maturity. ²⁷ This imbalance in osteoclast activity progressively leads to decreased bone density. Denosumab therefore inhibits RANKL to prevent osteoclast maturity in a manner akin to OPG but independent of estradiol.

Denosumab is packaged in pre-drawn syringes, which must be protected from light, freezing, and excessive vibration; they must be stored at 2–8 °C until use. ¹ Once removed from refrigeration, a syringe may be kept at room temperature <25 °C for up to 30 d. ¹ For osteoporosis, it is administered as a 60-mg subcutaneous injection every 6 mo. Following administration, CTX falls rapidly and stabilizes after 3 d. ¹ While the peak serum concentration is reached in 10 d and the half-life is 26 d, denosumab has been shown to control bone resorption for up to 6 mo, corresponding with suppressed CTX and maintenance of BMD during this period. ¹

During the literature search, no previous bed rest or in-flight studies involving denosumab were identified. However, denosumab is currently recommended as an alternative first-line option to bisphosphonates for fracture prevention in osteoporosis due to its efficacy. ^{1 , 27 , 46} The FREEDOM blind randomized control trial demonstrated significant reductions in relative risk of vertebral (68%), hip (40%), and nonvertebral (20%) fractures compared to placebo over 3 yr of treatment. ¹⁵ In addition, this was accompanied by a 9.2% and 6.0% relative increase in total vertebral and hip BMD compared to placebo. ¹⁵ In the phase III DECIDE double-blind randomized noninferiority trial, the denosumab group showed a further 0.9% and 1.1% absolute increase in BMD measured via DXA compared to treatment alendronate at the hip and lumbar spine, respectively. ⁹ The FREEDOM trial extension demonstrated that BMD continued to improve up to 10 yr, with up to a 21.7% increase in lumbar spine and 9.2% in total hip from study baseline. ⁷

Furthermore, the efficacy of denosumab over alendronate has been confirmed in additional independent studies. A 2017 retrospective analysis assessed both agents over a 12-mo period, with the denosumab group showing superior improvement in femoral neck density on DXA. ⁴ Though not seen in the alendronate group, denosumab showed significant increase in lumbar BMD. ⁴ A 2015 meta-analysis looking at multiple antiresorptive therapies showed that denosumab was as efficacious as bisphosphonates at preventing hip fractures (denosumab OR 0.60, alendronate OR 0.61); additionally, it demonstrated a significant decrease in the risk of vertebral fractures (OR 1.67), echoing denosumab’s real-world superiority and efficacy. ⁴⁸

Pooled data from phase III trials ¹⁵ showed that the most commonly reported side-effects in denosumab and placebo groups were: back pain (34.1% vs. 34.0%); arthralgia (20.4% in both); hypertension (15.3% vs. 16.1%); nasopharyngitis (14.8% vs. 15.6%); pain in the extremities (11.8% vs. 11.2%); osteoarthritis (10.9% vs. 11.1%); eczema (3.0% vs. 1.7%); and skin infections (0.4% vs. 0.1%). ¹ Pancreatitis was reported in 0.1% of cases in the denosumab group vs. 0.2% in the placebo group; however, most of these cases were due to pre-existing pathology, such as gallstones. ¹ Of note, there was no significant statistical difference in the occurrence rate of any serious side-effects between treatment and placebo groups in the FREEDOM trial (P = 0.91 & 0.61). ¹⁵ Anaphylaxis is rare with denosumab administration, with five reported cases in post-marketing surveillance and no fatal outcomes. ²¹ No evidence of neutralizing antibodies to denosumab have been reported. ^{7 , 15}

Hypocalcemia is a potential concern with a dose-dependent effect. A head-to-head trial of 5677 patients with bone metastases, in which denosumab was administered at a higher 120 mg dose monthly, showed that hypocalcemia occurred in 9.6% of denosumab patients compared to 5.0% of those treated with zoledronic acid. ²⁹ Severe symptomatic hypocalcemia (requiring treatment with IV calcium) occurred in 3.7% and 1.7% of cases in each respective treatment group. ²⁹ No cases were seen within the denosumab group in the initial FREEDOM trial, ¹⁵ with overall annual incidence remaining ≤0.1 per 100,000 in the 10 yr extension trial. ⁷ In 2013, post-marketing surveillance of denosumab use in osteoporosis patients only identified eight cases of severe symptomatic hypocalcemia, of which seven cases were associated with chronic kidney disease. ²¹ Though the risk of hypocalcemia is low in the dose used for osteoporosis, it is recommended that serum calcium be checked prior to commencement of denosumab, and adequate dietary intake or supplementation with calcium and vitamin D should be maintained during denosumab treatment. ^{1 , 18 , 27}

While there were no reported incidences of osteonecrosis of the jaw or atypical femur fractures in the DECIDE trial, ⁹ the FREEDOM extension trial recorded 14 cases of jaw osteonecrosis and 2 of confirmed atypical femur fractures, with an overall annual incidence of ≤0.1 per 100,000 for both conditions, respectively. ⁷

Discontinuation of denosumab in osteoporosis cases shows an elevation of resorption markers at 6 mo, exceeding pre-treatment levels at 12 mo, accompanied by a decline toward or below baseline BMD. ^{30 , 31} While these studies did not show an increased fracture risk in the groups that discontinued treatment, ^{30 , 31} a post hoc analysis of the 10-yr FREEDOM trial extension showed that there was a significant increase in single and multiple vertical fractures in the individuals who ceased denosumab, ¹⁴ confirming the findings in a series of case reports. ⁴⁴ However, it is noted that the rate does not exceed that of the placebo group and this was strongly associated with individuals who had a prior history of vertebral fractures. ¹⁴ While recommencing denosumab results in clinically significant improvements in BMD after a period of discontinuation, ⁷ treatment guidelines for osteoporosis warn against drug holidays and state that if a drug is being discontinued, bridging therapy such as a bisphosphonate should be considered. ^{18 , 27 , 46}

This may be a potential concern for astronauts returning after a long-duration spaceflight, who may need to take an antiresorptive agent for a period following a return to Earth. However, further data is needed in younger and healthier populations to confirm whether the discontinuation effect occurs outside of the elderly populations previously studied. A summary of denosumab compared to alendronate is presented in Table I .

Table I. Comparison of Alendronate and Denosumab in Spaceflight.

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Future Applications

Cirnigliaro et al. 2020 investigated the use of denosumab in maintaining lower limb BMD in total motor spinal cord injury patients. ¹¹ Even without exercise countermeasures, significant maintenance of bone density occurs when administered shortly after injury, with control groups showing >10% density loss and up to 43% increased absolute risk of lower limb fractures compared to the denosumab treatment group. ¹¹ While no bed rest studies have been performed on healthy neurologically intact individuals, this is the first study to show the role of denosumab in maintaining density in a low mechanical loading setting, which could be considered analogous to the unloading seen in microgravity. Further bed rest studies could be used to confirm this finding in healthy individuals.

Other Potential Therapies

During the literature review, an additional agent that has been trialed with denosumab was identified. Romosozumab is a novel monoclonal antibody that targets sclerostin and has recently been approved for treatment in postmenopausal osteoporosis. ⁵ Sclerostin is secreted by osteocytes and acts as an antianabolic agent via the Wnt signaling pathway, inhibiting osteoblast activity. ²⁷ Its secretion is regulated by mechanical loading as well as PTH and estrogen. ²⁷

A large international study assessing vertebral fracture risk randomized postmenopausal women to receive either 210 mg of romosozumab or a placebo monthly for 12 mo, followed by doses of denosumab every 6 mo for an additional year. ¹³ In the romosozumab group, vertebral fractures occurred in 0.6% of participants at 24 mo compared to 2.5% placebo group. ¹³

However, there are no published trials comparing it as a single agent head-to-head with other antiresorptive therapies. Also, unlike denosumab, it is administered by subcutaneous injection every 1 mo instead of every 6 mo. It is only effective for up to 12 mo, with 18% of patients being shown to develop neutralizing antibodies, which requires switching to a different antiresorptive agent. ⁵ Of concern is the reported increase in cardiovascular events with romosozumab, with a 2.5–4.9% incidence in treatment groups. ^{3 , 28 , 37} Thus, it is contraindicated in individuals with previous ischemic heart and cerebrovascular disease. ³ The elevated cardiovascular risk associated with romosozumab may preclude its utility in long-duration spaceflight.