Sunday, November 25, 2012

Prolia® (denosumab): Treatment for Osteoporosis--The Pros, Cons, and Unknowns

For more than 10 years, the use of bisphosphonate therapy has been the “osteoporosis treatment of choice” when medication is absolutely necessary. You have likely heard of the names alendronate (Fosamax®), residronate (Actonel®), ibandronate (Boniva®), and the most recent zoledronic acid (Reclast® approved in 2007). When the bone remodeling process has gone awry due to to chronic systemic inflammation, too much resorptive activity (bone degrading) by osteoclasts and too little bone formation by osteoblasts create the perfect storm for osteoporosis.  When bisphosphonates are used for therapy, they eliminate existing osteoclasts, thereby slowing bone loss and improving bone density. 

Over time, we have learned that there are adverse effects that need to be considered when using these medications. For instance, bisphosphonates have a high affinity for bone and are sequestered or held within calcium crystals, posing both beneficial and detrimental effects. Beneficial because bisphosphonates continue to suppress osteoclastic bone resorption long after a patient has discontinued their use. Detrimental because long-term suppression of osteoclasts can lead to low bone-turnover disorders such as osteonecrosis of the jaw and increased bone fragility with a heightened risk for atypical femur fractures. Stopping use of these drugs does nothing to help rectify these situations because of sequestered drug levels.
With new medication always comes new hope. 
Denosumab (Prolia®) was introduced for the treatment of osteoporosis in 2010, two years ago. Like bisphosphonates, this new medication limits the bone degrading activity of osteoclasts, but it uses a different mechanism. Denosumab is an antibody to RANKL, a natural signaling molecule in the body. When produced in normal amounts RANKL keeps the immune and skeletal systems functioning normally. But in excess (seen when the immune system gets out of balance), RANKL can cause havoc, spilling into the bone marrow and hyper-stimulating osteoclast cell formation/activation. Over time, this hyper-stimulation of osteoclastic bone resorption leads to osteoporosis. Being an antibody to RANKL, denosumab prevents bone loss by grabbing RANKL, neutralizing it, and preventing it from activating osteoclasts. (Note: If you want to learn more about RANKL and its involvement in bone loss, I give a more in-depth explanation in my book, The Whole-Body Approach to Osteoporosis.)
Unlike bisphosphonates, denosumab is not sequestered in bone and therefore does not have the same long-lasting effects after treatment has been discontinued. However, even with such a short track record, we are already beginning to see that denosumab has a dark side with long-term use.  So hopes for denosumab, like all medications, must be kept in perspective. We need to use medications to our benefit, but always be aware that they can carry risks. Here is what we have so far:
PROS
  • Denosumab is administered subcutaneously every 6 months (better than having to take bisphosphonates orally or through infusions). 
  • It works. Osteoclastic bone resorption is profoundly suppressed by denosumab and results in similar increases in bone mineral density when compared to bisphosphonates.  (Spine: 3% 5% after 1 year; 6% to 7% after two years; 8% to 10% after 3 years, 13% after 5 years) ( Hip: 1% to 3% after 1 year, 3% to 5% after 2 years, 5% to 6% after 3 years, 7% after 5 years)
  • Denosumab significantly reduces vertebral and nonvertebral fractures (similar to the rates of bisphosphonates).
  • Effective in patients with impaired kidney function.
CONS
  • Denosumab may disturb immune function and increase susceptibility to infections although in a two-year extension of the FREEDOM trial (a total treatment duration of seven years) researchers did not see an increase in infectious events.
  • There are concerns that denosumab may increase the risks of cancer. Anastasilakis et al. state, "numerically more cases of neoplasms, including those of the breast, ovary and gastrointestinal tract, have been reported." But since other trials have "failed to detect a statistically significant difference" with denosumab use compared to placebo, "long-term use of denosumab in a large post-marketing base would clarify this putative risk."
  • There is a significant risk of developing cutaneous allergic and inflammatory hypersensitivity skin reactions including eczema, dermatitis, and rashes.
  • There is a significant risk (7%) of developing high blood cholesterol levels.
  • Possible increased risk of vascular calcification and cardiovascular disease.
  • Increased risk for low blood calcium (hypocalcemia).
  • Because denosumab is such a potent suppressor of osteoclastic bone resorption and bone remodeling in general, it carries similar concerns of "frozen bone" types of adverse effects as the bisphosphonates. Atypical fractures of the femur and osteonecrosis of the jaw are of concern. In fact, Health Canada has just released a health advisory alerting the public of the association of Prolia with the increased risk of atypical femoral fractures.
UNKNOWNS
According to a review article in Therapeutics and Clinical Risk Management by Anastasilakis et al., when denosumab is discontinued "bone markers rise to above pretreatment levels within 12 months." In other words, if you stop using denosumab you quickly loose the density that was accrued. Not only that, but you loose it at a rate faster than that seen before treatment was initiated. This "rebound," or hyper-increase-in-bone-loss effect, lasts for approximately two years. Clinical studies have not determined the full implications of this rebound effect but when a person elects to begin treatment with denosumab they probably shouldn't stop. Any "drug holiday," such as that which is becoming common after 3 to 5 years of bisphosphonate therapy, may actually escalate fracture risk to a level even higher than that which the individual had before denosumab treatment was initiated. 
Anastasilakis et al. concluded "denosumab is reasonably safe for all subgroups of patients with postmenopausal osteoporosis, with the exception of those with hepatic or stage 5 renal insufficiency. However, given the lack of pharmacovigilance data for this agent as yet and its brief post-marketing period, it would be prudent to be vigilant for adverse events related to the putative effect of RANKL inhibition in tissues other than bone, as well as those related to bone turnover oversuppression."
Bottom line:  Osteoporosis medication should be used only when absolutely necessary – when fracture risk is moderately or severely elevated.
 
Anastasilakis et al., 2012. Long-term treatment of osteoporosis: safety and efficacy appraisal of denosumab. Therapeutics and Clinical Risk Management, 8:295-306.
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