Bisphosphonates define a class of drugs which are widely indicated since the 1990s to treat osteoporosis both in men and women. Their effectiveness to treat osteoporosis and other conditions is related to their ability to inhibit bone resorption.
Bisphosphonates have a structure similar to native pyrophosphate and are divided into two groups: nitrogen-containing and non-nitrogen containing bisphosphonates. Nitrogen-containing bisphosphonates include alendronate, risedronate, ibandronate, pamidronate, and zoledronic acid. Non-nitrogen containing bisphosphonates include etidronate, clodronate, and tiludronate. All bisphosphonates inhibit bone resorption by attaching to hydroxyapatite binding sites on the bone, particularly in areas with active resorption. As bone is resorbed by osteoclasts, the bisphosphonate that is embedded in the bone in released and impairs the osteoclast’s ability to continue bone resorption.
Nitrogen-containing bisphosphonates work by inhibiting farnesyl pyrophosphate synthase, which is important in promoting attachment of the osteoclast to the bone. As a result, the osteoclast detaches from the bone surface, thus inhibiting bone resorption.
Non-nitrogen containing bisphosphonates, on the other hand, are metabolized within the cell to substrates that replace the terminal pyrophosphate moiety of adenosine triphosphate, forming a nonfunctional molecule which competes with adenosine triphosphate in the energy metabolism of the cell. This initiates osteoclast apoptosis which in turn leads to an overall decrease in the bone breakdown.
Nitrogen-containing bisphosphonates are much more potent antiresorptive agents than the non-nitrogen-containing bisphosphonates. In addition, non-nitrogen containing bisphosphonates are found to have high potential to inhibit bone mineralization and can cause osteomalacia. For this reason, they are no longer used widely.
Alendronate, risedronate, and ibandronate are given orally, most commonly at weekly (alendronate, risedronate) or monthly (risedronate and ibandronate) intervals. Zoledronic acid and pamidronate are administered intravenously, and there is also an intravenous preparation of ibandronate that may be used. Intravenous preparations are beneficial in patients who cannot tolerate oral bisphosphonates or in whom oral bisphosphonates are contraindicated such as the presence or history of esophageal stricture.
Alendronate is given as 35 mg once weekly for the prophylaxis of osteoporosis in postmenopausal females and as 70 mg once weekly for the treatment of osteoporosis in men and women. For Paget disease of bone in males and females, alendronate is given orally as 40 mg once daily for 6 months, and risedronate is given 30 mg orally daily for 2 months. Risedronate is given as 35 mg once weekly and 150 mg once a month. Ibandronate sodium is given as 150 mg orally once a month or 3 mg intravenously every month. Zoledronic acid is given as 4 mg to 5 mg intravenously over at least 15 to 30 minutes every 12 months for the treatment of osteoporosis. Pamidronate is given as 30 mg to 60 mg by slow intravenous infusion every 3 to 6 months for the treatment of hypercalcemia of malignancy, Paget disease, and bone metastasis.
Patients should receive supplemental calcium 1000 to 1200 mg/day and vitamin D 800 to 1000 international units/day if dietary intake is inadequate. Bisphosphonates should be taken as first medication in the morning and more than 30 minutes before the first food intake, beverage (except water), or any other medication. Patients should be instructed to stay erect, not to lay down for at least thirty minutes or until the first food intake of the day to reduce esophageal irritation/ulceration. Milk and milk products, coffee, orange juice, and food may reduce the absorption of alendronate.
Side effects of bisphosphonates includes: acute phase reaction (10% to 30%) with transient pyrexia with myalgias, arthralgias, headaches, and influenza-like symptoms, transient hypocalcemia (18%), hypophosphatemia (10%), musculoskeletal pain (less than or equal to 6%), flatulence (less than or equal to 4%), headache and gastroesophageal reflux disease (3%), constipation or diarrhea (less than or equal to 3%), abdominal pain (2% to 7%), esophageal ulcer (2%), acid regurgitation (1% to 5%), abdominal distension, gastric ulcer, gastritis and muscle cramps (less than or equal to 1%).
Other Adverse Effects
One of the most severe adverse effects is bisphosphonate-related osteonecrosis of the jaw (BRONJ). BRONJ is diagnosed if all the following criteria are met:
The American Association of Oral and Maxillofacial Surgeons has further defined the stages of Bisphosphonate-related osteonecrosis of the jaw (BRONJ) as follows:
Dose modification or preferably stopping oral bisphosphonate therapy should be done in patients exhibiting systemic symptoms since bisphosphonates are known to affect the jaw completely and may lead to progression of BRONJ.
Another serious adverse effect of bisphosphonates is atypical femoral fracture typically involving diaphysis or sub-trochanteric region of the femur. It is caused by pathophysiological alterations of the bone quality and fracture repair process resulting in over-suppression of bone turnover.
Due to potential serious nature of BRONJ and atypical femur fracture, a two year drug holiday is recommended after 5 to 10 years for the oral bisphosphonates and 3 to 5 years for zoledronic acid.
Widely documented contraindications include hypersensitivity to the bisphosphonate, hypocalcemia, abnormalities of the esophagus such as achalasia, esophageal stricture, esophageal varices, Barrett's esophagus, inability to stand or sit upright for at least 30 minutes, history of bariatric surgery (Roux-en-Y gastric bypass) and in chronic kidney disease with glomerular filtration rate less than 30 to 35 mL/min.
Bone mineral density should be evaluated 1 to 2 years after initiating therapy and every two years or more frequently in patients deemed high risk. Annual measurements of height, weight, serum calcium, 25- hydroxyvitamin D and assessment of back pain for the development of compression fractures should be done.
In some circumstances, monitoring of biochemical markers of bone resorption such as N-telopeptide of type-1 collagen, C-terminal telopeptide of type 1-collagen and pyridinoline cross-links, and markers of bone formation such as bone-specific alkaline phosphatase, osteocalcin, and N-terminal propeptide of type 1 procollagen before and after 3 months after bisphosphonate initiation can be helpful in monitoring the effectiveness of the medication in inhibiting bone resorption and measuring compliance. However, these assays are fraught with significant variability within individual patients and poor standardization, so they are not routinely used.
There is no question that the bisphosphonates can reduce the risk of osteoporosis but the drugs are also expensive and have some serious adverse effects. Pharmacists should assist in monitoring the patient for adverse events and appropriate dosing. The key to treatment of osteoporosis is prevention, and it is here that the role of nurses is vital. The nurses should encourage the public to modify their lifestyle to reduce the risk of osteoporosis. This means discontinuing tobacco, becoming physically active and limiting the intake of caffeine, alcohol and animal protein and at the same time increasing the intake of dairy products. Individuals at risk for osteoporosis should be encouraged to undergo periodic bone densitometry studies to diagnose the condition early and take steps to prevent fractures. The patient with confirmed osteoporosis should have a dietary and physical therapy consult before discharge. Finally, low impact exercises such as walking and bicycling are recommended. (Level V)
The prognosis for patients who have early detection and treatment of osteoporosis is good. Patients can increase their bone mass and lower the fracture risk with bisphosphonates. But at the same time, they must make a positive change in their lifestyle. For those who have developed fractures, pain control and use of orthotic devices may be helpful. (Level V). Unfortunately, compliance with bisphosphonate therapy is not high due to the cost of the drugs and side effects. Hence, patient education is necessary.