Bisphosphonates inhibit bone breakdown or resorption making them useful for treating bone disorders such as podotrochlosis (navicular syndrome) in horses. In human medicine, patients with various bone fragility disorders, including osteoporosis, reap the rewards of bisphosphonate treatment.
In healthy animals, including horses, bone turns over continually. Cells called osteoclasts play a key role in breaking down old bone while another type of cell—osteoblasts—creates new bone. This natural process ensures bones remain strong and healthy and allows them to adapt to changes in exercise level or musculoskeletal system stress. Bisphosphonates bind to osteoclasts to block excess bone resorption. Currently, only two bisphosphonates have garnered U.S. Food and Drug Administration (FDA) approval for equine use. Both products are licensed for the control of clinical signs associated with navicular syndrome in horses.
Bisphosphonates appear to block osteoclast activity in horses with navicular syndrome, a common cause of chronic foot pain in horses resulting from the progressive degeneration of the navicular bone and its associated structures. Navicular syndrome has no cure, so current treatment options focus on controlling discomfort and potentially minimizing disease progression. Common management strategies include but are not limited to rest, corrective shoeing, and non-steroidal anti-inflammatory drugs (NSAIDs). In many cases, these treatment options provide inadequate control over navicular syndrome. Prior to bisphosphonate availability, many horses suffering from navicular syndrome had uncontrolled discomfort, decreased quality of life, and were unable to compete at their owners’ desired level, leading to retirement and attrition.
The two FDA-approved bisphosphonates for use in horses are clodronate disodium (Osphos) and tiludronate disodium (Tildren). Data support both products’ safety and efficacy in horses with navicular syndrome. The main difference between the two products is the administration route.
OSPHOS is administered intramuscularly. The dose is divided and injected into three distinct sites to minimize injection-site irritation or reaction. Manufacturer studies showed that: On day 28 after administration, 67.4% (60/89) of treatments were considered successes, defined as an improvement of at least one lameness grade and no worsening of lameness grade in the other forelimb on day 56 post-treatment as compared to the pre-treatment assessment.
Only 20.7% (6/29) of horses with navicular syndrome in the saline-treated control group demonstrated decreased lameness; On day 56, 74.7% (68/86) of treated horses and 3.6% (1/28) of control horses were treatment successes and at day 180, 85% (51) of the 60 horses that were deemed treatment successes on day 56 and were evaluable at day 180 remained treatment successes; however, 35% (21/60) of those evaluable horses had an increase in lameness grade at day 180 as compared to their day 56 evaluation. Including the 18 treatment failures at day 56, the estimated overall success rate for OSPHOS at day 180 is 65.4% (51/78). Horses treated with OSPHOS successfully enjoyed relief of clinical signs for approximately six months. This product can be administered every three to six months based on the recurrence of clinical signs of the navicular syndrome.
TILDREN is formulated for intravenous administration. Research showed treatment success (if the lameness in the primarily affected limb improved by at least one grade and there was no worsening of lameness grade in the other forelimb at two months post-treatment compared to pre-treatment) in 63.87% of 119 horses treated with Tildren. The maximum clinical effect does not occur immediately. Instead, clinical improvement typically occurs approximately two months after administration.
Several recent publications suggest that bisphosphonates could also be useful in managing horses diagnosed with hip and back pain and/or osteoarthritis of the back (i.e., between the individual vertebrae or bones that make up the spinal column and protect the spinal cord). Osteoarthritis, like navicular syndrome, currently lacks a definitive cure, and owners similarly rely on management techniques to help affected horses. Bisphosphonates could also help horses with osteoarthritis affecting other joints, such as hocks, although the evidence supporting this use is limited.