Research suggests that bisphosphantes are effective in controlling clinical signs associated with navicular syndrome in horses, but there's less robust scientific evidence on off-label use in horses. | Photo: Anne M. Eberhardt/The Horse

By Chris Riggs, BVSc, PhD, DEO, Dipl. ECVS, MRCVS, Hong Kong Jockey Club head of mainland veterinary operations and WEVA board member

Bisphosphonates are a class of drugs that have a similar molecular structure to naturally occurring pyrophosphates (chemical compounds that can inhibit calcification). Structural variations in a side chain attached to the base molecule determine the drugs’ potency.

These drugs are absorbed relatively poorly from the intestine, so they’re frequently administered by injection. Bisphosphonates have a strong affinity for the minerals found in bone and, while the horse excretes a large proportion of them in urine, a notable proportion remains bound to mineral within bone.

Bisphosphonates are toxic to cells, but their molecular structure is such that they are generally unable to cross cell membranes. A notable exception is the cells that digest bone, called osteoclasts