In 2014, the U.S Food and Drug Administration (FDA) approved the use of the bisphosphonates tiludronate (Tildren) and clodronate (Osphos) for controlling clinical signs of navicular disease in horses. Since then, however, more questions than answers have surfaced regarding this potentially ground-breaking class of drugs. Katja F. Duesterdieck-Zellmer, DMV, PhD, Dipl. ACVS, assistant professor of large animal surgery at Oregon State University’s Department of Clinical Sciences, in Corvallis, reviewed bisphosphonates and their effects at the 2016 American Association of Equine Practitioners Convention, held Dec. 3-7 in Orlando, Florida.

In humans, physicians use bisphosphonates to prevent loss of bone mass associated with osteoporosis. Among equine practitioners, however, “the use of bisphosphonates seems to be controversial, with many having strong opinions for or against the use of these drugs in equines,” said Duesterdieck- Zellmer. “Questions remain regarding the best choice of bisphosphonate, frequency, route and dose of administration, minimum age of the horse to be treated, what type of conditions may benefit from treatment, and how to select the best candidates from successful bisphosphonate treatment.”

The Pharmacokinetics

Veterinarians can administer bisphosphonates either intravenously (IV) or intramuscularly (IM). Once in the body, the drug binds to bone mineral, particularly at sites of active bone resorption.

“Here, bisphosphonates remain until being liberated via slowed bony resorption,” said Duesterdieck-Zellmer. This means the drug’s half-life (in the case of bisphosphonates, the time it takes for the drugs’ bone concentration to decrease by 50%) depends on the rate of bone turnover, which varies by location and disease state present. There’s currently no data available about bisphosphonates’ half-life in horses