Onsior is a non-steroidal anti-inflammatory drug (NSAID) for use in cats. Robenacoxib is part of the coxib class of NSAIDs and is recommended for the treatment of acute pain and inflammation associated with musculoskeletal disorders in cats.

Robenacoxib is a potent and selective inhibitor of the cyclooxygenase 2 enzyme (COX-2). The cyclooxygenase enzyme (COX) is present in 2 forms. COX-1 is the constitutive form of the enzyme and has protective functions, e.g. in the gastrointestinal tract and kidneys. COX-2 is the inducible form of the enzyme which is responsible for the production of mediators including prostaglandin E2 (PGE2) which induce pain, inflammation or fever.

In the in vitro whole blood assay in cats, robenacoxib was approximately 500 fold selective for COX-2 as compared to COX-1. 1-2 mg/kg robenacoxib tablets produced a marked inhibition of COX-2 activity in cats and had no effect on COX-1 activity.

After oral administration of robenacoxib tablets at approximately 2 mg/kg without food, peak blood concentrations are attained rapidly with a Tmax of 0.5 h. Co-administration of robenacoxib tablets with one third of the daily food ration produced no change in Tmax (0.5 h).

Robenacoxib is highly bound to plasma proteins (>99%) and extensively metabolised by the liver in cats. It is rapidly cleared from blood. After oral administration of tablets, the terminal half-life from blood was 1.7h. Robenacoxib persists longer and at higher concentrations at sites of inflammation that in blood. Robenacoxib is excreted predominantly via the biliary route (~70%) rather than via the kidneys (~30%).