The present invention relates to novel depot formulations (prodrugs) comprising an immobility promoting unit linked via an ester to an active pharmaceutical ingredient, i.a. common NSAIDs. The novel depot formulations are suitable for intra-articular injections and are soluble at slightly acidic pH to facilitate ease of injection, and sparingly soluble at physiological pH thereby precipitating at the site of administration. The precipitate will slowly dissolve and the active drug is released from dissolved depot formulation following esterase mediated cleavage of the ester link between the immobility promoting unit and the active pharmaceutical agent.