[en] Background: Recent innovations in the pharmaceutical drug discovery environment have generated new chemical entities with the potential to become disease modifying drugs for osteoarthritis (DMOAD's). Regulatory agencies acknowledge that such compounds may be granted a DMOAD indication, providing they demonstrate that they can slow down disease progression; progression would be calibrated by a surrogate for structural change, by measuring joint space narrowing (JSN) on plain X-rays with the caveat that this delayed JSN translate into a clinical benefit for the patient. Recently, new technology has been developed to detect a structural change of the OA joint earlier than conventional X-rays. Objective: The Group for the Respect of Ethics and Excellence in Science (GREES) organized a working party to assess whether these new technologies may be used as surrogates to plain x-rays for assessment of DMOADs. Methods: GREES includes academic scientists, members of regulatory authorities and representatives from the pharmaceutical industry. After an extensive search of the international literature, from 1980 to 2002, two experts meetings were organized to prepare a resource document for regulatory authorities. This document includes recommendations for a possible update of guidelines for the registration of new chemical entities in osteoarthritis. Results: Magnetic resonance imaging (MRI) is now used to measure parameters of cartilage morphology and integrity in OA patients. While some data are encouraging, correlation between short-term changes in cartilage structure observed with MRI and long-term radiographic or clinical changes are needed. Hence, the GREES suggests that MRI maybe used as an outcome in phase II studies, but that further data is needed before accepting MRI as a primary end-point in phase III clinical trials. Biochemical markers of bone and cartilage remodelling are being tested to predict OA and measure disease progression. Recently published data are promising but validation as surrogate end-points for OA disease progression requires additional study. The GREES suggests that biochemical markers remain limited to 'proof of concept' studies or as secondary end-points in phase II and III clinical trials. However, the GREES emphasizes the importance of acquiring additional information on biochemical markers in order to help better understand the mode of action of drugs to be used in OA. Regulatory agencies consider that evidence of improvement in clinical outcomes is critical for approval of DMOAD. Time to total joint replacement surgery is probably the most relevant clinical end-point for the evaluation of efficacy of a DMOAD. However, at this time, time to surgery can not be used in clinical trials because of bias by non disease-related factors like patient willingness for surgery or economic factors. At this stage, it appears that DMOAD should demonstrate a significant difference compared to placebo. Benefit should be measured by 3 co-primary end-points: JSN, pain and function. Secondary end-points should include the percentage of patients who are 'responder' (or 'failure'). The definition of a 'failure' patient would be someone with progression of JSN>0.5 mm over a period of 2-3 years or who has a significant worsening in pain and/or function, based on validated cut-off values. The definition of the clinically relevant cut-off points for pain and function must be based on data evaluating the natural history of the disease (epidemiological cohorts or placebo groups from long-term studies). These cut-offs points should reflect a high propensity, for an individual patient, to later require joint replacement. Conclusion: GREES has outlined a set of guidelines for the development of a DMOAD for OA. Although these guidelines are subject to change as new information becomes available, the information above is based on the present knowledge in the field with the addition of expert opinion. (C) 2004 OsteoArthritis Research Society International. Published by Elsevier Ltd. All rights reserved.