Inhibition of Ectopic Bone Formation by a Selective Retinoic Acid Receptor α-Agonist: A New Therapy for Heterotopic Ossification?

Heterotopic ossification (HO) consists of formation of ectopic cartilage followed by endochondral bone and is triggered by major surgeries, large wounds, and other conditions. Current therapies, including low-dose irradiation, are not always effective and do not target the skeletogenic process directly. Because chondrogenesis requires a decrease of nuclear retinoic acid receptor alpha (RAR alpha) action, we reasoned that pharmacologic activation of this receptor pathway should inhibit HO. Thus, we selected the synthetic retinoid NRX195183, a potent and highly selective RAR alpha-agonist, and found that it did inhibit chondrogenesis in mouse limb micromass cultures. We established a mouse HO model consisting of subcutaneous implantation of Matrigel mixed with rhBMP-2. Control mice receiving daily oral doses of vehicle (peanut oil) or retinol (a natural nonactive retinoid precursor) developed large HO-like masses by days 9-12 that displayed abundant cartilage, endochondral bone, vessels, and marrow. In contrast, formation of HO-like masses was markedly reduced in companion mice receiving daily oral doses of alpha-agonist. These ectopic masses contained sharply reduced amounts of cartilage and bone, blood vessels, and TRAP-positive osteoclasts, and expressed markedly lower levels of master chondrogenic genes including Sox9, cartilage genes such as collagen XI and X, and osteogenic genes including Runx2. The data provide proof-of-principle evidence that a pharmacological strategy involving a selective RAR alpha-agonist can indeed counteract an ectopic skeletal-formation process effectively and efficiently, and could thus represent a novel preventive treatment for HO. (C) 2009 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 28:271-277, 2010