VTP-195183

All-trans retinoic acid (ATRA) is used successfully in the treatment of acute promyelocytic leukemia (APL). ATRA enhances hematopoietic stem cell self-renewal through retinoic acid receptor (RAR)γ activation while promoting differentiation of committed myeloid progenitors through RARα activation. Its lack of success in the treatment of non-APL acute myeloid leukemia (AML) may be related to ATRA's non-selectivity for the RARα and RARγ isotypes, and specific RARα activation may be more beneficial in promoting myeloid differentiation. To investigate this hypothesis, the effects of ATRA and the specific RARα agonist NRX195183 was assessed in AML1-ETO (AE)-expressing murine bone marrow (BM) progenitors. ATRA potentiated the in vitro clonogenicity of these cells while NRX195183 had the opposite effect. Morphological and flow cytometric analysis confirmed a predominantly immature myeloid population in the ATRA-treated AE cells while the NRX195183-treated cells demonstrated an increase in the mature myeloid population. Similarly, NRX195183 treatment promoted myeloid differentiation in an AE9a in vivo murine model. In the ATRA-treated AE cells, gene expression analyses revealed functional networks involving SERPINE1 and bone morphogenetic protein 2; AKT phosphorylation was upregulated. Collectively, these findings confirm the contrasting roles of specific RARα and RARγ activation in the clonogenicity and differentiation of AE cells with potential significant implications in the treatment of non-APL AML using a specific RARα agonist.

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 α (RARα) 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 RARα‐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 α‐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 RARα‐agonist can indeed counteract an ectopic skeletal‐formation process effectively and efficiently, and could thus represent a novel preventive treatment for HO. © 2009 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 28:271–277, 2010