Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma of childhood, with an estimated five-year overall survival of less than 30% in patients with metastatic disease. Despite refinements in risk stratification and multimodality chemotherapy, improvements in RMS survival have been minimal and few molecularly targeted therapies are currently available. Previously, we demonstrated that RMS cells express estrogen receptor (ER) and are sensitive to 4-hydroxytamoxifen (4OHT), an active metabolite of tamoxifen. Here, we investigate the mechanisms by which 4OHT exert these effects. Previously, our group also found that 4OHT-induced apoptosis involves ER-dependent phosphorylation of c-Jun N-terminal kinase (JNK). Here, we found that inhibition of the pro-survival AKT/mTOR pathway is involved in 4OHT-induced apoptosis as well. These mechanisms can be exploited by combining 4OHT with other drugs acting on the JNK and AKT/mTOR pathways: vincristine (a core RMS chemotherapy agent) enhances JNK phosphorylation, while temsirolimus (an emerging RMS drug currently in phase III trials) inhibits mTOR complex 1. Combining tamoxifen with either of these drugs led to increased efficacy against cell proliferation in vitro. In an alveolar RMS (ARMS) xenograft mouse model, tamoxifen causes significantly reduced tumour growth and increased apoptosis. In contrast, long-term tamoxifen treatment led to enhanced tumour growth in an embryonal RMS (ERMS) xenograft model. A tolerable dose and schedule was optimized for tamoxifen in combination with vincristine and temsirolimus in vivo. However, preclinical phase II studies found that vincristine in combination with temsirolimus had similar efficacy with or without tamoxifen, at the given dose and schedule. Taken together, our work provides in vivo confirmation that tamoxifen may be an effective agent for treatment of ARMS, and provides a mechanistic framework that explains the anti-RMS effects of tamoxifen and its ability to potentiate vincristine and temsirolimus.Ph.D
