Small-cell lung cancer (SCLC) accounts for nearly 15% of lung cancer cases and exhibits aggressive clinical behavior characterized by rapid growth and metastatic spread to multiple organs. About 70% of patients with SCLC present with extensive disease and distant metastases at diagnosis. HSP90 is a 90-kDa molecular chaperone whose association is required for the stability and function of its numerous "client proteins." Here, we assessed the therapeutic potential of the HSP90 inhibitor 17-DMAG in SCLC. Notably, 17-DMAG hindered the viability of human SCLC cell lines-regardless of their chemosensitivityvia the decreased expression of client proteins, including the proto-oncogene c-Raf (also known as RAF1). In an in vivo imaging model of SCLC multiple-organ metastasis with the human SCLC cell line SBC-5, treatment with 17-DMAG remarkably inhibited the formation of metastatic sites in the liver, but was ineffective in hindering the progression of bone lesions. The latter was likely the result of activation of osteoclasts. IGF-1, which is supposed to be rich in bone environment, preserved c-Raf expression and maintained viability of SBC-5 cells treated with 17-DMAG. Furthermore, the combined use of a bisphosphonate with 17-DMAG significantly attenuated the progression of metastases in both the liver and the bone. These findings suggest that therapeutic effects of HSP90 inhibitors may be organ-specific and should be carefully monitored in SCLC clinical trials.Lung cancer is one of the most prevalent malignancies and the leading cause of cancer-related deaths worldwide.1 It is histologically classified in two types: non-small-cell lung cancer (NSCLC) and small-cell lung cancer (SCLC). While targeted molecular therapies have led to improved treatment outcomes for NSCLC, no progress has been made on therapeutic strategies for SCLC over the past two decades.2 SCLC accounts for nearly 15% of lung cancer cases and exhibits aggressive clinical behavior characterized by rapid growth and metastatic spread to multiple organs such as the brain, liver, and bones.3 Accordingly, about 70% of patients with SCLC present with extensive disease (ED) and distant metastases. Cisplatin-based chemotherapy is standard for patients with ED-SCLC, 4 but despite an initial response, most patients develop chemoresistant disease within 6 months and often become refractory to salvage chemotherapy.
4,5Thus, the median range for ED-SCLC survival is 8-13 months. 6 To improve the poor prognosis of SCLC, it is essential to develop novel effective therapies capable of limiting the progression of multiple-organ metastasis in chemorefractory SCLC. HSP90 is a 90-kDa molecular chaperone whose association is required for the stability and function of numerous "client proteins." 7 Because many of its client proteins-including EGFR (epithelial growth factor receptor), HER2 (human epithelial growth factor receptor 2), AKT and c-Raf (also known as RAF1)-are important participants in the pathways that drive tumor cell survival, proliferation and progression, HSP9...