PROJECT ABSTRACT Limited stage small cell lung cancer (LS SCLC), the only curable form of SCLC, is remarkably sensitive to etoposide plus cisplatin combined with thoracic radiotherapy with response rates > 70%; however, therapy- refractory recurrence is common. LS SCLC has less than a 25% 5-year overall survival (OS) and ultimately a strategy for improving long-term SCLC outcomes needs to successfully target tumor cell populations that survive standard therapy and give rise to recurrent disease. There is, however, a considerable gap in understanding the specific mechanisms responsible for chemoradiotherapy resistance in SCLC. Our project is unique among the current portfolio of SCLC funded programs in that we have focused on chemoradioresistance to increase cure rates in LS SCLC. Recently, our work has suggested using patient- derived xenograft (PDX) models of SCLC may be an important tool to elucidate mechanisms of therapy resistance. This approach was remarkably successful, identifying a tolerable and strongly synergistic anti- SCLC interaction that led to a CTEP-approved trial based on our preclinical data - (NCI #10070; Study Chair: Hann). In this research program, we will test key hypotheses via three specific aims that will provide more mechanistic insights into the rapidly emergent chemoradiation resistance observed in LS SCLC. One central hypothesis of this proposal is that adaptive gene expression changes mediate rapid emergence of the chemoradiation resistance phenotype in LS SCLC. We have developed a novel chemoradiation treatment regimen with SCLC PDX models to facilitate these studies. Development and characterization of this novel model involves a unique collaboration between medical oncologists, radiation oncologists, bioinformaticians, medical physicists, veterinarians and molecular/cell biologists that is extremely well suited to develop an integrated program dedicated to resolving questions of SCLC chemoradioresistance. Finally, we have already identified novel gene targets that are correlated with SCLC chemoradioresistance. Our research program is organized as follows: Aim #1: Characterize natural history of response of experimental models of SCLC to chemoradiation in vivo. We will determine response rates and recurrence patterns of a panel of SCLC PDXs and transgenic mouse models. Aim #2: Characterization of molecular underpinnings of SCLC chemoradiation resistance. We will reconstruct gene regulatory networks and gene expression profiles associated with chemoradiation resistance and develop small-scale predictive classifiers for therapy response to be validated in follow-up studies. Aim #3: Pharmacologic and genetic validation of candidate genes for SCLC chemoradiation resistance in vitro and in vivo. We will validate our novel gene candidates for conferring chemoradiation resistance using pharmacologic and genetic approach with SCLC PDX-derived organoids and SCLC transgenic mouse models.