PROJECT SUMMARY Despite the remarkable successes of targeted cancer therapies, certain cancers, including lung, breast, and prostate cancer and melanoma, invariably become resistant to therapy. One mechanism of secondary resistance—lineage plasticity—arises when cells transition into aggressive states, and, in the case of prostate cancer, acquire a neuroendocrine histology. This results in a rapid downhill course for a subset of the so–termed castrate–resistant prostate cancer (CRPC) patients. This, in essence, not only poses a clinical challenge, but also confronts us with a wide–open biological question—what are the molecular underpinnings of lineage plasticity, and importantly, can the process be reversed? We have very recently documented that the activation of JAK/STAT and FGFR signaling pathways promote lineage plasticity and result in complete insensitivity to androgen receptor signaling inhibitors (ARSIs) [*Chan, *Zaidi, et al., Science, 2022, PMID: 35981096, *co– first authors]. Importantly, we found that FDA–approved inhibitors of JAK/STAT (ruxolitinib) and FGFR (erdafitinib) synergize to reverse lineage plasticity and restore ARSI sensitivity. We therefore hypothesize that signals downstream of JAK/STAT and FGFR, including novel transcription factors, interact to promote lineage plasticity, and their timed perturbation can reverse plasticity and ARSI insensitivity. Thus, in Specific Aim 1, to study how FGFR signals synergize with JAK/STAT to impart plasticity, we will use chemical inhibitors and CRISPR–Cas9 to delete specific molecules in TP53/RB1–null mouse and human tumor organoids. Specific Aim 2 will focus on further deconvoluting the molecular complexity of lineage plasticity through unbiased single cell paired RNA and ATAC (multiome) sequencing in murine organoids. We expect to identify novel transcription factors and study their DNA accessibility post–TP53/RB1 deletion across the evolution of lineage plasticity, with and without ruxolitinib and/or erdafitinib. In Specific Aim 3, in proof–of–concept in vivo studies, we will examine the efficacy of combined treatment with ruxolitinib plus erdafitinib in reversing lineage plasticity and restoring ARSI sensitivity. For this, the ruxolitinib+erdafitinib combination will be studied in NOD SCID mice grafted either with TP53/RB1–null murine organoids, orthotopically, or with the human tumoroid MSK–PCA3, subcutaneously. These studies will not only inform future therapeutic strategies to subvert drug resistance in CRPC patients but should also provide a unique platform for my Training Aims. Under the tutelage of my primary mentor, Dr. Charles Sawyers, and my Advisory Committee, I expect to enhance my competencies in advanced computation, cancer modeling and genetic editing, and bedside translation. Together with the rich scientific environment and vast array of resources at MSKCC, my research and training should position me to achieve my goal of becoming an independently funded physician–...