Novel Therapeutic Strategies to Co-Target Undifferentiated Prostate Cancer (PCa) Stem Cells and Bulk PCa Cells

In this renewal R01A1 application (parent R01CA240290; “Novel Therapeutic Strategies to Co-Target Undifferentiated Prostate Cancer (PCa) Stem Cells and Bulk PCa Cells”), we continue to develop novel strategies to tackle a pervasive and intertwined problem of clinical significance in PCa, i.e., PCa cell heterogeneity and plasticity. PCa continues to claim a high mortality with >35,000 American men estimated to die from metastatic castration-resistant PCa (mCRPC) in 2024. This high mortality and increased diagnosis of advanced PCa have been linked to PCa cell heterogeneity (e.g., in androgen receptor; AR) and treatment- induced plasticity. It has become clear that the entire spectrum of primary PCa, CRPC and metastases harbors not only AR-expressing (AR+) cells but also a population of PCa cells with little/no AR (AR-/lo), the latter of which often become expanded in mCRPC. Clinically, since introduction of the new generation of antiandrogens such as enzalutamide (Enza), there has been a dramatic increase (up to 20-40%) in AR-/lo mCRPC. Compared to AR+ cells, the AR-/lo PCa cells are understudied and poorly understood. Our focused work over the past 22 years has advanced our understanding of AR-/lo PCa cells and the dynamic relationship between AR-/lo and AR+ cells in PCa ecosystem and during PCa progression. For example, we have demonstrated that the AR-/lo PCa cell population in treatment-naïve tumors harbors PCa stem cells (PCSCs), and we have identified novel targets in AR-/lo PCa cells that have been translated to clinical trials (e.g., NCT03751436). Herein, we have made new and unexpected findings that AR-/lo PCa cells preferentially express genes involved in homologous recombination (HR) based DNA damage response/repair (DDR) including ATR, ATM and BRCA2. Remarkably, many of the same HR genes are also highly expressed in AR-/lo genetically engineered mouse models (GEMMs) of PCa (Pten-/-;Rb1-/-; DKO and Pten-/-Rb1-/-p53-/-; TKO) as well as in patient AR- mCRPC that have undergone Enza-induced plasticity. In contrast, AR+ PCa cells mainly express PARP1 and NHEJ genes and utilize NHEJ for DDR. These exciting preliminary data led us to hypothesize that the AR-/lo PCa cells primarily employ the HR pathway to repair the DNA damage which makes these cells highly vulnerable to HR pathway inhibition. Therefore, combination of ATR inhibitor (ATRi) and ADT/Enza will elicit synthetic lethality and represents a novel therapeutic regimen to holistically target both AR-/lo and AR+ PCa cells. We test this hypothesis with 3 Aims: Aim 1: Test in PCa GEMMs that ATRi combined with castration/Enza will inhibit CRPC and ATRi alone will be highly effective against AR-lo CRPC/NEPC; Aim 2: Evaluate the therapeutic effects of ATRi alone on AR-/lo and ATRi/Enza combination on AR+ human CRPC/PDX models; and Aim 3: Define the DDR pathway dynamics and functions in AR+ and AR-/lo PCa cells using cell line and organoid models as well as patient tumor samples. These aims will be accomplished by therapeutic studies in the DKO/TKO GEMMs and human xenograft/PDX/organoid models combined with mechanistic investigations.