Tubo-ovarian high-grade serous carcinoma (HGSC) is the most common and lethal subtype of ovarian cancer, typically diagnosed at an advanced stage due to absence of specific symptoms and specific clinical biomarkers. Standard treatment has remained relatively unchanged for decades, consisting of primary debulking surgery and platinum-taxane chemotherapy. Usually, the initial treatment response is good, but relapse is common, leading to chemoresistance and death. Recent advances, especially the addition of PARP inhibitors to patients, whose tumors are homologous recombination (HR) deficient, have brought some benefit to patients. Yet, better understanding of the disease and translation of the extensive research into clinical practice is needed. Specifically, prognostic and predictive biomarkers that enable personalized treatment approaches are required to improve outcome of HGSC.

Therapy resistance in HGSC is multifactorial, with cancer stem cells (CSCs) purposed as key contributors. This thesis aimed to identify compounds targeting HGSC cancer cells and specifically CSCs by high-throughput drug screening of patient-derived cell cultures. In further studies, we focused on one of the potential drug compounds, a Wee1 inhibitor and studied its mode of action in more detail. A further objective was to evaluate the expression of four selected known CSC markers in patient tumors and their potential in tissue-based diagnostics by immunohistochemical and in situ -hybridization analysis.

Transcriptome analysis of cultured HGSC cells identified eight CSC markers that are associated with stemness features, and in clinical samples with poor prognosis. In the drug screen, the Wee1 inhibitor adavosertib emerged as an effective compound across most tested cell cultures, including those with CSC features. Functional in vitro assays revealed that adavosertib exerted broad cytotoxic effects. These effects were observed irrespective of the homologous recombination (HR) signature or functional HR status of the cells. We carried out further analysis of four CSC marker candidates: ALDH1A1, SOX2, MYC, and BMI1, in a tissue micro array to assess their potential as a diagnostic biomarker panel for HGSC. Notably, BMI1 and MYC emerged as individual markers independently associated with reduced platinum free interval and overall survival. Recent studies have indicated that BMI1 is involved in the control of HR which makes it as a potential target for novel therapeutic approaches.