| dc.description.abstract | Ex vivo tumor tissue cultures, such as organoids or explants, hold promise as a drug screening platform for functional precision diagnostics. In this project, we have employed such cultures to investigate functional and molecular properties of breast cancer (BC) tissue with distinct sensitivity to Enhertu, an antibody-drug conjugate (ADC). ADCs are a novel class of cancer therapy, that combines the specificity of monoclonal antibodies with the cytotoxic effects of chemotherapeutic drugs. As of now, three ADCs are approved for the treatment of BC, and two of these (Enhertu and Trodelvy) are tested in this project, using ex vivo tissue cultures. As a model, we used an isogenic pair of BC patient-derived xenografts (PDXs) consisting of the highly sensitive and resistant sublines, HBCx39 and HBCx39ER, respectively. We have shown that the cultured tissue from HBCx39ER was non-responsive to Enhertu, in contrast to the Enhertu-sensitive HBCx39, which validates the responses as seen in vivo. Furthermore, the cultured tissue from HBCx39ER was also less sensitive to another ADC, Trodelvy, and to the chemotherapeutic agent, paclitaxel. Thereby, we predicted that HBCx39ER tumors should be less sensitive or resistant to these drugs also in vivo, which was later validated by other members of the group. The poor response of HBCx39ER tissue to Enhertu, Trodelvy and paclitaxel suggested that the tissue had developed a multidrug resistance. To investigate the possible mechanisms of resistance, we compared the cultures from both PDXs with respect to: i) expression of the receptors that are targeted by the two ADCs (HER2 and Trop-2) and ii) sensitivity to topoisomerase I inhibition, which is the action of the payloads of both ADCs. We have not observed any difference between the two PDX cultures, suggesting that other resistance mechanisms reason the multidrug resistance. We have detected the presence of the early endosome marker, RAB5, a reported predictive biomarker for HER2-targeted ADC response, in cultures from the Enhertu-sensitive PDX. Unfortunately, the cultures from the Enhertu-resistant PDX were not investigated yet and thus, comparison was not possible. In conclusion, we have demonstrated that tumor tissue cultures are useful ex vivo tools for exploration of tumor drug sensitivity and investigation of resistance mechanisms. | eng |