dc.description.abstract
Therapy development in cancer involves several consecutive steps including (1) the identification of the unmet medical need (disease or clinical question), (2) the establishment of clinically relevant resources or models to address such need, (3) the evaluation of therapies or experimental questions in the established models, and (4) the design and execution of clinical trials or protocols. The ambition of my PhD work was to follow the mentioned steps to build a body of results with potential clinical impact in the pediatric oncology field. Research at our institution, SJD, focuses on several types of cancers with bad prognosis, such as pediatric sarcomas. These cancers belong to a rare and heterogeneous group of skeletal and soft tissue malignancies accounting for approximately 12% of all childhood solid tumors. The most frequently occurring ones are Ewing sarcoma, osteosarcoma and rhabdomyosarcoma. With the up-to-date treatment modalities, based on surgery, radiation and chemotherapy, the 5-year survival rates of these patients have improved up to 70-80% in the last decades. However, a substantial proportion of patients relapse and do not respond consistently to rescue therapy. Whether the lack of response to therapy of these patients is due to drug delivery issues in resistant tumor cells is not completely understood. The main question I wanted to address in my work was whether patients whose tumors stop responding to anticancer medicines show diminished intratumoral drug distribution. Because this experimental question was challenging to address in the clinical setting, we needed to generate the adequate laboratory models. Thus, we established PDX models in immunodeficient mice. There is compelling evidence suggesting that PDX, at least at initial passages, reproduce the clonal heterogeneity, genetics and histology of several types of cancer. Whether this holds true for pediatric sarcomas was not sufficiently clear. Thus, we embraced an ambitious project to address (1) the identification of clinical and technical factors involved in pediatric sarcoma PDX engraftment, (2) the characterization of the stability of the PDX during initial passages, and (3) the relationship between successful PDX and patient prognosis.
The main hypotheses of my work were:
(1) PDX models faithfully represent pediatric sarcoma tumors established from patients at different disease stages. PDX conserve the main histology, molecular and functional properties of human tumors over successive passages in mice.
(2) PDX engraftment correlates with patient prognosis. Successful establishment of PDX models helps identify patients with higher risk of recurrence and disease progression.
(3) Intratumoral distribution of anticancer drugs becomes restricted due to the evolution of patient disease. Cancer cells in relapsed tumors displace chemotherapeutics from the intracellular to the extracellular tumor compartment.
(4) The expression of multidrug resistance efflux transporters explains, at least partially, the shift of the intratumoral drug distribution towards the extracellular compartment upon tumor evolution in relapsed patients.
To address my hypotheses, the main objectives of my thesis were:
(1) To establish and characterize PDX models from biopsies and necropsies of pediatric patients diagnosed with Ewing sarcoma, osteosarcoma and rhabdomyosarcoma. To address this goal, we implanted patient samples obtained at Hospital Sant Joan de Déu in immunodeficient mice. Upon successful engraftment as PDX, we characterized the histology, genomics and response to model therapy (irinotecan) of the established tumor models over time in successive passages in mice.
(2) To identify factors favoring PDX engraftment and to study the association between PDX engraftment and prognosis in pediatric patients with Ewing sarcoma, osteosarcoma and rhabdomyosarcoma. To address this goal, we collected clinical and technical data from the patients and the PDX, and we studied the relationship between PDX engraftment and several patient and sample factors, including disease stage (diagnosis or relapse) and patient outcome (event free survival and overall survival).
(3) To detect and characterize changes in anticancer drug activity and intratumoral drug distribution during the evolution of Ewing sarcoma. To address this goal, we established pairs of PDX models from patients at diagnosis and late disease stages. In such paired PDX, we studied the activity and distribution of the model drug irinotecan and its active metabolite, SN-38. We applied the intratumoral microdialysis – tumor homogenate technique to obtain samples of PDX engrafted in mice receiving irinotecan infusions, and we calculated the unbound volume of distribution of SN-38 in the established PDX. We analyzed the expression and function of multidrug resistance efflux transporters, such as P-glycoprotein, as likely cause of the limited drug distribution and acquisition of resistance in relapsed tumors.
