Preclinical evaluation of inhibitors of the phosphatidylinositide 3-kinase/protein kinase B cell survival pathway in the treatment of pancreatic cancer

Pancreatic cancer is resistant to almost all classes of cytotoxic drugs. Gemcitabine appears to be the current drug of choice. However, even with this drug, the objective response rate is less than 20%. Patients often die within 6 months after diagnosis. Novel treatment strategies are urgently needed to improve quality of life and reduce mortality.; The hypothesis being tested in this thesis is that cell survival signals transmitted via phosphatidylinositide 3-kinase (PI3K) and protein kinase B (PKB/Akt) may play a significant role in mediating drug resistance seen in pancreatic cancers. Agents that perturb this survival pathway are therefore predicted to demonstrate therapeutic potential. To optimize the use of these experimental inhibitors in chemotherapy, a better understanding of their in vivo pharmacodynamics in pancreatic cancers is required. Studies were designed to examine the effects of the prototype PI3K inhibitors wortmannin and LY294002 on drug sensitivity in cultured human pancreatic cancer cell lines PK1 and PK8. Both agents suppressed PKB/Akt phosphorylation and enhanced gemcitabine-induced apoptosis. Subsequently, an orthotopic xenograft model was established by injecting PK1 cell suspensions into the pancreas of severely combined immunodeficient (SCID) mice, and used to determine the in vivo duration of action and dose-response profile of wortmannin. Phosphorylated PKB/Akt levels were decreased in a time- and dose-dependent manner by the drug in the orthotopic tumors in situ. Also, the combined use of wortmannin and gemcitabine produced more significant increase in apoptosis and inhibition of tumor growth than either agent used alone. To examine how far these findings can be generalized in the clinical situation, a panel of primary pancreatic cancer xenografts were established in SCID mice to study response profiles to the epidermal growth factor receptor inhibitor OSI-774 alone and in various drug combinations. OSI-774 significantly blocked epidermal growth factor receptor activation but failed to reduce phosphorylated PKB/Akt levels in the two patient xenografts tested, and decreased phosphorylated mitogen-activated protein kinase levels in one patient xenograft but not the other. The combination of OSI-774 and wortmannin plus gemcitabine further increased apoptosis compared to wortmannin plus gemcitabine. Such variability and complexity of responses to combinatorial treatments likely reflect the clinical situation, and may be attributed to differential genetic background of tumors from different patients.; Taken together, findings of this thesis have provided evidence on the role of the PI3K-PKB/Akt cell survival pathway in mediating drug resistance in pancreatic cancer in vivo and supported the use of its inhibitors as adjuncts to conventional chemotherapy in the treatment of this malignancy. By characterizing a patient's tumor, the efficacy of treatment regimens involving multiple signal transduction inhibitors may be further optimized.