Positron Emission Tomography Imaging of Platinum Resistant Ovarian Cancer And Drug Modulation

Abstract

Epithelial ovarian cancer is the commonest cause of death from gynaecological malignancy. Platinum based chemotherapy remains the cornerstone of first-line therapy for ovarian cancer, however relapse is common and acquired resistance is frequently observed on subsequent lines of platinum based treatment. Because activation of the PI3K/AKT pathway has been shown to play a role in acquired platinum resistance phenotype of ovarian cancer, combination of platinum with AKT pathway inhibitors holds promise for resensitising tumours to platinum. The aims of this thesis were i) to evaluate the effect of pharmacological AKT inhibitors AKT on the resensitisation of clinically-derived platinum resistant ovarian cancer cells to platinum, and ii) establish molecular imaging approaches for monitoring resensitisation to platinum in vivo combined with biochemical profiling of pathway activity. Treatment of platinum sensitive PEO1 ovarian cancer cells with cisplatin induced growth inhibition and apoptosis. The AKT inhibitor, API-2, resensitised paired platinum resistant PEO4 cells to cisplatin. These in vitro studies established 1h pre-treatment time point with API-2 followed by cisplatin as the most appropriate schedule for further studies. Resensitisation to platinum appeared to be a pathway effect rather than a specific effect of API-2. In order to establish the utility of PET imaging for therapy response, a bilateral tumour xenograft model comprising of platinum sensitive PEO1 and the platinum resistant cell line PEO4 was developed. Both [18F]fluorothymidine ([18F]FLT)-PET, which measures proliferation, and [18F]fluorodeoxyglucose ([18F]FDG)-PET, which measures glucose metabolism detected responses to cisplatin alone in PEO1 tumours and the combination of cisplatin and API-2 in both PEO1 and PEO4 tumours. Correlative reduction in phosphorylated PRAS40, a direct downstream marker of AKT activity indicated that in vivo changes in imaging variables of the combination treatment resulted from AKT inhibition. The imaging changes were also linked to histological reductions in Ki67 labelling index. AKT Biochemical profiling of tumours obtained after imaging confirmed that the changes in tumour [18F]FDG and [18F]FLT uptake were due, at least in part, to reductions in the expression of glucose transporter Glut-1 and hexokinase activity, as well as decreases in TK1 expression, respectively. These studies demonstrated that [18F]FDG-and [18F]FLT-PET hold promise for clinical evaluation of platinum resensitisation in patients with platinum resistant ovarian cancer. Given also that combination treatment with cisplatin and API-2 induced apoptosis in PEO4 cells in vitro, further studies were conducted with the aim of establishing [18F]ICMT11 PET, a specific cleaved caspase-3/7 radiotracer, for monitoring re-sensitisation to platinum in PEO4 tumours. [18F]ICMT11 uptake detected by PET increased after combination treatment but not with cisplatin alone. Parallel increases in TUNEL and cleaved caspase-3 staining by immunohistochemistry were observed consistent with the PET outcome. Thus, [18F]ICMT11 PET could also find utility in monitoring early responses to platinum therapy in combination with AKT inhibition. In addition to the 3 imaging agents described above, analysis of baseline levels of integrin αvβ3/5 expression, an index of angiogenesis, as well as myo-inositol uptake were investigated as potential discriminators of platinum resistant phenotype. These preliminary experiments were inconclusive. In summary PET imaging of proliferation, glucose metabolism and apoptosis were shown to be promising techniques for early detection of resensitisation to platinum therapy in platinum resistant ovarian cancer and warrant further investigation.