Using GEMMs to investigate novel therapeutic strategies for colorectal cancer
Despite recent advances in the clinic to integrate novel targeted therapeutic agents into standard therapy, colorectal cancer (CRC) remains a significant cause of mortality. The high attrition rate of novel compounds at phase III clinical trials for CRC, has been attributed to limited information from pre-clinical strategies, in particular, the use of inadequate xenograft models. In response, this thesis aimed to utilise robust and relevant genetically engineered mouse models of CRC to evaluate a number of novel therapeutic strategies.\ud Whilst mutations in the tumour suppressor APC are crucial for initiation of CRC, mutations which lead to activation of the PI3K and MAPK signalling pathways, such as through loss of the tumour suppressor protein PTEN and activation of oncogenic KRAS, have been implicated in promoting progression of CRC. As such, combinations of these genetic alterations within the murine intestine, using the Cre-LoxP system, lead to differing mouse models of invasive intestinal adenocarcinoma. This thesis reports therapeutic targeting of the PI3K and MAPK signalling pathways using the dual PI3K and mTOR inhibitor NVP-BEZ235, and the MEK inhibitor MEK162, respectively. For this, compounds were initially evaluated for pharmacodynamic and anti-tumour effects through short term exposure experiments. These analyses yielded a range of effects, some of which appeared predictive of long term efficacy, others which were contradictory and some which revealed novel feedback mechanisms. Furthermore, these agents were assessed in a long term therapeutic setting to evaluate the effect of continuous treatment on longevity and tumour burden of tumour models. Here, whilst dual PI3K/mTOR inhibition significantly increased longevity of all mouse models, MEK inhibition was only effective in the Apc and Apc Kras mutant settings, identifying Pten loss as a marker of non-response to MEK inhibition, independently and also in the Kras mutant setting. Furthermore, analysis of the combination therapy in short term settings identified scheduling of these agents to be key to achieve concomitant pathway inhibition, particularly in the Apc Pten deficient tumour setting. Ultimately, when evaluated in a long term setting, although the combination therapy displayed no further benefit in the Pten deficient setting, this had additive benefits in the Kras mutant setting and synergistic benefits in the Pten Kras mutant setting.\ud Nevertheless despite the promise of targeted therapy, standard chemotherapeutic agents such as 5-flurouracil (5-FU) remain the backbone of therapy for CRC, regardless of only moderate benefits of 10-15% in advanced tumour settings. Furthermore, resistance to 5-FU predominantly through upregulation of the enzyme thymidylate synthase (TS) frequently\ud 2\ud occurs in human tumours. Investigations reported here aimed to target tumours with upregulated TS using novel analogues of the anti-viral agent Brivudin (BVDU), metabolites of which are converted to anti-cancer metabolites by TS.\ud Initially, In vitro characterisation of a small library of compounds reported here identified a number of potent compounds. Following further in vitro characterisation and short term evaluation in the Apc Pten tumour model of invasive adenocarcinoma, two lead compounds: CPF472 and CPF3172 were taken forward for long term experiments. Subsequently, this study evaluated and identified compounds which showed increased efficacy in the TS upregulated setting.\ud Taken together, the investigations presented in this thesis highlight the utility of appropriate mouse models in evaluating novel therapeutic strategies and generating clinically relevant hypotheses.
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