Abstract 2930: Differential tumour response to birinapant and irinotecan revealed by non-invasive MRI
Authors: Papaevangelou Efthymia , Almeida Gilberto S. , Jamin Yann , Robinson Simon P. , deSouza Nandita M. 
Published: August 2015

Abstract
Background: Effective tumour therapy requires induction of cell death. Non-invasive serial imaging is desirable to detect processes such as necrotic and apoptotic cell death in cancer patients undergoing treatment with conventional as well as new drugs within clinical trials. This study investigated the use of diffusion-weighted magnetic resonance imaging (DW-MRI) for imaging cell death induced in human tumour xenografts by either the cytotoxic drug irinotecan, or the mimetic of the second mitochondria-derived activator of caspases (Smac) birinapant, which induces apoptosis by antagonising the inhibitors of apoptosis proteins (IAPs) and activating caspases.
Methods: Nude mice bearing human SW620 colon carcinoma xenografts were treated with vehicle, irinotecan or birinapant for 1, 3 or 5 days. DW-MRI was performed at days 1, 3 and/or 5 during treatment to assess tumour response in vivo. Assessment of apoptosis and necrosis ex vivo in tumour tissues was used to validate the imaging findings. Apoptosis was assessed by quantifying cleaved PARP expression, and cleaved caspase-3 and DNA fragmentation staining, whereas necrotic areas were measured on H&E stained tumour sections.
Results: Both birinapant and irinotecan induced significant tumour growth delay. Irinotecan induced a small increase in the tumour apparent diffusion coefficient (ADC) after 1 day, with a 20% and 30% increase at days 3 and 5 respectively. ADC was unchanged in the vehicle- and birinapant-treated tumours despite growth delay in the latter. Histological analysis showed that birinapant induced apoptosis after day 3, but had no effect on tumour necrosis compared to vehicle. Irinotecan increased necrosis at days 3 and 5, and induced apoptosis after 1 day.
Conclusion: Tumour water diffusion changes after irinotecan treatment were associated primarily with induction of necrosis with associated apoptotic cell death, and could be measured as early as 24 hours post-treatment prior to changes in tumour volume. Induction of apoptosis alone in the same tumour model as a result of treatment with birinapant was not sufficient to induce changes in tissue microstructure that were detectable with DW-MRI within 5 days of the treatment, despite the drug suppressing tumour growth. A possible explanation for the lack of changes on water diffusivity or tissue microarchitecture after birinapant treatment might be that removal of the dead cells is followed by tumour cell repopulation driven by apoptosis, and more particularly by activated caspase-3. ADC is a useful, clinically translatable, non-invasive biomarker for early detection of response to cytotoxic drugs. However, when considering apoptotic cell death in response to targeted therapeutics, use of ADC may result in false negatives, particularly if imaging at an early time-point before significant disruption to tissue microarchitecture has occurred.
Citation Format: Efthymia Papaevangelou, Gilberto S. Almeida, Yann Jamin, Simon P. Robinson, Nandita M. deSouza. Differential tumour response to birinapant and irinotecan revealed by non-invasive MRI. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2930. doi:10.1158/1538-7445.AM2015-2930