Tumour T1 changes in vivo are highly predictive of response to chemotherapy and reflect the number of viable tumour cells

Abstract

We have shown that chemotherapy rapidly reduces the spin-lattice relaxation of water protons (T1) in solid tumours and this change (T1) often precedes and strongly correlates with the eventual change in tumour volume [McSheehy et al 2010]. To understand the biological nature of T1, we have performed further studies in vivo and ex vivo with the allosteric mTOR inhibitor, everolimus. Mice bearing RIF-1 tumours were studied by magnetic resonance imaging (MRI) to determine TVol and T1, and MR spectroscopy (MRS) to determine levels of the proliferation marker choline and levels of lipid apoptosis markers, prior to and 5 days (endpoint) after daily treatment with vehicle or everolimus (10 mg/kg). At the endpoint, tumours were ablated and an entire section analyzed to permit full cellular and necrotic quantification as well staining for the proliferation antigen Ki67 and cleaved-caspase-3 as a measure of apoptosis. The number of blood-vessels per slice was evaluated by haematoxyline-eosin staining. Everolimus blocked tumour growth and significantly reduced the tumour T1 and total choline (Cho) levels, and increased polyunsaturated fattyacids (PUFA) which are markers of apoptosis. Histology and IHC showed that everolimus reduced the %Ki67+ cells and number of blood vessels but did not affect caspase-3 apoptosis, necrosis or cell density. The change in T1 (T1) strongly correlated with the TVol and also the Cho and the endpoint Ki67. In murine B16/BL6 melanoma tumours, everolimus also decreased T1 and this correlated with cell bioluminescence; another marker of cell viability. These studies suggest that T1 is not a measure of cell density but reflects the number of viable and proliferating tumour cells because cell and tissue destruction release factors that cause T1 relaxation. Receiver-operator-curves for everolimus on RIF-1 tumours showed that T1 had very high levels of sensitivity and specificity (AUC=0.91) and this was confirmed for the cytotoxic patupilone in the same tumour model (AUC=0.95)