Development of imaging techniques to assess tumour cell death

Abstract

Cell death is an important imaging target for assessing early tumour treatment response and the effectiveness of therapy. Traditionally, anatomical imaging modalities measure changes in tumour size (Response Evaluation Criteria in Solid Tumours (RECIST)). More recently these assessments have also included functional and molecular information, as these parameters can show earlier responses to chemotherapies, e.g. using PET measurements of 2-[¹⁸F]-fluoro-2-deoxy D-glucose uptake (FDG-PET). PET Response Criteria in Solid Tumours (PERCIST) was introduced as a potentially more sensitive method of assessing treatment response, particularly with therapies that stabilise disease. While metabolic imaging may indicate drug target engagement, and in some cases tumour cell death, there is a need for imaging methods that detect tumour cell death more directly post-treatment and that can give an indication of longer term treatment outcomes. In this project ²H magnetic resonance spectroscopic imaging with ²H-fumarate and PET-CT with ¹⁸F-C2Am are investigated as methods for the early detection of tumour cell death post-treatment. Fumarate, an intermediate in the tricarboxylic acid cycle, is hydrated in the reaction catalysed by the enzyme fumarase to produce malate. With loss of plasma membrane integrity in dying cells fumarate can rapidly gain access to the enzyme. By measuring increased malate production in treated tumours after fumarate injection, cell necrosis can be assessed. It could be established that ²H MRS and MRSI measurements at 7 T of [2,3-²H₂]fumarate conversion to ²H-labelled malate can be used to detect early cell death in various tumour models. ¹⁸F-C2Am is an ¹⁸F-labelled derivative of a phosphatidylserine-binding protein, the C2A domain of synaptotagmin-I (C2Am), which can be used to image tumour cell death in vivo using PET. C2Am binds in a Ca²⁺ -dependent manner with nanomolar affinity to phosphatidylserine (PS) on the outer leaflet of the plasma membrane of apoptotic cells or to PS on the inner leaflet following disruption of plasma membrane integrity in necrotic cells. The agent was tested in mice bearing human xenografts treated with a TRAIL-R2 agonist, MEDI3039, or with conventional chemotherapy. Both imaging markers may provide more specific and sensitive methods for detecting cell death, which can be used to rapidly assess treatment response and effectiveness at an early stage.