Bioluminescence imaging of apoptosis correlates with diffusion MRI detection of cell kill after treatment of subcutaneous D54 xenografts

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INTRODUCTION: A caspase 3 sensitive luciferase reporter [1] that enables non-invasive molecular imaging of apoptosis using in vivo bioluminescence (BLI) has been reported previously, but treatment validation and correlation with other readouts of efficacy is lacking. We sought to correlate the apoptosis readout measured by this reporter with diffusion MRI-measured apparent diffusion coefficient (ADC), and growth inhibition, after a standard treatment. METHODS: Nude mice were implanted subcutaneously with D54 tumors expressing the apoptosis reporter construct. Bioluminescence imaging and diffusion MRI (DMRI) measurement of ADC were used to characterize apoptosis and cell kill, respectively, during and after temozolomide treatment (120 mg/kg qdx5 ip). Tumor growth was also measured using the MRIs. RESULTS AND DISCUSSION: ADC showed an increase that was delayed by several days relative to the start of treatment, indicating delayed cell kill (see Figure). Bioluminescence imaging showed an early response in apoptosis (2x increase in light output), which increased after the end of treatment (3-4x increase), indicating a delayed onset of apoptosis (see Figure). Delayed divergence of the treated and control growth curves was observed at day 30. CONCLUSION: The caspase 3 apoptosis data correlated well with the ADC data, providing an early, sensitive indication of efficacy. This reporter may provide an efficient and non-invasive means for preclinical screening of drug candidates, or correlating apoptosis with growth inhibition. Further work is required to elucidate the complex mechanisms of cell kill that contribute to the Caspase-3 and ADC responses, and differences observed in their respective time courses. REFERENCE: [1] Laxman et al. PNAS, 2002, 99:16553