Pre-clinical quantitative imaging and mouse-specific dosimetry for <sup>111</sup>In-labelled radiotracers.
Da Pieve, C
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<h4>Background</h4>Accurate quantification in molecular imaging is essential to improve the assessment of novel drugs and compare the radiobiological effects of therapeutic agents prior to in-human studies. The aim of this study was to investigate the challenges and feasibility of pre-clinical quantitative imaging and mouse-specific dosimetry of <sup>111</sup>In-labelled radiotracers. Attenuation, scatter and partial volume effects were studied using phantom experiments, and an activity calibration curve was obtained for varying sphere sizes. Six SK-OV-3-tumour bearing mice were injected with <sup>111</sup>In-labelled HER2-targeting monoclonal antibodies (mAbs) (range 5.58-8.52 MBq). Sequential SPECT imaging up to 197 h post-injection was performed using the Albira SPECT/PET/CT pre-clinical scanner. Mice were culled for quantitative analysis of biodistribution studies. The tumour activity, mass and percentage of injected activity per gram of tissue (%IA/g) were calculated at the final scan time point and compared to the values determined from the biodistribution data. Delivered <sup>111</sup>In-labelled mAbs tumour absorbed doses were calculated using mouse-specific convolution dosimetry, and absorbed doses for <sup>90</sup>Y-labelled mAbs were extrapolated under the assumptions of equivalent injected activities, biological half-lives and uptake distributions as for <sup>111</sup>In.<h4>Results</h4>For the sphere sizes investigated (volume 0.03-1.17 ml), the calibration factor varied by a factor of 3.7, whilst for the range of tumour masses in the mice (41-232 mg), the calibration factor changed by a factor of 2.5. Comparisons between the mice imaging and the biodistribution results showed a statistically significant correlation for the tumour activity (r = 0.999, P < 0.0001) and the tumour mass calculations (r = 0.977, P = 0.0008), whilst no correlation was found for the %IA/g (r = 0.521, P = 0.29). Median tumour-absorbed doses per injected activity of 52 cGy/MBq (range 36-69 cGy/MBq) and 649 cGy/MBq (range 441-950 cGy/MBq) were delivered by <sup>111</sup>In-labelled mAbs and extrapolated for <sup>90</sup>Y-labelled mAbs, respectively.<h4>Conclusions</h4>This study demonstrates the need for multidisciplinary efforts to standardise imaging and dosimetry protocols in pre-clinical imaging. Accurate image quantification can improve the calculation of the activity, %IA/g and absorbed dose. Diagnostic imaging could be used to estimate the injected activities required for therapeutic studies, potentially reducing the number of animals used.
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Preclinical Molecular Imaging
Radiotherapy Physics Modelling
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EJNMMI research, 2016, 6 (1), pp. 85 - ?