Abstract
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Objectives: Camelidae possess an unusual class of antibodies devoid of light chains. Nanobodies™ are 15kDa intact antigen binding fragments that are stable, easily generated against multiple targets and fully functional. These properties favor their use as imaging agents. We compared intra-individually in vivo tumor uptake and biodistribution of 7C12 and 7D12 Nanobodies™ recognizing different epitopes on EGFR. Methods: Nanobodies™ were labeled via their hexahistidine tail with 99mTc-Tricarbonyl (Isolink, Mallinckrodt, the Netherlands). SPECT-CT imaging was performed at 1h post injection of 45-155 MBq 99mTc-7C12 or 99mTc-7D12 in A431 mice xenografts (n=7). Each animal was studied with both tracers on separate days (72h interval) in a crossover protocol. MicroCT imaging was followed by pinhole SPECT and both modalities were fused based on a rigid body transformation using 6 landmarks. Image quantification was performed using AMIDE. Ellipsoid regions of interest were drawn around tumor, muscle, liver, kidneys, lung, brain and total body. Tracer uptake is expressed as % injected activity/cm3 tissue (%IA/cm3). Results: Labeling efficiency was >90%. Tumor uptake of 99mTc-7C12 and 99mTc-7D12 was similar: 4.7±0.8 and 4.9±1.6 %IA/cm3 respectively. The tumor to muscle ratio was 24.1±9.3 for 99mTc-7C12 and 26.5±15.7 for 99mTc-7D12 . 99mTc-7C12 showed significantly higher kidney uptake (81.7±9.3 vs 54.9±12.9 %IA/cm3, p<0.001) and lower liver uptake (2.8±0.8 vs 7.2±2.5 %IA/cm3, p<0.001) compared to 99mTc-7D12. Uptake in lung was low for both tracers (0.6 %IA/cm3) and negligible in brain tissue (0.1 %IA/cm3). Tracer retention at 1h post injection was 80-85%. Conclusions: 7C12 and 7D12 show high and comparable tumor uptake but different biodistribution patterns within the same animal. Anti-EGFR specific Nanobodies™ are promising tools for in vivo detection of tumors expressing EGF receptor.
Research Support (if any): Supported by a grant from VUB-HOA.
- Society of Nuclear Medicine, Inc.