Anti-HER1 monoclonal antibody (mAb), panitumumab (Vectibix®) is a fully human mAb approved by the FDA for the treatment of epidermal growth factor receptor (EGFR, HER1)-expressing colorectal cancers. By combining the targeted specificity of panitumumab with the quantitative in vivo imaging capabilities of PET, we evaluated the potential of 89Zr-DFO-panitumumab PET/CT imaging and performed non-invasive, in vivo imaging of HER1 expression and estimated human dosimetry.
Panitumumab was radiolabeled with 89Zrusing a derivative of desferrioxamine (DFO-Bz-NCS) and with 111In using CHX-A″ DTPA as bifunctional chelators. Comparative biodistribution/dosimetry of both radiotracers was performed in non-tumor bearing athymic nude mice (n= 2 females and n=2 males) over 1-week following i.v. injection of either using 89Zr-DFO-panitumumab or 111In-CHX-A″-DTPA-panitumumab. Micro-PET/CT imaging of female athymic nude mice bearing human breast cancer tumors (n=5 per tumor group) with variable HER1-expression very low (BT-474), moderate (MDA-MB-231), and very high (MDA-MB-468) was performed at over 1 week following i.v. injection of 89Zr-DFO-panitumumab.
Radiochemical yield and purity of 89Zr-Panitumumab was > 70% and > 98% respectively with specific activity 150 ± 10 MBq/mg of panitumumab in a ~ 4 hr synthesis time. Biodistribution of 111In-CHX-A″ DTPA -panitumumab and 89Zr-DFO-panitumumab in athymic non-tumor bearing nude mice displayed similar percent injected dose per gram of tissue with prominent accumulation of both tracers in the lymph nodes, a known clearance mechanism of panitumumab. Also exhibited was prolonged blood pool with no evidence of targeted accumulation in any organ. Human radiation dose estimates showed similar biodistributions with estimated human effective doses of 0.578 and 0.183 mSv/MBq for 89Zr-DFO-panitumumab and 111In-CHX-A″-DTPA –panitumumab, respectively. Given the potential quantitative and image quality advantages of PET, imaging of tumor bearing mice was only performed using 89Zr-DFO-panitumumab. Immuno-PET imaging of 89Zr-DFO-panitumumab in mice bearing breast cancer xenograft tumors with variable HER1 expression showed high tumor uptake (SUV > 7) in the MDA-MB-468 high HER1-expressing mice and a strong correlation between HER1-expression level and tumor uptake (R2= 0.857, p<0.001).
89Zr-DFO-panitumumab can prepared with high radiochemical purity and specific activity. 89Zr-DFO-panitumumab microPET/CT showed uptake corresponding to HER-1 expression. Due to poor clearance, initial dosimetry estimates suggest that only a low dose 89Zr-DFO-panitumumab shows favorable human dosimetry; however due to high tumor uptake, the use of 89Zr-DFO-panitumumab is expected to be clinically feasible.