Radiolabeled cetuximab: Dose optimization for epidermal growth factor receptor imaging in a head‐and‐neck squamous cell carcinoma model

Noninvasive imaging of the epidermal growth factor receptor (EGFR) in head‐and‐neck squamous cell carcinoma could be of value to select patients for EGFR‐targeted therapy. We assessed dose optimization of 111Indium‐DTPA‐cetuximab (111In‐cetuximab) for EGFR imaging in a head‐and‐neck squamous cell carcinoma xenograft model. 111In‐cetuximab slowly internalized into FaDu cells in vitro, amounting to 1.0 × 104 molecules cetuximab per cell after 24 hr (15.8% of added activity). In nude mice with subcutaneous FaDu xenograft tumors, a protein dose escalation study with 111In‐cetuximab showed highest specific accumulation in tumors at protein doses between 1 and 30 μg per mouse (mean tumor uptake 33.1 ± 3.1%ID/g, 3 days postinjection (p.i.)). The biodistribution of 111In‐cetuximab and 125I‐cetuximab was determined at 1, 3 and 7 days p.i. at optimal protein dose. Tumor uptake was favorable for 111In‐cetuximab compared to 125I‐cetuximab. With pixel‐by‐pixel analysis, good correlations were found between intratumoral distribution of 111In‐cetuximab as determined by autoradiography and EGFR expression in the same tumor sections as determined immunohistochemically (mean r = 0.74 ± 0.14; all correlations p < 0.0001). Micro Single Photon Emission Computed Tomography (MicroSPECT) scans clearly visualized FaDu tumors from 1 day p.i. onward and tumor‐to‐background contrast increased until 7 days p.i. (tumor‐to‐liver ratios 0.58 ± 0.24, 3.42 ± 0.66, 8.99 ± 4.66 and 16.33 ± 11.56, at day 0, day 1, day 3 and day 7 p.i., respectively). Our study suggests that, at optimal cetuximab imaging dose, 111In‐cetuximab can be used for visualization of EGFR expression in head‐and‐neck squamous cell carcinoma using SPECT.