Estimation of Cumulative Activity of 177 Lu-Cetuximab from a Single Diagnostic 64 Cu-Cetuximab Scan.

Background: This study aimed at investigating whether a single diagnostic 64 Cu-cetuximab scan might predict therapeutic 177 Lu-cetuximab cumulative activity in a preclinical study. Methods: The framework of the reversible Patlak-Blasberg analysis was used to derive two independent equations of 177 Lu-cetuximab cumulative activity that involve the tumor-to-blood standard uptake value ratio (i.e., the SUR) assessed at peak-time of either decay-uncorrected or decay-corrected activity concentration of trapped 64 Cu-cetuximab, respectively. Applied to published data in TE-8 tumor-bearing mice, the two peak times and corresponding SUR values were assessed. Two estimates of 177 Lu-cetuximab cumulative activity were then computed for comparison with two previously published values obtained from different methods. Results: The two estimates of 177 Lu-cetuximab cumulative activity, computed at 14 and 59 h postinjection, were consistent with the previously published values: 4.7 × 10 12 and 5.5 × 10 12 versus 4.1 × 10 12 and 5.3 × 10 12 disintegrations/g. Conclusion: Two independent equations are proposed for estimating 177 Lu-cetuximab cumulative activity from an initial diagnostic 64 Cu-cetuximab scan, when acquired at two relevant injection acquisition times, respectively. Applied to published tumor-bearing mouse data, the 64 Cu-cetuximab SUR assessed at either 14 or 59 h postinjection turns out to be key metrics.