Reliable index of tumor targeting in RIT with 177Lu-J591 monoclonal antibody in patients with metastatic prostate cancer

1758 Objectives Based on clinical trials in patients with metastatic castrate-resistant prostate cancer (metCRPC), we have previously demonstrated that radioimmunotherapy (RIT) with 177Lu-J591 monoclonal antibody (mAb) results in a significant decline of PSA levels and tumor burden (TB). We have used tumor targeting index (TTI) and TB to quantify tumor targeting. We want to identify which index (TBI or TB) is more reliable and predicts treatment response. Methods In phase I clinical trial, 14 patients with metCRPC received repeated RITs with 177Lu-J591 (2 doses, 15-60 mCi/m2, 20mg mAb), imaging studies were performed on day 5-8 in each RIT. TTI was calculated as tumor count intensity corrected for background and total body count intensities. TB was calculated as a product of tumor count and pixel. TTI and TB of consecutive images were analysed with correlation coefficient and Bland-Altman plot. In phase II clinical trial, 34 patients with metCRPC received RIT with 177Lu-J591 (single dose, 65-70 mCi/m2, 20mg mAb), imaging studies were performed on day 6-7. Decline of PSA levels in phase II trial were compared to TTI and TB. Results Correlation coefficients of TTI and TB were r2=0.74 (p Conclusions This study clearly documents that TTI is a reliable index to quantify tumor targeting and may potentially be useful to predict treatment response in RIT with 177Lu-J591 monoclonal antibody in patients with metCRPC