Early Prediction of Tumor Response to Treatment: Preclinical Validation of 99mTc-Duramycin

Noninvasive imaging of cell death can provide an early indication of the efficacy of tumor treatment, aiding clinicians in distinguishing responding patients from nonresponding patients early on. Tc-99m-duramycin is a SPECT tracer for cell death imaging. In this study, our aim was to validate the use of Tc-99m-duramycin for imaging the early response of tumors to treatment. Methods: An in vitro binding assay was performed on COLO205 cells treated with 5-fluorouracil (3.1, 31, or 310 mu M) and oxaliplatin (0.7 or 7 mu M) or radiation (2 or 4.5 Gy). Tc-99m-duramycin cell binding and the levels of cell death were evaluated after treatment. In vivo imaging was performed on 4 groups of CD1-deficient mice bearing COLO205 human colorectal cancer tumors. Each group included 6 tumors. The first group was given irinotecan (100 mg/kg), the second oxaliplatin (5 mg/kg), the third irinotecan (80 mg/kg) plus oxaliplatin (5 mg/kg), and the fourth vehicle (0.9% NaCl and 5% glucose). For radiotherapy studies, COLO205 tumors received 4.5 Gy, 2 fractions of 4.5 Gy in a 24-h interval, pretreatment with an 80 mg/kg dose of irinotecan combined with 2 fractions of 4.5 Gy in a 24-h interval, or no treatment (n = 5-6/group). Therapy response was evaluated by Tc-99m-duramycin SPECT 24 h after the last dose of therapy. Blocking was used to confirm tracer specificity. Radiotracer uptake in the tumors was validated ex vivo using gamma-counting, cleaved caspase-3, and terminal deoxynucleotidyl transferase mediated deoxyuridine triphosphate nick-end labeling (TUNEL) histology. Results: Chemotherapy and radiotherapy increased Tc-99m-duramycin binding to COLO205 cells in a concentration/dose- and time-dependent manner, which correlated well with cell death levels (P < 0.05) as analyzed by annexin V and caspase 3/7 activity. In vivo, Tc-99m-duramycin uptake in COLO205 xenografts was increased 2.3- and 2.8-fold (P < 0.001) in mice treated with irinotecan and combination therapy, respectively. Blocking with unlabeled duramycin demonstrated specific binding of the radiotracer. After tumor irradiation with 4.5 Gy, Tc-99m-duramycin uptake in tumors increased significantly (1.24 +/- 0.07 vs. 0.57 +/- 0.08 percentage injected dose per gram in the unirradiated tumors; P < 0.001). gamma-counting of radioactivity in the tumors positively correlated with cleaved caspase-3 (r = 0.85, P < 0.001) and TUNEL (r = 0.81, P < 0.001) staining. Conclusion: We demonstrated that Tc-99m-duramycin can be used to image induction of cell death early after chemotherapy and radiotherapy. It holds potential to be translated into clinical use for early assessment of treatment response.