Early prediction of tumor response to treatment: Preclinical validation of 99mTc-Duramycin

Abstract

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. 99mTcduramycin is a SPECT tracer for cell death imaging. In this study, our aim was to validate the use of 99mTc-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 M) and oxaliplatin (0.7 or 7 M) or radiation (2 or 4.5 Gy). 99mTc-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 99mTc-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 γ-counting, cleaved caspase-3, and terminal deoxynucleotidyl transferase mediated deoxyuridine triphosphate nick-end labeling (TUNEL) histology. Results: Chemotherapy and radiotherapy increased 99mTc-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, 99mTc-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, 99mTc-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). γ-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 99mTc-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. 2016 by the Society of Nuclear Medicine and Molecular Imaging, Inc.