Monitoring Mitochondrial Complex-I Activity Using Novel PET Probe (18)F-BCPP-EF Allows Early Detection of Radiotherapy Effect in Murine Squamous Cell Carcinoma

OBJECTIVES: Aerobic glycolysis, the main pathway of energy production in tumors (Warburg effect) allows detection of tumors by positron emission tomography (PET) using (18)F-fluoro-2-deoxy-D-glucose ((18)F-FDG). Since ionizing radiation (IR) is reported to switch aerobic glycolysis to mitochondrial oxidative phosphorylation, radiotherapeutic efficacy was monitored by the activity of mitochondrial complex I (MC-I), using a new PET probe (18)F-BCPP-EF, (18)F-2-tert-butyl-4-chloro-5-{6-[2-(2-fluoro-ethoxy)-ethoxy] -pyridine-3-ylmethoxy}-2H-pyridazin-3-one, compared with (18)F-FDG uptake and the apoptosis index. METHODS: Tumor uptake of (18)F-BCPP-EF or (18)F-FDG was examined in C3H/HeN mice inoculated with murine squamous cell carcinoma SCCVII at various time points after a single dose of x-ray irradiation at 0, 6, 15, or 30 Gy. Apoptosis incidence was determined by TUNEL staining in excised tumor tissue. RESULTS: Tumor growth suppression was dose-dependent; tumor grew 10-fold (0 Gy), 5-fold (6 Gy), 2-fold (15 Gy), and reduced to half in its volume (30 Gy) 14 days after treatment. (18)F-BCPP-EF uptake was significantly increased as early as 3 days after 15 Gy or 30 Gy, when tumor size and apoptosis index showed no difference among radiation doses. In contrast, (18)F-FDG uptake was initially increased dose-dependently, remained elevated up to 7 days, and eventually decreased 10 days after 30 Gy and also 14 days after 15 Gy when tumor size was already reduced. Apoptosis index was increased after irradiation but failed to correlate with tumor response. CONCLUSION: Tumor uptake of (18)F-BCPP-EF was increased dose-dependently early after effective doses of IR when (18)F-FDG uptake as well as apoptosis incidence were not indicative of tumor response. The results suggest that (18)F-BCPP-EF is a promising “positive” MC-I imaging PET probe for early detection of efficacy of tumor radiotherapy.