Dual tracer evaluation of dynamic changes in intratumoral hypoxic and proliferative states after radiotherapy of human head and neck cancer xenografts using radiolabeled FMISO and FLT

Abstract

BackgroundRadiotherapy is an important treatment strategy for head and neck cancers. Tumor hypoxia and repopulation adversely affect the radiotherapy outcome. Accordingly, fractionated radiotherapy with dose escalation or altered fractionation schedule is used to prevent hypoxia and repopulation. 18F-fluoromisonidazole (FMISO) and 18F-fluorothymidine (FLT) are noninvasive markers for assessing tumor hypoxia and proliferation, respectively. Thus, we evaluated the dynamic changes in intratumoral hypoxic and proliferative states following radiotherapy using the dual tracers of 18F-FMISO and 3H-FLT, and further verified the results by immunohistochemical staining of pimonidazole (a hypoxia marker) and Ki-67 (a proliferation marker) in human head and neck cancer xenografts (FaDu).MethodsFaDu xenografts were established in nude mice and assigned to the non-radiation-treated control and two radiation-treated groups (10- and 20-Gy). Tumor volume was measured daily. Mice were sacrificed 6, 24, and 48 hrs and 7 days after radiotherapy. 18F-FMISO, and 3H-FLT and pimonidazole were injected intravenously 4 and 2 hrs before sacrifice, respectively. Intratumoral 18F-FMISO and 3H-FLT levels were assessed by autoradiography. Pimonidazole and Ki-67 immunohistochemistries were performed.ResultsIn radiation-treated mice, tumor growth was significantly suppressed compared with the control group, but the tumor volume in these mice gradually increased with time. Visual inspection showed that intratumoral 18F-FMISO and 3H-FLT distribution patterns were markedly different. Intratumoral 18F-FMISO level did not show significant changes after radiotherapy among the non-radiation-treated control and radiation-treated groups, whereas 3H-FLT level markedly decreased to 59 and 45% of the non-radiation-treated control at 6 hrs (p < 0.0001) and then gradually increased with time in the 10- and 20-Gy-radiation-treated groups. The pimonidazole-positive hypoxic areas were visually similar in both the non-radiation-treated control and radiation-treated groups. No significant differences were observed in the percentage of pimonidazole-positive cells and Ki-67 index.ConclusionIntratumoral 18F-FMISO level did not change until 7 days, whereas 3H-FLT level markedly decreased at 6 hrs and then gradually increased with time after a single dose of radiotherapy. The concomitant monitoring of dynamic changes in tumor hypoxia and proliferation may provide important information for a better understanding of tumor biology after radiotherapy and for radiotherapy planning, including dose escalation and altered fractionation schedules.

The reoxygenation of hypoxia and the reduction of glucose metabolism in head and neck cancer by fractionated radiotherapy with intensity-modulated radiation therapy

“…Another possibility is that radiotherapy may increase the FMISO washout ratio in the same hypoxic cells. However, these possibilities are not likely, as Fatema et al reported that the intratumoral FMISO level did not show a significant decrease after a single course of radiation to hypoxic tumors in mice [35]. Another, more likely, cause is that the number of hypoxic cells decreases without significant changes in the uptake or washout ability of each hypoxic cell.…”

“…Dose escalation requires an accurate determination of the hypoxic area in order to properly target the hypoxia, and the current FMISO-PET might not have sufficient spatial resolution and contrast to delineate this area. In terms of spatial resolution, new PET scanners with higher spatial resolution would have the potential to more accurately demarcate the hypoxic area [35]. In addition, the new generation of hypoxic tracers with lower lipid solubility (e.g., 18 F-fluoroazomycin arabinoside (18 F-FAZA)) might enable the detection of hypoxia with higher contrast [36].…”

Radiation-induced immune responses: mechanisms and therapeutic perspectives

“…Despite such obvious expectation, a number of recent studies indicate that this is not the case. It has been shown by using 18 F-misonidazole (F-MISO) radioactive tracer and positron emission tomography (PET) to monitor the dynamic changes in intratumoral hypoxia that 10 and 20 Gy ablative dose irradiated human head and neck squamous cell carcinoma xenografts had minimal changes in the intratumoral hypoxia [ 76 ]. Our recent work with F-MISO has also demonstrated that there is no immediate increase in tumor hypoxia following 15 Gy ablative radiation [ 77 ].…”

Imaging Monitoring of Kupffer Cell Function and Hepatic Oxygen Saturation in Preneoplastic Changes During Cholangiocarcinogenesis

“…If pimonidazole can directly demonstrate the area of hypoxia, it could be direct evidence of hypoxia in the carcinogenesis process. On the other hand, there were a few studies that demonstrated the possibility of imaging monitoring in animal models using invasive needle or radioisotope in terms of hypoxia monitoring 32 , 33 . Recent interest in tumor hypoxic conditions has shifted to the area of diagnostic imaging due to the rapid development of diagnostic methods.…”