Assessment of regional tumor hypoxia using ¹⁸F- fluoromisonidazole and ⁶⁴Cu(II)-diacetyl-bis(N4- methylthiosemicarbazone) positron emission tomography: Comparative study featuring microPET imaging, Po₂ probe measurement, autoradiography, and fluorescent microscopy in the R3327-AT and FaDu rat tumor models

Purpose: To compare two potential positron emission tomography (PET) tracers of tumor hypoxia in an animal model. Methods and Materials: The purported hypoxia imaging agents ¹⁸F-fluoromisonidazole (FMISO) and ⁶⁴Cu(II)-diacetyl-bis(N4-methylthiosemicarbazone) (Cu-ATSM) were compared by serial microPET imaging of Fisher-Copenhagen rats bearing the R3327-AT anaplastic rat prostate tumor. Probe measurements of intratumoral Po₂ were compared with the image data. At the microscopic level, the relationship between the spatial distributions of ⁶⁴Cu (assessed by digital autoradiography) and tumor hypoxia (assessed by immunofluorescent detection of pimonidazole) was examined. ¹⁸F-FMISO and ⁶⁴Cu-ATSM microPET images were also acquired in nude rats bearing xenografts derived from the human squamous cell carcinoma cell line, FaDu. Results: In R3327-AT tumors, the intratumoral distribution of ¹⁸F-FMISO remained relatively constant 1-4 h after injection. However, that of ⁶⁴Cu-ATSM displayed a significant temporal evolution for 0.5-20 h after injection in most tumors. In general, only when ⁶⁴Cu-ATSM was imaged at later times (16-20 h after injection) did it correspond to the distribution of ¹⁸F-FMISO. Oxygen probe measurements were broadly consistent with ¹⁸F-FMISO and late ⁶⁴Cu-ATSM images but not with early ⁶⁴Cu-ATSM images. At the microscopic level, a negative correlation was found between tumor hypoxia and ⁶⁴Cu distribution when assessed at early times and a positive correlation when assessed at later times. For the FaDu tumor model, the early and late ⁶⁴Cu-ATSM microPET images were similar and were in general concordance with the ¹⁸F-FMISO scans. Conclusion: The difference in behavior between the R3327-AT and FaDu tumor models suggests a tumor-specific dependence of Cu-ATSM uptake and retention under hypoxic conditions.