18F-Fluoromisonidazole Kinetic Modeling for Characterization of Tumor Perfusion and Hypoxia in Response to Antiangiogenic Therapy

Abstract

Multiparametric imaging of tumor perfusion and hypoxia with dynamic ¹⁸F-fluoromisonidazole (¹⁸F-FMISO) PET may allow for an improved response assessment to antiangiogenic therapies. Cediranib (AZD2171) is a potent inhibitor of tyrosine kinase activity associated with vascular endothelial growth factor receptors 1, 2, and 3, currently in phase II/III clinical trials. Serial dynamic ¹⁸F-FMISO PET was performed to investigate changes in tumor biomarkers of perfusion and hypoxia after cediranib treatment. Methods: Twenty-one rats bearing HT29 colorectal xenograft tumors were randomized into a vehicle-treated control group (0.5% methylcellulose daily for 2 d [5 rats] or 7 d [4 rats]) and a cediranib-treated test group (3 mg/kg daily for 2 or 7 d; 6 rats in both groups). All rats were imaged before and after treatment, using a 90-min dynamic PET acquisition after administration of 42.1 ± 3.9 MBq of ¹⁸F-FMISO by tail vein injection. Tumor volumes were delineated manually, and the input function was image-derived (abdominal aorta). Kinetic modeling was performed using an irreversible 1-plasma 2-tissue compartmental model to estimate the kinetic rate constants K₁, K₁/k₂, and k₃-surrogates for perfusion, ¹⁸F-FMISO distribution volume, and hypoxia-mediated entrapment, respectively. Tumor-to-blood ratios (TBRs) were calculated on the last dynamic frame (80-90 min). Tumors were assessed ex vivo by digital autoradiography and immunofluorescence for microscopic visualization of perfusion (pimonidazole) and hypoxia (Hoechst 33342). Results: Cediranib treatment resulted in significant reduction of mean voxelwise ¹⁸F-FMISO TBR, K₁, and K₁/k₂ in both the 2-d and the 7-d groups (P < 0.05). The k₃ parameter was increased in both groups but reached significance only in the 2-d group. In the vehicle-treated groups, no significant change in TBR, K₁, K₁/k₂, or k₃ was observed (P > 0.2). Ex vivo tumor analysis confirmed the presence of hypoxic tumor regions that nevertheless exhibited relatively lower ¹⁸F-FMISO uptake. Conclusion:¹⁸F-FMISO kinetic modeling reveals a more detailed response to antiangiogenic treatment than a single static image is able to reveal. The reduced mean K₁ reflects a reduction in tumor vascular perfusion, whereas the increased k₃ reflects a rise in hypoxia-mediated entrapment of the radiotracer. However, if only late static images are analyzed, the observed reduction in ¹⁸F-FMISO uptake after treatment with cediranib may be mistakenly interpreted as a global decrease, rather than an increase, in tumor hypoxia. These findings support the use of ¹⁸F-FMISO kinetic modeling to more accurately characterize the response to treatments that have a direct effect on tumor vascularization and perfusion.

Keywords: 18F-FMISO; antiangiogenesis; cediranib; hypoxia; kinetic modeling; perfusion.

FIGURE 1.

Experimental schematic. Animals bearing colorectal carcinoma xenografts were imaged with dynamic ¹⁸F-FMISO PET at baseline and subsequently randomized into cediranib and vehicle groups. After completion of treatment, second dynamic ¹⁸F-FMISO PET scan was obtained, followed by ex vivo analysis of tumor specimens.

FIGURE 2.

(A) Box-and-whisker plots summarizing mean intratumor values of TBR, k₃, K₁, and K₁/k₂ for animals treated 2 d with cediranib (left panels) or 2 d with vehicle (right panels). (B) Corresponding plots for animals treated for 7 d with cediranib (left panels) or 7 d with vehicle (right panels).

FIGURE 3.

(A) Single-slice, midtumor PET images obtained 90 min after injection from representative animal in 7-d cediranib group at baseline (left) and after treatment (right). (B) Intratumor voxelwise maps (coronal view) derived from dynamic PET images of same animal at baseline (top row) and after treatment (bottom row). DV = distribution volume.

FIGURE 4.

(A) Correlation between k₃ and TBR, two surrogate markers of tumor hypoxia, for representative animal in 7-d cediranib group. Pearson r between k₃ and TBR changed substantially, from 0.65 on baseline scan (blue) to −0.45 at follow-up (red), whereas T* increased from 22 to 60 min, respectively. (B) Corresponding scatterplot for representative animal in 2-d vehicle group. Pearson r between k₃ and TBR changed only slightly, from 0.85 on baseline scan (blue) to 0.73 at follow-up (red), whereas T* remained relatively unchanged (27 and 29 min, respectively).

FIGURE 5.

Hematoxylin and eosin (H&E), digital autoradiography (DAR), and immunofluorescence (IF) (green = pimonidazole, blue = Hoechst 33342) analysis of ex vivo tumor sections. Upper and lower zoom regions are indicated on whole-mount hematoxylin and eosin images. (A) Tumors in the cediranib group showed viable central regions positive for pimonidazole but lacking ¹⁸F-FMISO uptake (black arrows). (B) Tumors in the vehicle group showed strong correspondence between pimonidazole staining and ¹⁸F-FMISO uptake in both central and peripheral regions.