We compared the imaging characteristics and hypoxia selectivity of 4 hypoxia PET radiotracers (18F-fluoromisonidazole [18F-FMISO], 18F-flortanidazole [18F-HX4], 18F-fluoroazomycin arabinoside [18F-FAZA], and 64Cu-diacetyl-bis(N4-methylsemicarbazone) [64Cu-ATSM]) in a single murine xenograft tumor model condition using small-animal PET imaging and combined ex vivo autoradiography and fluorescence immunohistochemistry. Methods Nude mice bearing SQ20b xenograft tumors were administered 1 of 4 hypoxia PET tracers and images acquired 80–90 min after injection. Frozen sections from excised tumors were then evaluated for tracer distribution using digital autoradiography and compared with histologic markers of tumor hypoxia (pimonidazole, carbonic anydrase 9 [CA9]) and vascular perfusion (Hoechst 33342). Results The highest tumor uptake was observed with 64Cu-ATSM (maximum standardized uptake values [SUVmax], 1.26 ± 0.13) and the lowest with 18F-FAZA (SUVmax, 0.41 ± 0.24). 18F-FMISO and 18F-HX4 had similar intermediate tumor uptake (SUVmax, 0.76 ± 0.38 and 0.65 ± 0.19, respectively). Digital autoradiographs of hypoxia tracer distribution were compared pixel by pixel with images of immunohistochemistry stains. The fluorinated nitroimidazoles all showed radiotracer uptake increasing with pimonidazole and CA9 staining. 64Cu-ATSM showed the opposite pattern, with highest radiotracer uptake observed in regions with the lowest pimonidazole and CA9 staining. Conclusion The fluorinated nitroimidazoles showed similar tumor distributions when compared with immunohistochemistry markers of hypoxia. Variations in tumor standardized uptake value and normal tissue distribution may determine the most appropriate clinical setting for each tracer. 64Cu-ATSM showed the highest tumor accumulation and little renal clearance. However, the lack of correlation between 64Cu-ATSM distribution and immunohistochemistry hypoxia markers casts some doubt on the hypoxia selectivity of 64Cu-ATSM.

Purpose: Deregulated MYC drives oncogenesis in many tissues yet direct pharmacologic inhibition has proven difficult. MYC coordinately regulates polyamine homeostasis as these essential cations support MYC functions, and drugs that antagonize polyamine sufficiency have synthetic-lethal interactions with MYC. Neuroblastoma is a lethal tumor in which the MYC homologue MYCN, and ODC1, the rate-limiting enzyme in polyamine synthesis, are frequently deregulated so we tested optimized polyamine depletion regimens for activity against neuroblastoma.Experimental Design: We used complementary transgenic and xenograft-bearing neuroblastoma models to assess polyamine antagonists. We investigated difluoromethylornithine (DFMO; an inhibitor of Odc, the rate-limiting enzyme in polyamine synthesis), SAM486 (an inhibitor of Amd1, the second ratelimiting enzyme), and celecoxib (an inducer of Sat1 and polyamine catabolism) in both the preemptive setting and in the treatment of established tumors. In vitro assays were performed to identify mechanisms of activity.Results: An optimized polyamine antagonist regimen using DFMO and SAM486 to inhibit both rate-limiting enzymes in polyamine synthesis potently blocked neuroblastoma initiation in transgenic mice, underscoring the requirement for polyamines in MYC-driven oncogenesis. Furthermore, the combination of DFMO with celecoxib was found to be highly active, alone, and combined with numerous chemotherapy regimens, in regressing established tumors in both models, including tumors harboring highest risk genetic lesions such as MYCN amplification, ALK mutation, and TP53 mutation with multidrug resistance.Conclusions: Given the broad preclinical activity demonstrated by polyamine antagonist regimens across diverse in vivo models, clinical investigation of such approaches in neuroblastoma and potentially other MYC-driven tumors is warranted.

Purpose The behavior of 64Cu-ATSM in hypoxic tumors was examined through a combination of in vivo dynamic PET imaging, and ex vivo autoradiographic and histological evaluation using a xenograft model of H&N squamous cell carcinoma. Methods and Materials 64Cu-ATSM was administered during dynamic PET imaging, and temporal changes in 64Cu-ATSM distribution within tumors were evaluated for at least 1 h and up to 18 h. Animals were sacrificed at either 1 h (cohort A) or after 18 h (cohort B) post-injection of radiotracer and autoradiography performed. Ex vivo analysis of microenvironment sub-regions was conducted by immunohistochemical staining for markers of hypoxia (Pimonidazole Hydrochloride) and blood flow (Hoechst-33342). Results Kinetic analysis revealed rapid uptake of radiotracer by tumors. The net influx (Ki) constant was twelve-fold that of muscle, while the distribution volume (Vd) was five-fold. PET images showed large tumor-to-muscle ratios, which continually increased over the entire 18 h course of imaging. However, no spatial changes in 64Cu-ATSM distribution occurred in PET imaging after 20 minutes post-injection. Microscopic intratumoral distribution of 64Cu-ATSM and Pimonidazole were not correlated at 1 h or after 18 h post-injection, and neither was 64Cu-ATSM and Hoechst-33342. Conclusions The oxygen partial pressures at which 64CuATSM and Pimonidazole are reduced and bound in cells are theorized to be distinct and separable. However, this study demonstrated that microscopic distributions of these tracers within tumors are independent. Nonetheless, researchers have shown 64Cu-ATSM uptake to be specific to malignant expression, and this work has also demonstrated clear tumor targeting by the radiotracer.

Cediranib (AZD2171, AstraZeneca, UK) is a small-molecule pan-VEGFR inhibitor. The tumor response to short-term cediranib treatment was studied using dynamic contrast-enhanced (DCE) and diffusion-weighted (DW) MRI at 7 T as well as 18F-fluoromisonidazle (18F-FMISO) PET and histological markers. Rats bearing subcutaneous HT29 human colorectal tumors were imaged at baseline, then received three doses of cediranib (3 mg/kg per dose daily) or vehicle (dosed daily), with follow up imaging performed 2 hours after the final cediranib or vehicle dose. Tumors were excised and evaluated for the perfusion marker Hoechst 33342, endothelial cell marker CD31, smooth muscle actin (SMA), intercapillary distance (ICD) and tumor necrosis. DCE-MRI-derived parameters decreased significantly in cediranib-treated tumors relative to pre-treatment values: the muscle-normalized initial area under the gadolinium concentration curve (nIAUC90) by 48% (p = 0.002), the enhancing fraction (EnF) by 43% (p = 0.003) and Ktrans by 57% (p = 0.003), but remained unchanged in controls. No change between pre- and post-treatment tumor apparent diffusion coefficient (ADC) in either cediranib- or vehicle-treated group was observed over the course of this study. 18F-FMISO SUVmean decreased by 33% (p = 0.008) in the cediranib group, but showed no significant change in the control group. Histological analysis showed that the number of CD31-positive vessels (59 per mm2), the fraction of SMA-positive vessels (80 to 87%) and ICD (0.17 mm) were similar in cediranib- and vehicle-treated groups. The fraction of perfused blood vessels in cediranib-treated tumors (81±7%) was lower than in vehicle controls (91±3%, p = 0.02). The necrotic fraction was slightly higher in cediranib-treated rats (34±12%) than in controls (26±10%, p = 0.23). These findings suggest that short-term treatment with cediranib causes a decrease of tumor perfusion/permeability across the tumor cross-section, but changes in vascular morphology, vessel density or tumor cellularity do not manifest at this early time point.

<p>Supplementary Methods, Tables and Figures</p>