(89)Zr-labeled paramagnetic octreotide-liposomes for PET-MR imaging of cancer.

PURPOSE: Dual-modality PET/MR platforms add a new dimension to patient diagnosis with high resolution, functional, and anatomical imaging. The full potential of this emerging hybrid modality could be realized by using a corresponding dual-modality probe. Here, we report pegylated liposome (LP) formulations, housing a MR T(1) contrast agent (Gd) and the positron-emitting (89)Zr (half-life: 3.27 days), for simultaneous PET and MR tumor imaging capabilities. METHODS: (89)Zr oxophilicity was unexpectedly found advantageous for direct radiolabeling of preformed paramagnetic LPs. LPs were conjugated with octreotide to selectively target neuroendocrine tumors via human somatostatin receptor subtype 2 (SSTr2). (89)Zr-Gd-LPs and octreotide-conjugated homolog were physically, chemically and biologically characterized. RESULTS: (89)Zr-LPs showed reasonable stability over serum proteins and chelator challenges for proof-of-concept in vitro and in vivo investigations. Nuclear and paramagnetic tracking quantified superior SSTr2-recognition of octreotide-LP compared to controls. CONCLUSIONS: This study demonstrated SSTr2-targeting specificity along with direct chelator-free (89)Zr-labeling of LPs and dual PET/MR imaging properties.

A comparison of the imaging characteristics and microregional distribution of 4 hypoxia PET tracers.

UNLABELLED: We compared the imaging characteristics and hypoxia selectivity of 4 hypoxia PET radiotracers ((18)F-fluoromisonidazole [(18)F-FMISO], (18)F-flortanidazole [(18)F-HX4], (18)F-fluoroazomycin arabinoside [(18)F-FAZA], and (64)Cu-diacetyl-bis(N4-methylsemicarbazone) [(64)Cu-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 (64)Cu-ATSM (maximum standardized uptake values [SUV(max)], 1.26 ± 0.13) and the lowest with (18)F-FAZA (SUVmax, 0.41 ± 0.24). (18)F-FMISO and (18)F-HX4 had similar intermediate tumor uptake (SUV(max), 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. (64)Cu-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. (64)Cu-ATSM showed the highest tumor accumulation and little renal clearance. However, the lack of correlation between (64)Cu-ATSM distribution and immunohistochemistry hypoxia markers casts some doubt on the hypoxia selectivity of (64)Cu-ATSM.