Phillip S. Athey

Imaging of Vx‐2 rabbit tumors with ανβ3‐integrin‐targeted 111In nanoparticles

Earlier tumor detection can improve 5‐year survival of patients, particularly among those presenting with cancers less than 1 cm in diameter. ανβ3‐Targeted 111In nanoparticles (NP) were developed and studied for detection of tumor angiogenesis. Studies were conducted in New Zealand white rabbits implanted 12 days earlier with Vx‐2 tumor. ανβ3‐Targeted 111In/NP bearing ∼10 111In/NP vs. ∼1 111In/NP nuclide payloads were compared to nontargeted radiolabeled control particles. In vivo competitive binding studies were used to assess ligand‐targeting specificity. ανβ3‐Integrin‐targeted NP with ∼10 111In/NP provided better (p < 0.05) tumor‐to‐muscle ratio contrast (6.3 ± 0.2) than ∼1 111In/NP (5.1 ± 0.1) or nontargeted particles with ∼10 111In/NP (3.7 ± 0.1) over the initial 2‐hr postinjection. At 18 hr, mean tumor activity in rabbits receiving ανβ3‐integrin‐targeted NP was 4‐fold higher than the nontargeted control. Specificity of the NP for the tumor neovasculature was supported by in vivo competition studies and by fluorescence microscopy of ανβ3‐targeted fluorescent‐labeled NP. Biodistribution studies revealed that the primary clearance organ in rabbits as a %ID/g tissue was the spleen. Circulatory half‐life (t1/2β) was estimated to be ∼5 hr using a 2‐compartment model. ανβ3‐Targeted 111In perfluorocarbon NP may provide a clinically useful tool for sensitively detecting angiogenesis in nascent tumors, particularly in combination with secondary high‐resolution imaging modalities, such as MRI.

Targeted PARACEST nanoparticle contrast agent for the detection of fibrin.

A lipid‐encapsulated perfluorocarbon nanoparticle molecular imaging contrast agent that utilizes a paramagnetic chemical exchange saturation transfer (PARACEST) chelate is presented. PARACEST agents are ideally suited for molecular imaging applications because one can switch the contrast on and off at will simply by adjusting the pulse sequence parameters. This obviates the need for pre‐ and postinjection images to define contrast agent binding. Spectroscopy (4.7T) of PARACEST nanoparticles revealed a bound water peak at 52 ppm, in agreement with results from the water‐soluble chelate. Imaging of control nanoparticles showed no appreciable contrast, while PARACEST nanoparticles produced >10% signal enhancement. PARACEST nanoparticles were targeted to clots via antifibrin antibodies and produced a contrast‐to‐noise ratio (CNR) of 10 at the clot surface. Magn Reson Med, 2006.

High Sensitivity : High-Resolution SPECT-CT/MR Molecular Imaging of Angiogenesis in the Vx2 Model

Objectives:The use of antiangiogenic therapy in conjunction with traditional chemotherapy is becoming increasingly in cancer management, but the optimal benefit of these targeted pharmaceuticals has been limited to a subset of the population treated. Improved imaging probes that permit sensitive detection and high-resolution characterization of tumor angiogenesis could improve patient risk-benefit stratification. The overarching objective of these experiments was to develop a dual modality &agr;&ngr;&bgr;3-targeted nanoparticle molecular imaging agent that affords sensitive nuclear detection in conjunction with high-resolution MR characterization of tumor angiogenesis. Materials and Methods:In part 1, New Zealand white rabbits (n = 21) bearing 14d Vx2 tumor received either &agr;&ngr;&bgr;3-targeted 99mTc nanoparticles at doses of 11, 22, or 44 MBq/kg, nontargeted 99mTc nanoparticles at 22 MBq/kg, or &agr;&ngr;&bgr;3-targeted 99mTc nanoparticles (22 MBq/kg) competitively inhibited with unlabeled &agr;&ngr;&bgr;3-nanoparticles. All animals were imaged dynamically over 2 hours with a planar camera using a pinhole collimator. In part 2, the effectiveness of &agr;&ngr;&bgr;3-targeted 99mTc nanoparticles in the Vx2 rabbit model was demonstrated using clinical SPECT-CT imaging techniques. Next, MR functionality was incorporated into &agr;&ngr;&bgr;3-targeted 99mTc nanoparticles by inclusion of lipophilic gadolinium chelates into the outer phospholipid layer, and the concept of high sensitivity – high-resolution detection and characterization of tumor angiogenesis was shown using sequential SPECT-CT and MR molecular imaging with 3D neovascular mapping. Results:&agr;&ngr;&bgr;3-Targeted 99mTc nanoparticles at 22 MBq/kg produced the highest tumor-to-muscle contrast ratio (8.56 ± 0.13, TMR) versus the 11MBq/kg (7.32 ± 0.12) and 44 MBq/kg (6.55 ± 0.07) doses, (P < 0.05). TMR of nontargeted particles at 22.2 MBq/kg (5.48 ± 0.09) was less (P < 0.05) than the equivalent dosage of &agr;&ngr;&bgr;3-targeted 99mTc nanoparticles. Competitively inhibition of 99mTc &agr;&ngr;&bgr;3-integrin-targeted nanoparticles at 22.2 MBq/kg reduced (P < 0.05) TMR (5.31 ± 0.06) to the nontargeted control contrast level. Multislice CT imaging could not distinguish the presence of Vx2 tumor implanted in the popliteal fossa from lymph nodes in the same fossa or in the contralateral leg. However, the use of 99mTc &agr;&ngr;&bgr;3-nanoparticles with SPECT-CT produced a clear neovasculature signal from the tumor that was absent in the nonimplanted hind leg. Using &agr;&ngr;&bgr;3-targeted 99mTc-gadolinium nanoparticles, the sensitive detection of the Vx2 tumor was extended to allow MR molecular imaging and 3D mapping of angiogenesis in the small tumor, revealing an asymmetrically distributed, patchy neovasculature along the periphery of the cancer. Conclusion:Dual modality molecular imaging with &agr;&ngr;&bgr;3-targeted 99mTc-gadolinium nanoparticles can afford highly sensitive and specific localization of tumor angiogenesis, which can be further characterized with high-resolution MR neovascular mapping, which may predict responsiveness to antiangiogenic therapy.