Abstract A39: Multimodal imaging of blood-brain barrier disruption during brain metastatic progression in a relevant experimental mouse model (2013)

Abstract Introduction: Efficient drug delivery to brain metastases is difficult due to an often intact blood-brain barrier (BBB) around smaller lesions, and very few therapeutic drugs penetrate an intact BBB. Relevant experimental brain metastatic models are needed to study biological mechanisms and therapeutic responses of early brain lesions in relation to the BBB. Previous studies injecting small molecules such as sodium fluorescein (MW 376D) have indicated that the BBB is disrupted in experimental brain metastasis larger than 0.25 mm (1). However, little is known regarding at which time point in experimental brain metastasis development the BBB is disrupted, and to what extent larger sized molecules at that time are able to penetrate tumor tissue. We addressed these issues by performing multimodal imaging studies, after injecting contrast agents of various sizes into an established melanoma brain metastasis model (2, 3). Materials and Methods: All studies were approved by the UCD Animal Care and Use Committee. As in (3), we injected 5X105 human melanoma brain metastasis cells harboring the Luciferase and GFP genes, intracardially into 32 NOD/SCID mice. All animals were followed by bioluminescence imaging (BLI) weekly for 6 weeks. MRI (Bruker Biospec 70/30, RARE sequence, RARE factor=2, TE/TR/ST=9 ms/750 ms/1mm, FOV=2x2 cm, Matrix=256x256, NA=4) was performed at weeks 3, 4, 5 and 6 after tumor cell inoculation, before and after i.v. injections of either Gd-HPDO3A (Prohance, MW 559D, 0.5 μmol/g, 20 mice) or a newly synthetized Gadolinium contrast agent with 3 Gd(III) chelates, termed C3, (MW 2kD, 0.167 μmol/g, 5 mice). 64Cu-Albumin (MW 66.5kD) and 18F-FLT (MW 244D) was also injected i.v. prior to PET imaging at weeks 4 and 6. Results and Discussion: BLI visualized tumor cell spread in all animal brains 15mins after injections, and a gradual increase in tumor burden between weeks 1 to 6. T1w MRI at week 3 did not show any tumors, however BLI confirmed presence of metastasis. MRI detected tumors at week 4, and the mean, total number of tumors increased from 26 tumors at week 4, to 89 tumors at week 6. There was an exponential increase in tumor burden from week 4 to 6. The number of leaky tumors also increased exponentially, with mean diameters increasing from around 600 μm to around 700 μm. The smallest leaky tumors detectable by MRI were ~200 μm (week 5). The number of non-leaky tumors peaked at 5 weeks, and their mean diameters increased from 260 μm to 450 μm. Metastatic tumor cell proliferation was shown by 18F-FLT, which accumulated in the animal brains at weeks 4 and 6. 64Cu-Albumin PET also showed leakage of albumin into the metastatic lesions at week 6. Conclusions: Our study shows that T1w 7T MRI can detect leaky brain metastases down to ~200 μm in diameter. Molecules up to 2kD in size may penetrate brain tumor tissue early in tumor development. Our PET study indicates that larger molecules such as albumin may leak out of the metastatic lesions later in tumor development. References (1) Fidler IJ et al. Lancet Oncol 3:53-57, 2002. (2) Sundstrøm T et al, Cancer Res 2012 (In Press). (3) Wang J et al. Neuropathol Appl Neurobiol 37:189-205, 2011. Citation Format: Frits Thorsen, Brett Fite, Lisa Mahakian, Victoria Harrison, Sarah Johnson, Elizabeth Ingham, Shengping Qin, Jai W. Seo, Thomas Meade, Terje Sundstrøm, Katherine W. Ferrara. Multimodal imaging of blood-brain barrier disruption during brain metastatic progression in a relevant experimental mouse model. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Invasion and Metastasis; Jan 20-23, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;73(3 Suppl):Abstract nr A39.