P14.08.B COMPARING GD AND MN MRI CONTRAST AGENTS IN PRECLINICAL GLIOBLASTOMA FOR IMAGING BIOMARKER DISCOVERY: TISSUE VALIDATION OF IMAGING PARAMETERS USING AN INNOVATIVE SPATIAL PHENOMIC APPROACH
Authors: Icer Y , Vallatos A , Gallagher L , Birch J , Kinch K , Chalmers A , Waldman A , Thompson G 
Published: October 2024

Abstract

BACKGROUND
Glioblastoma (GBM) management relies on gadolinium-based contrast agent (GBCA)-enhanced magnetic resonance imaging (MRI). Manganese chelates might provide alternative paramagnetic enhancement which is safer (Mn is an essential nutrient) and more informative since Mn is a calcium analogue with potential for intracellular uptake while GBCA indicate the non-specific epiphenomenon of blood-brain barrier (BBB) breakdown confounded by treatment effects. Qualitative T1-weighted MRI paramagnetic enhancement reflects underlying quantitative longitudinal relaxation (T1) shortening.

METHODS
Gd and Mn chelate T1 effects were compared across three time points in n=7 G7 human GBM nude mice orthotopic xenografts. Twelve weeks post implantation, T1 relaxometry mapping MRI was conducted: pre- and post-Gd-DTPA (session 1); pre- and post-Mn-DPDP 2-3 days later (session 2); and 3-5 days later (session 3). Explanted flash-frozen brains at session 3 provided H&E histology and metal ion LA-ICP mass spectrometry imaging (MSI) with matched slice angulation to MRI using stacked in-plane histology. Controls received no Mn-DPDP. Mean tumour and contralateral thalamus T1 values were measured. MSI in Mn-injected and control mice were compared. H&E stacks and MSI were co-registered to match MRI spatial resolution. Resulting pseudo-MRI voxel values represent intravoxel cell number segmented in QuPath and metal ion content. The relationships between cell number, T1, and metal ion levels was explored through matched MRI voxels and regions of interest (ROI).

RESULTS
Gd and Mn chelates significantly shortened tumour T1 (p=0.002, p=0.001) but not contralateral thalamus. Tumour T1 values returned to pre-Gd baseline between sessions 1 and 2. Tumour T1 values post-Mn-DPDP remained significantly shortened for 3-5 days (p=0.023.) Mn levels in Mn-DPDP mice were significantly greater (p=0.0004) than controls. Positive spatial correlation was demonstrated between tissue cell number and [Mn]. The number of cells and [Mn] were higher in persistent T1 shortening ROIs at session 3 (p=0.0021, p=0.0404).

CONCLUSION
Gd-DTPA transiently shortens tumour T1 as expected due to BBB breakdown. Mn-DPDP shortens tumour T1 for 3-5 days indicating extravascular persistence. Tissue analysis demonstrates that: [Mn] are higher in Mn-DPDP administered tumours than controls; tumour [Mn] are spatially linearly correlated to cell number; and persistent T1 shortening post-Mn-DPDP relates to tissue [Mn] and cell number. We propose a non-invasive MRI paramagnetic agent biomarker of GBM cell number not simply representing BBB breakdown. This might distinguish active tumour from treatment change and support intraoperative MRI due to enhancement persistence. We are translating to humans and characterising tissue contributions to Mn accumulation and T1 shortening to understand underlying biological mechanisms.