Previous studies have demonstrated the feasibility of translocator
protein (TSPO) imaging to visualize and quantify human breast
adenocarcinoma (MDA-MB-231) cells in vivo using a TSPO-targeted near-infrared (NIR) probe (NIR-conPK11195). This study aimed to extend the use of the TSPO-targeted probe to a more biologically relevant and clinically important tumor microenvironment as well as to assess our ability to longitudinally detect the presence and progression of
breast cancer cells in the brain. The in vivo biodistribution and accumulation of
NIR-conPK11195 and free (unconjugated) NIR
dye were quantitatively evaluated in intracranial MDA-MB-231-bearing mice and non-
tumor-bearing control mice longitudinally once a week from two to five weeks post-inoculation. The in vivo time-activity curves illustrate distinct clearance profiles for
NIR-conPK11195 and free NIR
dye, resulting in preferential accumulation of the TSPO-targeted probe in the intracranial
tumor bearing hemisphere (TBH) with significant
tumor contrast over normal muscle tissue (p < 0.005 at five weeks; p < 0.01 at four weeks). In addition, the TSPO-labeled TBHs demonstrated significant contrast over the TBHs of mice injected with free NIR
dye (p < 0.001 at four and five weeks) as well as over the TSPO-labeled non-
tumor-bearing hemispheres (NTBHs) of control mice (p < 0.005 at four and five weeks). Overall, TSPO-targeted molecular imaging appears useful for visualizing and quantifying
breast cancer xenografts propagated in the murine brain and may assist in preclinical detection, diagnosis and monitoring of metastatic disease as well as drug discovery. Furthermore, these results indicate it should be possible to perform TSPO-imaging of
breast cancer cells in the brain using radiolabeled TSPO-targeted agents, particularly in light of the fact that [11C]-labeled TSPO probes such as [11C]-
PK 11195 have been successfully used to image
gliomas in the clinic.