. Author manuscript; available in PMC: 2021 Jul 1.

Published in final edited form as: Nat Rev Cancer. 2019 Oct 10;20(1):26–41. doi: 10.1038/s41568-019-0205-x

Table 2|.

Biomarkers of BBB/BTB permeability

Method	Agent or molecule	Measurement of BBB permeability	Clinical approval	Refs
Intravital microscopy	Fluorescent molecules	• Direct quantification of BBB/BTB permeability • Direct measurement of the extravasation of fluorescently labelled drugs and cells	No	^128,229
MRI (contrast enhanced)	Gadolinium or magnetic nanoparticles	• Semi-quantitative assessment of BBB permeability (K^trans) • Indirect assessment of the extravasation of drugs that have similar permeability with gadolinium or magnetic nanoparticles • Overall limited correlation with the extravasation of drugs with different physiochemical properties from those of the agents	Yes	^129,187
PET	¹⁸F	• Relative assessment of BBB permeability (extravasation of imaging tracer) • Direct measurement of radiolabelled drug and cell accumulation in the brain	Yes	^230–233

BBB, blood–brain barrier; BTB, blood–tumour barrier; K^trans, bulk transport parameter obtained using dynamic contrast-enhanced MRI that is dependent on both capillary permeability and perfusion; MRI, magnetic resonance imaging; PET, positron emission tomography.