Molecular MRI of neurovascular inflammation and angiogenesis

Abstract

This thesis describes the use of different MR contrast agent platforms for the direct detection of pathology-induced upregulation of intraluminally-expressed vascular entities by molecular MRI. We particularly focused on the in vivo target specificity and MR sensitivity of the agents. The biological targets of our molecular MRI approaches were associated with two types of vascular events that are critically involved in many pathologies; i.e., angiogenesis and neurovascular inflammation. In Chapter 2, we reported on the successful application of iron oxide-containing nanoemulsions targeted to angiogenesis-upregulated ανβ3-integrin to monitor the angiogenic activity of two different tumors. In the case of highly vascularized tumors, the accumulation was more confined to the periphery of the tumors, where angiogenesis is predominantly occurring. This in contrast to less vascularized tumors, where contrast was found throughout the tumor. This demonstrated that this targeted MR contrast agent platform can serve as an in vivo biomarker for anti-angiogenesis treatment and angiogenesis phenotyping. Chapter 3 also reported on molecular MRI of stroke-induced angiogenesis. In this study, PECAM-1-targeted MPIO were assessed for their use as contrast agent for molecular MRI of vascular remodeling after experimental stroke. Even though elevated levels of PECAM-1 were present in the infarcted tissue and in vitro essays showed effective labeling by αPECAM-1-MPIO, in vivo molecular MRI after stroke revealed no αPECAM-1-MPIO-based contrast enhancement and αPECAM-1-MPIO were absent in post mortem brain tissue. This indicated that this molecular MRI approach is not invariably effective for MRI-based assessment of stroke-induced alterations in expression of cerebrovascular markers. Assessment of the suitability of two ICAM-1-targeted MR contrast agents, i.e., based on Gd-liposomes or MPIO, for molecular MRI of upregulation of ICAM-1 after stroke, was described in Chapter 4. Both ICAM-1-targeted agents bound effectively to activated cerebrovascular cells in vitro, generating significant MRI contrast-enhancing effects. Direct in vivo MRI-based detection after stroke was only achieved with ICAM-1-targeted MPIO, although both contrast agents showed similar target-specific vascular accumulation. This study demonstrated the potential of in vivo MRI of post-stroke ICAM-1 upregulation, and signified target-specific MPIO as most suitable contrast agent for molecular MRI of cerebrovascular inflammation. In Chapter 5, these ICAM-1-targeted MPIO were administrated at different time points post-stroke in mice to determine to what extent different stages of endothelial activation and leukocyte infiltration could be visualized. αICAM-1-MPIO were shown to be suitable for in vivo MRI of stroke-induced ICAM-1 expression on vascular endothelium and leukocytes at different stages after stroke in mice. However, contrast effects from endogenous blood remains significantly hampered the detection of co-localized MPIO at later time points. Chapter 6 explored if ICAM-1-targeted MPIO were able to detect EAE-induced ICAM-1 upregulation with in vivo molecular MRI. ICAM-1-targeted MPIO-induced MR contrast was particularly detected in the subarachnoid space between the temporal lobe and the midbrain. These locations of ICAM-1-induced MR contrast were not necessarily associated with physical disruption of the BBB, which is clinically used for detection of MS lesions. This indicated its suitability as a new contrast agent to detect ongoing disease activity and the evaluation of treatment efficacy.