VEGFR have a major role in the development of pathological angiogenesis and therefore of cancer. In order to assess biochemical response to treatment and contribute to anti VEGFR drug development, advance in in-vivo molecular imaging methodology is mandatory.
Angiogenesis, the recruitment of new blood vessels, is a crucial mechanism in a number of physiological and pathological conditions. It is a tightly regulated, multiple step process, that results in the formation of blood vessels from pre-existing vasculature. Under normal conditions, angiogenesis occurs during embryonic development, wound healing and the female menstruation cycle.
Pathological angiogenesis occurs in disease states such as psoriasis, diabetic retinopathy, rheumatoid arthritis, chronic inflammation and cancer. In the process of angiogenesis, several events are included: proliferation, migration, and invasion of endothelial cells, organization of endothelial cells into functional tubular structures, maturation of vessels, and vessel regression. One of the major pathways for promotion of angiogenesis is the vascular endothelial growth factor (VEGF) family of growth factors and receptors.
Activation of the VEGF/VEGF-receptor (VEGFR) axis triggers multiple signaling networks that result in endothelial cell survival, mitogenesis, migration, differentiation, and vascular permeability.2 Over-expression of both VEGF and VEGFR mRNA has been associated with angiogenesis, tumor progression and poor prognosis in several tumor systems. While over-expression appears in tumor-associated endothelial cells, over-expression or upregulation was not found in the vasculature surrounding of normal tissues.
The over-expression of VEGFR2 (KDR) on tumor endothelial cells offers a unique target for specific anti-angiogenetic therapy. While angiogenetic vascularization collapse does not bring about a full recovery, it could help reduce tumor size to around 2 mm (the maximal tumor diameter without induction of angiogenesis), inhibit metastasis and enable more efficient therapy when administered concomitantly with known chemotherapeutic agents.
The current challenge in VEGFR targeted therapy is not only the inhibition of VEGF signaling via VEGFR antagonism, but the evaluation of anti angiogenetic treatment efficacy in the early stages in order to assess different dosing regiments. Tighter control over dosing could lead to personalized treatment and permit guided treatment over different stages of patient management.
Effective evaluation of VEGFR occupancy offers a tool for drug discovery and efficacy appraisal. Today, efficacy is evaluated by shrinkage of tumor size, microvessel density (MVD) and total vascular area (TVA), or evaluation of blood flow. Changes in tumor size are measurable by conventional imaging, however the time window post treatment is too long and is measured in months.
Determination of MVD and TVA is achieved by invasive methods and MRI evaluates blood flow and accumulation rather than microvessel integrity.
In order to explore the role of VEGFR in cancer, and the potential of VEGFR targeted therapy, there is a pressing need to develop specific and selective molecular imaging modalities that will enable the measurement of several important parameters such as concentration, and occupancy of the VEGFR in vivo with a noninvasive Nuclear Medicine modality. Specific labeled VEGFR2 antagonists could allow for selective, non-invasive quantitative imaging of angiogenetic blood vessels.
Several compounds of the quinoline and quinazoline families will be synthesized and for each one of them, the potency, affinity and selectivity towards the KDR (VEGFR-2) will be determined. A radiolabeling route for the most promising compounds will be developed and their potential as PET imaging agents will be evaluated. The compounds will be characterized in several biological tests including in vitro & in vivo testing. In vitro assays will be performed in a number of cell lines which over-express the VEGFR-2/KDR in comparison with these same cell lines without over-expression.
The in vitro experiments will include potency assays, specific binding assays & selectivity assays versus different Tyrosine kinase receptors including: VEGFR1, PDGFR, EGFR, INSULIN-R, FGFR, c-Kit & others. Following in vitro characterization, tumor cell lines will be injected into nude athymic mice and the PET imaging biomarker candidates will be evaluated in-vivo by biodistribution studies, and specific binding assays including Micro-PET imaging.