Targeting human cancer cells with VEGF receptor-2-directed liposomes.

Antibodies are among the most versatile tools used today to characterize and target molecules in cells and in biological tissues. The development of phage display libraries encoding a large repertoire of single chain antibodies, scFv, allows the rapid and efficient isolation of antibodies specific for almost any type of molecule. A great advantage of such recombinant antibodies is the possibility to functionalize them by introducing new amino acid sequences. This leads to new features that would be difficult to introduce into naturally occurring antibody molecules. This approach has been successfully applied to create molecules with new biological activities, e.g. by generating chimeric scFv antibodies carrying sequences derived from other biomolecules such as blood clotting factors or enzymes. Here, we describe a new antibody isolated from an M13 phage library that recognizes vascular endothelial growth factor receptor 2, VEGFR-2. This antibody, scFvVR-2H9 was coupled to liposomes and used to specifically target VEGFR-2-expressing human cancer cells in culture.

HIV TAT basic peptide is not a high-affinity ligand for VEGF receptor 2.

The 'transactivator of transcription' (TAT) protein of human immunodeficiency virus transforms cells in culture and promotes the development of tumors, so-called Kaposi's sarcoma, in AIDS patients. TAT induces growth and differentiation of blood vessels and has been suggested to directly activate VEGF receptor 2 expressed on endothelial cells through a peptide sequence located between amino acids 46 and 64, the so-called basic domain. This peptide mimics many aspects of TAT function when added to endothelial cells, even when expressed in the context of recombinant chimeric proteins. To define the exact sites of interaction between this peptide and VEGF receptor 2 we performed binding studies with recombinant proteins derived from the extracellular ligand binding domain of VEGF receptor 2. These in vitro binding studies showed that the TAT peptide binds with only low specificity to Ig-like domain 3 of the receptor, while VEGF interacts with receptor-derived proteins encompassing at least extracellular domains 1 through 3. The original concept that the angiogenic properties of TAT basic peptide result from specific, high-affinity interaction with VEGF receptor 2 must therefore be revised. Apparently this peptide interacts with cells in multiple ways: by directly activating acidic cell surface-exposed receptors, by releasing extracellular matrix-bound growth factors such as bFGF and VEGF which then bind to their cognate receptors, and by activating intracellular signalling molecules with which basic peptide interacts upon translocation into cells.