Heteromultivalent Ligands Directed Targeting of Cell-Surface Receptors - Implications in Cancer Diagnostics and Therapeutics

Abstract

Effective detection and treatment of tumor malignancies depends upon identifying targets – molecular markers that differentiate cancer cells from healthy cells. Current cancer therapies involve targeting overexpressed specific gene products. An alternative approach is proposed here: to specifically target combinations of cell-surface receptors using heteromultivalent ligands (htMVLs). There are about 2500 genes encoding for cellsurface proteins in the human genome that can potentially be targeted. Taken as sets, there can be ~ 10⁶ two-receptor combinations and ~ 10⁹ three-receptor combinations available. Our group envisions that using cell-surface protein combinations that are expressed on a cancer cell but not on a normal cell, multivalent constructs displaying complementary ligands of weak affinities can be assembled. These multivalent ligands should bind with high avidity to cancer populations in vivo, and provide a degree of specificity not seen with current approaches. As a proof-of-concept, a series of multivalent ligands were designed and synthesized for a model system consisting of the human Melanocortin subtype 4 receptor (hMC4R) and the Cholecystokinin subtype 2 receptor (CCK-2R). Modeling studies on GPCR dimers predicted that a minimum linker span of 20 - 50 Å would be required to non-covalently crosslink these two receptors. The multivalent ligands were assembled using a modular parallel synthesis approach and using solidphase chemistries. A variety of linkers were explored ranging from highly rigid to highly flexible, and using natural and/or synthetic building blocks. Ligand binding affinities were evaluated using a lanthanide based competitive binding assay in cells that expressed both receptors (bivalent binding) vs those that expressed only one of the receptors (monovalent binding), and were demonstrated to have enhanced binding affinities of up to nearly two orders of magnitude. The promising ligands were further explored by synthesizing fluorescently labeled and/or lanthanide chelate labeled monovalent and heterobivalent ligands designed for in vitro and in vivo studies. More explorative work using these labeled constructs is in progress. To the best of our knowledge, the author believes this is the first such demonstration of a 'synthetic htMVL' directed recruitment and crosslinking of two heterologous cell-surface receptors. This receptor combination approach opens up new possibilities for single cell imaging, cancer detection and therapeutic intervention, and can provide a revolutionary new platform technology with which to direct therapeutics to defined cell populations.