Targeted inhibition of tumor growth and angiogenesis

Abstract

Two main strategies have been pursued for the development of an effective and targeted anti-cancer treatment. The first strategy comprised the generation of a targeted nanomedicine for the inhibition of tumor cell proliferation by blocking growth factor receptor pathways. The epidermal growth factor receptor (EGFR) is a well-established anti-cancer target by virtue of its role in the proliferation and development of various tumor types. The clinical efficacy of EGFR inhibitors is limited due to resistance mechanisms of the tumor such as mutations or activation of compensatory pathways. Crosstalk between EGFR and insulin-like growth factor 1 receptor (IGF-1R) has been frequently described to be involved in tumor proliferation and resistance. To this aim, the developed targeted nanomedicine or Nanobullet consisted of anti-EGFR nanobody (Nb)-liposomes loaded with the anti-IGF-1R kinase inhibitor AG538. Nanobodies or VHHs, are small antigen binding fragments derived from the variable domain of camelidae heavy chain antibodies. The exploited anti-EGFR Nbs served as a targeting ligand for specific interactions with EGFR-positive cells, and as an EGFR inhibitor due to its antagonistic and downregulating properties. Nanobullet treatment effectively inhibited the activation of both EGFR and IGF-1R signaling in tumor cells. Inhibition of cell proliferation and survival pathways on a molecular level predicted the cytotoxic efficacy of the Nanobullets in a panel of tumor cell lines in vitro. In human xenograft models in mice, Nanobullet-induced anti-tumor effects corresponded with the cytotoxicity observed in the respective cell line. Nb-functionalized nanomedicines or Nanobullets are versatile systems that can be readily tailored for effective anti-cancer therapy by virtue of intrinsically active Nbs that serve as targeting ligands, in combination with the encapsulation of a therapeutic agent in a long-circulating drug delivery system The second strategy involved the inhibition of tumor angiogenesis using RNA interference. Angiogenesis, the formation of new blood vessels, is necessary for a tumor to overcome limiting concentrations of nutrients and oxygen in order to grow beyond microscopic size. Stimulation of endothelial cells (ECs) by vascular endothelial growth factor (VEGF) via its cognate receptor family (VEGFRs) is one of the critical events in angiogenesis. This signaling cascade can activate Rho GTPases, which are small proteins that function as molecular switches in the cell and are crucial regulators of various cellular processes. The Rho GTPase Rac1 was therefore explored as a potential target for anti-angiogenic therapy. Small interfering RNA (siRNA)-mediated knockdown of Rac1 in human ECs decreased the ability of the cells to transform into an angiogenic phenotype. Moreover, treatment with Rac1 siRNA inhibited angiogenesis and tumor growth in mice. Silencing of Rac1 may represent an attractive approach for anti-cancer therapy, but as Rho GTPases are ubiquitously expressed and involved in cellular household functions, it is crucial to inhibit these proteins at the site of disease using targeted nanomedicine approaches.