Endoglin-targeted cancer therapy

Abstract

Vascular-targeting antiangiogenic therapy (VTAT) of cancer can be advantageous over conventional tumor cell targeted cancer therapy if an appropriate target is found. Our hypothesis is that endoglin (ENG; CD105) is an excellent target in VTAT. ENG is selectively expressed on vascular and lymphatic endothelium in tumors. This allows us to target both tumor-associated vasculature and lymphatic vessels to suppress tumor growth and metastasis. ENG is essential for angiogenesis/vascular development and a co-receptor of TGF-β. Our studies of selected anti-ENG monoclonal antibodies (mAbs) in several animal models and in vitro studies support our hypothesis. These mAbs and/or their immunoconjugates (immunotoxins and radioimmunoconjugates) induced regression of preformed tumors as well as inhibited formation of new tumors. In addition, they suppressed metastasis. Several mechanisms were involved in the suppressive activity of the naked (unconjugated) anti-ENG mAbs. These include direct growth suppression of proliferating endothelial cells, induction of apoptosis, ADCC (antibody-dependent cell-mediated cytotoxicity) and induction of T cell immunity. To facilitate clinical application, we generated a human/mouse chimeric anti-ENG mAb termed c-SN6j and performed studies of pharmacokinetics, toxicology and immunogenicity of c-SN6j in nonhuman primates. No significant toxicity was detected by several criteria and minimal immune response to the murine part of c-SN6j was detected after multiple i.v. injections. The results support our hypothesis that c-SN6j can be safely administered in cancer patients. This hypothesis is supported by the ongoing phase 1 clinical trial of c-SN6j (also known as TRC105) in patients with advanced or metastatic solid cancer in collaboration with Tracon Pharma and several oncologists (NCT00582985).

Figures

Fig. 1

Reactivity of anti-ENG mAbs with malignant and normal breast tissues. Frozen breast carcinoma tissues were allowed to react with anti-ENG mAb SN6 (A) or an isotype-matched control IgG (IgG1-κ) (B) and stained with DAKO staining kits. In an additional test, formalin-fixed paraffin-embedded tissues of breast carcinoma (C) or normal breast (D) were allowed to react with anti-ENG mAb SN6h and stained with DAKO staining kits. An example of the stained blood vessels is indicated by an arrow in panels A and C. SN6 and SN6h show strong staining with multiple blood vessels in malignant breast tissue (A and C). Control IgG did not show any significant staining in each of tested tissues and an example is presented in panel B. SN6h shows marginal staining with a few blood vessels in normal breast tissues (D). It is important to note that reactivity of SN6 and SN6h with malignant breast tissues is restricted to vascular endothelium and no significant reactivity of these mAbs with tumor cells per se is detected.

Fig. 2

Effect of SN6j on the Matrigel plug angiogenesis in mice. Matrigel Matrix mixed with 1.25×105 colon 26 cells was injected s.c. in the left frank of BALB/c mice on day 0. The mice were treated with 1.8 μg/g body weight of SN6j or isotype-matched control IgG via tail vein on day 1, 4 and 7. The Matrigel plugs were fixed on day 10 for immunohistochemical staining. A, examples of rat anti-mouse CD31 (PECAM-1) mAb staining of Matrigel plug sections from SN6j-treated and control IgG-treated mice. The sections were counterstained with hematoxylin. Arrowheads in the figure indicate representative CD31 positive vessels. B, the CD31 positive vessels in hotspots were counted in four separate ×200 fields from 3 samples per group.

Fig. 3

Nucleotide sequence of the gene encoding VL of c-SN6j (also of SN6j) and the deduced amino acid sequence of VL of c-SN6j. FR and CDR denote the framework region and the complementarity-determining region of antibody, respectively.

Fig. 4

Nucleotide sequence of the gene encoding VH of c-SN6j (also of SN6j) and the deduced amino acid sequence of VH of c-SN6j. FR and CDR denote as described in the legend to Fig. 3.