ARGX-111 IS AN ANTI-C-MET ANTIBODY WITH BROAD APPLICABILITY IN CANCER Rotterdam, the Netherlands and Ghent, Belgium February 8, 2012–arGEN-X, a biopharmaceutical company focused on the discovery and development of human monoclonal antibodies from its proprietary SIMPLE Antibody™ platform, announces that it has progressed ARGX-111, its third therapeutic candidate, into formal preclinical development. ARGX-111 is a novel, fully human antibody specific for c-Met, a receptor tyrosine kinase involved in cell proliferation, angiogenesis and metastasis. c-Met is widely implicated in multiple cancers, including non small cell lung cancer (NSCLC), gastric, prostate and brain tumors. ARGX-111 has been shown in several preclinical models to outperform clinical-stage anti-c-Met antibodies in terms of potency and efficacy. ARGX-111 was selected from a panel of more than 60 antagonistic anti-c-Met antibodies generated from the SIMPLE Antibody™ platform, several of which have specificity for previously unknown epitopes on c-Met. ARGX-111 exhibits the unique ability to block both ligand-dependent and independent receptor activation, while avoiding agonistic activity. In addition, the antibody demonstrates potent cytotoxic effects against c-Met over-expressing tumor cells. Next to its ultra-high potency, ARGX-111 has other important characteristics that make it a very attractive development candidate. ARGX-111 has a fully human sequence and structure, and expression and stability studies indicate that it can be manufactured at consistently high quality and yield. arGEN-X anticipates filing an IND application for ARGX-111 in 2013. ARGX-111 is one of five therapeutic antibody programs under development at arGEN-X for the treatment of autoimmune disorders, inflammation and cancer. The initiation of formal preclinical development for ARGX-111 marks the third program to progress to this stage in only 26 months since the start of the Company’s operations. arGEN-X’ most advanced preclinical program is ARGX-110, a fully human antibody with unprecedented neutralization potency for a novel T- and B-cell receptor involved in haematological malignancies, solid tumors and autoimmune indications. An IND application for ARGX-110 is anticipated in early 2013. Commenting further on the Company’s progress, Tim Van Hauwermeiren, CEO of arGEN-X, said: “To have identified an antibody with such potency, specificity and excellent development potential for a target as important as c-Met is a major step in the validation of our unique SIMPLE Antibody™ platform. The highly attractive profile of ARGX-111 was recognized as an exciting investment proposition in our EUR27.5 million fundraising in December 2011. Our progress in successfully applying our transformational technology and rapidly building a best-in-class product pipeline puts arGEN-X in pole position to deliver its business plan in the coming years.” About arGEN-X – www.arGEN-X.com arGEN-X is a preclinical stage biopharmaceutical company with a broadly applicable, proprietary SIMPLE Antibody™ platform, a world-class source of fully human antibodies with differentiated qualities and outstanding therapeutic potential. The platform excels in delivering large panels of ultra-high potency, functionally diverse antibodies against complex disease targets, especially cell surface receptors and highly conserved proteins. Antibody leads generated from the SIMPLE Antibody™ platform have outstanding physicochemical properties and consistent manufacturability, thus enabling a seamless transition from discovery into development. Superior choice in discovery enables arGEN-X to prioritise leads with best-in-class therapeutic properties and thereby increase the probability of success later in development. The Company has validated its technology on almost a dozen diverse classes of target to date. arGEN-X’ SIMPLE Antibody™ platform is based on the active immunisation of Camelids with target antigens to deliver antibody variable regions that are essentially identical to those of human antibodies. The combination of those variable domains with human constant domains generates full size, human therapeutic antibodies. The SIMPLE Antibody™ platform is covered by broad patent claims, enjoys an independent, unencumbered patent position and is free of target gatekeeping restrictions. arGEN-X™ and SIMPLE Antibody™ are deposited trademarks of arGEN-X BV. SIMPLE stands for Superior Immunodiversity with Minimal Protein Lead Engineering.