Amgen has upped its investment in drugmaker Beigene to further access the increasingly large pharmaceutical market, China. Last year, Thousand Oaks, California-based Amgen closed a strategic collaboration with BeiGene, investing $2.8 billion to gain a presence in what’s becoming an ever-more essential market for global drug development. The investment earned it an R&D pact and ownership of about one-fifth of the Chinese oncology drug company. Now the firm has snapped up a chunk of additional shares in a move to maintain its…