Bifikafusp alfa (Darleukin) is a fusion protein of pro-inflammatory IL-2 and an L19 human mAb fragment (scFv domain) that binds the tumour microenvironment expressed fibronectin extradomain B (EDB) [1
]. Structurally it is a diabody with two IL-2 units per molecule. Incorporation of the L19 binding fragment targets the drug to the tumour, improving selective site accumulation and tumour killing and at the same time reducing the effective dose required to achieve a meaningful response, and reducing the otherwise dose-limiting toxicities associated with high-dose IL-2 administration that would be required to produce the same killing effect. Darleukin is one of a growing number of so-called 'immunocytokines' entering clinical development.