Case Study: Dual-specificity Fab engineering for conditional cytokine activation

IL-12 is among the most potent anti-tumour cytokines, but systemic toxicity has limited its clinical development for close to thirty years.

This case study sets out how FairJourney Bio engineered a dual-binding Fab that keeps IL-12 masked until it reaches the tumour.

Cytokines are potent immune activators, but systemic toxicity has confined their clinical use to a narrow therapeutic window. IL-12 is a prominent example: early trials were halted by severe immune-related toxicity despite signs of anti-tumour activity, and later strategies — intra-tumoral dosing, half-life extension, protease-cleavable pro-drugs — have had limited clinical success.

The approach described here keeps IL-12 inactive in systemic circulation and releases it locally only on binding Fibronectin Extra Domain B (FN-EDB), a matrix antigen abundant in the tumour extracellular matrix (up to 100 nM), largely absent from normal adult tissue, and retained rather than internalised. The sponsor, a venture-backed oncology company, designed the concept: a dual-binding Fab “switch” arm that binds IL-12 or FN-EDB competitively, paired with a targeting arm that drives tumour localization and avidity-driven unveiling. FJBio's Porto and Cambridge teams discovered and engineered that switch arm to a binding specification the sponsor fixed in advance.