Intracellular trafficking of furin enhances cellular intoxication by recombinant immunotoxins based on Pseudomonas exotoxin A

Furin is a mammalian serine protease with important roles in cellular homeostasis and disease. It cleaves and activates numerous endogenous and exogenous substrates, including the SARS-CoV-2 viral spike protein and protein toxins such as diphtheria toxin and Pseudomonas exotoxin A (PE). Recombinant immunotoxins (RITs) are toxin conjugates used as cancer therapeutics that connect tumor-directed antibodies with toxins for targeted cell killing. RITs based on PE have shown success in treating a variety of cancers, but often suffer from safety and efficacy concerns when used clinically. We have explored furin as a potential limiting factor in the intoxication pathway of PE-based RITs. Although the furin has widely recognized importance in RIT intoxication, its role is incompletely understood. Circumstantial evidence suggests that furin may act as a transporter for RITs in addition to its role of activation by cleavage. Here, we describe the creation of a CRISPR-engineered furin-deficient HEK293 cell line, {Delta}Fur293. Using {Delta}Fur293 and derivatives that express mutant forms of furin, we confirm the importance of furin in the PE RIT intoxication pathway and show that furin trafficking has a significant impact on RIT efficacy. Our data support the hypothesis that furin acts as a transporter during RIT intoxication, and suggest furin as a target to improve the effectiveness of RITs.