Legumain to the rescue: A new ADC linker strategy to address the limitations of cathepsin cleavage

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Project Summary: There are now 8 FDA-approved antibody-drug-conjugates (ADCs) on the market. ADC technology is being widely pursued for oncology, autoimmunity, ophthalmology, and infectious disease applications. In spite of tremendous success, however, the ADC development process is riddled with challenges associated with ADC aggregation, linker- stability concerns, unexpected toxicity, and poor pharmacokinetics. While it is impossible to lay the blame for this at the feet of a single phenomenon, one common factor shared by the vast majority of ADCs is the reliance on a hydrophobic peptide linker (i.e. ValCit-PABC) that is stable in plasma but rapidly cleaved by lysosomal cathepsins. Various studies have recently shown that this linker is more unstable in circulation was than previously believed, being susceptible to cleavage by both plasma esterases (Ces1C) and lymphocyte-associated proteases (neutrophil elastase). It is now widely believed that cleavage by neutrophil elastase is responsible for the dose-limiting neutropenia and thrombocytopenia so commonly seen for many ADCs. A new generation of cleavable linkers is urgently needed in order to overcome these liabilities. Herein, we propose the optimization and evaluation of “redesigned” ADC linkers that contain asparagine (Asn) motifs which are rapidly cleaved by asparaginyl endoprotease, also known as legumain. Our team has identified several Asn-containing sequences that are rapidly cleaved by lysosomal legumain while retaining stability in mouse and human plasma. Our preliminary data shows that ADCs that incorporate these linkers exhibit comparable in vitro cytotoxicity to classical ValCit-PABC linkers – but are not susceptible to cleavable by Ces1C or neutrophil elastase. We have designed three specific aims to optimize this linker and to demonstrate that model ADCs that use these linkers have improved therapeutic potential as compared to traditional ValCit-PABC linkers. Aim #1 is is focused on the optimization of the peptide linker itself. We will prepare a FRET-based library of Asn-containing peptides in order to identify robust linkers that can be incorporated into various cancer-targeting ADCs. Aim #2 is focused on the development of suitable self-immolative spacers for the legumain cleavable peptides. The spacer will be optimized in order to facilitate rapid cleavage in the lysosome and high stability in plasma. Aim #3 will establish the therapeutic utility of legumain-cleavable ADCs in a model of pancreatic cancer. Specifically, we will incorporate our novel linkers into anti-GCC ADCs that are of interest for the treatment of pancreatic cancer. The ADCs will be compared head-to-head with a traditional ADC (TAK-264) that failed phase 2 clinical trials due to dose limiting neutropenia. We anticipate that our novel linkers will impart an improved tolerability to this ADC while maintaining the efficacy. Accomplishment of these aims will firmly establish the suitability of legumain-cleavage for therapeutic applications – and will poise the technology for clinical application. Development of novel pancreatic-cancer targeting ADCs will pave the way towards clinical evaluation in an area of tremendous unmet medical need.