Evaluation of an ester-linked immunosuppressive payload: A case study in understanding the stability and cleavability of ester-containing ADC linkers

In spite of their value in prodrug applications, the use of esters in antibody-drug-conjugate (ADC) payloads and linkers has generally been avoided due to the ubiquitous and promiscuous nature of human esterases. ADCs generally have a long circulating half life (3–7 days) that makes them susceptible to esterase-mediated metabolism. Moreover, it is largely unclear whether lysosomal and cytosolic esterases cleave ester-containing linkers upon ADC internalization. Due to our interest in the targeted delivery of immune-modulators, our team has recently prepared a series of ester-linked dexamethasone ADCs. Herein, we report our studies of the functional activity of these ADCs, with a particular focus on their catabolism in various biological milieu. We found that esters are selectively but inefficiently cleaved upon cellular uptake, likely by cytosolic esterases. Lysosomal catabolism studies indicate that, in spite of the strong proteolytic activity, very little cleavage of ester-containing linkers occurs in the lysosome. However, ADCs bearing the ester-linked payloads are active in various immune-suppressive assays, suggesting that cytosolic cleavage is taking place. This was confirmed through LCMS quantitation of the payload following cell lysis. Finally, the stability of the ester linkage was evaluated in mouse and human plasma. We found, similar to other reports, there is a significant site-dependence on the cleavage. Esters attached at highly exposed sites, such as 443C, were rapidly cleaved in plasma while esters at more hindered sites, such at 334C, were not. Together, these results help to unravel the complexities of ester-incorporation into ADC linkers and pave a path forward for their utility in ADC applications.