Application of a bivalent “click” approach to target tyrosyl-DNA phosphodiesterase 1 (TDP1)†

Although inhibiting the DNA repair enzyme tyrosyl-DNA phosphodiesterase 1 (TDP1) synergizes with topoisomerase type I (TOP1) inhibitors in anticancer therapy, development of TDP1 inhibitors has been highly challenging. This may be due to the open and shallow nature of the TDP1 catalytic site and the necessity of competing with a large and highly extended substrate. The toolbox available to chemical biologists for studying TDP1 could be significantly enhanced by introducing the ability to selectively eliminate TDP1 using protein degraders. Our current work starts from phenyl imidazopyridine-based TDP1 inhibitors previously developed from small molecule microarrays (SMMs). Using crystal structures of lead inhibitors bound to TDP1, we designed and synthesized a series of bivalent proteolysis-targeting chimeras (PROTACs). The focus of our current work is to explore synthetic approaches that permit installation of E3 ligase-targeting functionality, while retaining the TDP1 binding. We employed copper-catalyzed azide–alkyne cycloaddition (CuAAC) “click” reactions to assemble PROTAC constituents with 1,2,3-triazole-containing linkers. With the addition of the relatively large parts of the linkers and E3-targeting moieties, we retained the ability to inhibit TDP1. The successful development of TDP1-directed PROTACS would yield a new therapeutic class that could potentially enhance the efficacy and selectivity of TOP1 inhibitors including those used as payloads in antibody drug conjugates (ADCs).