Design, Synthesis, and Evaluation of a Cross-Linked Oligonucleotide as the First Nanomolar Inhibitor of APOBEC3A

[Image: see text] Drug resistance is a major problem associated with anticancer chemo- and immunotherapies. Recent advances in the understanding of resistance mechanisms have revealed that enzymes of the APOBEC3 (A3) family contribute to the development of drug resistance in multiple cancers. A3 enzymes are polynucleotide cytidine deaminases that convert cytosine to uracil (C→U) in single-stranded DNA (ssDNA) and in this way protect humans against viruses and mobile retroelements. On the other hand, cancer cells use A3s, especially A3A and A3B, to mutate human DNA, and thus by increasing rates of evolution, cancer cells escape adaptive immune responses and resist drugs. However, as A3A and A3B are non-essential for primary metabolism, their inhibition opens up a strategy to augment existing anticancer therapies and suppress cancer evolution. To test our hypothesis that pre-shaped ssDNA mimicking the U-shape observed in ssDNA–A3 complexes can provide a better binder to A3 enzymes, a Cu(I)-catalyzed azide–alkyne cycloaddition was used to cross-link two distant modified nucleobases in ssDNA. The resultant cytosine-containing substrate, where the cytosine sits at the apex of the loop, was deaminated faster by the engineered C-terminal domain of A3B than a standard, linear substrate. The cross-linked ssDNA was converted into an A3 inhibitor by replacing the 2′-deoxycytidine in the preferred TCA substrate motif by 2′-deoxyzebularine, a known inhibitor of single nucleoside cytidine deaminases. This strategy yielded the first nanomolar inhibitor of engineered A3B(CTD) and wild-type A3A (K(i) = 690 ± 140 and 360 ± 120 nM, respectively), providing a platform for further development of powerful A3 inhibitors.