Structure activity relationship of 3-methylcytidine-5'-α,β-methylenediphosphates as CD73 inhibitors.

Adenosine in the tumor microenvironment acts on A and A adenosine receptors (ARs) on immune cells (T cells, dendritic cells, NK cells, macrophages and neutrophils) to prevent their activation. Therefore, a means of lowering extracellular adenosine would be beneficial in cancer immunotherapy either as a co-therapy or monotherapy. Inhibitors of CD73 (ecto-5'-nucleotidase, the main extracellular adenosine-forming enzyme) are already in clinical trials for cancer immunotherapy. Many purine (ADP) analogues have been shown to inhibit this enzyme, but we have explored pyrimidine (UDP/CDP) analogues. We recently reported N -substituted 3-methylcytidine-5'-α,β-methylenediphosphates as potent CD73 inhibitors. We now expand the structure activity relationship (SAR) of pyrimidine nucleotides as human CD73 inhibitors, focusing mainly on cytidine functionalization at the N position. 4-Chloro (MRS4598 16, K 0.673 nM) and 4-iodo (MRS4620 18, K 0.436 nM) substitution of the N -benzyloxy group increased inhibitory potency K by ~20-fold. Primary alkylamine derivatives coupled through a p-amido group and varying methylene chain length (24and 25) were functionalized congeners, for subsequent conjugation to carrier or reporter moieties. hCD73 X-ray structures with two of the newly synthesized inhibitors indicated a ribose ring conformational adaptation, and the benzyloxyimino group (E configuration) binds to the same region (between C-terminal and N-terminal domains) as N -benzyl groups in potent adenine-derived inhibitors. Molecular dynamics simulation identified stabilizing interactions and predicted conformational diversity around the N -benzyloxy moiety. Thus, by structure-guided substitution of the N -benzyloxy moiety, we have greatly enhanced their inhibitory potency and added functionality enabling molecular probes. Their potential as anticancer drugs was confirmed by blocking CD73 activity in human head and neck squamous cell carcinoma (HNSCC) and palatine tonsils in situ.