2-Substituted α,β-Methylene-ADP Derivatives: Potent Competitive Ecto-5′-nucleotidase (CD73) Inhibitors with Variable Binding Modes

CD73 inhibitors are promising drugs for the (immuno)therapy of cancer. Here, we present the synthesis, structure–activity relationships, and cocrystal structures of novel derivatives of the competitive CD73 inhibitor α,β-methylene-ADP (AOPCP) substituted in the 2-position. Small polar or lipophilic residues increased potency, 2-iodo- and 2-chloro-adenosine-5′-O-[(phosphonomethyl)phosphonic acid] (15, 16) being the most potent inhibitors with Ki values toward human CD73 of 3–6 nM. Subject to the size and nature of the 2-substituent, variable binding modes were observed by X-ray crystallography. Depending on the binding mode, large species differences were found, e.g., 2-piperazinyl-AOPCP (21) was >12-fold less potent against rat CD73 compared to human CD73. This study shows that high CD73 inhibitory potency can be achieved by simply introducing a small substituent into the 2-position of AOPCP without the necessity of additional bulky N6-substituents. Moreover, it provides valuable insights into the binding modes of competitive CD73 inhibitors, representing an excellent basis for drug development.