Structure–Activity Relationship of Purine and Pyrimidine Nucleotides as Ecto-5′-Nucleotidase (CD73) Inhibitors

Cluster of differentiation
73 (CD73) converts adenosine 5′-monophosphate
to immunosuppressive adenosine, and its inhibition was proposed as
a new strategy for cancer treatment. We synthesized 5′-O-[(phosphonomethyl)­phosphonic acid] derivatives of purine
and pyrimidine nucleosides, which represent nucleoside diphosphate
analogues, and compared their CD73 inhibitory potencies. In the adenine
series, most ribose modifications and 1-deaza and 3-deaza were detrimental,
but 7-deaza was tolerated. Uracil substitution with N3-methyl, but not larger groups, or 2-thio, was tolerated.
1,2-Diphosphono-ethyl modifications were not tolerated. N4-(Aryl)­alkyloxy-cytosine derivatives, especially with
bulky benzyloxy substituents, showed increased potency. Among the
most potent inhibitors were the 5′-O-[(phosphonomethyl)­phosphonic
acid] derivatives of 5-fluorouridine (4l), N4-benzoyl-cytidine (7f), N4-[O-(4-benzyloxy)]-cytidine (9h), and N4-[O-(4-naphth-2-ylmethyloxy)]-cytidine
(9e) (Ki values 5–10
nM at human CD73). Selected compounds tested at the two uridine diphosphate-activated
P2Y receptor subtypes showed high CD73 selectivity, especially those
with large nucleobase substituents. These nucleotide analogues are
among the most potent CD73 inhibitors reported and may be considered
for development as parenteral drugs.