NT5E and head and neck squamous cell carcinoma: Prior studieshave also highlighted PSB-12379 (2) and PSB-12489 (3) as potent, selective, and metabolically stable CD73 inhibitors.30,31 Compound 5, a pyrimidine nucleoside methylenediphosphonatederivative, was later identified as a novel CD73 inhibitor, exhibitingconcentration-dependent inhibition in vitro againsthuman head and neck squamous cell carcinoma.33 Replacement of the methylenediphosphonic acid moiety by both unmodifiedand modified methylenephosphonic acid moieties resulted in compounds 6 and OP-5244 (7), respectively.