Quantification of intracellular cAMP in RBCs and K562-Lu cells. cAMP levels were measured using an enzyme immunoassay. (A) cAMP levels in 4 AA and 7 SS RBC samples. Each dot represents the measured cAMP value in 10⁸ RBCs from one patient. The horizontal lines are medians. (B) cAMP values measured in 7 patients during HU treatment at D0, D15, M2, and more than M6. The decrease in cAMP levels was significant (P = .002; Friedman test). (C) Decreased cAMP values measured in K562-Lu cells incubated with HU for 24 and 48 hours.