ecto-5'-Nucleotidase (eN, CD73) catalyzes the hydrolysis of extracellular AMP to adenosine. eN inhibitors have potential for use as cancer therapeutics. The eN inhibitor alpha,beta-methylene-ADP (AOPCP, adenosine-5'-O-[(phosphonomethyl)phosphonic acid]) was used as a lead structure, and derivatives modified in various positions were prepared. Products were tested at rat recombinant eN. 6-(Ar)alkylamino substitution led to the largest improvement in potency. N-6-Monosubstitution was superior to symmetrical N-6,N-6-disubstitution. The most potent inhibitors were N-6-(4chlorobenzyl)-(10l, PSB-12441, K-i 7.23 n.M), N-6-phenylethyl(10h, PSB-12425, K-i 8.04 nM), and N-6-benzyl-adenosine-5'-O[(phosphonomethyl)phosphonic acid] (10g, PSB-12379, K-i 9.03 nM). Replacement of the 6-NH group in 10g by 0 (10q, PSB-12431) or S (10r, PSB-12553) yielded equally potent inhibitors (10q, 9.20 nM; 10r, 9.50 aM). Selected compounds investigated at the human enzyme did not show species differences; they displayed high selectivity versus other ecto-nudeotidases and ADP-activated P2Y receptors. Moreover, high metabolic stability was observed. These compounds represent the most potent eN inhibitors described to date.