Table 1. Heteroatom Analogues of STLC^a.

compd	X	R	inhibition of basal ATPase activity, Kiapp (nM)	ligand efficiency	K562 cells, GI50 (nM)
1 (STLC)	S	(R)-CO2H	135.9 ± 20.5	0.36	1452 ± 76
2	S	H	245.5 ± 51.2	0.39	2286 ± 213
6	O	H	2157.6 ± 342.2	0.34	nd
7	NH	(S)-CO2H	3018.5 ± 398.5	0.29	nd
8	NH	H	2659.4 ± 295.6	0.33	nd
rac-9	CH2	CO2H	311.6 ± 53.9	0.34	865 ± 131
(S)-9	CH2	(S)-CO2H	416.5 ± 64.2	0.33	2065 ± 168
(R)-9	CH2	(R)-CO2H	173.5 ± 24.5	0.35	776 ± 26
10	CH2	H	214.7 ± 30.1	0.40	577 ± 61

^and = not determined.