Table 1.

(Apparent) binding affinity (pK_i) of ligands for hH₃R expressed on HEK293T cell homogenates, and the amount of unlabeled receptors remaining after 1 h preincubation of hH₃R expressing membranes with 100 µM ligand and subsequent washout as determined by radioligand binding. Ligands are considered potential covalent binders when % unlabeled receptors remaining after 1 h pre-incubation with ligand was <50%. Data are mean ± S.E.M of 3 experiments performed in triplicate with the exception of 1 for which mean ± SD is reported for two experiments performed in triplicate.

Cmpd.	Central Ring	R1	R2	R3	pKi	% Unlabeled Receptors
2	Pyrimidine	A	Me	phenyl	7.1 ± 0.1	84 ± 16
1	Pyrimidine	A	CH=CH2	phenyl	< 5	41 ± 6 (SD)
10	Pyrimidine	B	NH2	iPr	7.3 ± 0.1	100 ± 0.0
34	Pyrimidine	B	Me	iPr	< 5	87 ± 13
35	Pyrimidine	B	Et	iPr	< 5	93 ± 7
21	Pyrimidine	B	CH=CH2	iPr	< 5	85 ± 15
11	Pyrimidine	C	NH2	iPr	6.7 ± 0.1	96 ± 4
36	Pyrimidine	C	Me	iPr	5.9 ± 0.1	96 ± 3
37	Pyrimidine	C	Et	iPr	5.5 ± 0.1	89 ± 11
22	Pyrimidine	C	CH=CH2	iPr	5.5 ± 0.0	41 ± 9
12	Pyrimidine	C	NH2	Me	6.5 ± 0.1	91 ± 5
23	Pyrimidine	C	CH=CH2	Me	5.4 ± 0.1	40 ± 8
20	Pyrimidine	C	C≡CH	Me	< 5	80 ± 2
42	Phenyl	C	NH2	-	5.9 ± 0.3	72 ± 8
43	Phenyl	C	NH-CO-CH2-Cl	-	6.2 ± 0.1	86 ± 5
44	Phenyl	C	N=C=S	-	6.5 ± 0.3	15 ± 2