Radiolabeled glycine transporter inhibitors

1. A compound of the formula:

wherein:

A is selected from N and CH;

R 2a and R 2b are independently selected from the group consisting of:

(1) hydrogen,

(2) fluoro,

(3) chloro, and

(4) bromo;

R 3 is C 1-6 alkyl, which is unsubstituted or substituted with fluoro;

R 4 is selected from the group consisting of:

(1) hydrogen, and

(2) C 1-3 alkyl;

one of X and Y is selected from the group consisting of:

(1) 18 F,

(2) —O( 11 CH 3 ), and

(3) —O(CD 2 18 F),

and the other of X and Y is hydrogen;

or a pharmaceutically acceptable salt thereof or an individual enantiomer or diastereomer thereof.

2. The compound of claim 1 of the formula Ia:

wherein R 4 is C 1-3 alkyl;

or a pharmaceutically acceptable salt thereof.

3. The compound of claim 1 of the formula Ib:

or a pharmaceutically acceptable salt thereof.

4. The compound of claim 1 wherein R 2a and R 2b are independently selected from the group consisting of:

(1) hydrogen,

(2) fluoro, and

(3) chloro.

5. The compound of claim 4 wherein R 2a is chloro and R 2b is chloro.

6. The compound of claim 4 wherein R 2a is chloro and R 2b is hydrogen.

7. The compound of claim 4 wherein R 2a is fluoro and R 2b is hydrogen.

8. The compound of claim 1 wherein R 3 is —CH 2 CH 3 .

9. The compound of claim 1 wherein X is 18 F and Y is hydrogen.

10. The compound of claim 1 wherein X is hydrogen and Y is 18 F.

11. The compound of claim 1 wherein X is —O( 11 CH 3 ) and Y is hydrogen.

12. The compound of claim 1 wherein X is hydrogen and Y is —O(CD 2 18 F).

13. A compound which is selected from the group consisting of:

or a pharmaceutically acceptable salt thereof.

14. A radiopharmaceutical composition which comprises the compound of claim 1 and a pharmaceutically acceptable carrier or excipient.