Alpha-synuclein ligands

1. A compound having the structure of Formula (I-a) or (I-b), or a pharmaceutically acceptable salt thereof:

wherein:

each R is hydrogen or substituted or unsubstituted C 1 -C 10 alkyl;

each A 1 is C—R 1 ;

each A 2 is C—R 2 ;

each A 3 is C—R 3 ;

each A 4 is C—R 4 ;

each A 5 is C—R 5 ;

each A 6 is C—R 6 ;

each A 7 is nitrogen;

each A 8 is C—R 8 ;

each A 9 is C—R 9 ;

each A 10 is C—R 10 ;

each A 11 is nitrogen;

each A 12 is C—R 12 ;

B is sulfur or oxygen;

each R 1 , R 2 , R 5 , R 6 , R 8 , R 9 , and R 12 is hydrogen;

each R 3 , R 4 , and R 10 is independently hydrogen, nitro, halo, cyano, hydroxy, carboxyl, substituted or unsubstituted C 1 to C 6 alkoxy, substituted or unsubstituted carboxylate, substituted or unsubstituted alkenyloxy, substituted or unsubstituted amino, substituted or unsubstituted thiourea, or substituted or unsubstituted amido,

wherein at least one of R 3 , R 4 , and R 10 is hydroxy or substituted or unsubstituted C 1 to C 6 alkoxy.

2. The compound of claim 1 wherein each R 4 is a halogen, a nitro, halo-substituted C 1 to C 6 alkoxy, or methoxy and each R 10 is methoxy.

3. The compound of claim 1 wherein each R 3 , R 4 , and R 10 is independently selected from the group consisting of hydrogen, a halo, a nitro, —CHO, —COCH 3 , —COOH, —CO 2 CH 3 , —OCH 3 , —OCH 2 CH 2 F, —OCH(CH 3 ) 2 , —OCH 2 OCH 3 , —NO 2 , —CN, —OH, and

4. The compound of claim 1 , selected from the group consisting of:

and pharmaceutically acceptable salts thereof.

5. The compound of claim 1 wherein the compound is radiolabeled with a synthetic radioactive isotope.

6. The compound of claim 5 wherein the synthetic radioactive isotope is selected from the group consisting of carbon-11, nitrogen-13, oxygen-15, fluorine-18, bromine-76, iodine-123, and iodine-125.

7. A pharmaceutical composition comprising a radiolabeled compound of claim 6 and at least one excipient.

8. A pharmaceutical composition comprising a compound of claim 1 wherein the composition comprises from about 0.001 mg to about 10 g of the compound and at least about 10 wt. % of the compound in the pharmaceutical composition is radiolabeled.

9. A method of diagnosing or monitoring a synucleinopathy in a subject comprising administering a pharmaceutical composition comprising the compound of claim 5 to a subject; and imaging the subject's brain by positron emission tomography.

10. The method of claim 9 wherein the subject is a mammal.

11. The method of claim 9 wherein the subject comprises a human suffering from Parkinson's disease.

12. The compound of claim 1 , having the structure corresponding to: