Metabolism probes for therapy and diagnosis

A compound of the following formula:

• wherein:
• R1 is a bond, optionally substituted C1-C6 alkyl, or optionally substituted SO2—(C1-C6 alkyl);
• R2 is optionally substituted aryl, optionally substituted heteroaryl, optionally substituted heterocycle, or optionally substituted heterocycloalkyl;
• R3 is H or optionally substituted C1-C6 alkyl;
• and
• R4 is NH2 or NHBoc;
• or pharmaceutically acceptable salts or derivatives thereof.