A compound of the formula or a pharmaceutically acceptable salt thereof: wherein: R 1 is selected from C 1-12 alkyl, C 3-12 alkenyl, C 3-12 alkynyl, C 3-8 cycloalkyl, and heterocyclyl, each of which is optionally substituted with one or more moieties selected from the group consisting of halo, OH, CN, hydroxy-C 1-12 alkyl, halo-C 1-12 alkyl, amino-C 1-12 alkyl, C 1-12 alkoxy, aryloxy, C 1-12 thioalkoxy, nitro, sulfonato, C 1-12 acyl, C 2-12 acyloxy, carboxyl, mercapto, C 1-12 alkoxy-carbonyl, C 1-12 alkoxy-carbonyloxy, amido, amino, C 1-12 alkylamino, and di-C 1-12 alkyl-amino; R 2 and R 3 are the same or different and each is selected from hydrogen, C 1-12 alkyl, C 2-12 alkenyl, C 2-12 alkynyl, C 3-8 cycloalkyl, heterocyclyl, aryl, and heteroaryl, each of which, other than hydrogen, is optionally substituted with one or more moieties selected from the group consisting of halo, OH, CN, hydroxy-C 1-12 alkyl, halo-C 1-12 alkyl, amino-C 1-12 alkyl, C 1-12 alkoxy, aryloxy, C 1-12 thioalkoxy, nitro, sulfonato, C 1-12 acyl, C 2-12 acyloxy, carboxyl, mercapto, C 1-12 alkoxy-carbonyl, C 1-12 alkoxy-carbonyloxy, amido, amino, C 1-12 alkylamino, and di-C 1-2 alkyl-amino; preferably wherein R 2 and R 3 are the same or different and each is hydrogen, C 1-12 alkyl, aryl, or heteroaryl, each of which, other than hydrogen, is optionally substituted; more preferably wherein R 2 and R 3 are hydrogen; and R 4 is -C(=O)R 5, wherein R 5 is selected from hydrogen, C 1-12 alkyl, C 2-12 alkenyl, C 2-12 alkynyl, C 3-8 cycloalkyl, heterocyclyl, aryl, and heteroaryl, each of which, other than hydrogen, is optionally substituted with one or more moieties selected from the group consisting of halo, OH, CN, hydroxy-C 1-12 alkyl, halo-C 1-12 alkyl, amino-C 1-12 alkyl, C 1-12 alkoxy, aryloxy, C 1-12 thioalkoxy, nitro, sulfonato, C 1-12 acyl, C 2-12 acyloxy, carboxyl, mercapto, C 1-12 alkoxy-carbonyl, C 1-12 alkoxy-carbonyloxy, amido, amino, C 1-12 alkylamino, and di-C 1-12 alkyl-amino; or R 4 is a non-steroidal anti-inflammatory drug (NSAID) moiety retaining its NSAID activity; preferably wherein R 4 is an NSAID moiety retaining its NSAID activity. R 1 is selected from C 1-12 alkyl, C 3-12 alkenyl, C 3-12 alkynyl, C 3-8 cycloalkyl, and heterocyclyl, each of which is optionally substituted with one or more moieties selected from the group consisting of halo, OH, CN, hydroxy-C 1-12 alkyl, halo-C 1-12 alkyl, amino-C 1-12 alkyl, C 1-12 alkoxy, aryloxy, C 1-12 thioalkoxy, nitro, sulfonato, C 1-12 acyl, C 2-12 acyloxy, carboxyl, mercapto, C 1-12 alkoxy-carbonyl, C 1-12 alkoxy-carbonyloxy, amido, amino, C 1-12 alkylamino, and di-C 1-12 alkyl-amino; R 2 and R 3 are the same or different and each is selected from hydrogen, C 1-12 alkyl, C 2-12 alkenyl, C 2-12 alkynyl, C 3-8 cycloalkyl, heterocyclyl, aryl, and heteroaryl, each of which, other than hydrogen, is optionally substituted with one or more moieties selected from the group consisting of halo, OH, CN, hydroxy-C 1-12 alkyl, halo-C 1-12 alkyl, amino-C 1-12 alkyl, C 1-12 alkoxy, aryloxy, C 1-12 thioalkoxy, nitro, sulfonato, C 1-12 acyl, C 2-12 acyloxy, carboxyl, mercapto, C 1-12 alkoxy-carbonyl, C 1-12 alkoxy-carbonyloxy, amido, amino, C 1-12 alkylamino, and di-C 1-2 alkyl-amino; preferably wherein R 2 and R 3 are the same or different and each is hydrogen, C 1-12 alkyl, aryl, or heteroaryl, each of which, other than hydrogen, is optionally substituted; more preferably wherein R 2 and R 3 are hydrogen; and R 4 is -C(=O)R 5, wherein R 5 is selected from hydrogen, C 1-12 alkyl, C 2-12 alkenyl, C 2-12 alkynyl, C 3-8 cycloalkyl, heterocyclyl, aryl, and heteroaryl, each of which, other than hydrogen, is optionally substituted with one or more moieties selected from the group consisting of halo, OH, CN, hydroxy-C 1-12 alkyl, halo-C 1-12 alkyl, amino-C 1-12 alkyl, C 1-12 alkoxy, aryloxy, C 1-12 thioalkoxy, nitro, sulfonato, C 1-12 acyl, C 2-12 acyloxy, carboxyl, mercapto, C 1-12 alkoxy-carbonyl, C 1-12 alkoxy-carbonyloxy, amido, amino, C 1-12 alkylamino, and di-C 1-12 alkyl-amino; or R 4 is a non-steroidal anti-inflammatory drug (NSAID) moiety retaining its NSAID activity; preferably wherein R 4 is an NSAID moiety retaining its NSAID activity.