A compound of formula: or a salt thereof, wherein: R 1 is 2-hydroxy phenyl that is optionally substituted with one or more groups independently selected from the group consisting of halo; R 2 is H, C 1-6 alkyl, or -C(=O)-C 1-6 alkyl; R 3 is H or C 1-6 alkyl; R 4 is selected from the group consisting of -R b, -O-R b, -N(R b) 2, -S 2 R b, and -C -N(R b) 2; each R b is independently selected from the group consisting of hydrogen, C 1-6 alkyl, C 2-6 alkenyl, C 2-6 alkynyl, 3-15 membered carbocyclyl, and 3-15 membered heterocyclyl, wherein each C 1-6 alkyl, C 2-6 alkenyl, C 2-6 alkynyl, 3-15 membered carbocyclyl, and 3-15 membered heterocyclyl is optionally substituted with one or more groups independently selected from the group consisting of R c, oxo, halo, -NO 2, -N(R c) 2, -CN, -C -N(R c) 2, -S -N(R c) 2, -S 2 -N(R c) 2, -O-R c, -S-R c, -O-C -R c, -C -R c, -C -OR c, -S -R c, -S 2 -R c, -N(R c)-C -R c, -N(R c)-S -R c, -N(R c)-C -N(R c) 2, and -N(R c)-S 2 -R c; each R c is independently selected from the group consisting of hydrogen, C 1-6 alkyl, C 2-6 alkenyl, C 2-6 alkynyl, C 1-6 alkoxy, 3-15 membered carbocyclyl, and 3-15 membered heterocyclyl, wherein each C 1-6 alkyl, C 2-6 alkenyl, C 2-6 alkynyl, C 1-6 alkoxy, 3-15 membered carbocyclyl, and 3-15 membered heterocyclyl is optionally substituted with one or more groups independently selected from R d; or two R c are taken together with the nitrogen to which they are attached to form a heterocyclyl that is optionally substituted with one or more groups independently selected from the group consisting of oxo, halo and C 1-3 alkyl that is optionally substituted with one or more groups independently selected from the group consisting of oxo and halo; each R d is independently selected from the group consisting of oxo, halo, -NO 2, -N(R e) 2, -CN, -C -N(R e) 2, -S -N(R e) 2, -S 2 -N(R e) 2, -O-R e, -S-R e, -O-C -R e, -C -R e, -C -OR e, -S -R e, -S 2-R e, -N(R e)-C -R e, -N(R e)-S -R e, -N(R e)-C -N(R e) 2, -N(R e)-S 2 -R e, C 1-6 alkyl, C 2-6 alkenyl, C 2-6 alkynyl, 3-15 membered carbocyclyl, and 3-15 membered heterocyclyl, wherein any C 1-6 alkyl, C 2-6 alkenyl, C 2-6 alkynyl, 3-15 membered carbocyclyl, and 3-15 membered heterocyclyl is optionally substituted with one or more groups independently selected from the group consisting of R e, oxo, halo, -NO 2, -N(R e) 2, -CN, -C -N(R e) 2, -S -N(R e) 2, -S 2 -N(R e) 2, -O-R e, -S-R e, -O-C -R e, -C -R e, -C -OR e, -S -R e, -S 2 -R e, -N(R e)-C -R e, -N(R e)-S -R e, -N(R e)-C -N(R e) 2, and -N(R e)-S 2 -R e; and each R e is independently selected from the group consisting of hydrogen, C 1-6 alkyl, C 2-6 alkenyl, C 2-6 alkynyl, carbocyclyl, and carbocyclyl(C 1-3 alkyl)-; or a compound of formula or or a salt thereof; provided the compound of formula is not 2-(6-aminopyridazin-3-yl)phenol or 2-(6-(methylamino)pyridazin-3-yl)phenol. R 1 is 2-hydroxy phenyl that is optionally substituted with one or more groups independently selected from the group consisting of halo; R 2 is H, C 1-6 alkyl, or -C(=O)-C 1-6 alkyl; R 3 is H or C 1-6 alkyl; R 4 is selected from the group consisting of -R b, -O-R b, -N(R b) 2, -S 2 R b, and -C -N(R b) 2; each R b is independently selected from the group consisting of hydrogen, C 1-6 alkyl, C 2-6 alkenyl, C 2-6 alkynyl, 3-15 membered carbocyclyl, and 3-15 membered heterocyclyl, wherein each C 1-6 alkyl, C 2-6 alkenyl, C 2-6 alkynyl, 3-15 membered carbocyclyl, and 3-15 membered heterocyclyl is optionally substituted with one or more groups independently selected from the group consisting of R c, oxo, halo, -NO 2, -N(R c) 2, -CN, -C -N(R c) 2, -S -N(R c) 2, -S 2 -N(R c) 2, -O-R c, -S-R c, -O-C -R c, -C -R c, -C -OR c, -S -R c, -S 2 -R c, -N(R c)-C -R c, -N(R c)-S -R c, -N(R c)-C -N(R c) 2, and -N(R c)-S 2 -R c; each R c is independently selected from the group consisting of hydrogen, C 1-6 alkyl, C 2-6 alkenyl, C 2-6 alkynyl, C 1-6 alkoxy, 3-15 membered carbocyclyl, and 3-15 membered heterocyclyl, wherein each C 1-6 alkyl, C 2-6 alkenyl, C 2-6 alkynyl, C 1-6 alkoxy, 3-15 membered carbocyclyl, and 3-15 membered heterocyclyl is optionally substituted with one or more groups independently selected from R d; or two R c are taken together with the nitrogen to which they are attached to form a heterocyclyl that is optionally substituted with one or more groups independently selected from the group consisting of oxo, halo and C 1-3 alkyl that is optionally substituted with one or more groups independently selected from the group consisting of oxo and halo; each R d is independently selected from the group consisting of oxo, halo, -NO 2, -N(R e) 2, -CN, -C -N(R e) 2, -S -N(R e) 2, -S 2 -N(R e) 2, -O-R e, -S-R e, -O-C -R e, -C -R e, -C -OR e, -S -R e, -S 2-R e, -N(R e)-C -R e, -N(R e)-S -R e, -N(R e)-C -N(R e) 2, -N(R e)-S 2 -R e, C 1-6 alkyl, C 2-6 alkenyl, C 2-6 alkynyl, 3-15 membered carbocyclyl, and 3-15 membered heterocyclyl, wherein any C 1-6 alkyl, C 2-6 alkenyl, C 2-6 alkynyl, 3-15 membered carbocyclyl, and 3-15 membered heterocyclyl is optionally substituted with one or more groups independently selected from the group consisting of R e, oxo, halo, -NO 2, -N(R e) 2, -CN, -C -N(R e) 2, -S -N(R e) 2, -S 2 -N(R e) 2, -O-R e, -S-R e, -O-C -R e, -C -R e, -C -OR e, -S -R e, -S 2 -R e, -N(R e)-C -R e, -N(R e)-S -R e, -N(R e)-C -N(R e) 2, and -N(R e)-S 2 -R e; and each R e is independently selected from the group consisting of hydrogen, C 1-6 alkyl, C 2-6 alkenyl, C 2-6 alkynyl, carbocyclyl, and carbocyclyl(C 1-3 alkyl)-; or a compound of formula or or a salt thereof; provided the compound of formula is not 2-(6-aminopyridazin-3-yl)phenol or 2-(6-(methylamino)pyridazin-3-yl)phenol.