A compound according to Formula I: or a pharmaceutically acceptable salt thereof, wherein: the moiety -X-Y- is selected from the group consisting of -O-C - and -C -O-; Z is selected from the group consisting of-O- and -S-; R 1 is selected from the group consisting of -C R 1a, H, C 1-8 alkyl, C 3-8 cycloalkyl, and C 6-10 aryl or benzyl; R 1a is selected from the group consisting of -NR 1b R 1c, C 1-8 alkyl, C 3-8 cycloalkyl, C 6-10 aryl, benzyl, 3- to 8-membered heterocyclyl, 5- to 12-membered heteroaryl, -OR 1b, and a solubilizing moiety; R 1b and R 1c are independently selected from H, C 1-8 alkyl, C 3-8 cycloalkyl, and a solubilizing moiety; R 2 and R 3 are taken together to form a fused tricyclic moiety, or R 2 and R 3 are optionally independently selected from C 6-10 aryl or benzyl; R 4, R 5, and R 6 are independently selected from the group consisting of H and C 1-6 alkyl; R 1, R 2, R 3, R 4, R 5, and R 6 are optionally and independently substituted with one or more R 7; each R 7 is independently selected from the group consisting of C 1-4 alkyl, halogen, -CN, -OR a, -C R b, -C OR a, -OC R b, -N(R a) 2, -NR a C R b, -C N(R a) 2, -S R b, -S 2 R b, -S 2 OR a, -S 2 N(R a) 2, and -NR a S 2 R b; each R a is independently selected from the group consisting of H and C 1-4 alkyl; each R b is C 1-4 alkyl; and subscript t is 0, 1, 2, or 3; wherein each cycloalkyl moiety is independently saturated or partially unsaturated; and each solubilizing moiety is independently a monosaccharide, an oligosaccharide, a polysaccharide, a polyol, an oligo(ethylene glycol), a poly(ethylene glycol), a poly-trimethylene glycol, a poly(N-isopropylacrylamide), a polyvinylpyrrolidone, a polyoxyethylene-polyoxypropylene block copolymer, a protein, starch, cellulose, heparin, and hyaluronic acid. the moiety -X-Y- is selected from the group consisting of -O-C - and -C -O-; Z is selected from the group consisting of-O- and -S-; R 1 is selected from the group consisting of -C R 1a, H, C 1-8 alkyl, C 3-8 cycloalkyl, and C 6-10 aryl or benzyl; R 1a is selected from the group consisting of -NR 1b R 1c, C 1-8 alkyl, C 3-8 cycloalkyl, C 6-10 aryl, benzyl, 3- to 8-membered heterocyclyl, 5- to 12-membered heteroaryl, -OR 1b, and a solubilizing moiety; R 1b and R 1c are independently selected from H, C 1-8 alkyl, C 3-8 cycloalkyl, and a solubilizing moiety; R 2 and R 3 are taken together to form a fused tricyclic moiety, or R 2 and R 3 are optionally independently selected from C 6-10 aryl or benzyl; R 4, R 5, and R 6 are independently selected from the group consisting of H and C 1-6 alkyl; R 1, R 2, R 3, R 4, R 5, and R 6 are optionally and independently substituted with one or more R 7; each R 7 is independently selected from the group consisting of C 1-4 alkyl, halogen, -CN, -OR a, -C R b, -C OR a, -OC R b, -N(R a) 2, -NR a C R b, -C N(R a) 2, -S R b, -S 2 R b, -S 2 OR a, -S 2 N(R a) 2, and -NR a S 2 R b; each R a is independently selected from the group consisting of H and C 1-4 alkyl; each R b is C 1-4 alkyl; and subscript t is 0, 1, 2, or 3; wherein each cycloalkyl moiety is independently saturated or partially unsaturated; and each solubilizing moiety is independently a monosaccharide, an oligosaccharide, a polysaccharide, a polyol, an oligo(ethylene glycol), a poly(ethylene glycol), a poly-trimethylene glycol, a poly(N-isopropylacrylamide), a polyvinylpyrrolidone, a polyoxyethylene-polyoxypropylene block copolymer, a protein, starch, cellulose, heparin, and hyaluronic acid.