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(O)- and -C(O)-O-; Z is selected from the group consisting of-O- and -S-; R 1 is selected from the group consisting of -C(O)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(O)R b , -C(O)OR a , -OC(O)R b , -N(R a ) 2 , -NR a C(O)R b , -C(O)N(R a ) 2 , -S(O)R b , -S(O) 2 R b , -S(O) 2 OR a , -S(O) 2 N(R a ) 2 , and -NR a S(O) 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(O)- and -C(O)-O-; Z is selected from the group consisting of-O- and -S-; R 1 is selected from the group consisting of -C(O)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(O)R b , -C(O)OR a , -OC(O)R b , -N(R a ) 2 , -NR a C(O)R b , -C(O)N(R a ) 2 , -S(O)R b , -S(O) 2 R b , -S(O) 2 OR a , -S(O) 2 N(R a ) 2 , and -NR a S(O) 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. <EOS>