A compound of formula: wherein: R 21 is selected from the group consisting of H, -CH 2 OH, -CH 2 CH 2 OH, -CH(CH 3)OH, -CH 2 OPO 2, -CH 2 C(=O)NH 2, -CH 2 CH 2 C(=O)OH and -CH 2 CH 2 C(=O)OR 8, wherein any one of the alkyl hydrogens can be replaced with a halogen; R 22 is H, C 1 -C 6 aliphatic, an amino protecting group, L 3 -C(=O)-, or A 2; L 1 and L 2 are each independently C 6 -C 21 aliphatic or C 5 -C 20 heteroaliphatic; L 3 is C 1 -C 21 aliphatic or C 2 -C 20 heteroaliphatic; A 2 is an amino acid or a peptide; R 23 is H or C 1 -C 6 aliphatic; R 24a and R 25a are each independently selected from C 1 -C 6 aliphatic and C 1 -C 6 heteroaliphatic and R 24b and R 25b are each independently selected from H, C 1 -C 6 aliphatic and C 1 -C 6 heteroaliphatic, or R 24a and R 24b together with the carbon atom to which they are attached form a C 3-8 cycloalkyl or 3-8 membered heterocyclyl group, and/or R 25a and R 25b together with the carbon atom to which they are attached form a C 3-8 cycloalkyl or 3-8 membered heterocyclyl group; X is selected from -S-, -S(=O)- and -S(=O) 2 -; v is an integer from 1-3 R 26 and R 27 are each independently selected from H and C 1 -C 6 aliphatic; Z 1 and Z 2 are independently selected from the group consisting of -C(=O)O-, -OC(=O)-, -C(=O)NR-, -NRC(=O)-, -C(=O)S-, -SC(=O)-, -OC(=O)O-, -NRC(=O)O-, -OC(=O)NR-, and -NRC(=O)NR-; PEG is a polyethylene glycol; wherein any aliphatic, heteroaliphatic, cycloalkyl and heterocyclyl present in any of R 21, R 22, R 23, R 24a, R 24b, R 25a, R 25b, R 26, R 27, L 1, L 2 and L 3 is optionally substituted or a pharmaceutically acceptable salt, solvate or prodrug thereof. R 21 is selected from the group consisting of H, -CH 2 OH, -CH 2 CH 2 OH, -CH(CH 3)OH, -CH 2 OPO 2, -CH 2 C(=O)NH 2, -CH 2 CH 2 C(=O)OH and -CH 2 CH 2 C(=O)OR 8, wherein any one of the alkyl hydrogens can be replaced with a halogen; R 22 is H, C 1 -C 6 aliphatic, an amino protecting group, L 3 -C(=O)-, or A 2; L 1 and L 2 are each independently C 6 -C 21 aliphatic or C 5 -C 20 heteroaliphatic; L 3 is C 1 -C 21 aliphatic or C 2 -C 20 heteroaliphatic; A 2 is an amino acid or a peptide; R 23 is H or C 1 -C 6 aliphatic; R 24a and R 25a are each independently selected from C 1 -C 6 aliphatic and C 1 -C 6 heteroaliphatic and R 24b and R 25b are each independently selected from H, C 1 -C 6 aliphatic and C 1 -C 6 heteroaliphatic, or R 24a and R 24b together with the carbon atom to which they are attached form a C 3-8 cycloalkyl or 3-8 membered heterocyclyl group, and/or R 25a and R 25b together with the carbon atom to which they are attached form a C 3-8 cycloalkyl or 3-8 membered heterocyclyl group; X is selected from -S-, -S(=O)- and -S(=O) 2 -; v is an integer from 1-3 R 26 and R 27 are each independently selected from H and C 1 -C 6 aliphatic; Z 1 and Z 2 are independently selected from the group consisting of -C(=O)O-, -OC(=O)-, -C(=O)NR-, -NRC(=O)-, -C(=O)S-, -SC(=O)-, -OC(=O)O-, -NRC(=O)O-, -OC(=O)NR-, and -NRC(=O)NR-; PEG is a polyethylene glycol; wherein any aliphatic, heteroaliphatic, cycloalkyl and heterocyclyl present in any of R 21, R 22, R 23, R 24a, R 24b, R 25a, R 25b, R 26, R 27, L 1, L 2 and L 3 is optionally substituted or a pharmaceutically acceptable salt, solvate or prodrug thereof.