A cytotoxic compound represented by the following formula: or a pharmaceutically acceptable salt thereof, wherein: L' is represented by the following formula: -NR 5 -P-C(=O)-(CR a R b) m -J; -NR 5 -P-C(=O)-Cy-(CR a R b) m' -J; or -NR 5 -P-C(=O)-(CR a R b) m -S-Z s; or wherein: P is an amino acid residue or a peptide containing between 2 to 20 amino acid residues; J is N-hydroxysuccinimde ester, N-hydroxy sulfosuccinimide ester, nitrophenyl ester, dinitrophenyl ester, sulfo-tetraflurophenyl ester, and pentafluorophenyl ester; R a and R b, for each occurrence, are each independently -H, (C 1 -C 3)alkyl or a charged substituent or an ionizable group Q, wherein Q is SO 3 H or a pharmaceutically acceptable salt thereof; m is an integer from 1 to 6; m' is 0 or an integer from 1 to 6; Cy is a cyclic alkyl having 5 or 6 ring carbon atoms optionally substituted with halogen, -OH, (C 1 -C 3)alkyl, (C 1 -C 3)alkoxy, or halo(C 1 -C 3)alkyl; and Z s is -H, -SR d, -C(=O)R d1 or is selected from any one of the following formulas: or wherein: q is an integer from 1 to 5; n' is an integer from 2 to 6; U is -H or SO 3 M; M is H +, Na + or K +; R d is a linear or branched alkyl having 1 to 6 carbon atoms or is selected from phenyl, nitrophenyl, dinitrophenyl, carboxynitrophenyl, pyridyl or nitropyridyl; and R d1 is a linear or branched alkyl having 1 to 6 carbon atoms; L" and L'" are both -H; the double line between N and C represents a single bond or a double bond, provided that when it is a double bond X is absent and Y is -H, or a linear or branched alkyl having 1 to 4 carbon atoms, and when it is a single bond, X is -H, Y is -OH or - SO 3 M; R 1, R 2, R 3, R 4, R 1 ', R 2 ', R 3 ' and R 4 ' are all -H; R 6 is -OMe; X' and Y' are both -H; A and A' are -O-; and M is H, Na + or K +; and R 5 for each occurrence is independently -H or an optionally substituted linear or branched alkyl having 1 to 10 carbon atoms. L' is represented by the following formula: -NR 5 -P-C(=O)-(CR a R b) m -J; -NR 5 -P-C(=O)-Cy-(CR a R b) m' -J; or -NR 5 -P-C(=O)-(CR a R b) m -S-Z s; or wherein: P is an amino acid residue or a peptide containing between 2 to 20 amino acid residues; J is N-hydroxysuccinimde ester, N-hydroxy sulfosuccinimide ester, nitrophenyl ester, dinitrophenyl ester, sulfo-tetraflurophenyl ester, and pentafluorophenyl ester; R a and R b, for each occurrence, are each independently -H, (C 1 -C 3)alkyl or a charged substituent or an ionizable group Q, wherein Q is SO 3 H or a pharmaceutically acceptable salt thereof; m is an integer from 1 to 6; m' is 0 or an integer from 1 to 6; Cy is a cyclic alkyl having 5 or 6 ring carbon atoms optionally substituted with halogen, -OH, (C 1 -C 3)alkyl, (C 1 -C 3)alkoxy, or halo(C 1 -C 3)alkyl; and P is an amino acid residue or a peptide containing between 2 to 20 amino acid residues; J is N-hydroxysuccinimde ester, N-hydroxy sulfosuccinimide ester, nitrophenyl ester, dinitrophenyl ester, sulfo-tetraflurophenyl ester, and pentafluorophenyl ester; R a and R b, for each occurrence, are each independently -H, (C 1 -C 3)alkyl or a charged substituent or an ionizable group Q, wherein Q is SO 3 H or a pharmaceutically acceptable salt thereof; m is an integer from 1 to 6; m' is 0 or an integer from 1 to 6; Cy is a cyclic alkyl having 5 or 6 ring carbon atoms optionally substituted with halogen, -OH, (C 1 -C 3)alkyl, (C 1 -C 3)alkoxy, or halo(C 1 -C 3)alkyl; and Z s is -H, -SR d, -C(=O)R d1 or is selected from any one of the following formulas: or wherein: q is an integer from 1 to 5; n' is an integer from 2 to 6; U is -H or SO 3 M; M is H +, Na + or K +; R d is a linear or branched alkyl having 1 to 6 carbon atoms or is selected from phenyl, nitrophenyl, dinitrophenyl, carboxynitrophenyl, pyridyl or nitropyridyl; and R d1 is a linear or branched alkyl having 1 to 6 carbon atoms; q is an integer from 1 to 5; n' is an integer from 2 to 6; U is -H or SO 3 M; M is H +, Na + or K +; R d is a linear or branched alkyl having 1 to 6 carbon atoms or is selected from phenyl, nitrophenyl, dinitrophenyl, carboxynitrophenyl, pyridyl or nitropyridyl; and R d1 is a linear or branched alkyl having 1 to 6 carbon atoms; L" and L'" are both -H; the double line between N and C represents a single bond or a double bond, provided that when it is a double bond X is absent and Y is -H, or a linear or branched alkyl having 1 to 4 carbon atoms, and when it is a single bond, X is -H, Y is -OH or - SO 3 M; R 1, R 2, R 3, R 4, R 1 ', R 2 ', R 3 ' and R 4 ' are all -H; R 6 is -OMe; X' and Y' are both -H; A and A' are -O-; and M is H, Na + or K +; and R 5 for each occurrence is independently -H or an optionally substituted linear or branched alkyl having 1 to 10 carbon atoms.