Cytotoxic agents

A compound having the structure:

• or a pharmaceutically acceptable salt thereof;
• wherein
• X and Z are members independently selected from O, S and NR23
• wherein
• R23 is a member selected from H, substituted or unsubstituted alkyl, substituted or unsubstituted heteroalkyl, and acyl;
• R1 is H, substituted or unsubstituted lower alkyl, or C(O)R , wherein R8 is a member selected from NR9R10 and OR9,
• in which
• R9, and R10 are members independently selected from H, substituted or unsubstituted alkyl and substituted or unsubstituted heteroalkyl;
• R2 is H or lower alkyl;
• R3 is a member selected from (═O), and OR11,
• wherein
• R11 is a member selected from H, substituted or unsubstituted alkyl, substituted or unsubstituted heteroalkyl, acyl, C(O)R12R13, C(O)OR12, C(O)NR12R13, P(O)(OR12)2, C(O)CHR12R13, and SiR12R13R14,
• in which
• R12, R13, and R14 are members independently selected from H, substituted or unsubstituted alkyl, substituted or unsubstituted heteroalkyl and substituted or unsubstituted aryl, wherein R12 and R13 together with the nitrogen atom to which they are attached are optionally joined to form a substituted or unsubstituted heterocycloalkyl ring system having from 4 to 6 members, optionally containing two or more heteroatoms;
• R4 and R5 are members independently selected from H, substituted or unsubstituted alkyl, substituted or unsubstituted aryl, substituted or unsubstituted heteroaryl, substituted or unsubstituted heterocycloalkyl, halogen, NO2, NR15R16, NC(O)R15, OC(O)NR15R16, OC(O)OR15, C(O)R15, OR15
• wherein
• R15 R16 are independently selected from H, substituted or unsubstituted alkyl, substituted or unsubstituted heteroalkyl, substituted or unsubstituted aryl, substituted or unsubstituted heteroaryl, and substituted or unsubstituted heterocycloalkyl, wherein R15 and R16 together with the nitrogen atom to which they are attached are optionally joined to form a substituted or unsubstituted heterocycloalkyl ring system having from 4 to 6 members, optionally containing two or more heteroatoms;
• R6 is a single bond which is either present or absent and when present R6 and R7 are joined to form a cyclopropyl ring;
• and
• R7 is CH2—X1 or —CH2— joined in said cyclopropyl ring with R6, wherein X1 is a leaving group;
• wherein at least one of R4, R5, R11, R12, R13, R15, and R16 has one or more cleavable groups, wherein the cleavable group is selected from peptides, amino acids, and disulfides.