Maytansinoid derivatives

A compound of Formula Ia:

• or a pharmaceutically acceptable salt or solvate thereof,
• wherein
• X is hydrogen or halo;
• Y is selected from the group consisting of hydrogen, C1-C6 alkyl, C3-C6 cycloalkyl, and —C(C═O)R5;
• R1 is selected from the group consisting of hydrogen, —OH, —OC(═O)R5 and —OR5;
• R2 is hydrogen or C1-C6 alkyl;
• R3 is methyl, —CH2OH, or —CH2C(═O)R6;
• R4 is —OH or —SH;
• R5 is C1-C6 alkyl or benzyl;
• R6 is C1-C6 alkyl, phenyl or benzyl;
• R7 is hydrogen, C1-C6 alkyl or an amino acid side chain;
• R8 is hydrogen or C1-6 alkyl;
• p is selected from 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10;
• L is selected from optionally substituted C1-C20 alkylene, optionally substituted C1-C20 alkylene wherein one or more of the —CH2— groups are independently replaced with —O—, —NR8—, —C(O)—, —C(C═O)NR8—, or —NR8C(═O)—;
• substituted C1-C20 alkylene is C1-C20 alkylene substituted with 1 to 4 R23, wherein each R23 is independently unsubstituted C1-6 alkyl;
• and
• Abu is an antigen binding unit.