Cytotoxic and anti-mitotic compounds, and methods of using the same

A compound having the following structure (I):

• or a stereoisomer, prodrug or pharmaceutically acceptable salt thereof, wherein:
• R1 is selected from the group consisting of optionally substituted alkyl, optionally substituted alkylamino, optionally substituted cycloalkyl, optionally substituted aryl, optionally substituted heterocyclyl and optionally substituted heteroaryl;
• R2 is selected from the group consisting of optionally substituted alkyl, optionally substituted alkylamino, optionally substituted cycloalkyl, optionally substituted aryl, optionally substituted heterocyclyl and optionally substituted heteroaryl;
• R3 is selected from the group consisting of H and C1-6alkyl;
• R4 is selected from the group consisting of H and C1-6alkyl;
• and
• R5 is selected from the group consisting of C1-6alkyl and —SH.