Microtubule polymerization inhibitor prodrugs and methods of using the same

A prodrug of formula:

• wherein R1 is a substituent selected from the group consisting of carboxyl and —C(═O)O-alkyl-OC(═O)-alkyl;
• R2 is a substituent selected from the group consisting of H, and optionally substituted alkyl, alkylaryl, alkylhetaryl, alkylheterocycloalkyl, alkenyl, alkenyl-cycloalkyl, alkynyl, alkynyl-cycloalkyl, cycloalkyl, cycloalkyl-alkenyl, cycloalkyl-heterocycloalkyl, cycloalkyl-heteroaryl, aryl, heteroaryl, acyloxy, acyl, aralkyl, ester, fluoroalkyl, heteroalkyl, heteroalkenyl, heteroalkynyl, heteroalkylaryl, heteroalkylheteroaryl, heteroalkylheterocycloalkyl, heteroalkylcycloalkyl, heteroarylalkyl, heterocycloalkyl, and heterocycloalkyl;
• R3a and R3b are substituents independently selected from the group consisting of H, and optionally substituted alkyl, alkylaryl, alkylhetaryl, alkylheterocycloalkyl, alkenyl, alkenyl-cycloalkyl, alkynyl, alkynyl-cycloalkyl, cycloalkyl, cycloalkyl-alkenyl, cycloalkyl-heterocycloalkyl, cycloalkyl-heteroaryl, aryl, heteroaryl, acyloxy, acyl, aralkyl, ester, fluoroalkyl, heteroalkyl, heteroalkenyl, heteroalkynyl, heteroalkylaryl, heteroalkylheteroaryl, heteroalkylheterocycloalkyl, heteroalkylcycloalkyl, heteroarylalkyl, heterocycloalkyl, and heterocycloalkyl;
• R4 is a substituent selected from the group consisting of H, and optionally substituted alkyl, alkoxy, -(alkoxy)n-alkyl, heteroalkyl, and -(alkyl)n-alkyl;
• n is an integer of 1-250;
• and the pharmaceutically acceptable salts thereof.