This invention relates to an oral formulation containing an effective amount of the compound of the following formula I: d-alpha-tocopheryl polyethylene glycol 1000 succinate (&ldquoTPGS&rdquo) and 2-(2-ethoxyethoxy)ethanol (&ldquoTranscutol&rdquo). R1 through R4 and n are defined herein. Also disclosed is a method of treating cancer by administering this formula to a subject orally.