The present invention pertains to a diagnostic and therapeutic method for assessing whether a patient is susceptible to the treatment of an ester-prodrug. The methods of the invention include the analysis of carboxyelesterase 2 (CES2)-expression in tumor samples as a predictive value for the assessment of treatment success with an ester-prodrug of a chemotherapeutic agent. Alternatively, the invention provides methods involving the analysis of the urinary ratio of the prodrug and the active therapeutic as another predictive value for assessing treatment susceptibility.