Until 10 to 15 years ago, oncology drug development was mainly conducted with compounds that were either genotoxic or cytotoxic. Determining the dose level that would provide the best ratio between efficacy and tolerability was mostly straightforward. As there is a clear justification for chemotherapeutics to destroy more cancer cells with an increasing dose, the highest tolerated dose level would also be the optimal dose to be taken forward. Rob Berg, MD, Ph.D., Senior Medical Director Oncology from INC research provides a medical oversight on oncology drug development.
 
https://www.jforcs.com/7/wp-content/uploads/2015/06/02.-Biomarker-based-Decsion-making-in-Early-stage….pdf