ABCG2 and neoplasm: Other reasons that have been mentioned by several authors for the disappointing results obtained in clinical trials were the existence of limitations and failures in the design of the studies, namely: the patients were not selected based on tumor expression of ABC transporters; in many studies the compounds evaluated were not tested at least against the three major ABC transporters; the lack of adequate preclinical models; the absence of clinically validated imaging assays to detect ABC transporter function, among others [9,31,52].