Multidrug resistance (MDR) occurring during cancer chemotherapy is a major obstacle for effectiveness and response to therapy and is often caused by ATP-binding cassette (ABC) efflux transporters. Belonging to the family of ABC transporters, breast cancer resistance protein is getting more and more in the spotlight of research. As a strategy to overcome MDR, inhibitors of ABC transporters were synthesized, which could be applied in combination with cytostatic drugs. For this purpose, 2,4-disubstituted pyridopyrimidine derivatives were synthesized. The investigations confirmed three key characteristics of good inhibitors: a low intrinsic cytotoxicity and a high potency and selectivity toward ABCG2. For selected compounds the interaction with ABCG2 was elucidated and their effect on ATPase activity and conformation sensitive 5D3 antibody binding was investigated. Their ability to reverse MDR in coadministration with the active metabolite of irinotecan and mitoxantron was confirmed.