The 2-aminopyrimidine ring as a new scaffold for potent MRP1 inhibitors

Multidrug resistance-associated protein 1 (MRP1) is a key ATP-binding cassette (ABCC1) transporter involved in the efflux of a wide range of substrates, including endogenous molecules and therapeutic drugs. Overexpression of MRP1 in certain cancers plays a significant role in the development of multidrug resistance (MDR), leading to reduced efficacy of chemotherapeutic agents. MRP1 inhibition offers a promising strategy to improve the therapeutic potential of standard-of-care chemotherapy in tumors exhibiting MDR phenotype. In this study, a series of novel polysubstituted 2-aminopyrimidine derivatives have been designed and synthesized to inhibit MRP1 transporter, guided by structure-activity relationship (SAR) studies based on a library of polysubstituted pyrimidines (423 compounds). Among the newly synthesized compounds, six out of eight 4,5,6-trisubstituted 2-aminopyrimidines demonstrated strong inhibition of calcein-AM efflux from MRP1-overexpressing doxorubicin (DOX)-resistant glioblastoma U87MG cells. Simultaneously, they sensitized the cells to the cytotoxic effect of DOX in a dose-dependent manner. Compound 21 (EC₅₀ = 177 nM) proved to be fivefold more potent than the reference MRP1/P-gp inhibitor Reversan in calcein accumulation assay and completely abolished DOX resistance at submicromolar concentrations. Thus, 4,5,6-trisubstituted 2-aminopyrimidines represent a promising class of MRP1 inhibitors that could be used as adjuvant therapy to overcome MDR in combination cancer therapy.