In Vitro: Ceefourin 2 is highly selective for MRP4 over other ABC transporters, including P-glycoprotein (P-gp), ABCG2 (Breast Cancer Resistance Protein; BCRP) and MRP1 (ABCC1). Ceefourin 2 has limited off-target effects and high stability.Ceefourin 2 (100 nM-100 μM) has low cellular toxicity (IC50 >50 μM) in two normal (HSF, MRC5), five neuroblastoma (BE(2)-C, IMR-32, IMR-32, SK-N-SH, SHEP) and four other human cancer cell lines (HEPG2, LNCap, SJ-G2, MCF7).