Rubipodanin B, a New Cytotoxic Cyclopeptide from Rubia podantha

Using the TLC cyclopeptide protosite detection method, a new cyclohexapeptide named rubipodanin B ( 1 ), together with 11 known Rubiaceae‐type cyclopeptides (RAs), RA‐X‐OMe ( 2 ), RA‐IV ( 3 ), RA‐XI ( 4 ), RA‐XIII‐OMe ( 5 ), rubiyunnanin C ( 6 ), RA‐I ( 7 ), RA‐III ( 8 ), RA‐V ( 9 ), RA‐VII ( 10 ), RA‐XII ( 11 ) and rubipodanin A ( 12 ), were obtained from the roots and rhizomes of Rubia podantha Diels . The structures were determined using various spectroscopic methods. Among them, 2 was firstly identified as a natural product, and 3–6 were firstly isolated from this species. Cytotoxicity and NF‐κB signaling pathway activity of 1 , 2 , 4 , 6 , 7 and 9 were evaluated. All these compounds showed cytotoxic activities against three human tumor cell lines, MDA‐MB‐231, SW620 and HepG2, with the IC 50 values between 0.015 and 10.27 μ m , and only 7 and 9 possessed NF‐κB inhibitory activities with the IC 50 values of 2.42 and 0.046 μ m , respectively, which demonstrated that 2‐alanine amino acid plays a key role to maintain the RAs bioactivity.