New cytotoxic bicyclic hexapeptides, RA-XXIII and RA-XXIV, from Rubia cordifolia L.

Two new bicyclic hexapeptides, RAAXIII and RA-XXIV, were isolated from the roots of Rubia cordifolia L. (Rubiaceae). Their structures were determined by the analysis of their 2D NMR spectra, chemical methods, and X-ray crystallography. The IC50 values of RA-XXIII and RA-XXIV against P-388 leukemia cells were 0.16 and 0.48 mu g/ml, respectively.