Urea Derivates of Ursolic, Oleanolic and Maslinic Acid Induce Apoptosis and Are Selective Cytotoxic for Several Human Tumor Cell Lines

2,3-Di-O-acetyl-maslinic acid benzylamide (5) has previously been shown to possess high cytotoxicity for a variety of human tumor cell lines while being of low cytotoxicity to non-malignant cells. Structural modifications performed on 5 revealed that the presence of these acetyl groups in 5 and the presence of (2β,3β)-configurated centers seems necessary for obtaining high cytotoxicity combined with best selectivity between malignant cells and non-malignant mouse fibroblasts. Compounds carrying an ursane skeleton showed weaker cytotoxicity than their oleanane derived analogs. In addition, the benzylamide function in compound 5 should be replaced by a phenylurea moiety to gain better cytotoxicity while retaining and improving the selectivity. Thus, maslinic acid derived N-[2β,3β-di-O-acetyl-17β-amino-28-norolean-12-en-17-yl]phenylurea (45) gave best results showing EC50 = 0.9 μM (for A2780 ovarian cancer cells) with EC50 > 120 μM for fibroblasts (NIH 3T3) and triggered apoptosis while caspase-3 was not activated by this compound.