Abstract B101: Caspase-dependent induction of apoptosis by novel lupane-type pentacyclic triterpenoids

Betulinic acid is a naturally occurring lupane-type pentacyclic triterpenoid with great activity in the treatment and prevention of cancer. As an apoptosis inducer, it inhibits cancer cell growth in human melanoma, glioblastoma, neuroblastoma, leukemia, prostate and ovarian carcinomas. Several studies have linked the anticancer property of betulinic acid to its ability to induce apoptosis in cancer cells by targeting the mitochondrial pathway of apoptosis. We have been interested in the structure -activity studies of modified lupane-type pentacyclic triterpenoids to develop new cancer preventive drugs with enhanced pro-apoptotic activities.

New compounds were prepared in five to seven synthetic steps. The panel of cell lines used in our investigation included SK-MEL-2 and A-375 from malignant melanoma of the skin, Daoy and LN-229 from cerebellar medulloblastoma, OVCAR-3 from ovarian, HT-29 from colorectal adenocarcinoma and MCF-7 from breast adenocarcenoma clinical samples. Cell growth inhibitory activity was measured by tetrazolium-based MTS assay and reported as IC₅₀ values. Homogeneous caspase activation was quantified by a fluorimetric assay and apoptosis induction was characterized by an annexin-type flow cytometry assay and measured as ratio of early-mid apoptotic to necrotic cells.

The structure -activity relationship analysis reveals that amino group incorporation has significant impact on the cytotoxicty of triterpenes. Mono-amino derivatives inhibited cancer cell growth with potency 2-6 times higher than betulinic acid, whereas bis-amino analogues exhibited inhibitory effect with IC₅₀ values of 10-100 times lower. Further mechanism of action studies revealed that these new amino pentacyclic triterpenoids induced apoptosis and activated caspases. In SK-MEL-2 (melanoma) cells, apoptosis was induced as early as 2 h at 15 μM and a similar amount of apoptosis induction was observed at 16 h in Daoy (glioblastoma) cells.

Our results show that incorporation of amino groups into the structure of lupane-type pentacyclic triterpenoids at various positions markedly improves cytotoxicity of these molecules with unique apoptosis induction ability. Further pharmacological evaluation on this new class of modified natural products is warranted to develop promising agents for cancer treatment and prevention.

Citation Information: Cancer Prev Res 2008;1(7 Suppl):B101.