Acetyl-boswellic acids are novel catalytic inhibitors of human topoisomerases I and II alpha

Acetyl-boswellic acids (acetyl-BA) are pentacyclic triterpenes derived from the gum resin of frankincense. We have previously shown that these compoun ds are effective cytotoxic agents, acting through a mechanism that appears to involve the inhibition of topoisomerase activity. We have now investigat ed the mechanism of action of acetyl-BA and show that these compounds are m ore potent inhibitors of human topoisomerases I and II a than camptothecin, and amsacrine or etoposide, respectively. Our data demonstrate that acetyl -BA and, to a lesser extent, some other pentacyclic triterpenes, such as be tulinic acid, ursolic acid, and oleanolic acid, inhibit topoisomer-ases I a nd II alpha through a mechanism that does not involve stabilization of the cleavable complex or the intercalation of DNA. Surface plasmon resonance an alysis revealed that topoisomerases I and II alpha bind directly to an immo bilized derivative of acetyl-BA. This acetyl-BA derivative interacts with h uman topoisomerases through high-affinity binding sites yielding KD values of 70.6 nM for topoisomerase I and 7.6 nM for topoisomerase II alpha. Based on our data, we propose that acetyl-BA inhibit topoisomerases I and II alp ha through competition with DNA for binding to the enzyme. Thus, acetyl-BA are a unique class of dual catalytic inhibitors of human topoisomerases I a nd II alpha.