We have shown the utility of a synthetic triterpenoid, CDDO-Me for inhibition of the process of carcinogenesis in a mouse model that is highly relevant to the development of invasive malignancy in women with a mutated BRCA1 gene. The suppression of malignancy in the mouse model (), although only partial, is nevertheless significant, and if it could be translated into clinical practice, would provide meaningful benefit to women who presently have no desirable therapeutic options. The dosage of triterpenoid that has been used in these experiments is apparently free of undesirable side effects in the mice, which continued to gain weight during the course of an almost year-long experiment.
Notably, a recent study has found that the EGFR inhibitor erlotinib delays tumor development in BRCA1
-mutant mice (17
), but erlotinib may be too toxic for clinical chemoprevention studies. CDDO-Me is not a conventional cytotoxic drug and has an excellent safety profile in humans (32
). The well-tolerated dose used in these studies was well below the maximum-tolerated dose or the doses successfully used previously for treatment of experimental lung or breast cancer (21
). All mice continued to gain weight throughout the study, but the mice fed CDDO-Me did not gain as much weight as the control mice. Although we have shown that the combination of CDDO-Me and LG268 delays tumor development in MMTV-neu mice (21
), this combination was not effective in this model, even though the weights of these mice were the same as mice fed CDDO-Me alone (data not shown). These data suggest that the delay in tumorigenesis in this model was not the result of toxicity or slower weight gain. Moreover, triterpenoids downregulate the fatty acid synthase pathway (34
), which contributes to weight gain and may contribute to the development of breast cancer driven by overexpression of ErbB2 (36
Mechanistically, we have shown important interactions between CDDO-Me and the protein target, ErbB2. ErbB2 is a validated target for drugs that are used in cancer chemotherapy, such as Herceptin. Many of the advances in chemotherapy of breast cancer have relied on the ability of drugs to modulate the activity of ErbB2, which undergoes phosphorylation to become biologically active. The biological relevance to carcinogenesis induced by BRCA mutation is shown here in which indicates that increased phosphorylation of ErbB2 occurs in mice as early as sixteen weeks of age. From a pharmacological perspective, we have shown that CDDO-Me inhibits phosphorylation of ErbB2 in cell cultures of breast cancer cells having a BRCA1
mutation; doses between 300-1000 nanoMolar are effective in vitro
, and concentrations of 1.5 μM can be obtained in the mammary glands in vivo
. Furthermore, CDDO-Me was able to suppress the expression of phospho-ErbB2 in actual tumors in mice that received this drug by chronic administration in the diet; levels of p-ErbB2 were reduced almost 50% in tumors obtained from treated mice (). Finally, in experiments with a biotinylated analog of CDDO-Me, we have also shown that the triterpenoid directly interacts with ErbB2, presumably by Michael addition with a reactive cysteine at the catalytically active ATP binding pocket of this protein. It is well established that CDDO-Me and related triterpenoids are potent agents for Michael addition, although this is not a random process. Rather, the pentacyclic scaffold of CDDO-Me, together with the exocyclic methyl groups of this triterpenoid, provide a highly stereospecific platform for interaction with unique cysteine residues on target proteins (20
). This is undoubtedly an important consideration in the relative safety of the use of these triterpenoids for chemoprevention.
In addition to the above mechanism, there are undoubtedly other mechanisms that contribute to the useful chemopreventive activity of CDDO-Me. It has previously been shown in both cell culture and in vivo
experiments that CDDO-Me is a potent anti-angiogenic agent (38
). Furthermore, recent proteomic studies have shown that the same biotinylated analog of CDDO-Me used here has multiple protein targets, most notably the mTOR complex and several of the nuclear receptors in the steroid receptor superfamily (39
). The separate and individual contributions of each of these targets are difficult to determine in an in vivo
context. Moreover, we cannot be certain how much of the chemopreventive activity of CDDO-Me is due to effects on stromal cells that comprise a particularly large fraction of the total cells in a carcinoma of the mammary gland (40
The ultimate application of the results we have obtained here to prevention of breast cancer in women at exceptionally high risk still requires further development. It is most likely that CDDO-Me will be most effective if used in combination with other chemopreventive agents (6
). Whether such agents will turn out to be PARP inhibitors, rexinoids, or other anti-inflammatory agents remains to be determined. The practical use of CDDO-Me (generically known as bardoxolone methyl) for successful treatment of advanced diabetic nephropathy in a Phase 2 clinical trial (32
) indicates that synthetic oleanane triterpenoids can be given safely to patients in a useful manner. Considering the clinical problems facing young women who are newly diagnosed with a BRCA mutation, there is now a compelling need to push this area of research to the point that it becomes clinically practical.