Synthetic Oleane Triterpenoids as Antioxidant Inflammation Modulators

          The synthetic oleane triterpenoid 6 (bardoxolone methyl) is currently in late-stage clinical trials as an orally bioavailable treatment of chronic kidney disease (CKD) in patients with type 2 diabetes. The compound is semi-synthetically derived from oleanolic acid (see Scheme above for the conversion of 1 into 6), which is produced by the fruit and leaves of the olive tree. Oleanolic acid itself is known to possess modest anti-inflammatory activity. However, when chemists at Dartmouth College installed a highly electrophilic enone system within the triterpenoid A-ring framework, in vitro potency increased by about 6 orders of magnitude relative to 1, as determined by an 'iNOS' assay. This assay quantitates inhibition of induction of 'inducible nitric oxide synthase' (iNOS), an enzyme that produces NO from arginine in macrophages and is recognized as playing a key role in inflammation. 

          The clinically relevant molecular target of 6 that is thought to mediate its therapeutic effects is the Kelch-like ECH-associated protein 1 or KEAP1, a repressor of another cytoplasmic protein, Nrf2. The oleane triterpenoids bind to KEAP1 and, in doing so, block the ubiquitination of Nrf2, which is a master regulator of the antioxidant and anti-inflammatory response. The ubiquitination of Nrf2 typically leads to sequestration and proteolysis of Nrf2, thereby preventing an aberrant anti-inflammatory response. Alternatively, Nrf2 activation results in nuclear translocation and subsequent induction of Nrf2 target genes that promote cellular control of oxidative or inflammatory stress. Hence, because Nrf2 activation leads to an antioxidant and anti-inflammatory response, and KEAP1 represses Nrf2 activation, KEAP1 is considered a promising drug target for a number of disease states including chronic kidney disease.

          A biotin-conjugated derivative of 6 (7) has been developed by the Dartmouth team in order to facilitate affinity chromatographic purification of target proteins. The detailed results of this effort have not been reported but it has been disclosed that "this compound can selectively bind to many different proteins in the cell with high affinity." It remains to be seen (pending the Phase 3 results expected in 2013) if this is a therapeutically beneficial quality of the clinical candidate (6). Structurally simplified tricyclic derivatives based on 6 have also been designed and evaluated as anti-inflammatory and cytoprotective agents. Compounds such as 8 are highly potent suppressors of induction of iNOS and are potent inducers of other cytoprotective enzymes. Given that the eastern substructure of 8 is enantiomeric relative to 6, it is clear that the presence of one or more reactive cyano enone systems is more important for biological potency than the intact triterpenoid carbon skeleton. Usually, the three-dimensional shape of a terpenoid framework, governed by ring-fusion stereochemistry, steric constraints and the pattern of oxygenation of a given molecule, is critical to the specificity of protein binding interactions that occur in a biological system. It will be interesting to see the pharmacokinetic properties and off-target binding profile of a relatively 'small molecule' such as 8, which bears two extremely reactive functional groups within its core structure. The authors note that Michael adducts between various thiol nucleophiles and 6 or 8 are not isolable due to reversibility of the conjugate addition. Perhaps this type of reactivity pattern is critical to the safety and bioavailability of these drug candidates to target proteins.