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.

An Approach to the Total Synthesis of Phragmalin-Type Limonoids

It has been argued that certain limonoid natural products constitute some of the most structurally complex and synthetically challenging targets for modern chemical synthesis. Relatively few organic chemists attempt to synthesize the tetranortriterpenoids (limonoids). Indeed, since 2010, only two groups have disclosed results in this field and, of those two reports, only one constituted a completed total synthesis of a targeted limonoid structure. For this reason, it was very exciting to see that one of the outstanding academic synthetic investigators, Richmond Sarpong, who has already provided brilliant solutions to the total synthesis of a selection of lycopodium alkaloids, is now working in the limonoid area. The Sarpong laboratory has recently identified phragmalin-type limonoids (e.g. xyloccensin O, shown above), putatively derived (biosynthetically) from the well-known mexicanolide terpenoids, as targets of interest. In Organic Letters (2012, Articles ASAP), Sarpong and co-workers recently disclosed the successful preparation of the tricyclic core of the phragmalin-type limonoids from a hydrindanone Diels-Alder cycloadduct. The key bond formation was accomplished by means of an irreversible intramolecular alkylative cyclization (see above, conversion of 1 to 2) or, with a more functionalized substrate (3), via a highly efficient Michael addition. To quote from a recent review of limonoid chemistry, "...certain oxidatively modified limonoids signify a frontier for chemical synthesis and present an educational platform to advance the development of organic chemistry." It is encouraging that a small number of today's synthetic chemists are willing to face the daunting scientific challenges that limonoid natural products present. There is little doubt that this type of work will lead to important advancements in organic methodology as well as pharmacology and drug development.

Semisynthetic Terpenoid Antifungal Agents from Scynexis and Merck

Medicinal chemists at Scynexis, Inc. and Merck have developed semisynthetic antifungal derivatives of the triterpenoid glycoside natural product enfumafungin (1) that exhibit oral efficacy in a murine model of disseminated candidiasis. Glucan synthase inhibitors 2 and 3 were prepared in 12 to 13 linear steps from 1. The critical transformation in this sequence is a chemoselective allylic oxidation of the congested C12 position of enfumafungin's carbocyclic framework. It was found that a C18 alpha-disposed primary carboxamide was required to facilitate the catalytic palladium(II)-mediated Corey-Yu allylic oxidation protocol. The optimized kilogram-scale procedure that was recently disclosed by process chemists from Merck Research Laboratories is illustrated above. The bulky oxidant cumene hydroperoxide was deployed in order to promote oxidation of a radical intermediate from the concave alpha-face. A related oxidation of the steroidal C12 position has been discussed here previously in the context of the partial synthesis of the hedgehog antagonist cyclopamine. Given that I have participated in a portion of this research, I will not comment further here on the enfumafungin medicinal chemistry program, except to say that it has culminated in the selection of a candidate for clinical development and that this unique antifungal agent (MK-3118) is the first orally active glucan synthase inhibitor.