Compound was generated using a literature procedure

Compound was generated using a literature procedure and required two-step Dess–Martin-periodinane/Pinnick oxidation prior to the methylation of the sulfonamide. Compound () was obtained from the Suzuki coupling of the methyl ester intermediate (en route to LEE011 ). Hydrolysis of all methyl ester intermediates with sodium hydroxide yielded compounds –, and which were assayed for binding on prostanoid receptors (). From the results shown in , only the 7-azaindoles , and demonstrated low nanomolar activity in the hCRTH2 binding assay. Similar results in the cAMP functional assay clearly indicated that this class of compounds behaves as a full antagonist. Additionally, compounds , and showed superior selectivity for hCRTH2 against DP and TP prostanoid receptors in comparison to compound MK-7246. Conversely, the 4- and 5-azaindole regioisomers and showed very low affinity on all prostanoid receptors tested, and although the 6-azaindole displayed some affinity for CRTH2 (139nM), we focused our efforts on the more promising 7-azaindole series. Finally, compound was found not to be a CYP2C9 inhibitor (IC >50μM) as compared to the lead MK-7246 (IC=9.4μM) but failed to address the in vivo covalent binding issue associated with MK-7246 (vide infra). Functionalization at the position-6 of the 7-azaindole with chlorine is well tolerated. In the EOS whole blood assay, the azaindole exhibited an activity similar compared to MK-7246. Since the presence of a 6-chloro substituent creates an electrophilic site on the azaindole core, we considered introducing a metabolically more robust substituent that would maintain the gain in potency while reducing the possibility of a nucleophilic addition mediated metabolism. With the introduction of an aromatic ring at position-6, as in compound (), the binding affinity for CRTH2 was conserved, but an eight fold shift in the whole blood EOS assay was observed. Although the overall profile of was encouraging, we pursued its optimization based on SAR studies imported from a novel class of compounds in which the sulfonamide moiety of MK-7246 was replaced by an amide group. Potential liabilities related to the presence of a sulfonamides group (i.e., allergic reaction to ‘sulfa’ drugs) as well as the potential for in vivo covalent binding found for compound MK-7246 and () were the main drivers for exploring this new chemical space. Following the sequence shown in , we converted the previously obtained sulfonamide intermediate into amides via a magnesium radical reduction of the sulfonamide to the secondary amine. The resulting amine was diversified using a series of different carboxylic acids using HATU coupling conditions followed by hydrolysis under standard conditions to obtain antagonists –. Most amides obtained through this process showed low nanomolar activity on both the binding and the whole blood assays (). They also showed excellent selectivity for the CRTH2 receptor over the other prostanoid receptors. Pharmacokinetic studies in rats on this class of compounds revealed that they were orally bioavailable and exhibited a wide range of clearance rates and half-lives (). From these studies, two compounds (, ) stood out for their low clearance and relatively high bioavailability, although, these results are based on only two animals. The improved CYP profile (i.e., lower CYP2C9 inhibition) observed with translated equally well in the amide series as shown by the similar results obtained for compound (). In addition, was found to display a lower activation constant (), a hallmark measurement of CYP3A4 time dependant inhibition (TDI) and a significantly lower in vivo covalent binding as compared to MK-7246 (). A reduced TDI of CYP3A4 for compound lowers the risks of potential drug–drug interaction caused by gradual accumulation of irreversible or quasi irreversible binders to the cytochrome active site. In conclusion, the indole core found in MK-7246 was effectively replaced by a 7-azaindole core while retaining affinity for the CRTH2 receptor and activity in the eosinophil shape change whole blood assay. The selectivity over other prostanoid receptors, mainly DP and TP, was improved as well as the CYP profile. The introduction of an amide group in lieu of the sulfonamide moiety reduced the shift in the EOS whole blood assay but more importantly solved the covalent binding issue that was observed with our lead compound. As a result of the SAR studies described in this manuscript, we identified the azaindole as a potential backup compound that addressed the potential liabilities associated with MK-7246.