Discovery of a novel tetrahydroimidazo[1,2-a]pyridine-5-carboxylic acid derivative as a potent and selective heparanase-1 inhibitor utilizing an improved synthetic approach.

Heparanase-1 (HPSE1) is an endo-ß-d-glucuronidase that catalyzes degradation of heparan sulfate proteoglycans. Inhibition of HPSE1 appears to be a useful therapeutic target against cancer and proteinuric kidney diseases. We previously reported tetrahydroimidazo[1,2-a]pyridine 2 as a potent HPSE1 inhibitor after optimization of the synthetic reaction. However, synthesis of 2 involves a total of 19 steps, including a cyclization process that accompanies a strong odor due to the use of Lawesson's reagent and an epimerization reaction; furthermore, 2 exhibited insufficient selectivity for HPSE1 over exo-ß-d-glucuronidase (GUSß) and glucocerebrosidase (GBA), which also needed to be addressed. First, the cyclization reaction was optimized to synthesize tetrahydroimidazo[1,2-a]pyridine without using Lawesson's reagent or epimerization, with reference to previous reports. Next, 16 and 17 containing a bulkier substituent at position 6 than the 6-methoxyl group in 2 were designed and synthesized using the improved cyclization conditions, so that the synthetic route of 16 and 17 was shortened by five steps as compared with that of 2. The inhibitory activities of 16 and 17 against GUSß and GBA were reduced as compared with those of 2, that is, the compounds showed improved selectivity for HPSE1 over GUSß and GBA. In addition, 16 showed enhanced inhibitory activity against HPSE1 as compared with that of 2. Compound 16 appears promising as an HPSE1 inhibitor with therapeutic potential due to its highly potent inhibitory activity against HPSE1 with high selectivity for HPSE1.

Structure-based lead optimization to improve potency and selectivity of a novel tetrahydroimidazo[1,2-a]pyridine-5-carboxylic acid series of heparanase-1 inhibitor.

Heparanase-1 (HPSE1) is an endo-ß-d-glucuronidase that is the only mammalian enzyme known to cleave heparan sulfate (HS) of heparan sulfate proteoglycans (HSPG), a key component of the glycocalyx layer of the vascular endothelium matrix. Inhibition of HPSE1 has therapeutic potential for cancer and proteinuric kidney diseases. We previously reported that 2 showed a moderate potency as an HPSE1 inhibitor and an issue of selectivity against exo-ß-d-glucuronidase (GUSß) and glucocerebrosidase (GBA) remained. A structure-based lead optimization of 2 using X-ray co-crystal structure analysis and fragment molecular orbital calculation resulted in 4e, which showed a more than 7-fold increase in HPSE1 inhibitory activity. The subsequent introduction of a methyl group into the 6-hydroxy group of 4e resulted in 18 with reduced inhibitory activities against GUSß and GBA while maintaining the inhibitory activity against HPSE1. The inhibitory activities of 18 against serum HPSE1 in mice were significant and lasted for 4 h at doses of 3, 30, and 100 mg/kg. Compound 18 could be a novel lead compound for HPSE1 inhibitors with improved inhibitory activity against HPSE1 and increased HPSE1 selectivity over GUSß and GBA.