Hedgehog Antagonist Pyrimidine-Indole Hybrid Molecule Inhibits Ciliogenesis through Microtubule Destabilisation.

One of the crucial regulators of embryonic patterning and tissue development is the Hedgehog-glioma (Hh-Gli) signalling pathway; its uncontrolled activation has been implicated in different types of cancer in adult tissues. Primary cilium is one of the important factors required for the activation of Hh signalling, as it brings the critical components together for key protein-protein interactions required for Hh pathway regulation. Most of the synthetic and natural small molecule modulators of the pathway primarily antagonise Smoothened (Smo) or other effectors like Hh ligand or Gli. Here, we report a previously described Hh antagonist, with a pyrimidine-indole hybrid (PIH) core structure, as an inhibitor of ciliogenesis. The compound is unique in its mode of action, as it shows perturbation of microtubule dynamics in both cell-based assays and in vivo systems (zebrafish embryos). Further studies revealed that the probable targets are α-tubulin and its acetylated form, found in the cytoplasm and primary cilia. PIH also showed axonal defasiculation in developing zebrafish embryos. We thus propose that PIH antagonises Hh signalling by repressing cilia biogenesis and disassembling α-tubulin from its stabilised form.

Discovery of Hedgehog Antagonists for Cancer Therapy.

BACKGROUND: The evolutionarily conserved Hedgehog (Hh) signaling cascade is one of the key mediators of embryonic development of many metazoans. This pathway has been extensively targeted by small molecule inhibitors as its misregulation leads to various malignancies and developmental disorders. Thus, blocking this pathway can be a novel therapeutic avenue for the treatment of Hedgehog-dependent cancers. This review covers the mechanism of hedgehog signaling in vertebrate cells, provides an overview of reported small molecule Hh pathway inhibitors, with the synthetic routes and SAR studies of some of them discussed briefly. METHODS: A comprehensive survey of literature related to synthetic and naturally occurring Hh signaling antagonists reported till date is presented. RESULTS: Given the selectivity of small molecules targeting, this pathway for cancer treatment compared to kinase, tubulin or HDAC inhibitors, several such antagonists have been discovered, of which some are in preclinical development and clinical studies. Most of the reported small molecules primarily antagonize the Smoothened receptor although agents targeting Gli1 transcription factor and Shh ligand have also been discovered. Till date, nine Smo antagonists have been evaluated in clinical trials.GDC- 0449/Vismodegib and NVP-LDE225/Erismodegib, were granted approval by the U.S. Food and Drug Administration (U.S. FDA) for the treatment of basal cell carcinoma. CONCLUSION: The challenge is to identify agents that target the pathway downstream of Smo and develop strategies to overcome acquired drug resistance to the current Smo inhibitors with deeper understanding of the resistance mechanisms.

Piperazic acid derivatives inhibit Gli1 in Hedgehog signaling pathway.

Piperazic acid, a non-proteinogenic amino acid, found in complex secondary metabolites and peptide natural substances, has shown down regulation of Gli1 expression in Hedgehog signaling pathway in cell based assays. Further structure activity relationship study indicated that amide derivatives of piperazic acid are more potent than piperazic acid itself, with little to no toxicity. However, other cellular components involved in the pathway were not affected. To the best of our knowledge, this is the first report on the inhibitory property of piperazic acid in this pathway. Hence, this molecule could serve as a useful tool for studying Hedgehog signaling.