Abstract C097: Pyrrolo[2′,3′:3,4]cyclohepta[1,2-d][1,2]oxazoles: A new class of antimitotic agents
Authors: Spanò Virginia , Giallombardo Daniele , Montalbano Alessandra , Carbone Anna , Gaudio Eugenio , Bortolozzi Roberta , Bai Ruoli , Viola Giampietro , Hamel Ernest , Bertoni Francesco , Barraja Paola 
Published: March 2020

Abstract
Tubulin-binding molecules constitute an important class of antineoplastic agents, with broad activity in both solid and hematologic malignancies. Oxazoles represent the core structure of many drug candidates with multiple targets, providing an attractive scaffold in medicinal chemistry. Diaryl[1,2]oxazoles have emerged as potent analogues of the antitubulin compound combretastatin A-4 (CA-4). Naphtylcombretastin and its derivatives incorporating the isoxazole moiety displayed potent cytotoxic effects and inhibition of tubulin polymerization. In particular, 5-(naphthalen-2-yl)-4-(TMP)-1,2-oxazole and 4-(naphthalen-2-yl)-5-(TMP)-1,2-oxazole showed the same inhibitory potency as naphtylcombretastatin and in the same order of magnitude as CA-4. We already reported the synthesis and biological evaluation of the [1,2]oxazolo[5,4-e]isoindole system (Spanò et al, E J Med Chem 2016), with in vitro and in vivo anti-tumor activity in diffuse malignant peritoneal mesothelioma as a consequence of inhibition of tubulin polymerization. On the basis of these results, we modified the original structure and synthetized 25 compounds belonging to a novel class of pyrrolo[2′,3′:3,4]cyclohepta[1,2-d][1,2]oxazoles. The new compounds were screened in a panel of 60 human cancer cell lines in the NCI drug screen showing potent activity with GI50values reaching the nanomolar level, with mean graph mid-points of 0.08–0.41 µM, and activity especially against models derived from prostate cancer (GI50 as low as 46 nM in the DU-145 cells), melanoma (GI50 as low as 19 nM in the MDA-MB-435 cells), and kidney cancer (as low as 20 nM in the A498 cells). All compounds were further tested on four cell lines derived from distinct lymphoma histotypes, plus two cell lines with secondary resistance to idelalisib and ibrutinib (Arribas at el, AACR 2018). Five of the 25 compounds showed potent growth inhibitory effects on some or all of the lymphoma cell lines, with IC50values lower than 500 nM. Investigation on the mechanism of action showed the ability of the new [1,2]oxazoles to impair cell cycle progression and induce apoptosis through the mitochondrial pathway. The most active compounds were able to arrest HeLa cells in the G2/M phase of the cell cycle in a concentration dependent manner. This effect was accompanied by apoptosis, mitochondrial depolarization, generation of reactive oxygen species and activation of PARP cleavage. These results indicate that the cellular actions of these agents involve mitotic arrest, due to interference with the functions of the mitotic spindle, and an apoptotic cell death. Finally, the effect of compounds on tubulin polymerization in vitro were evaluated, with IC50values of 1.9–8.2 µM. Inhibition of colchicine binding to tubulin was also demonstrated. In conclusion, pyrrolo[2′,3′:3,4]cyclohepta[1,2-d][1,2]oxazoles are a novel class of anti-mitotic compounds with anti-tumor activity in multiple cancer cell lines.
Citation Format: Virginia Spanò, Daniele Giallombardo, Alessandra Montalbano, Anna Carbone, Eugenio Gaudio, Roberta Bortolozzi, Ruoli Bai, Giampietro Viola, Ernest Hamel, Francesco Bertoni, Paola Barraja. Pyrrolo[2′,3′:3,4]cyclohepta[1,2-d][1,2]oxazoles: A new class of antimitotic agents [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr C097. doi:10.1158/1535-7163.TARG-19-C097