Developing an M3 Antagonist for Molecular Imaging of Muscarinic Acetylcholine Receptors in Breast Cancer

Abstract

The lack of the three targeted receptors, estrogen, progesterone, and the human epidermal growth factor receptor 2, causes difficulty in treating triple negative type cancers. Most often, patients with this type of malignancy have a low chance of survival. The focus of this work will be on constructing a molecular antagonist to detect the overexpression of a receptor subtype. The category of tumor under research is the triple negative type of breast cancer. Only about 30% of women are rated to survive this class of growth, especially at the metastatic stage when the cancer has spread to various parts of the body. Studies have shown that the M3 subtype of the muscarinic acetylcholine receptor is overexpressed at the fourth and last stages of the carcinoma. In the numerous efforts of developing an M3 antagonist, the greatest challenge has been identifying a compound with a high selectivity towards the M3 receptor antagonist due to its similarities with the M¬2 subtype in their binding pocket. Based on an article published in the “Journal of Medicinal Chemistry” in 2002 by Ohtake and his group at the Banyu Tsukuba Research Institute, a novel molecule with structural features like previously reported nonselective muscarinic antagonists was discovered showing high selectivity for M3 receptors over the other subtypes. The subtype of interest was that of M2/M3 = 98-fold. I adopted the structural features published in the article with alterations at the cationic site and the hydrophobic region to create a library of six compounds that had properties for potential radiolabelling in a future project. The six compounds were screened against all five subtypes of the muscarinic acetylcholine receptor for radioligand binding competition assay. The IC50 values to M3 receptors of two of the compounds, 47 (0.35 μM) and 45 (0.448 μM) had promising results. The two compounds, in particular 47, showed high selectivity for M3¬ receptors over the other muscarinic receptor subtypes: 47 (M1/M3 = 66-fold, M2/M3 = 73-fold, M4/M3 = 62-fold, M5/M3 = 37-fold) and 45 (M1/M3 = 31-fold, M2/M3 = 31-fold, M4/M3 = 30-fold, M5/M3 = 37-fold).