Antagonism of the Alpha-Helix Mediated Protein-Protein Interactions of the Bcl-2 and c-Myc Oncoprotein Families: Proteomimetic and Small-molecule Strategies

Abstract

The Bcl-2 oncoprotein family includes both anti- and pro-apoptotic proteins that are normally localized within the mitochondrial outer membrane. The over-expression of the anti-apoptotic proteins (such as Bcl-xL, Bcl-2, and Mcl-1) is associated with cancer and chemotherapeutic resistance. Pro-apoptotic Bcl-2 proteins (such as Bak and Bim) initiate the intrinsic apoptotic pathway via oligomerization at the mitochondrial membrane. However, in the presence of over-expressed anti-apoptotic Bcl-2 proteins, pro-apoptotic Bcl-2 proteins are sequestered and the intrinsic apoptotic pathway is antagonized. Specifically, the conserved BH3 alpha-helix of the pro-apoptotic proteins engage the hydrophobic binding crevices of the anti-apoptotic proteins largely through hydrophobic (i), (i + 3/4) and (i + 7) residues on one face of the helix. Though potent inhibitors of Bcl-2 and Bcl-xL have been identified, chemically diverse pan-Bcl-2 and Mcl-1 specific inhibitors are lacking. Inspired by the recent advances in alpha-helix mimicry and fragment-based drug design, we have successfully synthesized potent (Ki ~ 150 nM) pan-Bcl-2 inhibitors based on trisbenzamide and salicylate scaffolds and validated their activities in vitro. The c-Myc oncoprotein is an intrinsically disordered (ID) transcription factor of a vast number of genes that are involved in cell proliferation and growth. Similar to anti-apoptotic Bcl-2 proteins, overexpression of c-Myc is associated with a myriad of cancers such as prostate, breast, and lung tumors. Though biologically inactive in its ID monomeric form, the transcriptional activation of c-Myc is initiated upon binding its obligatory protein partner Max. The transcriptionally active c-Myc-Max heterodimers recognize and bind the hexanucleotide sequence 5'-CACGTG-3' on dsDNA, where the transactivation domain of c-Myc recruits additional transcriptional machinery. Owing to its ID properties, in the absence of Max, c-Myc does not exhibit any secondary structure that may function as a basis for drug design. While several c-Myc specific inhibitors have been identified through high-throughput screening, few structure-activity relationship (SAR) studies have been reported. Towards developing potent c-Myc inhibitors, we conducted an SAR study on the c-Myc inhibitor 10074-G5 (IC50 = 146 uM), which resulted in the discovery of an improved inhibitor, JY-3-094 (IC50 = 33 uM) whose ester prodrugs exhibited potent cell activities (IC50 < 10 uM).