Obatoclax is a direct and potent antagonist of membrane-restricted Mcl-1 and is synthetic lethal with treatment that induces Bim.

Abstract

BACKGROUND:

Obatoclax is a clinical stage drug candidate that has been proposed to target and inhibit prosurvival members of the Bcl-2 family, and thereby contribute to cancer cell lethality. The insolubility of this compound, however, has precluded the use of many classical drug-target interaction assays for its study. Thus, a direct demonstration of the proposed mechanism of action, and preferences for individual Bcl-2 family members, remain to be established.

METHODS:

Employing modified proteins and lipids, we recapitulated the constitutive association and topology of mitochondrial outer membrane Mcl-1 and Bak in synthetic large unilamellar liposomes, and measured bakdependent bilayer permeability. Additionally, cellular and tumor models, dependent on Mcl-1 for survival, were employed.

RESULTS:

We show that regulation of bilayer permeabilization by the tBid - Mcl-1 - Bak axis closely resemblesthe tBid - Bcl-XL - Bax model. Obatoclax rapidly and completely partitioned into liposomal lipid but also rapidly exchanged between liposome particles. In this system, obatoclax was found to be a direct and potent antagonist of liposome-bound Mcl-1 but not of liposome-bound Bcl-XL, and did not directly influence Bak. A 2.5 molar excess of obatoclax relative to Mcl-1 overcame Mcl-1-mediated inhibition of tBid-Bak activation. Similar results were found for induction of Bak oligomers by Bim. Obatoclax exhibited potent lethality in a cellmodel dependent on Mcl-1 for viability but not in cells dependent on Bcl-XL. Molecular modeling predicts that the 3-methoxy moiety of obatoclax penetrates into the P2 pocket of the BH3 binding site of Mcl-1. A desmethoxy derivative of obatoclax failed to inhibit Mcl-1 in proteoliposomes and did not kill cells whose survival depends on Mcl-1. Systemic treatment of mice bearing Tsc2(+) (/) (-) Em-myc lymphomas (whose cells depend on Mcl-1 for survival) with obatoclax conferred a survival advantage compared to vehicle alone (median 31 days vs 22 days, respectively; p=0.003). In an Akt-lymphoma mouse model, the anti-tumor effects of obatoclax synergized with doxorubicin. Finally, treatment of the multiple myeloma KMS11 cell model (dependent on Mcl-1 for survival) with dexamethasone induced Bim and Bim-dependent lethality. As predicted for an Mcl-1 antagonist, obatoclax and dexamethasone were synergistic in this model.

CONCLUSIONS:

Taken together, these findings indicate that obatoclax is a potent antagonist of membranerestricted Mcl-1. Obatoclax represents an attractive chemical series to generate second generation Mcl-1 inhibitors.

• Authors: Mai Nguyen, Regina Cencic, Franziska Ertel, Cynthia Bernier, Jerry Pelletier, Anne Roulston, John R Silvius, Gordon C Shore
• Journal: BMC cancer (BMC Cancer) Vol. 15 Pg. 568 (Aug 01 2015) ISSN: 1471-2407 [Electronic] England
• PMID: 26231047 (Publication Type: Journal Article, Research Support, Non-U.S. Gov't)

Chemical References

• Apoptosis Regulatory Proteins
• BCL2L11 protein, human
• Bcl-2-Like Protein 11
• Bcl2l11 protein, mouse
• Indoles
• MCL1 protein, human
• Membrane Proteins
• Myeloid Cell Leukemia Sequence 1 Protein
• Proto-Oncogene Proteins
• Pyrroles
• Doxorubicin
• obatoclax

Topics

• Animals
• Apoptosis Regulatory Proteins (metabolism)
• Bcl-2-Like Protein 11
• Cell Line, Tumor
• Disease Models, Animal
• Doxorubicin (administration & dosage)
• Drug Synergism
• Humans
• Indoles
• Lymphoma (drug therapy, metabolism)
• Membrane Proteins (metabolism)
• Mice
• Myeloid Cell Leukemia Sequence 1 Protein (antagonists & inhibitors, chemistry)
• Proto-Oncogene Proteins (metabolism)
• Pyrroles (administration & dosage, pharmacology)
• Xenograft Model Antitumor Assays