Targeting BCR-ABL1 in Chronic Myeloid Leukemia by PROTAC-Mediated Targeted Protein Degradation

• Published in: Cancer research (Chicago, Ill.) Ill.), 2019-09, Vol.79 (18), p.4744-4753
• Main Authors: Burslem, George M, Schultz, Anna Reister, Bondeson, Daniel P, Eide, Christopher A, Savage Stevens, Samantha L, Druker, Brian J, Crews, Craig M
• Format: Article
• Language: English
• Subjects: Animals; Apoptosis - drug effects; Cell Proliferation - drug effects; Drug Resistance, Neoplasm; Fusion Proteins, bcr-abl - antagonists & inhibitors; Humans; K562 Cells; Leukemia, Myelogenous, Chronic, BCR-ABL Positive - drug therapy; Leukemia, Myelogenous, Chronic, BCR-ABL Positive - metabolism; Leukemia, Myelogenous, Chronic, BCR-ABL Positive - pathology; Mice; Neoplastic Stem Cells - drug effects; Neoplastic Stem Cells - metabolism; Neoplastic Stem Cells - pathology; Protein Array Analysis; Protein Kinase Inhibitors - pharmacology; Proteolysis - drug effects; Tumor Cells, Cultured
• ISSN: 0008-5472; 1538-7445
• DOI: 10.1158/0008-5472.can-19-1236

Although the use of ATP-competitive tyrosine kinase inhibitors of oncoprotein BCR-ABL1 has enabled durable responses in patients with chronic myeloid leukemia (CML), issues of drug resistance and residual leukemic stem cells remain. To test whether the degradation of BCR-ABL1 kinase could offer improved response, we developed a series of proteolysis-targeting chimera (PROTAC) that allosterically target BCR-ABL1 protein and recruit the E3 ligase Von Hippel-Lindau, resulting in ubiquitination and subsequent degradation of the oncogenic fusion protein. In both human CML K562 cells and murine Ba/F3 cells expressing BCR-ABL1, lead compound GMB-475 induced rapid proteasomal degradation and inhibition of downstream biomarkers, such as STAT5, and showed increased sensitivity compared with diastereomeric controls lacking degradation activity. Notably, GMB-475 inhibited the proliferation of certain clinically relevant BCR-ABL1 kinase domain point mutants and further sensitized Ba/F3 BCR-ABL1 cells to inhibition by imatinib, while demonstrating no toxicity toward Ba/F3 parental cells. Reverse phase protein array analysis suggested additional differences in levels of phosphorylated SHP2, GAB2, and SHC associated with BCR-ABL1 degradation. Importantly, GMB-475 reduced viability and increased apoptosis in primary CML CD34 cells, with no effect on healthy CD34 cells at identical concentrations. GMB-475 degraded BCR-ABL1 and reduced cell viability in primary CML stem cells. Together, these findings suggest that combined BCR-ABL1 kinase inhibition and protein degradation may represent a strategy to address BCR-ABL1-dependent drug resistance, and warrant further investigation into the eradication of persistent leukemic stem cells, which rely on neither the presence nor the activity of the BCR-ABL1 protein for survival. SIGNIFICANCE: Small-molecule-induced degradation of BCR-ABL1 in CML provides an advantage over inhibition and provides insights into CML stem cell biology. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/79/18/4744/F1.large.jpg.