Breast cancer and its treatment hurdles
Breast cancer (BC) is the most common cancer in women, with over 2 million new cases diagnosed annually1. Being characterized by hormone receptor status into subtypes, the classic treatment is to administer receptor inhibitors2. In addition to complex molecular diversities in estrogen receptor (ER+), progesterone receptor (PR+), heregulin receptor (HER2+) positive tumors, or triple negative (TNBC) tumors, acquired resistance challenges treatment regimens. Resistance to inhibitors, like palbociclib, targeting cell cycle regulatory proteins, CDK4/6, has been a recurrent problem. Several bypass signaling mechanisms establishing resistance involves proteins like Cyclin B1 encoded by CCNB1 gene3, Cyclin B2 encoded by CCNB2 gene4, and CDC205, modulating G2/M arrest and anaphase exit respectively. This intricate scenario thus warrants the search of new therapeutic targets to maximize favorable treatment outcomes.
ACK1 inhibition arrests cell cycle and overcomes resistance
Upon extensive analysis of human tumor microarray, we identified ACK1 as a novel targetable oncoprotein to treat all breast cancer subtypes. ACK1 is observed to be activated by either gene amplification or by various receptor tyrosine kinase6,7. Development and use of a potent small molecular inhibitor, (R)-9b, to inhibit of ACK1 effectively lowered ACK1 transcript levels thereby impacting the cell cycle genes CCNB1, CCNB2 and CDC20 by removing the pY88-H4 epigenetic marks. It resulted in causing cell cycle arrest in the G2/M phase in all the four major subtypes of breast cancer cells. This simultaneous downregulation of these three crucial genes by (R)-9b uncovers a new therapeutic vulnerability for CDK4/6-inhibitor resistant cells. Evaluating (R)-9b against palbociclib-resistant cell lines triggered cell death in vitro and breast tumor reduction in pre-clinical mouse models. Our above observations suggest that resistant breast tumors can be rendered susceptible to ACK1 inhibitor, (R)-9b.
(R)-9b inhibits lung metastasis in mouse models
Breast cancer metastasis worsens patient prognosis leading to further complications. Recent studies have implicated CXCR4 gene8 in BC metastasis, making it a promising target for therapeutic intervention. Remarkably, (R)-9b not only reduced CXCR4 expression, but also circumvented breast cancer metastasis to lungs, underscoring the physiological relevance ACK1 inhibition in later stages of disease.
(R)-9b, a ‘safe compound’ with good drug-like properties
Evaluation of (R)-9b confirmed excellent tumor inhibition. The physicochemical and kinetic profiling showed high membrane permeability, low mitochondrial inhibition, target tissue specificity and high plasma distribution. (R)-9b exhibited significant stability in gastric and intestinal fluids, indicating that it can be orally administered. Above all, limited off-target activity of (R)-9b ensures high potency against cancer cells, with no side effects, thus making it a ‘safe compound’ with good drug-like properties.
ACK1 inhibitor, (R)-9b, moving forward
To our knowledge, this is the first report demonstrating the hyper-functioning of ACK1 kinase activity in breast cancer through epigenetic control of cell cycle regulating genes. Identification of (R)-9b as a novel small molecule inhibitor with excellent drug-like properties, eliminating off-target effects, opens up the potential of improving breast cancer therapy. Further validation of (R)-9b’s efficacy in clinical trials will improve treatment regimens, ensuring progression free survival in patients.
References:
1 Łukasiewicz, S. et al. Breast Cancer—Epidemiology, Risk Factors, Classification, Prognostic Markers, and Current Treatment Strategies—An Updated Review. Cancers 13, 4287 (2021).
2 Martin, M., Lopez-Tarruella, S. & Gilarranz, Y. J. Endocrine therapy for hormone treatment-naïve advanced breast cancer. Breast 28, 161-166 (2016).
3 Ding, K., Li, W., Zou, Z., Zou, X. & Wang, C. CCNB1 is a prognostic biomarker for ER+ breast cancer. Med Hypotheses 83, 359-364 (2014).
4 Shubbar, E. et al. Elevated cyclin B2 expression in invasive breast carcinoma is associated with unfavorable clinical outcome. BMC Cancer 13, 1 (2013).
5 Faesen, A. C. et al. Basis of catalytic assembly of the mitotic checkpoint complex. Nature 542, 498-502 (2017).
6 Mahajan, K. et al. Ack1 mediated AKT/PKB tyrosine 176 phosphorylation regulates its activation. PLoS One 5, e9646 (2010).
7 Mahajan, K. & Mahajan, N. P. ACK1/TNK2 tyrosine kinase: molecular signaling and evolving role in cancers. Oncogene 34, 4162-4167 (2015).
8 Chen, I. X. et al. Blocking CXCR4 alleviates desmoplasia, increases T-lymphocyte infiltration, and improves immunotherapy in metastatic breast cancer. Proc Natl Acad Sci U S A 116, 4558-4566 (2019).