Development of MKI-3: A Potent and Selective MASTL Inhibitor with Improved Efficacy for Cancer Treatment

Microtubule-associated serine/threonine
kinase-like (MASTL) is
a primary regulator of mitosis and a potential cancer target. We report
the discovery and optimization of MASTL kinase inhibitor-3 (MKI-3),
a potent and selective MASTL inhibitor with nanomolar cellular activity
and enhanced metabolic stability, suitable for in vivo assessment. Structure–activity relationship studies identified
MKI-3, a quinazoline-based scaffold that enhances MASTL binding, suppresses
endosulfine alpha (ENSA) phosphorylation, and activates protein phosphatase
2A. MKI-3 inhibited MASTL with a half-maximal inhibitory concentration
of 5.72 nM and exhibited strong selectivity over other AGC kinases.
In breast cancer cell lines (MCF7, MDA-MB-231, BT549, and 4T1), MKI-3
exhibited robust antiproliferative effects at nanomolar concentrations.
Mechanistically, MKI-3 disrupted the MASTL–ENSA–Aurora
A signaling axis, inducing chromosomal instability, mitotic catastrophe,
and apoptosis. MKI-3 significantly reduced tumor growth in treatment-refractory
4T1 triple-negative breast cancer without toxicity and outperformed
MKI-2 supporting MASTL-targeted therapy.