<p>Triple-negative breast cancer (TNBC) remains the most refractory breast cancer subtype because of its high invasiveness, lack of therapeutic targets and heterogeneity. Type I protein arginine methyltransferases (PRMTs) are important epigenetic enzymes that catalyze the methylation of arginine residues in various proteins, playing crucial roles in numerous cellular processes. Targeting type I PRMTs represents a promising strategy for TNBC. In this study we characterized a novel selective type I PRMTs inhibitor, SKLB06489. Compared with the precursor compound SKLB06329 (<i>F</i> = 0.2%), SKLB06489 exhibited a markedly enhanced oral bioavailability (<i>F</i> = 88.4%). SKLB06489 inhibited PRMT1, PRMT6, and PRMT8 with IC₅₀ values of 64.55, 4.21, and 51.27 nM, respectively. In TNBC cell lines MDA-MB-231, Hs578T, and BT549, SKLB06489 dose-dependently inhibited cell proliferation and colony formation with IC₅₀ values in the low micromolar range. In MDA-MB-231 subcutaneous xenograft models, administration of SKLB06489 (40, 80 mg·kg⁻¹·d⁻¹, i.g. for 33 days) dose-dependently suppressed tumor growth. RNA sequencing and in vitro validation revealed that SKLB06489 inhibited TNBC proliferation by impairing DNA replication, compromising DNA damage repair, and ultimately inducing G₀/G₁-phase cell cycle arrest and apoptosis. In addition, SKLB06489 (5, 10 μΜ) dose-dependently enhanced intracellular cholesterol efflux in MDA-MB-231 cells and Hs578T cells via upregulation of the ATP-binding cassette transporter A1 (ABCA1) and ATP-binding cassette transporter G1 (ABCG1), thereby disrupting cholesterol metabolic homeostasis. We conclude that SKLB06489 is a potent type Ⅰ PRMTs inhibitor with great therapeutic potential and is expected to overcome the TNBC treatment bottleneck. The discovery of SKLB06489-regulated cholesterol homeostasis provides a novel perspective on the biological function of type Ⅰ PRMTs, particularly their role in regulating metabolic pathway.</p><p></p>