Dual-targeting chromatin regulation and DNA damage repair signaling presents a promising avenue for cancer therapy. SP-1-303 acts as a class I isoform selective histone deacetylase (HDAC) inhibitor and an activator of the ataxia-telangiectasia mutated protein (ATM). SP-1-303 emerges as a novel second generation class I (HDAC1 and HDAC3) selective HDAC inhibitor, and ATM activator, capable of modulating estrogen receptor (ER) expression, and inhibiting growth of ER+ BC cells. Combined targeting of class I HDACs and ATM by SP-1-303 offers a promising therapeutic approach for treating ER+ breast cancers and supports further preclinical evaluation.