The BET bromodomain containing protein (BRD4) plays a key role in transcription regulation. Therefore, efforts to generate BRD4 inhibitors with excellent potency and DMPK properties are of clinical value. As a continuing work to improve the stability in in vitro metabolic experiments of liver microsomes of our previously reported 7-methylimidazo[1,5-<i>a</i>]pyrazin-8(7H)-one, our optimization of this poor pharmacokinetics focusing on the phenyl substituent is performed. Fortunately, compound <b>17</b> displayed subnanomolar potency (IC<sub>50</sub> = <b>30 nM</b>) against BRD4(1), and its liver microsome stability in human, rat, and mouse are more favorable than previously reported inhibitor <b>28</b>. Compound <b>17</b> exhibited antitumor efficacy with no significant toxicity in xenograft models of pancreatic cancer. In addition, fluorescent probe and nuclei-specific dye were utilized to verify apoptosis-inducing of compound <b>17</b> via intranuclear potency in BXPC-3 cell line.