Monastrol, a DHPM-based Eg5 inhibitor with well-known antiproliferative activity but limited therapeutic potential due to poor solubility and low bioavailability, was selected as the lead compound for the design of styryl-modified 3,4-dihydropyrimidin-2(1H)-ones with an improved pharmaceutical profile. Twelve derivatives (10–21) were synthesized via the Biginelli reaction and evaluated for cytotoxicity in HeLa and MCF-7 cells. Styryl derivatives 16 and 17 emerged as the most active. In HeLa cells, derivatives 17 (IC50 = 1.3 µM) and 16 (IC50 = 3.7 µM) were approximately 85-fold and 30-fold more potent than monastrol (IC50 = 111 µM), respectively. In MCF-7 cells, derivatives 16 and 17 displayed 18- to 20-fold higher potency than monastrol, respectively. Biological results also indicate that styryl derivatives 16 and 17 induce apoptosis in both HeLa and MCF-7 cells. In HeLa cells, activation of caspase-8, -9, and -3 suggests the involvement of both intrinsic and extrinsic pathways. In contrast, in MCF-7 cells, the increased expression of p53 and p21, together with PARP cleavage, suggests a p53-dependent apoptotic response. Derivatives 16 and 17 emerged as promising Eg5 inhibitors from docking studies, but their poor aqueous solubility (0.2–0.7 µM), despite high biological stability, highlights the need for formulation strategies to improve drug-like properties.