Background/Objectives: Chemoresistance of breast cancers (BCs) is a major impediment to current chemotherapeutics that urges the development of new drugs and new therapeutic approaches. To that end, phenyl 4-(2-oxo-3-alkylimidazolidin-1-yl)benzenesulfonates (PAIB-SOs) were recently prepared to fulfill some of the unmet needs with classic chemotherapeutics. PAIB-SOs are prodrugs bioactivated into potent antimitotics by the cytochrome P450 1A1 (CYP1A1), which is a frequent enzyme in resistant BC cells, but mostly missing in normal cells. Our screening program studies of PAIB-SO chemolibraries selected three prototypical PAIB-SOs as antimitotic prodrugs amenable for studies using BC animal models. Methods: Healthy female CD1^® IGS mice were treated with three prototypical PAIB-SOs, namely CEU-835, -934, and -938, for the determination of their toxicity and half-lives. Moreover, MCF7 tumor-bearing CD1-Foxn1^nu Nude female mice were treated with the three prototypical PAIB-SOs for the determination of their antitumor activity. Results: Herein, we show that multi-intravenous administrations of CEU-835, -934, and -938 at their maximal solubilities are well tolerated in healthy female CD1^® IGS mice, as depicted by the evaluation of distress behaviors, organ necropsy, total blood cell count, and histology. Moreover, the half-life of CEU-835, -934, and -938 administered intravenously in healthy CD1^® IGS female mice were 8.1, 23.2, and 21.5 h, respectively. Finally, their intravenous administrations of CEU-934 and -938 decreased MCF7 tumor growth as efficiently as paclitaxel in MCF7 tumor-bearing CD1-Foxn1^nu Nude mouse model. Conclusions: overall, our study demonstrated for the first time that pentyl-bearing PAIB-SOs are new CYP1A1-dependent prodrugs efficiently decrease breast cancer tumor growth, and show no side effects at their pharmacological concentration in mouse models.