Background: 1,2,3,4,5-pentathiepines (PTEs) are compounds originally identified in marine ascidians and are currently under investigation for their promising pharmacological properties, particularly as potential antineoplastic agents. Objectives: In this study, we investigated the antineoplastic properties of a series of ten indolizine-based PTEs, comprising eight previously reported compounds and two newly synthesized derivatives. Methods: These compounds were evaluated against a panel of human cancer cell lines of diverse tissue origins, as well as, for the first time, on non-cancerous CR9 fibroblasts to assess their cytotoxic selectivity. In addition, their effects were tested on 3D spheroid models, providing preliminary insights into their potential in vivo efficacy. Initial screening focused on cell viability, followed by a more detailed characterization of the most active compounds in terms of their ability to induce apoptosis, necrosis, cell cycle arrest, and reactive oxygen species (ROS) generation. The anti-migratory activity of PTEs and a newly adapted assay to confirm sulfur species release in the cells were also performed for the first time. Results and Conclusions: Our findings reveal that four PTEs bearing hydrophilic, hydrogen-bonding functional groups, particularly the two inspired by natural analogs, exhibited the most potent anticancer activity.

Prostate cancer (PCa) is the second most frequently diagnosed solid malignancy in men and a major cause of cancer-related mortality worldwide. While localized disease is associated with excellent long-term survival, advanced and castration-resistant PCa continues to represent a major therapeutic challenge. Current management ranges from active surveillance for indolent tumors to multimodal systemic approaches for metastatic disease. In this context, natural compounds are attracting increasing interest as adjunctive or novel therapeutic agents. Among these, silymarin, a Silybum marianum-derived flavonolignan complex, has shown promising antineoplastic activity in preclinical PCa models. In vitro, silymarin compounds consistently inhibit PCa cell proliferation by inducing G1 and G2/M cell cycle arrest, upregulating cyclin-dependent kinase inhibitors, and activating caspase-dependent apoptotic pathways. They also modulate key oncogenic signaling pathways involved in cell survival, proliferation, invasion, and metastasis. In vivo xenograft and transgenic models further show reduced tumor growth, angiogenesis, and metastatic spread with limited systemic toxicity. Emerging clinical evidence, including systematic reviews and meta-analyses, suggests translational potential; however, robust randomized trials are needed to define optimal formulations, dosing strategies, and therapeutic efficacy in PCa patients. This review provides a comprehensive overview of the molecular mechanisms, preclinical efficacy, and emerging clinical evidence supporting silymarin as a candidate for future PCa research.