Background/Objectives: Prostate cancer (PC) is among the most common malignancy in men. Several newly diagnosed patients have a locally advanced disease and distant metastasis at the initial diagnosis time. Castration-resistant PC (CRPC) patients have 100% recurrence incidence despite completing a therapeutic regimen, leading to high mortality. Androgen deprivation therapy and androgen inhibitors are initially effective, but resistance is inevitably developed. Epidemiological studies indicated that the Mediterranean diet, with high olive phenolic contents, is associated with a lower incidence of certain malignancies. This study aims at exploring the mCRPC progression and recurrence-suppressive and molecular effects of the major olive leaf phenolic glucoside (−)-oleuropein (OLE). Results: OLE downregulated the levels of proprotein convertase subtlisin/klexin type 9 (PCSK9) and normalized the low-density lipoprotein receptor (LDLR) in PC cells in vitro. Thus, a PCSK9-LDLR protein–protein interaction (PPI) in silico model was generated and used to assess OLE and its aglycone (OA) ability to bind at PCSK9 and thereby interfere with PCSK9-LDLR PPI. OLE perfectly filled the PCSK9 interface versus OA. Both OLE and OA showed virtual potential to interfere with PCSK9-LDLR PPI. OLE showed modest in vitro viability, migration, and clonogenicity suppressive effects on diverse human PC cell lines. OLE effectively suppressed mCRPC progression and recurrence in a nude mouse xenograft model. RNA-sequencing results proved the PCSK1, PCSK2, and PCSK9 downregulation in OLE-treated recurrent tumors versus vehicle control. Conclusions: Oleuropein is a novel lead useful for the control of mCRPC progression and the prevention of its recurrence via targeting PCSK9 expression and PPI with LDLR.

The proprotein convertase subtilisin kexin type 9 (PCSK9) emerged as a molecular target of great interest for the management of cardiovascular disorders due to its ability to reduce low density lipoprotein (LDL) cholesterol by binding and targeting at LDLR for lysosomal degradation in cells. Preliminary studies revealed that pseurotin A (PsA), a spiro-heterocyclic γ-lactam alkaloid from several marine and terrestrial Aspergillus and Penicillium species, has the ability to dually suppress the PCSK9 expression and protein–protein interaction (PPI) with LDLR, resulting in an anti-hypercholesterolemic effect and modulating the oncogenic role of PCSK9 axis in breast and prostate cancers progression and recurrence. Thus, a preliminary assessment of the PsA acute toxicity represents the steppingstone to develop PsA as a novel orally active PCSK9 axis modulating cancer recurrence inhibitor. PsA studies for in vitro toxicity on RWPE-1 and CCD 841 CoN human non-tumorigenic prostate and colon cells, respectively, indicated a cellular death shown at a 10-fold level of its reported anticancer activity. Moreover, a Western blot analysis revealed a significant downregulation of the pro-survival marker Bcl-2, along with the upregulation of the proapoptotic Bax and caspases 3/7, suggesting PsA-mediated induction of cell apoptosis at very high concentrations. The Up-and-Down methodology determined the PsA LD₅₀ value of >550 mg/kg in male and female Swiss albino mice. Animals were orally administered single doses of PsA at 10, 250, and 500 mg/kg by oral gavage versus vehicle control. Mice were observed daily for 14 days with special care over the first 24 h after dosing to monitor any abnormalities in their behavioral, neuromuscular, and autonomic responses. After 14 days, the mice were euthanized, and their body and organ weights were recorded and collected. Mice plasma samples were subjected to comprehensive hematological and biochemical analyses. Collected mouse organs were histopathologically examined. No morbidity was detected following the PsA oral dosing. The 500 mg/kg female dosing group showed a 45% decrease in the body weight after 14 days but displayed no other signs of toxicity. The 250 mg/kg female dosing group had significantly increased serum levels of liver transaminases AST and ALT versus vehicle control. Moreover, a modest upregulation of apoptotic markers was observed in liver tissues of both animal sexes at 500 mg/kg dose level. However, a histopathological examination revealed no damage to the liver, kidneys, heart, brain, or lungs. While these findings suggest a possible sex-related toxicity at higher doses, the lack of histopathological injury implies that single oral doses of PsA, up to 50-fold the therapeutic dose, do not cause acute organ toxicity in mice though further studies are warranted.

Elevated cholesterol levels play important mitogenic roles. Pseurotin A (PsA) is a fermentation product that has recently been reported as a dual inhibitor of proprotein convertase subtilisin/kexin type 9 (PCSK9) secretion and protein-protein interaction (PPI) with the LDLR. PsA showed a high acute safety profile and therapeutic potential against metastatic castration-resistant prostate cancer (mCRPC). The study aims to uncover the chronic safety, distribution, and anti-mCRPC genomic and molecular mechanistic insights of PsA. A 90-day chronic safety assessment of PsA up to 80 mg/kg in Swiss albino mice showed no signs of hematological, biochemical, or major organ toxicity. PsA demonstrated rapid intravenous distribution and elimination in Swiss albino mice. PsA is biodistributed to multiple key organs but was not detected in the brain, indicating its inability to cross the blood-brain barrier. PsA effectively suppressed the recurrence of nude mice xenografted mCRPC, which was subjected to a neoadjuvant docetaxel and enzalutamide regimen, followed by surgical excision. Collected PsA and vehicle control-treated recurrent tumors were subjected to RNA-sequencing and pathway enrichment analysis (PEA) of differentially expressed genes (DEGs). PsA-treated tumors revealed multiple significantly enriched pathways associated with promoting tumor apoptosis and inhibiting both invasion and migration. The PPI network analyses for the downregulated DEGs displayed prominent networks of genes associated with the ubiquitin-proteasome system. Results provide comprehensive mechanistic and preclinical validations for PsA’s potential as a novel PC recurrence suppressive lead entity.