A medicinal chemistry perspective on salinomycin as a potent anticancer and anti-CSCs agent

Effect of stereochemistry at position C20 on the antiproliferative activity and selectivity of N-acylated derivatives of salinomycin

Salinomycin (SAL), a natural polyether ionophore, exhibits a broad spectrum of pharmacological activities, including potent anticancer activity. Over the past decade, much effort has been put into developing methods for rational chemical modification of SAL to obtain semisynthetic analogs with higher anticancer activity than the native structure. In this paper, we describe an optimized procedure for synthesizing C20-aminosalinomycin 2 with native stereochemistry at position C20, which was confirmed by single-crystal X-ray diffraction analysis. We further transformed amine precursor 2 into a series of 48 C20-N-(thio)acylated products, including N-(sulfon)amides, N-(thio)ureas, and N-carbamates (urethanes), along with their sulfur analogs, i.e., S-substituted thiocarbamates and dithiocarbamates. This previously unreported class of derivatives showed superior cytotoxicity mostly in the nano- and subnanomolar concentration range and improved selectivity toward human cancer cells compared to those of chemically unmodified SAL and a commonly used oncological drug cisplatin. Of note, the obtained products inhibited the proliferation of reference cancer cells more effectively than their C20-epi-N-acylated counterparts, pointing out the pivotal role of stereochemistry at position C20. Our findings support the premise that the modification of SAL is a fruitful strategy for products with promising biological activity profiles. Moreover, the straightforward protocols should be of significant value for more elaborate modifications of SAL in the future.

Salinomycin inhibits porcine epidemic diarrhea virus infection by targeting Wnt/β-catenin pathway

Porcine epidemic diarrhea virus (PEDV) is a re-emerging pathogen that causes severe economic losses in the pig industry. Commercial PEDV vaccines provide limited protection against PEDV virulent strains. Therefore, the development of novel vaccines and antiviral drugs is urgently required. In this study, we investigated the inhibitory effects of Salinomycin (SLM) against PEDV infection in vitro. First, the half-maximal cytotoxic concentration (CC50) and half-maximal inhibitory concentration (IC50) of SLM were measured by a cell counting kit 8 (CCK-8) and cytopathic effect (CPE). The results showed that the CC50 of SLM on Vero cells was 7.698 μmol·L-1, and the IC50 for PEDV was 0.998 μmol·L-1. SLM dose-dependently suppressed the PEDV-QY strain infection in vitro. In addition, SLM mainly acted on the internalization and replication stages of the PEDV-QY strain, and had no significant effect on viral inactivation, attachment, and release. Finally, SLM inhibited PEDV infection by suppressing PEDV-induced Wnt/β-catenin activation. Collectively, these results suggest that SLM exerts anti-PEDV effects in vitro and presents a potential as an anti-PEDV drug.

Salinomycin inhibits SREBP1 to sensitize ferroptosis and ameliorate sorafenib resistance in clear cell renal cell carcinoma

Backgrounds Resistance to sorafenib, a frontline therapy for advanced ccRCC, is associated with decreased sensitivity to ferroptosis. Our research focuses on elucidating the mechanisms underlying ccRCC's resistance to sorafenib-induced ferroptosis and identifying potential new agents that could overcome this resistance.

METHODS

The silencing of SREBP1 was employed to evaluate the role of this key transcription factor in lipid synthesis and its contribution to ferroptosis resistance in sorafenib-treated ccRCC cells. The ATF4-mediated induction of SREBP1 following salinomycin treatment was assessed by western blot, RT-PCR, immunohistochemistry, chromatin immunoprecipitation, and dual-luciferase reporter assays. In cultured ccRCC cells, the combined effects of salinomycin and sorafenib on ferroptosis induction were evaluated by assessing cell viability, glutathione levels, malondialdehyde levels, BODIPY fluorescence, and intracellular Fe2+ concentration. In an orthotopic ccRCC mouse model, the synergistic effects of salinomycin and sorafenib on both ferroptosis and tumor progression were examined.

RESULTS

Overexpression of SREBP1 was observed in ccRCC tumor tissue, and induced by sorafenib treatment. Silencing SREBP1 reduced the resistance of ccRCC cells to ferroptosis induced by sorafenib. Salinomycin decreased ATF4 level, which in turn inhibited SREBP1 transcription. Treatment with salinomycin enhanced the sensitivity of ccRCC cells to sorafenib-induced ferroptosis. In the orthotopic xenograft mouse model of ccRCC, the combination of salinomycin and sorafenib showed a synergistic effect in inducing ferroptosis inhibiting tumor growth.

CONCLUSIONS

Salinomycin treatment mitigates resistance to sorafenib-induced ferroptosis by inhibiting SREBP1. The combination of salinomycin and sorafenib synergistically enhances ferroptosis and suppresses ccRCC growth.

Unveiling the microbial influence: bacteria's dual role in tumor metastasis

As cancer research advances, the intricate relationship between the microbiome and cancer is gaining heightened recognition, especially concerning tumor metastasis, where bacterial involvement becomes increasingly complex. This review seeks to systematically examine the dual roles of bacteria in the tumor metastasis process, encompassing both mechanisms that facilitate metastasis and the inhibitory effects exerted by specific microorganisms. We explore the mechanisms through which bacteria influence tumor cell migration by inducing chronic inflammation, evading host immune responses, and remodeling the ECM. Moreover, the immunomodulatory potential of probiotics and genetically engineered bacteria offers promising prospects for the prevention and treatment of tumor metastasis. This article elucidates the complexity and emerging frontiers of bacterial involvement in tumor metastasis by examining the clinical significance of bacteria as potential biomarkers and evaluating the effects of antibiotic usage on the metastatic process. We posit that comprehending the biological characteristics and clinical significance of bacteria, as a critical component of the tumor microenvironment, will offer innovative strategies and theoretical foundations for cancer treatment. Furthermore, this article explores future research directions, including the application of microbiome technologies and bacteria-based therapeutic strategies, thereby offering a valuable perspective for the development of novel anti-cancer approaches.

Anticancer activity of salinomycin quaternary phosphonium salts

In recent years salinomycin has emerged as a promising anticancer drug. Many literature reports have proved its remarkable antiproliferative activity. Moreover, chemical modifications of salinomycin lead to analogues with even higher cytotoxicity against cancer cell lines and a better selectivity index for malignant cells than those of the unmodified compound or a standard anticancer drug such as doxorubicin. In this paper we report the synthesis of a series of twelve novel salinomycin conjugates and their characterization by spectroscopic and spectrometric methods. Salinomycin was conjugated with different triphenylphosphonium cations in order to find out whether the conjugation with mitochondrial targeting vectors would have a beneficial impact on biological properties. Salinomycin and its novel conjugates were tested to determine their in vitro antiproliferative and antimicrobial activity. Taking into account the presence of triphenylphosphonium moiety, the impact of the obtained analogues on mitochondria activity was evaluated by MitoTrackers dyes, furthermore their apoptosis effect and cell cycle arrest were assessed. In addition, the changes in the mitochondrial membrane potential were measured and the ability to generate reactive oxygen species was assessed. Finally, we conducted biophysical studies to investigate the impact of the obtained salinomycin analogues on mitochondrial respiration rates and their electrophysiological properties. Results of this study have proved that conjugation of salinomycin with phosphonium cations leads to promising results in the search for promising anticancer agents.

Urea and Thiourea Derivatives of Salinomycin as Agents Targeting Malignant Colon Cancer Cells

BACKGROUND

Since it was discovered that a natural polyether ionophore called salinomycin (SAL) selectively inhibits human cancer cells, the scientific world has been paying special attention to this compound. It has been studied for nearly 15 years.

OBJECTIVE

Thus, a very interesting research direction is the chemical modification of SAL structure, which could give more biologically active agents.

METHODS

We evaluated the anticancer activity of (thio)urea analogues class of C20-epi-aminosalinomycin (compound 3b). The studies covered the generation of reactive oxygen species (ROS), proapoptotic activity, cytotoxic activity, and lipid peroxidation in vitro.

RESULTS

Thioureas 5a-5d showed antiproliferative activity against selected human colon cancer cell lines greater than that of chemically unmodified SAL, with a 2~10-fold higher potency towards a metastatic variant of colon cancer cells (SW620). Mechanistically, SAL derivatives showed proapoptotic activity in primary colon cancer cells and induced the production of reactive oxygen species (ROS) in these cells. In SW620 cells, SAL derivatives increased lipid peroxidation with a weak effect on apoptosis and low ROS formation with cytotoxic effects followed by cytostatic ones, suggesting different modes of action of the compounds against primary and metastatic colon cancer cells.

CONCLUSION

The results of this study suggested that urea and thiourea derivatives of SAL provide promising leads for the rational development of new anticancer active agents.

Effective Synthesis of C20-Epi-Isothiocyanato-Salinomycin and its Thiourea Derivatives as Potential Anticancer Agents

Salinomycin, a naturally occurring polyether ionophore antibiotic isolated from Streptomyces albus, has been demonstrated potent cytotoxic activity against a variety of cancer cell lines. In particular, it exhibits selective targeting of cancer stem cells. However, systemic toxicity, drug resistance and low bioavailability of the drug significantly limit its potential applications. In this study, the C20-epi-isothiocyanate of salinomycin was designed and synthesized, and then reacted with amines as a versatile synthon to assemble a series of salinomycin thiourea derivatives, which improved the druggability of salinomycin. The antiproliferative activities of the compounds were evaluated in vitro against A549, HepG2, HeLa, 4T1, and MCF-7 cancer cell lines using the CCK-8 assay. The pharmacological results showed that some salinomycin thiourea derivatives exhibited excellent inhibitory activity against at least one of the tested tumor cells and high selectivity. Further mechanistic studies showed that compound 9 f, containing a 3,5-difluorobenzyl moiety, could directly induce apoptosis, probably by increasing caspase-9 protein expression and cell cycle arrest in G1 phase in a concentration dependent manner.

Systematic review on the role of the gut microbiota in tumors and their treatment

Tumors present a formidable health risk with limited curability and high mortality; existing treatments face challenges in addressing the unique tumor microenvironment (hypoxia, low pH, and high permeability), necessitating the development of new therapeutic approaches. Under certain circumstances, certain bacteria, especially anaerobes or parthenogenetic anaerobes, accumulate and proliferate in the tumor environment. This phenomenon activates a series of responses in the body that ultimately produce anti-tumor effects. These bacteria can target and colonize the tumor microenvironment, promoting responses aimed at targeting and fighting tumor cells. Understanding and exploiting such interactions holds promise for innovative therapeutic strategies, potentially augmenting existing treatments and contributing to the development of more effective and targeted approaches to fighting tumors. This paper reviews the tumor-promoting mechanisms and anti-tumor effects of the digestive tract microbiome and describes bacterial therapeutic strategies for tumors, including natural and engineered anti-tumor strategies.

Drug Repurposing of Selected Antibiotics: An Emerging Approach in Cancer Drug Discovery

Drug repurposing is a method of investigating new therapeutic applications for previously approved medications. This repurposing approach to "old" medications is now highly efficient, simple to arrange, and cost-effective and poses little risk of failure in treating a variety of disorders, including cancer. Drug repurposing for cancer therapy is currently a key topic of study. It is a way of exploring recent therapeutic applications for already-existing drugs. Theoretically, the repurposing strategy has various advantages over the recognized challenges of creating new molecular entities, including being faster, safer, easier, and less expensive. In the real world, several medications have been repurposed, including aspirin, metformin, and chloroquine. However, doctors and scientists address numerous challenges when repurposing drugs, such as the fact that most drugs are not cost-effective and are resistant to bacteria. So the goal of this review is to gather information regarding repurposing pharmaceuticals to make them more cost-effective and harder for bacteria to resist. Cancer patients are more susceptible to bacterial infections. Due to their weak immune systems, antibiotics help protect them from a variety of infectious diseases. Although antibiotics are not immune boosters, they do benefit the defense system by killing bacteria and slowing the growth of cancer cells. Their use also increases the therapeutic efficacy and helps avoid recurrence. Of late, antibiotics have been repurposed as potent anticancer agents because of the evolutionary relationship between the prokaryotic genome and mitochondrial DNA of eukaryotes. Anticancer antibiotics that prevent cancer cells from growing by interfering with their DNA and blocking growth of promoters, which include anthracyclines, daunorubicin, epirubicin, mitoxantrone, doxorubicin, and idarubicin, are another type of FDA-approved antibiotics used to treat cancer. According to the endosymbiotic hypothesis, prokaryotes and eukaryotes are thought to have an evolutionary relationship. Hence, in this study, we are trying to explore antibiotics that are necessary for treating diseases, including cancer, helping people reduce deaths associated with various infections, and substantially extending people's life expectancy and quality of life.

Nanocarrier - Mediated Salinomycin Delivery Induces Apoptosis and Alters EMT Phenomenon in Prostate Adenocarcinoma

Salinomycin (Sal) has been recently discovered as a novel chemotherapeutic agent against various cancers including prostate cancer which is one of the most commonly diagnosed cancers affecting male populations worldwide. Herein we designed salinomycin nanocarrier (Sal-NPs) to extend its systemic circulation and to increase its anticancer potential. Prepared nanoform showed high encapsulation and sustained release profile for salinomycin. The present study elucidated the cytotoxicity and mechanism of apoptotic cell death of Sal-NPs against prostate cancer both in vitro and in vivo. At all measured concentrations, Sal-NPs showed more significant cytotoxicity to DU145 and PC3 cells than Sal alone. This effect was mediated by apoptosis, as confirmed by ROS generation, loss of MMP and cell cycle arrest at the G1 phase in both cells. Sal-NPs efficiently inhibited migration of PC3 and DU145 cells via effectively downregulating the epithelial mesenchymal transition. Also, the results confirmed that Sal-NPs can effectively inhibit the induction of Prostate adenocarcinoma in male Wistar rats. Sal-NPs treatment exhibited a decrease in tumour sizes, a reduction in prostate weight, and an increase in body weight, which suggests that Sal-NPs is more effective than salinomycin alone. Our results suggest that the molecular mechanism underlying the Sal-NPs anticancer effect may lead to the development of a potential therapeutic strategy for treating prostate adenocarcinoma.

pH-Responsive Polypropylene Sulfide Magnetic Nanocarrier-Mediated Chemo-Hyperthermia Kills Breast Cancer Stem Cells by Long-Term Reversal of Multidrug Resistance and Chemotherapy Resensitization

Cancer stem cells (CSCs) present a formidable challenge in cancer treatment due to their inherent resistance to chemotherapy, primarily driven by the overexpression of ABC transporters and multidrug resistance (MDR). Despite extensive research on pharmacological small-molecule inhibitors, effectively managing MDR and improving chemotherapeutic outcomes remain elusive. On the other hand, magnetic hyperthermia (MHT) holds great promise as a cancer therapeutic, but there is limited research on its potential to reverse MDR in breast CSCs and effectively eliminate CSCs through combined chemo-hyperthermia. To address these gaps, we developed tumor microenvironment-sensitive, drug-loaded poly(propylene sulfide) (PPS)-coated magnetic nanoparticles (PPS-MnFe). These nanoparticles were employed to investigate hyperthermia sensitivity and MDR reversion in breast CSCs, comparing their performance to that of small-molecule inhibitors. Additionally, we explored the efficacy of combined chemo-hyperthermia in killing CSCs. CSC-enriched breast cancer cells were subjected to low-dose MHT at 42 °C for 30 min and then treated with the chemical MDR inhibitor salinomycin (SAL). The effectiveness of each treatment in inhibiting MDR was assessed by measuring the efflux of the MDR substrate, rhodamine 123 (R123) dye. Notably, MHT induced a prolonged reversal of MDR activity compared with SAL treatment alone. After successfully inhibiting MDR, the breast CSCs were exposed to chemotherapy using paclitaxel to trigger synergistic cell death. The combination of MHT and chemotherapy demonstrated remarkable reductions in stemness properties, MDR reversal, and the effective eradication of breast CSCs in this innovative dual-modality approach.

Salinomycin disturbs Golgi function and specifically affects cells in epithelial-to-mesenchymal transition

Epithelial-to-mesenchymal transition (EMT) gives rise to cells with properties similar to cancer stem cells (CSCs). Targeting the EMT program to selectively eliminate CSCs is a promising way to improve cancer therapy. Salinomycin (Sal), a K+/H+ ionophore, was identified as highly selective towards CSC-like cells, but its mechanism of action and selectivity remains elusive. Here, we show that Sal, similar to monensin and nigericin, disturbs the function of the Golgi. Sal alters the expression of Golgi-related genes and leads to marked changes in Golgi morphology, particularly in cells that have undergone EMT. Moreover, Golgi-disturbing agents severely affect post-translational modifications of proteins, including protein processing, glycosylation and secretion. We discover that the alterations induced by Golgi-disturbing agents specifically affect the viability of EMT cells. Collectively, our work reveals a novel vulnerability related to the EMT, suggesting an important role for the Golgi in the EMT and that targeting the Golgi could represent a novel therapeutic approach against CSCs.

Secondary Metabolites and Biosynthetic Gene Clusters Analysis of Deep-Sea Hydrothermal Vent-Derived Streptomyces sp. SCSIO ZS0520

Streptomyces sp. SCSIO ZS0520 is a deep-sea hydrothermal vent-derived actinomycete. Our previous metabolism investigation showed that Streptomyces sp. SCSIO ZS0520 is a producer of cytotoxic actinopyrones. Here, another four types of secondary metabolites were identified, including six salinomycin isomers (2–7), the macrolide elaiophylin (8), the triterpene N-acetyl-aminobacteriohopanetriol (9), and the pyrone minipyrone (10). Among them, compounds 2–6 and 10 are new compounds. To understand the biosynthetic pathway of these compounds, a bioinformatic analysis of the whole genome was carried out, which identified 34 secondary metabolite biosynthetic gene clusters. Next, the biosynthetic pathways responsive to four types of products were deduced on the basis of gene function predictions and structure information. Taken together, these findings prove the metabolite potential of ZS0520 and lay the foundations to solve the remaining biosynthetic issues in four types of marine natural products.

Interaction of crown ethers with the ABCG2 transporter and their implication for multidrug resistance reversal

Overexpression of ABC transporters, such as ABCB1 and ABCG2, plays an important role in mediating multidrug resistance (MDR) in cancer. This feature is also attributed to a subpopulation of cancer stem cells (CSCs), having enhanced tumourigenic potential. ABCG2 is specifically associated with the CSC phenotype, making it a valuable target for eliminating aggressive and resistant cells. Several natural and synthetic ionophores have been discovered as CSC-selective drugs that may also have MDR-reversing ability, whereas their interaction with ABCG2 has not yet been explored. We previously reported the biological activities, including ABCB1 inhibition, of a group of adamantane-substituted diaza-18-crown-6 (DAC) compounds that possess ionophore capabilities. In this study, we investigated the mechanism of ABCG2-inhibitory activity of DAC compounds and the natural ionophores salinomycin, monensin and nigericin. We used a series of functional assays, including real-time microscopic analysis of ABCG2-mediated fluorescent substrate transport in cells, and docking studies to provide comparative aspects for the transporter-compound interactions and their role in restoring chemosensitivity. We found that natural ionophores did not inhibit ABCG2, suggesting that their CSC selectivity is likely mediated by other mechanisms. In contrast, DACs with amide linkage in the side arms demonstrated noteworthy ABCG2-inhibitory activity, with DAC-3Amide proving to be the most potent. This compound induced conformational changes of the transporter and likely binds to both Cavity 1 and the NBD-TMD interface. DAC-3Amide reversed ABCG2-mediated MDR in model cells, without affecting ABCG2 expression or localization. These results pave the way for the development of new crown ether compounds with improved ABCG2-inhibitory properties.

Expeditive Synthesis of Potent C20-epi-Amino Derivatives of Salinomycin against Cancer Stem-Like Cells

As a continuation of our studies toward the development of small molecules to selectively target cancer stem cells (CSCs), a library of 18 novel derivatives of salinomycin (Sal), a naturally occurring polyether ionophore, was synthesized with a good overall yield using a one-pot Mitsunobu-Staudinger procedure. Compared to the parent structure, the newly synthesized products contained the mono- or disubstituted C20-epi-amine groups. The biological activity of these compounds was evaluated against human mammary mesenchymal HMLER CD24^low/CD44^high cells, a well-established model of breast CSCs, and its isogenic epithelial cell line (HMLER CD24^high/CD44^low) lacking CSC properties. Importantly, the vast majority of Sal derivatives were characterized by low nanomolar activities, comparing favorably with previous data in the literature. Furthermore, some of these derivatives exhibited a higher selectivity for the mesenchymal state compared to the reference Sal and ironomycin, representing a promising new series of compounds with anti-CSC activity.

QAP14 suppresses breast cancer stemness and metastasis via activation of dopamine D1 receptor

Breast cancer is the second leading cause of cancer-related mortality in women, mainly due to metastasis, which is strongly associated with cancer stemness. Our previous studies showed that the eradication of cancer stem-like cells (CSCs) may be related to the activation of dopamine D1 receptor (D1DR). This study aimed to explicitly demonstrate the target-role of D1DR activation in antimetastatic therapy and to investigate the potential efficacy and the underlying D1DR-related mechanisms of QAP14, a new oral compound. 4T1, MDA-MB-231, and D1DR-knockout 4T1 (4T1-D1DR) cells were selected for in vitro study, while 4T1 and 4T1-D1DR cells were further used to establish a mouse allograft model for in vivo study. Our results showed that D1DR is abundantly expressed in both 4T1 and MDA-MB-231 cells and that knocking out D1DR in 4T1 cells accelerated migration and invasion in vitro as well as lung metastasis in vivo. QAP14 inhibited colony formation, cell motility, mammosphere formation and CSC frequency, induced CSC apoptosis and D1DR expression, and increased cAMP/cGMP levels. Additionally, QAP14 showed inhibitory effects on tumor growth and lung metastasis with acceptable safety in vivo. Knocking out D1DR almost completely abolished the efficacy, confirming that QAP14 exhibits its anti-CSC and antimetastatic effects through D1DR activation. The underlying mechanisms involved suppression of the nuclear factor κB (NF-κB)/protein kinase B (Akt) pathway and consequent downregulation of both epithelial-to-mesenchymal transition (EMT) process and cancer stemness. In summary, our findings suggest a potential candidate compound, QAP14, as well as a potential target, D1DR, for metastatic breast cancer therapy.

Endometrial Cancer Stem Cells: Where Do We Stand and Where Should We Go?

Endometrial cancer is one of the most common malignant diseases in women worldwide, with an incidence of 5.9%. Thus, it is the most frequent cancer of the female genital tract, with more than 34,000 women dying, in Europe and North America alone. Endometrial Cancer Stem Cells (CSC) might be drivers of carcinogenesis as well as metastatic and recurrent disease. Therefore, targeting CSCs is of high interest to improve prognosis of patients suffering of advanced or recurrent endometrial cancer. This review describes the current evidence of molecular mechanisms in endometrial CSCs with special emphasis on MYC and NF-κB signaling as well as mitochondrial metabolism. Furthermore, the current status of immunotherapy targeting PD-1 and PD-L1 in endometrial cancer cells and CSCs is elucidated. The outlined findings encourage novel therapies that target signaling pathways in endometrial CSCs as well as immunotherapy as a promising therapeutic approach in the treatment of endometrial cancer to impede cancer progression and prevent recurrence.

Drug Repurposing Strategies for Non-Cancer to Cancer Therapeutics

Global efforts invested for the prevention and treatment of cancer need to be repositioned to develop safe, effective, and economic anticancer therapeutics by adopting rational approaches of drug discovery. Drug repurposing is one of the established approaches to reposition old, clinically approved off patent noncancer drugs with known targets into newer indications. The literature review suggests key role of drug repurposing in the development of drugs intended for cancer as well as noncancer therapeutics. A wide category of noncancer drugs namely, drugs acting on CNS, anthelmintics, cardiovascular drugs, antimalarial drugs, anti-inflammatory drugs have come out with interesting outcomes during preclinical and clinical phases. In the present article a comprehensive overview of the current scenario of drug repurposing for the treatment of cancer has been focused. The details of some successful studies along with examples have been included followed by associated challenges.

Salinomycin as a potent anticancer stem cell agent: State of the art and future directions

Cancer stem cells (CSCs) are a small subpopulation of cells within a tumor that can both self‐renew and differentiate into other cell types forming the heterogeneous tumor bulk. Since CSCs are involved in all aspects of cancer development, including tumor initiation, cell proliferation, metastatic dissemination, therapy resistance, and recurrence, they have emerged as attractive targets for cancer treatment and management. Salinomycin, a widely used antibiotic in poultry farming, was identified by the Weinberg group as a potent anti‐CSC agent in 2009. As a polyether ionophore, salinomycin exerts broad‐spectrum activities, including the important anti‐CSC function. Studies on the mechanism of action of salinomycin against cancer have been continuously and rapidly published since then. Thus, it is imperative for us to update its literature of recent research findings in this area. We here summarize the notable work reported on salinomycin's anticancer activities, intracellular binding target(s), effects on tumor microenvironment, safety, derivatives, and tumor‐specific drug delivery; after that we also discuss the translational potential of salinomycin toward clinical application based on current multifaceted understandings.

Synthesis and Characterization of Salinomycin-Loaded High-Density Lipoprotein and Its Effects on Cervical Cancer Cells and Cervical Cancer Stem Cells

Background

Cervical cancer stem cells (CCSCs), a small part of tumor population, are one of the important reasons for metastasis and recurrence of cervical cancer. Targeting CCSCs may be an effective way to eliminate tumors. Salinomycin (Sal) has been proved to be an effective anticancer drug in many studies, especially for cancer stem cells (CSCs). However, the cytotoxicity of salinomycin limits its further research as an anticancer drug. High-density lipoprotein (HDL) nanoparticles are an excellent drug carrier, which can reduce the toxicity of Sal, have a certain targeting effect and improve the clinical benefit of Sal.

Methods

Salinomycin-loaded high-density lipoprotein (S-HDL) was synthesized and characterized by various analytical techniques. CD44^highCD24^low CCSCs were isolated from HeLa cells by magnetic separation. The uptake of HDL nanoparticles was observed by laser confocal microscopy, and the effect of S-HDL on the proliferation of CCCs and CCSCs was detected by cell viability analysis. Genome-wide analysis was used to analyze the effects of S-HDL on the biological processes of CCCs and then cell apoptosis, cell cycle and cell migration were selected for verification.

Results

S-HDL had a particle size of 38.98 ± 1.78 nm and an encapsulation efficiency of 50.73 ± 4.29%. Cell uptake analysis showed that HDL nanoparticles could enhance the drug uptake of CCCs and CCSCs and may target CCCs and CCSCs. In cell viability analysis, CCCs and CCSCs showed high sensitivity to S-HDL. S-HDL can more efficiently prevent CCSCs from developing tumorspheres than Sal in tumorsphere formation study. S-HDL had stronger ability to induce cell cycle arrest, promote cell apoptosis and inhibit cell migration compared with free Sal, which was consistent with the results of Genome Wide analysis.

Conclusion

S-HDL can effectively target and eliminate CCCs and CCSCs, which is a potential drug for the treatment of cervical cancer.

Anticancer Mechanisms of Salinomycin in Breast Cancer and Its Clinical Applications

Breast cancer (BC) is the most frequent cancer among women worldwide and is the leading cause of cancer-related deaths in women. Cancer cells with stem cell-like features and tumor-initiating potential contribute to drug resistance, tumor recurrence, and metastasis. To achieve better clinical outcomes, it is crucial to eradicate both bulk BC cells and breast cancer stem cells (BCSCs). Salinomycin, a monocarboxylic polyether antibiotic isolated from Streptomyces albus, can precisely kill cancer stem cells (CSCs), particularly BCSCs, by various mechanisms, including apoptosis, autophagy, and necrosis. There is increasing evidence that salinomycin can inhibit cell proliferation, invasion, and migration in BC and reverse the immune-inhibitory microenvironment to prevent tumor growth and metastasis. Therefore, salinomycin is a promising therapeutic drug for BC. In this review, we summarize established mechanisms by which salinomycin protects against BC and discuss its future clinical applications.

Simultaneous immunodetection of ionophore antibiotics, salinomycin and narasin, in poultry products and milk

Rabbit polyclonal antibodies were generated against the ionophore antibiotic salinomycin (SAL) as a determinant of the BSA-SAL conjugate. The homologous ELISA format was found to be preferred for similar recognition of SAL and narasin (NAR) with IC₅₀ values of 0.55 and 0.57 ng mL^-1, respectively. Both analytes could be determined in the range of 0.1-2.7 ng mL^-1 (IC₂₀-IC₈₀) with a detection limit of 0.03 ng mL^-1. To analyze matrices, individual pretreatment of samples was required. For chicken muscles, simple buffer extraction was sufficient to recover 87-110% of ionophores. Extraction with acetonitrile followed by evaporation of the solvent was best for recovering 67-108% SAL and NAR from egg homogenate. A feature of the extraction of ionophores from milk was the elimination of fat-mediated interference by organic solvation. It was found that the absence of Na⁺ and K⁺ ions during reconstitution of sample extracts was a key factor contributing to the increase in the average recovery of ionophores from 32% to 93%. Thanks to this special pretreatment and improved recovery, the developed immunoassay method was suitable for the analysis of ionophore antibiotics SAL and NAR in a milk matrix, which has not been previously reported.

Targeted Delivery of Combination Therapeutics Using Monoclonal Antibody 2C5-Modified Immunoliposomes for Cancer Therapy

PURPOSE

To develop immunoliposomes modified with monoclonal cancer-specific antibody (mAb) 2C5 and co-loaded with a combination of two chemotherapeutics, in order to simultaneously target bulk cancer cells using paclitaxel and cancer stem cells (CSCs) using salinomycin to prevent cancer growth and metastases.

METHODS

Breast cancer cells (MDA-MB-231 and/or SK-BR-3) were chosen as models for all in vitro testing. Liposomes composed of natural phospholipids co-loaded with salinomycin and paclitaxel were prepared and physically characterized. Immunoliposomes modified with mAb 2C5 coupled to polymeric conjugate were prepared and characterized for specific targeting. Wound healing assay was performed using the combination of free drugs in vitro. In vitro studies on cellular interaction and uptake were followed by holographic imaging to study cell-killing, cell-division and proliferation inhibiting effects of the formulation. Ex-vivo study on hemolysis was investigated to check possible toxicity of the formulation.

RESULTS

Physical characterization of the liposomes showed stable nanoparticles of consistent and desirable size range (170-220 nm), zeta potential (-13 mV to - 20 mV), polydispersity indices (<0.2) and drug encapsulation efficiencies (~150 μg per ml for salinomycin, ~210 μg/ml for paclitaxel and 1:1 for combination drug loaded liposomes). Combination therapy strongly affected cancer cell proliferation as shown by significant diminishing of artificial gap closure at the wound site on MDA-MB-231 cells in culture using wound healing assay. Quantitation of changes in wound widths showed ~219 μm for drug combination, ~104 μm for only paclitaxel, and ~ 7 μm for only salinomycin treatments. Statistically significant increase in cellular interaction and specific uptake of the targeted drug co-loaded liposomal nanopreparation (p value ≤ 0.05) by MDA-MB-231 and SK-BR-3 cells confirmed the effectiveness of the approach. Holographic imaging using MDA-MB-231 cells produced visible increase in cell-killing, proliferation and division in vitro. Ex-vivo experimentation showed reduced hemolysis correlating with low toxicity in athymic nude mice model.

CONCLUSION

The results demonstrated the enhanced therapeutic efficacy of a combination of salinomycin and paclitaxel delivered by mAb 2C5-modified liposomal preparation in cancer therapy.

Ester derivatives of salinomycin efficiently eliminate breast cancer cells via ER-stress-induced apoptosis

The polyether ionophore salinomycin (SAL) has been found to selectively target breast cancer cells, including those with stem-like phenotype. On the other hand, SAL amides and esters obtained through derivatisation of the C1 carboxyl of the ionophore were found to exhibit anticancer properties, whilst reducing potential toxicity issues which often occur during standard chemotherapy. However, the studies on the activity and especially on the mechanisms of action of this class of semi-synthetic products against breast cancer cells are very limited. Therefore, in this work, we confirmed the anti-breast cancer activity of SAL, and further investigated the potential of its selected C1 amide and ester analogs to destroy breast cancer cells, including the highly aggressive triple-negative MDA-MB-231 cells. Importantly, SAL esters were found to be more potent than the native structure and their amide counterparts. Our data revealed that SAL ester derivatives, particularly compounds 5 and 7 (2,2,2-trifluoroethyl and benzotriazole ester of SAL, respectively), increase the level of p-eIF2α (Ser51) and IRE1α proteins. Additionally, an increased level of DNA damage indicators such as γH2AX protein and modified guanine (8-oxoG) was observed. These findings suggest that the apoptosis of MCF-7 and MDA-MB-231 cells induced by the most promising esters derived from SAL may result from the interaction between ER stress and DNA damage response mechanisms.

Singly and doubly modified analogues of C20-epi-salinomycin: A new group of antiparasitic agents against Trypanosoma brucei

Polyether ionophores, with >120 molecules belonging to this group, represent a class of naturally-occurring compounds that exhibit a broad range of pharmacological properties, including promising activity towards a variety of parasites. In this context, salinomycin (SAL) seems to be interesting, as this ionophore has been found to be active against parasites that are responsible for a number of human and animal diseases. On the other hand, less explored is the investigation into the anti-parasitic activity of SAL derivatives. Recently, we identified C1 amides and esters of SAL and its analogue, C20-oxosalinomycin, as promising structures for trypanocidal drug candidates. In search for novel compounds effective against African trypanosomes, the synthetic access to a completely new series of C20-epi-salinomycin (compound 2) analogues is described in this paper. This series includes products obtained via derivatisation of either the C1 carboxyl or the C20 hydroxyl of 2, but also C1/C20 double modified derivatives. The anti-trypanosomal activity as well as the cytotoxic activity of these analogues were evaluated with bloodstream forms of T. brucei and human myeloid HL-60 cells, respectively. It was found that the C20 single modified derivatives 8, 12, and 18 (C20 decanoate, C20 ethyl carbonate, and C20 allophanate of 2, respectively) were the most active compounds in selectively targeting bloodstream-form trypanosomes, with 50% growth inhibition (GI₅₀) values of 0.027-0.043 μM and selectivity indices of 165-353. These results indicate that modification at the C20 position of C20-epi-salinomycin 2 can provide semi-synthetic products with enhanced trypanocidal activity that could be of great value for the development of new drugs to treat African trypanosomiasis.

Synthesis and Anticancer Activity of Dimeric Polyether Ionophores

Polyether ionophores represent a group of natural lipid-soluble biomolecules with a broad spectrum of bioactivity, ranging from antibacterial to anticancer activity. Three seem to be particularly interesting in this context, namely lasalocid acid, monensin, and salinomycin, as they are able to selectively target cancer cells of various origin including cancer stem cells. Due to their potent biological activity and abundant availability, some research groups around the world have successfully followed semi-synthetic approaches to generate original derivatives of ionophores. However, a definitely less explored avenue is the synthesis and functional evaluation of their multivalent structures. Thus, in this paper, we describe the synthetic access to a series of original homo- and heterodimers of polyether ionophores, in which (i) two salinomycin molecules are joined through triazole linkers, or (ii) salinomycin is combined with lasalocid acid, monensin, or betulinic acid partners to form 'mixed' dimeric structures. Of note, all 11 products were tested in vitro for their antiproliferative activity against a panel of six cancer cell lines including the doxorubicin resistant colon adenocarcinoma LoVo/DX cell line; five dimers (14-15, 17-18 and 22) were identified to be more potent than the reference agents (i.e., both parent compound(s) and commonly used cytostatic drugs) in selective targeting of various types of cancer. Dimers 16 and 21 were also found to effectively overcome the resistance of the LoVo/DX cancer cell line.

Emerging agents that target signaling pathways in cancer stem cells

Cancer stem cells (CSCs) contribute to the initiation, recurrence, and metastasis of cancer; however, there are still no drugs targeting CSCs in clinical application. There are several signaling pathways playing critical roles in CSC progression, such as the Wnt, Hedgehog, Notch, Hippo, and autophagy signaling pathways. Additionally, targeting the ferroptosis signaling pathway was recently shown to specifically kill CSCs. Therefore, targeting these pathways may suppress CSC progression. The structure of small-molecule drugs shows a good spatial dispersion, and its chemical properties determine its good druggability and pharmacokinetic properties. These characteristics make small-molecule drugs show a great advantage in drug development, which is increasingly popular in the market. Thus, in this review, we will summarize the current researches on the small-molecule compounds suppressing CSC progression, including inhibitors of Wnt, Notch, Hedgehog, and autophagy pathways, and activators of Hippo and ferroptosis pathways. These small-molecule compounds emphasize CSC importance in tumor progression and propose a new strategy to treat cancer in clinic via targeting CSCs.

Overcoming Resistance to Platinum-Based Drugs in Ovarian Cancer by Salinomycin and Its Derivatives-An In Vitro Study

Polyether ionophore salinomycin (SAL) and its semi-synthetic derivatives are recognized as very promising anticancer drug candidates due to their activity against various types of cancer cells, including multidrug-resistant populations. Ovarian cancer is the deadliest among gynecologic malignancies, which is connected with the development of chemoresistant forms of the disease in over 70% of patients after initial treatment regimen. Thus, we decided to examine the anticancer properties of SAL and selected SAL derivatives against a series of drug-sensitive (A2780, SK-OV-3) and derived drug-resistant (A2780 CDDP, SK-OV-3 CDDP) ovarian cancer cell lines. Although SAL analogs showed less promising IC₅₀ values than SAL, they were identified as the antitumor agents that significantly overcome the resistance to platinum-based drugs in ovarian cancer, more potent than unmodified SAL and commonly used anticancer drugs—5-fluorouracil, gemcitabine, and cisplatin. Moreover, when compared with SAL used alone, our experiments proved for the first time increased selectivity of SAL-based dual therapy with 5-fluorouracil or gemcitabine, especially towards A2780 cell line. Looking closer at the results, SAL acted synergistically with 5-fluorouracil towards the drug-resistant A2780 cell line. Our results suggest that combinations of SAL with other antineoplastics may become a new therapeutic option for patients with ovarian cancer.

Synthesis and Anticancer Activity of Tertiary Amides of Salinomycin and Their C20-oxo Analogues

The polyether ionophore salinomycin (SAL) has captured much interest because of its potent activity against cancer cells and cancer stem cells. Our previous studies have indicated that C1/C20 double-modification of SAL is a useful strategy to generate diverse agents with promising biological activity profiles. Thus, herein we describe the synthesis of a new class of SAL analogues that combine key modifications at the C1 and C20 positions. The activity of the obtained SAL derivatives was evaluated using primary acute lymphoblastic leukemia, human breast adenocarcinoma and normal mammary epithelial cells. One single- [N,N-dipropyl amide of salinomycin (5 a)] and two novel double-modified analogues [N,N-dipropyl amide of C20-oxosalinomycin (5 b) and piperazine amide of C20-oxosalinomycin (13 b)] were found to be more potent toward the MDA-MB-231 cell line than SAL or its C20-oxo analogue 2. When select analogues were tested against the NCI-60 human tumor cell line panel, 4 a [N,N-diethyl amide of salinomycin] showed particular activity toward the ovarian cancer cell line SK-OV-3. Additionally, both SAL and 2 were found to be potent ex vivo against human ER/PR⁺ , Her2^- invasive mammary carcinoma, with 2 showing minimal toxicity toward normal epithelial cells. The present findings highlight the therapeutic potential of SAL derivatives for select targeting of different cancer types.

Effects of salinomycin and niclosamide on small cell lung cancer and small cell lung cancer circulating tumor cell lines

Tumor dissemination and recurrence is attributed to highly resistant cancer stem cells (CSCs) which may constitute a fraction of circulating tumor cells (CTCs). Small cell lung cancer (SCLC) constitutes a suitable model to investigate the relation of CTCs and CSCs due to rapid tumor spread and a high number of CTCs. Expansion of five SCLC CTC lines (BHGc7, 10, 16, 26 and UHGc5) in vitro at our institution allowed for the analysis of CSC markers and cytotoxicity of the CSC-selective drugs salinomycin and niclosamide against CTC single cell suspensions or CTC spheroids/ tumorospheres (TOS). Salinomycin exerted dose-dependent cytotoxicity against the SCLC lines but, with exception of BHGc7 TOS, there was no markedly enhanced activity against TOS. Similarly, niclosamide exhibits high activity against BHGc7 TOS and UHGc5 TOS but not against the other CTC spheroids. High expression of the CSC marker CD133 was restricted to three SCLC tumor lines and the BHGc10 CTC line. All SCLC CTCs are CD24-positive but lack expression of CD44 and ABCG2 in contrast to the SCLC tumor lines which show a phenotype more similar to that of CSCs. The stem cell marker SOX2 was found in all CTC lines and SCLC GLC14/16, whereas elevated expression of Oct-3/4 and Nanog was restricted to BHGc26 and UHGc5. In conclusion, the SCLC CTCs established from patients with relapsed disease lack a typical CSC phenotype in respect to chemosensitivity to CSC-selective drugs, surface markers, expression of pluripotent stem cell and transcription factors.

Salinomycin and its derivatives - A new class of multiple-targeted "magic bullets"

The history of drug development clearly shows the scale of painstaking effort leading to a finished product - a highly biologically active agent that would be at the same time no or little toxic to human organism. Moreover, the aim of modern drug discovery can move from "one-molecule one-target" concept to more promising "one-molecule multiple-targets" one, particularly in the context of effective fight against cancer and other complex diseases. Gratifyingly, natural compounds are excellent source of potential drug leads. One of such promising naturally-occurring drug candidates is a polyether ionophore - salinomycin (SAL). This compound should be identified as multi-target agent for two reasons. Firstly, SAL combines a broad spectrum of bioactivity, including antibacterial, antifungal, antiviral, antiparasitic and anticancer activity, with high selectivity of action, proving its significant therapeutic potential. Secondly, the multimodal mechanism of action of SAL has been shown to be related to its interactions with multiple molecular targets and signalling pathways that are synergistic for achieving a therapeutic anticancer effect. On the other hand, according to the Paul Ehrlich's "magic bullet" concept, invariably inspiring the scientists working on design of novel target-selective molecules, a very interesting direction of research is rational chemical modification of SAL. Importantly, many of SAL derivatives have been found to be more promising as chemotherapeutics than the native structure. This concise review article is focused both on the possible role of SAL and its selected analogues in future antimicrobial and/or cancer therapy, and on the potential use of SAL as a new class of multiple-targeted "magic bullet" because of its multimodal mechanism of action.

Doxycycline, salinomycin, monensin and ivermectin repositioned as cancer drugs

Chemotherapy is one of the standard methods for the treatment of malignant tumors. It aims to cause lethal damage to cellular structures, mainly DNA. Noteworthy, in recent years discoveries of novel anticancer agents from well-known antibiotics have opened up new treatment pathways for several cancer diseases. The aim of this review article is to describe new applications for the following antibiotics: doxycycline (DOX), salinomycin (SAL), monensin (MON) and ivermectin (IVR) as they are known to show anti-tumor activity, but have not yet been introduced into standard oncological therapy. To date, these agents have been used for the treatment of a broad-spectrum of bacterial and parasitic infectious diseases and are widely available, which is why they were selected. The data presented here clearly show that the antibiotics mentioned above should be recognised in the near future as novel agents able to eradicate cancer cells and cancer stem cells (CSCs) across several cancer types.