A comprehensive review of salinomycin derivatives as potent anticancer and anti-CSCs agents

A highly selective C-H bond fluorination unlocks conformational reporting in a complex natural product derivative

The site-selective C-H bond fluorination of complex natural products is one of the more sought-after transformations in organic and medicinal chemistry. In many radical-based fluorinations, however, a tempest of poor regio- and stereoselectivity, multiple additions of fluorine, and difficult separations of products conspire to make selective monofluorination appear out of reach. In our fluorination of the antibiotic ionophore salinomycin and its simple derivatives, however, a chain of discoveries, including an unanticipated skeletal rearrangement, provided us a tortuous but unique path to a very selective result, unlocking low-noise conformational reporting by 19F NMR in a widely studied medicinal scaffold.

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.

Dichloroacetate and Salinomycin as Therapeutic Agents in Cancer

Cancer is the second leading cause of mortality worldwide. Despite the available treatment options, a majority of cancer patients develop drug resistance, indicating the need for alternative approaches. Repurposed drugs, such as antiglycolytic and anti-microbial agents, have gained substantial attention as potential alternative strategies against different disease pathophysiologies, including lung cancer. To that end, multiple studies have suggested that the antiglycolytic dichloroacetate (DCA) and the antibiotic salinomycin (SAL) possess promising anticarcinogenic activity, attributed to their abilities to target the key metabolic enzymes, ion transport, and oncogenic signaling pathways involved in regulating cancer cell behavior, including cell survival and proliferation. We used the following searches and selection criteria. (1) Biosis and PubMed were used with the search terms dichloroacetate; salinomycin; dichloroacetate as an anticancer agent; salinomycin as an anticancer agent; dichloroacetate side effects; salinomycin side effects; salinomycin combination therapy; dichloroacetate combination therapy; and dichloroacetate or salinomycin in combination with other agents, including chemotherapy and tyrosine kinase inhibitors. (2) The exclusion criteria included not being related to the mechanisms of DCA and SAL or not focusing on their anticancer properties. (3) All the literature was sourced from peer-reviewed journals within a timeframe of 1989 to 2024. Importantly, experimental studies have demonstrated that both DCA and SAL exert promising anticarcinogenic properties, as well as having synergistic effects in combination with other therapeutic agents, against multiple cancer models. The goal of this review is to highlight the mechanistic workings and efficacy of DCA and SAL as monotherapies, and their combination with other therapeutic agents in various cancer models, with a major emphasis on non-small-cell lung cancer (NSCLC) treatment.

From Pseudocyclic to Macrocyclic Ionophores: Strategies toward the Synthesis of Cyclic Monensin Derivatives

There has been a long search for a simple preparation of new cyclic analogues of ionophore antibiotics. We report a simple and general synthesis of three new cyclic derivatives of polyether ionophore, monensin A (MON). The application of the Huisgen 1,3-dipolar cycloaddition of azides and terminal alkynes to macrocyclization results in a concise, synthetic route to monensin lacton or lactam in only 4 steps. Additionally, macrolactamization by a simple amidation reaction using HATU, a commonly used conjugating agent, gives 72% yields and utilizes neither high dilution techniques nor template effects in the cyclization step. This in turn enables ready access to a range of unnatural MON analogues, showing the ability to form complexes with monovalent and divalent metal cations.

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.

A peptide-salinomycin conjugate with a bystander effect reduces the stemness characteristics of ovarian cancer cells and enhances drug sensitivity

Salinomycin (Sal) has attracted considerable attention in the field of tumor treatment, especially for its inhibitory effect on cancer stem cells (CSCs) and drug-resistant tumor cells. However, its solubility and targeting specificity pose significant challenges to its pharmaceutical development. Sal-A6, a novel peptide-drug conjugate (PDC), was formed by linking the peptide A6 targeting the CSC marker CD44 with Sal using a specific linker. This conjugation markedly enhances the physicochemical properties of Sal and compared to Sal, Sal-A6 demonstrated a significantly increased activity against ovarian cancer. Furthermore, Sal-A6, employing a disulfide bond as a linker, exhibited bystander killing effect. Moreover, it induces substantial cytotoxic effect on both cancer stem cells and drug-resistant cells in addition to enhance chemosensitivity of resistant ovarian cancer cells. In summary, the results indicated that Sal-A6, a novel PDC derived from Sal, has potential therapeutic applications in the treatment of ovarian cancer and drug-resistant patients. Additionally, this discovery offers insights for developing PDC-type drugs using Sal as a foundation.

Exploring Importance and Regulation of Autophagy in Cancer Stem Cells and Stem Cell-Based Therapies

Autophagy is a globally conserved cellular activity that plays a critical role in maintaining cellular homeostasis through the breakdown and recycling of cellular constituents. In recent years, there has been much emphasis given to its complex role in cancer stem cells (CSCs) and stem cell treatment. This study examines the molecular processes that support autophagy and how it is regulated in the context of CSCs and stem cell treatment. Although autophagy plays a dual role in the management of CSCs, affecting their removal as well as their maintenance, the intricate interaction between the several signaling channels that control cellular survival and death as part of the molecular mechanism of autophagy has not been well elucidated. Given that CSCs have a role in the development, progression, and resistance to treatment of tumors, it is imperative to comprehend their biological activities. CSCs are important for cancer biology because they also show a tissue regeneration model that helps with organoid regeneration. In other words, the manipulation of autophagy is a viable therapeutic approach in the treatment of cancer and stem cell therapy. Both synthetic and natural substances that target autophagy pathways have demonstrated promise in improving stem cell-based therapies and eliminating CSCs. Nevertheless, there are difficulties associated with the limitations of autophagy in CSC regulation, including resistance mechanisms and off-target effects. Thus, the regulation of autophagy offers a versatile strategy for focusing on CSCs and enhancing the results of stem cell therapy. Therefore, understanding the complex interactions between autophagy and CSC biology would be essential for creating therapeutic treatments that work in both regenerative medicine and cancer treatment.

Replenishment of mitochondrial Na+ and H+ by ionophores potentiates cutaneous wound healing in diabetes

Diabetic foot ulcer (DFU) is a highly morbid complication in patients with diabetes mellitus, necessitating the development of innovative pharmaceuticals to address unmet medical needs. Sodium ion (Na+) is a well-established mediator for membrane potential and osmotic equilibrium. Recently, Na+ transporters have been identified as a functional regulator of regeneration. However, the role of Na+ in the intricate healing process of mammalian wounds remains elusive. Here, we found that the skin wounds in hyponatremic mice display a hard-to-heal phenotype. Na+ ionophores that were employed to increase intracellular Na+ content could facilitate keratinocyte proliferation and migration, and promote angiogenesis, exhibiting diverse biological activities. Among of them, monensin A emerges as a promising agent for accelerating the healing dynamics of skin wounds in diabetes. Mechanistically, the elevated mitochondrial Na+ decelerates inner mitochondrial membrane fluidity, instigating the production of reactive oxygen species (ROS), which is identified as a critical effector on the monensin A-induced improvement of wound healing. Concurrently, Na+ ionophores replenish H+ to the mitochondrial matrix, causing an enhancement of mitochondrial energy metabolism to support productive wound healing programs. Our study unfolds a new role of Na+, which is a pivotal determinant in wound healing. Furthermore, it directs a roadmap for developing Na+ ionophores as innovative pharmaceuticals for treating chronic dermal wounds in diabetic patients.

MUC1-C is a target of salinomycin in inducing ferroptosis of cancer stem cells

The oncogenic MUC1-C transmembrane protein is a critical effector of the cancer stem cell (CSC) state. Addiction to MUC1-C for self-renewal in the progression of human cancers has emphasized the need for development of anti-MUC1-C agents. However, there are presently no approved small molecules for targeting MUC1-C-dependent CSCs. In screening for small molecules, we identified salinomycin (SAL), an inducer of ferroptosis, as a potent inhibitor of MUC1-C signaling. We demonstrate that SAL suppresses MUC1-C expression by disrupting a NF-κB/MUC1-C auto-inductive circuit that is necessary for ferroptosis resistance. Our results show that SAL-induced MUC1-C suppression downregulates a MUC1-C→MYC pathway that activates genes encoding (i) glutathione-disulfide reductase (GSR), and (ii) the LDL receptor related protein 8 (LRP8), which inhibit ferroptosis by generating GSH and regulating selenium levels, respectively. GSR and LRP8 contribute to the function of glutathione peroxidase 4 (GPX4), an essential negative regulator of ferroptotic cell death. We demonstrate that targeting MUC1-C genetically or with the GO-203 peptide inhibitor suppresses GPX4 expression and GPX activity in association with the induction of ferroptosis. Studies of CSCs enriched by serial passage as tumorspheres further demonstrate that the effects of SAL are mediated by downregulation of MUC1-C and thereby overcoming resistance to ferroptosis. As confirmation of these results, rescue of MUC1-C downregulation with the MUC1-C cytoplasmic domain (i) reversed the suppression of GSR, LRP8 and GPX4 expression, and (ii) attenuated the induction of ferroptosis. These findings identify SAL as a unique small molecule inhibitor of MUC1-C signaling and demonstrate that MUC1-C is an important effector of resistance to ferroptosis.

Spectroscopic, Spectrometric and Computational Studies of New Lasalocid Derivatives and Their Complexes with Selected Metal Cations

A series of five esters of lasalocid with neopentyl alcohol (LasNeo), geraniol (LasGeran), 2-ethylhexanol (LasEtHex), eicosanol (LasEico) and vanillyl alcohol (LasVanil) were synthesized and studied by NMR, FT-IR and ESI-MS. Then, their complexes with lithium, sodium and potassium cations were obtained and examined using FT-IR. The analysis of the products confirmed the synthesis of new esters with good yields. The newly obtained compounds, as well as their complexes with monovalent cations, were proved to be stabilized by a strong system of intramolecular hydrogen bonds. The PM6 semiempirical calculations provided information on the heat of formation (HOF) and permitted the making of visual representations of the structures of the newly synthesized esters and their complexes with the investigated cations. All the computational outcomes were consistent with the spectroscopic data.

Mechanisms and applications of ferroptosis-associated regulators in cancer therapy and drug resistance

Iron is an essential element for almost all living things. Both iron excess and iron deficiency can damage the body's health, but the body has developed complex mechanisms to regulate iron balance. The imbalance of iron homeostasis and lipid peroxidation are important features of ferroptosis. In this review, we summarize the latest regulatory mechanisms of ferroptosis, the roles of relevant regulators that target ferroptosis for cancer therapy, and their relationship to drug resistance. In conclusion, targeting ferroptosis is an important strategy for cancer therapy.

EGFR-targeting peptide conjugated polymer-lipid hybrid nanoparticles for delivery of salinomycin to osteosarcoma

CONTEXT

Salinomycin (SAL) is a chemotherapeutic drug with anti-osteosarcoma efficacy, but its hydrophobic properties have hindered its application. Nanoparticles have been widely used as drug carriers to improve the solubility of hydrophobic drugs. The dodecapeptide GE11 has been shown to have great binding affinity to the epidermal growth factor receptor (EGFR), which is highly overexpressed in osteosarcoma.

MATERIALS AND METHODS

We designed novel SAL-loaded GE11-conjugated polymer-lipid hybrid nanoparticles (GE11-NPs-SAL) to target osteosarcoma. The characterization and antitumor activity of GE11-NPs-SAL were evaluated both in vitro and in vivo.

RESULTS

The results showed that GE11-NPs-SAL had a size of ~100 nm with a high encapsulation efficacy of ~80%. Compared with the non-targeted nanoparticles, GE11-NPs-SAL showed increased internalization in osteosarcoma cells and improved therapeutic efficacy in osteosarcoma both in vitro and in vivo.

CONCLUSIONS

GE11-NPs-SAL is a promising treatment for osteosarcoma.

Screening of priority antibiotics in Chinese seawater based on the persistence, bioaccumulation, toxicity and resistance

Antibiotics are emerging pollutants that have detrimental effects on both target and non-target organisms in the environment. However, current methods for environmental risk assessment primarily focus on the risk to non-target organisms in ecosystems, overlooking a crucial risk of antibiotics - the induction of resistance in targeted bacteria. To address this oversight, we have incorporated resistance (R) risk with persistence, bioaccumulation and toxicity (PBT) to establish a more comprehensive PBTR (persistence, bioaccumulation, toxicity, and resistance) framework for antibiotic-specific risk assessment. Using the PBTR framework, we evaluated 74 antibiotics detected in Chinese seawater from 2000 to 2021, and identified priority antibiotics. Our analysis revealed that the priority antibiotics with R risk accounted for the largest proportion (50% to 70%), followed by P risk (40% to 58%), T risk (16% to 35%) and B risk (0 to 13%). To further categorize these priority antibiotics, we assigned them a risk level according to their fulfillment of criteria related to P, B, T, and R. Antibiotics meeting all four indicators were classified as Grade I, representing the highest risk level. Grade II and Grade III were assigned to antibiotics meeting three or two indicators, respectively. Antibiotics meeting only one indicator were classified as Grade IV, representing the lowest risk level. The majority of priority antibiotics fell into Grade IV, indicating low risk (55% to 79%), followed by Grade III (16% to 45%). The highest risk antibiotic identified in this study was clindamycin (CLIN), categorized as Grade II, in the East China Sea. Our findings aligned with previous studies for 25 antibiotics, affirming the validity of the PBTR framework. Moreover, we identified 13 new priority antibiotics, highlighting the advancement of this approach. This study provides a feasible screening strategy and monitoring recommendations for priority antibiotics in Chinese seawater.

Nanomedicine strategies to counteract cancer stemness and chemoresistance

Cancer stem-like cells (CSCs) identified by self-renewal ability and tumor-initiating potential are responsible for tumor recurrence and metastasis in many cancers. Conventional chemotherapy fails to eradicate CSCs that hold a state of dormancy and possess multi-drug resistance. Spurred by the progress of nanotechnology for drug delivery and biomedical applications, nanomedicine has been increasingly developed to tackle stemness-associated chemotherapeutic resistance for cancer therapy. This review focuses on advances in nanomedicine-mediated therapeutic strategies to overcome chemoresistance by specifically targeting CSCs, the combination of chemotherapeutics with chemopotentiators, and programmable controlled drug release. Perspectives from materials and formulations at the nano-scales are specifically surveyed. Future opportunities and challenges are also discussed.

Novel Cerium(IV) Coordination Compounds of Monensin and Salinomycin

The largely uncharted complexation chemistry of the veterinary polyether ionophores, monensic and salinomycinic acids (HL) with metal ions of type M⁴⁺ and the known antiproliferative potential of antibiotics has provoked our interest in exploring the coordination processes between MonH/SalH and ions of Ce⁴⁺. (1) Methods: Novel monensinate and salinomycinate cerium(IV)-based complexes were synthesized and structurally characterized by elemental analysis, a plethora of physicochemical methods, density functional theory, molecular dynamics, and biological assays. (2) Results: The formation of coordination species of a general composition [CeL₂(OH)₂] and [CeL(NO₃)₂(OH)], depending on reaction conditions, was proven both experimentally and theoretically. The metal(IV) complexes [CeL(NO₃)₂(OH)] possess promising cytotoxic activity against the human tumor uterine cervix (HeLa) cell line, being highly selective (non-tumor embryo Lep-3 vs. HeLa) compared to cisplatin, oxaliplatin, and epirubicin.

Ionophore Toxicity in Animals: A Review of Clinical and Molecular Aspects

For many years, ionophores have been used to control coccidiosis in poultry. However, misuse of ionophores can cause toxicity with significant clinical symptoms. The most critical factors influencing ionophores' toxicity are administration dose, species, and animal age. Although clinical signs of ionophore intoxication are well studied, the toxicity mechanisms of the ionophores at the molecular level still are not fully elucidated. This review summarizes the studies focused on polyether ionophores toxicity mechanisms in animals at the clinical and molecular levels. Studies show that ionophore toxicity mainly affects myocardial and skeletal muscle cells. The molecular mechanism of the toxication could be explained by the inhibition of oxidative phosphorylation via dysregulation of ion concentration. Tiamulin-ionophore interaction and the synergetic effect of tiamulin in ionophore biotransformation are discussed. Furthermore, in recent years ionophores were candidates for reprofiling as antibacterial and anti-cancer drugs. Identifying ionophores' toxicity mechanisms at the cellular level will likely help develop novel therapies in veterinary and human medicine.

Evidence of Metallic and Polyether Ionophores as Potent Therapeutic Drug Candidate in Cancer Management

Cancer remains one of the most crucial human malignancies with a higher mortality rate globally, and is predicted to escalate soon. Dysregulated ion homeostasis in cancerous cells prompted the researchers to investigate further ion homeostasis impeding agents as potent anticancerous agents. Reutilization of FDA-approved non-cancerous drugs has emerged as a practical approach to developing potent, cost-effective drugs for cancer treatment. Across the globe, most nations are incapable of fulfilling the medical demands of cancer patients due to costlier cancerous drugs. Therefore, we have inclined our review towards emphasizing recent advancements in cancer therapies involving ionophores utilization in exploring potent anticancer drugs. Numerous research reports have established the significant anticancerous potential of ionophores in several pre-clinical reports via modulating aberrant cell signaling pathways and enhancing antitumor immunity in immune cells. This review has mainly summarized the most significant ion homeostasis impeding agents, including copper, zinc, calcium, and polyether, that presented remarkable potential in cancer therapeutics via enhanced antitumor immunity and apoptosis induction. Altogether, this study could provide a robust future perspective for developing cost-effective anticancerous drugs rapidly and cost-effectively, thereby combating the limitations of currently available drugs used in cancer treatment.

Iron-Sensitive Prodrugs That Trigger Active Ferroptosis in Drug-Tolerant Pancreatic Cancer Cells

Persister cancer cells represent rare populations of cells resistant to therapy. Cancer cells can exploit epithelial-mesenchymal plasticity to adopt a drug-tolerant state that does not depend on genetic alterations. Small molecules that can interfere with cell plasticity or kill cells in a cell state-dependent manner are highly sought after. Salinomycin has been shown to kill cancer cells in the mesenchymal state by sequestering iron in lysosomes, taking advantage of the iron addiction of this cell state. Here, we report the chemo- and stereoselective synthesis of a series of structurally complex small molecule chimeras of salinomycin derivatives and the iron-reactive dihydroartemisinin. We show that these chimeras accumulate in lysosomes and can react with iron to release bioactive species, thereby inducing ferroptosis in drug-tolerant pancreatic cancer cells and biopsy-derived organoids of pancreatic ductal adenocarcinoma. This work paves the way toward the development of new cancer medicines acting through active ferroptosis.

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.

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.

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.

Synthesis and anti-tumor activity evaluation of salinomycin C20-O-alkyl/benzyl oxime derivatives

Seventeen C20-O-alkyl/benzyl oxime derivatives were synthesized by a concise and effective method. Most of these derivatives showed tens to several hundred nanomolar IC₅₀ values against HT-29 colorectal, HGC-27 gastric and MDA-MB-231 breast cancer cells, whose antiproliferative activity is 15-240 fold better than that of salinomycin. The C20-oxime etherified derivatives can coordinate potassium ions, and further adjust the cytosolic Ca²⁺ concentrations in HT-29 cells. The significant improvement of the potency should be attributed to the better ion binding and transport ability of the modified derivatives. In addition, the C20-O-alkyl/benzyl oxime derivatives showed much better selectivity indexes (SI) than salinomycin, indicating that they present lower neurotoxic risk.

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.

Evaluation of the anticancer activity of singly and doubly modified analogues of C20-epi-salinomycin

In developed countries, cancer is the second leading cause of death, with colon and prostate cancer belonging to the group of most often diagnosed types of neoplastic diseases. The search for new treatment strategies against these types of cancer is thus of top current interest. In this context, salinomycin (SAL), a naturally occurring polyether ionophore, has been identified recently as a very promising anticancer drug candidate towards several tumour cells. In the present work, a broad library of 24 derivatives of C20-epi-salinomycin (2), including C1 singly, C20 singly and C1/C20 doubly modified analogue structures, was screened to identify compounds with improved activity against colon and prostate cancer cells. Our study demonstrated that the growth inhibitory potency of the parent compound on both primary and metastatic colon cancer cells was similar to that of the semisynthetic products derived from SAL, and simultaneously the SAL analogues showed more potent toxic action on metastatic prostate cancer cells than that of the chemically unmodified ionophore. In contrast to the widely used oncological drug doxorubicin, some of the SAL derivatives demonstrated promising anticancer activity with no toxic effects on non-tumour cells, and with more favourable cytotoxicity than that of a reference agent 5-fluorouracil. Mechanistically, the SAL analogues induced late apoptosis in colon cancer cells and necrosis in prostate cancer cells, as well as reduced secretion of interleukin 6 (IL-6) in these cells.

The Regiocontrollable Enantioselective Synthesis of Chiral Trifluoromethyl-Containing Spiro-Pyrrolidine-Pyrazolone Compounds via Amino-Regulated 1,3-Proton Migration Reaction

An amino-controlled regiodivergent asymmetric synthesis of CF₃-containing spiro-pyrrolidine-pyrazolone compounds is described. With alkaloid-derived squaramide as catalyst, the 1,3-dipolar cycloaddition of α,β-unsaturated pyrazolone with diethyl 2-((2,2,2-trifluoroethyl)imino) malonate offered adducts in excellent yields, dr, and ee. While the cyclohexanediamine-derived squaramide was employed, the reaction afforded a series of structure isomers through a switched umpolung reaction.

Salinomycin-Based Drug Delivery Systems: Overcoming the Hurdles in Cancer Therapy

Cancer stem cells (CSCs) are reportedly responsible for the initiation and propagation of cancer. Since CSCs are highly resistant to conventional chemo- and radiotherapy, they are considered the main cause of cancer relapse and metastasis. Salinomycin (Sali), an anticoccidial polyether antibiotic, has emerged as a promising new candidate for cancer therapy, with selective cytotoxicity against CSCs in various malignancies. Nanotechnology provides an efficient means of delivering Sali to tumors in view of reducing collateral damage to healthy tissues and enhancing the therapeutic outcome. This review offers an insight into the most recent advances in cancer therapy using Sali-based nanocarriers.

Aptamer and Peptide-Modified Lipid-Based Drug Delivery Systems in Application of Combined Sequential Therapy of Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is known as the most common malignancy of the hepatobiliary system with a continued increase in incidence but limited therapeutic options. Nanomedicine has provided a promising strategy through engineered nanocarriers that are capable of targeting therapeutic agents specifically to tumor cells. In this research, two aptamer/peptide-modified lipid-based drug delivery systems (A54-PEG-SLN/OXA and A15-PEG-SLN/SAL) were developed as a sequential therapeutic strategy to conquer specific hepatocellular carcinoma. The nanomedicine A54-PEG-SLN/OXA was able to target specific hepatocellular carcinoma cell BEL-7402 and exhibited a strong targeting ability and antitumor efficiency both in vitro and in vivo. The A15-PEG-SLN/SAL could target and penetrate deeply to the spheroid composed of CD133+ cancer cells. In the study of developing a sequential therapeutic strategy, we demonstrated that A54-PEG-SLN/OXA could kill tumor cells and expose CD133+ cancer cells. After the administration of A15-PEG-SLN/SAL, the growth of the tumors was significantly inhibited. In conclusion, the aptamer/peptide-modified lipid-based drug delivery systems, A54-PEG-SLN/OXA and A15-PEG-SLN/SAL, could specifically target carcinoma cells and had an evident antitumor effect when administrated sequentially.

Drug repurposing approach to combating coronavirus: Potential drugs and drug targets

In the past two decades, three highly pathogenic human coronaviruses severe acute respiratory syndrome coronavirus (SARS-CoV), Middle East respiratory syndrome coronavirus, and, recently, SARS-CoV-2, have caused pandemics of severe acute respiratory diseases with alarming morbidity and mortality. Due to the lack of specific anti-CoV therapies, the ongoing pandemic of coronavirus disease 2019 (COVID-19) poses a great challenge to clinical management and highlights an urgent need for effective interventions. Drug repurposing is a rapid and feasible strategy to identify effective drugs for combating this deadly infection. In this review, we summarize the therapeutic CoV targets, focus on the existing small molecule drugs that have the potential to be repurposed for existing and emerging CoV infections of the future, and discuss the clinical progress of developing small molecule drugs for COVID-19.

Dichloroacetate attenuates the stemness of hepatocellular carcinoma cells via promoting nucleus-cytoplasm translocation of YAP

The antitumor effects of dichloroacetate (DCA) have been widely explored, however, its roles in hepatocellular carcinoma (HCC) progression are still unclear. In the current work, we found that DCA had little effects on HCC cell viability, but could attenuate the stemness of HCC cells, which is evident by decreasing the tumor sphere-formation ability, ALDH activity and the expression of stemness critical regulators. Mechanistic studies based on RNA-sequencing data showed that DCA activated the Hippo pathway. Furthermore, we indicated that DCA promoted the nucleus-cytoplasm translocation of YAP, but not TAZ, another critical executor of Hippo pathway. Moreover, suppressing of Hippo pathway using XMU-MP-1, an inhibitor of Hippo pathway, partially abrogated DCA-induced inhibitory effects on HCC cell stemness. This work suggests that DCA might be a potential inhibitor for HCC progression.

Dichloroacetate attenuates the stemness of colorectal cancer cells via trigerring ferroptosis through sequestering iron in lysosomes

Colorectal cancer stem cell (CSC) has been regarded to be the root of colorectal cancer progression. However, there is still no effective therapeutic method targeting colorectal CSC in clinical application. Here, we investigated the effects of dichloroacetate (DCA) on colorectal cancer cell stemness. We showed that DCA could reduce colorectal cancer cell stemness in a dose-dependent manner, which is evident by the decreased expression of stemness markers, tumor cell sphere-formation and cell migration ability. In addition, it was found that DCA trigerred the ferroptosis of colorectal CSC, which is characterized as the upregulation of iron concentration, lipid peroxides, and glutathione level, and decreased cell viability. Mechanistic studies demonstrated that DCA could sequester iron in lysosome and thus trigger ferroptosis, which is necessary for DCA-mediated attenuation on colorectal cancer cell stemness. Taken together, this work suggests that DCA might be a colorectal CSC-killer.

A novel 3D polycaprolactone high-throughput system for evaluation of toxicity in normoxia and hypoxia

Two-dimensional (2D) culturing of cancer cells has been indispensable for the development of anti-cancer drugs. Drug development, however, is lengthy and costly with a high attrition rate, calling to mind that 2D culturing does not mimic the three-dimensional (3D) tumour microenvironment in vivo. Thus, began the development of 3D culture models for cancer research. We have constructed a 3D 96-well plate using electrospun fibres made of biocompatible polycaprolactone (PCL). Finely-cut PCL fibre pieces in water/ethanol solution was pipetted to the wells of hydrophobic 96-well plates. A fibrous network of approximately 200 μm thickness and high porosity was formed after crosslinking and drying. Human JIMT-1 breast cancer cells or fibroblasts were seeded into the network. Confocal microscopy shows that the cells grow throughout the fibre network. The toxicity of paclitaxel and an experimental salinomycin analogue was assessed and compared in 2D and 3D cultures incubated under conditions of normoxia and hypoxia often found in tumours. The toxicity of both compounds is lower when the cells are cultured in 3D compared to 2D in either normoxia or hypoxia. We conclude that our 96-well assay is a cost-efficient tool that may be used for high-throughput pre-clinical screening of potential anti-cancer compounds.

Itraconazole attenuates the stemness of nasopharyngeal carcinoma cells via triggering ferroptosis

Radiotherapy is a common therapy method for nasopharyngeal carcinoma (NPC) treatment; however, radioresistance greatly limits the clinical efficiency and prognosis of NPC patients. Therefore, it is extremely urgent to reveal the underlying mechanism contributing to radioresistance and find possible diagnostic biomarkers. Here, we collected the spheroids formed by NPC cells, which had been confirmed to hold the stem cell-like traits, and found that these spheroids exhibited a certain degree of radioresistance. Additionally, NPC spheroids displayed a certain degree of ferroptosis resistance, as evident by the decrease of iron concentration in lysosomes and lipid peroxides oxygen, and increase of glutathione (GSH) level. Furthermore, we revealed that itraconazole triggered the ferroptosis of NPC spheroids, which is characterized as the increase of iron concentration and lipid peroxides oxygen, and decrease of GSH level, and decreased the cell viability of NPC spheroids. Notably, itraconazole partially reversed the radioresistance of NPC spheroids. Mechanistically, we found that itraconazole can sequester iron in lysosome and thus trigger ferroptosis; this is essential for itraconazole-mediated attenuation on NPC spheroid stemness. Therefore, this study provides evidences showing that itraconazole might be used for killing NPC stem cells and thus attenuate radioresistance.

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.

Design of naturally inspired jellyfish-shaped cyclopolylactides to manage osteosarcoma cancer stem cells fate

We report the synthesis, characterization and biological profile of new bis-triazoled cyclopolylactides (c-PLA, c-PLA-FA, c-PLA-Rhod) obtained by an optimized combination of ROP and click chemistry reactions. Cyclo-PLA having a number average molecular weight of 6000 g mol^-1 and a polydispersity index of 1.52 was synthetized by click ring-closure of well-defined α,ω-heterodifunctional linear precursors, followed by quaternarization of N₃-triazole nodes, and subsequent CuAAC with azido-folate and azido-rhodamine yielding jellyfish-shaped c-PLA-FA and c-PLA-Rhod. Salinomycin (Sal) was loaded into jellyfish-shaped c-PLA-FA and c-PLA-Rhod nanoparticles (NPs) by nanoprecipitation, with a good encapsulation efficiency (79% and 84%, respectively) and loading content (7.1% and 7.6%, respectively). The biological studies focused on their antiproliferative effects on osteosarcoma bulk MG63 and cancer stem cells (CSCs). The cycloPLA-based NPs, with a size ranging between 125 and 385 nm, killed CSCs and MG63, with a higher efficacy on CSCs; they (unloaded or Sal-loaded) evoked on CSCs a cellular response similar to the payload, with a higher effect than the free Sal. Internalization studies indicated a fast cellular uptake (within 2 h) and sarcospheres remained fluorescent till 72 h. To the best of our knowledge, this is the first study reporting anti-CSCs properties of cycloPLA with jellyfish architecture and we believe could contribute to the development of effective strategies for osteosarcoma targeting.

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.

A symbiotic bacterium of shipworms produces a compound with broad spectrum anti-apicomplexan activity

Apicomplexan parasites cause severe disease in both humans and their domesticated animals. Since these parasites readily develop drug resistance, development of new, effective drugs to treat infection caused by these parasites is an ongoing challenge for the medical and veterinary communities. We hypothesized that invertebrate-bacterial symbioses might be a rich source of anti-apicomplexan compounds because invertebrates are susceptible to infections with gregarines, parasites that are ancestral to all apicomplexans. We chose to explore the therapeutic potential of shipworm symbiotic bacteria as they are bona fide symbionts, are easily grown in axenic culture and have genomes rich in secondary metabolite loci [1,2]. Two strains of the shipworm symbiotic bacterium, Teredinibacter turnerae, were screened for activity against Toxoplasma gondii and one strain, T7901, exhibited activity against intracellular stages of the parasite. Bioassay-guided fractionation identified tartrolon E (trtE) as the source of the activity. TrtE has an EC50 of 3 nM against T. gondii, acts directly on the parasite itself and kills the parasites after two hours of treatment. TrtE exhibits nanomolar to picomolar level activity against Cryptosporidium, Plasmodium, Babesia, Theileria, and Sarcocystis; parasites representing all branches of the apicomplexan phylogenetic tree. The compound also proved effective against Cryptosporidium parvum infection in neonatal mice, indicating that trtE may be a potential lead compound for preclinical development. Identification of a promising new compound after such limited screening strongly encourages further mining of invertebrate symbionts for new anti-parasitic therapeutics.

Xenophagy in cancer

Macroautophagy (herein autophagy) is an intracellular pathway in which cytoplasmic components are captured by double-membrane vesicles (autophagosomes) that eventually fuse with lysosomes to degrade the cargo. Basal levels of autophagy in all eukaryotic cells maintain cellular homeostasis and under conditions of stress, organelles and proteins not essential for survival are degraded. Apart from these functions, cargoes like aggregated proteins, damaged organelles and intracellular pathogens, which are otherwise harmful to cells, are also selectively captured by autophagy and are destined for degradation. In terms of infectious diseases, pathogens are cleared by a specific form of autophagy known as xenophagy. This lysosomal mediated degradation of pathogens also increases the antigen presentation of cells thereby inducing a further immune response. The process of xenophagy provides a broad spectrum of defense mechanism to capture bacterial, viral and protozoan pathogens. However, pathogens have developed ingenious mechanisms to modulate xenophagy to enhance their intracellular survival. Meanwhile, certain pathogens also induce deleterious effects such as chronic inflammation and overexpression of oncogenes in the host system. This over time can increase the susceptibility of the host for tumorigenesis. Hence targeting tumor through anti-microbial mechanisms like xenophagy could be a novel strategy for combinatorial anti-cancer therapy. The recent developments in understanding the role of xenophagy in combating cancer causing pathogens will be discussed in this review.

Transition metal chelators, pro-chelators, and ionophores as small molecule cancer chemotherapeutic agents

Transition metal chelators and ionophores have shown promise as alternative chemotherapeutic strategies by selectively altering the concentrations of iron, copper, and zinc in cancer cells.

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.

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.