Bivalent polyether ionophores: Synthesis and biological evaluation of C 2 -symmetric salinomycin dimers

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.

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.

Recent Advances in the Use of the Dimerization Strategy as a Means to Increase the Biological Potential of Natural or Synthetic Molecules

The design of C₂-symmetric biologically active molecules is a subject of interest to the scientific community. It provides the possibility of discovering medicine with higher biological potential than the parent drugs. Such molecules are generally produced by classic chemistry, considering the shortness of reaction sequence and the efficacy for each step. This review describes and analyzes recent advances in the field and emphasizes selected C₂-symmetric molecules (or axial symmetric molecules) made during the last 10 years. However, the description of the dimers is contextualized by prior work allowing its development, and they are categorized by their structure and/or by their properties. Hence, this review presents dimers composed of steroids, sugars, and nucleosides; known and synthetic anticancer agents; polyphenol compounds; terpenes, known and synthetic antibacterial agents; and natural products. A special focus on the anticancer potential of the dimers transpires throughout the review, notwithstanding their structure and/or primary biological properties.

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.

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.