Anti-trypanosomal activity of doubly modified salinomycin derivatives

Suramin: Effectiveness of analogues reveals structural features that are important for the potent trypanocidal activity of the drug

Suramin was the first effective drug for the treatment of human African sleeping sickness. Structural analogues of the trypanocide have previously been shown to be potent inhibitors of several enzymes. Therefore, four suramin analogues lacking the methyl group on the intermediate rings and with different regiochemistry of the naphthalenetrisulphonic acid groups and the phenyl rings were tested to establish whether they exhibited improved antiproliferative activity against bloodstream forms of Trypanosomes brucei compared to the parent compound. The four analogues exhibited low trypanocidal activity and weak inhibition of the antitrypanosomal activity of suramin in competition experiments. This indicates that the strong trypanocidal activity of suramin is most likely due to the presence of methyl groups on its intermediate rings and to the specific regiochemistry of naphthalenetrisulphonic acid groups. These two structural features are also likely to be important for the inhibition mechanism of suramin because DNA distribution and nucleus/kinetoplast configuration analyses suggest that the analogues inhibit mitosis while suramin inhibits cytokinesis.

Synthesis of urea and thiourea derivatives of C20-epi-aminosalinomycin and their activity against Trypanosoma brucei

Salinomycin (SAL) is a natural polyether ionophore that exhibits a very broad spectrum of biological effects, ranging from anticancer to antiparasitic activities. Our recent studies have shown that the chemical modification of the SAL biomolecule is a fruitful strategy for generating lead compounds for the development of novel antitrypanosomal agents. As a continuation of our program to develop trypanocidal active lead structures, we synthesized a series of 14 novel urea and thiourea analogs of C20-epi-aminosalinomycin (compound 2b). The trypanocidal and cytotoxic activities of the derivatives were assessed with the mammalian life cycle stage of Trypanosoma brucei and human leukemic HL-60 cells, respectively. The most antitrypanosomal compounds were the two thiourea derivatives 4b (C20-n-butylthiourea) and 4d (C20-phenylthiourea) with 50% growth inhibition (GI₅₀) values of 0.18 and 0.22 μM and selectivity indices of 47 and 41, respectively. As potent SAL derivatives have been shown to induce strong cell swelling in bloodstream forms of T. brucei, the effect of compounds 4b and 4d to increase the cell volume of the parasite was also investigated. Interestingly, both derivatives were capable to induce faster cell swelling in bloodstream-form trypanosomes than the reference compound SAL. These findings support the suggestion that C20-epi-aminosalinomycin derivatives are suitable leads in the rational development of new and improved trypanocidal drugs.

Propargylamine: an important moiety in drug discovery

Propargylamine is a chemical moiety whose properties have made it a widely distributed group within the fields of medicinal chemistry and chemical biology. Its particular reactivity has traditionally popularized the preparation of propargylamine derivatives using a large variety of synthetic strategies, which have facilitated the access to these compounds for the study of their biomedical potential. This review comprehensively covers and analyzes the applications that propargylamine-based derivatives have achieved in the drug discovery field, both from a medicinal chemistry perspective and from a chemical biology-oriented approach. The principal therapeutic fields where propargylamine-based compounds have made an impact are identified, and a discussion of their influence and growing potential is included.

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.

Single and double modified salinomycin analogs target stem-like cells in 2D and 3D breast cancer models

Breast cancer remains one of the leading cancers among women. Cancer stem cells (CSCs) are tumor-initiating cells which drive progression, metastasis, and reoccurrence of the disease. CSCs are resistant to conventional chemo- and radio-therapies and their ability to survive such treatment enables tumor reestablishment. Metastasis is the main cause of mortality in women with breast cancer, thus advances in treatment will depend on therapeutic strategies targeting CSCs. Salinomycin (SAL) is a naturally occurring polyether ionophore antibiotic known for its anticancer activity towards several types of tumor cells. In the present work, a library of 17 C1-single and C1/C20-double modified SAL analogs was screened to identify compounds with improved activity against breast CSCs. Six single- and two double-modified analogs were more potent (IC50 range of 1.1 ± 0.1-1.4 ± 0.2 µM) toward the breast cancer cell line MDA-MB-231 compared to SAL (IC50 of 4.9 ± 1.6 µM). Double-modified compound 17 was found to be more efficacious than SAL against the majority of cancer cell lines in the NCI-60 Human Tumor Cell Line Panel. Compound 17 was more potent than SAL in inhibiting cell migration and cell renewal properties of MDA-MB-231 cells, as well as inducing selective loss of the CD44+/CD24/low stem-cell-like subpopulation in both monolayer (2D) and organoid (3D) culture. The present findings highlight the therapeutic potential of SAL analogs towards breast CSCs and identify select compounds that merit further study and clinical development.

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.

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.

The therapeutic potential of phytol towards Trypanosoma congolense infection and the inhibitory effects against trypanosomal sialidase

The search for novel therapeutic candidates against animal trypanosomiasis is an ongoing scientific endevour because of the negative impacts of the disease to the African livestock industry. In this study, the in vivo therapeutic potentials of phytol toward Trypanosoma congolense infection and the inhibitory effects on trypanosomal sialidase were investigated. Rats were infected with T. congolense and administered daily oral treatment of 50 and 100 mg/kg BW of phytol. Within the first 10 days of the treatment, no antitrypanosomal activity was recorded but a moderate trypanostatic activity was observed from day 17-day 21 pi. However, at 100 mg/kg BW, phytol demonstrated a significant (p < 0.05) ameliorative potentials toward T. congolense-induced host-associated pathological damages such as anaemia, hepatic and renal damages; and the data was comparable to diminazine aceturate. Moreover, the T. congolense caused a significant (p < 0.05) increase in free serum sialic acid level which was significantly (p < 0.05) prevented in the presence of phytol (100 mg/kg BW). In an in vitro analysis, phytol inhibited partially purified T. congolense sialidase using an uncompetitive inhibition pattern with inhibition binding constant of 261.24 μmol/mL. Subsequently, molecular docking revealed that the compound binds to homology modelled trypanosomal sialidase with a binding free energy of -6.7 kcal/mol which was mediated via a single hydrogen bond while Trp324 and Pro274 were the critical binding residues. We concluded that phytol has moderate trypanostatic activity but with a great potential in mitigating the host-associated cellular damages while the anaemia amelioration was mediated, in part, through the inhibition of sialidase.

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.