Novel Combretastatin A-4 Analogs-Design, Synthesis, and Antiproliferative and Anti-Tubulin Activity.

Combretastatins isolated from the Combretum caffrum tree belong to a group of closely related stilbenes. They are colchicine binding site inhibitors which disrupt the polymerization process of microtubules in tubulins, causing mitotic arrest. In vitro and in vivo studies have proven that some combretastatins exhibit antitumor properties, and among them, combretastatin A-4 is the most active mitotic inhibitor. In this study, a series of novel combretastatin A-4 analogs containing carboxylic acid, ester, and amide moieties were synthesized and their cytotoxic activity against six tumor cell lines was determined using sulforhodamine B assay. For the most cytotoxic compounds (8 and 20), further studies were performed. These compounds were shown to induce G0/G1 cell cycle arrest in MDA and A549 cells, in a concentration-dependent manner. Moreover, in vitro tubulin polymerization assays showed that both compounds are tubulin polymerization enhancers. Additionally, computational analysis of the binding modes and binding energies of the compounds with respect to the key human tubulin isotypes was performed. We have obtained a satisfactory correlation of the binding energies with the IC50 values when weighted averages of the binding energies accounting for the abundance of tubulin isotypes in specific cancer cell lines were computed.

Synthesis and evaluation of antibacterial and trypanocidal activity of derivatives of monensin A.

The synthesis and biological evaluation of eleven derivatives of the natural polyether ionophore monensin A (MON), modified at the C-26 position, is presented. Eight urethane and three ester derivatives were tested for their antimicrobial activity against different strains of Staphylococcus aureus, Staphylococcus epidermidis, Escherichia coli and Pseudomonas aeruginosa. In addition, their antiparasitic activity was also evaluated with bloodstream forms of Trypanosoma brucei. The majority of the modified ionophores were active against a variety of Gram-positive bacterial strains, including methicillin-resistant S. epidermidis, and showed better antibacterial activity than the unmodified MON. The phenyl urethane derivative of MON exhibited the most promising antibacterial activity of all tested compounds, with minimal inhibitory concentration values of 0.25-0.50 µg/ml. In contrast, none of the MON derivatives displayed higher antitrypanosomal activity than the unmodified ionophore.

Rate of translocation across lipid bilayer of triphenylphosphonium-linked salinomycin derivatives contributes significantly to their K+/H+ exchange activity on membranes.

Salinomycin (SAL), a polyether antibiotic exerting K+/H+-exchange on cellular membranes, effectively kills cancer stem cells. A series of cationic triphenylphosphonium (TPP+)-linked SAL derivatives were synthesized aiming to render them mitochondria-targeted. Remarkably, attaching a TPP+ moiety via a triazole linker at the C-20 position of SAL (compound 5) preserved the ion carrier potency of the antibiotic, while analogs with TPP+ linked at the C-1 position of SAL (6, 8) were ineffective. On planar bilayer lipid membranes (BLM), the SAL analogs 6 and 8 exhibited slow electrical current relaxation upon a voltage jump, similar to previously studied alkyl-TPP compounds. However, 5 demonstrated much faster current relaxation, which suggested its high permeability through BLM resulting in its pronounced potency to transport potassium and hydrogen ions across both artificial (liposomal) and mitochondrial membranes. SAL and 5 did not induce a steady-state electrical current through the planar lipid bilayer, thereby confirming that the transport mechanism is the electrically silent K+/H+ exchange. The ion exchange mediated by 5 in energized mitochondria was more active than that caused by SAL, which was apparently due to accumulation of 5 in mitochondria. Thus, compound 5 can be regarded as a promising lead compound for testing anticancer and antimicrobial activity.

Anti-glioblastoma activity of monensin and its analogs in an organoid model of cancer.

Glioblastoma (GBM) remains the most frequently diagnosed primary malignant brain cancer in adults. Despite recent progress in understanding the biology of GBM, the clinical outcome for patients remains poor, with a median survival of approximately one year after diagnosis. One factor contributing to failure in clinical trials is the fact that traditional models used in GBM drug discovery poorly recapitulate patient tumors. Previous studies have shown that monensin (MON) analogs, namely esters and amides on C-26 were potent towards various types of cancer cell lines. In the present study we have investigated the activity of these molecules in GBM organoids, as well as in a host:tumor organoid model. Using a mini-ring cell viability assay we have identified seven analogs (IC50 = 91.5 ± 54.4-291.7 ± 68.8 nM) more potent than parent MON (IC50 = 612.6 ± 184.4 nM). Five of these compounds induced substantial DNA fragmentation in GBM organoids, suggestive of apoptotic cell death. The most active analog, compound 1, significantly reduced GBM cell migration, induced PARP degradation, diminished phosphorylation of STAT3, Akt and GSK3ß, increased É£H2AX signaling and upregulated expression of the autophagy associated marker LC3-II. To investigate the activity of MON and compound 1 in a tumor microenvironment, we developed human cerebral organoids (COs) from human induced pluripotent stem cells (iPSCs). The COs showed features of early developing brain such as multiple neural rosettes with a proliferative zone of neural stem cells (Nestin+), neurons (TUJ1 +), primitive ventricular system (SOX2 +/Ki67 +), intermediate zone (TBR2 +) and cortical plate (MAP2 +). In order to generate host:tumor organoids, we co-cultured RFP-labeled U87MG cells with fully formed COs. Compound 1 and MON reduced U87MG tumor size in the COs after four days of treatment and induced a significant reduction of PARP expression. These findings highlight the therapeutic potential of MON analogs towards GBM and support the application of organoid models in anti-cancer drug discovery.

Antiproliferative activity of ester derivatives of monensin A at the C-1 and C-26 positions.

Monensin A (MON) is a polyether ionophore antibiotic, which shows a wide spectrum of biological activity, including anticancer activity. A series of structurally diverse monensin esters including its C-1 esters (1-9), C-26-O-acetylated derivatives (10-15), and lactone (16) was synthesized and for the first time evaluated for their antiproliferative activity against four human cancer cell lines with different drug-sensitivity level. All of the MON derivatives exhibited in vitro antiproliferative activity against cancer cells at micromolar concentrations. The majority of the compounds was able to overcome the drug resistance of LoVo/DX and MES-SA/DX5 cell lines. The most active compounds proved to be MON C-26-O-acetylated derivatives (10-15) which exhibited very good resistance index and high selectivity index.