Dual Antibiotic Approach: Synthesis and Antibacterial Activity of Antibiotic-Antimicrobial Peptide Conjugates.

In recent years, bacterial resistance to conventional antibiotics has become a major concern in the medical field. The global misuse of antibiotics in clinics, personal use, and agriculture has accelerated this resistance, making infections increasingly difficult to treat and rendering new antibiotics ineffective more quickly. Finding new antibiotics is challenging due to the complexity of bacterial mechanisms, high costs and low financial incentives for the development of new molecular scaffolds, and stringent regulatory requirements. Additionally, innovation has slowed, with many new antibiotics being modifications of existing drugs rather than entirely new classes. Antimicrobial peptides (AMPs) are a valid alternative to small-molecule antibiotics offering several advantages, including broad-spectrum activity and a lower likelihood of inducing resistance due to their multifaceted mechanisms of action. However, AMPs face challenges such as stability issues in physiological conditions, potential toxicity to human cells, high production costs, and difficulties in large-scale manufacturing. A reliable strategy to overcome the drawbacks associated with the use of small-molecule antibiotics and AMPs is combination therapy, namely the simultaneous co-administration of two or more antibiotics or the synthesis of covalently linked conjugates. This review aims to provide a comprehensive overview of the literature on the development of antibiotic-AMP conjugates, with a particular emphasis on critically analyzing the design and synthetic strategies employed in their creation. In addition to the synthesis, the review will also explore the reported antibacterial activity of these conjugates and, where available, examine any data concerning their cytotoxicity.

Solid-Phase Synthesis of Gly-Ψ[CH(CF3)NH]-Peptides.

The solid-phase synthesis of Gly-Ψ[CH(CF3)NH]-peptides is presented. In order to achieve this goal, the synthesis of Gly-Ψ[CH(CF3)NH]-dipeptides having the C-terminus unprotected, the N-terminus protected as Fmoc- or Teoc-, and possibly side chain functionalities protected with acid-labile protecting groups has been developed. A selected small library of six peptidomimetics, encompassing analogues of biological relevant peptides, have been obtained in high purity.

Synthesis of Orthogonally Protected Labionin.

We report the first synthesis of the complex amino acid labionin in a fully orthogonally protected and stereopure form. The structure-which incorporates five orthogonal protecting groups and three stereogenic centers-was assembled using two key synthetic steps: (1) a thia-Michael addition for installing the thioether bridge; (2) an electrophilic azidation for creating the central quaternary α-amino acid carbon in a stereochemically pure form. This work is expected to enable the solid phase synthesis of both natural and synthetic analogues labyrinthopeptins.

KEMTUB012-NI2, a novel potent tubulysin analog that selectively targets hypoxic cancer cells and is potentiated by cytochrome p450 reductase downregulation.

PURPOSE: There is an urgent need to develop effective therapies and treatment strategies to treat hypoxic tumors, which have a very poor prognosis and do not respond well to existing therapies. METHODS: A novel hypoxia-targeting agent, KEMTUB012-NI2, was synthesized by conjugating a 2-nitroimidazole hypoxia-targeting moiety to a synthetic tubulysin, a very potent antimitotic. Its hypoxic selectivity and mode of action were studied in breast cancer cell lines. RESULTS: KEMTUB012-NI2 exhibited a similar selectivity for hypoxic cells to that of tirapazamine, a well-established hypoxia-targeting agent, but was >1,000 times more potent in cell cytotoxicity assays. The hypoxia-targeting mechanism for both KEMTUB012-NI2 and tirapazamine was selective and mediated by one-electron reductases. However, while cytochrome p450 reductase (POR) downregulation could inhibit tirapazamine cytotoxicity, it actually sensitized hypoxic cells to KEMTUB012-NI2. CONCLUSION: KEMTUB012-NI2 is a potent new agent that can selectively target hypoxic cancer cells. The hypoxia selectivity of KEMTUB012-NI2 and tirapazamine appears to be differentially activated by reductases. Since reductases are heterogeneously expressed in tumors, the different activation mechanisms will allow these agents to complement each other. Combining POR downregulation with KEMTUB012-NI2 treatment could be a new treatment strategy that maximizes efficacy toward hypoxic tumor cells while limiting systemic toxicity.

Synthesis and Superpotent Anticancer Activity of Tubulysins Carrying Non-hydrolysable N-Substituents on Tubuvaline.

Synthetic tubulysins 24 a-m, containing non-hydrolysable N-substituents on tubuvaline (Tuv), were obtained in high purity and good overall yields using a multistep synthesis. A key step was the formation of differently N-substituted Ile-Tuv fragments 10 by using an aza-Michael reaction of azido-Ile derivatives 8 with the α,ß-unsaturated oxo-thiazole 5. A structure-activity relationship study using a panel of human tumour cell lines showed strong anti-proliferative activity for all compounds 24 a-m, with IC50 values in the sub-nanomolar range, which were distinctly lower than those of tubulysin A, vinorelbine and paclitaxel. Furthermore, 24 a-m were able to overcome cross-resistance to paclitaxel and vinorelbine in two tumour cell lines with acquired resistance to doxorubicin. Compounds 24 e and 24 g were selected as leads to evaluate their mechanism of action. In vitro assays showed that both 24 e and 24 g interfere with tubulin polymerization in a vinca alkaloid-like manner and prevent paclitaxel-induced assembly of tubulin polymers. Both compounds exerted antimitotic activity and induced apoptosis in cancer cells at very low concentrations. Compound 24 e also exhibited potent antitumor activity at well tolerated doses on in vivo models of diffuse malignant peritoneal mesothelioma, such as MESOII peritoneal mesothelioma xenografts, the growth of which was not significantly affected by vinorelbine. These results indicate that synthetic tubulysins 24 could be used as standalone chemotherapeutic agents in difficult-to-treat cancers.

Development of Fluorinated Analogues of Perhexiline with Improved Pharmacokinetic Properties and Retained Efficacy.

We designed and synthesized perhexiline analogues that have the same therapeutic profile as the parent cardiovascular drug but lacking its metabolic liability associated with CYP2D6 metabolism. Cycloalkyl perhexiline analogues 6a-j were found to be unsuitable for further development, as they retained a pharmacokinetic profile very similar to that shown by the parent compound. Multistep synthesis of perhexiline analogues incorporating fluorine atoms onto the cyclohexyl ring(s) provided a range of different fluoroperhexiline analogues. Of these, analogues 50 (4,4-gem-difluoro) and 62 (4,4,4',4'-tetrafluoro) were highly stable and showed greatly reduced susceptibility to CYP2D6-mediated metabolism. In vitro efficacy studies demonstrated that a number of derivatives retained acceptable potency against CPT-1. Having the best balance of properties, 50 was selected for further evaluation. Like perhexiline, it was shown to be selectively concentrated in the myocardium and, using the Langendorff model, to be effective in improving both cardiac contractility and relaxation when challenged with high fat buffer.

A Novel Functional Domain of Tab2 Involved in the Interaction with Estrogen Receptor Alpha in Breast Cancer Cells.

Tab2, originally described as a component of the inflammatory pathway, has been implicated in phenomena of gene de-repression in several contexts, due to its ability to interact with the NCoR corepressor. Tab2 interacts also with steroid receptors and dismisses NCoR from antagonist-bound Estrogen and Androgen Receptors on gene regulatory regions, thus modifying their transcriptional activity and leading to pharmacological resistance in breast and prostate cancer cells. We demonstrated previously that either Tab2 knock-down, or a peptide mimicking the Estrogen Receptor alpha domain interacting with Tab2, restore the antiproliferative response to Tamoxifen in Tamoxifen-resistant breast cancer cells. In this work, we map the domain of Tab2 responsible of Estrogen Receptor alpha interaction. First, using both co-immunoprecipitation and pull-down with recombinant proteins, we found that the central part of Tab2 is primarily responsible for this interaction, and that this region also interacts with Androgen Receptor. Then, we narrowed down the essential interaction region by means of competition assays using recombinant protein pull-down. The interaction motif was finally identified as a small region adjacent to, but not overlapping, the Tab2 MEKK1 phosphorylation sites. A synthetic peptide mimicking this motif efficiently displaced Tab2 from interacting with recombinant Estrogen Receptor alpha in vitro, prompting us to test its efficacy using derivatives of the MCF7 breast carcinoma cell lines that are spontaneously resistant to Tamoxifen. Indeed, we observed that this mimic peptide, made cell-permeable by addition of the TAT minimal carrier domain, reduced the growth of Tamoxifen-resistant MCF7 cells in the presence of Tamoxifen. These data indicate a novel functional domain of the Tab2 protein with potential application in drug design.

A critical review of both the synthesis approach and the receptor profile of the 8-chloro-1-(2',4'-dichlorophenyl)-N-piperidin-1-yl-1,4,5,6-tetrahydrobenzo[6,7]cyclohepta[1,2-c]pyrazole-3-carboxamide and analogue derivatives.

8-Chloro-1-(2',4'-dichlorophenyl)-N-piperidin-1-yl-1,4,5,6-tetrahydrobenzo[6,7]cyclohepta[1,2-c]pyrazole-3-carboxamide 9a was discovered as potent and selective CB1 antagonist by part of our group few years ago. In particular it was reported to have an affinity towards the CB1 cannabinoid receptor (CB1R), expressed as Ki, of 0.00035 nM. Nevertheless significantly divergent data were reported for the same compound from other laboratories. To unequivocally define the receptor profile of 9a, we have critically reviewed both its synthesis approach and binding data. Here we report that, in contrast to our previously reported data, 9a showed a Ki value for CB1R in the order of nanomolar rather than of fentomolar range. The new determined receptor profile of 9a was also ascertained for analogue derivatives 9b-i, as well as for 12. Moreover, the structural features of the synthesized compounds necessary for CB1R were investigated. Amongst the novel series, effects on CB1R intrinsic activity was highlighted due to the substituents at the position 3 of the pyrazole ring of the 1,4,5,6-tetrahydrobenzo[6,7]cyclohepta[1,2-c]pyrazole scaffold. Although the cannabinoid receptor profile of 9a was reviewed in this work, the relevance of this compound in CB1R antagonist based drug discovery is confirmed.

The tubulysin analogue KEMTUB10 induces apoptosis in breast cancer cells via p53, Bim and Bcl-2.

PURPOSE: Tubulysins are natural tetrapeptides that inhibit tubulin polymerisation. Tubulysins are very potent inhibitors of mammalian cancer cell growth, but restricted availability has limited their characterisation and development as anti-cancer compounds. KEMTUB10 was recently developed as a synthetic analogue of natural tubulysins. METHODS: The cell cytotoxicity of KEMTUB10 was studied in cell lines that represent the main breast cancer sub-types. The KEMTUB10 pro-apoptotic mechanism of action was studied in MCF7 and MDAMB231 cells. RESULTS: KEMTUB10 exerts a potent cytotoxic effect in cells representing the main breast cancer sub-types. KEMTUB10 blocks cells in the G2/M phase of the cell cycle and is a strong stimulator of apoptosis/cell death. KEMTUB10-induced apoptosis involves p53 and Bim, and to some extent Bcl-2 phosphorylation. CONCLUSIONS: KEMTUB10 is a promising new anti-mitotic that exerts a potent cytotoxic effect in breast cancer cells, blocks cells in the G2/M phase of the cell cycle and stimulates apoptosis/cell death. KEMTUB10 has a distinct mode of action to taxol, appears to be sensitive to different molecular factors in cells and is likely subject to different mechanisms of acquired resistance. KEMTUB10 has the potential to be an important addition to the anti-cancer therapeutic armoury.

Diversity oriented combinatorial synthesis of multivalent glycomimetics through a multicomponent domino process.

Both multicomponent reactions and diversity oriented synthesis are indispensable tools for the modern medicinal chemist. However, their employment for the synthesis of multivalent glycomimetics has not been exploited so far although the importance that such compounds play in exploring multivalency on glycoside inhibition. Herein, we report the combinatorial synthesis of diversity oriented hetero di- and trivalent glycomimetics through a multicomponent domino process. The process is high yielding and very general, working efficiently with easily accessible sugar starting materials such as glycosylamines, glycosylazides, and glycosylisothiocyanates, having the reactive functional groups tethered either directly to the anomeric carbon, through a suitable linker, or to the primary 6 position of hexoses (or 5 position of pentoses), leading, in the latter case, to glycomimetics with artificial enzymatically stable backbone. The process has been also exploited for the multicomponent synthesis of aminoglycoside (neomycin) conjugates.

Development of novel ADCs: conjugation of tubulysin analogues to trastuzumab monitored by dual radiolabeling.

Tubulysins are highly toxic tubulin-targeting agents with a narrow therapeutic window that are interesting for application in antibody-drug conjugates (ADC). For full control over drug-antibody ratio (DAR) and the effect thereof on pharmacokinetics and tumor targeting, a dual-labeling approach was developed, wherein the drug, tubulysin variants, and the antibody, the anti-HER2 monoclonal antibody (mAb) trastuzumab, are radiolabeled. (131)I-radioiodination of two synthetic tubulysin A analogues, the less potent TUB-OH (IC50 > 100 nmol/L) and the potent TUB-OMOM (IC50, ~1 nmol/L), and their direct covalent conjugation to (89)Zr-trastuzumab were established. Radioiodination of tubulysins was 92% to 98% efficient and conversion to N-hydroxysuccinimide (NHS) esters more than 99%; esters were isolated in an overall yield of 68% ± 5% with radiochemical purity of more than 99.5%. Conjugation of (131)I-tubulysin-NHS esters to (89)Zr-trastuzumab was 45% to 55% efficient, resulting in ADCs with 96% to 98% radiochemical purity after size-exclusion chromatography. ADCs were evaluated for their tumor-targeting potential and antitumor effects in nude mice with tumors that were sensitive or resistant to trastuzumab, using ado-trastuzumab emtansine as a reference. ADCs appeared stable in vivo. An average DAR of 2 and 4 conferred pharmacokinetics and tumor-targeting behavior similar to parental trastuzumab. Efficacy studies using single-dose TUB-OMOM-trastuzumab (DAR 4) showed dose-dependent antitumor effects, including complete tumor eradications in trastuzumab-sensitive tumors in vivo. TUB-OMOM-trastuzumab (60 mg/kg) displayed efficacy similar to ado-trastuzumab emtansine (15 mg/kg) yet more effective than trastuzumab. Our findings illustrate the potential of synthetic tubulysins in ADCs for cancer treatment.

Novel phage display-derived neuroblastoma-targeting peptides potentiate the effect of drug nanocarriers in preclinical settings.

Molecular targeting of drug delivery nanocarriers is expected to improve their therapeutic index while decreasing their toxicity. Here we report the identification and characterization of novel peptide ligands specific for cells present in high-risk neuroblastoma (NB), a childhood tumor mostly refractory to current therapies. To isolate such targeting moieties, we performed combined in vitro/ex-vivo phage display screenings on NB cell lines and on tumors derived from orthotopic mouse models of human NB. By designing proper subtractive protocols, we identified phage clones specific either for the primary tumor, its metastases, or for their respective stromal components. Globally, we isolated 121 phage-displayed NB-binding peptides: 26 bound the primary tumor, 15 the metastatic mass, 57 and 23 their respective microenvironments. Of these, five phage clones were further validated for their specific binding ex-vivo to biopsies from stage IV NB patients and to NB tumors derived from mice. All five clones also targeted tumor cells and vasculature in vivo when injected into NB-bearing mice. Coupling of the corresponding targeting peptides with doxorubicin-loaded liposomes led to a significant inhibition in tumor volume and enhanced survival in preclinical NB models, thereby paving the way to their clinical development.

Synthesis and structure-activity relationship studies of novel tubulysin U analogues--effect on cytotoxicity of structural variations in the tubuvaline fragment.

Tubulysins are cytotoxic natural products with promising anti-cancer properties, originally isolated from myxobacterial cultures. Structurally, tubulysins are tetrapeptides, incorporating three unusual (Mep, Tuv and Tup) and one proteinogenic amino acid (Ile). Here we describe the synthesis and structure-activity relationship studies of novel tubulysin U and V analogues, with variations in the central Tuv fragment, which is known to be of paramount importance for tubulysins' potency and hence cytotoxicity, but has seldom been modified in previous studies. Specifically, we replaced the natural iso-propyl and acetoxy functionalities with other structurally related groups. In general, the new analogues showed much lower potency relative to native tubulysin U. However, one of the synthetic analogues (1f) having a MOM function replacing the acetyl group exhibited a 22 nM IC50 on the HT-29 cell line which is comparable to the IC(50) displayed by tubulysin U (3.8 nM). Furthermore, the synthetic methodology reported herein was found to be flexible enough to deliver different core-modified tubulysin analogues and hence may be regarded as a scalable and convenient strategy for the chemical generation of novel tubulysin analogues.

Novel pyrazole derivatives as neutral CB1 antagonists with significant activity towards food intake.

In spite of rimonabant's withdrawal from the European market due to its adverse effects, interest in the development of drugs based on CB1 antagonists is revamping on the basis of the peculiar properties of this class of compounds. In particular, new strategies have been proposed for the treatment of obesity and/or related risk factors through CB1 antagonists, i.e. by the development of selectively peripherally acting agents or by the identification of neutral CB1 antagonists. New compounds based on the lead CB1 antagonist/inverse agonist rimonabant have been synthesized with focus on obtaining neutral CB1 antagonists. Amongst the new derivatives described in this paper, the mixture of the two enantiomers (±)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-3-(2-cyclohexyl-1-hydroxyethyl)-4-methyl-1H-pyrazole ((±)-5), and compound 5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-3-[(Z)-2-cyclohexyl-1-fluorovinyl]-4-methyl-1H-pyrazole ((Z)-6), showed interesting pharmacological profiles. According to the preliminary pharmacological evaluation, these novel pyrazole derivatives showed in fact both neutral CB1 antagonism behaviour and significant in vivo activity towards food intake.

Role of cysteine residues and disulfide bonds in the activity of a legume root nodule-specific, cysteine-rich peptide.

The root nodules of certain legumes including Medicago truncatula produce >300 different nodule-specific cysteine-rich (NCR) peptides. Medicago NCR antimicrobial peptides (AMPs) mediate the differentiation of the bacterium, Sinorhizobium meliloti into a nitrogen-fixing bacteroid within the legume root nodules. In vitro, NCR AMPs such as NCR247 induced bacteroid features and exhibited antimicrobial activity against S. meliloti. The bacterial BacA protein is critical to prevent S. meliloti from being hypersensitive toward NCR AMPs. NCR AMPs are cationic and have conserved cysteine residues, which form disulfide (S-S) bridges. However, the natural configuration of NCR AMP S-S bridges and the role of these in the activity of the peptide are unknown. In this study, we found that either cysteine replacements or S-S bond modifications influenced the activity of NCR247 against S. meliloti. Specifically, either substitution of cysteines for serines, changing the S-S bridges from cysteines 1-2, 3-4 to 1-3, 2-4 or oxidation of NCR247 lowered its activity against S. meliloti. We also determined that BacA specifically protected S. meliloti against oxidized NCR247. Due to the large number of different NCRs synthesized by legume root nodules and the importance of bacterial BacA proteins for prolonged host infections, these findings have important implications for analyzing the function of these novel peptides and the protective role of BacA in the bacterial response toward these peptides.

Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis.

Sinorhizobium meliloti differentiates into persisting, nitrogen-fixing bacteroids within root nodules of the legume Medicago truncatula. Nodule-specific cysteine-rich antimicrobial peptides (NCR AMPs) and the bacterial BacA protein are essential for bacteroid development. However, the bacterial factors central to the NCR AMP response and the in planta role of BacA are unknown. We investigated the hypothesis that BacA is critical for the bacterial response towards NCR AMPs. We found that BacA was not essential for NCR AMPs to induce features of S. meliloti bacteroids in vitro. Instead, BacA was critical to reduce the amount of NCR AMP-induced membrane permeabilization and bacterial killing in vitro. Within M. truncatula, both wild-type and BacA-deficient mutant bacteria were challenged with NCR AMPs, but this resulted in persistence of the wild-type bacteria and rapid cell death of the mutant bacteria. In contrast, BacA was dispensable for bacterial survival in an M. truncatula dnf1 mutant defective in NCR AMP transport to the bacterial compartment. Therefore, BacA is critical for the legume symbiosis by protecting S. meliloti against the bactericidal effects of NCR AMPs. Host AMPs are ubiquitous in nature and BacA proteins are essential for other chronic host infections by symbiotic and pathogenic bacteria. Hence, our findings suggest that BacA-mediated protection of bacteria against host AMPs is a critical stage in the establishment of different prolonged host infections.