Toxicological evaluation of microbial secondary metabolites in the context of European active substance approval for plant protection products

Risk assessment (RA) of microbial secondary metabolites (SM) is part of the EU approval process for microbial active substances (AS) used in plant protection products (PPP). As the number of potentially produced microbial SM may be high for a certain microbial strain and existing information on the metabolites often are low, data gaps are frequently identified during the RA. Often, RA cannot conclusively clarify the toxicological relevance of the individual substances. This work presents data and RA conclusions on four metabolites, Beauvericin, 2,3-deepoxy-2,3-didehydro-rhizoxin (DDR), Leucinostatin A and Swainsonin in detail as examples for the challenging process of RA. To overcome the problem of incomplete assessment reports, RA of microbial AS for PPP is in need of new approaches. In view of the Next Generation Risk Assessment (NGRA), the combination of literature data, omic-methods, in vitro and in silico methods combined in adverse outcome pathways (AOPs) can be used for an efficient and targeted identification and assessment of metabolites of concern (MoC).

Oleic acid-based vesicular nanocarriers for topical delivery of the natural drug thymoquinone: Improvement of anti-inflammatory activity

The topical administration of a drug compound remains the first choice for the treatment of many local skin ailments. Many skin diseases can be treated by applying the active formulation directly to the skin, but unfortunately some drugs are unable to overcome the stratum corneum and exert their pharmacological action. An example is thymoquinone, a naturally derived drug obtained from Nigella sativa L. and potentially effective in the treatment of inflammatory and oxidative skin conditions. Since its physico-chemical properties are not suitable for overcoming the stratum corneum, we wanted to circumvent the problem by proposing new lipid-based nanovesicles called "oleoethosomes", made up of naturally derived ingredients, for its delivery. Among several formulations of oleoethosomes, the sample made up of 2% (w/w) oleic acid:PL90G 1:1 (molar ratio), and ethanol 15% showed the best physico-chemical characteristics and above all it showed the capacity to contain a suitable amount of thymoquinone (2 mg/ml). The formulation was tested in vitro on stratum corneum and viable epidermis membranes confirming its ability to induce the passage of thymoquinone through the human stratum corneum and to act as a permeation enhancer. In fact, it showed thymoquinone permeation values of 22.63 ± 1.49% regarding the applied drug amount. Oleoethosomes were compared with oleosomes, another kind of naturally derived nanosystems but free of ethanol. The experimental data confirmed that ethanol was an important component that enhanced the activity of the oleoethosomes when tested on the skin of healthy volunteers. The thymoquinone-loaded oleoethosome treatment demonstrated a significantly greater extent of anti-inflammatory activity than the treatment with thymoquinone-loaded oleosomes or the conventional dosage form of the drug. These in vivo results confirmed the synergic effect between oleic acid and ethanol on the lipid and protein compartments of the outermost skin layer, thus promoting a greater penetration capacity.

Antiprotozoal Structure-Activity Relationships of Synthetic Leucinostatin Derivatives and Elucidation of their Mode of Action

Leucinostatin A is one of the most potent antiprotozoal compounds ever described, but little was known on structure-activity relationships (SAR). We used Trypanosoma brucei as a protozoal model organism to test synthetically modified derivatives, resulting in simplified but equally active compounds 2 (ZHAWOC6025) and 4 (ZHAWOC6027), which were subsequently modified in all regions of the molecule to gain an in-depth SAR understanding. The antiprotozoal SAR matched SAR in phospholipid liposomes, where membrane integrity, leaking, and dynamics were studied. The mode of action is discussed based on a structure-activity analysis of derivatives in efficacy, ultrastructural studies in T. brucei, and artificial membrane models, mimicking membrane stability and membrane potential. The main site of antiprotozoal action of natural and synthetic leucinostatins lies in the destabilization of the inner mitochondrial membrane, as demonstrated by ultrastructural analysis, electron microscopy and mitochondrial staining. Long-time sublethal exposure of T. brucei (200 passages) and siRNA screening of 12'000 mutants showed no signs of resistance development to the synthetic derivatives.

Organic Antifungal Drugs and Targets of Their Action

In recent decades, there has been a significant increase in the number of fungal diseases. This is due to a wide spectrum of action, immunosuppressants and other group drugs. In terms of frequency, rapid spread and globality, fungal infections are approaching acute respiratory infections. Antimycotics are medicinal substances endorsed with fungicidal or fungistatic properties. For the treatment of fungal diseases, several groups of compounds are used that differ in their origin (natural or synthetic), molecular targets and mechanism of action, antifungal effect (fungicidal or fungistatic), indications for use (local or systemic infections), and methods of administration (parenteral, oral, outdoor). Several efforts have been made by various medicinal chemists around the world for the development of antifungal drugs with high efficacy with the least toxicity and maximum selectivity in the area of antifungal chemotherapy. The pharmacokinetic properties of the new antimycotics are also important: the ability to penetrate biological barriers, be absorbed and distributed in tissues and organs, get accumulated in tissues affected by micromycetes, undergo drug metabolism in the intestinal microflora and human organs, and in the kinetics of excretion from the body. There are several ways to search for new effective antimycotics: - Obtaining new derivatives of the already used classes of antimycotics with improved activity properties. - Screening of new chemical classes of synthetic antimycotic compounds. - Screening of natural compounds. - Identification of new unique molecular targets in the fungal cell. - Development of new compositions and dosage forms with effective delivery vehicles. The methods of informatics, bioinformatics, genomics and proteomics were extensively investigated for the development of new antimycotics. These techniques were employed in finding and identification of new molecular proteins in a fungal cell; in the determination of the selectivity of drugprotein interactions, evaluation of drug-drug interactions and synergism of drugs; determination of the structure-activity relationship (SAR) studies; determination of the molecular design of the most active, selective and safer drugs for the humans, animals and plants. In medical applications, the methods of information analysis and pharmacogenomics allow taking into account the individual phenotype of the patient, the level of expression of the targets of antifungal drugs when choosing antifungal agents and their dosage. This review article incorporates some of the most significant studies covering the basic structures and approaches for the synthesis of antifungal drugs and the directions for their further development.

Synthesis of AHMOD-containing aminolipopeptides, unique bioactive peptaibiotics

An interesting family of bioactive aminolipopeptides contain the unusual building block 2-amino-6-hydroxy-4-methyl-8-oxodecanoic acid (AHMOD).

Chitosan-coated microvesicles: Effect of polysaccharide-phospholipid affinity on decafluorobutane dissolution

The stability of perfluorinated microvesicles is mainly determined by the presence of interfacial materials and their ability to hinder the gas component diffusibility into the bloodstream. The goal of this study is to increase the persistence of the gaseous-core by introducing chitosan-coated 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) microvesicles, reducing gas diffusion from microvesicles, and increasing for a long time ultrasonic signals. Our hypothesis was based on the irreversible adhesion of chitosan towards DSPC head groups observed in thin-films models. This affinity enhanced the stabilization of gaseous-core microvesicles, in which the polysaccharide effectively reduced the phospholipid phase transition enthalpy from 383±5.5Jmg(-1) for plain to 150±9.7Jmg(-1) for chitosan-coated microvesicles, providing a more stable structure that diminished the gaseous component lost and provided the persistence of intense (19)F-NMR signals after 48h, twice as long compared to plain samples. As a result, stronger and long-lasting ultrasonic signals were produced by the more stable chitosan-containing microvesicles, thus, presenting great potential to increase the diagnostic and therapeutic applications of perfluorocarbon carries.

Folate-targeted supramolecular vesicular aggregates as a new frontier for effective anticancer treatment in in vivo model

Supramolecular vesicular aggregates (SVAs), made up by self-assembling liposomes and polyasparthydrazide co-polymers conjugated to folic acid molecules were extensively investigated in this manuscript as potential active targeting formulation for anticancer drug delivery. Folate-targeted systems (FT-SVAs) were used to treat breast cancer and to further proof the potential in vivo administration of these systems for the therapeutic treatment for several aggressive solid tumors. The physicochemical and technological parameters of FT-SVAs are suitable for their potential in vivo administration. The chemotherapeutic activity of GEM-loaded FT-SVAs was increased during in vivo experiments. NOD-SCID mice bearing MCF-7 human xenograft is used as breast cancer model. The measurement of the volume and weight of tumor masses decreased when animal models are treated by using GEM-loaded FT-SVAs, compared to data obtained by using GEM-loaded mPEG-SUVs and the free form of GEM. An almost complete regression of the tumor (≈ 0.2 cm(3)) was observed in NOD-SCID mice bearing MCF-7 human xenografts treated by GEM-loaded FT-SVAs due to the noticeable improvement of GEM pharmacokinetic parameters provided by FT-SVAs with respect to native anticancer drug. The obtained data showed that supramolecular systems could represent an innovative drug delivery system by self-assembling liposomes and biocompatible polymers to be potentially used for anticancer treatment.

Liposomal delivery improves the growth-inhibitory and apoptotic activity of low doses of gemcitabine in multiple myeloma cancer cells

Gemcitabine-loaded pegylated unilamellar liposomes (200 nm) were proposed for the treatment of multiple myeloma cancer disease. Physicochemical and technological parameters of liposomes were evaluated by using laser light scattering and gel permeation chromatography. The growth-inhibitory activity of gemcitabine-loaded liposomes compared to the free drug was assayed in vitro on U266 (autocrine, interleukin-6-independent) and INA-6 (IL-6-dependent) multiple myeloma cell lines. Liposomes noticeably improved the growth-inhibitory activity of gemcitabine in terms of both dose-dependent and incubation-time effects. Liposomal delivery of gemcitabine consistently and significantly increased induction of apoptosis and caused a complete inhibition of proliferation. Liposomes were able to interact with multiple myeloma cells as demonstrated by confocal laser scanning microscopy and hence to improve the intracellular gemcitabine delivery. Gemcitabine-loaded liposomes were much more effective in vitro than the free drug. This formulation may offer even more in vivo advantages both in terms of drug pharmacokinetic and biodistribution.

In vitro and in vivo evaluation of Bola-surfactant containing niosomes for transdermal delivery

A novel niosome formulation is proposed for topical drug delivery of ammonium glycyrrhizinate, a natural compound with an efficacious anti-inflammatory activity. Niosomes were made up of a new non ionic surfactant, alpha,omega-hexadecyl-bis-(1-aza-18-crown-6) (Bola-surfactant)-Span 80-cholesterol (2:3:1 molar ratio). Niosome vesicles were prepared with the thin layer evaporation method and were physico-chemically characterized. The tolerability of Bola-surfactant both as free molecules or assembled ion niosome vesicles was evaluated in vitro on cultured of human keratinocyte cells (NCTC2544). Human tolerability was evaluated on volunteers. The ability of Bola-niosomes to promote intracellular delivery was evaluated by confocal laser scanning microscopy (CLSM) studies. Human stratum corneum and epidermis (SCE) membranes were used in vitro to investigate the percutaneous permeation. The anti-inflammatory activity of ammonium glycyrrhizinate was evaluated in vivo on human volunteers with a chemically induced erythema. Experimental data show that Bola-niosomes are characterized by a mean size of approximately 400 nm and are able to provide an encapsulation efficiency of 40% with respect to the drug amount used during preparation. CLSM showed that Bola-niosomes were able to promote the intracellular uptake of the delivered substances. Bola-niosomes were also able to significantly improve (p<0.001) the percutaneous permeation of ammonium glycyrrhizinate with respect to both the aqueous drug solution and a physical mixture between unloaded Bola-niosomes and the aqueous drug solution. Bola-niosomes showed a suitable tolerability both in vitro and in vivo. Ammonium glycyrrhizinate-loaded Bola-niosomes determined a significant (p<0.001) and noticeable improvement of the in vivo anti-inflammatory activity of the drug. An effective example of conjugating innovative colloidal carriers, coming from pharmaceutical nanotechnology, and therapeutically effective natural compounds, coming from traditional medicine, was reported.

Interaction between O-carboxymethylchitosan and dipalmitoyl-sn-glycero-3-phosphocholine bilayer

O-carboxylmethylchitosan (OCMCS), a chitosan derivative, has emerged as a strong polymeric biomembrane perturbant. In this study, the interaction between OCMCS and dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) was examined with cross-polarization microscopy, differential scanning calorimetry (DSC) and the surface pressure-area isotherms techniques. Cross-polarized light images showed that OCMCS induced the fusion of small DPPC multilamellar vesicles (MLV) to form large lamellar structures. From DSC measurement, the highest degree of fusion was found at the optimum OCMCS concentrations between 0.0625 and 0.2 mg/ml which are orders of magnitude lower than those required for similar reductions with unmodified chitosan as perturbant. At these concentrations, the association of DPPC and OCMCS enhances the fusion of DPPC vesicles. Surface pressure-area isotherms of DPPC monolayer in the presence of OCMCS imply interactions between OCMCS and DPPC in neutral condition. In comparison with chitosan, OCMCS proved to be a more effective membrane perturbant not only in neutral but also in acidic and basic conditions. The physical driving forces for OCMCS-induced perturbation of DPPC bilayer in neutral conditions are mainly hydrogen bonding and hydrophobic interactions. In acidic or basic conditions, the physical driving forces are dominated by the electrostatic interactions. The strong OCMCS-DPPC interaction will potentially increase the effectiveness of OCMCS for gene or drug delivery.

Interaction of poly-L-arginine with dihexadecyl phosphate/phosphatidylcholine liposomes

In the present study, mixed liposomes of dihexadecyl phosphate sodium salt:phosphatidylcholine:cholesterol at a 1:19:9.5 molar ratio were allowed to interact with poly-L-arginine at temperatures below and above the main phase transition of the liposomal membrane. The interaction led to the formation of aggregates, which gradually increased in size and eventually precipitated. It was, however, possible, during the initial stage of the experiments, when the ratio of guanidinium group relative to phosphate was smaller than ca. 40%, to determine their size and charge and observe their morphology in aqueous dispersion. Fluorescence experiments established that the liposomes are not ruptured during their interaction with poly-L-arginine. Instead, they are attached at the polypeptide chain through the guanidinium-phosphate complementary pair. Fluorescence quenching experiments indicated that the poly-L-arginine chain is accessible for interaction with iodides dissolved in the aqueous phase when the temperature of the liposomal dispersion is below the main lipid phase transition. It is, however, partitioned in the interior of the membrane at temperatures exceeding this main lipid phase transition.

Shape Recovery of an Optically Trapped Vesicle: Effect of Flow Velocity and Temperature

A new biophysical approach based on optical tweezers is developed to measure the time-dependent shape transformation and recovery of a single liposome, which is induced by the sudden stop of a moving liposome from various flow velocities at constant temperature. A simple viscoelastic model has been applied to correlate the temporal geometric parameter of the deformed liposome with a characteristic time constant, i.e., the ratio of membrane viscosity to elasticity. Our results show that membrane viscosity becomes dominant in governing the shape recovery rate when sample temperature goes beyond the main phase transition temperature of the phospholipid bilayer. More importantly, flow speed and vesicle size are demonstrated as key physical determinants for the shape recovery of liposome.

Leucinostatin-A loaded nanospheres: characterization and in vivo toxicity and efficacy evaluation

Leucinostatin A (Leu-A) is a nonapeptide exerting a remarkable activity especially against Candida albicans and Cryptococcus neoformans; nevertheless, its employment is limited due its toxicity. Therefore, we recently developed liposomal formulations, as suitable delivery systems, in order to increase its therapeutic index. However, liposomes present disadvantages related to their long-term instability. For this reason poly(lactic-co-glycolic) nanospheres (NS) were chosen as alternative colloidal carriers for Leu-A delivery. NS were formulated by spontaneous emulsification solvent diffusion method. This study investigates the effects of different parameters on drug encapsulation efficiency and particle size as well. The best preparation obtained was also characterized for its in vitro release, in vivo acute toxicity (LD50), and effectiveness against C. albicans in mice. In vitro release was performed over 100 h and resulted sufficiently sustained with more than 93% of the peptide released. Acute toxicity showed that the LD50 was increased more than 18-fold and the study on systemic candidiasis models revealed high effectiveness of the NS in reducing either the growth of fungal colonies in infected mice liver or in the mortality index. In conclusion, we can propose that Leu-A loaded NS could represent a new promising therapeutic system against Candida infection.

Phospholipid-stabilized nanoparticles of cyclosporine A by rapid expansion from supercritical to aqueous solution

The purpose of this research was to form stable suspensions of submicron particles of cyclosporine A, a water-insoluble drug, by rapid expansion from supercritical to aqueous solution (RESAS). A solution of cyclosporine A in CO2 was expanded into an aqueous solution containing phospholipid vesicles mixed with nonionic surfactants to provide stabilization against particle growth resulting from collisions in the expanding jet. The products were evaluated by measuring drug loading with high performance liquid chromatography (HPLC), particle sizing by dynamic light scattering (DLS), and particle morphology by transmission electron microscopy (TEM) and x-ray diffraction. The ability of the surfactant molecules to orient at the surface of the particles and provide steric stabilization could be manipulated by changing process variables including temperature and suspension concentration. Suspensions with high payloads (up to 54 mg/mL) could be achieved with a mean diameter of 500 nm and particle size distribution ranging from 40 to 920 nm. This size range is several hundred nanometers smaller than that produced by RESAS for particles stabilized by Tween 80 alone. The high drug payloads (approximately 10 times greater than the equilibrium solubility), the small particle sizes, and the long-term stability make this process attractive for development.

The peptide antibiotic clavanin A interacts strongly and specifically with lipid bilayers

In this study the interaction of the antimicrobial peptide clavanin A with phosphatidylcholine bilayers is investigated by DSC, NMR, and AFM techniques. It is shown that the peptide interacts strongly and specifically with the lipids, resulting in increased order-disorder phase transition temperatures, phase separation, altered acyl chain and headgroup packing, and a drastically changed surface morphology of the bilayer. These results are interpreted in terms of clavanin-specific interactions with lipids and are discussed in the light of the different mechanisms by which clavanin A can destroy the barrier function of biological membranes.

Interactions of phospholipid bilayer with chitosan: effect of molecular weight and pH

Chitosan has demonstrated its potentials as a gene carrier and a membrane perturbant for subsequent drug delivery to cells. However, there is currently a lack of experimental correlation between the physiochemical properties of chitosan and the resulting degree of lipid bilayer destabilization. In this study, the effect of pH and chitosan molecular weight on the interaction between chitosan and dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayer was examined with cross-polarization microscopy, differential scanning calorimetry (DSC), and Fourier transform- (FT-) Raman spectroscopy. Cross-polarized images showed that the direct hydration of the DPPC/chitosan mixture led to the formation of larger DPPC multilamellar vesicles (MLV), and pure chitosan also induced fusions of individual MLV. Under the influence of chitosan, the calorimetric enthalpy of DPPC was reduced in a concentration-dependent manner, and a new phase appeared at 28 degrees C during sample cooling. Even the lowest chitosan mole fraction of 0.04% reduced the cooperative unit of the DPPC bilayer by more than 70%. In addition, the electrostatic effect between chitosan and DPPC tuned the degree of membrane bilayer perturbation. Reduction of pH increased the number of protonated amines on the chitosan backbone and caused further disruption on the membrane organization. Mixing DPPC with chitosan in an organic medium before hydration enhanced the hydrophobic interactions between the two molecules and greatly reduced the cooperative unit among individual lipids during the main phase transition. The increase of chitosan molecular weight also affected the cooperativity in the thermotropic transition of DPPC bilayer. FT-Raman spectroscopy provided additional evidence that chitosan directly perturbed the organizations of the hydrophobic inner core of the DPPC bilayer.

Interaction of liposome with immobilized chitosan during main phase transition

It has been recently demonstrated that chitosan in aqueous solution alters the phase behavior and structure of a phospholipid bilayer (Fang, N.; et al. Biomacromolecules 2001, 2, 1161-1168). Until now, the physical driving forces between chitosan and the phospholipid bilayer upon their initial encounter remains unknown. In this study, confocal reflectance interference contrast microscopy (C-RICM), phase contrast microscopy and bioadhesion modeling are concurrently applied to probe the interaction of phospholipid vesicle with immobilized chitosan at various temperatures, pH, and osmotic stress. First, the successful immobilization of chitosan on amino-silanized glass is indicated by the increases in both the degree of vesicle deformation and adhesion energy of vesicles adhering on chitosan modified substrate in comparison with those on amino-silanized glass. Second, the phase transition of a phospholipid bilayer does not modulate the adhesion strength at the chitosan-biomembrane interface at pH 7.4. With increase of the degree of protonation on the chitsoan backbone at pH 4, the adhesion energy is increased by 5-fold for vesicles of all sizes compared to that in pH 7.4. Furthermore, pH reduction amplifies the thermal-induced response of larger vesicles on the immobilized chitosan layer. Interestingly, a moderate increase of osmotic stress maximizes the degree of vesicle deformation and adhesion energy at 23 degrees C and dampens the effect of phase transition on vesicle adhesion. Overall, this study demonstrates the quantitation of chitosan-biomembrane interactions that will be critical for future applications of chitosan in biological systems.

Thermal induced modification of the contact mechanics of adhering liposomes on cationic substrate

The correlation between the mechanical property and the thermotropic transition of the phospholipid bilayer has been recently demonstrated (Chem. Phys. Lipids 110 (2001) 27). However, the role of thermal induced mechanical responses of phospholipid bilayer on the contact mechanics of liposome adhering on a cationic substrate has not been determined. In this study, confocal-reflectance interference contrast microscopy, phase contrast microscopy and contact mechanics modeling are applied to probe the adhesion mechanisms of liposomes in the presence of electrostatic interactions during the thermotropic transition of the lipid bilayer. When temperature increases from 23 to 49 degrees C at pH 7.4, the degree of liposome deformation (a/R) and adhesion energy of dipalmitoyl-sn-glycero-3-phosphocholine liposome increases by 10% and remains constant, respectively, on 3-amino-propyl-triethoxy-silane (APTES) modified substrate. The extents of increase in these two parameters are highly dependent on the physicochemical properties of the rigid substrate. At pH 4, the adhesion energies above and below the phase transition temperature (T(m)) are increased by one order of magnitude due to the formation of the free silanol groups on APTES substrate. In hypotonic condition, the degree of vesicle deformation remains constant and the adhesion energy reduces by 20% during sample heating. Under all conditions, the adhesion energy of the adhering liposome spans a few orders of magnitude against the increase of liposome size as the surface area to volume ratio is maximized in smallest vesicle.

Microbiological and densitometric TLC analyses for peptides in liposomes

Quantitative determination of Leucinostatins and/or of similar peptides, such as Peptaibols, is sometimes quite difficult to perform especially when they are entrapped in vectors, i.e. liposomes, whose components display UV absorbances that may obscure those of the active principle. Therefore, in these cases, it is useful to find alternative ways, especially when high pressure liquid chromatography (HPLC) is difficult to perform or needs long procedure times. In the present paper, the use of microbiological and densitometric methods for quantitative analysis of Leucinostatin A (Leu-A) are described and the results compared with those from HPLC analyses. The use of microbiological and densitometric assays, furnished results comparable with those obtained by HPLC. Of the two methods used, the microbiological procedure appeared to be less accurate and precise.