Synthesis and characterisation of a new amphotericin B-methoxypoly(ethylene glycol) conjugate

Searching for Conjugates as New Structures for Antifungal Therapies

The progressive increase in fungal infections and the decrease in the effectiveness of current therapy explain research on new drugs. The synthesis of compounds with proven antifungal activity, favorable physicochemical and pharmacokinetic properties affecting their pharmaceutical availability and bioavailability, and limiting or eliminating side effects has become the goal of many studies. The publication describes the directions of searching for new compounds with antifungal activity, focusing on conjugates. The described modifications include, among others, azoles or amphotericin B in combination with fatty acids, polysaccharides, proteins, and synthetic polymers. The benefits of these combinations in terms of activity, mechanism of action, and bioavailability were indicated. The possibilities of creating or using nanoparticles, "umbrella" conjugates, siderophores (iron-chelating compounds), and monoclonal antibodies were also presented. Taking into account the role of vaccinations in prevention, the scope of research related to developing a vaccine protecting against fungal infections was also indicated.

Amphotericin B-PEG Conjugates of ZnO Nanoparticles: Enhancement Antifungal Activity with Minimal Toxicity

Amphotericin B (AMB) is commonly used to treat life-threatening systemic fungal infections. AMB formulations that are more efficient and less nephrotoxic are currently unmet needs. In the current study, new ZnO-PEGylated AMB (ZnO-AMB-PEG) nanoparticles (NPs) were synthesized and their antifungal effects on the Candida spp. were investigated. The size and zeta potential values of AMB-PEG and ZnO-AMB-PEG NPs were 216.2 ± 26.9 to 662.3 ± 24.7 nm and −11.8 ± 2.02 to −14.2 ± 0.94 mV, respectively. The FTIR, XRD, and EDX spectra indicated that the PEG-enclosed AMB was capped by ZnO, and SEM images revealed the ZnO distribution on the surface NPs. In comparison to ZnO-AMB NPs and free AMB against C.albicans and C.neoformans, ZnO-AMB-PEG NPs significantly reduced the MIC and MFC. After a week of single and multiple dosage, the toxicity was investigated utilizing in vitro blood hemolysis, in vivo nephrotoxicity, and hepatic functions. ZnO-AMB-PEG significantly lowered WBC count and hematocrit concentrations when compared to AMB and ZnO-AMB. RBC count and hemoglobulin content, on the other hand, were unaltered. ZnO-AMB-PEG considerably lowered creatinine and blood urea nitrogen (BUN) levels when compared to AMB and ZnO-AMB. The difference in liver function indicators was determined to be minor by all formulae. These findings imply that ZnO-AMB-PEG could be utilized in the clinic with little nephrotoxicity, although more research is needed to determine the formulation’s in vivo efficacy.

Polymer–Drug Conjugates for Treating Local and Systemic Fungal Infections

In immunocompromised patients, fungal infections are the major cause of morbidity and mortality. Currently, three major classes of drugs—polyenes, azoles, and echinocandins—with different mechanisms of action are used as antifungals for systemic infections. However, these conventional drugs were reported to induce toxic effects due to their low specificity, narrow spectrum of activity and drug–drug interactions. Some of these limitations could be overcome by altering the properties of existing drugs through physical and chemical modifications. For example, modification of amphotericin B (AmB), a polyene antibiotic includes the micellar suspension of AmB in deoxycholic acid (Fungizone®), non-covalent AmB lipid complexes (ABLC™), liposomal AmB (AmBisome®), and AmB colloidal dispersion (Amphocil™). All these formulations ensure the smoother release of AmB accompanied by its restricted distribution in the kidney, thereby lowering its nephrotoxicity. Although various methods such as polymeric micelles, nanoparticles and dendrimers were explored for enhancing the efficacy of the antifungal drugs, polymer–drug conjugates of antifungal drugs have received more attention in recent years. Polymer–drug conjugates improve the aqueous solubility of water-insoluble drugs, are stable in storage and reduce the toxicity of highly toxic drugs and are capable of releasing the drug at the site of action. This chapter discusses the polymer conjugates of antifungal drugs, their merits, and demerits. Studies reported so far show that the polymer–drug conjugates have significant advantages compared to conventional dosage forms for antifungal therapy.

Characterization of a Polyethylene Glycol-Amphotericin B Conjugate Loaded with Free AMB for Improved Antifungal Efficacy

Amphotericin B (AMB) is a highly hydrophobic antifungal, whose use is limited by its toxicity and poor solubility. To improve its solubility, AMB was reacted with a functionalized polyethylene glycol (PEG), yielding soluble complex AmB-PEG formulations that theoretically comprise of chemically conjugated AMB-PEG and free AMB that is physically associated with the conjugate. Reverse-phase chromatography and size exclusion chromatography methods using HPLC were developed to separate conjugated AMB-PEG and free AmB, enabling the further characterization of these formulations. Using HPLC and dynamic light scattering analyses, it was observed that the AMB-PEG 2 formulation, having a higher molar ratio of 2 AMB: 1 PEG, possesses more free AMB and has relatively larger particle diameters compared to the AMB-PEG 1 formulation, that consists of 1 AMB: 1 PEG. The identity of the conjugate was also verified using mass spectrometry. AMB-PEG 2 demonstrates improved antifungal efficacy relative to AMB-PEG 1, without a concurrent increase in in vitro toxicity to mammalian cells, implying that the additional loading of free AMB in the AMB-PEG formulation can potentially increase its therapeutic index. Compared to unconjugated AMB, AMB-PEG formulations are less toxic to mammalian cells in vitro, even though their MIC₅₀ values are comparatively higher in a variety of fungal strains tested. Our in vitro results suggest that AMB-PEG 2 formulations are two times less toxic than unconjugated AMB with antifungal efficacy on Candida albicans and Cryptococcus neoformans.

Polymer–drug conjugates for novel molecular targets

Polymer therapeutics can be already considered as a promising field in the human healthcare context. The discovery of the enhanced permeability and retention effect by Maeda, together with the modular model for the polymer–drug conjugate proposed by Ringsdorf, directed the early steps of polymer therapeutics towards cancer therapy. Orthodox anticancer drugs were preferentially chosen in the development of the first conjugates. The fast evolution of polymer chemistry and bioconjugation techniques, and a deeper understanding of cell biology has opened up exciting new challenges and opportunities. Four main directions have to be considered to develop this ‘platform technology’ further: the control of the synthetic process, the exhaustive characterization of the conjugate architectures, the conquest of combination therapy and the disclosure of new therapeutic targets. We illustrate in this article the exciting approaches offered by polymer–drug conjugates beyond classical cancer therapy, focusing on new, more effective and selective targets in cancer and in their use as treatments for other major human diseases.

Norfloxacin-Poly(l-Lysine Citramide Imide) Conjugates and Structure-dependence of the Drug Release

Norfloxacin (Nflx), an antibiotic which is active against some intracellular bacteria, was coupled to a polymeric carrier, namely poly(L-lysine citramide) via a lysine or an ethylcarbamate spacer to obtain a macromolecular prodrug. The carrier, which derived from the two metabolites citric acid and L-lysine, is known to be biocompatible and slowly degradable under slight acidic conditions. Conjugates were characterised by UV, 1H and 13C NMR and SEC. The presence of Norfloxacin and the lysine type spacer caused chain aggregation, due to a probable physical cure. The release of Norfloxacin from these prodrugs and from a prodrug where Norfloxacin is bound to the carrier backbone without spacer arm was investigated comparatively in vitro. Conjugation via a carbamate-type linkage appeared as a method to achieve the release of Norfloxacin from a PLCA-type conjugate at neutral.

Complex formation of amphotericin B in sterol-containing membranes as evidenced by surface plasmon resonance

Amphotericin B (AmB) is a membrane-active antibiotic that increases the permeability of fungal membranes. Thus, the dynamic process of its interaction with membranes poses intriguing questions, which prompted us to elaborate a quick and reliable method for real-time observation of the drug's binding to phospholipid liposomes. We focused on surface plasmon resonance (SPR) and devised a new modification method of sensor chips, which led to a significant reduction in the level of nonspecific binding of the drug in a control lane. With this method in hand, we examined the affinity of AmB for various membrane preparations. As expected, AmB exhibited much higher affinity for sterol-containing palmitoyloleoylphosphatidylcholine membranes than those without sterol. The sensorgrams recorded under various conditions partly fitted theoretical curves, which were based on three interaction models. Among those, a two-state reaction model reproduced well the sensorgram of AmB binding to an ergosterol-containing membrane; in this model, two states of membrane-bound complexes, AB and AB*, are assumed, which correspond to a simple binding to the surface of the membrane (AB) and formation of another assembly in the membrane (AB*) such as an ion channel complex. Kinetic analysis demonstrated that the association constant in ergosterol-containing POPC liposomes is larger by 1 order of magnitude than that in the cholesterol-containing counterpart. These findings support the previous notion that ergosterol stabilizes the membrane-bound assembly of AmB.

Synthesis of pH-sensitive amphotericin B–poly(ethylene glycol) conjugates and study of their controlled release in vitro

New intravenous conjugates of amphotericin B (AMB) with poly(ethylene glycols) (PEG) (M=5000, 10,000, 20,000) have been synthesized and characterised. The intermediate PEGs possess a 1,4-disubstituted benzene ring with aldehyde group at the end of the chain. The benzene ring is connected with PEG at its 4-position (with respect to the aldehyde group) by various functional groups (ether, amide, ester). Reaction of terminal aldehyde group of the substituted PEGs with AMB gave conjugates containing a pH-sensitive imine linkage, which can be presumed to exhibit antimycotic effect at sites with lowered pH value. All types of the conjugates are relatively stable in phosphate buffer at physiological conditions of pH 7.4 (37 degrees C), less than 5 mol% AMB being split off from them within 24 h. For a model medium of afflicted tissue was used a phosphate buffer (pH 5.5, 37 degrees C), in which controlled release of AMB from the conjugates takes place. The imine linkage is split to give free AMB with half-lives of 2-45 min. The rate of acid catalysed hydrolysis depends upon substitution of the benzene ring; however, it does not depend on molecular weights of the PEGs used. The conjugates with ester linkage undergo enzymatic splitting in human blood plasma and/or blood serum at pH 7.4 (37 degrees C) with half-lives of 2-5 h depending on molecular weights of the PEGs used (M = 5000, 10,000, 20,000). At first, the splitting of ester linkage produces the relatively stable pro-drug, that is, 4-carboxybenzylideniminoamphotericin B, which is decomposed to AMB and 4-formylbenzoic acid in a goal-directed manner only at pH 7 (t1/2 = 2 min, pH 5.5, 37 degrees C). A goal-directed release of AMB is only achieved by acid catalysed hydrolysis of imine linkage, either from the polymeric conjugate or from the pro-drug released thereof. The LD50 values determined in vivo (mouse) are 20.7 mg/kg and 40.5 mg/kg for the conjugates with ester linkage (M = 10,000 and 5000, respectively), which means that they are ca. 6-11 times less toxic than free AMB.

Synthesis and characterisation of a new pH-sensitive amphotericin B--poly(ethylene glycol)-b-poly(L-lysine) conjugate

This paper reports on the synthesis, characterisation, and efficiency of a new intravenous conjugate of amphotericin B (AMB). Twelve molecules of AMB were attached to block copolymer poly(ethylene glycol)-b-poly(L-lysine) via pH-sensitive imine linkages. In vitro drug release studies demonstrated the conjugate (M(w)=26,700) to be relatively stable in human plasma and in phosphate buffer (pH 7.4, 37 degrees C). Controlled release of AMB was observed in acidic phosphate buffer (pH 5.5, 37 degrees C) with the half-life of 2 min. The LD(50) value determined in vivo (mouse) is 45 mg/kg.

Pharmacological agents in development for invasive aspergillosis

The urgent medical need for new potent antifungal agents in the management of invasive aspergillosis (IA) has resulted in the development of several compounds which may be of value in the future for the treatment or prophylaxis of IA. In the past years, several novel types of drugs have been discovered and developed, some of which are already in late-stage clinical trials and ready to enter the market. This paper discusses the antifungal agents, classified by their mode of action, that are currently available and the agents which are still in development for treatment or prevention of IA.

Recent Advances in Chemistry and Applications of Substituted Poly(ethylene glycol)s

Poly(ethylene glycol)s are well defined and easily accessible macromolecules with suitable properties for applications in chemistry, biotechnologies and medicine. The terminal hydroxy groups of poly(ethylene glycol)s can easily be converted into reactive functional groups by a number of routine reactions of organic chemistry. The chemical reagents or catalysts anchored to poly(ethylene glycol) chains were used in a number of syntheses including the enantioselective ones. Poly(ethylene glycol)s serve as carriers in combinatorial syntheses in the liquid phase. Coupling of poly(ethylene glycol)s with other polymers was used to prepare series of block copolymers having numerous applications. From the point of view of medical applications it is significant that substituted poly(ethylene glycol)s are non-toxic and resistant to recognition by the immunity system. That is why they are often used as carriers of many low-molecular-weight as well as high-molecular-weight medical drugs (drug delivery systems). In the conjugates with drugs their biological activity increases and their toxicity decreases. A review with 41 references.

Conjugation of amino-containing drugs to polysaccharides by tosylation: amphotericin B–arabinogalactan conjugates

The coupling of amphotericin-B (AmB), a water-insoluble antifungal and antileishmanial agent, to arabinogalactan (AG) via tosylate or mesylate derivatives was investigated as a method for the conjugation of amino-containing drugs to polysaccharides. In the first step, AG was reacted with tosyl- or mesyl-chloride at different ratios to obtain tosylate or mesylate AG derivatives. AmB was conjugated to AG derivatives in either aqueous or organic media via an amine bond. AG-AmB conjugates were soluble in water and exhibited improved stability in aqueous solutions as compared to the unbound drug. The conjugates showed comparable inhibitory concentration values against the pathogenic yeast Candida albicans, and against Leishmania major parasites. They were about 60 times less hemolytic against sheep erythrocytes than the free drug, and less toxic when injected i.v. to BALB/c mice.

Utility of poly(ethylene glycol) conjugation to create prodrugs of amphotericin B

This paper reports on the synthesis, safety, and efficacy of a series of water-soluble derivatives of poly(ethylene glycol) (PEG)-conjugated amphotericin B (AmB). PEG 40 000 attached to the sugar amino group of AmB via labile carbamate and carbonate linkages was examined. The synthetic program conducted for this investigation provided a series of disubstituted PEG-AmB derivatives which had in vitro PEG half-life of hydrolyses rates in rat plasma varying between 1 and 3 h. Importantly, all conjugates demonstrated less than 6% hydrolysis following 24 h incubation in pH 7.4 phosphate buffer at 25 degrees C and showed solubilities greater than 46 mg/mL in aqueous solutions. The solubility of AmB in the conjugates increased up to approximately 200 times compared to unmodified AmB in saline. As a major finding, this investigation demonstrated that conjugation of PEG to AmB could produce conjugates that were significantly (6x) less toxic than AmB-deoxycholate and maintained, or even had enhanced, in vivo antifungal activity.

Effective drug delivery by PEGylated drug conjugates

The current review presents an update of drug delivery using poly(ethylene glycol) (PEG), that focuses on recent developments in both protein and organic drugs. Certainly the past 10 years has resulted in a renaissance of the field of PEG drug conjugates, initiated by the use of higher molecular weight PEGs (M(w)>20,000), especially 40,000 which is estimated to have a plasma circulating t(1/2) of approximately 10 h in mice. This recent resuscitation of small organic molecule delivery by high molecular weight PEG conjugates was founded on meaningful in vivo testing using established tumor models, and has led to a clinical candidate, PEG-camptothecin (PROTHECAN), an ester based prodrug currently in phase II trials. Additional applications of high molecular weight PEG prodrug strategies to amino containing drugs are presented: similar tripartate systems based on lower M(w) PEG and their use with proteins is expounded on. The modification of a benzyl elimination tripartate prodrug specific for mercaptans is presented, and its successful application to 6-mercaptopurine giving a water soluble formulation is discussed. Recent novel PEG oligonucleotides and immunoconjugates are also covered. Clinical results of FDA approved PEGylated proteins are also presented.