Spectroscopic investigation of the molecular state of nystatin encapsulated in liposomes

Effect of antibiotic amphotericin B on structural and dynamic properties of lipid membranes formed with egg yolk phosphatidylcholine

Amphotericin B (AmB) is a popular antibiotic applied in treatment of deep-seated mycotic infections. The mode of action of AmB is based upon interactions with biomembranes but exact binding properties of the antibiotic to the lipid membranes still remain obscure. Effect of incorporation of AmB into egg yolk phosphatidylcholine membranes in the concentration range from 0.01 to 5 mol% on structural and dynamic properties of lipid bilayers was studied with application of small-angle neutron scattering, X-ray diffractometry and Fourier-transform infrared spectroscopy (FTIR). The results of the experiments show that AmB is located predominantly in the headgroup region of the membranes at concentrations below 1 mol%. The process of AmB aggregation, at concentrations above 1 mol%, is associated with ordering effect within the acyl chain region and therefore indicates incorporation of AmB into the hydrophobic membrane core.

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.

Interaction between nystatin and natural membrane lipids in Langmuir monolayers--the role of a phospholipid in the mechanism of polyenes mode of action

Nystatin (NYS), a polyene antifungal antibiotic, has been investigated in Langmuir monolayers alone and in mixtures with mammalian and fungi membrane sterols (cholesterol and ergosterol, respectively) as well as with a model phospholipid (DPPC). The interactions between film molecules have been examined both in a qualitative and quantitative way with the excess area per molecule (AExc), excess free energy of mixing (DeltaGExc) and the interaction parameter (alpha). The obtained results have been compared with those previously reported for another polyene antimycotic: amphotericin B (AmB) mixed with lipids. Higher affinity of NYS has been observed for ergosterol vs. cholesterol, however, the strongest attractions were found for its mixtures with DPPC. The obtained results have been verified with biological studies reported previously for both antibiotics (NYS and AmB). A thorough analysis of the Langmuir experiment results performed for both polyenes enabled us to conclude that the presence of DPPC can be considered as a key factor affecting their antifungal activity as well as their toxicity towards host cells.

Nystatin in Langmuir monolayers at the air/water interface

The paper presents a thorough characteristics of Langmuir monolayers formed at the air/water interface by a polyene macrolide antibiotic-nystatin. The investigations are based on the analysis of pi/A isotherms recorded for monolayers formed by this antibiotic at different experimental conditions. A significant part of this work is devoted to the stability and relaxation phenomena. It has been found that nystatin forms at the air/water interface monolayers of the LE state. A plateau region, observed during the course of the isotherm compression, is suggested to be due to the orientational change of nystatin molecules from horizontal to vertical position. Quantitative analysis of the desorption of the monolayer material into bulk water indicates that the solubility of nystatin monolayers increases with surface pressure. At low surface pressures, the desorption of nystatin from a monolayer is controlled both by dissolution and by diffusion. However, at the plateau and in the post-plateau region, the desorption does not achieve a steady state and the monolayer is less stable than in the pre-plateau region. However, the presence of membrane lipids, even at a low mole fraction, considerably increases the stability of nystatin monolayers. This enables the application of the Langmuir monolayer technique to study nystatin in mixture with cellular membrane components, aiming at verifying its mode of action and the mechanism of toxicity.

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.

Entrapment of peptidoglycans and adamantyltripeptides into liposomes: an HPLC assay for determination of encapsulation efficiency

The encapsulation of different immunomodulating peptides, the peptidoglycan monomer, its semisynthetic derivatives (Adamant-1-yl)-acetyl-peptidoglycan monomer and Boc-Tyr-peptidoglycan monomer, respectively, and of two diastereoisomers of adamantyltripeptides into the large negatively charged multilamellar liposomes was investigated. The reproducible quantitative method using HPLC was established for the determination of the entrapped compounds. It was shown that the tested compounds could be efficiently incorporated into liposomes using either the film or modified film method. The results confirmed that the peptidoglycans with lipophilic substituents and particularly the adamantyltripeptides were incorporated into liposomes with higher efficiency than the peptidoglycan monomer using either of the described methods. Liposome preparations were stable at 4 degrees C up to seven days as shown by minimal leaking of the entrapped material.

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

The reaction of methoxypoly(ethylene glycol)-4-nitrophenyl carbonate with amphotericin B has been used to prepare a new conjugate of amphotericin B (mPEG-AmB). A preliminary screening of in vitro antifungal activity has suggested that mPEG-AmB possesses a similar effect and a similar spectrum of activity as the conventional amphotericin B formulated with sodium desoxycholate.