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Biomedical subjects

Alfred Fahr

Publications and source records attributed to Alfred Fahr.

12 recordsLinked to original sources
PubMed2006

Adaptive neuro-fuzzy modeling of poorly soluble drug formulations.

PURPOSE: The purpose of this study was to evaluate the efficiency of a neuro-fuzzy logic-based methodology to model poorly soluble drug formulations and predict the development of the particle size that has been proven to be an important factor for long-term stability. METHODS: An adaptive neuro-fuzzy inference system was used to model the natural structures within the data and construct a set of fuzzy rules that can subsequently used as a predictive tool. The model was implemented in Matlab 6.5 and trained using 75% of an experimental data set. Subsequently, the model was evaluated and tested using the remaining 25%, and the predicted values of the particle size were compared to the ones from the experimental data. The produced adaptive neuro-fuzzy inference system-based model consisted of four inputs, i.e., acetone, propylene glycol, POE-5 phytosterol (BPS-5), and hydroxypropylmethylcellulose 90SH-50, with four membership functions each. Moreover, 256 fuzzy rules were employed in the model structure. RESULTS: Model training resulted in a root mean square error of 4.5 x 10(-3), whereas model testing proved its highly predictive efficiency, achieving a correlation coefficient of 0.99 between the actual and the predicted values of the particle size (mean diameter). CONCLUSIONS: Neuro-fuzzy modeling has been proven to be a realistic and promising tool for predicting the particle size of drug formulations with an easy and fast way, after proper training and testing.

Acetone↗
PubMed2006

Electrostatic model for mixed cationic-zwitterionic lipid bilayers.

The current interest in mixed cationic-zwitterionic lipid membranes derives from their potential use as transfer vectors in nonviral gene therapy. Mixed cationic-zwitterionic lipid membranes have a number of structural properties that are distinct from the corresponding anionic-zwitterionic lipid membranes. As known from experiment and reproduced by computer simulations, the average cross-sectional area per lipid changes nonmonotonically with the mole fraction of the charged lipid, passing through a minimum at a roughly equimolar mixture. At the same time, the average orientation of the zwitterionic headgroup dipoles changes from more parallel to the membrane plane to more perpendicular. We suggest a simple mean-field model that reveals the physical mechanisms underlying the observed structural properties. To backup the mean-field calculations, we have also performed Monte Carlo simulations. Our model extends Poisson-Boltzmann theory to include (besides the cationic headgroup charges) the individual charges of the zwitterionic lipid headgroups. We model these charges to be arranged as dipoles of fixed length with rotational degrees of freedom. Our model includes, in a phenomenological manner, the changes in steric headgroup interactions upon reorientation of the zwitterionic headgroups. Our numerical results suggest that two different mechanisms contribute to the observed structural properties: one involves the lateral electrostatic pressure and the other the zwitterionic headgroup orientation, the latter modifying steric headgroup interactions. The two mechanisms operate in parallel as they both originate in the electrostatic properties of the involved lipids. We have also applied our model to a mixed anionic-zwitterionic lipid membrane for which neither a significant headgroup reorientation nor a nonmonotonic change in the average lateral cross-sectional area is predicted.

Cations↗
PubMed2006

Stearylated octaarginine and artificial virus-like particles for transfection of siRNA into primary rat neurons.

RNA interference (RNAi) provides a powerful experimental tool for sequence-specific gene silencing, allowing efficient analysis of gene function in a multitude of cell types. However, application of RNAi in primary mammalian neurons has been limited by low-transfection efficiency and considerable toxicity of conventional transfection methods. In this study, we evaluated a peptide-mediated and a polymer/lipid-based cellular delivery method for siRNA into rat primary neurons and compared the results with a commonly used liposomal transfection reagent. Stearylated octaarginine (Stearyl-R8) was used as polypeptide and artificial virus-like particles (AVPs) were used as a combined liposomal-polymeric vector, since both reagents have been previously shown to successfully transfect DNA into cell lines. Stearyl-R8 and AVPs both promoted siRNA transfection into primary hippocampal neurons via the endosomal pathway. SiRNA-mediated gene silencing could be effectively induced in primary neuron cultures. In comparison with the commonly used cationic liposome transfection agent, both novel reagents were less detrimental to cell metabolic activity. We conclude that these novel transfection methods yield performances comparable to cationic liposome-mediated transfection for siRNA, while being less cytotoxic in primary neurons. Stearyl-R8 and AVPs may therefore represent novel and more cost-efficient alternatives to conventional siRNA-transfection reagents.

Animals↗
PubMed2006

Lipophilic drug transfer between liposomal and biological membranes: what does it mean for parenteral and oral drug delivery?

This review presents the current knowledge on the interaction of lipophilic, poorly water soluble drugs with liposomal and biological membranes. The center of attention will be on drugs having the potential to dissolve in a lipid membrane without perturbing them too much. The degree of interaction is described as solubility of a drug in phospholipid membranes and the kinetics of transfer of a lipophilic drug between membranes. Finally, the consequences of these two factors on the design of lipid-based carriers for oral, as well as parenteral use, for lipophilic drugs and lead selection of oral lipophilic drugs is described. Since liposomes serve as model-membranes for natural membranes, the assessment of lipid solubility and transfer kinetics of lipophilic drug using liposome formulations may additionally have predictive value for bioavailability and biodistribution and the pharmacokinetics of lipophilic drugs after parenteral as well as oral administration.

Administration, Oral↗
PubMed2006

In vitro characterization of binding and stability of single-chain Fv Ni-NTA-liposomes.

Recently, we presented a new method for the generation of single-chain Fv (scFv) immunoliposomes, which circumvents the necessity to introduce additional reactive groups in the protein. This method is based on immobilizing scFv fragments via their C-terminal hexahistidyl-tag on liposomes containing nickel-complexed dioleoyl-glycero-succinyl-nitrilotriacetic acid (Ni-NTA-DOGS) as an anchor lipid within the lipid bilayer. Here, we have extended this approach to various other scFv fragments and further demonstrate strong and selective binding of these liposomes to target cells in vitro. In order to evaluate suitability for in vivo applications, we investigated the influence of human plasma on stability and binding behaviour of scFv Ni-NTA-liposomes in vitro using scFv A5 directed against human endoglin (CD105) as a model antibody. We could show that the binding activity to target cells is rapidly lost in the presence of human plasma. Incorporation of polyethylene glycol (PEG) chains into the lipid bilayer did not protect against loss of binding capability. Further studies showed that loss of binding is mainly due to displacement of Ni-NTA-bound scFv fragments caused by plasma proteins. In conclusion, the system allows for a rapid and flexible generation of target cell specific immunoliposomes for in vitro applications but lacks stability for in vivo applications.

Antigens, CD↗
PubMed2005

Transfer of lipophilic drugs between liposomal membranes and biological interfaces: consequences for drug delivery.

This review paper describes the present knowledge on the interaction of lipophilic, poorly water soluble, drugs with liposomal membranes and the reversibility of this interaction. This interaction is discussed in the context of equilibrium and spontaneous transfer kinetics of the drug, when the liposomes are brought in co-dispersion with other artificial or natural phospholipid membranes in an aqueous medium. The focus is on drugs, which have the potential to partition (dissolve) in a lipid membrane but do not perturb membranes. The degree of interaction is described as solubility of a drug in phospholipid membranes and the kinetics of transfer of a lipophilic drug between membranes. Finally, the consequences of these two factors on the design of lipid based carriers for oral, as well as parenteral use, for lipophilic drugs and lead selection of oral lipophilic drugs is described. Since liposomes serve as model-membranes for natural membranes, the assessment of lipid solubility and transfer kinetics of lipophilic drug using liposome formulations may additionally have predictive value for bioavailability and biodistribution and the pharmacokinetics of lipophilic drugs after parenteral as well as oral administration.

Amphotericin B↗
PubMed2005

Generation of immunoliposomes using recombinant single-chain Fv fragments bound to Ni-NTA-liposomes.

Recombinant single-chain Fv antibody fragments (scFv) can be combined with liposomes to generate immunoliposomes for targeted drug delivery. Recent studies have shown that scFv molecules modified to express a C-terminal cysteine residue can be used for site-directed chemical conjugation. Here, we present a new method by immobilizing scFv fragments via their C-terminal hexahistidyl-tag on liposomes containing Ni-NTA-lipids (Ni-NTA-DOGS) in their lipid bilayer without the need to introduce additional reactive groups in the protein. Using an anti-endoglin scFv as a model antibody, we could show that scFv molecules are efficiently immobilized on the liposome surface and that these immunoliposomes bind specifically and strongly to endoglin-expressing endothelial cells. This approach allows for a rapid and flexible generation of target cell-specific immunoliposomes.

Drug Delivery Systems↗
PubMed2004

Encapsulation of gadobutrol in AVE-based liposomal carriers for MR detectability.

Artificial virus-like envelopes (AVEs) are liposomal carriers that may be useful for target-site-specific delivery of contrast agents. We speculated that T(1) relaxation times of a suspension of Gadolinium-filled AVEs might be shortened after internalization and lysosomal breakdown. To test this hypothesis we evaluated the T(1) relaxation times of Gadobutrol-containing AVEs before and after degradation in vitro and after receptor-mediated cellular uptake. AVEs were filled with 1 M Gadobutrol (Gadovist; Schering AG, Berlin, Germany) yielding Gd-chelate-AVEs. T(1)-relaxation times were calculated using an inversion recovery technique for different concentrations of the liposomal suspension. AVEs were degraded in vitro to mimic the release of the encapsulated Gadolinium in cells and to determine a putative increase of the T(1)-effect. Finally, Gd-chelate-AVEs where equipped with integrin-binding RGD ligands and the T1 relaxation times of these Gd-chelate-RDG-AVEs were determined after cellular uptake into endothelial or melanoma cells. Gadobutrol could be encapsulated into AVEs at a high concentration of 1 M (Gd-chelate-AVEs). The Gd-chelate-AVEs could be visualized by MRI. Concentrations down to 1:4 x 10(3) showed a significant T(1)-shortening effect. The degradation of the liposomes with Triton X-100 resulted in a further reduction down to concentrations of 1:10 x 10(3). In addition, cellular uptakes of Gd-chelate-RGD-AVEs also lead to a significant T(1)-shortening. Our study shows that Gadolinium can be efficiently encapsulated into AVEs and that Gd-chelate-AVEs can be detected by MRI T(1)-weighted measurements. The MRI detectability is enhanced by degradation. Gd-chelate-RGD-AVEs can be used to enhance the Gd uptake in cells expressing the alpha(v)beta(3) receptor.

Biological Transport↗
PubMed2004

Efficient gene delivery with serum into human cancer cells using targeted anionic liposomes.

Success of human gene therapy depends upon the development of delivery vehicles or vectors, which can selectively deliver therapeutic genes to target cells with efficiency and safety. Previous studies have shown an efficient, systemic trans-gene expression in many cell lines (in vitro) by using an anionic liposomal vector, based on the composition of retroviral envelopes (artificial viral envelopes, AVEs). The AVE-liposomes and their complexes with plasmid (DNA) were characterized according to zeta potential measurements and transmission electron microscopy (TEM). We successfully demonstrated that AVE liposomes, dispersed in 10% serum-containing growth medium, efficiently delivered plasmid DNA to HuH-7 (human hepatoma cell line) cells. We assessed the utility of liver-targeted vesicles as a drug/gene delivery system for the treatment of liver diseases. We found that small unilamellar AVE vesicles containing 15 mol% digalactosyl diglyceride (DGDG) are efficiently targeted to the liver via the hepatic asialoglycoprotein receptor.

Carbohydrates↗
PubMed2002

Sustained-release injectables formed in situ and their potential use for veterinary products.

The controlled drug delivery of hydrophilic and lipophilic drug substances via the parenteral route has gained increasing importance in the development of pharmaceutical dosage forms. In particular, the animal health industry has generated strong interest in long-term drug delivery for both companion and farm animals during the past few years. At present sustained-release injectables formed in situ for s.c./i.m. administration have become an attractive alternative to common slow release technologies such as microspheres or standard implants. In this context, technologies based on PLA/PLGA, sucrose acetate isobutyrate (SAIB) and the amphipathic molecules Poloxamer, glycerol monooleate or PEG-PLA-PEG copolymers, are discussed. Release periods from hours to months can be obtained by choosing one of these drug delivery technologies. The release times are strongly dependent on the biodegradation of the polymers and the physico-chemical properties of the drug substance used. Furthermore, the use of different solvents for the matrix-forming agents and the individual loading capacity are critically assessed. Additionally acceptance of the excipients for parenteral use by the regulatory authorities is closely considered. Scientific articles as well as patent publications are reviewed to give a wide overview of the existing approaches and their future potential for animal health products.

Animals↗
PubMed2002

Interactions of cyclosporines with lipid membranes as studied by solid-state nuclear magnetic resonance spectroscopy and high-sensitivity titration calorimetry.

Cyclosporin A (CyA) interacts with lipid membranes. Binding reaction and membrane location of CyA and analogs were examined with 2H-NMR, high-sensitivity isothermal titration calorimetry (ITC), and CD spectroscopy. Effects of CyA and charged analogs on the phosphocholine head group and on the membrane interior were investigated using selectively deuterated phospholipids. Incorporation of cyclosporin generated small disordering of the lipid acyl chains. Binding of CyA and neutral and positively charged analogs to lipid membranes showed endothermic heats of reaction between + 5.9 and + 11.3 kcal/mol, whereas enthalpy of binding was close to zero for the negatively charged derivative. Binding constants of cyclosporines to liposomal membranes were in the range of K(P) = 1650-5560 M(- 1) depending on the cholesterol content. (2)H-NMR provides evidence that CyA is essentially located in the interior of the bilayer membrane. For the charged analogs an additional interaction occurs at the head group level, placing the polar groups of these CyA analogs in the vicinity of the phosphocholine dipoles. The association of CyA and its analogs is accompanied by a positive enthalpy change, which is overcompensated by positive entropy changes. Binding of CyA to lipid membranes thus follows the classical hydrophobic effect, which is in contrast to many other peptide-lipid binding reactions.

Algorithms↗