The lipoidal permeability barriers of the skin and alimentary tract

Best practices in current models mimicking drug permeability in the gastrointestinal tract - an UNGAP review

The absorption of orally administered drug products is a complex, dynamic process, dependent on a range of biopharmaceutical properties; notably the aqueous solubility of a molecule, stability within the gastrointestinal tract (GIT) and permeability. From a regulatory perspective, the concept of high intestinal permeability is intrinsically linked to the fraction of the oral dose absorbed. The relationship between permeability and the extent of absorption means that experimental models of permeability have regularly been used as a surrogate measure to estimate the fraction absorbed. Accurate assessment of a molecule's intestinal permeability is of critical importance during the pharmaceutical development process of oral drug products, and the current review provides a critique of in vivo, in vitro and ex vivo approaches. The usefulness of in silico models to predict drug permeability is also discussed and an overview of solvent systems used in permeability assessments is provided. Studies of drug absorption in humans are an indirect indicator of intestinal permeability, but in vitro and ex vivo tools provide initial screening approaches are important tools for direct assessment of permeability in drug development. Continued refinement of the accuracy of in silico approaches and their validation with human in vivo data will facilitate more efficient characterisation of permeability earlier in the drug development process and will provide useful inputs for integrated, end-to-end absorption modelling.

Metabolic engineering of the non-conventional yeast Pichia ciferrii for production of rare sphingoid bases

The study describes the identification of sphingolipid biosynthesis genes in the non-conventional yeast Pichia ciferrii, the development of tools for its genetic modification as well as their application for metabolic engineering of P. ciferrii with the goal to generate strains capable of producing the rare sphingoid bases sphinganine and sphingosine. Several canonical genes encoding ceramide synthase (encoded by PcLAG1 and PcLAF1), alkaline ceramidase (PcYXC1) and sphingolipid C-4-hydroxylase(PcSYR2), as well as structural genes for dihydroceramide Δ(4)-desaturase (PcDES1) and sphingolipid Δ(8)-desaturase (PcSLD1) were identified, indicating that P. ciferrii would be capable of synthesizing desaturated sphingoid bases, a property not ubiquitously found in yeasts. In order to convert the phytosphingosine-producing P. ciferrii wildtype into a strain capable of producing predominantly sphinganine, Syringomycin E-resistant mutants were isolated. A stable mutant almost exclusively producing high levels of acetylated sphinganine was obtained and used as the base strain for further metabolic engineering. A metabolic pathway required for the three-step conversion of sphinganine to sphingosine was implemented in the sphinganine producing P. ciferrii strain and subsequently enhanced by screening for the appropriate heterologous enzymes, improvement of gene expression and codon optimization. These combined efforts led to a strain capable of producing 240mgL(-1) triacetyl sphingosine in shake flask, with tri- and diacetyl sphinganine being the main by-products. Lab-scale fermentation of this strain resulted in production of up to 890mgkg(-1) triacetyl sphingosine. A third by-product was unequivocally identified as triacetyl sphingadienine. It could be shown that inactivation of the SLD1 gene in P. ciferrii efficiently suppresses triacetyl sphingadienine formation. Further improvement of the described P. ciferrii strains will enable a biotechnological route to produce sphinganine and sphingosine for cosmetic and pharmaceutical applications.

Synthesis and immobilization of ceramide analogs on silica particles

Ceramides are the major lipid components of the stratum corneum, the major permeability barrier of the skin. Here we report a chemical synthesis of ceramide analogs covalently bonded on the silica particles, that can be used to predict the skin permeability of chemicals via HPLC methods.

Topical anesthetic agents in dermatologic surgery. A review

BACKGROUND

The ideal topical anesthetic agent is one that provides 100% anesthesia in a short period of time, work on intact skin without systemic side effects, and invokes neither pain nor discomfort. The quest to find such an agent continues today. Because a topical anesthetic agent will induce anesthesia painlessly, the need for an effective agent is clear. This will serve to eliminate painful injections with lidocaine prior to many dermatologic procedures.

OBJECTIVE

To provide a review of topical agents used in the past, to present products that are being used today, and to look to the future of topical anesthesia. CONCLUSIVE: During the last three decades a variety of methods have been employed to administer topical anesthesia. Presently, EMLA (eutectic mixture of local anesthetics) is the most often used method among practicing dermatologists. However, iontophoresis and the anesthetic patch are equally effective with a few notable advantages over EMLA. Liposomal agents show promise as we enter into a new millennium.

Enhancing effects of lipophilic vehicles on skin penetration of methyl nicotinate in vivo

Vehicle effects may be caused by thermodynamic effects and by specific (penetration enhancing) effects. To investigate the effects of various lipophilic vehicles on drug penetration, an in vivo permeability study was conducted with methyl nicotinate as the model drug. The drug was dissolved in the respective vehicles at concentrations that provide equal drug escaping tendencies. Drug solutions were applied to the upper arms of volunteers with a glass chamber system. To avoid drug depletion effects, drug disappearance rates were measured under steady-state conditions by the difference method. Enhancement factors were calculated from the steady-state flux values (i.e., drug disappearance rates per area unit) and compared with results from non-steady-state experiments. Significant enhancing effects (p < 0.01) were observed with dibutyl adipate, caprylic/capric acid triglycerides containing 5% phospholipids, isopropyl myristate, and mineral oil. Caprylic/capric acid triglycerides, cetearyl isooctanoate, and the standard vehicle dimethicone 100 were without effect on drug penetration. The explanation for the observed enhancing effects may be an interaction of the lipophilic liquids with the lipid bilayers of the stratum corneum that leads to a decrease of the barrier resistance.

Membrane structures in normal and essential fatty acid-deficient stratum corneum: characterization by ruthenium tetroxide staining and x-ray diffraction

Despite the importance of intercellular lamellar bilayers for stratum corneum (SC) barrier function, knowledge about the structure of these bilayers is limited due to their poor visualization and/or retention. Whereas substitution of ruthenium tetroxide (RuO4) for osmium tetroxide fixation provides clear images of these bilayers, the usefulness of RuO4 has been limited by its slow penetration and cytotoxicity. Utilizing a new fixation protocol for RuO4, we obtained clear images of lamellar domains at all levels of murine SC. Computer-aided image reconstructions demonstrated a lamellar spacing of 129 +/- 2 A, which agreed with x-ray diffraction data from parallel, unfixed samples (131 +/- 2 A), a spacing not affected by hydration. Furthermore, novel structures were seen in the intercellular spaces of normal SC. Finally, in murine essential fatty acid deficiency (EFAD), the overall lamellar spacing is comparable to normal [127 +/- 7 A by computer transform vs. 131.9 +/- 2 A (hydrated) and 129.6 +/- 2.2 A (dry) by x-ray diffraction]. Yet, these domains are structurally abnormal, displaying regions with either an excess or absence of lamellae. The new RuO4 protocol provides quantitative information about SC lamellar dimensions and morphologic abnormalities in bilayer distribution and substructure in EFAD stratum corneum that are not detected by either x-ray diffraction or computer-aided image reconstruction. Thus, the barrier abnormality in EFAD stratum corneum can be ascribed either to focal depletion of lamellae or abnormalities in lamellar substructure.

Oleic acid: its effects on stratum corneum in relation to (trans)dermal drug delivery

Calorimetric studies with porcine stratum corneum (SC) have shown that the lipid phase transitions associated with the intercellular bilayers are markedly affected by treatment with oleic acid. Specifically, the transition temperatures (Tm) and cooperativity are reduced, whereas no effect was observed on the endotherm associated with keratin denaturation, suggesting that oleic acid primarily affects the SC lipids. The decrease in the lipid-associated Tm's was further correlated with the amount of oleic acid taken up by the SC. Parallel experiments with silastic implied that the uptake is dependent on the thermodynamic activity of oleic acid in the vehicle itself. The in vitro transport of Piroxicam across human and hairless mouse skin (HMS) was significantly enhanced by oleic acid, as a function of the extent of oleic acid uptake, with an attendant change in Tm. These results emphasize the role of SC lipids in percutaneous absorption. Transport also depended on the donor concentration of ionized drug suggesting that the enhanced transport mechanism cannot be accounted for solely on the principles of the classical pH-partition hypothesis. Accordingly, a model of skin permeability enhancement involving solid-fluid phase separation within the SC lipids is proposed for oleic acid, consistent with the existing phospholipid literature. In conjunction with the use of oleic acid as an enhancer, very soluble hydrophilic salts were recognized as key factors in attaining maximum delivery. Oleic acid uptake, lipid delta Tm, and enhanced drug flux were all found to correlate, exhibiting a bell-shaped curve as a function of the ethanol vehicle concentration. Therefore, uptake and/or DSC experiments are useful for formulating enhanced topical delivery systems.