Left-handed and right-handed metamaterials composed of split ring resonators and strip wires.

The behavior of two structures composed of split ring resonators (SRRs) and strip wires (SWs) is examined through full wave simulations. It is shown that both structures exhibit a transmission peak in the region where the real parts of the electric permittivity and magnetic permeability are presumed to be negative, a property which is usually assumed to imply a negative index of refraction. However, an analysis of the dispersion characteristics and insertion phase of the two structures shows that the first structure, in which the SRRs and SWs are printed on opposite sides of a dielectric substrate, is a left-handed medium in the passband, whereas the second structure, in which SRRs and SWs are printed on the same side, is a right-handed medium in the passband. Hence the transmission magnitude alone does not provide sufficient evidence of a negative index of refraction. To determine the sign of the index correctly, the insertion phase for propagation through several lengths of the structure or calculations of dispersion diagrams are necessary. The impact of the unit cell size on the "handedness" of the structure is also examined.

Negative group velocity and group delay in left-handed media.

The dynamics of wave propagation in media with negative index of refraction is analyzed through analytical calculations, simulations, and experiments. Using a free space setup, the transmission characteristics of two split ring resonator and strip wire left-handed media (LHM), designed for operation at K -band frequencies (18-26 GHz), are measured. The first LHM, which is 3 unit cells long in the propagation direction, exhibits a maximum negative group delay of -0.9 ns . The second LHM (4 unit cells long) exhibits a maximum negative group delay of -1.2 ns . For both LHM the bandwidth of the negative group delay (and hence the negative group velocity) region was approximately 250 MHz.

Effect of polyoxyl 35 castor oil and Polysorbate 80 on the intestinal absorption of digoxin in vitro.

Surfactants are classically used to improve the solubilization of lipophilic drugs such as digoxin. Polysorbate 80 and Cremophor EL (polyoxyl 35 castor oil) are such surfactants but they may also modulate the action of P-glycoprotein, an energy-dependent "counter-transport" system implicated in the phenomenon of multidrug resistance in cancer cells. P-glycoprotein is also present in the intestine on the apical membrane of mature enterocytes and can potentially reduce the absorption of a wide range of drugs. In this study, using the improved everted gut sac method, the effects of Polysorbate 80, Cremophor EL and cyclosporin on the absorption of digoxin were studied. An increase in the uptake of digoxin in the presence of these three products could be shown with our in vitro model. Cremophor EL and Polysorbate 80 had no toxic effects at the concentrations used. These results suggest that surfactants such as Cremophor EL and Polysorbate 80 should not only support solubilization but can also modulate the P-glycoprotein system to improve the bioavailability of poorly absorbed drugs.

Lectins for gastrointestinal targeting--15 years on.

In the mid-1980s, the concept of bioadhesion using synthetic polymers emerged, and brought with it the promise of improved efficiency for the delivery of drugs via mucosal surfaces. Studies in the author's laboratory concentrated on 'biological' bioadhesion using the naturally-occurring proteins, lectins, which recognise and bind sugars in glycoconjugates, such as those found on the surfaces of cells. Tomato Lectin (TL) was extensively studied as a putative non-toxic lectin with potential for drug targeting/delivery to the gastrointestinal (GI) tract. In vitro, the TL displayed impressive binding to the intestinal mucosa, but in vivo failed to significantly modify intestinal transit. A number of research groups have coupled the TL to microparticles, and significant systemic uptake of these has been observed in animal studies. Polymers with pendant sugars have also been shown to be bioadhesive, by interacting with endogenous lectins present on the cells of the GI tract. The use of lectins to target to Peyer's patches and diseased tissues in the colon is an interesting development, but much work remains to be done. Lectins also have potential in mucosal vaccines. Before advanced drug delivery systems using lectins can be realised, rigorous evaluation of their toxicity and immunogenicity will be required, but they clearly offer a number of possibilities for GI drug targeting systems in the future.

Studies on the uptake of tomato lectin nanoparticles in everted gut sacs.

Tomato lectin (TL) is a bioadhesive glycoprotein that has been shown to bind selectively to the small intestine epithelium. When bound to polystyrene microspheres, intestinal uptake occurs not only through the gut associated lymphoid tissue (GALT) but also through normal enterocytes. In this study, the everted gut sac model was used to compare the rates and quantities of intestinal uptake of tomato lectin and that of TL coupled to microspheres. Using bovine serum albumin (BSA) and BSA coupled to microspheres as comparators. Uptake is time and concentration dependent. Transfer of TL from the lumen to the serosa was 3.9 ng/mg per h whereas that of BSA was 0.5 ng/mg per h. Hence uptake of tomato lectin was 7-fold higher than BSA. The rate of uptake of TL coupled microspheres was 41.5 ng/mg per h, which was 4-fold higher than microspheres coupled to BSA (11.8 ng/mg per h). The uptake of TL conjugated microspheres was shown to be inhibited by N-acetyl-d-glucosamine tetramer [GlcNac]4.

An improved everted gut sac as a simple and accurate technique to measure paracellular transport across the small intestine.

An improved everted gut sac system has been developed in which the sacs were carefully prepared from rat small intestine and incubated in tissue culture medium. Under these conditions, the tissue showed good morphology at the electron microscope level, and was metabolically active for up to 2 h at 37 degrees C. Mannitol, an established probe of paracellular transport, was transported from the mucosal to the serosal side of the sac tissue. Excellent kinetic data showed that transport was linear up to 75 min and over a wide range of concentrations (0.025 - (10 mM). Mannitol was not detected in the tissue and transport was enhanced by EGTA, confirming the paracellular route of passage. Sacs prepared from colon also showed mannitol transport, but at a slower rate. Comparisons with Caco-2 cell monolayers showed that the everted sacs exhibited higher levels of paracellular transport than the cultured cell line. The improved everted gut sac system is an inexpensive and relatively simple technique with considerable potential as an in vitro tool to study the mechanisms, kinetics and enhancement of drug absorption across the small intestine at different sites and in the colon.

Enzymatic barriers for GI peptide and protein delivery.

The oral delivery of therapeutic peptides and proteins is a major challenge to pharmaceutical science. The gastrointestinal (GI) tract contains many endo- and exopeptidases, enzymes that hydrolyze peptide bonds and act synergistically to degrade proteins and peptides. It is important to have both qualitative and quantitative data on these peptidases when devising strategies for oral peptide and protein delivery. The greatest threat to therapeutic peptides lies in the lumen of the small intestine, which contains gram quantities of peptidases secreted from the pancreas, as well as cellular peptidases from the mucosal cells, which are constantly sloughed off from the villi. The second major enzymatic barrier is the brush border membrane of the epithelial cells, which contains at least 15 peptidases that together have a broad specificity and can degrade both proteins and peptides. Lysosomal peptidases will also present a barrier to any peptides or proteins endocytosed by the epithelial cells. Although the colon has received some attention as a possible site for peptide delivery, evidence shows that the lumen of the colon contains substantial amounts of peptidase activity, largely because of enzyme production by microorganisms. From a knowledge of the enzymatic barrier, the strategies for oral peptide delivery of enzyme inhibition and the synthesis of enzyme-resistant peptide analogues are logical developments. The latter approach is the most promising.

Putative immunolocalization of the mechanoelectrical transduction channels in mammalian cochlear hair cells.

Hair cells bear an apical bundle of stereocilia arranged in serried rows. Deflection of the bundle controls the opening and closing of mechanoelectrical transduction channels, thereby altering the conductance across the apical plasma membrane. Two locations for these channels have been proposed in the bundle, either near the bases of the stereocilia or towards their tips. One hypothesis that is consistent with the latter possibility suggests that fine extracellular filaments, which run between the tips of the shorter stereocilia and the sides of the taller stereocilia behind, operate the channels. Determining the precise position of the channels is essential to test this hypothesis. We have therefore attempted to localize them immunocytochemically. Because hair-cell transduction is amiloride sensitive, the channels may have an amiloride-binding site associated with them. We have therefore used a polyclonal antibody raised against another amiloride-sensitive ion channel to hunt for them. This antibody recognizes a 62-64 kDa band in immunoblots of cochlear tissue, and produces discrete labelling in the hair bundle. This is most concentrated just below the tips of the shorter stereocilia, coinciding with a region of specialization in the closely apposed membranes of the short and tall stereocilia but not with either end of the tip link.

Mucosal peptide hydrolase and brush-border marker enzyme activities in three regions of the small intestine of rats with experimental uraemia.

1. The activities of nine peptide hydrolases and three non-peptidase brush-border marker enzymes have been quantified in crude homogenates prepared from the proximal, mild and distal regions of small-intestinal mucosa for sham-operated (n = 9) and uraemic (n = 14) rats. Abnormalities in enzyme activities were observed in all regions studied in the uraemic group, although no reduction in food intake occurred. 2. The proximal region of the small intestine from uraemic rats showed a general fall in enzyme activities associated with the brush-border. This fall was combined with a decline in mucosal protein content. In contrast, the mid and distal regions showed increased activity against the dipeptide tyrosyl-glycine. 3. It is proposed that the fall in brush-border enzyme activities in the proximal small intestine of uraemic rats is a response to the increased water intake associated with this, and presumably other, rat models of uraemia. The increased enzyme activity against tyrosyl-glycine found in the mid and distal regions of the small intestine of uraemic rats may be caused by an increased small-intestinal transit rate, but could be an attempt to maximize tyrosine absorption in response to decreased plasma tyrosine levels. 4. This study casts doubt on specific activities being the most useful units of enzyme activity, when measured in crude homogenates prepared from the proximal small intestine of uraemic rats. It also demonstrates that enzyme activities measured at a single site in the small intestine of uraemic rats may not be representative of the enzymatic changes occurring in the small-intestinal mucosa as a whole.

Experimental uraemia with associated plasma amino acid abnormalities but without retarded food intake and weight gain.

A rat model of moderate uraemia is described in which uraemic animals attain normal food intake and weight gain. Despite the low levels of the induced uraemia, which has been well defined, changes in plasma amino acid concentrations associated with experimental uraemia still occur. It appears from this study that a reduction in food intake is not a major factor in the aetiology of the plasma amino acid changes seen in uraemia and that the model may prove useful in future studies of experimental chronic renal failure.

A fluorimetric assay for aminopeptidase W.

A novel two-step enzyme-linked assay for aminopeptidase W is described and validated by comparison with other assays. L-alpha-Glutamyl-L-tryptophan (Glu-Trp) is a favoured substrate for this enzyme. With the use of glutamate dehydrogenase (EC 1.4.1.2) in a second step, the assay measured the release of free glutamate from L-alpha-glutamyl-L-tryptophan by the increase in NADH fluorescence. In the presence of 5 mM-1,10-phenanthroline and 50 microM-cilastatin the contribution of other membrane peptidases, in particular aminopeptidases N and A and microsomal dipeptidase in kidney, was very small. Residual cytosolic activities hydrolysing Glu-Trp were sensitive to inhibition by 2.5 mM-N-ethylmaleimide. The activity of aminopeptidase W was unaffected by these inhibitors. There was good correlation between the fluorimetric assay and those in which the free tryptophan released by kidney membrane fractions was determined by h.p.l.c. or the aminopeptidase W was measured immunoradiometrically with a monoclonal antibody.

Role of rat intestinal brush-border membrane angiotensin-converting enzyme in dietary protein digestion.

The role of rat intestinal angiotensin-converting enzyme (ACE; E.C 3.4.15.1) in the digestion and absorption of dietary protein was investigated. Enzyme activity was associated with the brush-border membrane fraction, with the highest activity in the proximal to midregion of the small intestine. Preliminary enzyme characterization studies were carried out using purified brush-border membrane preparations. When a variety of N-blocked synthetic peptides were used as potential substrates for ACE, activity was highest with those containing proline at the carboxy terminal position. The hydrolytic rates observed with these prolyl peptides were comparable to those observed when major digestive peptidases of the brush-border membrane such as aminopeptidase N and dipeptidyl aminopeptidase IV were assayed. When isolated rat jejunum was perfused in vivo with solutions of Bz-Gly-Ala-Pro, the dipeptide Ala-Pro was the main hydrolytic product detected in the perfusates. Absorption rates of the constituent amino acids, alanine and proline, depended on the concentration of peptide perfused. Captopril, an active site specific ACE inhibitor, significantly inhibited hydrolysis and absorption of constituent amino acids from Bz-Gly-Ala-Pro. These results show that intestinal brush-border membrane ACE functions as a digestive peptidase in addition to its role as a regulator of biologically active peptides in other tissues.

Liposomes for oral administration of drugs.

In the late 1970s liposome-entrapped insulin was administered by the oral route to both normal and diabetic animals. Results showed that small but significant amounts of insulin could reach the circulation. However, different liposome compositions gave varied results and no mechanism of absorption was elucidated. Subsequent in vitro studies suggested that many liposome compositions used were unstable in the conditions prevailing in the gastrointestinal tract. Using more stable liposomes in an everted gut system, it has been demonstrated that liposomes were pinocytosed by intestinal epithelial cells and transferred to the serosal side of the gut. Recent studies both in vitro and in vivo show that there may be the possibility of enhancing the uptake process to deliver a range of drugs by the oral route.

Breakdown of gliadin peptides by intestinal brush borders from coeliac patients.

The 'missing peptidase' hypothesis to explain the aetiology of coeliac disease has never been satisfactorily resolved and recent reports suggest that coeliac brush borders may have depressed levels of specific peptidase enzymes. It has been inferred from these studies that the subsequent brush border digestion of gliadin peptides may therefore be defective. In this present study a sensitive fluorometric assay was used to measure the hydrolysis of a peptic-tryptic digest of gliadin by both normal and coeliac brush borders. The coeliac brush borders were as efficient as the normals in hydrolysing gliadin peptides and showed no depression of any specific peptidase activity.

An accurate fluorometric method to measure the breakdown of gliadin and gliadin peptides.

A simple and accurate method is described to measure the breakdown of gliadin and gliadin peptides. It involves measuring the release of the predominant amino acids glutamine and glutamic acid using a fluorometric double enzyme assay and contains none of the problems normally associated with previously used techniques. The assay is highly accurate in that small concentrations of the free amino acids can be measured with no interference from the peptide bound amino acids. The assay system was used to study the breakdown of: whole gliadin, a peptic/tryptic digest of gliadin (PT gliadin), and a peptic/tryptic/chymotryptic digest of gliadin (PTC gliadin) using a rat intestinal brush border fraction. Both PT and PTC gliadin are hydrolysed at higher rates than is whole gliadin. Exhaustive hydrolysis shows that a brush border fraction can totally break down PT gliadin while an initial linear rate of breakdown is observed (up to 60 min).

Uptake of free and liposome-entrapped 125I-labelled PVP by rat intestinal sacs in vitro: evidence for endocytosis.

The uptake of free and liposome-entrapped 125I-labelled PVP was measured in and intestinal-sac preparation from adult rats. Uptake of the free macromolecule was directly proportional to the substrate concentration and was reduced by colchicine and sodium azide. Uptake of the liposome-entrapped macromolecule was greater than that of the free macromolecule, was not directly proportional to substrate concentration, showed signs of saturation at high liposome concentrations, and was also reduced by sodium azide and colchicine. These results suggest that gut epithelial cells take up the free macromolecule by fluid-phase endocytosis and the liposome-entrapped macromolecule by adsorptive endocytosis.

Uptake of free and liposome-entrapped insulin by rat intestinal sacs in vitro.

Entrapment of insulin within distearoylphosphatidylcholine and cholesterol liposomes can protect the hormone from degradation in an in vitro gut-sac system and can increase the quantity of immunoreactive insulin that reaches the serosal fluid 26-fold. 95% of the insulin that reaches the serosal fluid is present within relatively intact liposomes. However, the presence of hydrolytic agents, probably enzymes, within the serosal fluid of everted gut sacs makes it necessary to separate the liposomes from the serosal fluid, by centrifugation, prior to release of the entrapped insulin with Triton X-100. This procedure prevents hydrolysis of the released insulin which can then be measured by radioimmune assay.

The uptake of distearoylphosphatidylcholine/cholesterol liposomes by rat intestinal sacs in vitro.

The uptake of free and liposome-entrapped 125I-labelled polyvinylpyrrolidone was measured in an intestinal sac preparation from adult rats. At an equal concentration of 125I-labelled polyvinylpyrrolidone, the rate of uptake of the liposome-entrapped macromolecule by the tissue was over 4-times that of the free macromolecule. The quantity of 125I-labelled polyvinylpyrrolidone present in the serosal fluid of gut sacs, cultured for 2 h, was 1.8-times greater when the macromolecule was entrapped in liposomes than when it was free in the culture medium. When gut sacs were cultured with liposome-entrapped macromolecule, approx. 50% of the total 125I-labelled polyvinylpyrrolidone present in the serosal fluid was associated with a 100 000 X g liposomal pellet.

The stability of liposomes in vitro to pH, bile salts and pancreatic lipase.

If liposomes are to be effective as carriers for the oral administration of insulin they must be able to withstand passage through the stomach and small intestine. Multilamellar liposomes, some identical in composition to those used in reported in vivo studies on the uptake of orally administered insulin, were tested in vitro for their stability in the presence of bile salts, pancreatic lipase, and variations in pH. While low or high pH had little effect on most liposomes, 10 mM bile salts caused the release of over 80% of entrapped marker from all liposomes tested except those composed of distearoyl phosphatidylcholine/cholesterol and those composed of dipalmitoyl phosphatidylethanolamine/cholesterol/dicetylphosphate. However, the latter were unstable at low pH. The distearoyl phosphatidylcholine/cholesterol liposomes were also resistant to pancreatic lipase, and therefore may be suitable for use in the oral administration of therapeutic agents.