Indication of transcytotic movement of insulin across human bronchial epithelial cells

This study was performed to evaluate insulin permeability across human bronchial epithelial cell lines and investigate if insulin is transported via the paracellular or transcellular pathway. The movement of insulin across two bronchial epithelial cells, 16HBE14o- and Calu-3, was studied in the presence or absence of octylmaltoside. Mannitol and propanolol have been used as paracellular and transcellular marker, respectively, and transepithelial electrical resistance (TEER) was determined to investigate the tight junctional integrity of the monolayers. The possible endocytotic mechanism of insulin across these two cell lines was studied by confocal laser scanning microscopy after incubating the cells with fluorescent-labeled insulin. The TEER values for both cell monolayers were >400 Omega cm2 at confluency. There was a decrease in the TEER values when octylmaltoside was added to the apical side of transwells. Similarly, the apparent permeability coefficient (P(app)) values of insulin, mannitol and propanolol, showed an increase with the rise in the concentration of octylmaltoside. In the absence of octylmaltoside, the P(app) values for insulin and the markers were in the following order: propanolol > mannitol > insulin. Confocal microscopic studies revealed that the uptake of insulin by the bronchial epithelial cells perhaps occurs via translocation across the cell. The data presented in this study demonstrate that insulin perhaps moves across the bronchial cells via both paracellular and transcellular pathways.     

Serially Passaged Human Nasal Epithelial Cell Monolayer for in Vitro Drug Transport Studies

Purpose. To evaluate the feasibility of using a serially passaged culture of human nasal epithelial cell monolayers on a permeable support for in vitro drug transport studies. The optimum conditions for passaged culture as well as the correlation between the transepithelial electrical resistance (TEER) value and drug permeability (P_app) were evaluated. Methods. Fresh human nasal epithelial cells were collected from normal inferior turbinates and were subcultured repeatedly in serum-free bronchial epithelial cell growth media (BEGM) in petri dishes. The subcultured cells of each passage were seeded onto permeable supports at 5 × 10⁵ cells/cm² and grown in Dulbecco's modified Eagle medium (DMEM). Morphologic characteristics were observed by scanning electron microscopy (SEM). To verify the formation of tight junctions, actin staining and transmission electron microscopy (TEM) studies were conducted. In the drug transport study, [¹⁴C]mannitol and budesonide were selected as the paracellular and the transcellular route markers, respectively. Results. Serially passaged cells were successfully cultured on a permeable support and showed significantly high TEER values up to passage 4. After 14 days of seeding, SEM showed microvilli, and protrusions of cilia and mucin granules were detected by TEM. The paracellular marker [¹⁴C]mannitol showed a nearly constant permeability coefficient (P_app) when the TEER value exceeded 500 ·cm² regardless of the passage number. However, as expected, budesonide showed a higher permeability coefficient compared to [¹⁴C]mannitol and was less affected by the TEER value. Conclusions. Human nasal epithelial cell monolayers were successfully subcultured on a permeable support up to passage 4. These cell culture methods may be useful in high-throughput screening of in vitro nasal transport studies of various drugs.     

Engineering of Dendrimer Surfaces to Enhance Transepithelial Transport and Reduce Cytotoxicity

Purpose. To evaluate the cytotoxicity, permeation, and transport mechanisms of PAMAM dendrimers and surface-modified cationic PAMAM dendrimers using monolayers of the human colon adenocarcinoma cell line, Caco-2. Methods. Cytotoxicity was determined using the MTT assay. The effect of dendrimers on monolayer integrity was determined from measurements of transepithelial electrical resistance (TEER) and [¹⁴C]mannitol apparent permeability coefficient (P_app). The P_app of dendrimers through monolayers was measured in both the apical (A)-to-basolateral (B) and BA directions at 4°C and 37°C and also in the presence and absence of ethylenediamine tetraacetic acid (EDTA) and colchicine. Results. The cytotoxicity and permeation of dendrimers increased with both concentration and generation. The cytotoxicity of cationic dendrimers (G2, G3, G4) was greater than that of anionic dendrimers (G2.5, G3.5) but was reduced by conjugation with lauroyl chloride; the least cytotoxic conjugates were those with six attached lauroyl chains. At 37°C the P_app of cationic dendrimers was higher than that of anionic dendrimers and, in general, increased with the number of attached lipid chains. Cationic dendrimers decreased TEER and significantly increased the P_app of mannitol. Modified dendrimers also reduced TEER and caused a more marked increase in the P_app of mannitol. The P_app values of dendrimers and modified dendrimers were higher in the presence of EDTA, lower in the presence of colchicine, and lower at 4°C than at 37°C. Conclusions. The properties of dendrimers may be significantly modified by surface engineering. Conjugation of cationic PAMAM dendrimers with lauroyl chloride decreased their cytotoxicity and increased their permeation through Caco-2 cell monolayers. Both PAMAM dendrimers and lauroyl-PAMAM dendrimer conjugates can cross epithelial monolayers by paracellular and transcellular pathways.     

Mechanism of Intestinal Absorption of Ranitidine and Ondansetron: Transport Across Caco-2 Cell Monolayers

We have investigated the transport of ranitidine and ondansetron across the Caco-2 cell monolayers. The apparent permeability coefficients (P _app) were unchanged throughout the concentration range studied, indicating a passive diffusion pathway across intestinal mucosa. No metabolism was observed for ranitidine and ondansetron during the incubation with Caco-2 cell monolayers. P _app values for ranitidine and ondansetron (bioavailability of 50 and 100% in humans, respectively) were 1.03 ± 0.17 × 10^–7 and 1.83 ± 0.055 × 10^–5 cm/sec, respectively. The P _app value for ranitidine was increased by 15- to 20-fold in a calcium-free medium or in the transport medium containing EDTA, whereas no significant change occurred with ondansetron, indicating that paracellular passive diffusion is not rate determining for ondansetron. Uptake of ondansetron by Caco-2 cell monolayers was 20- and 5-fold higher than that of ranitidine when the uptake study was carried out under sink conditions and at steady state. These results suggest that ranitidine and ondansetron are transported across Caco-2 cell monolayers predominantly via paracellular and transcellular pathways, respectively.     

Passive Transepithelial Absorption of Thyrotropin-Releasing Hormone (TRH) via a Paracellular Route in Cultured Intestinal and Renal Epithelial Cell Lines

Transport studies using intestinal brush-border membrane vesicles isolated from rats and rabbits have failed to demonstrate proton- or Na⁺-dependent carrier-mediated transport of thyrotropin-releasing hormone (TRH), despite a pharmacologically relevant oral bioavailability. To examine the hypothesis that reported levels of oral bioavailability reflect predominately a paracellular rather than transcellular route for transepithelial transport of TRH, we have studied TRH transport in cultured epithelial cell types of intestinal (Caco-2 and T₈₄) and renal (MDCK I, MDCK II, and LLC-PK₁ origin, whose paracellular pathways span the range of permeability values observed in natural epithelia. Transport of TRH across monolayers of intestinal Caco-2 cells was similar to the flux of mannitol (1–4% per 4 hr), and unlike other putative substrates for the di-/tripeptide carrier, apical-to-basolateral transport was not increased by the presence of an acidic pH in the apical chamber. TRH transport did not show saturation, being uneffected in the presence of 20 mM cold TRH. In each cell type studied TRH and mannitol transport were similar and positively correlated with the conductance of the cell layers, consistent with a passive mechanism of absorption. This evidence suggests that, providing that a peptide is resistant to luminal hydrolysis, small but pharmacologically significant amounts of peptide absorption may be achieved by passive absorption across a paracellular route.     

Functional and cytometric examination of different human lung epithelial cell types as drug transport barriers

To develop inhaled medications, various cell culture models have been used to examine the transcellular transport or cellular uptake properties of small molecules. For the reproducible high throughput screening of the inhaled drug candidates, a further verification of cell architectures as drug transport barriers can contribute to establishing appropriate in vitro cell models. In the present study, side-by-side experiments were performed to compare the structure and transport function of three lung epithelial cells (Calu-3, normal human bronchial primary cells (NHBE), and NL-20). The cells were cultured on the nucleopore membranes in the air–liquid interface (ALI) culture conditions, with cell culture medium in the basolateral side only, starting from day 1. In transport assays, paracellular transport across all three types of cells appeared to be markedly different with the NHBE or Calu-3 cells, showing low paracellular permeability and high TEER values, while the NL-20 cells showed high paracellular permeability and low TEER. Quantitative image analysis of the confocal microscope sections further confirmed that the Calu-3 cells formed intact cell monolayers in contrast to the NHBE and NL-20 cells with multilayers. Among three lung epithelial cell types, the Calu-3 cell cultures under the ALI condition showed optimal cytometric features for mimicking the biophysical characteristics of in vivo airway epithelium. Therefore, the Calu-3 cell monolayers could be used as functional cell barriers for the lung-targeted drug transport studies.     

Correlation Between Oral Drug Absorption in Humans,and Apparent Drug Permeability in TC-7 Cells,A Human Epithelial Intestinal Cell Line: Comparison with the Parental Caco-2 Cell Line

Purpose. To determine and compare the relationship between in vivo oral absorption in humans and the apparent permeability coefficients (P _app ) obtained in vitro on two human intestinal epithelial cell lines, the parental Caco-2 and the TC-7 clone. Methods. Both cell lines were grown for 5–35 days on tissue culture-treated inserts. Cell monolayers were analysed for their morphology by transmission electron micrography, and for their integrity with respect to transepithelial electrical resistance, mannitol and PEG-4000 transport, and cyclosporin efflux. P _app were determined for 20 compounds exhibiting large differences in chemical structure, molecular weight, transport mechanisms, and percentage of absorption in humans. Results. The TC-7 clone exhibits morphological characteristics similar to those of the parental Caco-2 cell line, concerning apical brush border, microvilli, tight junctions and polarisation of the cell line. The TC-7 clone however appeared more homogenous in terms of cell size. Both cell lines achieved a similar monolayer integrity towards mannitol and PEG-4000. Monolayer integrity was achieved earlier for the TC-7 clone, mainly due to its shorter doubling time, i.e. 26 versus 30 hours for parental Caco-2 cells. When using cyclosporin A as a P-glycoprotein substrate, active efflux was lower in the TC-7 clone than in the parental Caco-2 cells. The P_app and mechanisms of transport (paracellular or transcellular routes, passive diffusion and active transport) were determined for 20 drugs. A relationship was established between the in vivo oral absorption in humans and P_app values, allowing to determine a threshold value for P_appof 2 10^–6 cm/sec, above for which a 100% oral absorption could be expected in humans. Both correlation curves obtained with the two cell types, were almost completely superimposable. These studies also confirmed that the dipeptide transporter is underexpressed in both cell lines. Conclusions. On the basis of morphological parameters, biochemical activity and drug transport characteristics, the TC-7 clone appeared to be a valuable alternative to the use of parental Caco-2 cells for drug absorption studies.     

The Effect of β-Turn Structure on the Permeation of Peptides Across Monolayers of Bovine Brain Microvessel Endothelial Cells

Purpose. To investigate the effects of the -turn structure of a peptide on its permeation via the paracellular and transcellular routes across cultured bovine brain microvessel endothelial cell (BBMEC) monolayers, an in vitro model of the blood-brain barrier (BBB). Methods. The effective permeability coefficients (P_eff) of the model peptides were determined across BBMEC monolayers. The dimensions of the aqueous pores in the tight junctions (TJs) of the BBMEC monolayers were determined using a series of hydrophilic permeants. This value and the molecular radius of each peptide were used to calculate the theoretical paracellular (P_P ^*) and transcellular (P_T ^*) permeability coefficients for each peptide. Results. A comparison of the theoretical P_P ^* values with the observed P_eff values was made for a series of model peptides. For the most hydrophobic peptides (Ac-PheProXaaIle-NH₂ and Ac-PheProXaaIleVal-NH₂; Xaa = Gly, Ile), it was concluded that the Gly-containing peptide of each pair more readily permeates BBMEC monolayers via the transcellular pathway than the Ile-containing analog. In addition, the Gly-containing peptides, which exhibit more -turn structure, were shown to be more lipophilic than the Ile-containing peptides as estimated by the log of their l-octanol:HBSS partition coefficients (log P_o/w). However, the three hydrophilic peptide pairs (Ac-TyrProXaaAspVal-NH₂, Ac-TyrProXaaAsnVal-NH₂, and Ac-TyrProXaaIleVal-NH₂; Xaa = Gly, Ile) were found to permeate BBMEC monolayers predominantly via the paracellular pathway. No differences were observed in the P_eff values of the hydrophilic peptides having higher -turn structures as compared to the peptides lacking these structural features. In addition, the Ile-containing peptides exhibited significantly higher log P_o/w values than the Gly-containing hydrophilic peptides. Conclusions. Hydrophobic peptides that exhibit significant -turn structure in solution are more lipophilic as measured by log P_o/w, and more readily permeate BBMEC monolayers via the transcellular route than hydrophobic peptides that lack this type of solution structure. Similar secondary structural features in hydrophilic peptides do not appear to sufficiently alter the physicochemical properties of the peptides so as to alter their paracellular flux through BBMEC monolayers.     

Application of biopharmaceutics classification system (BCS) in drug transport studies across human respiratory epithelial cell monolayers

Transport studies of model drugs were conducted across the human nasal epithelial (HNE) and normal human bronchial epithelial (NHBE) cell monolayers cultured by air?Cliquid interface method. Physicochemical properties (e.g., molecular weight, calculated partition coefficient, dose number) of model drugs were quoted from literatures and apparent permeability coefficients (P _app) across the HNE and NHBE cell monolayers were directly measured. A linear relationship was observed between the P _app values of model drugs in the HNE and NHBE cell monolayers. As the molecular weight of model drugs increased, the P _app showed a decreasing pattern while the increase of partition coefficients resulted in the increment of P _app. These results indicated that the transport of model drugs across both cell monolayers followed mainly the passive diffusion mechanism, although substrates mediated by drug transporters showed a deviating pattern. It was also interesting to note that almost all model drugs could be grouped into the same biopharmaceutics classification system as that classified by the human intestinal permeability when the P _app was plotted as a function of dose number (D ₀) of each drug.     

Myosin Light Chain Kinase Inhibition: Correction of Increased Intestinal Epithelial Permeability <Emphasis Type="BoldItalic">In Vitro</Emphasis>

Purpose To examine whether myosin light chain kinase (MLCK) inhibitors can reduce intestinal epithelial permeability increases in vitro. Materials and Methods Isolated rat, mouse and human colonic tissue mucosae and Caco-2 monolayers were exposed to cytochalasin D (cD) and sodium caprate (C₁₀), in the absence and presence of the MLCK inhibitors, ML-9 and D PIK. Transepithelial electrical resistance (TEER) and Papp of [¹⁴C]-mannitol or FITC-dextran 4000 (FD-4) were measured. Western blots were used to measure MLC phosphorylation. Results Increases in Papp of [¹⁴C]-mannitol and decreases in TEER were induced by tight junction openers. These changes were attenuated by ML-9. D-PIK offset the FD-4 Papp increase induced by C₁₀ in Caco-2 only, while ML-9 and PIK inhibited MLC directly. cD induced constriction of peri-junctional actin in Caco-2 monolayers, but this was prevented by ML-9. Although mannitol fluxes across colonic mucosae from dextran-sulphate (DSS)-treated mice were higher than control, they were not ameliorated by either ML-9 or PIK in vitro. Conclusions ML-9 inhibits paracellular permeability increases in several intestinal epithelial models. D-PIK reduced stimulated paracellular fluxes in Caco-2 monolayers, but not in tissue. Pre-established increases were not modified by two MLCK inhibitors in a mouse model of IBD.     

Modulation of tight junctions does not predict oral absorption of hydrophilic compounds: Use of caco-2 and calu-3 cells

Permeability estimates using Caco-2 cells do not accurately predict the absorption of hydrophilic drugs that are primarily absorbed via the paracellular pathway. The objective of this study was to investigate whether modulation of tight junctions would help differentiation of paracellularly absorbed compounds. Tight junctions in Caco-2 cell monolayers were manipulated using calcium depletion approaches to decrease the transepithelial electrical resistance (TEER) of the monolayers, and permeability of hydrophilic compounds were measured under these conditions. Permeability of these compounds were also measured in Calu-3 cells, which have tighter junctions than Caco-2 cells. Calcium depletion loosened the tight junctions of Caco-2 cells to varying levels as measured by the decrease in TEER values of the monolayers. While the absolute permeability of all the model compounds increased as the tight junctions were loosened, the ratios of their permeability relative to mannitol permeability were similar. The permeability of these compounds in the tighter Calu-3 cells were also found to be similar to each other. Altering the tight junctions of Caco-2 cells to obtain leakier cell monolayers, or using a cell line with tighter junctions like Calu-3 cells, did not improve differentiation between well absorbed and poorly absorbed hydrophilic drugs. Mere manipulation of the tight junctions to increase or decrease transepithelial electrical resistance does not appear to be a viable approach to predict human absorption for hydrophilic compounds that are primarily absorbed via the paracellular pathway.     

Modulation of the Tight Junctions of the Caco-2 Cell Monolayers by H2-antagonists

Purpose. The tight junctions in the intestinal epithelium represent highly specialized intercellular junctions. Ranitidine, an H₂-antagonist, causes a tightening of the tight junctions. Hence, we have investigated the effect of ranitidine and other H₂-antagonists on the function of the intestinal tight junctions. Methods. Effect of the H₂-antagonists on the tight junctions has been investigated using the transepithelial electrical resistance (TEER) and the transport of mannitol across the Caco-2 cell monolayers. Results. Four different H₂-antagonists caused an increase in the TEER across the Caco-2 cell monolayers, accompanied by a decrease in the permeability for mannitol. The effect was concentration-dependent and saturable. Ranitidine and famotidine, caused a decrease in their own transport rate across the Caco-2 cells. Ranitidine competitively inhibited the increase in TEER caused by famotidine, whereas compounds which represent molecular fragments of ranitidine had no effect. The relative potency of the four H₂-antagonists in causing an increase in the TEER correlated inversely with the oral bioavailability of these compounds in humans. Conclusions. We hypothesize that the H₂-antagonists exert their effect on the tight junctions of Caco-2 cells by modulation of interactions among proteins associated with the tight junctional complex.     

Effect of Size and Charge on the Passive Diffusion of Peptides Across Caco-2 Cell Monolayers via the Paracellular Pathway

Purpose. To evaluate the effect of size and charge on the permeation characteristics of peptides across the intestinal mucosa. Methods. The lipophilicities of neutral, positively and negatively charged capped amino acids (Asn, Lys, Asp), tripeptides (Ac-Gly-X-Ala-NH₂; X = Asn, Lys, Asp) and hexapeptides (Ac-Tip-Ala-Gly-Gly-X-Ala-NH₂; X = Asn, Lys, Asp) were estimated using an immobilized artificial membrane. The diffusion coefficients used to calculate the molecular radii were measured by NMR. The transport characteristics of the model peptides were determined across Caco-2 cell monolayers. Results. When model compounds having the same charge were compared, permeation was highly size-dependent (capped amino acids > tripeptides > hexapeptides), suggesting transport predominantly via the paracellular route. For example, the flux of the negatively charged Asp amino acid (P_app = 10.04 ± 0.43 × 10^–8 cm/s) was 3 times greater than that observed for the Asp-containing hexapeptide (P_app = 3.19 ± 0.27 × 10^–8 cm/s). When model compounds of the same size were compared, permeation across the cell monolayer was charge-dependent (negative < positive neutral). For example, the neutral, Asn-containing tripeptide (P_app = 25.79 ± 4.86 × 10^–8 cm/s) was substantially more able to permeate the Caco-2 cell monolayer than the negatively charged Asp-containing tripeptide (P_app = 7.95 ± 1.03 × 10^–8 cm/s) and the positively charged Lys-containing tripeptide (P_app = 9.86 ± 0.18 × 10^–8 cm/s). The permeability of the cell monolayer to peptides became less sensitive to net charge as the size of the peptides increased. Conclusions. A positive net charge of hydrophilic peptides enhances their permeation across the intestinal mucosa via the paracellular pathway. With increasing molecular size, molecular sieving of the epithelial barrier dominates the transport of peptides, and the effect of the net charge becomes less significant.     

Enhanced permeability of the antimicrobial agent 2,5-bis(4-amidinophenyl)furan across Caco-2 cell monolayers via its methylamidoidme prodrug

Purpose. DB75 [2,5-bis(4-amidinophenyl)furan] is a promising antimicrobial agent although it has poor oral potency. In contrast, its novel prodrug, 2,5-bis(4-amidinophenyl)furan-bis-O-methyl- amidoxime (DB289), has excellent oral potency. The mechanisms of transport of DB289 and DB75 across intestinal epithelium have been investigated in these studies to understand differences in their oral potency. Methods. Caco-2 cell monolayers were used as an in vitro model to examine the mechanisms of transport of DB289 and DB75. Samples collected from the transport studies were quantified using high-performance liquid chromatography with ultraviolet and fluorescence detection. Results. A low permeability coefficient (3.8 × 10^–7 cm/s for transport in apical [AP] to basolateral [BL] direction) and high sensitivity to extracellular Ca²⁺ suggest that AP to BL transport of DB75 across Caco-2 cell monolayers occurs predominantly via a paracellular route. DB289 has an 85-fold higher transport rate (322.0 × 10^–7 cm/s for transport in the AP to BL direction) across Caco-2 monolayers than that of DB75. This, with its insensitivity to extracellular Ca²⁺ indicates that AP to BL transport of DB289 across Caco-2 cell monolayers occurs predominantly via a transcellular route. Conclusions. DB75 is transported across Caco-2 cell monolayers predominantly via paracellular pathways, whereas the prodrug DB289 is transported via transcellular pathways. This could account for the much higher oral activity of DB289 over DB75.     

自微乳化系统对细胞紧密连接蛋白的影响

目的从分子细胞水平考察正、负电荷自微乳化系统对细胞紧密连接蛋白复合体的影响。方法建立了模拟小肠上皮细胞结构的Caco-2细胞模型;测定对跨膜电阻和细胞间转运物质甘露醇转运评价制剂对细胞完整性的影响。采用免疫荧光法评价两种自微乳化系统不同稀释倍数对细胞紧密连接蛋白ZO-1和细胞骨架肌动蛋白(actin)的影响。结果负电荷自微乳化系统在不同稀释倍数对跨膜电阻都无显著性影响。正电荷处方在考察的3种稀释倍数显著性降低了跨膜电阻(P<0.05)。细胞单分子层经正电荷自微乳制剂处理2 h，再经过48 h培养，较高稀释倍数跨膜电阻能够完全恢复；50倍稀释的处方不能完全恢复(81.3%)。两种制剂在不同稀释倍数都能显著提高甘露醇的渗透系数(2.9～64.6倍)(P<0.05)，作用与自微乳稀释倍数具有相关性。免疫荧光结果表明，经制剂处理后，影响了actin和ZO-1细胞分布，呈现不连续性。正电荷处方由于静电吸引，可能对细胞膜产生压力，比负电荷处方更能影响actin的分布。对细胞紧密连接影响具有制剂稀释倍数的依赖性。结论自微乳化系统能够提高甘露醇的细胞间转运。通过影响actin和ZO-1的细胞膜分布的作用机制打开细胞紧密连接。     

Transport of Surface Engineered Polyamidoamine (PAMAM) Dendrimers Across IPEC-J2 Cell Monolayers

The aim of our study was to prepare arginine-and ornithine-conjugated Polyamidoamine (PAMAM) dendrimers and study their permeability across IPEC-J2 cell monolayers, a new intestinal cell line model for drug absorption studies. Arginine and ornithine were conjugated to the amine terminals of the PAMAM_G4 dendrimers by Fmoc synthesis. The apical-to-basolateral (AB) and basolateral-to-apical (BA) apparent permeability coefficients (P_app) for the PAMAM dendrimers increased by conjugating the dendrimers with both of these polyamines. The enhancement in permeability was dependent on the dendrimer concentration and duration of incubation. Correlation between monolayer permeability and the decrease in transepithelial electrical resistance (TEER) with the PAMAM dendrimers and the polyamine-conjugated dendrimers suggests that paracellular transport is one of the mechanisms of transport across the epithelial cells. Cytotoxicity of these surface-modified dendrimers was evaluated in IPEC-J2 cells by MTT (methylthiazoletetrazolium) assay. Arginine-conjugated dendrimers were insignificantly more toxic than PAMAM dendrimer as well as ornithine-conjugated dendrimers. Though investigations on the possible involvement of other transport mechanisms are in progress, results of the present study suggest the potential of dendrimer-polyamine conjugates as the carriers for antigen/drug delivery through the oral mucosa.     

Voltage Dependence of Transepithelial Guanidine Permeation Across Caco-2 Epithelia Allows Determination of the Paracellular Flux Component

Purpose The aim of this study was to investigate transepithelial ionic permeation via the paracellular pathway of human Caco-2 epithelial monolayers and its contribution to absorption of the base guanidine. Methods Confluent monolayers of Caco-2 epithelial cells were mounted in Ussing chambers and the transepithelial conductance and electrical potential difference (p.d.) determined after NaCl dilution or medium Na substitution (bi-ionic conditions). Guanidine absorption (J_a–b) was measured ± transepithelial potential gradients using bi-ionic p.d.'s. Results Basal NaCl replacement with mannitol gives a transepithelial dilution p.d. of 28.0 ± 3.1 mV basal solution electropositive (P_Cl/P_Na = 0.34). Bi-ionic p.d.'s (basal replacements) indicate a cation selectivity of NH₄⁺ > K⁺∼Cs⁺ > Na⁺ > Li⁺ > tetraethylammonium⁺ > N-methyl-d-glucamine⁺∼choline⁺. Transepithelial conductances show good correspondence with bi-ionic potential data. Guanidine J_a–b was markedly sensitive to imposed transepithelial potential difference. The ratio of guanidine to mannitol permeability (measured simultaneously) increased from 3.6 in the absence of an imposed p.d. to 13.8 (basolateral negative p.d.). Conclusions Hydrated monovalent ions preferentially permeate the paracellular pathway (Eisenman sequence 2 or 3). Guanidine may access the paracellular pathway because absorptive flux is sensitive to the transepithelial potential difference. An alternative method to assess paracellular-mediated flux of charged organic molecules is suggested.     

Intestinal Absorption of Miltefosine: Contribution of Passive Paracellular Transport

Purpose This study aimed to characterize the transepithelial transport of miltefosine (HePC), the first orally effective drug against visceral leishmaniasis, across the intestinal barrier to further understand its oral absorption mechanism. Materials and Methods Caco-2 cell monolayers were used as an in vitro model of the human intestinal barrier. The roles of active and passive mechanisms in HePC intestinal transport were investigated and the relative contributions of the transcellular and paracellular routes were estimated. Results HePC transport was observed to be pH-independent, partially temperature-dependent, linear as a function of time and non-saturable as a function of concentration. The magnitude of HePC transport was quite similar to that of the paracellular marker mannitol, and EDTA treatment led to an increase in HePC transport. Furthermore, HePC transport was found to be similar in the apical-to-basolateral and basolateral-to-apical directions, strongly suggesting that HePC exhibits non-polarized transport and that no MDR-mediated efflux was involved. Conclusions These results demonstrate that HePC crosses the intestinal epithelium by a non-specific passive pathway and provide evidence supporting a concentration-dependent paracellular transport mechanism, although some transcellular diffusion cannot be ruled out. Considering that HePC opens epithelial tight junctions, this study shows that HePC may promote its own permeation across the intestinal barrier.     

Evaluation of human nasal RPMI 2650 cells grown at an air-liquid interface as a model for nasal drug transport studies

This study tests the hypothesis that human nasal RPMI 2650 cells grown at an air-liquid interface is a feasible model for drug transport studies via the nasal route. RPMI 2650 cells were cultured in Eagle's minimal essential medium (MEM) at both air-liquid and liquid-liquid interfaces. For each culture regimen, monolayer integrity was tested by measuring the transepithelial resistance (TEER) as well as the transport of paracellular and transcellular markers across the monolayer. The expression of tight junction proteins-differentiation markers-in cells of the different monolayers was studied by western blot analysis and confocal microscopy. The highest TEER values (192 +/- 3 Omega . cm2) were observed for RPMI 2650 cells seeded onto collagen-coated permeable polytetrafluoroethylene inserts and grown at an air-liquid interface for 10 days; a seeding density of 4 x 10(5)/cm2 generated and maintained a cell monolayer with suitable barrier properties at days 9-12. Microscopic examination showed that RPMI 2650 cells grown on filter inserts formed a fully confluent monolayer. The apparent permeability coefficients of the paracellular marker, [14C] mannitol, and the transcellular marker, [3H] propranolol, were 5.07 +/- 0.01 x 10(-6) cm/s and 16.1 +/- 0.1 x 10(-6) cm/s, respectively. Western blot analysis indicated the presence of four tight junction proteins: ZO-1, occludin, claudin-1 and E-cadherin; and the quantities of ZO-1, occludin, and E-cadherin were significantly higher in cells grown at an air-liquid interface than in cells grown at a liquid-liquid interface. Confocal microscopic studies showed ZO-1, F-actin, occludin and claudin-1 proteins at cell-cell contacts and revealed significant differences in the distributions and densities of ZO-1 protein in cells grown at the two types of interface. The data indicate that RPMI 2650 cells grown at an air-liquid interface form polarized monolayers with the cells interconnected by tight junction proteins. This human nasal cell line model could provide a useful tool for in vitro screening of nasal drug candidates.     

Effect of Rhamnolipids on Permeability Across Caco-2 Cell Monolayers

Purpose

This report describes the effect of rhamnolipids (RLs), an amphiphilic biosurfactant produced by the bacterium Pseudomonas aeruginosa, on the integrity and permeability across Caco-2 cell monolayers.

Methods

We measured the trans-epithelial electrical resistance (TEER) and permeability of [¹⁴C]mannitol across Caco-2 cell monolayers upon incubation with 0.01–5.0% v/v RLs as a function of incubation time (30, 60, 90, and 120 min). We also studied the recovery of RL-treated Caco-2 cell monolayers upon incubation with Kaempferol, which is a natural flavonoid that promotes the assembly of the tight junctions.

Results

TEER of Caco-2 cell monolayers incubated with 0.01–5.0% v/v RLs solution dropped to 80–28% of that of untreated cells. Decline in TEER was associated with an increase in [¹⁴C]mannitol permeability as a function of RLs concentration and incubation time with Caco-2 cells. Incubation of RLs-treated Caco-2 cell monolayers with normal culture medium for 48 h did not restore barrier integrity. Whereas, incubation of a RLs-treated Caco-2 cells with culture medium containing Kaempferol for 24 h restored barrier function indicated by the higher TEER and lower [¹⁴C]mannitol permeability values.

Conclusions

These results show the ability of RLs to modulate the integrity and permeability of Caco-2 cell monolayers in a concentration- and time-dependent fashion, which suggest their potential to function as a non-toxic permeation enhancer.