The mucosa of the gastrointestinal tract, eyes, nose, lungs, cervix and vagina is lined by epithelium interspersed with mucus-secreting goblet cells, all of which contribute to their unique functions. This mucus provides an integral defence to the epithelium against noxious agents and pathogens. However, it can equally act as a barrier to drugs and delivery systems targeting epithelial passive and active transport mechanisms. This review highlights the various mucins expressed at different mucosal surfaces on the human body, and their role in creating a mucoid architecture to protect epithelia with specialized functions. Various factors compromising the barrier properties of mucus have been discussed, with an emphasis on how disease states and microbiota can alter the physical properties of mucus. For instance, Akkermansia muciniphila, a bacterium found in higher levels in the gut of lean individuals induces the production of a thickened gut mucus layer. The aims of this article are to elucidate the different physiological, biochemical and physical properties of bodily mucus, a keen appreciation of which will help circumvent the slippery slope of challenges faced in achieving effective mucosal drug and gene delivery.

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Intestinal efflux transporters affect the gastrointestinal processing of many drugs but further data on their intestinal expression levels are required. Relative mRNA expression and relative and absolute protein expression data of transporters are commonly measured by real-time polymerase chain reaction (RT-PCR), Western blot and mass spectrometry-based targeted proteomics techniques. All of these methods, however, have their own strengths and limitations, and therefore, validation for optimized quantification methods is needed. As such, the identification of the most appropriate technique is necessary to effectively translate preclinical findings to first-in-human trials. In this study, the mRNA expression and protein levels of the efflux transporter P-glycoprotein (P-gp) in jejunal and ileal epithelia of 30 male and female human subjects, and the duodenal, jejunal, ileal and colonic tissues in 48 Wistar rats were quantified using RT-PCR, Western blot and liquid chromatography-tandem mass spectrometry (LC-MS/MS). A similar sex difference was observed in the expression of small intestinal P-gp in humans and Wistar rats where P-gp was higher in males than females with an increasing trend from the proximal to the distal parts in both species. A strong positive linear correlation was determined between the Western blot data and LC-MS/MS data in the small intestine of humans ( R 2 = 0.85). Conflicting results, however, were shown in rat small intestinal and colonic P-gp expression between the techniques ( R 2 = 0.29 and 0.05, respectively). In RT-PCR and Western blot, an internal reference protein is experimentally required; here, beta-actin was used which is innately variable along the intestinal tract. Quantification via LC-MS/MS can provide data on P-gp expression without the need for an internal reference protein and consequently, can give higher confidence on the expression levels of P-gp along the intestinal tract. Overall, these findings highlight similar trends between the species and suggest that the Wistar rat is an appropriate preclinical animal model to predict the oral drug absorption of P-gp substrates in the human small intestine.

The rat is one of the most commonly used animal models in pre-clinical studies. Limited information between the sexes and the effect of food consumption on the gastrointestinal (GI) physiology, however, is acknowledged or understood. This study aimed to investigate the potential sex differences and effect of food intake on the intestinal luminal fluid and the efflux membrane transporter P-glycoprotein (P-gp) along the intestinal tract of male and female Wistar rats. To characterise the intestinal luminal fluids, pH, surface tension, buffer capacity and osmolality were measured. Absolute P-gp expression along the intestinal tract was quantified via liquid chromatography-tandem mass spectrometry (LC-MS/MS). In general, the characteristics of the luminal fluids were similar in male and female rats along the GI tract. In fasted male rats, the absolute P-gp expression gradually increased from the duodenum to ileum but decreased in the colon. A significant sex difference (p < 0.05) was identified in the jejunum where P-gp expression in males was 83% higher than in females. Similarly, ileal P-gp expression in male rats was approximately 58% higher than that of their female counterparts. Conversely, following food intake, a significant sex difference (p < 0.05) in P-gp expression was found but in a contrasting trend. Fed female rats expressed much higher P-gp levels than male rats with an increase of 77% and 34% in the jejunum and ileum, respectively. A deeper understanding of the effects of sex and food intake on the absorption of P-gp substrates can lead to an improved translation from pre-clinical animal studies into human pharmacokinetic studies.

Active pharmaceutical ingredients are routinely formulated with a range of excipients in the manufacture of drug products. Excipients are considered to be inert components of the formulations, although recent research has contested its inactive behaviour. This study investigated the effect of the excipient polyethylene glycol 400 (PEG 400) on the oral bioavailability and intestinal permeability of cimetidine in male and female human volunteers.Aqueous solutions of cimetidine with pharmaceutically relevant concentrations of PEG 400 at 0% w/v (control), 0.3% w/v, 0.5% w/v, 0.7% w/v and 1.0% w/v were orally administered to both sexes. Urine samples were then collected and assayed for the determination of cimetidine which reflected oral bioavailability. This human study showed that PEG 400 at 0.3% w/v, 0.5% w/v and 0.7% w/v concentrations significantly increased cimetidine bioavailability by 34%, 58% and 41% respectively, although this enhancement was only demonstrated in men and not women (p < 0.05). Ussing chamber transport studies with male human jejunal tissues revealed that cimetidine permeability increased by 26%, 48% and 29% with PEG 400 at 0.3% w/v, 0.5% w/v and 0.7% w/v respectively (p < 0.05). No such enhancement was demonstrated in female tissues (p > 0.05). We have shown that PEG 400 interacts with intestinal P-glycoprotein (P-gp) expression differently in males and females. The mechanistic action of PEG 400 at gut level was further investigated on human jejunal tissues following the pre-treatment of the P-gp inhibitor PSC 833 (valspodar) on the transport of cimetidine. When intestinal P-gp was inhibited, the sex-and dose-dependent modulatory effect of PEG 400 with cimetidine was completely eradicated, thus confirming that PEG 400 has a modulatoryrather than inhibitory effect on P-gp. In sum, the widely used excipient PEG 400 is not inert at pharmaceutically relevant concentrations and its modulatory effect is demonstrated at a human clinical level. Such pharmacological effects, however, are sex-and dose-dependent via its modulation on intestinal P-gp, as evidenced by the boost in cimetidine bioavailability only in male human volunteers. As such, these findings should be carefully considered towards the co-formulation of PEG 400 with drugs that are P-gp substrates.

Intestinal interactions with nutrients, xenobiotics and endogenous hormones can influence the expression of clinically relevant membrane transporters. These changes in the gastrointestinal (GI) physiology can in turn affect the absorption of numerous drug substrates. Several studies have examined the effect of food on intestinal transporters in male and female humans and animal models. However, to our knowledge no studies have investigated the influence of a non-nutritive fibre meal on intestinal efflux transporters and key sex and GI hormones. Here, we show that a fibre meal increased the acute expression of P-glycoprotein (P-gp), breast cancer resistance protein (BCRP), and multidrug-resistance-associated protein-2 (MRP2) in small intestinal segments in both male and female Wistar rats. Enzyme-linked immunosorbent assays were used for the protein quantification of efflux transporters and hormonal plasma concentration. In male rats, the fibre meal caused the plasma concentration of the GI hormone cholecystokinin (CCK) to increase by 75% and the sex hormone testosterone to decrease by 50%, whereas, in contrast, the housing food meal caused a decrease in CCK by 32% and testosterone saw an increase of 31%. No significant changes in the hormonal concentrations, however, were seen in female rats. A deeper understanding of the modulation of efflux transporters by sex, food intake and time can improve our understanding of inter- and intra-variability in the pharmacokinetics of drug substrates.

Active pharmaceutical ingredients are routinely formulated with a range of excipients in the manufacture of drug products. Excipients are considered to be inert components of the formulations, although recent research has contested its inactive behaviour. This study investigated the effect of the excipient polyethylene glycol 400 (PEG 400) on the oral bioavailability and intestinal permeability of cimetidine in male and female human volunteers. Aqueous solutions of cimetidine with pharmaceutically relevant concentrations of PEG 400 at 0% w/v (control), 0.3% w/v, 0.5% w/v, 0.7% w/v and 1.0% w/v were orally administered to both sexes. Urine samples were then collected and assayed for the determination of cimetidine which reflected oral bioavailability. This human study showed that PEG 400 at 0.3% w/v, 0.5% w/v and 0.7% w/v concentrations significantly increased cimetidine bioavailability by 34%, 58% and 41% respectively, although this enhancement was only demonstrated in men and not women (p < 0.05). Ussing chamber transport studies with male human jejunal tissues revealed that cimetidine permeability increased by 26%, 48% and 29% with PEG 400 at 0.3% w/v, 0.5% w/v and 0.7% w/v respectively (p < 0.05). No such enhancement was demonstrated in female tissues (p > 0.05). We have shown that PEG 400 interacts with intestinal Pglycoprotein (P-gp) expression differently in males and females. The mechanistic action of PEG 400 at gut level was further investigated on human jejunal tissues following the pre-treatment of the Pgp inhibitor PSC 833 (valspodar) on the transport of cimetidine. When intestinal P-gp was inhibited, the sex-and dose-dependent modulatory effect of PEG 400 with cimetidine was completely eradicated, thus confirming that PEG 400 has a modulatory-rather than inhibitory-effect on Pgp. In sum, the widely used excipient PEG 400 is not inert at pharmaceutically relevant concentrations and its modulatory effect is demonstrated at a human clinical level. Such pharmacological effects, however, are sex-and dose-dependent via its modulation on intestinal Pgp, as evidenced by the boost in cimetidine bioavailability only in male human volunteers. As such, these findings should be carefully considered towards the co-formulation of PEG 400 with drugs that are P-gp substrates.

Most marketed peptide drugs are administered parenterally due to their inherent gastrointestinal (GI) instability and poor permeability across the GI epithelium. Several molecular design techniques, such as cyclisation and D-amino acid (D-AA) substitution, have been proposed to improve oral peptide drug bioavailability. However, very few of these techniques have been translated to the clinic. In addition, little is known about how synthetic peptide design may improve stability and permeability in the colon, a key site for the treatment of inflammatory bowel disease and colorectal cancer. In this study, we investigated the impact of various cyclisation modifications and D-AA substitutions on the enzymatic stability and colonic tissue permeability of native oxytocin and 11 oxytocin-based peptides. Results showed that the disulfide bond cyclisation present in native oxytocin provided an improved stability in a human colon model compared to a linear oxytocin derivative. Chloroacetyl cyclisation increased native oxytocin stability in the colonic model at 1.5 h by 30.0%, whereas thioether and N-terminal acetylated cyclisations offered no additional protection at 1.5 h. The site and number of D-AA substitutions were found to be critical for stability, with three D-AAs at Tyr, Ile and Leu, improving native oxytocin stability at 1.5 h in both linear and cyclic structures by 58.2% and 79.1%, respectively. Substitution of three D-AAs into native cyclic oxytocin significantly increased peptide permeability across rat colonic tissue; this may be because D-AA substitution favourably altered the peptide’s secondary structure. This study is the first to show how the strategic design of peptide therapeutics could enable their delivery to the colon via the oral route.