Alternative approaches

As much as the simplicity of the dialysis diffusion technique was an advantage to meet the objectives of the present study it is in several ways inferior to methods, which more closely resemble the properties of the intestinal barrier in vivo. Such methods include in vitro, in situ and in vivo models, which have recently been reviewed in detail by Le Ferrec et al. (2001) and will briefly be discussed in the following. In vitro methods for drug diffusion experiments can be divided into cell culture and tissue culture studies. Various cell culture models exist deriving cells from different tissues such as canine kidney (MDCK), human colorectal adenocarcinoma (Caco-2) or others. In culture, they differentiate spontaneously into intestinal epithelial cells possessing an apical brush border as well as tight junctions between adjacent cells and also express certain metabolic enzymes and typical microvillar transporters. Tissue culture studies such as everted gut sac and Ussing chamber technique, on the other hand, involve the use of sheets of membrane tissue stripped of rat intestine. In comparison to cell culture studies they offer the advantage for measuring the absorption at different sites in the small intestine. In situ methods including open or closed loop perfusion studies of rat small intestine resemble the in vivo conditions even more closely than in vitro methods as here the uptake of drug compounds into the systemic circulation can be monitored. A disadvantage of these techniques is, however, that the animal is anaesthetised, which sometimes may have a significant effect on intestinal drug absorption (Yuasa et al., 1993). In vivo methods for investigating drug diffusion include perfusion experiments such as the Loc-I-gut technique, which involves the isolation of a specified part of the small intestine with inflatable balloons. The advantage of such experiments is that it allows the assessment of drug permeation in humans and it has been stated, “the only real model for man is man” (Davis and Wilding, 2000). All of the discussed methods also have limitations. The more closely a model reflects the in vivo situation the more complex it becomes, which makes it rather difficult to separate the variables involved in the process of drug absorption, i.e. it does not allow the identification of the individual

rate-limiting factors. In addition to an increasing complexity of the various methods in the order of in vitro, in situ and in vivo experiments is the increasing expenditure of cost and time involved.

3.5. Conclusions

The primary objective of the present study was to investigate the influence of different solubilizing agents on the passive diffusion of ranitidine. The presence of PEG 400 at concentrations of 5, 10 and 30 % (w/w) was found to have no major effect on the diffusion rate of ranitidine. A slight delay in drug diffusion, however, was observed at PEG 400 concentrations of 30 % (w/w) in the initial 6 hrs of the experiment. Solutions containing relatively high amounts of propylene glycol, 30, 50 and 70 % (w/w) also resulted in a slight delay in the diffusion of ranitidine across the dialysis membrane. The effect, however, did not appear to be dependent on the dose of propylene glycol present in the donor solution. It is suggested that these findings for both cosolvents are partly the consequence of an increased osmotic pressure within the donor compartment eliciting an adverse flux of water, which is more apparent in the case of propylene glycol having a considerable osmotic activity. The main impact of a delaying effect of the presence of high cosolvent concentrations is expected to be an increased viscosity of the solution, which results in a decrease in the thermodynamic activity of ranitidine. PEG 400 and propylene glycol were both shown to move across the artificial hydrophilic membrane. Therefore, an increased osmolarity and/or viscosity of the donor preparations is expected to be decreased during the course of the experiment as a result of the diffusion of the cosolvents into the sink solution. No interaction or complex formation between the drug and either cosolvent was observed. For the passive diffusion of the hydrophilic compound ranitidine in the presence of increasing concentrations of non-ionic surfactant it can be concluded that drug diffusion was not compromised through micelle formation in solutions containing 0.2 and 2 % (w/w) VitE-TPGS. At concentrations as high as 20 % (w/w), VitE-TPGS had a marked negative effect on drug diffusion showing a considerable decrease in the diffusion rate of ranitidine. This effect is thought to be primarily the consequence of the increased viscosity of the continuous aqueous phase in the presence of high micellar concentrations, which leads to a reduction in the

thermodynamic activity of ranitidine and hence a decrease in the passive diffusion of the drug.

The results obtained from the present diffusion experiments using artificial hydrophilic membranes lead to the assumption that the considerable reduction in ranitidine bioavailability, which was observed after the oral administration of an oral solution containing relatively high amounts of PEG 400 in healthy volunteers in vivo (Chapter 2) was not caused by physicochemical interactions of the polymer with the active agent.