A study to correlate drug absorption in humans (Fa) to in vitro intestinal permeation using the Sweetana-Grass diffusion model
Background: The oral route still remains the most popular method for drug delivery as it is convenient, cost efficient and has high patient compliance. However, for a drug to be successfully administered via this route it must be able to cross the intestinal membranes with a rate high enough (permeability) in order to enter the blood stream to produce the desired reaction. So, although a new chemical entity may possess pharmacological activity, it may not have the necessary physicochemical properties to be absorbed to such an extent that it will be of clinical value. For this reason it is necessary to have the means to accurately predict the in vivo permeability of drugs in vitro in a cost efficient manner. Permeability can be predicted by: (i) physicochemical characterisation, (ii) in silico, (iii) in vitro, (iv) in situ or (v) in vivo methods. Of these methods, in vivo studies are the most accurate, however, they have several drawbacks such as being time consuming, have high costs and ethical approval has to be obtained. Physicochemical characterisation and in silico methods are the easiest and cheapest to perform, but lack the physical interactions that occur at the membrane interface, thus lacking in reliability and close correlation with the in vivo situation. In vitro methods using intestinal tissue provides a compromise between the physicochemical characterisation and in vivo testing, as it is easy to perform and mimics the membrane interface found in vivo. Aim: The aim of this study was to determine the permeability of selected drugs using rat intestine mounted in Sweetana-Grass diffusion chambers and compare the permeability observed with the absorption of the same drugs in humans using the Fa parameters in order to determine the feasibility of using this in vitro method to predict in vivo permeability. Two different methods of intestinal tissue preparation were also investigated to optimise the method further. Methods: Validations of the various analytical methods used to determine the selected drugs quantitatively were performed. Jejunal tissue was prepared in one of two ways. It was either used as is (unstripped) or the serosal muscle layer was removed (stripped). The transport of caffeine, furosemide, verapamil, ketoprofen, propranolol, carbamazepine, promethazine, paracetamol, acyclovir and ranitidine was determined using a vertical diffusion chamber system, mounted with either stripped or unstripped tissue. All the studies were done in the apical to basolateral direction. Permeability was expressed by the calculation of the apparent permeability co-efficient (Papp). Results: The permeability of all drugs studied was lower than found in previous studies with Caco-2 cells. This can be expected because of the difference in the composition of the rat intestine and the Caco-2 cell monolayer as well as the longer path length the drugs have to travel during the absorption process through the jejunal tissue. The data show a non-linear relationship between Fa and Papp. For the stripped method of tissue preparation the r² value obtained was 0.0579 indicating poor correlation. For the unstripped method of tissue preparation the r² value was 0.2877, indicating poor but improved correlation compared to the stripped method of tissue preparation. Graphs used to indicate a correlation between Papp and Fa showed aciclovir to be an outlier. Removal of this drug from the equation gave an r² of 0.4013 for the stripped tissue preparation and 0.4623 for unstripped tissue preparation indicating a much better correlation between transport across rat intestine and fraction absorbed in humans. Conclusion: It was possible to determine the Papp values for ten selected drugs in this study by using the in vitro Sweetana-Grass diffusion method. These Papp values showed a nonlinear relationship between Fa and Papp in rat jejunum, as has been observed in other studies. When comparing the methods of tissue preparation, the unstripped method of tissue preparation gave a better correlation between Papp and Fa (r² = 0.0579 compared to r² = 0.2877). If the outlier aciclovir is excluded, the correlation improves significantly (stripped r² = 0.4013 unstripped r² = 0.4623). A possible reason for the high permeability of aciclovir observed in this study may be the fact that its transport is concentration dependant. Also, the absorption of aciclovir varies across species, thus not making it ideal for a study comparing transport across rat intestine to absorption in humans. This method of intestinal transport prediction does show a correlation with fraction absorbed in humans, however a larger number of drugs, with a wider spread of Fa values should be evaluated before it can be used to predict in vivo absorption with confidence.
- Health Sciences