Absorption in vivo methods

OECD. Test guideline 427 Skin Absorption In Vivo Method, OECD, Paris, 2004. 

OECD (2004b). Skin absorption in vivo method, OECD Guideline 427, Guideline for the Testing of Chemicals, Environment Directorate, Organization for Economic Cooperation and Development, Paris, France. 

OECD (2(X)4c). OECD Guidelines for the Testing of Chemicals. Test No. 427. Skin absorption In vivo method. Paris OECD. 

OECD, 2004a, Test Guideline 427, Skin Absorption In vivo Method (Rat Protocol Only). 

One of the most common in vivo methods used to assess the permeability of the buccal mucosa is the buccal absorption test of Beckett and Triggs [13]. In this test, a known volume of a drug solution is introduced into the oral cavity of a subject, who swirls it around for a specified period of time and then expels it. The subject then rinses his or her mouth with an aliquot of distilled water or buffer solution, and the expelled drug solution and rinse are combined and analyzed for drug content. The difference between the initial and final drug concentration in the solution is assumed to be the amount of drug taken up into the oral mucosa. 

OECD, Skin absorption in vitro method. OECD Test Guideline 428, Paris, 2004. Wagner, H., et ah. Drug distribution in human skin using two different in vitro test systems comparison with in vivo data. Pharm. Res., 17, 1475-81, 2000. 

A hybrid approach combining in vitro and in vivo methods is the everted sac technique of WTTson and Wiseman ( ), which has been used to estimate both digestibility and absorption. 

Rougier A, Dupuis D, Lotte C, Maibach HI (1999) Stripping method for measuring percutaneous absorption in vivo. Robert L et al. (eds) Percutaneous Absorption, 3rd edn. Marcel Dekker, New York... 

Absorption of drugs through the vagina is an important parameter to be evaluated, particularly when a systemic effect is required. Also, assessment of the absorption potential of drugs intended to locally exert their effects needs to be evaluated, as this event can lead to unwanted systemic effects. The evaluation of both formulated and unformulated drugs can be performed by in vitro or in vivo methods. 

Higo, N. et al. Validation of reflectance infrared spectroscopy as a quantitative method to measure percutaneous absorption in vivo. Pharmaceutical Research 70(10) 1500-1506, 1993. 

Systemic bioavailability is the product of fraction of dose absorbed (/a), fraction of dose escaping gut metabolism (/g), and fraction of dose escaping first-pass metabolism (F ). Permeability class is based upon /a, which may be estimated either in vivo or in vitro by direct measurement of mass transfer across human intestinal epithelium. In vivo methods include (i) mass balance studies using unlabeled, stable-isotope labeled, or a radiolabeled drug substance (ii) oral bioavailability using a reference intravenous dose or (iii) intestinal perfusion studies either in humans or an acceptable animal model. Suitable in vitro methods involve the use of either excised human/animal intestinal tissues or cultured epithelial monolayers. All of these methods are deemed appropriate for drugs whose absorption is controlled by passive mechanisms. 

Among these testing methods, the small animal GI model and the Caco-2 cell culture model have shown the best correlation with oral absorption in vivo. The Caco-2 culture system consists of a monolayer of human intestinal epithelial cells grown on semipermeable supports such as polycarbonate membranes. Because the cells are human in origin, they exhibit many characteristics of the human small intestinal epithelium. The permeability coefficients relative to the extents of human drug absorption are listed here ... 

In vivo evaluation of formulations thus requires a high level of knowledge regarding the drug substance absorption properties obtained both by in vitro and in vivo methods described in this chapter and in Chapter 4, and also by basic pharmacokinetic studies, a review of which is outside the scope of the present chapter. 

Dermal absorption of agricultural chemicals and animal drugs in food-producing animals must be considered as a potential route from which tissue residues of drugs and chemicals may occur. This has been supported in studies of topical pesticide exposure in cows and sheep. Despite the many advances made in in vitro and in vivo techniques for assessing percutaneous absorption in laboratory animals and man, very little systematic attention has been focussed on food-producing animals. The only exception is the pig since it is an accepted model for human studies. The purpose of this manuscript is to overview the literature on dermal xenobiotic absorption in food-producing animals to illustrate the risk that is present, and to outline how in vitro and in vivo methods could be applied to this problem. 

Various in vitro and in vivo methods have been used to predict drug absorption including Caco-2 cells, in situ intestinal permeability, whole-animal studies, and more recently chromatographic methods. Compared with in vivo absorption studies, evaluation of intestinal permeability in vitro requires less compound is relatively easy to study, often avoiding complicated surgery is rapid and can allow a wider variety of variables to be controlled. ... 

There are numerous approaches for assessing absorption in vivo in animals and humans. Although the common thread between them is their use in intact organisms, they do assess different components of the dermal absorption and penetration process. These are briefly introduced here in the context of what they are actually measuring and how a specific method s limitations may affect interpretation. Figure 4.1 illustrates the relationship between the fate of a compound within the skin and the in vivo methods discussed in this chapter. The primary developments in this area over recent years have been in response to developing more noninvasive... 

Figure 4.1 Fate of a topically applied compound in skin relative to in vivo methods used to assess absorption.

Advances of in vivo methods for dermal absorption studies have shown promising results. Microdialysis has bear ranployed successfully for measuring dermal penetration of a wide variety of compounds. This technique has been used extensively in topieal bioavaUabUity and bioequivalenee studies. The noninvasive techniques for percutaneous absorption studies such as tape-stripping, ATR-FTIR, and skin surface biopsies arc also becoming popular because these can be used safely in humans. Regulatory bodies rely on mass balance approaches because all of the topically... 

Wester, R.C. and Maibach, H.l. (1999). In vivo methods for percutaneous absorption measurements, in R.L. Bronaugh and H.l. Maibach (eds.). Percutaneous Absorption. Drugs—Cosmetics—Mechanisms—Methodology, New York Dekker, pp. 215-227. 

Tetiachloroethylene applied to a patch of abdominal skin of ICR mice for 15 minutes/week resulted in an in vivo absorption rate of 0.24 mg/cm%our (Tsuruta 1975). An in vitro study using excised rat (SD-JCL) skin demonstrated that the rate of penetration by tetiachloroethylene was much slower than that of several other halogenated hydrocarbons (i.e., 2,070 times slower than that of the fastest compound, dichloromethane), and the measured rate for tetiachloroethylene penetration was 0.005 mg/cm%our. The penetration rate observed in the in vitro method was 44 times slower than that observed in the in vivo method. The difference may result from the solubility of the substance in 0.9% sodium chloride and its solubility in body fluids (Tsumta 1977). 

---