Efflux mechanisms

Poorly absorbed compounds have been identified as those with a PSA>140Af Considering more compounds, considerable more scatter was found around the sigmoidal curve observed for a smaller set of compounds [74]. This is partly due to the fact that many compounds do not show simple passive diffusion only, but are affected by active carriers, efflux mechanisms involving P-glycoprotein (P-gp) and other transporter proteins, and gut wall metabohsm. These factors also con-... 

In conclusion, there are several drawbacks to the use of Caco-2 cells in studies of active drug transport. Despite these drawbacks, we note that a recent comprehensive study comparing various P-glycoprotein drug efflux assays in drug discovery came to the conclusion that the Caco-2 transport assay is the method of choice, since it displays a biased responsiveness towards compounds with low or moderate permeability - in other words, towards compounds whose intestinal permeability is most likely to be significantly affected by drug efflux mechanisms [101]. 

Hidalgo, I. J., Li, J., Carrier-mediated transport and efflux mechanisms in Caco-2 cells, Adv. Drug Delivery Rev. 1996, 22, 53-66. 

Kamm, W., Hauptmann, J., Behrens, I., Sturzebecher, J., Dullweber, F., Gohlke, H., Stubbs, M., Klebe, G., Kissel, T., Transport of peptido-mimetic thrombin inhibitors with a 3-amidino-phenylalanine structure permeability and efflux mechanism in monolayers of a human intestinal cell line (Caco-2), Pharm. Res. 2001, 18, 1110-1118. 

Measurement of absorption can be complicated by efflux mechanisms. It is clear that many compounds are actively transported back into the GIT, into the bile or into the urine by efflux proteins. In the case of those in the GIT these may have an impact on the apparent absorption of a compound. Some understanding of the substrate specificity for one of these proteins, P-glycoprotein, is becoming apparent [8, 9], but currently the understanding is limited. At the moment there are no published reliable methods either in vivo or in vitro for predicting the importance of efflux mechanisms for a particular compound in man [10-12],... 

A direct in vivo assessment was carried out with the single-pass perfusion approach in the human jejunum by using the Loc-I-Gut technique with R/S-verapamil (log D6 5 2.7, octanol/water pH 7.4 MW 455 Da) as the model compound for CYP 3A4 and P-gp-mediated local intestinal kinetics [2, 34, 35, 122] (see Figs. 7.7 and 7.9). The Peff for both enantiomers at each of the concentrations (4.0, 40, 120, and 400 mg L-1) was 2.5 x 10 4, 4.7 x 105.5 x 104 and 6.7 x 104 cm s-1, respectively (Fig. 7.15) [34, 35], A luminal concentration of 400 mg L 1 is expected to be achieved in the upper part of the small intestine after oral administration of a 100-mg dose of verapamil in an immediate-release dosage form [1, 34, 35], The three other perfusate concentrations represent fractions of the dose when 30%, 10%, and 1%, respectively are left to be absorbed [34, 35], The increased in vivo jejunal Peff of R/S-vcrapamil, along with its increased luminal perfusate concentration, is in accordance with a saturable efflux mechanism mediated by... 

Palm and co-workers, as part of their research regarding the applicability of PSA, selected 20 compounds (Table 16.1) with quite reliable values for the percent fraction absorbed (%FA) in humans that were devoid of, or compensated for, problems such as active transport and efflux mechanisms [2]. The dataset consists of a variety of compounds drugs, drug-like compounds, as well as sugar-like structures. A number of studies based on this dataset using various structural descriptions have been performed [2, 17-22],... 

Reduced Culture Time Caco-2 cells are usually grown for at least 20 days to form differentiated monolayers on a porous filter support, forming an apical and a basolateral compartment. The 20-day culture time is required to obtain tight junctions, cell polarity, and an adequate expression of drug efflux mechanisms. For economical reasons, various attempts have... 

The applicability of an in vitro assay to classify drug substances is verified by demonstrating a rank-order relationship between the extent of human absorption and experimental permeability values with 20 model drugs. The model drugs should represent ranges of 0-50%, 50-89%, and 90-100% absorption (/a) and the results should clearly differentiate between HP and LP drugs. The model can also be characterized for the presence of functional active transporters (e.g., amino acids, di/tripeptides, monocarboxylic acids, nucleosides) and efflux mechanisms (e.g., P-gp, MRP). 

Active transport mechanisms exist to remove exogenous substances from gastrointestinal epithelial cells (efflux mechanisms) thereby limiting entry into the systemic circulation. It is not possible to identify, which substances could be removed by efflux mechanisms from physicochemical data. 

Cell culture models are routinely used to assess permeability of new potential drug candidates. The simplicity and higher throughput of these models makes them a useful alternative to in vivo studies. These models are used to predict absorption in vivo, rank order compounds and examine absorption mechanism. Transcellular, paracellular, active uptake and efflux mechanisms can be studied with these models. 

The availability of human Pgfr data is more limited than Fa% data and is biased towards highly absorbed compounds. Therefore, the use of in vivo P ff data is more limited for understanding incompletely absorbed compounds which may be subject to paracellular transport or active uptake and efflux mechanisms. 

The use of in vitro models for prediction of compounds that are predominantly absorbed passively by the transcellular route is generally good with these models. Predicting compounds which are absorbed paracellularly or via active uptake or efflux mechanisms is more difficult. There is a lack of understanding of expression levels of transporters in the gut, which makes in vivo predictions difficult. 

A number of microorganisms have evolved mechanisms to overcome the inhibitory actions of the p-lactam antibiotics. There are four major mechanisms of resistance inactivation of the p-lactam ring, alteration of PBPs, reduction of antibiotic access to PBPs, and elaboration of antibiotic efflux mechanisms. Bacterial resistance may arise from one or more than one of these mechanisms. 

Finally, some gram-negative organisms demonstrate a fourth mechanism of resistance. For example, strains of P. aeruginosa produce xenobiotic efflux pumps to eject antibiotics. Drug efflux mechanisms are associated with multiple drug resistance, including resistance to (3-lactam antibiotics. 

G.D. Albertson, M. Niimi, R.D. Cannon, and H.F. Jenkinson, Multiple efflux mechanisms are involved in Candida albicans fluconazole resistance, Antimicrob. Agents Chemother., 40, 2835, 1996. 

Wasaki, S., Sakaida, I., Uchida, K., Kimura, T., Kayano, K., and Okita, K., 1997, Preventive effect of cyclosporin A on experimentally induced acute liver injury in rats, Liver 17, pp. 107-114 Weiss, J.N., Korge, P., Honda, H. M., and Ping, P., 2003, Role of the mitochondrial permeability transition in myocardial disease, Circ. Res 93, pp. 292-301 Wingrove, D.E. and Gunter, T. E., 1986a, Kinetics of mitochondrial calcium transport. I. Characteristics of the sodium-independent calcium efflux mechanism of liver mitochondria, J. Biol. Chem. 261, pp.15159-15165... 

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