Process passive transport

Transport across biological membranes is classified according to the thermodynamics of the process. Passive transport is a thermodynamically downhill process the species move toward the equilibrium. The driving force for the passive transport is the potential difference between the two sides of the membrane. Active transport is a thermodynamically uphill process, it is coupled to a chemical reaction and is driven by it. The following transport mechanisms have been recognized ... 

Permeation across the intestinal wall involves both passive and facilitated processes. Passive transport includes passive transcellular permeation and paracellular diffusion across cell junctions, while facilitated transport includes active influx and efflux processes that pump molecules in and out of the cells. In order to improve absorption of a molecule, engineering passive permeability is a preferred strategy over engineering affinity to an influx transporter. The expression levels of active transporters vaiy significantly between different tissues and individuals, and the specificities and expression levels vaiy among mammals, which decreases the ability to predict human PK based on animal studies. In contrast, passive membrane permeability basically works the same with any eukatyotic membrane, although there may be minor quantitative differences due to different membrane compositions. As paracellular permeation is mainly pertinent to small and polar molecules, passive membrane permeability, which is crucial to transcellular diffusion, is therefore one of the kty properties that needs to be optimized for developing bioavailable macrocycles. 

From a thermodynamic and kinetic perspective, there are only three types of membrane transport processes passive diffusion, faeilitated diffusion, and active transport. To be thoroughly appreciated, membrane transport phenomena must be considered in terms of thermodynamics. Some of the important kinetic considerations also will be discussed. 

Thus the picture that emerges at this point is that in the membrane probably a diprotomeric (a) )2 structure (oc is 114 kDa, P is 60-80 kDa) is responsible for the active transport process. On the other hand the minimal structure for K -ATPase activity, K -pNPPase activity, passive transport and phosphorylation is either an otp protomer or another larger oligomeric structure. 

ATPase also catalyzed a passive Rb -Rb exchange, the rate of which was comparable to the rate of active Rb efflux. This suggested that the K-transporting step of H,K-ATPase is not severely limited by a K -occluded enzyme form, as was observed for Na,K-ATPase. Skrabanja et al. [164] also described the reconstitution of choleate solubilized H,K-ATPase into phosphatidylcholine-cholesterol liposomes. With the use of a pH electrode to measure the rate of H transport they observed not only an active transport, which is dependent on intravesicular K, but also a passive H exchange. This passive transport process, which exhibited a maximal rate of 5% of the active transport process, could be inhibited by vanadate and the specific inhibitor omeprazole, giving evidence that it is a function of gastric H,K-ATPase. The same authors demonstrated, by separation of non-incorporated H,K-ATPase from reconstituted H,K-ATPase on a sucrose gradient, that H,K-ATPase transports two protons and two ions per hydrolyzed ATP [112]. 

Models of lipid bilayers have been employed widely to investigate diffusion properties across membranes through assisted and non-assisted mechanisms. Simple monovalent ions, e.g., Na+, K+, and Cl, have been shown to play a crucial role in intercellular communication. In order to enter the cell, the ion must preliminarily permeate the membrane that acts as an impervious wall towards the cytoplasm. Passive transport of Na+ and Cl ions across membranes has been investigated using a model lipid bilayer that undergoes severe deformations upon translocation of the ions across the aqueous interface [126]. This process is accompanied by thinning defects in the membrane and the formation of water fingers that ensure appropriate hydration of the ion as it permeates the hydrophobic environment. 

The enthusiasm for using Caco-2 cells and other epithelial cell cultures in studies of drug transport processes has been explained by the ease with which new information can be derived from these fairly simple in vitro models [7]. For instance, drug transport studies in Caco-2 cells grown on permeable supports are easy to perform under controlled conditions. This makes it possible to extract information about specific transport processes that would be difficult to obtain in more complex models such as those based on whole tissues from experimental animals. Much of our knowledge about active and passive transport mechanisms in epithelia has therefore been obtained from Caco-2 cells and other epithelial cell cultures [10-15]. This has been possible since Caco-2 cells are unusually well differentiated. In many respects they are therefore functionally similar to the human small intestinal enterocyte, despite the fact that they originate from a human colorectal carcinoma [16, 17]. 

For weak acids, e.g., salicylic acid, the dependency on a pH gradient becomes complex since both the passive diffusion and the active transport process will be dependent on the proton concentration in the apical solution [61, 63, 98, 105] and a lowering of the pH from 7.4 to 6.5 will increase the apical to basolateral transport more than 20-fold. Similarly, for weak bases such as alfentanil or cimetidine, a lowering of the pH to 6.5 will decrease the passive transport towards the basolateral side [105]. The transport of the ionizable compound will, due to the pH partition hypothesis, follow the pKa curve. 

Both active and passive transport occur simultaneously, and their quantitative roles differ at different concentration gradients. At low substrate concentrations, active transport plays a major role, whilst above the concentration of saturation passive diffusion is the major transport process. This very simple rule can be studied in an experimental system using cell culture-based models, and the concentration dependency of the transport of a compound as well as asymmetric transport over the membrane are two factors used to evaluate the presence and influence of transporters. Previous data have indicated that the permeability of actively absorbed compounds may be underestimated in the Caco-2 model due to a lack of (or low) expression of some uptake transporters. However, many data which show a lack of influence of transporters are usually derived from experiments... 

Let us conclude this section by proposing that provided that the drug is sufficiently soluble in the gastrointestinal fluids, the complex process of intestinal drug absorption can often be satisfactorily described by focusing on passive transport across the cell membrane, and that the development of models that predict passive transcellular permeability is particularly important. Such models are the focus of the remaining part of this chapter. 

Antipyrine Passively transported compound Steady-state permeability values for antipyrine indicate that passive transport processes are operational. Stable transmucosal transport of antipyrine during the experiment indicates that the mesenteric blood flow is stable [33, 60]... 

Transport across the cell membrane may occur via different routes. Some of these transport processes are energy dependent and therefore termed active others are independent from energy, thus passive. Passive transport phenomena, for example, transcellular transport, are triggered by external driving forces, such as concentration differences, and do not require metabolic activity. However, generally, they are restricted to small lipophilic compounds. In contrast, active transport phenomena, such as active carrier-mediated transport or vesicular pathways, take course independent from external driving... 

In a recent review of pharmacokinetics in drug discovery, Ruiz-Garcia et al. [81] compiled an exhaustive list of software resources for absorption prediction. The main topic in the described databases is transporters, in particular the ATP-binding cassette, of which the efflux transporter P-gp and the peptide transporter PEPTl are well known examples. These examples show that science is moving away from the simplistic passive transport view of permeability and towards an all-inclusive, mechanism-understanding model of absorption, which takes account of all the interactions between the agents involved in the specific permeation process. 

FIGURE 11-34 Three general classes of transport systems. Transporters differ in the number of solutes (substrates) transported and the direction in which each is transported. Examples of all three types of transporters are discussed in the text. Note that this classification tells us nothing about whether these are energy-requiring (active transport) or energy-independent (passive transport) processes. 

---