Across Membrane Barriers

Drugs and other substances that pass through biologic membranes usually do so via passive diffusion, active transport, facilitated diffusion, or some special process such as endocytosis (Fig. 2-2). Each of these mechanisms is discussed here. 

Drugs and other substances will pass through a membrane by way of diffusion providing two essential criteria are met. First, there must be some type of difference or gradient on one side of the membrane 

Passive Diffusion Active Transport Facilitated Endocytosis 

Effect of Ionization on Lipid Diffusion. Passive lipid diffusion of certain drugs is also dependent on whether or not the drug is ionized. Drugs will diffuse more readily through the lipid layer if they are in their 

FIGURE 2-3 Effect of phi and ionization on absorption of drugs from the gastrointestinal tract. Weak acids and bases are absorbed from the stomach and duodenum, respectively, when they are in their neutral, nonionized form. 

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The equations used to calculate permeability coefficients depend on the design of the in vitro assay to measure the transport of molecules across membrane barriers. It is important to take into account factors such as pH conditions (e.g., pH gradients), buffer capacity, acceptor sink conditions (physical or chemical), any precipitate of the solute in the donor well, presence of cosolvent in the donor compartment, geometry of the compartments, stirring speeds, filter thickness, porosity, pore size, and tortuosity. 

Pharmaceutical scientists assess and express drug permeation across membrane barriers in terms of flux. Flux measures the molar unit of a drug that permeates a resistant barrier (e.g., skin or gastrointestinal epithelial cells) per unit time and surface area (Box 13.1). Permeation enhancers, such as alcohols and surfactants, increase flux by modulating resistance factors that counteract drug diffusion across barriers at the site of administration. 

In former times, chewing or sniffing tobacco were quite popular, further illustrating that nicotine can be taken up across membrane barriers. 

Foged C, Nielsen HM. Cell penetrating peptides for drag delivery across membrane barriers. Expert Opin Drag Deliv 2008 5(1) 105-117. 

Castanospermine has been screened for efficacy against simian immunodeficiency virus (265), and has been shown to prevent syncytium formation in feline astrocyte cultures infected with the feline immimodeficiency virus by modifying the viral cell envelope (266). It suppressed syncytium formation and hemolytic activity in baby hamster kidney cells infected with Newcastle disease virus however, synthesis and cell surface expression of the hemagglutinin-neuraminidase glycoprotein in the viral envelope were not affected, which strengthens the hypothesis that poor transport of the parent alkaloid across membrane barriers may limit its therapeutic use (267). Both 239 and its 6-0-butanoyl ester had comparable relative toxicities and antiviral effects on Rauscher murine leukemia virus (268), but the ester was more potent than the parent alkaloid in inhibiting replication of Moloney murine leukemia virus (258). The ester was also active against herpes simplex viruses types 1 and 2 (269,270). In the latter case, conclusive evidence was provided for intracellular hydrolysis to 239. 

Without exception, proteins carry out their functions in living organisms via direct interactions with themselves or through interactions with other proteins or biomolecules. The processes are likely to involve multiple binding partners in multiple binding steps, and may or may not take place across membrane barriers between cells or cell organelles. Because these processes involve complex biological pathways, they and the phenotypes they produce are rarely understood in detail. 

One mechanism for the transfer of particular translation products across membrane barriers which has received extensive experimental support is the vectorial release of secretory proteins into the cisternal space of the endoplasmic reticulum. In this system, secretory proteins are synthesized selectively on polysomes attached to the rough endoplasmic reticulum and are subsequently released from the membrane-bound ribosomes through the adjacent membrane into the cisternum of the reticulum for later transport from the cell. ... 

The organic cation transporters (OCTs) facilitate the uptake of many cationic drugs across different barrier membranes from kidney, liver, and intestine... 

The evaluation of the apparent ionization constants (i) can indicate in partition experiments the extent to which a charged form of the drug partitions into the octanol or liposome bilayer domains, (ii) can indicate in solubility measurements, the presence of aggregates in saturated solutions and whether the aggregates are ionized or neutral and the extent to which salts of dmgs form, and (iii) can indicate in permeability measurements, whether the aqueous boundary layer adjacent to the membrane barrier, Umits the transport of drugs across artificial phospholipid membranes [parallel artificial membrane permeation assay (PAMPA)] or across monolayers of cultured cells [Caco-2, Madin-Darby canine kidney (MDCK), etc.]. 

In practice, estimation of Laq requires information on the rate of solute removal at the membrane since aqueous resistance is calculated from experimental data defining the solute concentration profile across this barrier [7], Mean /.aq values calculated from the product of aqueous diffusivity (at body temperature) and aqueous resistance obtained from human and animal intestinal perfusion experiments in situ are in the range of 100-900 pm, compared to lumenal radii of 0.2 cm (rat) and 1 cm (human). These estimates will necessarily be a function of perfusion flow rate and choice of solute. The lower Laq estimated in vivo is rationalized by better mixing within the lumen in the vicinity of the mucosal membrane [6],... 

Possible driving forces for solute flux can be enumerated as a linear combination of gradient contributions [Eq. (20)] to solute potential across the membrane barrier (see Part I of this volume). These transbarrier gradients include chemical potential (concentration gradient-driven diffusion), hydrostatic potential (pressure gradient-driven convection), electrical potential (ion gradient-driven cotransport), osmotic potential (osmotic pressure-driven convection), and chemical potential modified by chemical or biochemical reaction. 

Mucosal brush border membrane vesicles and basolateral membrane vesicles can be isolated to study solute uptake across specific enterocyte boundaries. These more isolated vesicle systems allow for investigation of solute transport across a particular membrane barrier and permit separation of membrane trans-... 

Raub TJ, CL Barsuhn, LR Williams, DE Decker, GA Sawada, NFH Ho. (1993). Use of a biophysical-kinetic model to understand the roles of protein binding and membrane partitioning on passive diffusion of highly lipophilic molecules across cellular barriers. J Drug Targeting 1 269-286. 

Fig. 7.1. The intestinal permeability of drugs in vivo is the total transport parameter that may be affected by several parallel transport mechanisms in both absorptive and secretory directions. Some of the most important transport proteins that may be involved in the intestinal transport of drugs and their metabolites across intestinal epithelial membrane barriers in humans are displayed.

There appear to be two major ways by which ionophores aid ions to cross membrane barriers. Ionophores such as valinomycin and nonactin enclose the cation such that the outside of the complex is quite hydro-phobic (and thus lipid-soluble). The transport behaviour thus involves binding of the cation at the membrane surface by the antibiotic, followed by diffusion of the complexed cation across the membrane to the opposite surface where it is released. Such carrier type ionophores can be very efficient, with one molecule facilitating the passage of thousands of ions per second. A prerequisite for efficient transport by this type of ionophore is that both the kinetics of complex formation and dissociation be fast. 

Generally, to produce a biological response, a drug molecule must first cross at least one biological membrane. The biological membrane acts as a lipid barrier to most drugs and permits the absorption of lipid-soluble substances by passive diffusion while lipid-insoluble substances can diffuse if at all across the barrier only with considerable difficulty. The interrelationship of the dissociation constant, lipid solubility, and pH at the absorption site and absorption characteristics of various drugs are the basis of the pH-partition theory. 

Hynn, G.L. and Yalkowsky, S.H. 1972, Correlation and prediction of mass transport across membranes I Influence of alkyl chain length on flux-determining properties of barrier and diffusant. J. Pharm. Sci. 61 838-852. 

The meaning of this term is shown by Figure 2.5 and it is essentially the time required to attain steady state flux across a barrier. When the resistance in the boundary layer is negligible, the lag-time equation provides a convenient means of calculating membrane or polymer-diffusion coefficients. 

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