Passage of drugs across membranes
Before studying the passage of drugs across membranes it is necessary to recall the composition and structure of the membrane.
Composition and structure of membranes
Plasma membrane which surrounds each cell consists of approximately 60% of phospholipids and 40% of protein.
The lipids entering the composition of membranes are amphipathic, they consist of molecules containing a polar head and a non polar or hydrophobic tail. They are primarily:
- glycerophospholipids (substituted glycerol).
- sphingolipids (i.e. derived from sphingosine which is an alcohol substituted by a fatty acid and a polar group). The sphingolipids are divided into three groups: sphingomyelins, cerebrosides and gangliosides.
- cholesterol consisting of a sterane nucleus substituted by a polar group (OH) and a non polar flexible chain. Cholesterol, intercalated between other lipids, reinforces the membrane structure.
These amphipathic lipids (glycerophospholipids and sphingolipids) are naturally oriented to form a bilayer: polar groups located on both sides and the non polar tails in the middle.
The fluidity of the bilayer depends on temperature, but also on its composition. The unsaturated fatty acids which form less linear chains than the saturated fatty acids increase membrane fluidity.
The proteins found in membranes are either inserted in the lipid bilayer, inside or outside, or transmembrane on both sides,.
These proteins constitute:
- cell-surface receptors (generally glycoproteins, which ensure the intercellular communications),
- structures which ensure the exchange of ions and certain molecules between the cell and its environment: pumps such as Na+ /K+-ATPase, channels, “exchangers”.
Transfer of molecules and drugs across membranes
To penetrate into the cell, the drug must cross the cytoplasmic membrane. To pass from a compartment to another, the drug must cross one or several membranes. The membranes consist of cells bound to each other more or less tightly. These cells are located on a basement membrane, itself more or less permeable to molecules. One distinguishes:
- transcellular transfer
When the cells are tightly against each other, as in the endothelium of the cerebral capillaries, the drug must cross the cells themselves, i.e. the cytoplasmic membrane, to pass from one compartment to the other.
- paracellular transfer:
When the epithelial cells are separated from each other by looser junctions, the molecules can pass through these junctions called “gap junctions”. The paracellular transfer of a molecule depends primarily on its molecular weight and its flexibility.
- porous filter:
Certain epithelia, like that of the renal glomerulus, bear pores permeable to molecules smaller than the pores. At the level of the renal glomerulus, the molecules whose molecular weight is lower than 68 000 can theoretically pass, but more the molecular weight of a molecule approaches 68 000, more its transfer becomes difficult. Other parameters than the molecular weight (directly linked to the size), such as charges or flexibility, are also to be taken into consideration.
Transfer through the lipid bilayer: passive diffusion
The transfer through a lipid bilayer is passive without requiring energy. The lipid bilayer membrane constitutes a barrier:
- impermeable to ions such as Na+, K+, Cl - , to polar molecules even non charged, i.e. non-ionic, like glucose, and proteins.
- permeable to non polar molecules (liposoluble or hydrophobic) of low or middle molecular weight, and to molecules at the gas state and small molecules of low polarity.
Passive diffusion through a lipid bilayer
The passage through the membrane is directed from the most concentrated solution towards the least concentrated solution until obtaining an equilibrium. The rate of transfer depends on the area S on the membrane, on the concentrations C1 and C2 on both sides of the membrane and on a diffusion constant K linked to the liposolubility and the size of the molecule (the smaller the molecule, the easier the transfer).
V = K.S. (C2 - C1)
The liposoluble character of a molecule is determined by the measurement of its partition coefficient between an aqueous solvent and an organic solvent like hexane. The liposoluble or non-polar molecules accumulate in organic solvent and the polar molecules in water.
The polarity of a drug depends on its ionization and one distinguishes three categories:
- Permanently ionized molecules, whatever the pH, for example those with a quaternary ammonium. These molecules, in theory, do not cross the lipid bilayer by passive diffusion.
- Neutral molecules, not ionized, whatever the pH. It is the case of the organic solvents which cross the lipid bilayer easily. This rule has limitations, in particular concerning intestinal absorption because it takes place from an aqueous medium. A very lipophilic molecule but almost insoluble in water can be poorly absorbed. A low availability can be the consequence of a low solubility in water.
- Molecules whose ionization depends on pH: in a neutral state, they cross the lipid bilayer , but not in the ionized state. The acid drugs dissociate in a basic medium and the bases in an acidic medium, to give ionized molecules. The pKa of an acid is the pH at which it is 50% dissociated. For example, an acid drug R-COOH partially dissociated in R-COO - + H + , neutral form R-COOH crosses membranes but not the ionized form R-COO - .
Consequently, when two compartments at different pH are separated by a membrane, the equilibrium of the concentrations of a drug R-COOH on both sides of the membrane will be moved and drug R-COOH will accumulate in the basic compartment where it dissociates.
Transmembrane transfer of an acid drug according to pH
Binding of the drugs to plasma proteins, in particular to albumin, which can go from 0% to 99% modifies their transfer through the membranes. In the free state, i.e. unbound to plasma proteins, the liposoluble drugs, if there is a gradient of favorable concentration, cross the lipid membrane, whereas the fraction bound to plasma or tissue proteins does not cross it.
Transmembrane transfer of a drug according to its protein binding
Albumin is a protein with a molecular weight of 68000 daltons, present in plasma at a concentration approximately 50 g/L. It is synthesized by the liver and degraded primarily by the vascular endothelium. Its plasma half-life is approximately 20 days. In addition to its role in oncotic pressure, it binds various endogenous molecules and drugs. There are on a molecule of albumin six different types of sites with different affinities for drugs.
The equilibrium for a drug between the free fraction, D, and the bound fraction, DP, is reversible and non-static in the body because the blood circulation causes dynamic changes.
Blood cells, mainly red cells, can play a role comparable to that of albumin for drugs that they bind or incorporate.
Transfer through membrane protein structures
The transfer through the protein structures of membranes is carried out by active transport, i.e. using energy provided by cellular metabolism.
Direct active transport by pumps
The active transport requires energy, generally provided by ATP hydrolysis. The Na+/K+-ATPASE pump, Mg2 + dependant, uses the energy of ATP to polarise the cell by extruding three Na+ ions and incorporating two K+ ions, which creates a potential difference between the intracellular medium and the extracellular medium. There are other pumps: a Ca2+-ATPASE pump localized on the level of the cytoplasmic membrane and at the level of the endoplasmic reticulum and an H+ /K+-ATPASE pump.
A particular membrane protein, P-glycoprotein, or P170 because its molecular weight is 170 Kd, uses, like the Na+/K+-ATPASE pump , the energy supplied by the hydrolysis of ATP to drive certain drugs out of the cell. This glycoprotein, also called Multi drug transporter, protects the cell against xenobiotics but, as it is expressed in the majority of cancerous cells, it also expel antineoplastic drugs out of cancerous cells, thus explaining their resistance to chemotherapy which is called MDR “Multi drug resistance”. Drugs such as verapamil or quinidine inhibit P-glycoprotein but have other properties making their use as inhibitors of P-glycoprotein difficult. P-glycoprotein present in the blood-brain barrier reduces penetration of certain drugs into the brain. It can also reduce their digestive absorption.
Various microorganisms have in their plasma membrane, in addition to P-glycoprotein, proton-dependant pumps which drive toxic molecules out of their cytoplasm.
Indirect active transport
The energy necessary for facilitated diffusion, or secondary active transport, is supplied by the ion gradients on both sides of the membrane. It is not carried out through the lipid bilayer but through the protein structures.
It relates to only a small number of compounds:
- molecules implicated in metabolism, like glucose, amino acids, certain transmitters and drugs of close chemical structure. The energy necessary for their transport can be supplied by the sodium gradient.
The kinetics of transfer is of Michaelis Menten type with maximum rate and possibility of competition between molecules. When sodium and the substrate cross the membrane in the same direction, the transport is called symport and “antiport” when they cross in opposite directions.
- peptides formed of 2 or 3 acids amino are partially absorbed by the digestive tract and reabsorbed in the nephron by specific carriers using the H+ gradient as source of energy. These carriers are involved in the digestive absorption of drugs with a chemical peptidic-like structure such as beta-lactamines. A polypeptide such as insulin can cross the blood-brain barrier but the responsible mechanisms are poorly understood.
- ions like Na+, K+, Ca2+, Cl _ , whose membrane transporters are channels and exchangers:
- channels whose opening and closure depend either on the intra/extracellular potential difference (opening during depolarization), or on the presence of receptors which can be activated or inhibited by various transmitters
- exchangers, Na+/Ca2+ for example.
Transport by exocytosis and endocytosis
Exocytosis consists of the exit out of the cell of molecules contained in vesicles which fuse with the plasma membrane and release their contents outside. It is the mode of release of transmitters.
Endocytosis consists of the absorption by a cell of an extracellular molecule. After its inclusion in a vesicle formed by an invagination of the plasma membrane, the molecule penetrates into the cytoplasm. It is, for example, the process used by hepatocytes to take up lipoproteins and transferrin,.
Oligonucleotides of 10 to 20 units, such as those used in antisense therapy (oligonucleotides complementary to the mRNA on which they bind) penetrate into cells by various mechanisms but primarily by endocytosis. In fact, an oligonucleotide penetrates better into cells that an isolated nucleotide.