1. The fundamental processes which determine the pharmacokinetics The two fundamental processes that determine the concentration of a drug at any moment and in any region of the body are:  translocation (movement)  chemical transformation (metabolism)

2. General principles of drug movement in the body translocation (movement)  translocation (movement)  bulk flow transfer over relatively long distances in the blood and lympha, similar for different drugs  diffusion in body fluids (extracellular, intracellular) over short distances, influenced by physico-chemical characteristics  transfer accross barriers (epithelial, endothelial, cytoplasmic membranes) very different depending on the physico-chemical characteristics of a molecule

3. 1/ Plasma membrane of a cell behaves as a lipid barrier. Aqueous pores are very small (< 1 nm = 10 A) and only very small hydrophilic molecules (water, urea,…) can pass through. Membrane is permeable for lipohilic molecules. Larger hydrophilic molecules can use specialized membrane transport proteins. Cytoplasmic membrane of a cell: Lipid bilayer consists of phospholipids with proteins interspersed. Approximately 70-100 Angstroms (A) thick. Barriers to drug movement

4. Epithelial barriers to drug movement 2/ Epithelial barriers  Single cell thickness, e.g., gastrointestinal mucosa or renal tubule.  The drug has to cross two cytoplasmic membranes.  Permeable for lipohilic molecules.  Intestinal brush-border epithelium is specialized for transport and is equiped with many membrane transporters

5. Epithelial barriers to drug movement 2/ Epithelial barriers  The small intestine represents the principle site of absorption for any ingested compound, whether dietary, therapeutic, or toxic.  Drug absorption occurs predominantly on the outer surface of the GI epithelium (specifically, enterocytes).  The highly differentiated villi of these cells have an absorptive function.  The role of the colon in drug absorption is limited, even though it has the capacity to be an absorption site for certain types of drugs.

6. Barriers to drug movement 3/ Vascular endothelium  Cell-cell adhesions may be:  tight - capillaries in some locations of body, e.g., CNS  very leaky - capillaries in such locations as the liver, spleen.  Gaps between cells are packed with a loose matrix of proteins which act as aqueous pores  Water and small-to-moderate molecules move quickly, macrormolecules of Mr 80 000 - 100 000 transfer slowly.

7. Vascular endothelium

8. Blood capillaries and renal glomerular membranes. These membranes are quite porous allowing non-polar and polar molecules (up to a fairly large size, just below that of albumin, Mr 69,000) to pass through. This is especially useful in the kidney since it allows excretion of polar (drug and waste compounds) substances. Renal tubules. In the kidney, there are a number of regions important for drug elimination. In the tubules, drugs may be reabsorbed. However, because the membranes are relatively non-porous, only lipid compounds or non-ionized species (dependent of pH and pKa) are reabsorbed.

9. Blood-brain barrier. The membranes between the blood and brain have effectively no pores. This will prevent many polar materials (often toxic materials) from entering the brain. However, lipid soluble molecules, such as diethyl ether, halothane, can easily enter. the brain.

10. Drugs may move (passively diffuse + be filtered with water) through aqueous channels in the intercellular junctions (A, PARACELLULAR TRANSPORT) or through lipid cell membranes (B, TRANSCELLULAR TRANSPORT). Drugs with the appropriate characteristics may be transported by carriers into or out of cells (C). Very impermeant molecules may also bind to cell surface receptors (dark binding sites), be engulfed by the cell membrane (endocytosis), and then released inside the cell or expelled via the membrane-limited vesicles out of the cell into the extracellular space (exocytosis, D).

11. 1/ Filtration implies movement with water through pores, fenestrae between cells, etc. in blood capillaries. Does NOT occur through cytoplasmic membranes. It is not saturable (capacity-limited) because the drug is transported with water. The driving force is the difference in hydrostatic and osmotic pressures. 2/ Passive diffusion through the lipid membranes or aqueous pores from the side of higher conc. to the side of lower conc. The process occurs due to the concentration gradient, it requires no external energy 2a/ passive diffusion 2b/ facilitated diffusion (passive but requires the carrier) 3/ Active carrier-mediated transport – consumes external energy 4/ Pinocytosis and receptor-mediated endocytosis (large molecules, drug-protein adducts) Movement acrross the barriers

12. Simple passive diffusion Diffusion occurs in a homogenous aqueous and lipoid medium as a random Brownian motion from the site of a high to the side of a low concentration. Regarding the transmembrane transport, lipid soluble molecules dissolve in the lipoid membrane, travel through it and escape into the aqueous cytosol. Drug diffuses across the membrane to equalize the drug concentration on both sides of the membrane. Fick's first law: Rate of diffusion = D × Area × (C 1 -C 2 ) / dif. distance D…diffusion coefficient (size and shape of the molecule, characteristics of the medium)

13. Simple passive diffusion Among different drug molecules, the diffusion coefficient varies only slightly (the influence of chemical structure is low) so the most important variable is the ability to dissolve in the lipoid membrane (which can be expressed as a partition coefficient for the substance distributed between the membrane phase and the aqueous environment) For this reason, lipid solubility is one of the most important determinants of the pharmacokinetic characteristics of a drug, and many properties-such as the rate of absorption from the gut, penetration into the brain and other tissues, and the extent of renal elimination-can be predicted from knowledge of a drug's lipid solubility.

14. Some molecules are too big and polar (amino acids, penicillin, frusemide, morphin etc.) to cross the membrane via passive diffusion. Facilitated diffusion goes down the gradient (requires no energy) but uses a carrier protein: It is substrate- specific and can be saturated. Facilitated diffusion

15. Carrier-mediated active transport Carrier mediated transport (important in the gastrointestinal tract, renal tubule, billiary tract, blood brain barrier) is important for endogeneous compounds and for many drugs which are chemically related. It contains a binding step, it is substrate- specific, and can be saturated at high drug concentrations (is capacity-limited) It can be a site of competition between several molecules. Carrier mediated transport can occur against the concentration gradient but this requires energy in the form of ATP (primary active transport) or coupling to the transport of other molecules which occurs in the direction of its concentration gradient (secondary active or coupled transport).

16. Transporters for drugs and endogenous compounds

17. Drug transport through the intestinal epithelium The drug transporters are members of two superfamilies: the ABC (ATP-binding cassette) superfamily are responsible for primary active transport, while the members of the SLC (solute carrier) superfamily are involved in secondary active transport. (A) passive diffusion via tight junctions; (B) passive lipid diffusion; (C) endocytosis; (D) carrier-mediated transport; (E) carrier- mediated uptake; (F) carrier-mediated efflux.

18. Energy of transport Passive diffusion Facilitated passive diffusion (carrier-mediated) Active carrier-mediated transport

19. The pH-partition theory and ion trapping Many drugs behave as weak acids or weak bases. They can exist in both ionized and non-ionized states. The ratio of the nonionized to ionized drug concentrations depends on: - pH of the medium -pK a of the drug (Henderson-Hasselbach equation) Acidic drug:Basic drug:

20. Bases are protonated at more acidic pH (cations cannot passively diffuse through lipid membranes) Acids dissociate at more alkaline pH (anions cannot passively diffuse through lipid membranes)

21. Acid-base characteristics of drugs

22. b Only the no-ionized molecule can diffuse through a lipoid membrane! b Therefore, diffusion is driven by the concetration gradient of unionized molecules only! b pH partition means that weak acids tend to accumulate in compartments with a high pH, whereas weak bases accumulate in compartments with a low pH. b The non-ionized molecule crosses the membrane and the ions are trapped in more acidic (drugs-bases) or more alkaline fluids (drugs-acids). The pH-partition theory and ion trapping

23. b Drug HA is a weak acid with pKa = 4.4 (aspirin). Its equilibrium concentration is much higher in the plasma (pH=7.4, total concentration of 1001) than in gastric juice (pH=1.4, 1.001). The ionized form A - accumulates in the plasma. The concetration of HA, i.e. the form capable of diffusion, is equal on both sides of the membrane (1). The pH-partition theory and ion trapping

24. b pH of body fluids: b plasma 7.4 b plasma of the fetus7.3 b urine 4 - 8 b cerebrospinal fluid7.3 b human milk7.0 – 7.3 b gastric juice 1-3 b duodenal content 5-6 b ileum, colon8.0 b acidic drugs are better absorbed in the stomach, while basic drugs in the ileum b acidic drugs are more rapidly excreted by the kidneys if the urine is alkaline. They are filtered, dissociate in urine and can not diffuse back to the blood in the tubulus. Alkaline drugs are more protonated and their urinary excretion is faster if urine is more acidic. Importance of acid-base characteristics of the drug