The document discusses various mechanisms of drug transport across biomembranes in the gastrointestinal tract. The major pathways include paracellular transport between cells and transcellular transport across cells. Transcellular transport can occur through passive diffusion, carrier-mediated transport, active transport, facilitated transport, and endocytosis. Efflux transporters like P-glycoprotein can also pump drugs back into the gut lumen, reducing their oral bioavailability.

1. TRANSPORT ACROSS BIOMEMBRANES

2. MECHANISMS OF DRUG TRANSPORT ACROSS BIOMEMBRANES Introduction The apical cell membrane of the columnar absorption cell behaves as a ‘lipoidal’ membrane, interspersed by sub-microscopic water-filled channels or pores. Water soluble substances of small molecular size (radius 0.4 nm) such as urea are absorbed by simple diffusion through the water-filled channels.

3. MECHANISMS OF DRUG TRANSPORT ACROSS BIOMEMBRANES Most drugs molecules are too large to pass through the aqueous channels. The apical cell membrane of the g.i.-blood barrier allows the passage of lipid-soluble drugs in preference to lipid-insoluble drugs. 2 Main Mechanisms Paracellular pathway: between cells Transcelular pathway

4. MECHANISMS OF DRUG TRANSPORT ACROSS BIOMEMBRANES Paracellular Pathway may be divided into: Convective – “Solvent drag” Diffusive component

5. MECHANISMS OF DRUG TRANSPORT ACROSS BIOMEMBRANES Transcellular pathway includes: Simple passive diffusion Carrier-mediated transport Active transport Facilated transport Endocytosis Refer to table 16.2 page 231 of Pharmaceutics: The Science of Dosage Form Design (2 nd ed) by M. Aulton

6. From: Hunter J, Hirst BH. Intestinal secretion of drugs. The role of P-glycoprotein and related drug efflux systems in limiting oral drug absorption. Advanced Drug Delivery Reviews 25:129-157, 1997.

7. MECHANISMS OF DRUG TRANSPORT ACROSS BIOMEMBRANES – Paracellular Pathway Transport of materials in the aqueous pores between cells rather than across them. The epithelia of the small intestine has more pores than others. This pathway is important for the absorption of ions such as calcium, sugars, amino acids and peptides at concentrations above the capacity of their carriers. Small hydrophillic and charged drugs also use this route. Transport molecules up to 200 Da mol. Wt.

8. MECHANISMS OF DRUG TRANSPORT ACROSS BIOMEMBRANES Convective absorption Compound is carried across the epithelium by the water flux. By this mechanism, very small molecules such as water, urea and low molecular weight sugars and organic electrolytes are able to cross cell membranes through aqueous filled channels or pores. The effective radii of these channels are small (≈ 0.4 nm) such that the mechanism is of little significance in the absorption of large, water-insoluble drug molecules or ions. It is the mechanism involved in the renal excretion of drugs and the uptake of drugs into the liver.

9. Trancellular Pathway Passive Diffusion Involves the movement of drug molecules from region of relatively high to low concentration without expenditure of energy. Movement continues until equilibrium has been reached between both sides of the membrane and the equilibrium tend to be achieved faster with highly permeable (i.e. lipid soluble drugs) and when membrane has a large surface area (e.g. intestine vs stomach or duodenum). The apical cell membrane plays only a passive role in the passive diffusion transport process.

10. Trancellular Pathway Passive Diffusion The main factors determining the rate of drug transport are: Physicochemical properties of the drug i.e. particle size, solubility, partition coefficient, pH and pKa. The nature of the membrane, and The concentration gradient of drugs across the membrane.

11. Diagrammatic representation of g.i. absorption by passive diffusion h Partition Partition Diffusion Drug in solution Drug in solution carried away by circulating blood G.I FLUID BLOOD G.I. MEMBRANE

12. Fick’s Law of diffusion Where dQ/dt = rate of appearance of drug in the blodd at the site of absorption D = the effective diffusion coefficient of the drug in the gi membrane A = the surface area of g.i. membrane available for absorption by passive diffusion k 1 = the apparent PC of drug between g.i. ‘membrane’ & the g.i. fluid.

13. Fick’s Law of diffusion (Cont.) C g is the concentration of drug in solution in the g.i. fluid at the site of absorption k 2 is the apparent PC of drug between the g.i. membrane & the blood C b is the concentration of drug in the blood at the site of absorption h is the thickness of the g.i. membrane.

14. Fick’s Law of diffusion (Cont.) The drug in blood vessel is rapidly cleared away and the blood thus serves as a “sink” for absorbed drug as a result of: Distribution in a large volume of blood i.e. systemic circulation Distribution into body tissues and other fluids of distribution Metabolism and excretion Protein binding Hence, a large concentration gradient is always maintained across the g.i. membrane during absorption process and this conc. gradient becomes the sole driving force behind drug absorption by passive diffusion mechanism.

15. Specialized Transport Mechanisms Active transport Facilitated transport Active transport Substances are transported against their concentration gradient (i.e. from low to high regions of concentration) across a cell membrane. It is an energy-consuming process and involves active participation of the apical cell membrane of the columnar absorption cell.

16. Specialized Transport Mechanisms Active transport Drug molecule or ion forms a complex with a “carrier” which, may be an enzyme or some other components of the cell membrane, to form a “drug-carrier” complex. This complex then moves across the membrane, liberates the drug on the other side and the carrier returns to the original state and surface to repeat the process. As for g.i absorption, transfer occurs only in the direction of g.i. lumen to the blood i.e. not normally against the conc. gradient, the carrier being generally a ‘one-way’ transport system.

17. Specialized Transport Mechanisms Active transport Several carrier-mediated transport systems exist in the small intestine and each is highly selective with respect to the structure of substances it transports. Drugs resembling such substances can be transported by the same carrier mechanism. E.g. Levodopa resembles tyrosine and phenylalanine and is absorbed by the same mechanism. Active transport proceeds at a rate directly proportional to the concentration of the absorbable species only at low concentration and the mechanism becomes saturated at high concentrations.

18. Proposed Model for Carrier-Mediated Transport

19. Membrane Transporters and Their Substrates Transporter Substrates Amino acid transporters baclofen, cyclosporin, L-dopa, gabapentin, methyldopa Peptide transporters  -lactam antibiotics, ACE inhibitors, (hPEPT1, HPT1) cephalexin, cyclosporin, methyldopa Nucleoside transporters zidovudine, zalcitabine, dipyridamole (CNT1, CNT2) Organic anion transporters ceftriaxone, benzoic acid, methotrexate (OATP1, OATP3, OATP8) pravastatin Organic cation transporters thiamine, desipramine, quinidine, (OCT1,OCT2) midazolam, verapamil Bile acid transporters chlorambucil, thyroxine (IBAT/ISBT)

20. Illustration of Specialized Transport

21. Specialized Transport Mechanisms Facilitated Transport Differs from active transport in that it can not transport a substance against its concentration gradient Does not require energy input. Its driving force is the concentration gradient. Another transport facilitator is required in addition to the carrier molecule.

22. Facilitated Transport of Vit. B 12 Carrier B12 IF B12-IF Transported Vit. B12

23. Transcellular Pathway Endocytosis Definition The process by the plasma membrane of the cell invaginates and the invagination becomes pinched off. Small intracellular membrane-bound vesicles are formed and enclose a volume of material. Material can then be transported into the cell

24. Transcellular Pathway Endocytosis After invagination the material is transferred to othe vesicles or lysosomes and digested. This is an energy dependent process. May be divided into 4 main processes Pinocytosis or fluid-phase endocytosis Receptor-mediated endocytosis Phagocytosis transcytosis

25. Transcellular Pathway Pinocytosis Substance does not have to be in aqueous solution to be absorbed. Like phagocytosis, it involves invagination of the material by the apical cell membrane of the columnar absorption cell lining the g.i.t. to form vacuoles containing the material. These vacuoles then cross the columnar absorption cells. It is the main mechanism for the absorption of macromolecules such as proteins and water-insoluble substances like vit. A, D, E and K.

26. Receptor-mediated endocytosis Process of ligand movement from the extracellular space to the inside of the cell by the interaction of the ligand with a specific cell-surface receptor. The receptor binds the ligand at its surface Internalizes it by means of coated pits and vesicles The coated cell lose their coat once transported to the cytoplasm of the cell.

27. Receptor-mediated Endocytosis The ligand is delivered to lysosomes (spherical or oval cell organelles surrounded by a single membrane) The lysosomes contain digestive enzymes which break down bacteria and large molecules such as protein, polysaccharides and nucleic acids.

28. Receptor-mediated endocytosis Cell membrane Free drug Released drug

29. Phagocytosis Phagocytosis is: The engulfment by the cell membrane of particles larger than 500 nm This is the process of absorption of polio and other vaccines from the GIT.

30. Transcytosis The process by which the material internalized by the membrane domain is transported through the cell and secreted on the opposite side.

31. Ion-pair transport In this mechanism, some ionized drug species interact with endogeneous organic ions of opposite charge to form absorbable neutral specie i.e. an ion-pair. The charges are “buried” in ion pair and the complex can now partition into the lipoidal cell membrane lining the g.i.t. and be absorbed by passive diffusion. A suitable mechanism for the absorption of quaternary ammonium compounds and tetracyclines which are ionized over the entire g.i. pH range. Ion pair ≡ Organic anions + Organic cations = Neutral molecules (crossing lipoidal membrane by passive diffusion.

32. Efflux of Drugs from the Intestine Countertransport efflux proteins expel specific drugs back to the lumen of the GIT after absorption. The main countertransport protein is P-glycoprotein. High levels of glycoprotein are expressed in the jejunum. The efflux reduces bioavailability Drugs with wide structural diversity are affected eg. cycolsporine.

33. Consequence of the Efflux Transporter P-glycoprotein 1) Limited drug absorption enterocyte pgp Gut lumen 2) Enhanced drug elimination Proximal tubule cells Tubule lumen hepatocytes bile 3) Limited distribution Endothelial cells capillary Brain or testes syncytiotrophoblast Maternal blood lymphocyte Adapted from: Fromm MF. Trends in Pharmacol Sci 25:423, 2004