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Introduction of Biopharmaceutics & Pharmacokinetics Drug Transport What’s the plan for today • We will look at barriers to drug entry in human body • Close look at physiology of cell membrane • Passage of drugs across cell membrane • Different mechanism of drug transport Barriers in Human body • Our body has barriers that prevent direct entry in to blood • Epithelia act as a barrier • Epithelial cells cover the entire external as well as internal surface of body • They have 2 major role 1. Protect internal environment 2. Allow exchange of nutrients, gases, solutes etc Epithelial Tissue Gastro-intestinal Epithelia For drugs to be absorbed they have to pass through biological membranes Cell Membrane Cross section of Intestine Cell membrane acts as “gate Keeper” Cell membrane plays a very important role of controlling the movement of substance across cell Structure of Cell membrane • Lipid Bilayer “fluid mosaic model” • The cell membrane consists double layer of amphipathic phospholipids • Hydrocarbon chains face inwards, polar heads face outside • Create a inner hydrophobic core & outside hydrophilic Cell Membrane: “Fluid Mosaic Model” • Attached to lipids are proteins • Membrane is dynamic with changing composition of lipids and proteins • Crystalline solid-liquid state Composition of cell membrane Lipids • Phospholipids, glycolipids, and sterols • The entire membrane is held together via noncovalent interaction of hydrophobic tails Carbohydrates • Mostly found on outer surface • Glyco+proteins • Glyco+lipids • Short polysaccharides forming a cell coat Proteins • 50% of membrane volume • Important different functions • cell–cell contact, surface recognition, cytoskeleton contact, signaling, enzymatic activity, or transporting substances across the membrane Role of Proteins in Cell Membrane • Structural Role • Link cytolskeleton to membrane • Cell to Cell connections • Act as enzymes, catalyse reactions near cell membrane • Receptors • These receptors sense specific signals • Transporters • Channels • Act as carrier How are drugs transported across cell membrane ? Two major mechanism by which the drugs are transported across cell membrane TRANSCELLULAR (INTRA) Transport ACROSS the membrane PARACELLULAR (INTER) Transport BETWEEN the membrane cells Classification of Trans-cellular Pathway TRANSCELLULAR PASSIVE TRANSPORT Transport ACROSS the membrane ACTIVE TRANSPORT PASSIVE TRANSPORT PASSIVE DIFFUSION PORE TRANSPORT ION PAIR TRANSPORT FACILITATED OR MEDIATED DIFFUSION Passive Diffusion • Movement of drugs across membrane • Driving force to move drug is concentration difference across cell membrane • Also called Non-ionic diffusion • Almost 90% of drugs absorbed by passive diffusion Passive Diffusion • Passive diffusion best explained by “Ficks 1st Law of diffusion” • Drug molecule diffuse from a region of higher concentration to one of lower concentration until equilibrium is attained and • that the rate of diffusion is directly proportional to the concentration gradient across the membrane Ficks st 1 law of Diffusion • Mathematically 𝑑𝑄 • 𝑑𝑡 • • • • • • = 𝐷𝐴𝐾𝑚 𝑤 / ℎ 𝐶𝑔𝑢𝑡 − 𝐶 Where 𝑑𝑄 = Rate of diffusion 𝑑𝑡 D = diffusion coefficient, A = area of absorbing membrane Km/w = Partition coefficient (Lipoidal membrane/Aq water) 𝐶𝑔𝑢𝑡 − 𝐶 = concentration difference across cell membrane h = thickness of membrane Ficks 𝑑𝑄 • 𝑑𝑡 = st 1 law of Diffusion 𝐷𝐴𝐾𝑚 𝑤 / ℎ 𝐶𝑔𝑢𝑡 − 𝐶 • Based on the Fick’s law, what general conclusions can we say for Passive diffusion of Drugs ?? Ficks • 𝑑𝑄 𝑑𝑡 • 𝑑𝑄 𝑑𝑡 • 𝑑𝑄 𝑑𝑡 = st 1 law of Diffusion 𝐷 𝐴 𝐾𝑚 𝑤 / ℎ 𝐶𝑔𝑢𝑡 − 𝐶 Under usual conditions, these are all constants = 𝑃 𝐶𝑔𝑢𝑡 − 𝐶 , where P is permeability coeff (ease with which drug moves across membrane) = 𝑃 𝐶𝑔𝑢𝑡 under sink conditions 𝐶𝑔𝑢𝑡 >> 𝐶 • Diffusion is a 1st order process Pore Transport Other Names: Convective, bulk flow, filtration • Transport through protein channels • Driving force is hydrostatic pressure or osmotic that leads to water flow • Small solute molecule move along water (solvent drag) • Which drugs use this pathway • • • • Low molecular weight (<100) Low molecular size Water soluble drugs through aqueous filled channels Ex. Urea, Water, sugars Ion-Pair transport Endogenous anion - + Neutral complex - Membrane + + - + Cationic drug - • Drugs that are ionized under all pH • Combine with endogenous ions at GIT (Mucin) • Form reversible-neutral complex • Complex sufficiently lipophilic • Passive diffusion occurs • After crossing, drugs dissociate • Ex. Propranolol “Carrier” Mediated Transport (Features) • Specialized transport mechanism • Transport done by Special Proteins • Transport proteins have non-polar region on the outside as a result they can merge with membrane lipids • Binds to drug (non-covalent forces) & transport across the membrane “Carrier” Mediated Transport Most of vitamin B group is transported by carrier proteins. Does it make sense to prescribe a SINGLE LARGE dose of these vitamins ???? Rate of Diffusion • Move in both directions • “Structure specific” binding of substrates by proteins • Process is capacity limited • Competitive process since transporters limited Drug Conc “Carrier” Mediated Transport • Carrier mediated transport can be used to transport drugs as well • Drugs similar to “internal Substrates” are also transported by proteins • Anti-cancer drugs such as 5-FU, 5-BU are transported by this mechanism • Important mode of transport for Monosacharides, amino acids, vitamins “Carrier” Mediated Transport Is Energy Involved in Transport ?? NO Facilitated Diffusion YES Active Transport “Carrier” Mediated Transport Facilitated Diffusion • Carrier proteins move substance along the concentration gradient • NO energy is involved • Driving force is concentration gradient • Process not inhibited by chemicals blocking energy production • Limited importance for transport of drugs • Examples: • Entry of glucose in to RBC • Intestinal absorption of Vitamin B1, B2 • Intrinsic factor (Transporter glycoprotein secreted by stomach) complex with B12, transported across intestine “Carrier” Mediated Transport Is Energy Involved in Transport ?? YES NO Facilitated Diffusion Active Transport YES Primary Active Transport Is ATP directly needed for transport ?? NO Secondary Active Transport “Primary” Active Transport • Primary active transport use ATP energy to move substances across membrane • Move only in one direction (uniporter) “Primary” Active Transport • Primary active transport use ATP energy to move substances across membrane • Move only in one direction (uniporter) • ABC (ATP-binding cassette) Transporters • • • • • Mechanism to protect from outside molecules Transport small molecules (Drugs & toxins) Mostly efflux out of cells P-glycoprotein well known efflux transporter Known to efflux drugs out leading to drug resistance “Active Transporters” Active Transport Features • Molecule transported against concentration gradient • Need energy to transport (typically provided by ATP) • Process is faster that passive diffusion Endogenous molecules Transported • Na, K, Fe • Glucose • Certain Amino Acids Drugs Transported by Active route • 5-FU, 5-BU via pyrimidine transport • Methydopa, levodopa via Amino acid transporter • Enalapril via peptide transporter “Carrier” Mediated Transport Is Energy Involved in Transport ?? YES NO Facilitated Diffusion Active Transport YES Primary Active Transport Is ATP directly needed for transport ?? NO Secondary Active Transport “Secondary” Active Transport Driving ion Driven ion • No Direct requirement of ATP energy • Energy obtained from moving “driving ion” along conc gradient • Movement of “driven” or “passenger ion” can either direction “Secondary” Active Transport • SYMPORT • Direction of Driving & Driven ion is same • Ex. Na+/glucose cotransporter (SGLT1), found in the small intestine and kidney proximal tubules, simultaneously transports 2 Na+ ions and 1 glucose molecule into the cell across the plasma membrane • Transporter uses the conc gradient energy of Na to move glucose against conc gradient New Drugs developed that inhibit “Secondary Active Transport” FORXIGA® in Type 2 diabetes FORXIGA® is indicated in adults aged 18 years and older with Type 2 diabetes mellitus to improve glycaemic control as: Monotherapy: When diet and exercise alone do not provide adequate glycaemic control in patients for whom use of metformin is considered inappropriate due to intolerance. Add-on combination therapy: In combination with other glucose-lowering medicinal products including insulin, when these, together with diet and exercise, do not provide adequate glycaemic control. FORXIGA® is not indicated for the management of obesity or high blood pressure. The use of FORXIGA® with pioglitazone is not recommended. FORXIGA® has not been studied in combination with glucagon-like peptide-1 analogues. Reference: 1. FORXIGA®. Summary of product characteristics. Bristol-Myers Squibb/AstraZeneca EEIG, 2013. Cookie notice Privacy notice Terms of use Compliance Site map © 2014 Bristol-Myers Squibb | Last updated: January 2014 | 732HQ13PR09408 “Secondary” Active Transport • ANTI-PORT • If direction of Driving & Driven ion is OPPOSITE • Ex. Na+/Ca2+ exchanger (NCX), found in cardiac muscle cells and elsewhere • Transports 3 Na+ ions into the cell in exchange for 1 Ca2+ion transported out of the cell How are drugs transported across cell membrane ? Two major mechanism by which the drugs are transported across cell membrane TRANSCELLULAR (INTRA) Transport ACROSS the membrane PARACELLULAR (INTER) Transport BETWEEN the membrane cells Paracellular Transport PARACELLULAR (INTER) Transport BETWEEN the membrane cells • Transport of drugs through the junctions between the GI epithelial cells • Which drugs ?? • Only for Small molecules • Mostly for lipophilic drugs • Tight junctions are negatively charged & prefer positive charged molecules Paracellular Transport • Two major mechanism of transport between the cells 1. Transport BETWEEN the membrane cells Tight Junctions 2. Transport BETWEEN the membrane cells at temporary opening due to shedding of cells (Persorption) Paracellular Transport Can we modify these tight junctions to increase drug transport ??? Transport BETWEEN the membrane cells Endocytosis, Vesicular Transport • Transport by engulfing extracellular material & forming vesicle • Energy dependent process • Trans-cellular route • Phagocytosis (cell eating) is for solid particulates • Pinocytosis (cell drinking) is for liquids • Examples: • Macromolecules : Starch, fats • Oil soluble vitamins • Drugs such as Insulin Summary of Different Transport Process Passive Diffusion Drugs with high lipophilicity, Most drugs use this mechanism (100-400 MW range) Pore Transport Water soluble drugs (<100 MW) Ion-Pair transport Drugs that ionize at all pH, complex with other ions, gets transported Carrier mediated Structure specific transport by specific proteins from specific sites rich in these proteins Endocytosis Macromolecular drugs (large drugs) and nutrients Single drug can be transported by multiple mechanism for ex, Vitamins are transported by passive diffusion as well as carrier mediated process Road to systemic circulation for Oral drugs • Hepatic portal vein carries blood from GI tract to liver • Drug undergoes first pass metabolism before entering general circulation • Minor lymphatic pathway for substances absorbed at GI • More important for large molecules (>10,000 Da) THANK YOU -PHARMA STREET