Pharmacokinetics - PHARMACOKINETICS Page 1 PK sometimes described as what the body does to a drug refers to the movement of drug into through and out of

Pharmacokinetics - PHARMACOKINETICS Page 1 PK sometimes...

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Page 1 PHARMACOKINETICS PK, sometimes described as what the body does to a drug , refers to the movement of drug into, through, and out of the body the time course of its absorption, bioavailability, distribution, metabolism and excretion (see Drug Excretion ).
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Page 2 DRUG PERMEATION or DRUG UPTAKE It is the passage of drug molecules across biological membranes Process of Uptake of Drug or Drug permeation 1. in between cells (paracellular transport) 2. through cells (transcellular transport) 3. via transporter or carrier proteins a. active transport b. facilitated transport 4. via endocytosis and exocytosis or trancytosis Drug permeation In-between cells Drug Uptake Through Cells It is an aqueous diffusion allowing water soluble, large (abt 30,000Da) and charged drug molecules to pass in between cells It is a lipid diffusion diffusion of lipophilic drugs thru the lipid bilayer Conc. driven a passive diffusion frm high conc. to low conc. Also conc. driven a passive diffusion frm high conc. to low conc. Does not cost the cell energy Does not energy to the cell No competition; so transport of the drug is not limited No competition; so transport of the lipophilic drug is not limited Does not involve the use of aided transport (i.e. carrier transport) Also does not involve the use of aided transport (i.e. carrier transport) Occurs through every biological memb. except thru physiological barriers (Blood-testis barrier, Blood- Brain Barrier) Occurs thru biological membranes including physiological barriers Determined by membrane potential Determined by lipid to aqueous partition coefficient; High lipid to aq. PC favours lipid diffusion Drug permeation via transporters The transporters move molecules into or out of a cell Characteristics a. It is selective ; it can transport only a specific group of drug because of their structure b. There is competition btw drugs of similar structure for uptake by the transporter. So drug uptake is limited c. it is saturable d. E.g. of the transporters ABC (ATP-Binding Cassette) family of transporter esp. p-glycoprotein in the brain, testis and other tissue. it is also used in tumor cells resistant to some chemotherapeutics e. divided into (i) Active transport and (ii) facilitated diffusion Active transport Facilitated Transport a. Not conc. driven ; movt. of molecules occur against a conc. gradient or electrochemical gradient b. It is the MOST RAPID mode of membrane permeation c. it cost the cell energy , making use of Primary transporters (energy-dependent transporters coupled to ATP hydrolysis) and Secondary transporters d. Site of active transport include: Neuronal membranes, Choroid plexus, renal tubular cells, Hepatocytes etc… a. is conc. driven ; movt of molecules is along conc. or electrochemical gradient b. Does not require energy from ATP c. e.g. include movt. of water soluble nutrients into cells (e.g. sugar,, purines, pyrimidine, …) Drug uptake via Endocytosis + Exocytosis (=Transcytosis) A process in which extracellular Macromolecules (MW > 1000) captured
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