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Pharmacology and Toxicology
PHAR 100
Hisham Elbatarny

Drug Absorption, Distribution, Metabolism and Excretion Nature of Drugs • Receptors – a specific macromolecule that has a regulatory role in the homeostatic processes • most drugs influence these receptors • in a few cases, drugs may be non-specific in their interaction – for example, osmotic diuretics and general anesthetics. • enzyme inhibitors → enzyme = receptor Ex:MichaelJackson, both drugs allosteric activators of eachother Definitions • agonist: produces a response • competitive inhibitor: blocks the response to an agonist (compete for receptor) • allosteric activator: alters conformation of the receptor, allows agonist to be more effective • allosteric inhibitor: changes conformation of receptor so agonist less effective • drugs are administered to achieve a therapeutic response • drugs bind to receptors in a reversible manner • drug effect is related to concentration of the drug at the site of action– receptor. • cannot measure drug concentration at receptor – hence, use plasma concentration and relate it to response Drug distribution Toxic Concentrations Onset ofToxicity Drug Concentration Therapeutic Range in Plasma Minimum Therapeutic Concentration Ineffective Concentrations Drug concentration-pharmacologic effect relationship • Dosage regimens are established to produce plasma drug concentrations in the therapeutic range • Interindividual differences in: absorption, distribution, biotransformation and excretion require dosage adjustment to maintain therapeutic drug concentration. Concentration at site of action determined by • Amount to be administered • Extent and rate of absorption • Redistribution and localization in other tissues: how much gets into fat, muscle, etc. • Biotransformation (inactivation) • Excretion: two majo mechanisms ◦ bio transformation / metabolism: drug is taken by liver (usually) metabolized and made more water soluble → puts back into blood ◦ blood passes through kidney, b/c water soluble, kidney can now excrete in kidney Membrane permeation Absorption, distribution and excretion all require drugs to cross membranes. (lipoid in nature) Processes are: A- Filtration via pores B - Passive diffusion C - Active transport D – Pinocytosis Simple diffusion Drug less than Has to be Carrier mediated molecular weight of sufficiently transport: requires Pinocytosis: drug engulfed 150 lipid soluble ATP and then released on the other side Major way Active transport 2 major route • filtration via pores: Small molecules can pass through spaces between cells • passive diffusion: membrane is lipid in nature and drugs dissolve in the lipid and move across the membrane ◦ go from high concentration to low concentration (conc gradient) • active transport: carrier proteins transport drugs across the membrane, needs energy (ATP) ◦ important for kidney and liver • pinocytosis: cell membrane engulfs the drug and carries it to the other side of the membrane and releases drug Absorption by Lung • drugs which are gases can be absorbed by the lung • anesthetics are administered by this route (at least some are) • transferred from lung to the blood and carried to the brain until the conc. in the brain is sufficient for anesthesia • also a route for toxicants – pollutants in the air • may also administer a drug for local effect and sometimes systemic effect Absorption by the Enteral Route • by rectum: most drugs not well absorbed via this route – use when oral route not available • by mouth: most drugs administered by this route – most convenient (~90%) • steps for a tablet – disintegration, dissolution, and absorption (usually passive diffusion) • food may interfere with absorption • small intestine has large surface area – villi Oral Dosage forms • to exert effect a drug must: ◦ be taken: compliance → do they take their drug the way its supposed to be taken (60-65%)
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