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Drug absorption Drugs need to … • Be effectively absorbed • Be properly distributed • Remain to produce an effect Developments towards efficacy… • Capsules – against degradation by gastric acid • Modified –release tablets- extend action duration • Prodrugs- precursor (forerunner) of a drug. It must undergo • • • • • chemical conversion by metabolic processes before becoming an active pharmacological agent using body processes to convert agents into active compounds patches for trans-dermal delivery Dermal implants Inhalers Intrauterines – long term drug delivery in women Dosage- adjustable self- injection devices Examples - prodrugs • E.g. Sulfasalazine is a prodrug. It is not active in its ingested form. It has to be broken down by bacteria in the colon into two products -- 5aminosalicylic acid (5ASA) and sulfapyridine -before becoming active as a drug. MR Tablets • Gliclazide 30mg • used in diabetes mellitus • increases the amount of insulin released by the pancreas and helps the body use insulin more efficiently. Dosage- adjustable self- injection Nutropin AQ Pen® • delivers liquid GH (growth hormone) from a prefilled cartridge • For children/ teenagers with growth failure How drugs are absorbed… • • • • • Passive diffusion down a concentration gradient Cell membrane and fat-solubility of drugs Active transport Disintegration and dissolution of tablets Presystemic metabolism Passive diffusion • Drugs are absorbed from intestine, skin, mucous membranes by diffusion across cell membranes through the concentration gradient ( the movement of a substance from a higher to lower place concentration) Cell membrane and fat-solubility of drugs • Oral drugs must cross the cell membranes of the intestinal villi • Fat soluble molecules can pass directly through the cell membrane. • E.g. instantaneous absorption of anti-anginal drug GTN ( glyceryl trinitrate )across the buccal mucosa - lining of the cheeks and the back of the lips, inside the mouth where they touch the teeth. • The spray is absorbed almost as fast as an i.v. injection Active transport • Cells have active transport mechanismscarriers transporting ions, sugars, amino acids in and out of cells that can move drugs from low to high concentration • E.g. levodopa (to treat Parksinson’s disease) • Fluorouracil ( anti-cancer drug) are actively transported across the intestinal mucosa. • N.B – important to know that some cardiac drugs act by increasing or decreasing cellular transport mechanisms Disintegration and dissolution of tablets • Plain tablets disintegrate then dissolve in stomach • For maximal concentration in small intestine, a drug should be taken before food Presystemic metabolism • Break down of drugs by enzymes- mainly in liver and some in intestinal mucosa, lungs and skeletal muscle • Extensive metabolism results in decreased plasma concentration of drug. • Drug dosage already accounts for the calculation
