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WEEK 3:
Transfer of drugs across biological membranes
| Question | Answer |
|---|---|
| residence time meaning | time that drug stays within the membrane |
| pKa | ionisation constant/ acid dissociated constant (determining strength of an acid) |
| weak acid drugs examples | aspirin, amoxicillin, theophylline and warfarin |
| weak base drugs examples | codeine, cocaine, atropine, adrenaline |
| lower pKa | acidic |
| higher pKa | basic |
| weak acids | good absorption from stomach and intestine and is largely present in ionised form at blood pH (unlikely to enter brain easily) |
| weak bases | negligible absorption from stomach but good absorption from intestine and largely present in unionised form at blood pH (likely to enter brain and likely to be secreted into stomach and saliva) |
| weak bases are likely to be secreted into | stomach and saliva |
| weak bases are likely to enter | brain |
| weak bases are largely present in what form at blood pH | unionised |
| what organ has good absorption with weak bases | intestine |
| what organ has negligible absorption with weak bases | stomach |
| what organ has good absorption with weak acids | stomach and intestine |
| weak acids are largely present in what form at blood pH | ionised |
| weak acids are unlikely to enter | brain |
| stomach area for absorption | smallish surface area for absorption |
| pH in stomach | gastric pH of 1.5 |
| weak acids in stomach are predominantly | unionised and absorbed |
| weak bases in stomach are predominantly | ionised and not absorbed |
| presence of food increases what | time in stomach |
| acid stable drugs have | slowed absorption |
| acid labile drugs | stay longer and so more readily broken down |
| PPIs | acid labile and so are given in gastro-resistance capsules |
| examples of drugs taken before food | flucloxacillin and omeprazole (preferably in the morning) |
| omeprazole are coated in | acid resistance capsule (enteric coated) |
| residence time in small intestines | long |
| pH in small intestines | 5.7 to 7.3 |
| what happens to most weak acids and weak bases in small intestines | most weak acid and weak bases absorbed as high levels of uncharged drugs |
| buccal absorption | under the tongue (sublingual) giving rapid absorption for rapid effects and avoid first past metabolism |
| example of drug given buccally | glyceryl trinitrate |
| percutaneous absorption (absorption through skin) | dermis almost totally permeable to drugs and epidermis behaves as a simple lipid barrier so drugs intended for topical use may be absorbed |
| important of dermis integrity | can affect drug absorption as it is almost totally permeable to drugs and epidermis behaves as a simple lipid barrier |
| example of drug with problems with percutaneous absorption | doxepin cream (drowsiness) |
| difference between inflamed and disease skin with normal skin | inflamed/ diseased skin is more permeable to drugs that normal skin |
| examples of passive diffusion for drugs | inhaled anaesthetic gases (systemic effect) and drugs delivered as aerosols or as particulate inhalation (usually intended for local effect) |
| aerosols | dosage levels usually small (bronchodilators) |
| why is size of particulates important | determines site at which particle is deposited |
| more than 90% of all Beta receptors are located in | alveolar wall (no smooth muscle exists here) |
| salbutamol requires | more peripheral delivery to medium and small airways |
| drug absorption from muscles depends on | drug solubility in intestinal fluid and blood flow |
| patients suffering from anaphylaxis are benefited from | IM route due to systemic vasodilation, where absorption from IM site is more reliable (so given IM adrenaline) |
| most common mechanism of drug transfer | passive diffusion |
| pKa of drug influences | lipophilicity, solubility, permeability, which results in affecting ADME |
| main site of absorption in GIT | small intestines |
| limiting factors for drug factors | pH, lipid solubility and residence time |
| why do weak base drugs have good CNS penetration | |
| omepazole |
Created by:
22tango
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