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2. Neuropharmacology Intro.docx

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Kristin Hillman

8/23/2013 2:21:00 PM Lipophilic = lipid based (fat loving) – want to be this to get through BBB Drugs and the Body:  Drugs job:  Get the right binding site  Fit snugly  Turn receptor on/off or just occupy the site  PHARMACODYNAMICS  Body’s job:  Try to prevent drug entry to tissue  Manage drug if it does get in (break it down and eliminate it)  PHARACOKENETICS Body does not want drug – is going to try and prevent drug to get in in the first place, if it does manage to get in, its going to try and break you down etc to get you out of the body Pharmacokenetics:  Important for learning how to administer drugs effectively  How much to we need to give (dose?)  How long do we need to wait to  4 subdivisions of pharmacokinetic:  ABSORBTION: o Getting drug into bloodstream (definition) o Measurement for absorption = bioavailability (portion of administered drug that reaches systemic circulation) o What factors affect absorption?  Drug solubility (particle size, ionization, lipophilic) – very soluble = high bioavailability (easily gets in)  Route of administration (e.g. IV injection = 100% bioavailability because it goes straight into the blood)  First-pass metabolism (break down of drug before it even gets into system)  Has to do with taking drugs orally (first-pass)  Take drug  gets to stomach, physical breakdown/enzymatic breakdown/protein- peptidases.  Carries on down GI tract into intestines  Small intestine (SI) = absorption (getting things broken down even further into things that can be absorbed) – important part because this is where drug gets into system  Large intestine (LI) = bringing water absorption  Drug diffuses across intestinal epithelium into hepatic portal circulation.  Blood circulates to liver first (protective part on behalf of body because liver will break down things that shouldn’t be there – safety mechanism – first-pass metabolism = dumped into liver which starts to detox it…this liver stage can really reduce the bioavailability (amount of drug that gets in) – when taken orally  ‘detox’ of blood begins  its up to the drug to make it past the liver stage (sneak past) to make it into the systemic circulation  (e.g.) alcohol dehydrogenases and cytochrome P450 enzymes  DISTRIBUTION: o Movement of drug from bloodstream into tissue o What factors affect distribution:  Drug solubility  Perfusion rate of tissue  Rate of blood flow  Easily distributed in highly perfused organs such as the liver, heart and kidney.  It is distributed in small quantities through less perfused tissues like muscle, fat and peripheral organs  Protein binding (e.g. Albumin)  A drug in blood exists in two forms:  Bound and unbound  Only unbound drug molecules are available to diffuse to tissue sites where the effects of the drug occur, therefore, the unbound drug concentration in systemic circulation typically determines drug concentration at the active site and thus efficacy.  When a drug is free, it can pass back and forth. When its bound up, its not going to be able to push through – its stuck and its not doing to distribute (its not going to have an effect)  Some drugs don’t bind at all.  Tissue sequestering (e.g. Adipose)  If we’ve made our drug purposefully small/lipophilic, it will preferentially go to places that lipophilic is (distributes to fat places etc.)  METABOLISM: o Enzymatic breakdown of drug o First-pass metabolism (one type of metabolism) o Phase 1 (initial phase to break drug so its not active) and phase 2 (stage where something is added on/prep the drug to be eliminated – usually via kidneys) metabolism  Metabolism mostly to prep drug for elimination  But can create toxic intermediates (e.g. paracetamol over dose – enzyme that create toxicity builds up and can kill you)  Can be harmful  Also can create active intermediates  Can also be beneficial  Active drugs = pro-drug (has to be activated)  E.g. metabolism of codeine – way it works is to be metabolized into morphine – helps pain relief. o Half-life (t ½ )  Measurement of metabolism  Time taken to metabolism 50% of drug  Given in time amounts o What factors affect metabolism:  Liver function:  Have to have good function liver to be able to break down drugs appropriately.  Enzyme function:  Genetics (some of us have certain enzymes others don’t have – e.g. by race – genetic background dictates how much and what type of enzymes we do have)  Enviro influences  Workload (if you’re a smoker, it is going to up- regulate enzymes to make up for nicotine effects) – you will have up-regulated enzymes if you have smoked for a while in the liver – why doctors ask if you smoke or not if giving a prescription for drugs.  Drug-drug interactions:  If there is other drugs being taken it is going to affect the metabolism of other drugs taken – might be a lot of workload which affects the metabolism of drugs being taken (some might get though the barrier because workload is too much) o Half-life  Always 50%...  Important because:  Key to effective dosing regimes  Key to effective drug prescribing  Lorazeapam (larger half-life)  Triazolam (smaller half-life/6 hours) o Need something that lasts for the night and not the next day…  Effect which drug you would take for certain needs… e.g. need sleeping drug – wouldn’t take one that has a small half- life or it wouldn’t last the night.  ELIMINATION: o Routes of elimination:  Renal – majority of drugs go through urine  Biliary  Exhalation  Lactation (most drugs can go through breast milk – breastfeeding can affect baby as well)  Perspiration (some things can be eliminated by sweat but would need to sweat liters and litres for it to be effective) o What factors might influence elimination?  Perspiration  Hydration Understanding pharacokenetics key to administering drugs effectively  Minimum
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