----start---- kotlikoff 2/20 NSAIDS these area mixed group of agents with different chemical structures, all having similar effects. most of these act by a single mechanism (cox inhibition) but there are some exceptions we'll discuss later. Actions of NSAIDs: they all have peripheral actions at site of inflammation, they're non habit forming. some of them have antipyretic and analgesic effects as well. they relieve pain of low to moderate intensity - they aren't strong analgesics. they relieve pain associated with inflammation, not other kinds of pain. they are antipyretic also by virtue of inhibiting hypothalamic things we'll discuss later. -antiinflammatory - reduce amount of inflammation, probably mainly associated with inhibition of formation of PGE2 and PGI2 at site of inflammation. these two factors do cause erythema and edema. -analgesic - relieve the hyperalgesia associated with inflammation, the sensitization of nerves associated w/inflammation. aspirin and aspirin like compounds inhibit cox at low concentrations and their antiinflammatory potency is associated with inhibition of cox. different agents have different molecular modes of action - aspirin acetylates cox, other drugs do different things, but generally they inhibit cox in some way. these drugs do not inhibit cellular influx associated with inflammation like steroids do, b/c they don't inhibit formation of the 5' lipoxygenase products, the leukotrienes. they're mild analgesics. type of pain is important. they're good at getting rid of pain associated iwth inflammation, musculoskeletal pain, pain associated with mechanical trauma, arthritis. they are acting peripherally at site of inflammation. not centrally. they alter the production of products which sensitize nerve endings. they're used often for postoperative analgesia, not useful as analgesics in relieving visceral pain, pain associated with GI disturbances, mechanical blockages. those cases aren't inflammatory per se, they're more due to mechanical distension, and require other analgesics. -antipyresis: these drugs, some of them, are used mainly as antipyretics, lowering fever associated iwth inflammation, tissue damage, and infxn. neutrophils release pyrogens at site of inflammation which travels to hypothalamus, binds there, and the transducing signal is production of prostaglandin in hypothalamus - and this is inhibited by the NSAID. treatment of animals with prostaglandins are associated with fevers, btw. the NSAIDs don't inhibit fever associated with giving prostaglandins, because they don't get rid of the prostaglandins already made - their antipyretic action is based on blocking the synthesis of prostaglandins in response to endogenous pyrogen release. side effects of NSAIDS: gastric irritation platelet inhibition alteration of renal function it's in these systems, in the gut, platelets, and kidney, that the endogenous production of prostaglandins is important for normal function. in gut, production of PGI2 by mucosa is cytoprotective and is required for normal mucosal barrier. by inhibiting production of this prostaglandin, NSAIDs will tend to render the gastric mucosa more sensitive to low pH, and lead toward gastric ulceration. additionally, most of the NSAIDs themselves are weak acids. these get trapped in the gastric mucosa cells because the intracellular pH is about 7.2, and then they will dissociate in those cells, putting free H+ into those cells. so you're eliminating the cytoprotective effect of local prostaglandins AND you are putting free H+ into the mucosal cells. platelets - aggregation is associted with thromboxane A2 which makes them sticky so they can group together. since platelets have no nuclei, and only have a limited amount of cox, they are particularly sensitive - they can't make more cox to replace that which is inhibited by the NSAIDs. this exquisite sensitivity is exploited by use of low dose NSAIDS to selectively inhibit platelets and have minimal other effects - EOD aspirin dosing to reduce risk of heart attack, for example. reduces risk of thrombus formation. renal function - renal blood flow is normally regulated by prostaglandin formation. this is a common form of side effect, not only w/ aspirin but many other NSAIDs, associated w/disruption of osmotic gradient of kidney. may induce renal failure in those with compromised kidneys, may induce renal abnormalities, prerenal azotemia, decreased renal function in normal individuals. sensitivity to this effect varies. rarely a nephropathy, a tubular nephropathy, is also seen with NSAIDs. to summarize then, the side effects - we see that normally prostaglandin production results in thromboxane A2 in platelets, PGI2 and maybe PGE2 in gut, and PGI2 and E2 in kidney - these are all important for normal physiological function, and blocking their production with NSAIDs can have serious effects as discussed above. now, we know from the last few years of research that there are two types of cox, cox1 and cox2. cox1 is the normal constitutive enzyme found in nearly all cells in the body. it's the one that promotes production of the normally required prostaglandins. regulation of GI protection, platelet and kidney function is done by this cox1. now, the other cox, cox2, is induced in inflammatory cells at the time of inflammation. signals induced attime of inflammation cause inflammatory cells to make cox2, and then to produce prostaglandins, and release them into the inflammatory site. these then cause inflammatory reactions in tissues. we'd like to just block cox2, and not block cox1. this would give us NSAIDs with far fewer side effects. currently, most drugs are more selective and potent at cox1 than cox2. see p 3 d in handout - cox1 is the place where most NSAIDs are currently more potent, cox2 is what we'd prefer to selectively inhibit. please correct your handout. IC50 - concentration at which it causes inhibition of 50% of enzyme aspirin IC50 is 1.67 at cox1, and 278 at cox2- you need 200 times the dose to inhibit cox2. it's much more potent at cox1. this is true of pretty much all the NSAIDs currently in use. there are knockout mice now, missing cox1 or missing cox2, and their phenotypes are not what you'd expect with respect to normal function and inflammation, so this cox1/cox2 thing may not carry over completely into all domestic spp either. aspirins: aspirin is the rodney dangerfield of pharmacology - it gets no respect. it's used commonly, often dismissed b/c it's so commonly used. the clinical differences between the NSAIDs are very slight. the main reason aspirin is a problem in people is GI irritation - but a lot of people can take aspirin with no problem. if you are able to tolerate aspirin, it's very effective, and you can use it instead of more expensive drugs. it has a long history - was used by that guy Celsius in 30 AD to treat his rubor,dolor, etc.it's rapidly absorbed by the stomach. it's excreted by the kidney, and excretion can be markedly increased - changing the urinary pH from 6.4 to 8 results in 4-6 fold increase in excretion of aspirin. aspirin is the classic pharmacokinetic problem in vet med. it varies in half life of elimination from horse, where it's so short as to be virtually ineffective, to the cat, where it's so long that it's very dangerous. t1/2 horse: 1 hr - not effective dog 8 hr - similar to people cat 38 hrs - toxic the problem is, it can saturate the elimination processes quickly and then move into zero order kinetics. as the concentration of a drug increases, normally the rate of elimination increases, in a linear fashion - until you saturate the elimination mechanisms. then you go to zero order, and have a fixed rate of elimination no matter how much more drug you add.this is true for aspirin esp in the cat. repeated dosing in the cat is quite dangerous. see p 4 of handout for table of doses for various spp. toxicity is as you'd predict - gastric ulceration, bleeding, renal dysfunction. at high concentrations, can be lethal - renal and respiratory failure, vasomotor collapse, coma. children who think aspirin is candy develop metabolic acidosis, vomiting, CNS signs, and death. aspirin IS a safe NSAID in the cat, if used at proper dosage. all the problems result from improper doses. usually the dose in the cat is a tiny amount every other day. it's a recommended drug for thromboembolic disease, hypertrophic cardiomyopathy. it is safe when used properly in cat and is probably the ONLY NSAID safe in the cat! aspirin in the horse isn't very effective. other compounds are better. in cow, high rumen pH renders absorption so poor that it's also not very effective. other NSAIDS: ibuprofen, naproxen, piroxicam, etc. most non-aspirin NSAIDs cost 10x or more the cost of aspirin. many of these drugs in domestic spp are also associated with toxicity. in dogs, aspirin is associated with high incidence of GI ulceration. but, you can give prostagandins for cytoprotection....however, the other NSAIDs are also associated with some toxicity,and there isn't really good clinical data in vet med to guide us about relative incidence of toxicity with these various drugs. ibuprofen vs naproxyn vs aspirin - we're not sure. there are incidental reports associating ibuprofen with toxicity in dogs - but these are often involving inappropriate doses. so. starting with proprionic acid derivatives like ibuprofen and naproxyn - toxicity has been reported. it's approved for use in the horse, and has relatively short half life in the horse, but a long half life in the dog - if dosed at human doses ibuprofen quickly results in toxicity in the dog. these are cox inhibitors with higher affinity for cox1 than cox2, and are generally associated with similar side effects, although in people and perhaps dogs, gastric irritation may be less common. another common class of drugs are phenylbutazone type drugs. bute isn't used in humans because it may cause aplastic anemia in people. aplastic anemia is rare but fatal, so you shouldn't take bute. it's an effective drug in the horse and dog. it's associated with some mystique - jockeys often take it because they think it's "better" than aspirin - but it has the same mechanism of action and the same toxicity. high doses cause a high incidence of gastric and oral lesions in the horse. at high doses, or a bit above the recommended dose, ulcers and even fatality are common. not for use in food producing animals. dipyrone is related to this drug - is used princicpally as an antipyretic. paraaminophenyls (??)- acetominophen/tylenol - mild analgesic. toxic in cats under all circumstances - never for use in cats. unsafe at any dose. read handout for more info on that. tx of toxicity is to try to give cysteine,which is the precursor of glutathione, to help the cat metabolize it more quickly. fetamates - flunixin meglamine/banamine - commonly used in horse. advantages over bute are that it can be used IM, whereas bute is only IV or oral and is irritating to tissue. also, banamine has longer duration of action than bute. potency is something you'll see referred to a lot - banamine is twice as potent as bute, or whatever - this isn't an advantage. side effects will vary along with potency. consider relative costs and effective doses. if your effective dose costs the same, there's no advantage to potency. DMSO: dimethylsulfoxide. not approved for use in humans except to tx chronic cystitis. used in vet med a lot, as locally applied substance - it's an industrial solvent that increases local blood flow. mechanism of action poorly understood,but not a cox inhibitor. approved for use in dogs and horse. human athletes have approached vets to get it. it may be a free radical scavenger. has been associated with teratogenesis in pregnant animals. use gloves with it. it's so lipid soluble that if you touch it you immediately achieve blood levels. pouring it on an animal is basically equivalent to giving it IV. summary: NSAID mechanism of action - cox inhibitors. know about cox1 (constitutively expressed) and cox2 (induced by inflammation) gastric/platelet/renal side effects of all these drugs (except DMSO) - why they occur - inhibition of prostaglandin formation. aspirin - pharmacokinetics vary greatly - slow metabolism in cat but safe w/appropriate doses; rapid metabolism in horse, ineffective. high cost benefit ratio in patients that tolerate it. bute dipyrone both important in vet med. NSAIDs which are similar to the others, but with relative adv/disadv. flunixin meglumine - should know, commonly used in lg animals acetaminophen - mild analgesic, not much antiinflammatory property, mainly analgesia. lots of toxicity in cats. DMSO - odd man out. not cox inhibitor. ---break--- fluharty Autacoids: brief overview- handout that we have isn't complete. we're going to get another handout that should be integrated into this handout. autacoid is a term that describes a diversity of chemical compounds that regulate locally tissue and organ function, usually related to things like inflammation, control of circulation, glandular secretions. often used interchangably with paracrine. really, all chemical compounds have a spectrum of activity - at one end, is the classic NT that is released from a neuron into a synapse to regulate function, and at the other end is the hormone that gets released and goes all over.those two classes of chemical messengers have existed for a long time. but there are also these intermediate compounds of things that aren't NTs, and aren't hormones - we call them all autacoids. like all classification systems, they do break down at some point. many compounds fall into more than one classification - may act as a hormone and as a paracrine secretion. or may have well established role as NT and also act as autacoid - like serotonin/5 hydroxytryptophan. so boundaries are artificial. most compounds have multiple roles. regardless, in general, we're talking about compounds that locally as well as globally to participate in circulatory control, glandular secretions, inflammation, and so forth. Renin/AT system: a classic hormonal system known for at least 100 years. in 1900 or so, a lot of groups demonstrated that renal extracts contained a pressor substance - they called it renin. this was a landmark discovery. everyone knew the kidney was the major exocrine gland of the body - but now, we found out that there is also an endocrine role for the kidney! they found something that was produced in the kidney that was released into circulation and elevated blood pressure. they knew it was a protein based on tests they did. it turned out to be an enzyme. we now call this renin. later in the 1940s at the cleveland clinic led by irving page, they proved it was an enzyme and that it produced a biologically active peptide called Angiotensin II which circulated in the body. still, we didn't know the physiological context in which this hormone acted. it wasn't until later that people showed this peptide hormone was ideally positioned to control all aspects of fluid homeostasis - by causing release of aldosterone from adrenal cortex, promoting sodium reabsorption, for one thing. but also there are a whole host of functions that defend ECF volume and composition. so, why are we talking about it as an autacoid? well, it's getting increasingly clear that many other tissues posses their own intrinsic renin/AT system. there are tissues that make their own ATII locally - the heart, some blood vessels, probably the kidney, salivary glands, and also the brain. so this is a classic hormonal system and a paracrine/autacoid system. what is the renin/AT system? AT II synthesis begins with a large prohormone called angiotensinogen which is biologically inert. this is an abundant protein in the plasma,made mostly in the liver and which is also found in those other tissues we discussed that have the intrinsic system. it's an alpha globulin protein. in the amino terminus of this large protein is a stretch of 14 AAs, a tetradecapeptide, where renin acts. renin is the enzyme that starts the enzymatic cascade. it's not only the first but also the rate limiting enzyme in the system. production of AT II in circulation and tissues is governed by activity of the enzyme renin. renin generates AT I, a decapeptide, from the aminoterminus tail of the angiotensinogen. renin release occurs mainly from juxtaglomerular cells of the kidney, where the ascending loop of henle bends back right by the glomerulus before diving back down. these jg cells release the renin under various circumstances. control of renin secretion is partly governed by intrarenal pressure. if BP drops, the stretch receptors detect a diminution in RBF, which represents a drop in BP, and trigger renin release. JG cells themselves may be sensitive to stretch as well. in addition, when BP drops, that activates the sympathetic NS, resulting in renin release, because there's sympathetic innervation of the JG cells. finally, we know the JG cells sense sodium levels in the lumenal fluid of the tubular network. if sodium drops, that also triggers renin release. JG cells sense blood volume, pressure, and sodium levels and respond by releasing renin, the rate limiting enzyme in angiotensin production. this system functions to defend ECF volume and composition in pretty much any imaginable way. anyway, AT I is a precursor with no biological activity. the common route for production of AT II is to have AT I acted on by an enzyme called ACE - angiotensin converting enzyme. that name is kind of a misnomer, in that it implies specificity -but ACE is really a dipeptide carboxypeptidase. it can remove AAs in a pair from a whole variety of compounds, by acting at the carboxy terminus. it's not specific to this renin/ATsystem. it is also involved in regulating bradykinin functions. however, it does function to cleave off the two peptide pair from AT I to make AT II, an octapeptide. ACE is found in many places, esp the lumen of endothelial cells, and all the organs where AT is locally generated. it's very abundant. that's the traditional route of making AT II. there's another way - it involves action of an aminopeptidase immediately on ATI, which makes a nonapeptide by cleaving an aspartate from the first position of AT I. this compound then, when acted on by ACE,bypasses production of AT II, and directly makes AT III, a 7 AA heptapeptide, lacking the aspartate in the first position. most tissues make AT III from AT II by removing the aspartate after ACE does its thing. so, now we know that AT III has some biological activity.in some tissue,like the zona glomerulosa cells of adrenal cortex, AT III is as potent as AT II - for promoting aldosterone release. AT III is not as potent in making vasoconstriction. It is as potent as AT II in the brain though. note that in both instances, the converting enzyme is critical in formation of peptides with biological activity. if you block ACE< you block production of AT II and all that follows, rendering the whole system biologically inert. if you block ACE and the alternative pathway is being employed to make AT III, you still make it void of biological activity, because it is ACE that makes the heptapeptide out of the nonapeptide. blockade of ACE causes substantial blockade of activity in any/all tissues where AT acts. angiotensin II is a really cool peptide - it's like a shoestring, it can form all these different shapes, it's very elastic. it later gets broken down into inactive peptide fragments, which may act on different receptors in different tissues to produce various responses. so that's the system. we know without a doubt that most of the ability of AT to stimulate receptor is in the carboxy terminus. that's why AT III is sometimes as potent - because the amino terminus has little important information. in fact, the phenylalanine in the 8 position is essential for agonist activity - if you replace it with any other AA, you will produce a receptor antagonist. for years,this was the approach taken to produce drugs to treat cardiovascular disease. of course, peptides are lousy drugs, because when given orally, they're broken down by proteases and so forth. the first position has a lot to do with conveying stability - because in most tissues, the aminopeptidase acts first. if you replace the aspartate, it's more stable. if you replace 1 and 8 you get a stable antagonist. but it's still a peptide. we're going to talk mostly about AT II but do not forget that some actions are mediated by AT III. unquestionably, AT II action on vascular smooth muscle is one of the most important actions. on a molar basis, AT is the most potent pressor we know of - causes vasoconstriction and increases total peripheral resistance.when BP is low,renin release is high, AT production is high, and this helps to restore or maintain BP in conditions of hypovolemia/hemorrhage. also acts on adrenal cortex to promote release of aldosterone, which promotes Na+ conservation in late distal tubule. also can act directly in kidney to promote Na+ retention and water retention in proximal tubule independent of aldosterone. and, in brain, either b/c locally generated AT is at work, or because it's crossing BBB, it causes thirst, salt appetite in most animals except carnivores, a pronounced central pressor response, and release of vasopressin (ADH) which acts on kidney to promote H2O reapsorption, and release of ACTH, which is critical for allowing aldosterone release to continue. these effects fit into some complex sequence of events involving the overal defense of ecf volume, composition,and so forth. all the known stimuli for aldosterone also promote angiotensin formation. this helps maintain BP and BV - removing the stimulus for renin release. AT may also act directly on kidney to promote reabsorption of H2O andsodium. AT by itself has small effects on cardiac contractility - works mostly on blood vessels, and if anything causes a reflexive bradycardia. AT plays a role in regulation of BP even when renin levels are normal. it's tonically active at low levels. therefore, drugs blocking this system are useful in people with low renin hypertension as well. so, the system is clearly involved in facilitating sympathetic activity. so, sympathetic neurons cause renin release, and AT II enhances release of norepi and epi from postganglionic neurons and chromaffin cells, and sensitizes tissues to the catecholamines. this is important positive feedback, key in handling extreme hypotensive hemorrhage situations. but when inappropriately activated, it's a disaster. if you start w/normotensive state, you get dangerously high BP. renin/ATsystem affects CNS function, and all these functions are equally important - behavioral changes - thirst, salt appetite (in all animals but carnivores) and release of vasopressin and ACTH which are critical for preservation of blood volume. then, there's the central pressor response which isn't fully understood - it elevates sympathetic function in a descending fashion, ultimately promoting endocrine driven increase in BP. it does all these things by acting at two main types of receptors. Type 1 and type 2. forget about 2 - expressed in brain but we don't know what it does or how it works. so, type 1 AT1a and 1b areexpressed a lot in peripheral tissues, and is the target for many drugs. AT binding here produces vasoconstriction and increases BP. all these receptors are G protein coupled receptors with various transduction mechanisms including PI hydrolysis, adenylate cyclase, and many others. DRUGS... drugs that inhibit the renin AT system. there is no reason to facilitate the system - it's pretty capable on its own and doens't seem to need any help from us. the problem is when it's inappropriately hyperactive and you have volume expansion and elevations in BP that are dangerous. the point is to antagonise or turn off the system. early drugs were ones that blocked the converting enzyme, ACE. several years ago - ok, 20 yrs ago, scientists at Squibb found that pit viper venom contained a peptide that blocked action of renin/AT system, and simultaneously potentiated bradykinin activity. they took that peptide and looked at it to figure this out. they were able to literally develop a nonpeptide drug that did the same thing. this was called captopril. there are now many ACE inhibitors based on this type of drug. these drugs are used to treat high blood pressure. in veterinary medicine, we need these drugs as well. captopril was the first highly specific ACE blocker. it's useful in all forms of hypertension, not just high renin hypertension. often combined with diuretic. all diuretics facilitate antihypertensives. also,useful in CHF - clear cardioprotective effect - reduces afterload and preload as all vasodilators do, but better, not fully understood why. there are also drugs that block renin/AT system - non peptides that interact with AT1 receptor, preventing AT II from interacting with it. there are no renin inhibitors. ----end---