Chemistry 106: Drugs in Society Lecture 20: How do Drugs Elicit an Effect? Interactions between Drugs and Macromolecular Targets II 5/11/18

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1 Chemistry 106: Drugs in Society Lecture 20: How do Drugs Elicit an Effect? Interactions between Drugs and Macromolecular Targets II 5/11/18 By the end of this session, you should be able to 1. Define enzyme inhibitor and know some drugs act as enzyme inhibitors 2. Know competitive inhibition can occur at both enzymes and receptors our 2 main targets for drugs and that since both are proteins with complementary binding sites for the molecules they are meant to bind, they both show similar inhibition curves 3. Know some drugs do not fully activate receptors while they are in the receptor binding site; these partial agonists may actually reduce the effect of full agonists Rationalize why Talwin should not be given to individuals with break through pain who are on morphine Rationalize why Buprenex might be a better choice for managing heroin withdrawal than either methadone or Narcan 4. Of the following classes of antihypertensive drugs: ACE inhibitors, AT blockers, and -blockers, know which work by blocking enzymes and which work by blocking receptors

2 Since we now have a feel for how enzymes and receptors function naturally, let s consider what happens when we start raising the concentration of drug in the [noncellular] portion of the bloodstream the plasma (Cp). Competitive Enzyme Inhibition We typically block enzymes, and the percentage of the total enzyme available that is blocked is a function of Cp, and may be described by a Ki for a given drug V velocity S S max K m Where Km = Km(1 + I/Ki), and I is inhibitor concentration, Ki tells what inhibitor concentration is necessary for inhibition essentially how well the inhibitor binds If concentration of inhibitor I is 0 then K m = K m and the enzyme functions based on the amount of substrate available If concentration of natural substrate is huge by comparison, you compete away any inhibiting drug If K i is tiny, then any concentration of I will generate a much larger K m and the enzyme would stop working since S would be small by comparison Here s how it looks graphically

3 Go figure it looks just like the curve for inhibiting receptors on the next page. Both are proteins, both have specific sites that bind to naturally occurring molecules Receptor Inhibition Considerations For receptors the situation is a bit more complicated, since drugs can act as full mimics of the receptor and activate it (agonist), bind and prevent the binding of agonists and endogenous ligands (antagonists), or bind and activate the receptor but not very well (partial agonist) By looking at cellular effect, we account for spare receptors and can focus on what the drug is doing to the cell (and thus tissue, organ, system, person) Compare the concentration that provides an effect in 50 % of the cells C and the concentration needed with an inhibitor C = C(1 + I/Ki) to that for an enzyme inhibitor in the graph below once again, we are blocking the site where the natural molecule would bind o If one where to consider the binding of inhibitor to a receptor as temporarily removing the receptor, then this would be akin to instantaneous receptor down-regulation (though there are no instances I know of where antagonizing a receptor acts to raise the number of those receptors odd, eh?)

4 If concentration of inhibitor I is 0 then C = C and the receptor functions based on the amount of ligand available If concentration of natural substrate is huge by comparison, you compete away any antagonizing drug If Ki is tiny, then any concentration of I will generate a much larger C and the receptor would stop working since C would be small by comparison The effect of adding a partial agonist is complex, but is easily understood when you consider o The effect on the cell should initially decrease, since you are replacing a strongly activating molecule with a weakly activating molecule o The effect will never be completely lost as the partial agonist will exert an effect

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6 Q: Pentazocine (Talwin ) is a partial opioid agonist that can be used in the treatment of pain. Why should pentazocine not be administered with a strong opioid agonist such as morphine? Q: Buprenorphine (Buprenex) is a partial opioid agonist that has been shown to be effective in the treatment of heroin withdrawal. Why would this compound be a better choice than an antagonist such as naloxone? Q: Methadone is considered a full opioid mu receptor agonist. Which do you think would be a better choice for opioid succession, buprenorphine or methadone?

7 As an example of drugs that act on receptors and enzymes, let s return to the problem of hypertension (high blood pressure) and its treatment

8 Not only does cortisol promote hypertension, so does adrenaline, and is useful in treating patients who are suffering a hypotensive crisis (blood pressure too low). It constricts the smooth muscle of the cardiovascular system by activating the - adrenergic receptor (which we used as an example of receptor desensitization) However, with the prevalence of hypertension (blood pressure too high) in western societies, some of the most widely prescribed drugs are the -blockers, such as atenolol (Tenormin ), metoprolol (Lopressor ), and propranolol (Inderal ) Clearly, appropriate use of drugs is circumstance dependant give someone who is having a hypertensive crisis (sustained systolic blood pressure > 180 mm Hg, or diastolic blood pressure > 120 mm Hg) adrenaline and you could kill them In keeping with our previous discussion on receptor reregulation, one of the main causes for hypertensive crisis is discontinuation of antihypertensive medications (another is the use of stimulant drugs as a rule, adrenergic agonists) What do you think would happen if you gave someone stimulants (or adrenaline) and -blockers?

9 More recently, we have learned how intercede in the main regulatory pathway for the regulation of fluid balance, as shown below Angiotension Converting Enzyme (ACE) Inhibitors ADH = anti-diuretic hormone; blocking this system results in lowering of blood pressure The angiotensins are peptide signaling molecules, with angiotensin II producing the bulk of the effects, they bind to AT receptors of which there are 2 flavors, AT1 & AT2 o So there is a potential to block the enzyme that converts angiotensin I to angiotensin II - Enalapril (Vasotec ) does this - or to directly block the receptors angiotensin II binds - Losarten (Cozaar ) does this. In either case, the signaling pathway leading to the release of ADH is inhibited, more fluid is excreted, and blood pressure goes down

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11 ACE (B below) belongs to the (large) family of enzymes known as the serine proteases, Carboxypeptidase (A below) being a classic, well defined member and very similar The ACE inhibitors are able to act as transition state mimics; as such ACE binds them tightly There are other drugs that are routinely used to affect hypertension by acting directly on the [calcium channel receptors in] heart (e.g. s diltiazem (Cardizem ), amlodipine (Norvasc ), Nifedipine (Procardia ) and some that promote fluid loss directly from the kidney such as furosemide (Lasix ). All represent attempts to intervene on the major cause of morbidity and mortality in western civilization, heart attack and stroke. The question then becomes have we become overly reliant on these medications since we know they are available? Recall how Ayurvedic medicine and the Asclepiads empirically administered changes to diet, exercise, meditation and rest - after all this time, lifestyle changes are still some of the best advice medicine has to offer