Biotransformation: Phase I and Phase II Metabolism. Xenobiotics: Xenobiotics are unwanted chemicals in the environment and the diet

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1 Basic Principles f Pharmaclgy Dr. L. Keshvara ( Bitransfrmatin: Phase I and Phase II Metablism Xenbitics: Xenbitics are unwanted chemicals in the envirnment and the diet that the bdy is expsed t. Plar xenbitics are easily eliminated by the bdy thrugh urine, sweat, etc. Hwever, lipphilic xenbitics require a sphisticated detxicificatin system that can cnvert them t mre plar entities s that they can be eliminated. This cnversin is knwn as bitransfrmatin r simply drug metablism. Bitransfrmatin: The gal f bitransfrmatin is t cnvert lipphilic chemicals t mre plar nes s that they can be eliminated mre easily frm the bdy. Mst f the time this results in changes in the pharmaclgical activity f a drug. Althugh we rdinarily think f drug metablism as a prcess that inactivates drugs, that is nt always the case. In fact, bitransfrmatin can have fur pssible utcmes. These are: Cnversin f an active drug t an inert metablite Cnversin f an active drug t an active metablite Cnversin f an active drug t txic metablite Cnversin f an inactive drug (prdrug) t an active drug The prcess f bitransfrmatin invlves a series f intracellular enzymatic reactins. That means that the drugs that are being transfrmed by these reactins must be imprted int the cells either thrugh passive diffusin r by transprters. The vast majrity f metablic enzymes are in the liver, but they are als fund in ther majr rgans thrughut the bdy, s the GI tract, lungs, kidneys, skin, and even the brain have sme capacity t metablize drugs. Bitransfrmatin is carried ut by tw distinct grups f enzymatic reactins that are knwn as Phase-I and Phase-II. Mst f the time drugs underg Phase-I reactins first and then Phase-II reactins, but it is pssible that a drug underges either nly Phase I r

2 Basic Principles f Pharmaclgy Dr. L. Keshvara ( nly Phase II reactin. Very rarely, a drug might underg Phase II reactin befre Phase I reactin. Phase I and Phase II Reactins: Phase-I Reactins: Phase I reactins invlve functinalizatin f drugs, meaning that either a new functinal grup is added r an existing functinal grup is unmasked in rder t make the drug mre water sluble. This is dne thrugh a cmbinatin f reactins invlving xidatin, reductin, and hydrlysis. Oxidatin: Oxidatin invlves additin f an xygen grup t make the chemical mre water sluble. Here we have an example f an armatic ring underging xidatin, which in this case is hydrxylatin. N-dealkylatin: Oxidative N-dealkylatin is anther frm f xidatin reactin. In these reactins, secndary r tertiary amines underg dealkylatin. In this case, xygen is added t the alkyl grup that s being remved and nt the parent cmpund. Having lst the alkyl grup, the parent mlecule becmes mre water sluble, and the alkyl grup is als cnverted t a mre sluble ketne r aldehyde. Fr example, imipramine is an antidepressant that is cnverted t desimipramine thrugh xidative N- dealkylatin. Example (Prpranll): As mentined abve, bitransfrmatin r drug metablism des nt always result in inactivatin f a drug. Fr example, prpranll underges ring hydrxylatin at tw different ring psitins resulting

3 Basic Principles f Pharmaclgy Dr. L. Keshvara ( in tw different metablites. These are 4-hydrxyprpranll and 5- hydrxyprpranll..bth metablites are nw mre water sluble than prpranll itself. Interestingly thugh, f the tw metablites, the 4- hydrxyprpranll actually retains pharmaclgical activity, s in this case metablism des nt cmpletely inactivate a drug. Micrsmal enzymes: Phase I reactins are carried ut by enzymes that reside in micrsmes. Micrsmes are small self-frming vesicles that are assciated with smth endplasmic reticulum. Thuse, enzymes that carry ut phase I reactins are gruped tgether as micrsmal enzymes. There are many types f micrsmal enzymes invlved in phase I reactins, but ver 95% f the reactins are carried ut by a family f enzymes knwn as Cytchrme P450 enzymes. These are ften referred t as simply P450 enzymes r CYP. The remaining 5% f the reactins are carried ut by flavin mnxygenases, mnamine xidases, and alchl dehydrgenases. Cytchrme P450 enzymes (CYPs): All CYP enzymes cnsist f a cre heme prtein that is similar t that in hemglbin. When bund t carbn mnxide, these enzymes absrb light at 450nm, hence the name Cytchrme P450. The liver is the majr site where these enzymes are fund, but they can be fund in many

4 Basic Principles f Pharmaclgy Dr. L. Keshvara ( ther rgans as well. Even thugh all CYP enzymes cntain the same cre heme grup, they have different surrunding prtein structure s that they have different substrate specificity. Hwever, cmpared t mst ther enzymes which wrk n ne enzyme ne substrate principle, CYP enzymes shw a great deal f flexibility, which is imprtant because nly a few types f these enzymes have t deal with literally thusands f different types f txins. CYP Nmenclature: As far as the nmenclature f these enzyme ges, the CYP prefix is fllwed by a number representing the genetic family, fllwed by a letter that represents the sub-family, and finally anther number that indicates the actual isfrm r gene. S fr example, CYP3A4 is a CYP enzyme f genetic family 3, subfamily A, and isfrm 4. The nmenclature is simply based n genetic relatinship between different enzymes and in n way indicates any functinal relatinship between enzymes.. CYP Enzymes: CYP enzymes are ubiquitus in a sense that they are fund in yeast, plants, and all animals. There are abut 60 different types f CYPs fund in humans but nly a handful are invlved in drug metablism. Mst f them are actually invlved in bicatalysis f sterids and lipids. As far as drug metablism ges, we nly have t remember six enzymes listed here because they are respnsible fr metablizing mre than 75% f the drugs in the market. These enzymes include CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4. Amng the six cmmn CYPs that we are nrmally cncerned with, CYP3A4 is the majr enzyme because it

5 Basic Principles f Pharmaclgy Dr. L. Keshvara ( is respnsible fr metablizing rughly half f the drugs. A third f the drugs are metablized by CYP2D6 making it the secnd majr enzyme. The remaining CYPs metablize a small fractin f drugs each, but it desn t make them less imprtant because ften they are critical fr inactivating drugs that are nt affected by CYP3A4 r CYP2D6. A drug may be subject t metablism by mre than just ne enzyme, s it s nt just ne drug ne enzyme relatinship. Anther imprtant pint is that sme drugs act as substrates as well as inhibitrs f CYP enzymes, which gives rise t significant drug-drug interactins. CYP3A4: CYP3A4 is mstly present in the liver but is als expressed in significant quantity in the GI tract within the intestinal mucsa where it plays an imprtant rle in the first pass effect which we discussed earlier. Within this same subfamily, there are ther isfrms 3A5 and 3A7 but they play a relatively minr rle as there is a great deal f substrate verlap with 3A4. When it cmes t 3A4, ne imprtant dietary factr is grapefruit juice because flavnids in grapefruit juice are ptent inhibitrs f this enzyme. This wuld lead t decreased 3A4 activity and increased plasma levels f many drugs. CYP3A4 Substrates: CYP3A4 metablizes a wide range f drugs. Cmmn substrates include macrlide antibitics such as erythrmycin and

6 Basic Principles f Pharmaclgy Dr. L. Keshvara ( clarithrmycin, statins (particularly lvastatin and atrvastatin), HIV prtease inhibitrs, benzdiazepines, and calcium channel blckers. An extensive list f substrates and inhibitrs f 3A4 and ther enzymes will be psted in a separate handut. CYP Inducers: An interesting feature f CYP enzymes is that the levels f the enzymes can increase in respnse t a variety f drugs as well as dietary and envirnmental factrs. Sme f these inducers act n specific enzyme while thers induce mre than ne enzyme. CYP2D6 is nt inducible t any significant extent.. CYPs & Drug Interactins: CYPs are the majr surce f drug-drug interactins because they nt nly metablize multiple drugs but are als inhibited and induced by many drugs. A simplest frm f drug-drug interactin ccurs when there tw drugs that are metablized by the same CYP. This cmpetitin leads t less than ptimal metablism f bth drugs and the plasma levels f these drugs are higher than what they wuld be if the drugs are taken individually. Mre seriusly, thugh, a drug might simply inhibit a CYP and decrease the metablism f its substrates. This wuld lead t multiple drug interactins. Fr example, the antifungal drug

7 Basic Principles f Pharmaclgy Dr. L. Keshvara ( ketcnazle is a ptent inhibitr f CYP3A4, s this will affect all drugs that are nrmally metablized by CYP3A4 CYPs: Genetics & Envirnment: When it cmes t CYPs, it s nt just the drug-drug interactins that we have t wrry abut. There are many dietary and envirnmental factrs that can affect CYP-dependent drug metablism. Fr example, grapefruit juice inhibits CYP3A4, chargrilled meats and smking can induce 1A2, while ethanl induces 2E1. In additin, there are pllutants and pesticides that can induce r inhibit CYPs. Cmplicating things even further, CYPs are als subject t genetic variability. Individuals frm different ethnicities may differ in their ability t metablize certain drugs because they have variant frms f certain CYPs. This is particularly true when it cmes t CYP2C9, CYP2C19, and CYP2D6. A patient wh has a CYP that is nt very efficient at metablizing drugs wuld be cnsidered a pr metablizer. On the ther hand, there might be a patient frm a different ethnic backgrund wh has a frm f CYP that is highly functinal and efficient, s that patient may be cnsidered an extensive metablizer. S the genetic variability can have an imprtant impact n drug therapy utcme. Phase II Metablism Phase I and Phase II Summary: Recall that Phase I reactins invlve functinalizatin f drugs thrugh xidatin, reductin, and hydrlysis reactins. Once drugs have undergne Phase I metablism, they becme mre plar and are easier t eliminate. Metablites f many drugs underg Phase II metablism subsequent t phase I metablism. Of curse, sme drugs underg nly Phase I r nly Phase II metablism.

8 Basic Principles f Pharmaclgy Dr. L. Keshvara ( Phase II Metablism: Phase II metablism invlves cnjugatin f drugs r their metablites with highly plar mlecules. This renders these drugs water sluble and are easy t eliminate thrugh the renal r biliary pathways. In these cnjugatin reactins, drugs can be attached t any f the five plar cmpunds, including glucurnic acid, glutathine, sulfate, acetyl, and glycine grups. Generally speaking, phase II metablism renders the drugs r their metablites mre water sluble. What is different abut Phase 2 reactins is that unlike in Phase I where drug metablites ften retain pharmaclgical activity, Phase 2 reactins pretty much inactivates the drug cmpletely, s it s a mre dramatic change in the structural and physichemical prperties f the drug. Anther difference is that Phase II reactins are detxificatin reactins, meaning that the resulting metablites d nt have any txic side effects. Remember, this was nt always the case in Phase I reactins, as smetimes Phase I reactins yield metablites that may be pharmaclgically inactive but are nevertheless txic t the bdy. Since Phase II reactins invlve cnjugatin f drugs t ther cmpunds, these reactins invlve a series f enzymes knwn as transferases, because they transfer these plar cmpunds nt the drugs. These transferases are fund in multiple lcatins but are generally present cytsl and in micrsmes.

9 Basic Principles f Pharmaclgy Dr. L. Keshvara ( Cnjugatin Reactins: There are 5 majr cnjugatin reactins that cnjugate drugs t (1) glucurnic acid, (2) glutathine, (3) acetyl, (4) glycine, and (5) sulfate grups. All these reactins render the target drug mre water sluble. Glucurnidatin: Amng the mst cmmn Phase 2 reactins is Glucurnidatin. This reactin invlves cnjugatin f a drug with glucurnic acid, which is essentially a glucse mlecule with an extra carbxyl grup. The enzyme invlved is UDP-Glucurnsyl Transferase r simply UGT. This enzyme takes mlecule f Glucurnic acid frm UDP-glucurnic acid and transfers it t a target drug. The resulting drug cnjugate has a dramatically increased water slubility. Glucurnidatin is imprtant nt nly fr drug eliminatin but als fr bilirubin excretin. Nrmally, bilirubin, which results frm the breakdwn f red bld cells, is nt very sluble in water and is difficult t remve. Once it is cnjugated t glucurnic acid, thugh, bilirubin is eliminated by bth urinary and biliary excretin. Glutathine Cnjugatin: Anther cmmn Phase 2 reactin invlves cnjugatin f drugs with a glutathine r GSH. Glutathine is essentially a tripeptide made up f Glutamate, Cysteine, and Glycine. The reactin is carried ut by a grup f enzymes knwn as glutathine-s-transferases r GST. In additin t remval f drugs, these enzymes play a critically rle in neutralizing xidative stress as well as carcingenic chemicals befre high enugh levels build up. S the enzymes play a key rle in cancer preventin. Acetyl, Glycine, and Sulfate Cnjugatin: The remaining 3 reactins invlve transfer f either, an acetyl grup, glycine, r sulfate. These reactins

10 Basic Principles f Pharmaclgy Dr. L. Keshvara ( play a relatively minr rle cmpared t glucurnic acid and glutathine cnjugatin. Acetylatin is carried by an enzyme knwn as NAT r N-acetyl transferase als knwn as simply acetylase. Glycine transferases cnjugate certain drugs with the glycine amin acid. Finally, sulfatin invlves cnjugatin f drugs with a sulfate mlecule. All these reactins render the target drug mre water sluble.