Processes of Drugs Metabolism in the Body

Processes of Drugs Metabolism in the bodyAbstr flakeMetabolism of medicates is a complex and study movement deep down the body, occurring primarily in the liver. The aim of metabolic process is to make the dose much(prenominal) frosty to enable excretion via the kidneys. The basic abjectstanding of drug metamorphosis is paramount to ensure drug optimisation, maximum healing(predicate) benefits and a reduction in unbecoming effects. Essenti wholey drug metabolic process is broken down into twain anatomys, seduce I and Phase II. Phase I is c oncerned with the biotrans com smear of compounds, and then broadcastred to Phase II. However, for rough drugs this is the end of their metabolic journey in the body, as they adduce more paired compounds which be readily excreted. Phase II answers be where compounds argon conjugated to produce more pissing meltable compounds for easy excretion. Phase I receptions atomic number 18 dominated by the Cytochrome-450 en zyme superfamily. These enzymes atomic number 18 fix predominantly in the liver, which is the study site of drug trans nameuration. However, drug metabolic process is non localised merely to the liver, there argon other study sites at which this process occurs. Some of these sites intromit the skin, lungs, gastro-intestinal tract and the kidneys close to tout ensemble tissues get hold of the cleverness to metabolise drugs referable to the presence of metabolising enzymes. The nigh strategic enzymes ar the cytomchrome-450 superfamily, which argon abundant in about tissues.In energetic drugs with the energy to reconvert to the spry enkindle drug once metabolised to exert their redress actions ar defined as prodrugs. They atomic number 18 classified depending on the site of vicissitude and actions (gastrio-intestinal fluids, intracellular tissues or blood). This report gives different study examples of such prodrugs and how their metabolism differs within th e body, compargond to their brisk metabolites. Individual drug metabolism whitethorn be affected by variant factors, such as, age or sex. Drug metabolism can cause an maturation in toxcity. The bioactivation of a parent compound can form electrophiles that bind to proteins and DNA. Some of this perniciousness can occur in Phase I metabolism e.g. acet aminic chemical multitudephen. However, in some set toxicity occurs in Phase II e.g. zomepirac, polymorphism can overly cause idiosyncracity of received drugs to be toxic.1.1 Phase IPhase unmatchable, differently know as drug bio change nerve tract is princip every(prenominal)y broken into oxidization, reduction and hydrolysis. A reaction under this phase involves an addition of root O tittle aiming to improve the water solubility of drugs. As the result some metabolites from this phase can be extracted immediately if they are charged enough moreover at measure a single addition of free radical O is non sufficient enough to overcome the lipophilicity of indisputable drugs and thence their metabolite from this phase has to be carried onto phase II for get along reactions. study example of OxidationAccounting for roughly 20 complex reactions the more or less substantial oxidative metabolic course dominating phase I is the cytochrome-P450 (CYP450) monotype Oase carcass touch by C-P450. Located primarily in the liver CYP450 was found to be redeem in all forms of organisms, including humans, plant and bacteria. It is important to note that the liaison of CYP450 goes beyond drug metabolism just now it is excessively touch on in metabolism of xenobiotics, robust soluble vitamin and synthesis of steroids. With substratum specificity of more than 1000 and its ability to produce activated metabolites such as epoxide are the underlying reason for its dominance and grandness in drug discovery. The general appliance the CYP450 monooxygenase oxidization isR + O2 + NADPH + H+ ROH + H2O + NA DP+ ( anatomy 2)From the above formula it can be this reaction is of NADPH (Nicotinamide type A dinucleotide phosphate) and an oxygen molecule dependent. As menti unrivaledd above oxygen is important to increase the water solubility and in the same manner NADPH is in like manner important for oxygen activation and source of electron. excessively important for activation of oxygen is the presence of cystine amino paneling located near the protein terminal carboxyl of CYP450. Among the 500 amino pane of glassulent present in CYP450, cystine has proven to be most important as it activates the oxygen to a greater extend. This is due to the fact that it contains a thiol assort as one of its ligand and it is the thiol which alerts the re occupation.Highlighting the numerous mediate structures snarled as well as range of iron, oxygen and proton (Figure) shows the catalytic conversion required for cp450 oxidation reaction to place. The bandaging of the subst ordination with low birle ferrous CYP450 enzyme induces a change in its active site. This testament effects the stability of the water ligand and de vocalism displace it (shown in the draw from a-b). Containing a high up spin heme iron the enzyme and substratum form a ferric complex. The change in electronic state impart result in the release and transfer of one electron from NADPH via electron transfer chain ( reduction ferric heme iron to ferric state) and thus reduction of the complex. The second electron is transferred when the complex reacts covalently with the oxygen forming a invigorated ternanry complex. Initially the complex is an unstable oxy-P450(diagram d), besides this is reduced to produce ferric peroxide by a loss of an electron. This intermediate is goldbrick lived and sustaines protonation twice resulting in a release one water molecule. Out of the oxygen molecules released one in incorporated in this water molecule and the rest into the substratum. Another method of form ing the iron-oxo intermediate is via the peroxide shunt which elimited steps from C to F.Some of the park addition of oxygen molecule reactions which CYP450 dependent are cognize as epoxidation (of double bond), N-hydroxylation, oxygen/nitrogen/ sulfur dealkylation, s-oxidation, dechlorination, oxidative desulfurisation and aromatic hydroxylation. Note they all follow the same principle of adding oxygen molecule to the substrate. The diagram below provides an example of how these reactions are processedAromatic hydroxylation substrate generally produces phenols such as that seen on figure 3. The production of Phenol can be either via a non enzymatic rearrangement or by Epoxide hydrolase and cytosolic dehydrogenase which entrust ultimately give advance a catechol. The position of hydroxylation depends greatly on the nature of the R- group attached to the ring an electron withdrawing group will position the -OH group on the metha man the electron donating will position it on t he para or ortha. Aromatic hydroxylation also involves a change in NIH shift, which involves the movement and shifting of the R group to an adjacent position during the oxidation. It is important to note that certain substrate for aromatic hydroxylation can also be oxidized via the aliphatic (C-H) hydroxylation. Under such terminal figure the aliphatic C-H) hydroxylation will oxidize it. Aliphatic dehydrogenation can also occur involving electron transfer to the CYP450.Currently more than 50 CYP-450 has been identified in human, withal the bulk of drug metabolism is essentially carried by CYP1, CYP2 and CYP3 families, specially the CYP450-3A. The diagram on the right hand side clearly demonstrate just how much of drug metabolism is CYP450 3A responsibility in comparison to other, business relationship for roughly 50%. Metabolism of drugs given orally are greatly find by CYP450-3A primarily because this enzyme is present in both the liver and catgut and thus providing a barrie r for all drugs before they can enter the systemic circulations, otherwise ordinarily known as number one pass effect. Upon entranceway the drugs are taken up via passive diffusion and/or facilitated diffusion or active transport into the entercocyte where they can be metabolized by CYP450-3A. They can once again be metabolized by the very same enzyme when they enter the liver (hepatocyte) ,which unlike the intestine in order to reach the systemic circulation it is unavoidable. This family of enzymes are also known to be cause of m either serious adverse effects as they are influenced by diet and drug components, hence drug-drug and drug-food interactions is an important factor.Flavin monooxygenases mistakable to cytochrome p450 monooxygenases system,Flavin monooxygenasesalso plays a major office staff in metabolism of drugs, carcinogens and Nitrogen/ sulfur/ phosphorous containing compounds. Also oxygen and NAPDH dependent, Flavin monooxygenases has much broader substrate speci ficity than CYP450. Once they have obtain associated with substrate the flavin monooxygenases is activated into 4-hyroperoxyflavin and unlike CYP450 the oxygen activation takes place without the need for substrate to bind to the intermediate. This pre-activated oxygen means that any compound binding to the intermediate is a substrate to be metabolized. The fact that this enzyme is able to remain stable and lacks any need for correct arrangement and disorientation of the substrate gives it ability to withhold all the energy required for the reaction to takes place and hence as short as appropriate lipophilic substrate beats available it starts the process immediately. Adverse side effects are rarely associated with these enzymes.The binding of oxygen to the reduced flavin is processed via a non-radical nucleophilic displacement. The substrate is oxidized via a nucleophilic attack by the oxygen that is located at end of 4-hyroperoxyflavin. This is then followed by partition of per oxide. The flavin monooxygenase catalytic cycle is finished once the original form of 4-hyroperoxyflavin has been regained apply NADPH, oxygen and hydrogen proton. Note the metabolite product can at any times allow reduction back to its original parent form. intoxicant dehydrogenase and aldehyde dehydrogenaseThese families of enzymes are both zinc containing NAD specific and catalyze the reversible oxidation of alcoholic drink and aldehydes respectively. Grouped into 1-6 intoxicant dehydrogenase, are homodimer that exist in the soluble section of the tissue. It is involved in metabolism of some drugs such as cetirizine up to now it is more predominantly known as alcohol metabolism enzyme specifically ethanol, whether products of peroxides or that of exogenous (i.e administered drugs). It is important to note that although alcohol dehyrogenase is the main metabolic parcel of land for ethanol, however CYP2E1 also plays in its metabolism. CYP2E1 can be induced by ethanol resulti ng in adverse side effects between alcohol and with certain analgesics drugs. alcohol dehydrogenase also metabolizes ethylene glycol and methanol. With a prospicienter half(prenominal) life and rapid absorption from the gut, methanol can result in series of displeasing side effects and metabolic acidosis, hence highlighting the immenseness of alcohol dehydrogenase. Similarly, aldehyde dehydrogenase catalysis the oxidation of aldehyde to its corresponding carboxylic acid. Class one of alcohol dehydrogenase plays a major role in detoxification of anti cancer drugs. Alcohol dehydrogenase is also involved in reduction highway of aldehyde or ketone back to its pharmacologically active alcohol form.Monoamine oxidase and diamineLocated in liver, intestine and kidney as some of its site, this membrane bound enzyme is divided into deuce classes in accordance to their substrates specificity, they are monoamines-A and monoamine-B. Responsible for metabolizing amines via deaminization t o aldehyde, these enzymes are flavin containing enzymes and within their cysteinyl residue the flavin is linked to the covalently bounded flavin via a thioether. Monoamine oxidase has several substrates, ranging from secondary to tertiary amines that have alky group smaller than methyl radical. The general mechanism for this enzyme is the two electron oxidation shown belowR.CH2.NH2 + O2 + H2O R.CHO + NH3 + H2O2 (fig 7)As it can be seen this reaction requires oxygen to react and a hydrogen peroxide is produced as for all one molecule of oxygen is cloaked for every molecule of substrate oxidized (Principle of drug metabolism, 2007). Proportional to the rate of oxygen uptake this is unremarkably use to deduce the rate of reaction. Research has shown that monoamines-A is more commonly involved in oxidation of endogenous substrates such as noradrenalin part monoamine-B which is found mostly in platelets appears to catalyses exogenous substrates such as phenylethylamines. Their comm on substrate is dopamine. Inhibition of monoamine oxidase has long been of an interest for scientist in treatment of several of illness such as depression.Present in liver, lungs and kidney as few of its locations diamine oxidase also catalyses the organic law of aldehyde from histamine and diamines in the same manner.ReductionThis passageway of metabolism is enzymatically the least studied in phase I and yet it plays an important role in metabolism of disulfides and double bonds of for example progestational steroids as well as dehydroxylation of aliphatic and aromatic compounds. In general ketone containing xenobiotics are more readily metabolized and eliminated via this pathway in the mammalian tissue. This is due to the fact that the carbonyl group is very lipophilic, thus the lipophilicity will be reduced and excreta is ensured as ketone is converted to alcohol.One of the major enzymes involved in this pathway is the NADPH cytochrome P450 reductase. Containing flavin adenine dinucleotide and flavin mononucleotide is an electron donor playing an important role in the metabolism of drugs such as chloramphenicol by reducing its nitro group.HydrolysisAs the name suggests this pathway uses water to cause a breakage of a bond. Major enzymes under this pathway are the amide and ester hydrolysis and hence amide and esters are the common substrates. Naturally esters are much easier targets to esterase hydrolysis than amides. A very common amide substrate is a local anesthetic, Lidocaine and an antiepileptic drug known as levetiracetam. Catalyzing ester and certain type of amides are the group of enzymes referred to as carboxylesterase. This enzyme hydrolysis choline like ester substrate and procaine. As a rule, the more lipophilic the amide the better it be use uped as a substrate for this enzyme and thus eliminated. Esters that are sterically hindered are however much harder and unwinder to be hydrolysed and will usually be eliminated unchanged at a high per centage such as that for atropine, eliminated 50% unchanged.A very good example of esterase enzyme is the paraoxonase. The hydrolysis of substrate such as phenyl acetate and other acyl group esters are catalyzed by this. For hydrolases and substrate to be involved in this pathway certain criterias are haughty for a fast reaction rate, these include having a electrophilic group a nucleophile that will attack the carbon attached to the oxygen resulting in a formation of tetrahedral orientation. The presence of a hydrogen donor to the improvers the leaving group abilities is the final requirement.1.2 Phase II (Second part of drug metabolism)Second part of drug metabolism, involves introduinh of new ionic chemicals on to the substrate (including the metabolites from phase I) in order to increase its water solubilyt for elimination. This phase is usually refered to as amalgamation reaction and its products are generally inactive unlike those of phase 1. The following reaction are major brotherhood of phase II.Methylation is the transfer of methyl group to the substrate from cofactor s-adenosyl-L-methionine (fig 9). S-adenosyl-L-methione is an active intermediate that receives a transferred methyl group from methionine after its gene linkage with adenosine triphosphate in presence of adenosine transferase enzyme. Itis this methyl group that is ultimately transferred on to the substrate. S-adenosyl-L-methionine methyl group suffers attached to the sulfonium center marking electrophilic character (Principle of drug metabolism, 2007). Depending on the available group present on the substrate Conjugation via methylation is broken down to nitrogen, oxygen and sulfate methylation.O-methylationO-merthylation is the most common reaction that occurs for substarte containing the organic (formally known as pyrocatechol compound, catechol moiety) hence why the enzyme obligated for this type of reaction is called catechol O-methyltransferase. This Magnesium dependent, foun d cyclic but also, less frequently, as a membrane bound enzyme, is found commonly in liver and kidney among other tissues. Common drug for this type reaction are L-DOPA, where generally the methyl is transferred on to the substrate in meta position and less commonly para, depending the substituent (R group) that is attached on the ring. According to Principle of drug metabolism the rate of reactivity of O-methylation is decreased in accordance to size of the substituted group, the bigger it is the slower the rate of reaction degree of acidity of the catechol group itself.N-methylationNaturally this reaction has substrate specificity of amine, involving however primary and seconday only. Unlike the above reaction, N-methylation consists of several enzymes, all of which are categorized in accordance to the specific type of amine substrate which they catalyze. Enzymes such as amine-N-Methyltransferase, nicotinamide-N-methyltransferase and histamine-N-methyltransferase are few examples. Despite the substrate specificity all the enzymes involved do however follow the same principle of transferring methyl fromcofactor s-adenosyl-L-methionine to the substrate.With drug substrates such as captoril, reactions of N-methylation can be broken down into two distinct types as illustrated in Fig 11. Reactions that have a low pharmacological monumental yield an ineffective n-methylation as the substrate and the product have a same electrical state thus the metabolites are usually less hydrophilic than parent. As it can be seen from fig 7a, in thesereactions one proton is exchange for a methyl group. On the other hand a more hydrophilic product and an effective reaction of detoxification is achieved with pyridine type (nitrogen atom) substrate. These substrate will result in a creation of positive change on the product (fig 7b) quite a than an exchange process.Sulfate and phosphate labor unionSulphate uniting is one of the most important reactions in biotransformation of s teroids, effecting its biological activates and decreasing its ability for its receptor. Nucleophilic hydroxyl groups such as alcohol and phenol, primary or seconday amine and drug containing a SO-3 group are the common substrates for this pathway. slackly sulphate are transferred via a membrane bound enzyme named sulfotransferase (located in golgi apparatus) from their cyclic cofactor 3-phosphoadenosine 5 (shown in fig 8 ) to substrate. 3-phosphoadenosine 5 is organize in a reaction between adenosine triphosphate and inorganic sulfate where the sulfate/phosphate group are bonded via a anhydride linkage which gives rise an exothermic reaction when broken, hence providing the energy for the reaction. In human there is two class, SULT 1A- 1E and SULT 2A-2B, each of which will have different specificity yet with overlaps. This enzyme acts on both endogenous as well as exogenous compounds as long as they possess an alcohol (less affinity with varying product stabilities) or phenol (pr oducts are stable arly sulfate esters with a high affinity). Substrates are generally of mediocre sized, highly ionized and hydrophilic, hence excreted easier via urine. The rate of this pathway is determined by the lipophilicity and nature of amino acid present on the substrate. Interestingly phenol is also of an interest for the Glucoronic conjugation pathway and are metabolized by this when they are at high concentration and 3-phosphoadenosine 5 becomes rate limiting. The sulfate conjugation will produce ester sulfate or sulfamide some of which will undergo further heterolytic reaction head teachering to electrophilic substrate and hence toxicity.Unlike the sulfate conjugation the phosphate conjugation is less common unless the drug in question is anticancer or antiviral. Catalyzed phosphotransferases.conjugation The most important and major occurring metabolic pathway of phase II is the glucoronic conjugation, accounting for the largest share of conjugated metabolite in the u rine. This pathway is important due to the fact there is a high availability of glucucronic acid, huge substrate specificity and the large-minded range of poorly reabsorbed metabolite. The glucoronic conjugation takes place as the glucoronic acid is transferred to the acceptor molecule from its cofactor uridine-5-diphosphh-alpha-glucoronic acid (fig 9 ) of which glucoroniuc acid is attached in 1 configuration. However products produced are in -configuartion. This is due to the nucleophilicity of the functional groups of the substrate. To be able to undergo this pathway of metabolism the functional group of drugs in question must have nucleophilic characteristics. Generally the drug that are at high affinity for this pathway is firstly phenol (paracetamol) and then alcohol (primary, secondary or tertiary) such a morphine. The transformation of the drugs involves a condensation reaction and hence release of water, while the conjugate replaces the hydrogen on the -OH group. Present i n the ER uridine-5-diphosphae-alpha-D glucoronic acid is produced due to oxidation of carbon position six of UDP--D-glucose. Interaction of this co factor with the substrates is catalysed by one the two classes of UGT1 or UGT 2, present mostly in liver however still found in brain and lungs.As this pathway produces a wide compartmentalisation of procucts, work has been done to divide them into four groups of O/S/C/N glucoronides, with the o-glucoronides being the most important forming a reactive metabolite known as acyl-glucuronides. Generally drugs containing functional groups such as carboxylic acid, alcohol and phenol give rise more examples shown in fig 10.AcetylationInvolving a transferring of an active acetyl linked via a thioester bridge to acetyl-coenzyme A (fig below) to a nucleophilic function group of substrate this metabolic pathway mainly occurs in liver involving amino groups of medium basic properties. One of the common drug metabolized by this pathway is the para-a minosalicly. Large group of enzymes known as acetyltransferase are enzymes involved in catalyzing this pathway, among these are the aromatic-hydroxylamine O-acetyltransferase and the arylamine N-acetyltransferase.Interestingly, genetic polymerization of acetylation function has meant that the rate of reaction and occurrence of toxicity will differ in accordance to the polymers. Fast acetylation will have result in a fast conversion and elimination while slow acetylators will have the opposite effect and will tercet to build of unconjugated compounds in the blood and hence leading to toxicity.Conjugation with co-enzyme ACommonly using this pathway are the carboxylic containing which are activated into an negociate and eventually forming a acetyl-CoA conjugate It is important to note that primary metabolites from this reaction do not show up in vivo and only in vitro, however some of its secondary and stable metabolites that have undergone further reactions do. A factor that seems t o cause problems with this pathway is the occurrence of toxicity, rare but serious as it the conjugates interject with normal endogenous pathway. A common example was seen with NSAID which have now been long removed from market.Conjugation with amino acidThis metabolic pathway is the most important for carboxcylic drugs where they form conjugate with the most common amino acid, glycine. Products are non-toxic (with no exception) and more water soluble than their parent compound. The drugs first become activated to the co- enzyme A before forming an amide or peptide bond between its carboxylic group and amino acid. The enzymes that facilitate this reaction are those of N-acyl transferases, such as glutamine N-acyltransferase. carboxylic substrate for this pathway are also of an competition for the glucoronic conjugation, at high concentration if drugs glucoronic conjugation is preferred due to high availability, while at low concentration conjugation with amino acid is used for the metabolism.Conjugations with GlutathioneConjugation with glutathione has a wide variety of substrate specificity this is partly due to the fact that in vivo glutathione exists as in equilibrium between its oxidised and reduced form hence enabling it to accept a wider range of substrate. The reduced form of glutathione is able to act as a protecting agent as it removes free radicals while the oxidised form oxidizes peroxides. A thiol, the glutathione contains a tripeptide and with a pka of 9.0, allowing it to be an excellent nucleophile agents, due to the increase in the ionization due to the thiol group. As the result of these electrophilic groups are easily attacked, usually on the most electrophilic carbon (commonly sp3 or sp2 hybridised) that contains the functional group. Enzymes responsible for catalyzing these reactions are known as glutathione transferase, seven of which are found in human. They also run an important role apart from catalysing as upon binding of the active side with the glutathione will results in a decrease in pka value and hence an increase in acidity (the thiol is deprotonated thiolate), thus enhancing the nucleophilic abilities.Depending on the substrate in question the conjugation with glutathione can be divided into forms, nucleophilic substation or nucleophilic addition. During the nucleophilic addition, an addition followed by an elimination reaction occurs. Attack occur at the activate electron lacking CH2 group, which the glutathione substitutes as it becomes added on to the carbonyl as shown in fig 12. Nucleophilic substitution reaction is much more common with xenobiotic than drugs although it is seen with chloramphenicol, where its -CHCL2 becomes electrophilic due to a electron withdrawing group.One of the most important conjugation in relation to glutathione is with epoxides giving rise to a protective mechanism of liver. The more chemically active epoxide undergo this reaction are attacked at carbon sp3 hybridised via n ucleophilic addition. The metabolite will lose a water molecule via dehydration catalyzed by acid giving rise to a GSH aromatic conjugate. As a final metabolite a mercapturic acid (a condensation reaction exerted by urine) as shown in (fig below) is formed via a series reactions including cleavage and n-acetylation .2.1 Metabolism in the liverWhen a drug can be cleaved by enzymes or biochemically transformed, this is referred to as drug metabolism. The main site of drug metabolism within the body occurs in the liver, however, this is not the only site in which metabolism of drugs occurs, this will be discussed later. The liver ensures drugs are subjected to attack by various metabolic enzymes the main purpose of these enzymes is to convert a non-polar drug into more polar molecules, thereby increasing elimination via the kidneys. The polar molecules formed are known as metabolites, these lose a certain degree of activity compared to the original drug. metabolic enzymes, cytochrome P450 enzymes enable the addition of a polar compound to circumstance drugs, making them now polar and more water-soluble. On the other hand, some drugs may become activated and then have the desired effect within the body, these are referred to as pro-drugs and will be considered in greater detail later.Drug metabolism is split into two stages known as Phase I reaction and Phase II reaction, both of which have been discussed earlier. Certain oral drugs undergo a first pass effect in the liver, thereby reducing bioavailablity of the drug. This can lead to numerous problems, such as, individual variation that can then lead to unpredictable drug action, and a marked increase in metabolism of the drug. These problems related to the first pass effect may hinder the desired therapeutic effects from being fully achieved. Many drugs undergo first pass metabolism, previously seen as a disadvantage, but now due to a greater understanding of hepatic metabolism it can be used advantageously, f or example Naproxcinod. Naproxcinod is related to naproxen, which will be discussed below, we will also be examining the metabolism of propanolol.Naproxcinod is derived from the non-steroidal anti-inflammatory drug (NSAID), naproxen. First we will examine the metabolism of Naproxen (6-methoxy-a-methyl-2-naphthyl acetic acid). Naproxen is a widely used NSAID, possible of blocking both cyclo-oxygenase isoforms 1 and 2, therefore making it a non-selective inhibitor of these isoforms. Rheumatoid arthritis and osteoarthritis are the main reason for use of naproxen, which is administered orally as the S-enantiomer.This particular drug is well absorbed by the body and is metabolised in vivo to form various metabolites, the major metabolites being naproxen-b-1-O-acylglucuronide (naproxen-AGLU) and desmethyl-naproxen (DM-naproxen).Naproxen is conjugated in a Phase II reaction with glucuronic acid to form an acyl glucuronide ( plot 2), with the intermediate being DM-naproxen. Usually conjuga tion reactions produce inactive metabolites, however with glucuronic acid the metabolite formed can occasionally become active. This reaction is facilitated by the superfamily UDP-glucuronosyl transferase (UGT) enzymes. The major UGT isoforms found in the liver are 1A1, 1A3, 1A4, 1A6, 1A9, 2B4, 2B7 2B10, 2B15, 2B17 and 2B28. The isoform 2A1 is found mainly in the nasal epithelium, while 1A7, 1A8 and 1A10 are only localised to the gastro-intestinal tract. UGT acts as a catalyst enabling glucuronic acid to bind to naproxen at the carboxylic acid group via covalent bonding.It has been found that all UGT isoforms contribute to the conversion of naproxen to its metabolite naproxen-AGLU, except UGT-1A4, 2B4, 2B15, and 2B171. This reaction produces a highly polar glucuronic acid molecule bound to naproxen. Its main mode of elimination is through the urine. The next major metabolite of naproxen is, DM-naproxen. Demethylation of naproxen forms DM-naproxen, via removal of a single methyl grou p, as shown in Diagram 3. An unstable metabolite is formed during this process, however it is hydrolysed immediately to DM-naproxen. The enzymes involved in this reaction are cytochrome P450 1A2 and 2C9 from Phase I.Once DM-naproxen has formed it is glucuronidated with the helper of UGT enzymes 1A1, 1A3, 1A6, 1A9 and 2B7 and converted to its acyl glucuronide. UGT-2B7 is a high affinity enzyme and so has a high activity in this process, as does UGT-1A6. UGT-1A4, 2B15 and 2B17 do not contribute to the acyl glucuronidation process1. DM-naproxen is also converted to phenolic glucuronide this is formed by the UGT enzymes 1A1 and 1A9. Enzymes UGT 1A3, 1A6 and 2B7 appear to play no part in this reaction. UGT 2B7 works well in glucuronidating the carboxylic acid moiety in particular drugs however it is unable to glucuronidate the phenolic group, so for this reason is not involved in forming phenolic glucuronide.The aim of hepatic metabolism is to ensure metabolites are made more water-solu ble hence easily excreted. All metabolites formed from naproxen are water soluble and easily eliminated from the body. However, there are two metabolites that have been found to be far more water soluble, these are naproxen-AGLU and acyl glucuronide2. Huq (2006) explains this is due to the high solvation energy of both metabolites compared to naproxen and its other metabolites.Metabolites of NaproxenNaproxen is a widely prescribed NSAID and works extraordinarily well however there are several hateful adverse effects, which precipitate after an extended period of use, such as increase in blood pressure. A new drug has been derived from naproxen without this effect, Naproxcinod. From Diagram 19 it is possible to see that the hydrogeProcesses of Drugs Metabolism in the BodyProcesses of Drugs Metabolism in the BodyAbstractMetabolism of drugs is a complex and major process within the body, occurring primarily in the liver. The aim of metabolism is to make the drug more polar to enable e xcretion via the kidneys. The basic understanding of drug metabolism is paramount to ensure drug optimisation, maximum therapeutic benefits and a reduction in adverse effects. Essentially drug metabolism is broken down into two phases, Phase I and Phase II. Phase I is concerned with the biotransformation of compounds, and then transferred to Phase II. However, for some drugs this is the end of their metabolic journey in the body, as they produce more polar compounds which are readily excreted. Phase II reactions are where compounds are conjugated to produce more water soluble compounds for easy excretion. Phase I reactions are dominated by the Cytochrome-450 enzyme superfamily. These enzymes are found predominantly in the liver, which is the major site of drug metabolism. However, drug metabolism is not localised merely to the liver, there are other major sites at which this process occurs. Some of these sites include the skin, lungs, gastro-intestinal tract and the kidneys close to all tissues have the ability to metabolise drugs due to the presence of metabolising enzymes. The most important enzymes are the cytomchrome-450 superfamily, which are abundant in most tissues.Inactive drugs with the ability to reconvert to the active parent drug once metabolised to exert their therapeutic actions are defined as prodrugs. They are classified depending on the site of conversion and actions (gastrio-intestinal fluids, intracellular tissues or blood). This report gives different study examples of such prodrugs and how their metabolism differs within the body, compared to their active metabolites. Individual drug metabolism may be affected by variant factors, such as, age or sex. Drug metabolism can cause an increase in toxcity. The bioactivation of a parent compound can form electrophiles that bind to proteins and DNA. Some of this toxicity can occur in Phase I metabolism e.g. acetaminophen. However, in some circumstances toxicity occurs in Phase II e.g. zomepirac, po lymorphism can also cause idiosyncracity of certain drugs to be toxic.1.1 Phase IPhase one, otherwise known as drug biotransformation pathway is generally broken into oxidation, reduction and hydrolysis. A reaction under this phase involves an addition of oxygen molecule aiming to improve the water solubility of drugs. As the result some metabolites from this phase can be extracted immediately if they are polar enough however at times a single addition of oxygen is not sufficient enough to overcome the lipophilicity of certain drugs and hence their metabolite from this phase has to be carried onto phase II for further reactions.Major example of OxidationAccounting for roughly 20 complex reactions the most important oxidative metabolic pathway dominating phase I is the cytochrome-P450 (CYP450) monooxygenase system processed by C-P450. Located primarily in the liver CYP450 was found to be present in all forms of organisms, including humans, plant and bacteria. It is important to note that the function of CYP450 goes beyond drug metabolism but it is also involved in metabolism of xenobiotics, fat soluble vitamin and synthesis of steroids. With substrate specificity of more than 1000 and its ability to produce activated metabolites such as epoxide are the underlying reason for its dominance and importance in drug discovery. The general mechanism the CYP450 monooxygenase oxidation isR + O2 + NADPH + H+ ROH + H2O + NADP+ (fig 2)From the above formula it can be this reaction is of NADPH (Nicotinamide adenine dinucleotide phosphate) and an oxygen molecule dependent. As mentioned above oxygen is important to increase the water solubility and in the same manner NADPH is also important for oxygen activation and source of electron. Also important for activation of oxygen is the presence of cystine amino acid located near the protein terminal carboxyl of CYP450. Among the 500 amino acid present in CYP450, cystine has proven to be most important as it activates the oxygen to a greater extend. This is due to the fact that it contains a thiol group as one of its ligand and it is the thiol which alerts the reactivity.Highlighting the numerous intermediate structures involved as well as function of iron, oxygen and proton (Figure) shows the catalytic conversion required for cp450 oxidation reaction to place. The binding of the substrate with low spin ferric CYP450 enzyme induces a change in its active site. This will effects the stability of the water ligand and will displace it (shown in the diagram from a-b). Containing a high spin heme iron the enzyme and substrate form a ferric complex. The change in electronic state will result in the release and transfer of one electron from NADPH via electron transfer chain (reducing ferric heme iron to ferrous state) and thus reduction of the complex. The second electron is transferred when the complex reacts covalently with the oxygen forming a new ternanry complex. Initially the complex is an unstable oxy-P450( diagram d), however this is reduced to produce ferrous peroxide by a loss of an electron. This intermediate is short lived and undergoes protonation twice resulting in a release one water molecule. Out of the oxygen molecules released one in incorporated in this water molecule and the remaining into the substrate. Another method of forming the iron-oxo intermediate is via the peroxide shunt which elimited steps from C to F.Some of the common addition of oxygen molecule reactions which CYP450 dependent are known as epoxidation (of double bond), N-hydroxylation, oxygen/nitrogen/ sulfur dealkylation, s-oxidation, dechlorination, oxidative desulfurisation and aromatic hydroxylation. Note they all follow the same principle of adding oxygen molecule to the substrate. The diagram below provides an example of how these reactions are processedAromatic hydroxylation substrate mostly produces phenols such as that seen on figure 3. The production of Phenol can be either via a non enzymatic rear rangement or by Epoxide hydrolase and cytosolic dehydrogenase which will ultimately give rise a catechol. The position of hydroxylation depends greatly on the nature of the R- group attached to the ring an electron withdrawing group will position the -OH group on the metha while the electron donating will position it on the para or ortha. Aromatic hydroxylation also involves a change in NIH shift, which involves the movement and shifting of the R group to an adjacent position during the oxidation. It is important to note that certain substrate for aromatic hydroxylation can also be oxidized via the aliphatic (C-H) hydroxylation. Under such condition the aliphatic C-H) hydroxylation will oxidize it. Aliphatic dehydrogenation can also occur involving electron transfer to the CYP450.Currently more than 50 CYP-450 has been identified in human, however the bulk of drug metabolism is essentially carried by CYP1, CYP2 and CYP3 families, especially the CYP450-3A. The diagram on the right ha nd side clearly demonstrate just how much of drug metabolism is CYP450 3A responsibility in comparison to other, accounting for roughly 50%. Metabolism of drugs given orally are greatly determined by CYP450-3A primarily because this enzyme is present in both the liver and intestine and thus providing a barrier for all drugs before they can enter the systemic circulations, otherwise commonly known as first pass effect. Upon entering the drugs are taken up via passive diffusion and/or facilitated diffusion or active transport into the entercocyte where they can be metabolized by CYP450-3A. They can once again be metabolized by the very same enzyme when they enter the liver (hepatocyte) ,which unlike the intestine in order to reach the systemic circulation it is unavoidable. This family of enzymes are also known to be cause of many serious adverse effects as they are influenced by diet and drug components, hence drug-drug and drug-food interactions is an important factor.Flavin monooxy genasesSimilar to cytochrome p450 monooxygenases system,Flavin monooxygenasesalso plays a major role in metabolism of drugs, carcinogens and Nitrogen/ sulfur/ phosphorous containing compounds. Also oxygen and NAPDH dependent, Flavin monooxygenases has much broader substrate specificity than CYP450. Once they have become associated with substrate the flavin monooxygenases is activated into 4-hyroperoxyflavin and unlike CYP450 the oxygen activation takes place without the need for substrate to bind to the intermediate. This pre-activated oxygen means that any compound binding to the intermediate is a substrate to be metabolized. The fact that this enzyme is able to remain stable and lacks any need for correct arrangement and disorientation of the substrate gives it ability to withhold all the energy required for the reaction to takes place and hence as soon as appropriate lipophilic substrate becomes available it starts the process immediately. Adverse side effects are rarely associat ed with these enzymes.The binding of oxygen to the reduced flavin is processed via a non-radical nucleophilic displacement. The substrate is oxidized via a nucleophilic attack by the oxygen that is located at end of 4-hyroperoxyflavin. This is then followed by cleavage of peroxide. The flavin monooxygenase catalytic cycle is finished once the original form of 4-hyroperoxyflavin has been regained using NADPH, oxygen and hydrogen proton. Note the metabolite product can at any times undergo reduction back to its original parent form.Alcohol dehydrogenase and aldehyde dehydrogenaseThese families of enzymes are both zinc containing NAD specific and catalyze the reversible oxidation of alcohol and aldehydes respectively. Grouped into 1-6 Alcohol dehydrogenase, are homodimer that exist in the soluble section of the tissue. It is involved in metabolism of some drugs such as cetirizine however it is more predominantly known as alcohol metabolism enzyme specifically ethanol, whether products of peroxides or that of exogenous (i.e administered drugs). It is important to note that although alcohol dehyrogenase is the main metabolic pathway for ethanol, however CYP2E1 also plays in its metabolism. CYP2E1 can be induced by ethanol resulting in adverse side effects between alcohol and with certain analgesics drugs. Alcohol dehydrogenase also metabolizes ethylene glycol and methanol. With a longer half life and rapid absorption from the gut, methanol can result in series of unpleasant side effects and metabolic acidosis, hence highlighting the importance of alcohol dehydrogenase. Similarly, aldehyde dehydrogenase catalysis the oxidation of aldehyde to its corresponding carboxylic acid. Class one of alcohol dehydrogenase plays a major role in detoxification of anti cancer drugs. Alcohol dehydrogenase is also involved in reduction pathway of aldehyde or ketone back to its pharmacologically active alcohol form.Monoamine oxidase and diamineLocated in liver, intestine and kidney a s few of its site, this membrane bound enzyme is divided into two classes in accordance to their substrates specificity, they are monoamines-A and monoamine-B. Responsible for metabolizing amines via deamination to aldehyde, these enzymes are flavin containing enzymes and within their cysteinyl residue the flavin is linked to the covalently bounded flavin via a thioether. Monoamine oxidase has several substrates, ranging from secondary to tertiary amines that have alky group smaller than methyl. The general mechanism for this enzyme is the two electron oxidation shown belowR.CH2.NH2 + O2 + H2O R.CHO + NH3 + H2O2 (fig 7)As it can be seen this reaction requires oxygen to react and a hydrogen peroxide is produced as for every one molecule of oxygen is absorbed for every molecule of substrate oxidized (Principle of drug metabolism, 2007). Proportional to the rate of oxygen uptake this is commonly used to deduce the rate of reaction. Research has shown that monoamines-A is more commonly involved in oxidation of endogenous substrates such as noradrenalin while monoamine-B which is found mostly in platelets appears to catalyses exogenous substrates such as phenylethylamines. Their common substrate is dopamine. Inhibition of monoamine oxidase has long been of an interest for scientist in treatment of several of illness such as depression.Present in liver, lungs and kidney as few of its locations diamine oxidase also catalyses the formation of aldehyde from histamine and diamines in the same manner.ReductionThis pathway of metabolism is enzymatically the least studied in phase I and yet it plays an important role in metabolism of disulfides and double bonds of for example progestational steroids as well as dehydroxylation of aliphatic and aromatic compounds. In general ketone containing xenobiotics are more readily metabolized and eliminated via this pathway in the mammalian tissue. This is due to the fact that the carbonyl group is very lipophilic, thus the lipophili city will be reduced and elimination is ensured as ketone is converted to alcohol.One of the major enzymes involved in this pathway is the NADPH cytochrome P450 reductase. Containing flavin adenine dinucleotide and flavin mononucleotide is an electron donor playing an important role in the metabolism of drugs such as chloramphenicol by reducing its nitro group.HydrolysisAs the name suggests this pathway uses water to cause a breakage of a bond. Major enzymes under this pathway are the amide and ester hydrolysis and hence amide and esters are the common substrates. Naturally esters are much easier targets to esterase hydrolysis than amides. A very common amide substrate is a local anesthetic, Lidocaine and an antiepileptic drug known as levetiracetam. Catalyzing ester and certain type of amides are the group of enzymes referred to as carboxylesterase. This enzyme hydrolysis choline like ester substrate and procaine. As a rule, the more lipophilic the amide the better it be accepted a s a substrate for this enzyme and thus eliminated. Esters that are sterically hindered are however much harder and slower to be hydrolysed and will usually be eliminated unchanged at a high percentage such as that for atropine, eliminated 50% unchanged.A very good example of esterase enzyme is the paraoxonase. The hydrolysis of substrate such as phenyl acetate and other acyl esters are catalyzed by this. For hydrolases and substrate to be involved in this pathway certain criterias are imperative for a fast reaction rate, these include having a electrophilic group a nucleophile that will attack the carbon attached to the oxygen resulting in a formation of tetrahedral orientation. The presence of a hydrogen donor to the improvers the leaving group abilities is the final requirement.1.2 Phase II (Second part of drug metabolism)Second part of drug metabolism, involves introduinh of new ionic chemicals on to the substrate (including the metabolites from phase I) in order to increase its water solubilyt for elimination. This phase is usually refered to as conjugation reaction and its products are generally inactive unlike those of phase 1. The following reaction are major conjugation of phase II.Methylation is the transfer of methyl group to the substrate from cofactor s-adenosyl-L-methionine (fig 9). S-adenosyl-L-methione is an active intermediate that receives a transferred methyl group from methionine after its linkage with ATP in presence of adenosine transferase enzyme. Itis this methyl group that is ultimately transferred on to the substrate. S-adenosyl-L-methionine methyl group becomes attached to the sulfonium center marking electrophilic character (Principle of drug metabolism, 2007). Depending on the functional group present on the substrate Conjugation via methylation is broken down to nitrogen, oxygen and sulfate methylation.O-methylationO-merthylation is the most common reaction that occurs for substarte containing the organic (formally known as pyrocat echol compound, catechol moiety) hence why the enzyme responsible for this type of reaction is called catechol O-methyltransferase. This Magnesium dependent, found cyclic but also, less frequently, as a membrane bound enzyme, is found commonly in liver and kidney among other tissues. Common drug for this type reaction are L-DOPA, where generally the methyl is transferred on to the substrate in meta position and less commonly para, depending the substituent (R group) that is attached on the ring. According to Principle of drug metabolism the rate of reactivity of O-methylation is decreased in accordance to size of the substituted group, the larger it is the slower the rate of reaction degree of acidity of the catechol group itself.N-methylationNaturally this reaction has substrate specificity of amine, involving however primary and seconday only. Unlike the above reaction, N-methylation consists of several enzymes, all of which are categorized in accordance to the specific type of am ine substrate which they catalyze. Enzymes such as amine-N-Methyltransferase, nicotinamide-N-methyltransferase and histamine-N-methyltransferase are few examples. Despite the substrate specificity all the enzymes involved do however follow the same principle of transferring methyl fromcofactor s-adenosyl-L-methionine to the substrate.With drug substrates such as captoril, reactions of N-methylation can be broken down into two distinct types as illustrated in Fig 11. Reactions that have a low pharmacological significant yield an ineffective n-methylation as the substrate and the product have a same electrical state thus the metabolites are usually less hydrophilic than parent. As it can be seen from fig 7a, in thesereactions one proton is exchange for a methyl group. On the other hand a more hydrophilic product and an effective reaction of detoxification is achieved with pyridine type (nitrogen atom) substrate. These substrate will result in a creation of positive change on the produ ct (fig 7b) rather than an exchange process.Sulfate and phosphate conjugationSulphate conjugation is one of the most important reactions in biotransformation of steroids, effecting its biological activates and decreasing its ability for its receptor. Nucleophilic hydroxyl groups such as alcohol and phenol, primary or seconday amine and drug containing a SO-3 group are the common substrates for this pathway. Generally sulphate are transferred via a membrane bound enzyme named sulfotransferase (located in golgi apparatus) from their cyclic cofactor 3-phosphoadenosine 5 (shown in fig 8 ) to substrate. 3-phosphoadenosine 5 is formed in a reaction between adenosine triphosphate and inorganic sulfate where the sulfate/phosphate group are bonded via a anhydride linkage which gives rise an exothermic reaction when broken, hence providing the energy for the reaction. In human there is two class, SULT 1A- 1E and SULT 2A-2B, each of which will have different specificity yet with overlaps. This enzyme acts on both endogenous as well as exogenous compounds as long as they possess an alcohol (less affinity with varying product stabilities) or phenol (products are stable arly sulfate esters with a high affinity). Substrates are generally of medium sized, highly ionized and hydrophilic, hence excreted easier via urine. The rate of this pathway is determined by the lipophilicity and nature of amino acid present on the substrate. Interestingly phenol is also of an interest for the Glucoronic conjugation pathway and are metabolized by this when they are at high concentration and 3-phosphoadenosine 5 becomes rate limiting. The sulfate conjugation will produce ester sulfate or sulfamide some of which will undergo further heterolytic reaction leading to electrophilic substrate and hence toxicity.Unlike the sulfate conjugation the phosphate conjugation is less common unless the drug in question is anticancer or antiviral. Catalyzed phosphotransferases.conjugation The most important and major occurring metabolic pathway of phase II is the glucoronic conjugation, accounting for the largest share of conjugated metabolite in the urine. This pathway is important due to the fact there is a high availability of glucucronic acid, huge substrate specificity and the wide range of poorly reabsorbed metabolite. The glucoronic conjugation takes place as the glucoronic acid is transferred to the acceptor molecule from its cofactor uridine-5-diphosphh-alpha-glucoronic acid (fig 9 ) of which glucoroniuc acid is attached in 1 configuration. However products produced are in -configuartion. This is due to the nucleophilicity of the functional groups of the substrate. To be able to undergo this pathway of metabolism the functional group of drugs in question must have nucleophilic characteristics. Generally the drug that are at high affinity for this pathway is firstly phenol (paracetamol) and then alcohol (primary, secondary or tertiary) such a morphine. The transformation of th e drugs involves a condensation reaction and hence release of water, while the conjugate replaces the hydrogen on the -OH group. Present in the ER uridine-5-diphosphae-alpha-D glucoronic acid is produced due to oxidation of carbon position six of UDP--D-glucose. Interaction of this co factor with the substrates is catalysed by one the two classes of UGT1 or UGT 2, present mostly in liver however still found in brain and lungs.As this pathway produces a wide variety of procucts, work has been done to divide them into four groups of O/S/C/N glucoronides, with the o-glucoronides being the most important forming a reactive metabolite known as acyl-glucuronides. Generally drugs containing functional groups such as carboxylic acid, alcohol and phenol give rise more examples shown in fig 10.AcetylationInvolving a transferring of an active acetyl linked via a thioester bridge to acetyl-coenzyme A (fig below) to a nucleophilic function group of substrate this metabolic pathway mainly occurs in liver involving amino groups of medium basic properties. One of the common drug metabolized by this pathway is the para-aminosalicly. Large group of enzymes known as acetyltransferase are enzymes involved in catalyzing this pathway, among these are the aromatic-hydroxylamine O-acetyltransferase and the arylamine N-acetyltransferase.Interestingly, genetic polymerization of acetylation function has meant that the rate of reaction and occurrence of toxicity will differ in accordance to the polymers. Fast acetylation will have result in a fast conversion and elimination while slow acetylators will have the opposite effect and will lead to build of unconjugated compounds in the blood and hence leading to toxicity.Conjugation with co-enzyme ACommonly using this pathway are the carboxylic containing which are activated into an Intermediate and eventually forming a acetyl-CoA conjugate It is important to note that primary metabolites from this reaction do not show up in vivo and only in vitro, however some of its secondary and stable metabolites that have undergone further reactions do. A factor that seems to cause problems with this pathway is the occurrence of toxicity, rare but serious as it the conjugates interfere with normal endogenous pathway. A common example was seen with NSAID which have now been long removed from market.Conjugation with amino acidThis metabolic pathway is the most important for carboxcylic drugs where they form conjugate with the most common amino acid, glycine. Products are non-toxic (with no exception) and more water soluble than their parent compound. The drugs first become activated to the co- enzyme A before forming an amide or peptide bond between its carboxylic group and amino acid. The enzymes that facilitate this reaction are those of N-acyl transferases, such as glutamine N-acyltransferase. Carboxylic substrate for this pathway are also of an competition for the glucoronic conjugation, at high concentration if drugs glucoronic conjugation is preferred due to high availability, while at low concentration conjugation with amino acid is used for the metabolism.Conjugations with GlutathioneConjugation with glutathione has a wide variety of substrate specificity this is partly due to the fact that in vivo glutathione exists as in equilibrium between its oxidised and reduced form hence enabling it to accept a wider range of substrate. The reduced form of glutathione is able to act as a protecting agent as it removes free radicals while the oxidised form oxidizes peroxides. A thiol, the glutathione contains a tripeptide and with a pka of 9.0, allowing it to be an excellent nucleophile agents, due to the increase in the ionization due to the thiol group. As the result of these electrophilic groups are easily attacked, usually on the most electrophilic carbon (commonly sp3 or sp2 hybridised) that contains the functional group. Enzymes responsible for catalyzing these reactions are known as glutathione transferase, seven of which are found in human. They also serve an important role apart from catalysing as upon binding of the active side with the glutathione will results in a decrease in pka value and hence an increase in acidity (the thiol is deprotonated thiolate), thus enhancing the nucleophilic abilities.Depending on the substrate in question the conjugation with glutathione can be divided into forms, nucleophilic substation or nucleophilic addition. During the nucleophilic addition, an addition followed by an elimination reaction occurs. Attack occur at the activate electron lacking CH2 group, which the glutathione substitutes as it becomes added on to the carbonyl as shown in fig 12. Nucleophilic substitution reaction is much more common with xenobiotic than drugs although it is seen with chloramphenicol, where its -CHCL2 becomes electrophilic due to a electron withdrawing group.One of the most important conjugation in relation to glutathione is with epoxides giving rise to a protectiv e mechanism of liver. The more chemically active epoxide undergo this reaction are attacked at carbon sp3 hybridised via nucleophilic addition. The metabolite will lose a water molecule via dehydration catalyzed by acid giving rise to a GSH aromatic conjugate. As a final metabolite a mercapturic acid (a condensation reaction exerted by urine) as shown in (fig below) is formed via a series reactions including cleavage and n-acetylation .2.1 Metabolism in the liverWhen a drug can be cleaved by enzymes or biochemically transformed, this is referred to as drug metabolism. The main site of drug metabolism within the body occurs in the liver, however, this is not the only site in which metabolism of drugs occurs, this will be discussed later. The liver ensures drugs are subjected to attack by various metabolic enzymes the main purpose of these enzymes is to convert a non-polar drug into more polar molecules, thereby increasing elimination via the kidneys. The polar molecules formed are kn own as metabolites, these lose a certain degree of activity compared to the original drug. Metabolic enzymes, cytochrome P450 enzymes enable the addition of a polar compound to particular drugs, making them now polar and more water-soluble. On the other hand, some drugs may become activated and then have the desired effect within the body, these are referred to as pro-drugs and will be considered in greater detail later.Drug metabolism is split into two stages known as Phase I reaction and Phase II reaction, both of which have been discussed earlier. Certain oral drugs undergo a first pass effect in the liver, thereby reducing bioavailablity of the drug. This can lead to numerous problems, such as, individual variation that can then lead to unpredictable drug action, and a marked increase in metabolism of the drug. These problems related to the first pass effect may hinder the desired therapeutic effects from being fully achieved. Many drugs undergo first pass metabolism, previously seen as a disadvantage, but now due to a greater understanding of hepatic metabolism it can be used advantageously, for example Naproxcinod. Naproxcinod is related to naproxen, which will be discussed below, we will also be examining the metabolism of propanolol.Naproxcinod is derived from the non-steroidal anti-inflammatory drug (NSAID), Naproxen. First we will examine the metabolism of Naproxen (6-methoxy-a-methyl-2-naphthyl acetic acid). Naproxen is a widely used NSAID, possible of blocking both cyclo-oxygenase isoforms 1 and 2, therefore making it a non-selective inhibitor of these isoforms. Rheumatoid arthritis and osteoarthritis are the main reason for use of naproxen, which is administered orally as the S-enantiomer.This particular drug is well absorbed by the body and is metabolised in vivo to form various metabolites, the major metabolites being naproxen-b-1-O-acylglucuronide (naproxen-AGLU) and desmethyl-naproxen (DM-naproxen).Naproxen is conjugated in a Phase II reaction with glucuronic acid to form an acyl glucuronide (Diagram 2), with the intermediate being DM-naproxen. Usually conjugation reactions produce inactive metabolites, however with glucuronic acid the metabolite formed can occasionally become active. This reaction is facilitated by the superfamily UDP-glucuronosyl transferase (UGT) enzymes. The major UGT isoforms found in the liver are 1A1, 1A3, 1A4, 1A6, 1A9, 2B4, 2B7 2B10, 2B15, 2B17 and 2B28. The isoform 2A1 is found mainly in the nasal epithelium, while 1A7, 1A8 and 1A10 are only localised to the gastro-intestinal tract. UGT acts as a catalyst enabling glucuronic acid to bind to naproxen at the carboxylic acid group via covalent bonding.It has been found that all UGT isoforms contribute to the conversion of naproxen to its metabolite naproxen-AGLU, except UGT-1A4, 2B4, 2B15, and 2B171. This reaction produces a highly polar glucuronic acid molecule bound to naproxen. Its main mode of elimination is through the urine. The next major m etabolite of naproxen is, DM-naproxen. Demethylation of naproxen forms DM-naproxen, via removal of a single methyl group, as shown in Diagram 3. An unstable metabolite is formed during this process, however it is hydrolysed immediately to DM-naproxen. The enzymes involved in this reaction are cytochrome P450 1A2 and 2C9 from Phase I.Once DM-naproxen has formed it is glucuronidated with the help of UGT enzymes 1A1, 1A3, 1A6, 1A9 and 2B7 and converted to its acyl glucuronide. UGT-2B7 is a high affinity enzyme and so has a high activity in this process, as does UGT-1A6. UGT-1A4, 2B15 and 2B17 do not contribute to the acyl glucuronidation process1. DM-naproxen is also converted to phenolic glucuronide this is formed by the UGT enzymes 1A1 and 1A9. Enzymes UGT 1A3, 1A6 and 2B7 appear to play no part in this reaction. UGT 2B7 works well in glucuronidating the carboxylic acid moiety in particular drugs however it is unable to glucuronidate the phenolic group, so for this reason is not invo lved in forming phenolic glucuronide.The aim of hepatic metabolism is to ensure metabolites are made more water-soluble hence easily excreted. All metabolites formed from naproxen are water soluble and easily eliminated from the body. However, there are two metabolites that have been found to be far more water soluble, these are naproxen-AGLU and acyl glucuronide2. Huq (2006) explains this is due to the high solvation energy of both metabolites compared to naproxen and its other metabolites.Metabolites of NaproxenNaproxen is a widely prescribed NSAID and works extraordinarily well however there are several undesirable adverse effects, which precipitate after an extended period of use, such as increase in blood pressure. A new drug has been derived from naproxen without this effect, Naproxcinod. From Diagram 19 it is possible to see that the hydroge