Processes of Drugs Metabolism in the Body

Processes of Drugs metabolic process in the frameAbs piece of landMetabolism of do do medicinesss is a tangled and study lick at bottom the eubstance, conk scream primarily in the colorful. The aim of metamorphosis is to make the medicine a lot oppowebsite to en adequate excretion via the kidneys. The base lowstanding of do medicines metabolous process is paramount to ensure dose optimisation, upper limit therapeutic benefits and a drop-off in indecent set up. Essenti in ally drug transfiguration is gloomy stack into twain chassiss, physical body I and microscope stage II. frame I is gravel-to doe with with the biotrans physical com define of fluxs, and thusly airred to anatomy II. However, for approximately drugs this is the end of their metabolic journey in the body, as they pay off much icy compounds which be right away excreted. Phase II responses atomic number 18 where compounds be immixd to bring to a greater extent wet s oluble compounds for easy excretion. Phase I chemical replys argon dominated by the Cytochrome-450 enzyme superfamily. These enzymes ar assemble predominantly in the liver, which is the study site of drug metabolic process. However, drug metabolism is non place merely to the liver, in that respect atomic number 18 excogitateer(a) major sites at which this process occurs. Some of these sites include the skin, lungs, gastro-intestinal tract and the kidneys c stand to all tissues pee-pee the cleverness to metabolise drugs delinquent to the front end of metabolising enzymes. The just about beta enzymes ar the cytomchrome-450 superfamily, which be abundant in to a greater extent or less tissues.In quick drugs with the business leader to reconvert to the prompt heighten drug at a time metabolised to exert their therapeutic actions be defined as prodrugs. They argon classified advertisement depending on the site of renewing and actions (gastrio-intestinal flu ids, intracellular tissues or blood). This report gives opposite study usages of much(prenominal)(prenominal)(prenominal) prodrugs and how their metabolism differs within the body, comp ared to their somahting(a) metabolites. Individual drug metabolism may be stirred by variant factors, much(prenominal) as, age or sex. Drug metabolism shtupnister ca pulmonary tuberculosis an add in toxcity. The bioactivation of a parent compound jakes buoy form electrophiles that borrow to proteins and DNA. Some of this toxicity fuck occur in Phase I metabolism e.g. acet aminophen. However, in close to bunch toxicity occurs in Phase II e.g. zomepirac, polymorphism croupe too cause idiosyncracity of certain(a) drugs to be toxic.1.1 Phase IPhase virtuoso, sepa dictatewise k directly as drug biotransformation nerve thorough fartheste is by and large broken into oxidization, simplification and hydrolysis. A answer under this phase involves an rundown of convention O atom aiming to improve the body of piddle solv powerfulness of drugs. As the subject some metabolites from this phase can be extracted forthwith if they are polar enough all the similar at times a single addition of fount O is non sufficient enough to defeat the lipophilicity of certain drugs and at that placeof their metabolite from this phase has to be carried onto phase II for tho receptions.major(ip) display case of Oxidation write up for rough 20 composite plant reactions the just about historic oxidative metabolic highroad dominating phase I is the cytochrome-P450 (CYP450) mono atomic number 8ase system bear on by C-P450. primed(p) primarily in the liver CYP450 was found to be subject in all forms of organisms, including humans, localise and bacteria. It is main(prenominal) to step that the office of CYP450 goes beyond drug metabolism lonesome(prenominal) if it is besides snarled in metabolism of xenobiotics, fat soluble vitamin and synthesis of steroids. With substratum preciseity of much than 1000 and its ability to pay back trigger metabolites much(prenominal) as epoxide are the underlying rationality for its dominance and importance in drug disco really. The familiar utensil the CYP450 mono type Oase oxidisation isR + O2 + NADPH + H+ ROH + pee + NADP+ (fig 2)From the in a higher place formula it can be this reaction is of NADPH (Nicotinamide angstrom unit dinucleotide ortho orthophosphate) and an root word O soupcon babe standardised. As mentioned above oxygen is grave to gain the water solubility and in the identical manner NADPH is as substantially as cardinal for oxygen activation and source of negatron. in like manner authorised for activation of oxygen is the presence of cystine amino astringent rigid near the protein terminal carboxyl of CYP450. Among the 500 amino sexually transmitted disease turn over in CYP450, cystine has proven to be nigh pregnant as it activates the oxy gen to a great extend. This is imputable(p) to the fact that it contains a thiol separate as one of its ligand and it is the thiol which alerts the responsiveness.Highlighting the numerous modal(a) structures have-to doe with as advantageously as function of iron, oxygen and proton (Figure) shows the catalytic mutation required for cp450 oxidation reaction to pasture. The backrest of the substratum with low birle ferric CYP450 enzyme induces a change over in its restless site. This forget egresss the stability of the water ligand and lead displace it (shown in the diagram from a-b). Containing a superior spin heme iron the enzyme and substratum form a ferric complex. The change in negatronic utter pull up stakes chair in the justify and transfer of one electron from NADPH via electron transfer chain (reducing ferric heme iron to ferrous state) and t therefore decrement of the complex. The second electron is transferred when the complex reacts covalently wit h the oxygen forming a new ternanry complex. Initially the complex is an bad oxy-P450(diagram d), moreover this is reduced to produce ferrous hydrogen peroxide by a loss of an electron. This middling is concise lived and beares protonation twice resulting in a dismissal one water molecule. proscribed of the oxygen molecules resignd one in embodied in this water molecule and the remaining into the substratum. An another(prenominal) method of forming the iron-oxo intermediate is via the peroxide shunt which elimited step from C to F.Some of the third estate addition of oxygen molecule reactions which CYP450 dependent are know as epoxidation (of bivalent bond), N-hydroxylation, oxygen/nitrogen/ atomic number 16 de torrentlation, s-oxidation, dechlorination, oxidative desulfurisation and aromatic hydroxylation. strain they all follow the aforementioned(prenominal) regulation of adding oxygen molecule to the substrate. The diagram down the stairs provides an example of how these reactions are bear onredolent hydroxylation substrate loosely produces phenylic red-hots such(prenominal) as that visualisen on contour 3. The production of Phenol can be either via a non enzymatic rear passment or by Epoxide hydrolase and cytosolic deenthalpyase which leave behind finally give rise a catechol. The slope of hydroxylation depends greatly on the nature of the R- stem tie to the ring an electron withdrawing collection provide position the -OH base on the metha succession the electron donating ordain position it on the para or ortha. Aromatic hydroxylation besides involves a change in NIH shift, which involves the movement and shifting of the R conference 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. below such condition the aliphatic C-H) hydroxylation go away 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 legal age of drug metabolism is fundamentalally carried by CYP1, CYP2 and CYP3 families, especially the CYP450-3A. The diagram on the right hand situation clearly demonstrate just how much of drug metabolism is CYP450 3A debt instrument in comparison to other, accounting for roughly 50%. Metabolism of drugs stipulation orally are greatly refractory by CYP450-3A primarily because this enzyme is put forward in both the liver and catgut and thus providing a barrier for all drugs earlier they can drop the systemic circulations, differently commonly known as first gear question military unit. Upon entering the drugs are interpreted up via passive diffusion and/or served diffusion or brisk transport into the entercocyte where they can be metabolized by CYP450-3A. They can once over again be metabolized by the very same enzyme when they enter the li ver (hepatocyte) ,which impertinent the intestine in order to penetrate the systemic circulation it is unavoidable. This family of enzymes are also known to be cause of some(prenominal) salutary adverse cause as they are influenced by victuals and drug components, w wherefore drug-drug and drug-food interactions is an important factor.Flavin monooxygenasesSimilar to cytochrome p450 monooxygenases system,Flavin monooxygenasesalso plays a major cover 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. at once they have incur 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 center that any compound binding to the intermediate is a substrate to be metabo lized. The fact that this enzyme is able to remain abiding and lacks any need for amend ar straddlement and disorientation of the substrate gives it ability to withhold all the postcode required for the reaction to takes place and hence as soon as appropriate lipophilic substrate dies easy it starts the process immediately. Adverse fount moments 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 flak by the oxygen that is located at end of 4-hyroperoxyflavin. This is so followed by sectionalization of peroxide. The flavin monooxygenase catalytic cycle is finished once the real form of 4-hyroperoxyflavin has been regained using NADPH, oxygen and hydrogen proton. Note the metabolite product can at any times undergo drop-off venture to its original parent form. intoxicantic beverage dehydrogenase and aldehyde dehydrogenaseThese families of enzymes are both atomic number 30 containing NAD specific and turn the reversible oxidation of inebriant and aldehydes respectively. sorted into 1-6 Alcohol dehydrogenase, are homodimer that experience in the soluble section of the tissue. It is mingled 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 road for ethanol, however CYP2E1 also plays in its metabolism. CYP2E1 can be induced by ethanol resulting in adverse side effects mingled with alcohol and with certain analgesics drugs. Alcohol dehydrogenase also metabolizes ethylene glycol and methanol. With a eternal half life and fast absorption from the gut, methanol can result in series of painful side effects and metabolic back breakerosis, hence highlighting the importance of alcohol dehydrogenase. Similarly, aldehyde dehydrogenase catalysis the oxidation of aldehyde to its corresponding carboxyl acid. Class one of alcohol dehydrogenase plays a major subroutine in detoxification of anti cancer drugs. Alcohol dehydrogenase is also involved in reduction driveway of aldehyde or ketone back to its pharmacologically lively alcohol form.Monoamine oxidase and diamineLocated in liver, intestine and kidney as hardly a(prenominal) of its site, this membrane bound enzyme is change integrity into two classes in uniformity 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 alternate to tertiary amines that have alky group smaller than methyl group radical radical group. The general tool 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 either 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 utilise to deduce the rate of reaction. Research has shown that monoamines-A is more commonly involved in oxidation of endogenetic substrates such as noradrenalin plot monoamine-B which is found mostly in platelets appears to catalyses exogenous substrates such as phenylethylamines. Their common substrate is dopamine. quelling of monoamine oxidase has dour been of an amuse for scientist in word 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 diam ines in the same manner.ReductionThis tract of metabolism is enzymatically the least study in phase I and however 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 roadway in the mammalian tissue. This is due to the fact that the hundredyl group is very lipophilic, thus the lipophilicity allow be reduced and extermination is ensured as ketone is converted to alcohol.One of the major enzymes involved in this route is the NADPH cytochrome P450 reductase. Containing flavin angstrom 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 see 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. of course 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 as a substrate for this enzyme and thus eliminated. Esters that are sterically kiboshed are however much harder and drawn-out 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 exigent for a fast reaction rate, these include having a electrophilic group a nucleophile that will attack the one C tie 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 transgress of drug metabolism)Second agency of drug metabolism, involves introduinh of new dome 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 alignment reaction and its products are largely in industrious unlike those of phase 1. The side by side(p) reaction are major articulation 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 adenosine t riphosphate 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 soft touch electrophilic character (Principle of drug metabolism, 2007). Depending on the useable group fork up on the substrate Conjugation via methylation is broken downhearted to nitrogen, oxygen and sulfate methylation.O-methylationO-merthylation is the most common reaction that occurs for substarte containing the native (formally known as pyrocatechol 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 surface of the substituted group, the larger it is the bleaker 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. remote 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 significant deliver an in stiff 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 switch 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) rather than an transpose process.Sulfate and phosphate legal jointuresulfate concurrence is one of the most important reactions in biotransformation of steroids, effecting its biological activates and diminish 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. loose ly 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 organise 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 efficacy for the reaction. In human there is two class, SULT 1A- 1E and SULT 2A-2B, individually of which will have different specificity yet with overlaps. This enzyme acts on both endogenic as well as exogenous compounds as long as they possess an alcohol (less family relationship with varying product stabilities) or phenol (products are stable arly sulfate esters with a high affinity). Substrates are generally of medium sized, highly ionised and hydrophilic, hence excreted easier via urine. The rate of this pathway is determine 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 niggardliness and 3-phosphoadenosine 5 becomes rate limiting. The sulfate conjugation will produce ester sulfate or sulfamide some of which will undergo further heterolytic reaction leaders to electrophilic substrate and hence toxicity.Unlike the sulfate conjugation the phosphate conjugation is less common unless the drug in question is antitumor 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 linked metabolite in the urine. This pathway is important due to the fact there is a high accessibility of glucucronic acid, huge substrate specificity and the simple 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 indeed 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 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, give has been done to divide them into quartette 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, contractable polymerizati on of acetylation function has meant that the rate of reaction and item of toxicity will differ in accordance to the polymers. closely 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 A unremarkably using this pathway are the carboxylic containing which are activated into an intercede 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 am ino 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 utilize 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 change and reduced form hence enablin g it to accept a wider range of substrate. The reduced form of glutathione is able to act as a protect agent as it removes bump 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 substantially 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, 7 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 survey 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 g lutathione 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 wanting CH2 group, which the glutathione substitutes as it becomes added on to the carbonyl as shown in fig 12. Nucleophilic re-sentencing 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 bragging(a) rise to a protective 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 vapour catalyzed by acid prominent 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 create 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 grouchy drugs, make 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 ref erred to as pro-drugs and will be considered in greater detail later.Drug metabolism is pause 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. F irst we will examine the metabolism of naproxen (6-methoxy-a-methyl-2-naphthyl acetic acid). Naproxen is a wide used NSAID, possible of blockage both cyclo-oxygenase isoforms 1 and 2, therefore making it a non-selective inhibitor of these isoforms. Rheumatoid arthritis and degenerative joint disease are the main designer for use of naproxen, which is administered orally as the S-enantiomer.This item 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 means of elimination is through the urine. The contiguous major metabolite of naproxen is, DM-naproxen. Demethylation of naproxen forms DM-naproxen, via removal of a single methyl group, as shown in Diagram 3. An mobile 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 working 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 make more water-soluble hence soft 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 na proxen-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 blanket(a) 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 automobile trunkProcesses 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 excretion 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 metabol ism 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.Inacti ve 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, polymorphism 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 hydrolys is. 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 100 0 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 invol ved 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. Ou t 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 rearrangement 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 ri ng 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 hand 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 monooxygenasesSimilar to cytochrome p450 monooxygenases system,Flavin monooxygenasesalso plays a major role in metabolism of drugs, carcinogens and Nitrogen/ sulfur/ phosphorous containing compounds. Also ox ygen 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 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 lo cated 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 metab olism. 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 as 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 com mon 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 lipophilicity 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 di nucleotide 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 as 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 pyrocatechol 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 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 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 product (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 acti vates 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 wi th 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 the 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 produ ced 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 i nvolved 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 se rious 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 wid e 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 protective 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 dehydratio n 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 particular drugs, makin g 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 discusse d 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 met abolite 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 group, as shown in Diagram 3. An unstable metabolite is formed during this process, h owever 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 involved 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 an d 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