The function of drug metamorphosis is to modify the functional group of a drug molecule to do it more H2O soluble ( e.g. by doing it more polar ) , which makes it more readily excreted by the kidneys ( i.e. the organic structure seek to acquire rid of the “ chemical encroacher ” ) . As the drugs molecule enters the organic structure, it will be attacked by a scope of metabolic enzymes, and the reaction consequence in the formation of metabolites. These metabolites can either lose the activity of the parent drug, or possess different activity which may do toxicity or unwanted side effects. Besides, the drug metamorphosis may trip a drug molecule in the organic structure, which is known a pro-drug scheme.
One of the enzymes involved in drug metamorphosis is cytochrome P450 ( found in the liver ) , which is able to add a polar group to a scope of drugs. Other enzymes may uncover a cloaked polar group that is already present in the drug ( e.g. demethylation of methyl quintessence to uncover a hydroxyl group ) .
The terminal consequence of drug metamorphosis is to extinguish the drug from the organic structure as either unchanged signifier or through transition to metabolites. This procedure where the drug is converted to a metabolite is called biotransformation, and they are classified as Phase I ( functionalization reactions ) and Phase II ( junction reactions ) .
Phase I reactions involve the debut of a functional group to the drug molecule, which result in either loss pharmacological activity or active intermediate metabolites ( as in the instance of produrgs ) . Phase I by and large involve oxidization, hydrolysis and decrease. The bulk of these reactions happen in the liver, although some of them ( e.g. hydrolysis of amides and esters ) can happen in a different topographic point, such as blood plasma or intestine walls. The reactions can go on inside the cell, i.e. endoplasmic Reticulum and cytosol, but other topographic points are besides common such as the plasma membrane or chondriosome.
On the other manus, stage II reactions form a covalent nexus between the functional group on the original molecule with glucuronic acid, glutathione, acetate etc. These conjugates are inactive and really polar which means they will be excreted easy in the piss or fecal matters. Most of the reactions are conjugation reactions by transferase enzymes, and glucuronic acid junction is the most common reaction.
It is now a demand to set up surveies on drug metamorphosis to place all its metabolites before the drug can be approved. This is due to the fact that some drugs can organize more than two metabolites while other drugs might non be metabolised at all. Knowledge of the possible metabolic reactions for assorted functional groups aid the medicative chemist to foretell the metabolic tract or merchandise, and hence let him to plan new drugs which do non hold unwanted metabolites.
Hydroxylation
Oxidation is the most common type of biotransformation in drug metamorphosis ; it includes aromatic hydroxylation, aliphatic hydroxylation, epoxidation and other reactions. All the reactions involve an initial interpolation of an O atom into the drug molecule followed by rearrangement. Here are some illustrations of hydroxylation reaction that occurs in the organic structure:
1 ) Conversion of lignocaine ( antidysrhythmic and local anesthetic ) to the 3-hydroxy derived function
2 ) Hydroxylation of pentobarbitone
3 ) Epoxidation of benzo ( a ) pyrene to its 4,5-epoxide
Aliphatic hydroxylation
Normally, aliphatic compounds do non acquire oxidized unless a primary/secondary intoxicant or aromatic side concatenation formed. If a drug can be metabolized either by aromatic hydroxylation or aliphatic hydroxylation so aliphatic hydroxylation is ever prevailing.
Aromatic Hydroxylation
The mechanisms of aromatic hydroxylation involve an epoxide intermediate as the first measure, followed by NIH displacement ( NIH is called after the U.S. National Institute of Health where it was discovered ) . The reaction is catalysed by cytochrome P450.
A extremist iron-Oxo species delivers oxygen. The rate of ring-opening depends on the intermediates stableness. In add-on, aromatic hydroxylation is faster for electron-rich rings than electron-poor rings.
The NIH displacement is an intramolecular migration of a substituent group at the site of oxidization which moves to an next ring place, and can impact halogenated substituent ( e.g. chloro- and fluro- ) . This can happen in both enzymatic and chemical hydroxlations, where the NIH displacement derives from rearrangement of arena oxide intermediate in enzymatic reactions, ( although other tracts can happen ) . Hydroxylation at the D place can take to a migration of the substituent ( if the R group is non readily ionziable e.g. CH3 and OCH3 ) . It can besides do loss of substituent, ( the viing tract without D ) .
None of merchandise has D atom
Major merchandise has D atom in meta postion
National institutes of health
When a drug molecule contains a benzine ring, it can undergo aromatic hydroxylation to acquire metabolised to an epoxide intermediate foremost. After that, there are two reactions than can happen. In the first reaction, the epoxide undergo non-enzymatic rearrangement to phenol as the major metabolite phenol ( with pKa=10 and less than 1 % ionized at pH 7.4 ) . the phenol the undergo glucuronidation to phenyl glucuroide ( with pHa=3.4 ) , which is really H2O soluble because more than 99 % ionized at pH 7.4.
In the 2nd reaction, the epoxide gets attacked epoxide hydrolyase to acquire 1,2-dihydro-1,2-diol which so get converted to catechol.
Competing tract
In recent studied, it has been shown than benzine can be a possible carcinogen, due to the production of a reactive O species from one of its metabolites. A recent survey reviewed the mechanism if antiapoptotic effects ( that leads to protract cell endurance ) by the benzine metabolite, hydroquinone and p-benzoquinone. These metabolites were found to do serum famishment and deficiency of an extracellular matrix which consequence in inhibit of apoptotic decease of NIH3t3 cells. It has besides been reported that benzine metabolites can do carcinogenesis by bring oning deregulating of programmed cell death which is due an suppression of caspase-3 ( caspase is protein that plays a cardinal function in executing stage of cell programmed cell death ) .
Hydroxylation of phenols is rather interesting, as when the the free postion is para to the first hydroxyl group, it will be hydroxylated faster than the ortho-postion. An illustration of a drug than behave like this form is salicylamid, where around half of the administered dosage is hydroxylated to 5-hydroxylate metabolite and 20 % is 3-hydroxylated.
Hydrolysis
Mammalian carboxylesterases ( CESs ) play an of import function in the hydrolytic biotransformation of assorted curative agents that contain an ester or amide bond. They can be found in assorted mammals and are classified into 5 groups ( CES1-5 ) , where the bulk belong to CES1-2. CES1 and CES2 are significantly different, e.g. CES chiefly hydrolyses a substrate with little intoxicant group and big acyl group, while CES2 hydrolyse big intoxicant groups and little acyl groups. The substrate specificity is due to limitation by the capableness of acyl-enzyme conjugate formation due to the presence of conformational intervention in the active pocket. For illustration, Oseltamivir, antiviral ester prodrug, accomplish its activity through its hydrolytic metabolite ( oseltamivir carboxylate ) which is the merchandise from the hydrolysis of CES1, which besides hydrolyse other drugs such as delapril, methyl ester of cocaine, Demerol and the ethyl esters of temocapril.
The mechanism of CES can be divided into the undermentioned stairss:
1. In the first measure, the substrate enters the active site of the enzyme and signifiers an enzyme-substrate composite. This will place the substrate in a right orientation ready for the reaction.
2. Thereafter, the hydrolysis starts by a nucleophilic onslaught from the O of the hydroxyl group of Ser203 on the carbonyl of the ester bond.
3. Gly123 and Gly124 stabilise the negatively charged O through H bonding of the N-H groups and the O of the tetrahedral intermediate. The H bonding of two NH groups to negatively charged carboxyl O is called oxyanion hole.
The acid catalysed measure consequences in interrupting the ester bond and the go forthing group picks up a proton from the imidazolium ion of the Hi450. The acyl portion of the original ester remains bonded to the enzyme as an acyl-enzyme intermediate. The acylation measure is completed when the intoxicant group ( R’-OH ) spread off.
4. In this measure, a 2nd tetrahedral intermediate is formed following an onslaught from a H2O molecule on the acyl-enzyme intermediate.
5. Thereafter, His450 donates a proton to the O of Ser203. This will ensue in the release of the acerb part of the substrate, which so diffuse off and the enzyme is ready for contact action. Stabilization of the tetrahedral passage province is due low barrier H bonds formed between His450 and Glu336. This low barrier H adhering promote weak H adhering between the O- and NH groups ( from Gly123 and 124 ) , which stabilize the tetrahedral adduct on the substrate side of the passage province.
In the following measure remotion of a proton from His450 interrupt the tetrahedral intermediate which consequences in the formation of acyl-enzyme intermediate. A 2nd measure, deacylation, occurs when the unbound part of the intoxicant group of the first merchandise of the substrate diffuse off. Deacylation is the contrary of the acylation where a H2O molecule substitutes the intoxicant group of the original substrate.
