1. Various theories involved in drug action.There are various theories that involved in drug action which are Occupancy theory, Charnière theory, rate theory, induced-fit theory, macromolecular perturbation theory and occupation-activation theory of “two-state” model.Occupancy theoryOccupancy theory, which is also known as template theory, states that the intensity of pharmacological effect is directly proportional to the number of receptors occupied by the drug. This theory explains the interactions between drug and receptor which is closely related to Langmuir adsorption isotherm. Drug-receptor interactions governed by the law of mass action which can be expressed by the following equation: Once the drug (ligand) binds to the receptor, drug-receptor complex will be formed. However, this complex may not sufficient to elicit pharmacological effects. Therefore, drug must undergoes conformation changes in receptor in order to produce the effect. When all the receptors are occupied by the specific drug, maximum effect with be produced. Thus, the structure of the drug must acquires high affinity towards the specific receptor. However, this theory does not explain both partial agonists and inverse agonists. The theory was then modified to account for partial agonist, that the interaction between drug and receptor involve two stages, which are affinity and intrinsic activity. Affinity is the ability of drug to bind to the particular receptor while intrinsic activity is the ability to produce pharmacological effect. Both agonist and antagonist have strong affinity towards the receptor but only agonist can produce the pharmacological effect. The example of agonist is adrenaline which contains polar groups but it can be transformed into antagonist by introducing aromatic rings in the structure to form propranolol (Korolkovas, A. and Burckhalter, J.H. 1988). Adrenaline Propranolol Charnière theoryNext is Charnière theory. There are two sites available in the pharmacological receptor. The first one is a specific site which binds by the pharmacophoric groups of the agonist while the second site is the nonspecific site. It complexes with the nonpolar groups of the antagonist. This theory states that interaction of both agonist and antagonist with specific site is by forming weak reversible bonds. However, antagonist can forms strong hydrophobic and van der Waals interactions with nonspecific site. Agonist competes for binding to specific site of receptor with antagonist. Due to weakly binds to specific site, antagonist can easily displaces agonist from specific site but it can be also easily dislodge from the specific site. On the other hand, when antagonist strongly binds to the nonspecific site, it is difficult to dislodge from there even with the excess of agonist. The example can be seen in the competition between diphenhydramine and histamine (Anuar, H. 2011). Diphenhydramine Histamine Rate theory Then is rate theory. This theory states that the pharmacological activity is a function of the rate of association and dissociation of the drug with the receptor, and not the number of occupied receptors. The association and dissociation rates are fast in agonist but for antagonist, rate of association is fast but rate of dissociation is slow. For partial agonist, it has intermediate dissociation rate. Antagonist has low dissociation rate because it has relatively larger in size than agonist and partial agonist which makes it to have strong affinity to receptor and hard to dislodge from it. For occupancy theory, the rate theory does not explain why the different types of compounds exhibit the characteristics that they do (Korolkovas, A. and Burckhalter, J.H. 1988). Morphine and naloxone are the agonist and antagonist respectively. Naloxone is structurally similar to morphine. It is the competitive antagonist which competes for binding with morphine for the same receptor. Naloxone has slower dissociation rate than morphine. Morphine Naloxone Induced-fit theory Induced-fit theory also involved in drug action. This theory states that there is not necessary only for the receptor to undergo conformation in order to bind the drug. Conformation change of receptor is induced when the drug binds to the specific receptor to produce the pharmacological effect. Therefore, the receptor is considered to be elastic as it can undergoes conformation and can returns to its original shape once the drug detached from the receptor (Figure 1). Besides, the drug also can undergoes deformation once it is binding to the specific receptor which indicates that it has a flexible structure. Conformation change will be induced and produces the pharmacological response in case of agonist. For partial agonist, it will only cause a partial conformation change. However, for antagonist, it will not undergo conformation change once binding. Example of induced-fit is adenylate kinase. When the substrates, adenosine triphosphate (ATP) and 1-methyl-2-pyrrolidone (NMP) bind to this enzyme, this enzyme will undergo conformation change (Berg et al., 2002) Figure 1: Diagram of induced-fit interaction between enzyme and substrate. Adenosine triphosphate (ATP) 1-methyl-2-pyrrolidone (NMP)Macromolecular perturbation theoryMoreover, macromolecular perturbation theory is another theory involved in drug action. This theory states that specific conformational perturbation and nonspecific conformational perturbation will be occurred when a drug interacts with the receptor. For specific conformation perturbation, it refers to the agonist that able to produce a pharmacological response. For nonspecific conformational perturbation, it refers to the antagonist that will not produce a pharmacological response. When the drug has both macromolecular perturbations, it refers to the partial agonist in which it can forms mixture of two complexes. The example of this theory can be seen in the interaction of trimethylammonium alkyl derivatives which is butyltrimethylammonium with muscarinic receptor (Korolkovas, A. and Burckhalter, J.H. 1988). ButyltrimethylammoniumOccupation-activation theoryLastly is occupation-activation theory of “two-state” model. This theory states that unoccupied receptor can occur in two states, which are nonactivated and activated states. These states are in equilibrium in which activated state (R*) can produce a biological response while at nonactivated state (R), it cannot. For agonist, it has high affinity for activated form of the receptor and therefore, shift the equilibrium to the activated state. For antagonist, it has high affinity for nonactivated state and therefore, shift the equilibrium to the inactivated state. In short, the agonist need to shift the equilibrium to the activated state in order to activate the receptor. The example can be seen in norepinephrine and propranolol. Noradrenaline which is the agonist binds to the receptor and causes the activation of the enzyme which is adenylate cyclase. In the presence of propranolol which acts as a antagonist, it binds to the adenylate cyclase which causes the receptor not being activated (Korolkovas, A. and Burckhalter, J.H. 1988). Noradrenaline Propranolol 2. Bio-oxidation of acetanilide and metabolic conversion of phenylbutazone gave rise to two better tolerated drug molecule used frequently and profusely in the therapeutic armamentarium. Acetanilide Acetaminophen Aniline Acetanilide was accidentally discovered which has antipyretic properties. However, it possess high toxicity. Acetanilide undergoes hepatic metabolism through aromatic hydroxylation to form acetaminophen. This metabolism is the major pathway. The minor pathway of this metabolism is by forming aniline. Only small amount of acetanilide can be hydroxylated into aniline, which can be further hydroxylated to phenylhydroxylamine. Therefore, acetaminophen and aniline are the metabolites of acetanilide. Aniline is responsible for acetanilide toxicity because it can cause the formation of methemoglobin as well as causes hemolytic anemia. However, the formation of acetaminophen is safer and used as a better tolerated pain medication. It has been used widely as analgesic and antipyretic agent to relieve pain and fever. The advantage of this metabolite is that it does not cause ulceration or increase bleeding time. This is mainly due to acetaminophen is less acidic in nature toward the stomach (Beale, J.M. and Block, J.H. 2011). Phenylbutazone Oxyphenylbutazone Gamma-hydroxyphenylbutazone Phenylbutazone is a nonsteroidal anti-inflammatory drug (NSAID) as well as acts as antipyretic and analgesic drug which can be used to relieve fever and pain in short-term treatment. It also provides uricosuric activity. Two metabolites can be formed from phenylbutazone, which are oxyphenylbutazone and gamma-hydroxyphenylbutazone. Oxyphenylbutazone which is formed from aromatic hydroxylation of phenylbutazone, is found to be more active than phenylbutazone, which is a parent drug. Gamma-hydroxyphenylbutazone which is formed from aliphatic hydroxylation of phenylbutazone, is also a active metabolite but it has different activity from oxyphenylbutazone. While oxyphenylbutazone has anti-inflammatory activity, gamma-hydroxyphenylbutazone possess uricosuric effects (Caira, M.R. and Ionescu, C. 2006). When phenylbutazone has both anti-inflammatory and uricosuric effects, its metabolites split these two major effects into two, in which oxyphenylbutazone gives only anti-inflammatory activity to relieve pain and inflammation. On the other hand, gamma-hydroxyphenylbutazone gives only uricosuric activity with limited anti-inflammatory effects to relieve gout by increasing the excretion of uric acid in urine and reduce uric acid concentration in blood plasma (Kelley, W.N. and Weiner, I.M. 2012). Phenylbutazone is no longer available in market because it will cause gastrointestinal side effects such as peptic ulcer to patient. Therefore, these two metabolites of phenylbutazone have been used as separate drug entities to reduce the side effects caused by their parent drug. 3. Preclinical experimental models of drug metabolism and disposition in drug discovery and development.Preclinical experimental models used in the drug metabolism and disposition in drug discovery and development involved the application of in vitro and in vivo experimental models. These models can be varied from cell culture to, animal studies. It is important to establish these models in order to determine the efficacy and safety in drug design. Absorption, distribution, metabolism and excretion (ADME) parameters can be obtained from in vitro and in vivo models which is important in the process of drug discovery and development. Drug-drug interaction, partition coefficient between blood and plasma, stability are ADME parameters generated by in vitro models. For in vivo models, information such as drug distribution, clearance, bioavailability can be obtained (Zhang et al, 2012). In the beginning of drug design, high throughput screening (HTS) is carried out. It aids in the identification of lead compound. The next step is lead optimisation as the lead compound and drug target have been discovered. During lead optimisation, the chosen lead compound are screened with the use of in vitro tests. Lead optimisation is used to select preclinical candidates which can produce pharmacological response in humans (Gunaratna, C. 2000). Therefore, experimental models of in vitro and in vivo studies in drug metabolism play an important role in the development of drug candidate. For in vitro metabolic assays, it can determine the potency and chemical properties of drug in a simple and fast way. Little amount of test compound also is one of the advantage in these assays. in vitro models involve tests in enzymes or specific tissues or cells (Patrick. G. L, 2009). Human-based in vitro assays can made the outcomes in clinical trial to be more accurate. These assays also can obtain information on structure-activity relationship (SAR) in metabolic stability which may found to be difficult to obtain in animal studies. For example, by using primary hepatocyte cultures in negative induction of human in vitro assays, it can link to negative induction used in the clinical trial. Therefore, there is correlations between in vitro and in vivo studies which can be establish in human. in vivo correlation can be used as a parameter to establish human outcomes. Such parameters include prediction in efficacy, toxicology and pharmacokinetics properties in human (Zhang et al, 2012). Expressed enzymes can be used in conducting reaction-phenotyping for drug candidates. The mostly used enzymes for drug metabolism are CYP enzymes. Therefore, identification of CYP isoforms is important for drug metabolism. Non-metabolic pathways, for example, renal and biliary clearance can be used in drug metabolism. By combining both CYP metabolism data and non-metabolic pathway data, it can be used in the estimation of CYP isoform that involved. Examples of CYP isoform are CYP3A4 and CYP2C9. Therefore, metabolic pathway of drug candidate which involved CYP450 enzymes can use expressed enzyme for its identification. For in vitro transporter models, transporters is essential for drug-drug interaction, disposition and toxicity of drug. The transporters can be divided into two families, which are ATP-binding cassette (ABC) transporter and solute carrier (SLC) transporter. In order for the substrate to pump out of the cell, it requires ABC transporters. On the other hand, in order for the substrate to be taken into the cells, it requires SLC transporter. These substrates can be xenobiotics or endogenous compounds. in vitro experimental models are required to determine the status of drug candidate (Zhang et al, 2012). For in vivo models, it provides permeability, distribution, metabolism and excretion effects as well as toxicity and pharmacokinetic parameters. Toxicities and exposure to drug can be determined using animal studies. For example, ADME problems such as high clearance can be identified by using in vivo rat studies. Through these studies, closer estimations in the clinical outcomes of human can be obtained. Besides, pharmacokinetic studies are important for lead selection as well as for lead optimisation. After pharmacokinetic screening, it proceeds with lead optimisation in which it can produce drug candidate though rapid elimination before it proceeds to preclinical testing. For ex vivo models, liver is mainly used in this model. Liver is important in drug metabolism and disposition because liver contains many metabolic enzymes and transporters. Toxicity can be studied by the use of liver tissues or liver organ. Therefore, hepatocytes play an important role in determining hepatic metabolism and clearance mediated by liver enzymes.Transgenic animals mainly mouse is used as in vivo models. This is due to genes in mouse is having similarities to human’s genes. Alteration in mouse’s genes can performs similar effects in human when tested for particular disease (Patrick. G. L. 2009). Therefore, engineered mouse models can be used to determine toxicity and drug metabolism. For example, by replacing mouse’s gene into human’s gene, involvement of specific CYP enzyme in ADME can be determined. For dispositional studies, it provide data on mass balance, biliary excretion and tissue distribution. By conducting with non-radiolabeled compounds, mass balance and ADME studies can produce limited quantitation data for parent drug and metabolites (Zhang et al, 2012). Mass balance studies often designed to resemble toxicology studies in which dose and route of administration are closely correlated between both studies. Mass balance also provides information on metabolism and route of excretion for both parent drug and metabolites. This information helps the scientists to understand the differences between qualitative and quantitative studies in metabolism among different species (Zhang et al, 2012). ReferenceKorolkovas, A. and Burckhalter, J.H. 1988, Essential of Medicinal Chemistry, 172-178, 2nd Edition, New York, USA, John Wiley and Sons. Anuar, H. 2011, Functional Antagonism. https://www.scribd.com/doc/53700982/Functional-Antagonism.html (accessed 23 April 2011).Berg, J.M., Tymoczko, J.L. and Stryer, L. 2002, Biochemistry, 203-205, 5th Edition, New York, USA, W.H Freeman and Company. 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