Drug Toxicology and Pharmacology - Assignment Example

Drug Toxicology and Pharmacology - Drug 1a. Lead compounds are actually the ‘leading’ organic molecules with some pharmacological activity. It can be viewed as the skeletal structure that can be further modified to formulate a potent drug. Therefore, lead compounds are the starting point in a drug designing process. These compounds are mostly obtained from natural sources as a primary molecule or a secondary metabolite. For example, lovastatin is a fungal metabolite that acted as a leading compound in development of many cholesterol lowering drugs. 1b. Analogue came from the Greek ‘analogia’ and is used to describe functional or structural similarities. In the context of drugs, direct analogue has a same chemical structure and pharmacological action as another drug. Similarly, functional analogue is only similar in its function or pharmacological action but not in structure. Structural analogue has same structure as other drug but not the same pharmacological action. (Wermuth 2006, pp.348-54). 1c i. 1c ii. In the first step benzoyl chlorides are reacted with the scaffold molecule to form an intermediate that is further reacted with aliphatic alcohols to give the analogue molecule. 1c iii. As there are 5 substituted benzoyl chloride and three aliphatic alcohols, a total of 15 derivatives can be formed. 1c IV. 1d. One of the major advantages of solid phase approach is that the whole process can be automatic without much human intervention required. The use of beads as attachment base for substrates makes it easier to purify the products at the end of the process. Apart from these advantages, there are certain downfalls to this approach. One bead can only attach one only type of compound but it is very difficult to identify it. In case of large libraries the production quantity can be very low. 1e. Rational drug design is a technique or step wise formulated protocols used in the development of a drug. The design work by identifying the target molecule that can be modulated to reverse the disease. A ligand or small molecule corresponding to that target molecule is than developed that can either stimulate or inhibit the target molecule. With the advent of technology, more sophisticated instruments are available for the visualization of the target molecule. It is important to understand the precise structure so that a complementary ligand can be designed. 3D x-rays and NMR are the latest technologies that have revolutionized the drug discovery process with the additional help of computer aided methodologies. (Mandal et al 2009, pp.90-91). 2a. A small library of analogue of the lead compound can be easily formulated for the given lead compound using the parallel synthesis approach. A close analysis of lead compound reveals that it is formed by three different groups of molecules; amino ester, acid chloride and phenol. Based on the available reagents, one ester, 3 acid chlorides and 3 different phenols, 3 intermediate molecules will be formed after reacting ester with acid chloride. Subsequent addition of 3 different phenols to these intermediated will result in formation of 9 different structures. Following is the structure of a representative member of the library. 2b. When selecting a new drug discovery program few important aspects should be considered. Firstly, a molecular target should have already been identified that is associated with the disease for which drug discovery program has been initiated. Secondly, through experimentations it should have been confirmed that modulation or any intervention of the molecular target results in reversal of the disease. Once these aspects have been considered, a hunt for molecule that can modulate the target should be carried out in the drug discovery program. (Mandal et al 2009, pp.90-91). 2c. Lapinski’s rule of five includes certain criteria that if present would most likely result in increase membrane permeability of a substance or drug with increase absorption in the body. This rule was based on 90 percentile values of the drugs property distribution. Most importantly, this rule of five only applies to the molecules that are passively absorbed and do not pass through cell membrane via special carrier proteins. 2d. Rational drug design is a technique or step wise formulated protocols used in the development of a drug. The design work by identifying the target molecule that can be modulated to reverse the disease. A ligand or small molecule corresponding to that target molecule is than developed that can either stimulate or inhibit the target molecule. With the advent of technology, more sophisticated instruments are available for the visualization of the target molecule. It is important to understand the precise structure so that a complementary ligand can be designed. 3D x-rays and NMR are the latest technologies that have revolutionized the drug discovery process with the additional help of computer aided methodologies. (Mandal et al 2009, pp.90-91). 2e. Major advantage of solution phase approach is that there is no requirement of reoptimising the reaction on solid phase and there are no competing reactions. Moreover, in a solution phase approach only two steps are required instead of four as attachment and cleavage steps are not required. But there are also some disadvantages of this approach. In comparison with solid phase libraries, the purity of the product may be less in solution phase approach. Short step reactions with high yielding sequences are best for solution phase but longer step reactions with a low yielding sequence can lead to impurity formations. (Combinatorial Chemistry 1998, pp.57-58). 2f. Bioisosteric replacement is the exchange of a single atom or a group of atoms from a molecule and replacing it with a relatively similar single atom or group of atoms. The resulted molecule is structurally little different from the parent molecule but has the same biological activity. Most of these modifications result in reduced toxicity of drug molecules (leading compound) and initiate more specific activity. For example, hydrogen molecules in a lead compound can be replaced by fluorine that results in metabolic modulation. Toxicology 1a. Contamination of food by fungus may not be very common, mostly due to high standards of hygiene and food control, but still it is a potential threat to human health. According to a research study conducted on 166 different samples of commercially available yoghurt with variable flavors, 7.2% of them were positive for fungi. The percentage of contamination may be higher in non processed or raw foods such as fruits and vegetables. Ochratoxin A (OTA) is a secondary metabolite of fungi and have stirred some major concerns due to its association with cancer. It is globally found as a contaminant of various foods such as cereal products, nuts, coffee, spices, raisins and wine. OTA is produced by various species of Aspergillus and Penicillium. Aspergillus flourishes and contaminate food mostly in the warm climate, whereas, penicillium are frequently isolated in colder regions. OTA has also shown nephrotoxic, teratogenic and immunotoxic properties. (Magan & Olsen 2004, pp 307-319). Other fungal toxins that are potentially poisonous to humans include aflatoxins, fusarium toxin and citrinin. 1b. The word ‘antidote’ originated from the Greek antidoton, meaning countermeasures. The existence of ‘poison’ was acknowledged by humans long ago and various mythological and archaeological evidences are there that suggest use of poison by Greeks and Romans. Consequently, there was a need to develop some potion that can counter the effect of those poisons. Those assigned with this task would wander deep forests to look for herbs that can serve as an antidote. With the advent of technology, understanding of how various poison works also aid in the development of its antidote. Antidotes are mostly used after identification of the exact poison. This is because some of these antidotes can act as a poison themselves. Antidotes work by either binding to the poison, decreasing their absorption, or inactivating and detoxifying poison into a harmless substance. (Chemie 2009, pp. 6-18). Cyanide poisoning is fatal and can result in death within minutes if ingested in large dose. Antidotes containing nitrites (amyl nitrate) and thiosulfate (STS) have long been used for the reversal of cyanide toxicity. 1c. In the context of chemical exposure, TWA stands for ‘Time Weighted Average’ and is used to measure the daily exposure of workers to various hazardous substances such as chemicals, fumes, dust, gases etc. It is especially suitable when duration and concentration of chemical exposure is not constant over time. The measurement is obtained by multiplying duration of time period exposed by concentration of chemical substance exposed during the period and adding all the durations. That value is then divided by average working hours per day, mostly 8 hours. TWA is used to determine permissible exposure limits (PELs) for workers. 2a. LD 50 test is the standard method to determine the toxicity of a chemical substance since 1920. The test is performed on laboratory animals that are exposed to relevant chemical being tested. LD 50 is the lethal dose at which 50% of the subjects in an animal group are killed by administered substance. The value obtained can be compared with LD 50 of other substances and a relative toxicity of a substance can be determined. This test is becoming increasingly unpopular because of its various limitations. Firstly, different species react variably to a particular toxin. For example, when compared to dogs, guinea pigs are 10,000 times more sensitive to dioxin. Therefore, LD 50 measured for lab animals may not apply for humans. Furthermore, method used to administer chemical varies among different animal that produce variable LD 50 results. (Girard 2010, p472) 2b. Worker 1: This worker was exposed for 40 minutes of his 480 minutes of daily work. According to the formula: TWA = 40/480 x 320 TWA = 26.667 ppm Worker 2: This worker was exposed for all the 480 minutes of his work. According to the formula: TWA = 480/480 x 10 TWA = 10 ppm Although worker 1 was only exposed for 40 minutes of his daily working hours to the paint fumes, still he has a higher TWA as compared to worker 2 who was exposed for the whole duration of his work. Based on these results it can be concluded that the time alone does not determine the level of toxic exposure but the product of time and concentration together are the significant determinants. 2c. There are various methods by which gastrointestinal decontamination can be achieved to remove toxins from the body. Ipecac syrups became popular in the late 1970s and early 1980s to induce vomiting after oral poisoning. It works by expelling the toxin before it is completely ingested in the gastrointestinal tract. Gastric lavage (GL) is another method of GI decontamination and is particularly beneficial in altered mental status patients. It is commonly used along with activated charcoal for additional benefits. Charcoal, when activated, is a highly absorptive material and therefore take in the poisonous substance inhibiting its absorption in the gastrointestinal tract. Another method for GI decontamination includes whole bowel irrigation (WBI). According to latest research studies WBI is the most effective way of decontaminating a poisoned patient. 3a. Occupational exposure limits (OEL) are determined by competent authorities to control exposure to harmful or toxic substances in a work place. The measurements are taken using sophisticated instruments and a limit is set by national scientific institutes and scientific committees. The concept used behind setting up this limit is ‘no observed adverse effect levels’ (NOAELs). Various cases of previous exposure, epidemiological studies and experimental results are used for setting up the criteria. Air limit values are most commonly determined OEL values whereby acceptable limit of a substance concentration in the breathing air is predicted. OEL has certain advantages over LD50 values. OEL is mostly based on retrograde analysis of cases with toxic exposure to humans. On the other hand LD 50 is solely based on animal studies and as discussed earlier, lethal dose varies among animals and humans may not have the same sensitivity. 3b. There are many incidences in the historical timeline that demonstrates the abilities of ‘professional poisoner’ at work. Assassination of Chechen extremist commander Ibn-al-Khattab by the Federal Security Services of the Russian Federation (FSB) is one such example. FSB hired the services of professional poisoner to prepare a poisoned letter that was delivered to Ibn-al-Khattab by one of his own turned against Russian agent. (Atkins 2004, p167). According to various sources, poison used in assassination was sarin, a colorless and odorless nerve agent that blocks cholinesterase enzyme and induce cholinergic crisis. Because of these disguised properties of the poison, the assassination was successful. 3c. Antidotes are mostly used after identification of the exact poison. This is because some of these antidotes can act as a poison themselves. Antidotes work by either binding to the poison, decreasing their absorption, or inactivating and detoxifying poison into a harmless substance. (Chemie 2009, pp. 6-18). Cyanide poisoning is fatal and can result in death within minutes if ingested in large dose. Antidotes containing nitrites (amyl nitrate) and thiosulfate (STS) have long been used for the reversal of cyanide toxicity. 3d. Mostly, appropriate antidote is given after identification of the poison. But treatment of patients suffering from poisoning should be initiated as soon as possible without waiting for the lab analysis. Appropriate initial management options include gastric lavage, activated charcoal and other methods of gastrointestinal decontamination. These protocols are followed to decrease mortality rate and toxicity of poison by limiting its absorption. Research has shown an increase efficacy of these measures when performed within first hour of poisoning. Pharmacology 1. Radioligand Binding: Most of the drugs work by interacting with certain receptors in our body. Physiologically, these receptors are the target site for endogenous transmitters such as hormones. But certain drugs can also bind (affinity) to these receptors and can have same effect (efficacy) as endogenous transmitters. Such drugs are called receptor agonist. Compounds with affinity to a receptor but no efficacy compete for site with endogenous transmitters and are called antagonist. For simplicity in assay studies both of these types of drug compounds are termed as ligands. To determine the specificity of a ligand with its receptor, it can be radiolabelled and characterized in vitro. Radiolabelling is the most complicated and technical part and can be achieved by two ways. In few cases the ligand already has a suitable place where radiolabelling can be done. But mostly ligand has to be structurally modified first in order to incorporate a binding site required for radiolabelling. During this modification some of the specificity and selectivity of a ligand to a particular receptor may alter which is a complication. In a net shell, effect of radiolabelling a ligand is not very predictable but its specificity and selectivity can be determined using two different techniques; saturation and competition binding assays. KD is the concentration of ligand required to bind 50% of the receptors and is calculated using saturation binding analysis. Using this vital information and by determining the maximum density of receptors (Bmax), accurate final binding parameters can be calculated. (Mather 1996, pp.169-179) Isolated Tissue testing: This type of testing is carried out on isolated cells, tissues and sometimes on whole organ. A cellular or tissue extract bath is prepared using various apparatuses and scientific techniques. Studies are usually conducted in groups on variable number of samples with varying concentration of drug and other factors. These types of testing are more commonly conducted to evaluate the physiological functioning of cardiovascular, gastrointestinal, pulmonary and renal tissues. Contraction relaxation measurements of muscle tissues obtained from various organs can reflect drug interaction with the nervous system and sometime provide certain additional information. Effect of drugs on isolated cells and tissues can be better appreciated as compared to in vivo testing and the effect produced can be solely attributed to the administered drug without any interference of the systemic factors. (Mather 1996, pp.169-179) 2a. Although, in vitro drug testing has various advantages but still in vivo animal testing is crucial before it can be tested on human subjects. This is because in vitro testing has its limitations and may not reveal everything about drug interaction. Most importantly, in vitro techniques such as isolated tissue sampling cannot perfectly simulate human kinetics. There are various endogenous factors that may interfere with the normal functioning of the drug and are impossible to detect in vitro. Pharmacodynamic (PD) actions and Pharmacokinetic actions can be well demonstrated using alive animal models. Once sufficient in vivo animal testing results have been obtained, drug can be further tested in humans. 2b. There are many advantages and disadvantages of using rodents as lab animals for experimentation. They are inexpensive and easily available unlike other animals that may require far greater capital. Most of the experiments require large number of subjects for testing, owing to their small size, rodents are much easier to house. Rats have similar metabolic physiology as humans making them suitable for the studies of pharmacokinetic and pharmacodynamic actions of drugs. Other physiological similarities in CVS and immunology further facilitate experimentation. 2c. Animal model represents or mimics diseased and pathological conditions of humans and other animals. These animal models may inherit the abnormal conditions or they may appear spontaneously. Another type of model is the induced type, whereby, abnormal diseased conditions are produced by laboratory manipulations that includes feeding modified diets, administration of certain drugs or chemicals and performing special surgeries. Diseases such as hypertension, diabetes insipidus and retinal degeneration are inherited by certain animals. Tumors and hemorrhagic strokes can be induced or caused by external manipulation. (University of Washington 2000, p7) 2d. Two methods by which analgesics drug can be tested in animals includes Haffner’s method and multi toe pinch method. These two methods are more or less similar to each other. In Haffner’s method, pain is induced by applying an artery clip at the root of mouse tail. Dosages at which insensitivity to this stimulus is observed are recorded. (Bianchi & Franceschini, 1954). In multi toe pinch method, light artery clip is applied to the base of a toe to induce pain. Animals respond by producing an immediate squeak as a sign of pain. (Collier et al 1961). In the control group, physiological saline is administered instead of analgesics to reduce the chance findings. 3a. Acute toxicity testing: Acute toxicity tests are the common initial tests conducted and reveals the relative toxicity that is evident after administration of a single dose of tested drug. In this study, several groups of animals are administered with graduated dose (three to four dose levels) of test substance, each group receiving the same single dose. Tested substance can be administered by intravenous, intraperitoneal, subcutaneous and oral dose. The test is helpful in determining the gross behavior and LD50 values after drug intoxication. The major aim of acute toxicity test is to devise a therapeutic index; a ratio between lethal dose and effective dose in a specie. (Bharadwaj 2012, pp. 103-129) Subacute toxicity testing: Unlike acute toxicity testing, subacute tests are used to determine the toxicity of a drug administered over a period of several weeks to several months. Three dose levels are calibrated and same dose is received by a group of tested animals with one group acting as a control. These types of tests are often conducted after initial acute toxicity test results have revealed a therapeutic dose. That dose is the starting point and is increased stepwise until toxic signs are evident. (Bharadwaj 2012, pp. 103-129) Chronic toxicity testing: Repeat dosing toxicity studies are conducted to observe the effect of long term use of tested substance. The doses used in chronic types of tests are usually less than used in acute types. Chronic toxicity testing is divided into various stages and duration for each stage varies from 2 week to a couple of years. The result of each stage is used in the subsequent stages. This type of testing are mostly always conducted in at least two different species such as rats and dogs. This is because different species responds variably to the toxic effect of a drug. But if the same toxicity is evident in both, there is a greater probability that same effect will be evident in humans as well. Chronic toxicity testing helps to determine the nontoxic effect dosage in animals which can further be adjusted for humans. For example, non toxic dosage of rats is adjusted for humans by dividing it by 10 and of dogs is divided by 6. (Bharadwaj 2012, pp. 103-129). After this chronic toxicity test, when no overt sign of toxicity is observed, drugs can be administered to humans. 3b. In phase I testing, experiments are mostly conducted on healthy volunteers. But, sometimes based on the type of drug under analysis, relevant diseased patients may also be made part of the testing. Number of subjects may vary depending upon the drug but are usually between the ranges of 20 to 80. The aim of these testing is to provide some crucial information about the drug such as its pharmacokinetics and pharmacodynamics. It is important to evaluate how drug interact with the systemic environment of human body and how body reacts towards the drug. Drug’s half life and clearance are important entities that are measured during the experimentation. Reference (1998). Combinatorial chemistry & high throughput screening. Hilversum, Netherlands, Bentham Science Publishers. ATKINS, S. E. (2004). Encyclopedia of modern worldwide extremists and extremist groups. Westport, Conn, Greenwood Press. Bharadwaj, S. (2012). STUDIES OF ACUTE, SUBACUTE AND CHRONIC TOXICITY TEST. International journal of Advanced Research in Pharmaceutical and Bio sciences. Volume.1, 103-129 BIANCHI, C., & FRANCESCHINI, J. (1954). EXPERIMENTAL OBSERVATIONS ON HAFFNERS METHOD FOR TESTING ANALGESIC DRUGS. British Journal of Pharmacology and Chemotherapy. 9, 280-284. CHEMIE FACHINFORMATIONSZENTRUM. (2009). +report+. Berlin, Germany, Fachinformationszentrum Chemie GmbH (FIZ CHEMIE Berlin - The Chemistry Information Center). COLLIER HO, WARNER BT, & SKERRY R. (1961). Multiple toe-pinch method for testing analgesic drugs. British Journal of Pharmacology and Chemotherapy. 17, 28-40. GIRARD, J. (2010). Principles of environmental chemistry. Sudbury, Mass, Jones and Bartlett Publishers. MAGAN, N., & OLSEN, M. (2004). Mycotoxins in food: detection and control. Cambridge, England, Woodhead Publishing Limited. MANDAL, S., MOUDGIL, M., & MANDAL, S. K. (2009). Rational drug design. European Journal of Pharmacology. 625, 90-100. MATHER, S. J. (1996). Current directions in radiopharmaceutical research and development. Dordrecht, Kluwer Academic Publishers. UNIVERSITY OF WASHINGTON, & AMERICAN COLLEGE OF LABORATORY ANIMAL MEDICINE. (2000). Laboratory animal medicine & science series II. [Chester, NH], American College of Laboratory Animal Medicine. WERMUTH CG. (2006). Similarity in drugs: reflections on analogue design. Drug Discovery Today. 11, 7-8. Read More