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Transcript
Lecture #13: Enzyme Mechanisms-III: Techniques for Drug Discovery A. Drug Design: Improvements in medical care are largely attributed to development of wide variety of drugs including: -antibiotics, anti-inflammatory agents, analgesics and anesthetics, antidepressants, antipsychotics, immunosuppressants, cancer drugs, cardiovascular and stroke drugs, etc. B. Techniques of Drug Discovery: -most drugs act by modifying the function of a particular receptor in the body or an invading pathogen -receptor: enzyme, channel or signalling protein -compounds that modify the receptor function = agonists -compounds that bind without modifying function, but block agonist binding = antagonists -biochemical and physiological effects of a drug and its mechanism of action are called pharmacodynamics 1 1. Complexity of Drug Discovery -most drugs were discovered by screening large numbers of synthetic compounds and natural products for the desired effect -natural products are discovered by fractionation of the organism in which they occur and isolating the “active ingredient” -in vitro screens are initially used such as the binding of a drug to a target enzyme implicated in the disease -as the number of drug candidates is reduced then testing in animals are employed -a drug candidate that exhibits a desired effect is called a lead compound -a good lead compound binds to its target with a Kd < 1 M -high affinity is necessary to minimize nonspecific binding to other macromolecules to ensure that low doses are required Re: for enzyme inhibitors the Kd = KI 2 Measures of a Drug’s effectiveness IC50: the [inhibitor] at which an enzyme exhibits 50% of its maximal activity (vi/vo = 0.5) ED50: the effective dose of a drug to produce a therapeutic effect in 50% of a test sample TD50: the toxic dose of a drug to produce a particular toxic effect in animals LD50: mean lethal dose of drug required to kill 50% of a test sample therapeutic index: is the TD50/ED50 ratio Drugs that have high therapeutic indices are the best 2. SARS and QSARS -lead compound is the starting point to design more efficacious compounds -even minor modifications to a drug candidate can result in major changes in its pharmacological properties -might place methyl, chloro, hydroxyl, or benzyl groups at various places on the lead compound -most successful drugs today are the result of 5,00010,000 related compounds that were synthesized/tested -process has been made systematic through structureactivity relationships (SARS) -SARS: the determination by synthesis and screening of which groups on a lead compound are important for drug function 3 QSAR-quantitative structure-activity relationship -there is a simple mathematical relationship between the biological activity of a drug and its physiochemical properties -e.g., if the hydrophobicity of a drug is important for its biological activity then changing the substituents on the drug to alter its nonpolar character will affect its activity -hydrophobicity is measured by P (partition coefficient) between two immiscible solvents where P = [drug]oct/[drug]H20 -if C is [drug] to achieve a specified level of biological function then activity may be expressed as 1/C -plot of 1/C vs log P for a series of derivatives having a small range of log P values often indicates a linear relationship log(1/C) = k1logP + k2 where k1 and k2 are constants whose optimum values in QSAR can be determined by computerized curve fitting -for compounds with a larger range of logP values, the plot of log1/C vs log P will have a maximum value and will be described by a quadratic equation log(1/C) = k1(logP)2 +k2logP + k3 4 -typically QSAR depends on the chemical properties of a variety of subsituents V&V:F15-29 -pK values, van der Waals radii, H-bonding energy, and conformation 3. Structure-based drug design -also called rational drug design uses the structure of a receptor in complex with a drug candidate to guide the development of more efficacious compounds -method is possible due to dramatic advances in the speed and precision with which a macromolecular structure can be determined by X-ray crystallography and NMR -structures reveal positions of H-bonding donors/acceptors in receptor’s binding site and cavities where substituents may be placed on a drug candidate to increase its binding affinity -may be supplemented with molecular modelling tools 5 such as energy minimization, quantum mechanical calculations, docking simulations -process is iterative: structure of receptor in complex with improved properties is determined to further improve its properties 4. Combinatorial Chemistry and High-Throughput Screening -recent advances in combinatorial chemistry to rapidly and inexpensively synthesize large numbers of compounds and development of robotic high-throughput screening techniques has facilitated this make-manycompounds-and-see-what-they-do approach V&V:F15-30 -previously, investigations of a hydrophobic substituent might have involved ethyl, propyl, benzyl derivatives -through combinatorial synthesis perhaps 100 different groups may be added to a single position 6 C. Introduction to Pharmacology -in vitro development of an effective drug candidate is only the first step in the development process -a useful drug must also be delivered in sufficiently high concentration to the target receptor in the human body without causing unacceptable side effects 1. Pharmacokinetics -refers to the ways in which a drug interacts with various cell barriers -most convenient form of drug delivery is orally -drugs must pass a series of barriers -chemically stable in the acidic pH of stomach and not be degraded by digestive enzymes -must be absorbed into the bloodstream and pass several membranes -must not bind too tightly to other substances in the body -must survive modification by a battery of enzymes (mainly in the liver) -avoid rapid excretion by the kidneys -must pass from capillaries to the target tissue -if target is in brain then must pass blood-brain barrier -if target is intracellular then must pass through the plasma membrane 7 -bioavailability of a drug (extent to which it reaches its site of action) depends on both the dose given and its pharmacokinetics -Lipinski’s rule of five states that a compound is likely to exhibit poor absorption or permeation if: -its molecular weight is greater than 500 Da -it has more than 5 H-bond donors -it has more than 19 H-bond acceptors -its value of log P is greater than 5 CA Lipinski, Adv. Drug Del. Rev. 1997, 23, 3 -most drugs are a compromise: they are neither too lipophilic nor too hydrophilic -pK values usually in the range of 6 - 8 2. Toxicity and Adverse Reactions Eliminate Most Drug Candidates -final criteria for a drug candidate are that its use be safe and efficacious in humans -tests initially carried out in animals but since humans and animals often react quite differently then ultimately the drug must be tested in clinical trials -in US, clinical trials are monitored by the FDA and have 3 phases 8 a. Phase I -designed to test the safety of a drug candidate but also to determine the dosage range and optimal dosage method -carried out on a small number of normal, healthy volunteers except for highly toxic drugs (e.g., a cancer chemotherapeutic agent) where it is carried out on volunteer patients with the target disease b. Phase II -tests the efficacy of the drug against the target disease in 100 to 500 volunteer patients but also refines the dosage range and checks for side effects -usually tested via single blind tests in which the patient is unaware of whether he/she has received the drug or a placebo (an inert substance) c. Phase III -monitors adverse reactions from long-term use as well as confirming efficacy in 1000 to 5000 patients -tests the drug candidate against control substrances through statistical analysis of carefully designed double blind tests in which neither the patients or the investigators know whether a given patient has received the drug or a control substance 9 3. Drug Candidate Statistics -5 drug candidates in 5000 that enter preclinical trials reach clinical trials -1 of the 5 is approved for clinical use -40% of candidates pass Phase I trials -50% of those passing Phase I trials pass Phase II trials -most drugs that enter Phase III pass this trial -preclinical portion of drug discovery process averages 3 years -successful clinical trials usually require 7 – 10 years -becoming increasingly expensive to bring a drug to market, on average the cost is $300 million USD -not uncommon for a drug to be withdrawn some months or years later when it is found to have caused unanticipated life-threatening side effects in as few as 1 in 10,000 individuals 4. Cytochrome P450 Metabolize Drugs -why is it that drug that is well tolerated by most patients can pose a danger to others? -differences in reactions to drugs arise from genetic differences , different disease states, other drugs that they are taking, age, sex, and environmental factors 10 -cytochrome P450 role is to detoxify xenobiotics and assist in metabolic clearance of drugs -humans express ca. 100 isoenzymes that catalyze the general reaction: RH + O2 + 2H+ + 2e- ROH + H2O V&V:F15-31 -many cytochrome P450 in humans are polymorphic with several common alleles -gives rise to tremendous variation in drug metabolism among individuals V&V:F15-32 11 -acetaminophen is a widely used analgesic and antipyretic (fever reducer) -safe in therapeutic doses (1.2g/day for an adult) but in large doses (> 10g) is highly toxic -usually 95% of acetaminophin is enzymatically glucuronidated or sulfated at its OH group to form conjugates which are readily excreted -remaining 5% is converted by cytochrome P450 (CYP2E1) to acetimidoquinone which is conjugated with glutathione -upon large doses of acetaminophin, the glucuronidation and sulfation pathways become saturated and the cytochrome P450-mediated pathway becomes more significant -if hepatic glutathione is depleted faster than it can be replaced, acetimidoquinone, a reactive compound, instead conjugates with the sulfhydryl groups of cellular proteins causing often fatal hepatotoxicity 12 5. Many Drugs are Enzyme Inhibitors -modern drug therapy is based on concepts of enzyme inhibition -toxicity is unavoidable because, with the exception of cell wall synthesis in bacteria, most drugs that kill tumors, viruses, and bacteria also kill normal eukaryotic cells within the host -relatively short generation time of the microbial organisms and tumor cells can be exploited -these cells will be more sensitive to metabolic inhibitors than the host eukaryotic cells a. Sulfa Drugs -sulfanilamide is an antibacterial agent because it completes (competitive inhibitor) with paminobenzoic acid (PABA), essential for bacterial growth 5,6,7,8-tetrahydrofolate -bacteria can’t absorb folic acid but must synthesize it -bacterial dihyropteroate synthase is tricked into making an intermediate containing sulfaniliamide that can’t be converted to folate -bacteria is starved of the required folate and can’t grow or divide -since humans obtain folate from dietary sources, the sulfanilimide is not harmful at doses that kill the bacteria 13 b. Viagra®-an unexpected outcome in a drug-design program -sometimes the outcome of a drug design program is unanticipated -e.g., penicillin discovery by Fleming -relaxation of smooth muscle cells of blood vessels is controlled by intracellular [Ca2+] as triggered by [cGMP] -cGMP is hydrolyzed by phosphodiesterases (PDE) to form 5’-GMP and let muscles contract again -Pfizer designed PDE inhibitor against PDE5 isotype, a dominant form in human vascular tissue -no significant benefits for angina or hypertension but some men in clinical trials reported penile erection -apparently, Viagra® causes an increase in [cGMP] in penile vascular tissue allowing vascular muscle relaxation, improved blood flow and erection. A drug was born! 5’-GMP Viagra® 14