1 - Intro to Pharmacodynamics.pdf

FSYD-Pharmacology Lesson 1 Uccellini - Palumbo Introduction to pharmacology (pharmacodynamics 1) The main objectives of this course of pharmacology are: - introduction to the pharmacological concepts that constitute the base of this science: pharmacokinetics and pharmacodynamics - illustrating th...

FSYD-Pharmacology Lesson 1 Uccellini - Palumbo Introduction to pharmacology (pharmacodynamics 1) The main objectives of this course of pharmacology are: - introduction to the pharmacological concepts that constitute the base of this science: pharmacokinetics and pharmacodynamics - illustrating the general principles of chemotherapy of infections Chemotherapy is a concept that has not to be restricted only to the pharmacology of cancer; it originated as a branch of pharmacology able to target the specific kinds of cells (different from the other cell types present in the body, and therefore specifically targetable) that are the leading causes of disorders: microbes and tumor cells. Pharmacokinetics and pharmacodynamics These are the two main principles on which pharmacology is based. Every time a person takes a drug, a bidirectional array of interactions is initiated between the drug and the organism: on one hand there is the effect produced by the drug over the body (associated to the amelioration of symptoms or resolution of disease) and on the other there is the effect produced by the body over the drug (as soon as a drug is introduced in the organism the drug is distributed and metabolized by the elements present in the living system). In particular: • Pharmacodynamics deals with the molecular mechanisms through which a drug is able to exert its effects on the body. • Pharmacokinetics studies what the body does to the drug: it studies the movement of the drugs throughout the body. A really clear exemplification of this concept is given by the route taken by an orally administered pill: - Upon administration, the drug is initially absorbed at the level of the GI (moves from the site of administration to the bloodstream) - then it moves from the bloodstream to the tissues (process known as diffusion) - in the meanwhile, it passes through organs such as the liver in which the drug is metabolized, undergoing a series of enzymatic reactions that are able to modify its chemical structure resulting in an enhancement or decrease of its effect - eventually, it is excreted. (note that these general steps are common to all drugs despite the method of administration) Definition of a drug There are two possible of definitions of drug: - general definition: any chemical or natural agent able to generate a biological response - restricted definition: any chemical or natural agent that can be used for diagnostic, prophylactic or therapeutic purposes (in the clinic) 1 FSYD-Pharmacology Lesson 1 Uccellini - Palumbo The first definition is opened to all agents that affect living, including both substances that are beneficial (clinically used molecules such as benzodiazepines, that targets GABA, or phenytoin, which is an antiepilectic drug that targets sodium channels) and detrimental (such as cocaine or heroin) to the organism. The second definition is a bit more appropriate since it excludes purely detrimental/recreational agents. History of pharmacology Since the beginning humans have tried to exploit substances of natural origin (mainly plant derived solutions) to treat medical disorders, principally to ameliorate medical symptoms. The Ebers Papyrus (1500 BC) is the first written evidence containing information on the use of several plant-derived preparations as purgative, diuretics or sedative. It also contains a detailed description about the use of opium both as curative and as poison; in particular to treat diarrhea (morphine is able to ameliorate several of the related symptoms), increase the mood and as a sedative. At this point in time there was only a basic knowledge about symptoms (fever, nausea… etc.) but there was no kind of notion about disorder, its ethology and all the mechanisms underlying the symptoms. In the middle of 1700 among scientist, rose the conviction that most probably it is not the mixture that produces a beneficial effect but that a specific component of the mixture is responsible for it. From this assumption efforts were put in the attempt of extracting these specific molecules; which resulted in the extraction/identification of active principles such as quinine, strychnin (which are both alkaloids) and digitalis (a glycoside). Among them quinine is particularly important: this alkaloid can be used as an antimalaric drug, however the exact mechanism of action of this drug is not known yet. It is thought that quinine can modify the intracellular environment of RBCs in order to make it not suitable for the parasite (that is the site where, in one of the phases of its life cycle, it replicates), in particular in relation to heme Iron that is toxic for Plasmodium. At the beginning of the 1800 the production of principles in the lab initiated (dawn of chemical synthesis), at the beginning of 1900 industrial scale production started (aspirin was one of the first drugs to be produced on industrial scale). Drug discovery processes (discovery vs invention) There are two possible processes of drug discovery, very different from the conceptual point of view. 1) The first and older is discovery (performed until 20 years ago), this method is mainly based on the concept of serendipity which refers to “an accidental discovery” (finding one thing while looking for something else); consisting on the capacity to recognize that previously used agents (mainly drugs used in the treatment of specific disorders) possess additional effects to the specific one they were used for. The most famous example of drug discovery concerns Iproniazid, a drug that was originally developed for the treatment of tuberculosis and successively discovered by researchers to possess an antidepressant effect simply by noticing that patients undergoing treatment became inappropriately happy. This drug possess this additional effect because it is able to block the monoxydase enzymes (he described these enzymes with this name but searching the action of this active principle on the web, the enzymes that are described to be blocked are the monoaminooxydases MAO), which are important for the metabolism (and in particular degradation) of various neurotransmitters affecting mood among which noradrenalin, adrenalin and serotonin are the most important ones; therefore resulting in a positive effect on mood. 2 FSYD-Pharmacology Lesson 1 Uccellini - Palumbo 2) The second and newer one is drug invention, that is a process of rational based drug discovery: a target centered drug design based on the process of high-throughput screening, in which candidate drugs (which structure is known and contained in a huge molecular library) are identified based on a robotic automation to test many thousands of compounds rapidly. For this process to be carried out plenty of information about the specific receptor to be targeted are required; in particular about the structure of the specific receptor. The appropriate drug candidate is discovered starting from the 3-D structure of the target receptor that is obtained through NMR or x-ray crystallography. Concept of receptor One of the most important revolution in the field of pharmacology, and in particular of pharmacodynamics, is the concept of receptor. 80 to 90% of drugs work by interacting with a specific receptor, this implies the concept of drug specificity and the capacity to selectively target one aspect of the disease and not the possible others (meaning that you can avoid any side of side effect/withdrawal effect by simply targeting a specific kind of receptor). In pharmacology, the term receptor is used in a wider way compared to the strict molecular definition, any cell constituent that can be specifically influenced by a drug is considered a pharmacological receptor (they are mainly proteins, with different function and localization) This concept has been introduced at the beginning of the 19th century by Paul Ehrlich, scientist that by working with specific dyes recognized that these molecules when applied to a system infected with specific bacteria were able to recognize selectively (and stain) the microbial cells and not the other cells. He thought that the dye must function by specifically interacting and bonding to specific structures that are present in the microbial cell only. He then realized that the dye could probably be used as a pharmaceutical drug (however the compound had a lot of side effects and could not be used in the clinic). He also introduced one of the main concepts at the base of the idea of a good antimicrobial drug: a drug should be like a magic bullet, able to target selectively structures and biological mechanisms possessed only by the invading microbe. Drug receptor interaction Another fundamental concept at the basis of pharmacology (and in particular pharmacodynamics) is the interaction between the drug and the receptor. This reaction is effectively exemplified by the “lock and key” metaphor in which the receptor is represented by the lock and the key represents the drug. As already described, 80 to 90% of drugs used in the clinic work by interacting with a specific receptor, however there are some exceptions. The remaining 10 to 20% is represented by five classes of drugs that work thanks to their specific physical/chemical properties. They are: - Absorbents: such as activated carbon that is used upon poisoning (for example by barbiturates). It’s used to prevent the absorption of the harmful drug (it can be used to treat only poisoning occurred by mouth and to be effective it has to be administered within a short time of the poisoning. It works by directly sequestering the poison of interest. In rare and extreme situations it can also be used in a 3 FSYD-Pharmacology Lesson 1 Uccellini - Palumbo hemoperfusion system to remove the toxins directly from the blood) in order to prevent its negative effects. - Disinfectants: antimicrobial agents able to eliminate microbes based on their general chemical properties (es. release of H2O2). - Antacids: as sodium bicarbonate, they act thanks to their acid-basic properties. - Laxatives and osmotic diuretics: act thanks to their osmotic properties. The following figure shows the chain of events regulating the drug-receptor dynamic of interaction. Once the receptor interacts with the drug, the drug binds to the receptor and the drug receptor complex is created. This complex is able to produce the biological effect of the drug, which is mediated by an effector that is activated/generated by the active receptor. To sum up, the importance of the receptor and of the drug-receptor interaction in pharmacodynamics is summarized by three points: - the concept of selectivity: receptors are responsible for selectivity of drug action. The drug in order to be effective should interact with high affinity with a receptor; this selective binding is determined by several factors (molecular shape of the drug, electrical charge, conformation and structure … etc) - receptors largely determine the quantitative relationship between drug concentration and evoked biological effect (based on receptor affinity and total number of receptors) - the receptor mediates the action of pharmacologic agonists and antagonists Consideration on the size of clinical drugs The dimension of drugs used in the clinic is very heterogeneous: there are very small drugs (such as lithium, with is a metal ion used in the treatment of bipolar mood disorders) and very big drugs (such as really big anti-aggregant drugs), however the vast majority of drugs used in the clinic (90%) are between 100 and 1000 kilodalton. To be biologically effective a drug needs to have upper and lower size limits: the lower limit is determined by the receptor site of interaction (the drug has to fit precisely the active site of the receptor) while the upper limit is determined by the necessity of diffusion (drugs that are too big can’t be absorbed at the level of the GI and can’t diffuse to tissues). Drugs can interact with their specific receptor either via weak bonds or strong bonds. Most drugs interact with the receptor via weak bonds, consisting of electrostatic interactions, van der Waals forces or salt bridges. This kind of interaction is reversible, implying that the activation of the receptor can be competed by other molecules (es. the biological action of a drug acting via 4 FSYD-Pharmacology Lesson 1 Uccellini - Palumbo weak bonds can be antagonized by simply giving an antagonist to the patient); even if this kind of bond is described as weak it is still extremely selective and generally at very high affinity. On the other hand, drugs acting via strong bonds form covalent bonds with the receptor. This interaction is irreversible (no molecular competitor can be used to make the drug detach, it is only possible to wait that the degradation of the receptors) and therefore the action of these drugs is long lasting. Examples of drugs that act via strong bonds are omeprazole, aspirin, reserpine, penicillin and nerve agents. The very strong interaction can be an advantage, especially when the aim of the drug is a very long lasting effect. Sarin, was a nerve agent used in concentration camps in WW2 which works by blocking acetylcholinesterase (enzyme required to degrade acetylcholine). When this enzyme is blocked acetylcholine remains in the synaptic/neuromuscular clefts of the target and produces a constitutive stimulation resulting in several symptoms among which the most threatening one is the impairment of the muscle contraction in the respiratory system (particularly diaphragm) resulting in death from suffocation. One of the crucial most important factors regulating the drug-receptor interaction is the Kd, which is the dissociation constant and therefore is an index of the affinity of interaction between the drug and the receptor (the lower the Kd, the stronger the interaction). Another important aspect regulating the interaction between drug and receptor is drug specificity in relation to shape. A clear example of this concept is the differential pharmacological action of chiral active principles, of which the two enantiomers generally have very different specificity of interaction for a receptor. This is due to the different molecular interactions deriving from the different geometric properties of the enantiomers, even though the overall molecular structure is the same. An example of this aspect is the drug carvedilol, a modulator of beta adrenergic receptor used in many different disorders (hypertension, arrhythmia… etc.); in the vast majority of cases this drug is administered as a racemic mixture (containing both enantiomers), with the two enantiomers that have very different affinities for the receptor. There are specific drugs in which the racemic mixture can’t be used because while one enantiomer is curative the other is extremely toxic. The most common example is thalidomide, drug used in the 50s to treat a lot of symptoms present in pregnancy but that was removed from the markets because only one enantiomer is beneficial while the other one is teratogenic. Additionally, enantiomers may have also different metabolism: the activity of the enzymes involved in their metabolism could be affected by the geometrical structure resulting in a very high rate of degradation for one enantiomer and a very low rate for the other. Teratogen = an agent that causes abnormality following foetal exposure in pregnancy Occupancy theory Theory that regulates the drug-receptor dynamics, it is a quantitative analysis of drug-receptor interaction that is valid for both endogenous and exogenous ligands. It has to be kept in mind that there is a fixed number of receptors (of a certain type) in a biological system, when a particular receptor is 5 FSYD-Pharmacology Lesson 1 Uccellini - Palumbo targeted by a drug the biological effect produced depends on the saturation level (how many receptors are occupied) of the receptor repertoire. Since the receptor number is stable, the strength of the evoked effect is dependent on the dosage of the drug. Once administered and delivered to the site of activity a drug interacts with the receptor forming the drug-receptor complex, at the equilibrium it is possible to calculate the affinity between drug and receptor represented by the Kd. At equilibrium, if the affinity is high, the concentration of the ligand-receptor complex will be higher than the one of the drug and receptor alone. This (as showed by the first equation on the right) will result in a very low Kd; the lower is the Kd, the higher is the affinity of interaction. The previous equation can be rearranged for the concentration of the ligand receptor complex (which indicates the most important factor regulating the response). Assuming that the total amount of receptors in the biological system is the sum of the free receptors and the receptors bound to the ligand, it is possible to substitute this equation with the first one. The obtained equation can be solved for [R] and be re-substituted into the initial one; a relation between the fraction of occupied receptors and the concentration of the drug is obtained (last equation). The obtained equation describes the drug-receptor binding curves (it regulates binding between drug and receptor). *1 The demonstration of this equation with all passages is at the end of the sbobina If the [LR]/[Ro] ratio is 1 it means that all the receptor are bond to the drug, if it is 1\2 it means that 50% of receptors are bond to the drug; it reflects the level of saturation of the receptor. The curve described by the equation is a hyperbolic curve; increasing the total concentration of the drug, the concentration of occupied receptors will also increase; using a high drug amount, all the receptors will be eventually bond. Since the curve is hyperbolic, in the initial phase of increase of drug concentration things change very rapidly (the curve rises very steeply). In order to expand the first and most rapidly variable part of the curve at the expense of a loss in the description at high drug concentrations of the last part (that however changes very slowly), the curve can be converted to a semi-logarithmic curve; in doing so the description becomes a sigmoid curve, it is the same but it’s a little bit easier to be studied (you can see precisely what happens in the first part). Since the drug effect takes place upon binding, this fraction (and curve) will also represent the total response. In addition, the fraction bond/not bond will represent the % of the possible maximal effect generated; every time this ratio is equal to 1 the max effect of the drug is obtained. When 50% (when the fraction reaches 0,5) of the receptors occupied the Kd is equal to concentration of the drug. 6 FSYD-Pharmacology Lesson 1 Uccellini - Palumbo *1 7