General principles of pharmacology

Chemical structure

Organic molecules consist of functional groups organised around a carbon skeleton. These structures may adopt different conformations allowing specific and non-specific interactions with receptors and binding with proteins and other molecules within the body. The physical organisation of the drug's molecules, functional groups and charge determines further interactions between groups on the same or other molecules. The exposure of charges and hydrophobic or hydrophilic groups may influence a drug's ability to cross membranes, reach a site of action or be excreted.

Isomerism

Isomers are molecules sharing the same chemical formula but with a differing physical arrangement of atoms. Different forms (enantiomers) may interact with other molecules in a variable way and this, therefore affects function. There are several different classes of isomer ( Fig. 1.1 ).

Structural isomerism describes the organisation of functional groups within an organic molecule, where either the carbon skeleton, functional group or position of functional groups along a chain differs. Isoflurane and enflurane share the formula C ₃ H ₂ ClF ₅ O but have differences in solubility, metabolism, potency (as defined by the minimum alveolar concentration, or MAC) and other characteristics. The different positions of the highlighted F and Cl atoms in Fig. 1.1 distinguish these different isomers. Because of the molecular size, spatial conformation and charges contained within these functional groups, the different configurations can affect end function and particularly interaction with receptors and target molecules.

Stereoisomerism refers to molecules with identical chemical and molecular structures but a different spatial organisation of groups around a chiral atom (usually carbon). Chiral atoms are present in all stereoisomers and have bonds entirely occupied by dissimilar functional groups. Stereoisomers are subclassified by the direction in which they rotate plane polarised light (optical isomerism) and/or by the spatial arrangement of atomic groups around the chiral centre. When polarised light is travelling towards the viewer, it can be rotated clockwise (termed dextrorotatory ( d ) or +) or anticlockwise (laevorotatory ( l ) or −). The dextro (+) and laevo (−) enantiomers of a compound are non-superimposable geometric mirror images of each other. The alternative R/S classification organises compounds according to the atomic number of groups of atoms attached directly to the chiral centre. When viewed with the lowest priority (lowest atomic number) facing away, the priority of the other groups can decrease clockwise (R form) or anticlockwise (S form). These correspond to ‘right-handed’ (rectus) or ‘left-handed’ (sinister) forms. Note that the laevo/dextro (or +/−) and R/S properties of a drug are independent.

The different configurations are known as enantiomers. Naturally occurring compounds (including some drugs) contain R and S forms in equal proportions – a racemic mixture . However, as enantiomers may have different properties (including receptor binding, activity and effects) it may be advantageous to use an enantiopure drug formulation that contains a single enantiomer. Examples include S(+) ketamine and levobupivacaine, as discussed later.

Molecules containing more than one chiral centre are described as diastereomers, which may form geometric isomers, based on the orientation of functional groups around a non-rotational carbon-carbon double bond. Fig. 1.1 shows the example of cis- and trans-geometric isomers. Atracurium contains four chiral centres, resulting in 10 stereoisomers present in solution. The enantiopure form, cisatracurium, causes less histamine release than the non-pure form, theoretically conferring greater haemodynamic stability.

Tautomerism describes dynamic structural isomerism, where drug structure may change according to the surrounding environment. For example, midazolam has an open, water-soluble ring structure in storage at pH 3 but undergoes a conformational change at body pH of 7.4 to become a completed ring structure, which is lipid soluble and passes through the blood–brain barrier.

Transport of drugs

Most drugs must pass from their site of administration to their site of action (effect site) before metabolism and elimination, usually via the kidneys or liver. This occurs by local diffusion across plasma membranes, and subsequently mass transport in the blood through dissolution or carriage by transport proteins.

Transmembrane movement is either active or passive. The former is undertaken by transporter proteins with the hydrolysis of adenosine triphosphate (ATP) or movement of other molecules, and the latter occurs by diffusion with no net utilisation of energy. Facilitated diffusion utilises carrier proteins within the cell membranes to increase diffusion along a concentration gradient.

The rate of passive diffusion is determined by the nature of the drug, barriers or membranes, concentration gradient and temperature. The presence of any transporter molecules or ion channels enhance diffusion. The rate of diffusion is inversely proportional to the square of the molecular weight (Graham's law). The relationship between rate of diffusion and concentration gradient, membrane permeability, thickness and surface area is described by Fick's law. Drug solubility, by virtue of size, charge, and the presence of hydrophilic or lipophilic groups, also determines ease of movement across membranes.

Active transport describes the expenditure of energy to facilitate the movement of molecules, for example via symports, antiports or cotransporters, either at the expense of other molecules or ATP. Bacteria may develop antibiotic resistance by actively extruding antibiotic molecules from their cells via such mechanisms.

Ionisation and equilibria

An acid is a proton donor and a base is a proton acceptor; many drugs are weak acids or bases, partially dissociating to exist in equilibria between ionised and unionised forms. Strong acids and bases are substances that dissociate completely.

The relative proportions of ionised and unionised forms for a given drug may be derived from the Henderson–Hasselbalch equation ( Eq. 1.1 ) and depend on the environmental pH and the p K a, the pH at which a given drug exists as equal proportions of its ionised and unionised forms. The p K a is determined by the chemical nature of a drug and is independent of whether it is acidic or basic.

Only the unionised forms of a drug or molecule are highly lipid soluble and can cross cell membranes easily, so the environmental pH determines the action of many drugs.

The Henderson–Hasselbalch equation (see Eq. 1.1 ) can be rearranged to derive the relative proportion of ionised and unionised forms of a drug, given a known p K a and pH.

For a weakly acidic drug dissociating to H ⁺ and A ⁻ , substituting these into Eq. 1.1 ( Eq. 1.2 ) and rearranging gives the relative proportions of the ionised, A ⁻ , and unionised form, HA ( Eq. 1.2C ). Similarly for a base, accepting a proton to form BH ⁺ , the same principles apply.

The equilibria given are dynamic and follow the law of mass action depending on the prevailing [H ⁺ ]. The p K a is the negative logarithm to the base 10 of the dissociation constant. It is an intrinsic property of a drug or molecule and is also the pH at which half of all molecules are ionised. The ionised (BH ⁺ ) form of a weak base predominates at a pH lower than its p K a, whereas the converse is true for weak acids. The principles of this are shown in Fig. 1.2 .

Local anaesthetics are weak bases (i.e. proton acceptors) with p K a values of approximately 8. Therefore at physiological pH (7.4) the environment is relatively acidic compared with the drug, and this renders local anaesthetics ionised. The addition of alkali favours the unionised form (by mass effect) and aids passage of local anaesthetic molecules across the neuronal membrane. Once inside the neuronal tissue, the molecule then becomes charged inside the nerve and is trapped, facilitating interaction with its target, the sodium channel. Acidic environments (in infected tissue) promote the ionised form, preventing entry into nerve tissues, rendering local anaesthetics less effective.

Weak acids, such as thiopental, have the opposite relationship, with the proportion of unionised molecules decreasing with increasing pH (see Fig. 1.2A ).

Urinary alkalinisation traps the ionised form of acidic compounds in the renal tubules, preventing reabsorption, and is hence sometimes used in the management of poisoning (e.g. by salicylic acid).

Receptor-mediated effects

Most drugs act at specific receptors. A receptor is a protein that produces an intracellular response when a ligand (which may be a drug or an endogenous molecule) binds to it. In the unbound state, receptors are functionally silent. Increasing knowledge of the molecular structure of the binding regions of receptors and the complementary region on drug molecules facilitates the design of specific, targeted drugs and opens the possibility of reverse pharmacology – the discovery of new ligands for a given receptor structure.

Receptors enable cells to adapt to environmental conditions outside (in the case of membrane-bound receptors) or inside (in the case of nuclear receptors). Receptors may initiate responses locally (such as the opening or closure of ligand-gated ion channels) or via secondary messenger systems. Through activating second messengers, events on the cell surface may be amplified and cause intracellular changes.

The structure of the receptor is fundamental to its function and production of a specific response to ligand binding. The binding regions are unique to each receptor, but there are some common motifs between classes ( Table 1.2 ).

G protein–coupled receptors

The largest superfamily of receptors contains 7-transmembrane domains and couple to G-proteins. G protein–coupled receptors act as adapters between extracellular signalling and intracellular downstream second messenger systems, the endpoint of receptor activation ( Fig. 1.4 ). Upon activation, the receptor undergoes a conformational change, causing the α subunit of the G protein to exchange bound guanosine diphosphate (GDP) for guanosine triphosphate (GTP) (see Fig. 1.4A ), which then causes it to dissociate from the G protein–coupled receptor and move to an effector molecule (see Fig. 1.4B ). Whilst influencing the effector the bound GTP is hydrolysed to GDP by the GTPase of the α subunit (see Fig. 1.4C ), which then returns to the intracellular terminus of the receptor and reassociates with the βγ subunits (see Fig. 1.4D ).

There are three major types of G protein ( Table 1.3 ) capable of interacting with the second messengers adenylate cyclase (G _i/o and G _s ) and phospholipase C (G _q ). A single receptor may activate many G proteins, and this amplifies the initial signal. Drugs can target different parts of this receptor cascade by interacting with the receptor (e.g. opioids), the G protein (e.g. botulinum toxin) or further downstream (e.g. Ca ²⁺ ).

Second messenger systems

Second messenger systems are the endpoint of a number of different inputs. They are a further level of amplification, enabling a small extracellular signal to effect a large and significant change to the intracellular environment.

Second messengers can enable other functions, such as protein phosphorylation. The result of protein phosphorylation can influenceion transport (e.g. acetylcholine, glutamate, γ-aminobutyric acid (GABA) receptors), and receptor activation state.

What influences the response of a receptor?

Ligands bind to receptors and are classified according to their effect, either to increase (agonist) or to have no effect (antagonist) on receptor response. Ligand-receptor kinetics, biological reactions and enzyme kinetics are all subject to the law of mass action, which states that the rate of reaction is proportional to the concentration of biologically active species present at a given time.

The classic rectangular hyperbolic dose–response relationship of an agonist is shown in Fig. 1.5A . As the number of agonist molecules increases, the receptor response increases until all binding sites are fully occupied and response is maximal (see Fig. 1.5A ). This rectangular hyperbolic shaped curve is conventionally transformed to a semilogarithmic scale to produce a sigmoid-shaped curve that is approximately linear between 20% and 80% of maximum effect, allowing comparison of the effects of different agonists (see Fig. 1.5B ). Agonists are described by the size of the effect they can produce (efficacy) and the dose required to elicit a given effect (potency).

The pEC ₅₀ is a log-transformed measure of the ligand concentration required to elicit a half-maximal response for the drug, a measure of potency. A rightward shift on the logarithmic scale indicates that the drug has lower potency; therefore an increased dose is required to achieve the same effect. Efficacy is the size of the response produced by ligand-receptor interaction. E _MAX is the maximal response. Therapeutic ligand-receptor interactions may produce unwanted adverse effects (such as the respiratory depression caused by agonism at the μ opioid receptor).

Full agonists can elicit a maximal effect, whereas partial agonists can never achieve maximum effect irrespective of dose and so have lower efficacy. Fig. 1.5B shows the effects of agonists A, B and C. B is less potent than A, but both are full agonists. B and C have the same potency, but C is a partial agonist as it is not able to elicit a full response, irrespective of concentration.

Antagonists bind to the receptor but produce no intrinsic effect. Competitive antagonists bind to the receptor, and displace endogenous ligands or other drugs therefore reducing receptor response. Non-competitive antagonists do not compete with endogenous ligands for the same binding site. They chemically or structurally modify the receptor, reducing the response induced by ligands occupying the binding site. Unlike competitive antagonists they cannot be overcome by increasing agonist concentration.

In the presence of a competitive antagonist, the log[dose]–response curve of an agonist is shifted to the right but the maximum effect remains unaltered (curves A and B in Fig. 1.5C ). The effects of these antagonists can be overcome with an increasing agonist concentration. Examples of this effect include the displacement of morphine by naloxone and endogenous catecholamines by β-blockers.

A non-competitive (irreversible) antagonist also shifts the dose–response curve to the right but, with increasing concentrations, reduces the maximum effect (curve D in Fig. 1.5C ) as the receptor pool is effectively limited. Examples of irreversible antagonism include the platelet inhibitors clopidogrel and aspirin. Organophosphates cause irreversible modification of acetylcholinesterase, causing increased cholinergic effects.

Drug actions on enzymes

An enzyme is a protein to which other molecules (substrate) may bind and undergo chemical modification in some way, such as breakdown or synthesis of other products. Drugs may act by binding reversibly or irreversibly to the active site of the enzyme and acting as substrates or to block binding of endogenous substrate. Some drugs bind to an alternative (allosteric) site to modify the activity of the enzyme.

Enzyme kinetics

The relationship between an enzyme, substrate and rate of reaction is described in Fig. 1.6A .

When an enzyme joins with appropriate substrate at the active site, the complex undergoes chemical modification to yield products. This equation ( Eq. 1.3 ) is governed by the law of mass action, and therefore, where substrate concentrations are high, the equilibrium shifts to the right, favouring the generation of products.

$\mathrm{Enzyme}+\mathrm{Substrate}\rightleftharpoons \mathrm{Complex}\rightleftharpoons \mathrm{Enzyme}+\mathrm{Products}$

Where the substrate is at low concentration, its availability is rate limiting, whereas where substrate exceeds the enzyme's capacity, then enzymatic activity is the limiting factor.

Fig. 1.6A and B shows how, at low substrate concentrations, a small increase in substrate produces a significant increase in enzymatic activity, whereas at high substrate concentrations, the activity remains maximal and independent of substrate concentration. The effects of different enzymes are compared and modelled using the V _MAX (the maximal enzyme reaction rate) and k _M (substrate concentration at which half V _MAX is achieved).

Knowledge of enzyme kinetics is useful for predicting drug metabolism and pharmacokinetics. Drugs processed by first-order kinetics are metabolised by a large pool of enzymes and therefore overall enzyme activity is increased at high drug concentration; this increases the rate of metabolism and also reduces the chance of toxicity ( Fig. 1.6C ). However, where the enzymatic pool is small or the dose so high that the available enzymes become saturated, the rate of breakdown becomes fixed irrespective of drug concentration ( Fig. 1.6D ). This is known as zero order, or saturation kinetics, and is an important consideration in the metabolism of phenytoin, paracetamol and alcohol, sometimes requiring the monitoring of plasma drug concentrations.

Drugs may modulate enzymes by increasing or decreasing intrinsic activity or by competing with endogenous substrate molecules at the active site. As with receptor kinetics, reversible inhibition is caused by competition for the active site and may be overcome by an increase in the endogenous substrate. Examples include the reversible antagonism of acetylcholinesterase (by neostigmine), phosphodiesterase (by aminophylline), and angiotensin converting enzyme (by lisinopril). Irreversible enzyme inhibition occurs when a stable chemical bond is formed between drug and enzyme, resulting in prolonged or permanent inactivation. Examples include the irreversible inhibition of gastric hydrogen-potassium-ATPase (by omprazole), cyclo-oxygenase (by aspirin) and acetylcholinesterase (by organophosphates). Allosteric modulation refers to binding to a site other than the active site to influence enzyme activity – for example, the antiretroviral reverse transcriptase inhibitor efavirenz.

Physicochemical properties

The physicochemical properties of drugs produce other effects outside receptor, enzyme or secondary messenger pathways. These non-specific mechanisms often rely on physical properties such as pH, charge or physical interactions with other molecules (via chelation or adsorption).

pH-based interactions include the neutralisation of acid with alkali. Sodium citrate, aluminium and magnesium hydroxide neutralise gastric acid via this mechanism.

Chelating agents combine chemically with compounds, reducing their toxicity and enhancing elimination of the inactive complex. Such drugs include sugammadex (chelates rocuronium), deferoxamine (chelates iron and aluminium), dicobalt edetate (cyanide toxicity), sodium calcium edetate (lead) and penicillamine (copper and lead).

Molecular adsorption describes the interaction and binding of a molecule to the surface of another, reducing the free fraction available for absorption from the gastrointestinal tract. This mechanism may be useful in the treatment of drug toxicity to prevent an oral overdose from being absorbed (activated charcoal) or in the management of hyperkalaemia (calcium resonium reduces the GI absorption of potassium).

Absorption

Absorption describes the process by which a drug is taken up from the initial site of administration into the blood. The rate and amount of absorption affects the final plasma concentration and therefore drug concentration at the effect site. For drugs requiring multiple doses, these principles will also affect peak plasma concentration and time to maximal concentration.

Absorption is influenced by factors specific to both drug and patient, as discussed earlier. The pathway from site of administration to final effect site includes passage across membranes and blood transport (see Transport of drugs and Routes of administration ). Drugs may have specific formulations which affect the rate of drug release or facilitate delivery to the target site. These are covered in more detail in Pharmacokinetic principles .

Distribution

Metabolism

Most drugs are lipid soluble, and the majority are metabolised in the liver. Metabolites are mostly pharmacologically inactive, ionised (water soluble) compounds which are then excreted in bile or by the kidneys. However, some metabolites may be active and cause prolonged clinical effect after the parent compound has been broken down or removed from the circulation. Some drugs are metabolised outside the liver (by kidneys, lungs, plasma and tissues).

Medications that are absorbed enterally undergo first-pass metabolism before passing from the portal circulation into the systemic circulation.

First-pass metabolism may increase or decrease drug effect to a variable extent. A substance is termed a prodrug if it is inactive in the form administered and its pharmacological effects depend on the formation of active metabolites. Codeine is a prodrug, undergoing metabolism via gluconuridation (50%–70%), N -demethylation (10%–15%) and O -demethylation (0%–15%). Morphine, resulting from O -demethylation of codeine by CYP2D6, is the most active metabolite and has greater activity at the opioid receptor. CYP2D6 exists as slow, rapid and ultra-rapid phenotypes, which affect the therapeutic response to codeine. Those with inactive CYP2D6 may derive no analgesia from codeine, whereas ultra-rapid metabolisers may have significant drowsiness, respiratory depression and features of opiate toxicity.

Drugs undergo two types of reactions during metabolism: phase I and phase II. Phase I reactions include reduction, oxidation and hydrolysis. Drug oxidation occurs in the smooth endoplasmic reticulum, primarily by the cytochrome P450 enzyme system. This system and other enzymes also perform reduction reactions. Hydrolysis is a common phase I reaction in the metabolism of drugs with ester or amide groups (e.g. meperidine). Amide drugs often undergo hydrolysis and oxidative N -dealkylation (e.g. lidocaine, bupivacaine).

Phase II reactions involve conjugation of a metabolite or the drug itself with an endogenous substrate. Conjugation with glucuronic acid is a major metabolic pathway, but others include acetylation, methylation and conjugation with sulphate or glycine.

Extra-hepatic or extra-renal metabolism is independent of liver or renal function. Typically this leads to a rapid offset of drug action because of the abundance of enzyme sites for metabolism. Drugs metabolised via these routes can be useful in those with hepatic or renal failure. For example, suxamethonium and mivacurium are metabolised by plasma cholinesterase, esmolol by erythrocyte esterases, remifentanil by tissue esterases and, in part, dopamine by the kidney and prilocaine by the lungs. Occasionally drugs will undergo spontaneous degradation to generate active or inactive metabolites. These processes are also independent of hepatic or renal pathways – such as the spontaneous breakdown of atracurium by Hofmann degradation.