Pharmacology

How drugs act - general principles

Protein targets for drug binding

Four main kinds of regulatory protein are commonly involved as primary drug targets, namely:

• receptors
• enzymes
• carrier molecules (transporters)
• ion channels
• plasma proteins and other tissue proteins

Receptors are protein molecules whose function is to recognise and respond to endogenous chemical signals. Most drugs affect multiple receptors other than their principal ones.

The tendency of a drug to bind to receptors is governed by it’s affinity, whereas the tendency for it, once bound, to activate the receptor is denoted by its efficacy. Agonists activate the receptors, and antagonists combine to the same sites without causing activation, and block the effects of agonists on that receptor. Agonists and antagonists both have high affinity, the former has high efficacy while the latter low. Partial agonists have intermediate levels of efficacy so even with 100% receptor occupation the tissue response is sub-maximal, while full agonists can elicit a maximal tissue response. Receptors can accommodate only one molecule at a time, so a competitive antagonist can reduce the effects of an agonist.

If a ligand reduces the level of constitutive activation; such drugs are known as inverse agonists. Neutral antagonists, by binding to the agonist binding site, will antagonise both agonists and inverse agonists. It turns out that most of the receptor antagonists in clinical use are actually inverse agonists when tested in systems showing constitutive receptor activation. However, most receptors show a preference for the inactive state, and for these there is no practical difference between a competitive antagonist and an inverse agonist.

In addition to the agonist binding site (now referred to as the orthosteric binding site), to which competitive antagonists also bind, receptor proteins possess many other (allosteric) binding sites through which drugs can influence receptor function in various ways, by increasing or decreasing the affinity of agonists for the agonist binding site, by modifying efficacy or by producing a response themselves. Depending on the direction of the effect, the ligands may be allosteric antagonists or allosteric facilitators of the agonist effect.

Other forms of drug antagonism:

• chemical antagonism
  • uncommon situation where the 2 substances combine in a solution thus the effect of the drug is lost
• pharmacokinetic antagonism
  • the ‘antagonist’ effectively reduces the concentration of of the active drug at its site of action e.g. by increasing metabolic degradation or reduce rate of absorption from the gastrointestinal tract
• block of receptor–response linkage
  • the antagonist blocks at some point downstream from the agonist binding site on the receptor, and interrupts the chain of events that leads to the production of a response by the agonist
• physiological antagonism
  • the interaction of two drugs whose opposing actions in the body tend to cancel each other

Desensitisation and tolerance

Often, the effect of a drug gradually diminishes when it is given continuously or repeatedly. Desensitisation and tachyphylaxis are synonymous terms used to describe this phenomenon, which often develops in the course of a few minutes. The term tolerance is conventionally used to describe a more gradual decrease in responsiveness to a drug, taking hours, days or weeks to develop, but the distinction is not a sharp one.

• change in receptors
  • The desensitised state is caused by a conformational change in the receptor, resulting in tight binding of the agonist molecule without the opening of the ionic channel. Phosphorylation of intracellular regions of the receptor protein is a second, slower mechanism by which ion channels become desensitised.
• translocation of receptors
  • Prolonged exposure to agonists often results in a gradual decrease in the number of receptors expressed on the cell surface, as a result of internalisation of the receptors. This is a slower process than the uncoupling described above.
• exhaustion of mediators
  • In some cases, desensitisation is associated with depletion of an essential intermediate substance. Drugs such as amphetamine, which acts by releasing amines from nerve terminals show marked tachyphylaxis because the amine stores become depleted.
• increased metabolic degradation of the drug
  • Repeated administration of the same dose produces a progressively lower plasma concentration, as a result of increased metabolic degradation.
• physiological adaptation
  • Diminution of a drug’s effect may occur because it is nullified by a homeostatic response. For example, the blood pressure-lowering effect of thiazide diuretics is limited because of a gradual activation of the renin–angiotensin system.

How drugs act

• receptors
• ion channels
• enzymes
• transporters (carrier molecules)

Receptors

Receptors are the sensing elements in the system of chemical communications that coordinates the function and responses of all the different cells in the body, the chemical messengers being the various hormones, transmitters and other mediators.

Ion channels

Ion channels are essentially gateways in cell membranes that selectively allow the passage of particular ions, and that are induced to open or close by a variety of mechanisms. Two important types are ligand-gates channels and voltage-gates channels. The former open only when one or more agonist molecules are bound, and are properly classified as receptors, since agonist binding is needed to activate them. Voltage-gates channels are gated by changes in the transmembrane potential rather than by agonist binding. Drugs affect ion channels in several ways:

• By binding to the channel protein itself, either to the ligand-binding (orthosteric) site of ligand-gated channels, or to other (allosteric) sites, or in the simplest case, exemplified the by the action of local anaesthetics on the voltage-gates sodium channel, the drug molecule plugs the channel physically, blocking ion permeation.
• By an indirect interaction, involving an activates G protein subunit or other intermediary.
• By altering the level of expression of ion channels on the cell surface. For example, gabapentin reduces the insertion of neuronal calcium channels into the plasma membrane.

Enzymes

Many drugs target enzymes. Often, the drug molecule is a substrate analogue that acts as a competitive inhibitor of the enzyme. Drugs may also act as false substrates, where the drug molecule undergoes chemical transformation to form an abnormal product that subverts the normal metabolic pathway.

Transporters

The movement of ions/molecules across cell membranes generally either occurs through channels, or through the agency of a transport proteins, because the permeating molecules are often insufficiently lipid-soluble to penetrate lipid membranes on their own.

Receptor proteins

Much information has been gained by introducing the cloned DNA encoding individual receptors into cell lines, producing cells that express the foreign receptors in a functional form. Such engineered cells allow much more precise control of the expressed receptors than is possible with natural cells or intact tissues, and the technique is widely used to study the binding and pharmacological characteristics of cloned receptors.

Types of receptors

Based on molecular structure and the nature linkage between receptor occupation and ensuing response (the transduction mechanism), we can distinguish four receptor types, or superfamilies:

• ligand-gated ion channels (ionotropic)
  • membrane, ion channel, direct coupling
• G protein-coupled receptors
  • membrane, ion channel, G protein or arrestin coupling
• kinase-linked and related receptors
  • membrane, protein kinases, direct coupling
• nuclear receptors
  • intracellular, gene transcription, DNA coupling

Absorption and distribution of drugs

The major compartments are:

• plasma (5% of body weight)
• interstitial fluid (16%)
• intracellular fluid (35%)
• transcellular fluid (2%)
• fat (20%)

Volume of distribution is defined as the volume of solvent that would contain the total body content of the drug at a concentration equal to the measured plasma concentration. Lipid-insoluble drugs are mainly confined to plasma and interstitial fluids; most do not enter the brain following acute dosing. Lipid-soluble drugs reach all compartments and may accumulate in fat. For drugs that accumulate outside the plasma compartment (e.g. in fata or by being bound to tissues), volume of distribution may exceed total body volume.

Approaches to improve drug delivery or localize the drug to target tissue:

• prodrugs
  • inactive precursors that are metabolized to active metabolites
• antibody-drug conjugates
• packaging in liposomes
• coated implantable devices

Noradrenergic transmission

Adrenoceptor subtypes:

• two main α-adrenoceptor subtypes, α1 and α2, each divided into three further subtypes (α1A,α1B, α1D and α2A, α2B, α2C)
• three β-adrenoceptor subtypes (β1, β2, β3)
• all belong to the superfamily of G protein–coupled receptors
• α1 receptors:
  • vasoconstriction, relaxation of gastrointestinal smooth muscle, salivary secretion and hepatic glycogenolysis
• α2 receptors:
  • inhibition of: transmitter release (including noradrenaline and acetylcholine release from autonomic nerves) caused by opening of K+ channels and inhibition of Ca2+ channels; platelet aggregation; vascular smooth muscle contraction; inhibition of insulin release
• β1 receptors:
  • increased cardiac rate and force
• β2 receptors:
  • bronchodilatation; vasodilatation; relaxation of visceral smooth muscle; hepatic glycogenolysis; muscle tremor
• β3 receptors:
  • lipolysis and thermogenesis; bladder detrusor muscle relaxation.

Dopamine in the CNS

Dopamine is a neurotransmitter as well as being the precursor for noradrenaline. It is degraded in a similar fashion to noradrenaline, giving rise mainly to dihydroxyphenylacetic acid and homovanillic acid, which are excreted in the urine.

• There are four main dopaminergic pathways:
  • nigrostriatal pathway, important in motor control;
  • mesolimbic pathway, running from groups of cells in the midbrain to parts of the limbic system, especially the nucleus accumbens, involved in emotion and drug-induced reward;
  • mesocortical pathway, running from the midbrain to the cortex, involved in emotion;
  • tuberohypophyseal neurons, running from the hypothalamus to the pituitary gland, whose secretions they regulate.
• There are five dopamine-receptor subtypes.
  • D1 and D5 receptors are linked to stimulation of adenylyl cyclase.
  • D2, D3 and D4 receptors are linked to activation of K+ channels and inhibition of Ca2+ channels as well as to inhibition of adenylyl cyclase.
  • D2 receptors may be implicated in the positive symptoms and D1 receptors in the negative symptoms of schizophrenia.
  • Parkinson’s disease is associated with a deficiency of nigrostriatal dopaminergic neurons.
  • Hormone release from the anterior pituitary gland is regulated by dopamine, especially prolactin release (inhibited) and growth hormone release (stimulated).
  • Dopamine acts on the chemoreceptor trigger zone to cause nausea and vomiting.

5-Hydroxytryptamine in the CNS

• 5-HT1 receptors (5-HT1A, 5-HT1B, 5-HT1D, 5-HT1E, 5-HT1F) are predominantly inhibitory in their effects.
  • 5-HT1A receptors are expressed on the soma and dendrites of 5-HT neurons in the raphe nuclei and are activated by locally released 5-HT. This inhibitory effect tends to limit the rate of firing of these cells. They are also widely distributed in the limbic system, and are believed to be a major target for drugs used to treat anxiety and depression.
  • 5-HT1B and 5-HT1D receptors are found mainly as presynaptic inhibitory receptors on both 5-HT-containing and other nerve terminals in the basal ganglia and cortex. Agonists acting on 5-HT1B and 5-HT1D receptors such as sumatriptan are used to treat migraine.
• 5-HT2 receptors (5-HT2A, 5-HT2B and 5-HT2C) are abundant in the cortex and limbic system, where they are located at both pre- and postsynaptic sites. They can exert excitatory or inhibitory effects by enhancing the release of glutamate and GABA. They are believed to be the target of some antidepressants and antipsychotic drugs as well as various hallucinogenic drugs. Lorcaserin, a 5-HT2C agonist is an anti-obesity drug. The use of 5-HT2 receptor antagonists such as methysergide in implicated in treating migraine.
• 5-HT3 receptors are pentameric ligand-gated cation channels that can be either homomeric or heteromeric complexes of different 5-HT3 receptor subunits. While 5-HT3A and 5-HT3B subunits are the most extensively studied, the roles of other subunits remain to be fully investigated. In the brain, 5-HT3 receptors are found in the area postrema (a region of the medulla involved in vomiting) and other parts of the brain stem, extending to the dorsal horn of the spinal cord. They are also present in certain parts of the cortex, as well as in the peripheral nervous system. They are excitatory ionotropic receptors, and specific antagonists (e.g. granisetron and ondansetron) are used to treat nausea and vomiting.