Molecular targets (receptors etc)

• What are the %s of drugs that target major families:

  -Kinases -3%

  -Ion channels - 18%

  -Nuclear receptors -16%

  -GPCRs - 33%
• What is the intracellular concentration of potassium?

  -150mM greater than extracellular potassium between 3-5 mmol
• Potassium channels:


  -Most diverse family of ion channels.

  -More than 70 different genes encoding K channels α subunits in the human genome.
• What is the function of potassium channels?

  Regulate cell excitability through different modalities:

  Frequency and shape of action potentials.

  Secretion of hormones and neurotransmitters.

  Membrane potential.
• What is the function of Minoxidil?


  -Opens potassium channels

  -Causes hyperpolarisation in smooth muscle cells

  -Muscle relaxation and hence vasodilation.

  -Indication: Can be used in the treat of hypertension in combination with a diuretic and β adrenoreceptor blocker

  -Recent evidence states it can stimulate hair growth
• Sodium channels:

  -The first members of the ion channel superfamily to be discovered.

  -Voltage-gated sodium-selective ion channels present in the membrane of most excitable cells.

  -Comprise of one pore-forming α subunit, which may be associated with either one or two β subunits.

  -Encoded by at least 10 genes.

  -nAChR, glutamate & 5HT3 are all excitatory receptors
• What is lidocaine?


  Blocks voltage-gated sodium channels (Nav1.5, Nav1.7, Nav1.9).

  Primary target is Nav1.5 which is the main cardiac sodium channel.

  Indication: ventricular arrhythmias, especially after myocardial infarction. In addition to local anaesthesia.
• Calcium channels:


  -Voltage-gated ion channels present in the membrane of most excitable cells.

  -Ca2+ channels form hetero-oligomeric complexes.

  -The α1 subunit is pore-forming and provides the extracellular binding site(s) for practically all agonists and antagonists.

  -Glycine and GABA receptors are inhibitory
• What are the 3 calcium channel families?High-voltage activated dihydropyridine-sensitive (L-type, CaV1.x) channels; 2.High-voltage activated dihydropyridine-insensitive (CaV2.x) channels 3.Low-voltage-activated (T-type, CaV3.x) channels.

  -The α2-δ1 and α2-δ2 subunits bind gabapentin and pregabalin
• What is verapamil?


  -Blocks L types voltage gated calcium channels

  -Blocks calcium influx in myocardial and vascular smooth muscle cells.

  -Leads to a reduction in cardiac and VSM contraction bringing about a dilation in coronary and systemic arteries.

  -Indication: supraventricular arrhythmias, angina, hypertension (dose and preparation dependent)
• Sodium channel channelopathy:


  -Epilepsy

  -Pain
• Potassium channels channelopathy:


  -Epilepsy

  -Diabetes

  -Ataxia
• Calcium channels channelopathy:


  -Pain

  -Ataxia

  -Epilepsy
• Potassium ATP channels channelopathy:

  -Diabetes
• What are ligand gated ion channels?

  -Membrane proteins that contain a pore which allows regulated flow of selected ions across plasma membrane

  -Link ligand binding to an ionic flux

  -Ion flux is passive and driven by electrochemical gradient

  -Heteromeric assemblies with 4-5 subunits.

  -Mediate fast synaptic transmission in milliseconds in CNS and somatic NMJ

  -Nicotinic ACh receptor first to be cloned.
• Acetylcholine receptors


  -Nicotinic

  -Structure solved by X-ray crystallography

  -Each subunit has 4 transmembrane (TM) domains

  -TM2 from each subunit lines the ion channel pore

  -Sodium flows along its concentration gradient

  -Causes depolarisation of cell

  -In muscle

  = contraction

  -In neurons = increases probability of neurone firing action potential

  -Action is FAST, within milliseconds of ligand binding

  -Different nAChRs exist, depending on which subunits are assembled into the complex:

  -(a1)2b1dγ = muscle subtype

  -(a4)2(b2)3 = one of the CNS subtypes
• What is Succinylcholine?


  -Full agonist of the nACh receptor

  -Mimics the effects of ACh but is not subject to breakdown by AChE.

  -Indication: neuromuscular blockade (short duration) for surgery
• What is memantine?


  -Non-competitive NMDA receptor antagonist

  -Blocks overstimulation of NMDA receptors evident in Alzheimer's disease.

  -Indication: moderate to severe dementia of the Alzheimer's type.
• What is Granisteron?


  -5-HT3 antagonist

  -5-HT mediates synaptic transmission to the medullary vomiting centre and area postrema.

  -Indication: Nausea and vomiting induced by cytotoxic chemotherapy or radiotherapy (oral or transdermal). Postoperative nausea and vomiting (intravenous).
• What is diazepam?


  -Full agonist of the GABAA receptor

  -Potentiates GABA (inhibitory) activity, increasing Cl- influx.

  -Indication: short-term use in anxiety or insomnia, life-threatening acute drug-induced dystonic reactions, adjunct in acute alcohol withdrawal, status epilepticus, febrile convulsions, muscle spasm.
• What is Ivermectin?


  -Irreversible agonist of the glycine receptor

  -Activates glycine receptor, hyperpolarisation of cells (influx of Cl-).

  -Indication: For the treatment of intestinal strongyloidiasis due to the nematode parasite Strongyloides stercoralis.

  -Also for the treatment of river blindness due to the nematode parasite Onchocerca volvulus.
• What is Suramin?


  -Broad spectrum P2X receptor antagonist.

  -Blocks ATP activation of P2X receptors, leading to a change in intracellular Ca2+.

  -Used for parasite infections.
• What are the major targets of drugs?

  -Enzymes

  -Transporters

  -Ion channels

  -Receptors
• Define receptor:


  -Target molecules through which soluble physiological mediators can produce their desired biological effects
• What are the 4 different types of receptors?

  -G-Protein coupled

  -Ligand-gated ion channels e.g., Nicotinic acetylcholine receptors

  -Enzymes coupled receptors

  -Nuclear receptors
• What is a G-Protein coupled receptor?


  -Coupled to G proteins to indicate signal transduction

  -7 transmembrane domains

  -Largest family of cell-surface receptors with 800 genes

  -Represent 50% of current drug targets for stomach ulcers, allergies. hypertension. migraines and glaucoma.
• What signals activate GPCRs?

  -Photons

  -Hormones

  -Peptides

  -Peptidases
• Class A GPCRs


  -Rhodopsin like

  -short N-terminus

  -Agonists binds with extracellular loops & transmembrane domains

  -Majority of prescribed GPCR drugs are class A
• Which GPCR receptors belong to class A?

  -B-adrenoceptors,

  -Histamine receptors

  -Dopamine receptors
• Class B GPCRs


  -Secretin like

  -Larger, globular N-terminus; plays role in agonist binding

  -Currently no small molecule drugs on market
• Which GPCR receptors belong to class B?

  -Secretin receptors

  -Calcitonin receptors

  -Glucagon receptors
• Class C GPCRs


  ➟Metabotropic glutamate receptors

  ➟very large N-terminal domain binds agonists; form obligatory dimers

  ➟few small molecule drugs on market
• Which GPCR receptors belong to class C?

  -Metabotropic glutamate receptors

  -GABA receptors
• Second messengers example:


  -3'-5'-cyclic adenosine monophosphate (cyclic AMP)
• Desensitisation of GPCRs


  -Molecular mechanism that prevents continuous activation of GPCRs; usually occurring within seconds

  -Achieved by phosphorylation and/or internalization

  -Critical to prevent uncontrolled signalling
• What is homologous Desensitisation?

  -Effects are restricted to agonists acting through a specific receptor
• What is heterologous Desensitisation?

  -Effects can affect receptors that share a component of the same signalling cascade
• What is downregulation?


  -Reduction in the numbers of a functional receptor
• What are the molecular events that occur after agonist binding which

  lead to internalisation of a GPCR?

  -Confirmational change

  -Phosphorylation

  -Binding of B-arrestins

  -Recruitment of clathrin
• What is tolerance?


  -The progressive reduction in the effectiveness of a drug; usually occurring over days or hours

  -Cellular and molecular mechanisms are not necessarily specified and are often difficult to resolve

  -Does not involve change in receptor number

  -Problem in health care industry e.g., morphine