Receptors

• Receptor → protein-based moiety that contains a region that binds a natural ligand (or drug) to bring about a response
• They are located either:
  • Within the cell membrane → ligand is poorly lipid-soluble (Ie. cannot cross cell
    membrane)
  • Intracellularly → ligand is lipid-soluble (Ie. can cross cell membrane)
• Drug-receptor binding can invoke 3 responses:
  • Elicits an effect → agonist response
  • Prevent the action of a natural ligand → antagonist response
  • Reduce a constitutive effect of a receptor → inverse agonist response
• There are 5 classes of receptors:
  • Ligand-gated ion channels
  • G-protein coupled receptors (GPCR)
  • Membrane guanylyl cyclase
  • Tyrosine kinase receptors
  • Intracellular receptors

Drug – Receptor Response

Agonist:

Defined as a ligand or drug that binds to a receptor and alters the receptor state resulting in a biological response

Antagonist:

Defined as a ligand that binds to a receptor but does not activate it, instead blocking that receptor to a natural agonist

Efficacy:

• Defined as the ability of drug to elicit the maximal effect (EMAX) when bound to receptor
• Measured by height of plateau phase (or EMAX) in “log dose-response curve” → ↑ height of plateau phase (or EMAX) = ↑ efficacy
• It reflects “intrinsic activity” of the drug (Ie. magnitude of effect drug has once bound)
  • Full agonists → 100% efficacy (or IA = 1)
  • Partial agonists → efficacy b/t 0 and 100% (or 0 < IA < 1)
  • Antagonists → 0% efficacy (or IA = 0)
• It is vital when selecting drugs (Ie. paracetamol and morphine are both analgesics but with different efficacy)

Potency:

• A comparative measure b/t drugs that have the same action on a receptor (Ie. have same log dose-response curve slopes) → refers to the different doses of two drugs needed to produce the same drug effect (Ie. more potent drug evokes a ↑ response at a ↓ dose)
• Measured by the EC50 in “log dose-response curve” → ↑ EC50 = ↓ potency
• It reflects the “affinity” of the drug for the receptor → ↑ receptor affinity = ↓ KD or ↑ KA = ↑ potency
• It is not as vital when selecting drugs (cf. efficacy), as long as the effective dose can be administered conveniently

Pharmacological effect of drug-receptor binding depends on:

• Properties of the drug:
  • Affinity (Ie. how avidly a drug binds to a receptor)
    → determined by KA or KD of drug (such that ↑ KA or ↓ KD affinity
  • Intrinsic activity (IA; Ie. magnitude of effect drug has once bound)
    → drugs have IA b/t 0 and 1 (Nb. inverse agonists have IA b/t -1 and 0)
• State of receptor activation:
  • Receptors exist in an equilibrium b/t “active form” and “inactive form”, which is altered by the presence of a drug

2024A 01: 28% of candidates passed this question.
A good answer provided a definition of a receptor as a protein/glycoprotein that undergoes a conformational change upon ligand binding. They then gave an overview of where these are found and the mechanisms of activation/downstream processing. A definition of each type (agonist, antagonist, partial agonist and inverse agonist) was expected with reference to the intrinsic activity at the receptor and the affinity for the receptor. A good answer also touched upon law of mass action (D+R=DR) and how this equilibrium is altered by each. Concepts such as reversible and irreversible binding and its implications as well as competitive and non-competitive antagonism would elevate the answer.

2022B 19: 31% of candidates passed this question.
The description of a receptor was worth 20% thus it was expected that detailed information on the different forms of receptors, their structure, the resultant conformational change when activated and where they are found would be provided for full marks. Most candidates were able to correctly define an agonist, antagonist, partial agonist and inverse agonist. Unfortunately, this was the limit of most answers. Candidates were expected to provide details of drug or agonist/receptor interaction discussing the terms affinity/intrinsic activity and how different mechanisms of binding and interacting with the receptor alters these terms.

7 methods by which drugs cause their action include: PEFsPdCpRsig (almost PEEPd CpR sig)

1. PHYSICOCHEMICAL ACTIONS
   • Antacids exert their effects by neutralizing gastric acid
   • Chelating agents reduce the concentration of certain metallic ions within the body (eg, desferoxamine and iron, or activated charcoal)
   • CaCl3, KCl, blood, Mg, replase alter or supplement endogenous primary substrates
2. ACTIONS ON ENZYMES
   • Enzymes are biological catalysts, and most drugs that interact with enzymes are inhibitors
   • Increased concentration of the substrate and decreased concentration of the product eg. ACE inhibitors (captopril, enalapril) prevent the conversion of ACEI to ACEII and bradykinin to various fragments
   • Neostigmine inhibits acetylcholinesterase reversibly
3. DRUGS WHICH ACT AS FALSE SUBSTRATES FOR ENZYMES
   • Fluorouracil act as “false substrate”, replaces uracil as an intermediate in purine biosynthesis but cannot be converted into thymidylate 
4. PRODRUGS
   • Require conversion to activated form by metabolic pathway
   • Levodopa → Dopamine
   • Parecoxib → Valdecoxib
5. ALTERATION OF CARRIER PROTEIN PROPERTIES
   • Cardiac glycosides (such as digoxin) inhibits Na-K pump
   • Loop diuretics inhibits Na/K/Cl co-transporter in loop of Henle
   • Cocaine inhibits noradrenaline re-uptake
6. VOLTAGE GATED ION CHANNELS
   • Involved in the conduction of action potentials in excitable tissues
   • Several groups of drugs have specific blocking actions at these ion channels
     • local anaesthetics (eg, lignocaine) block Na channels,
     • calcium channel blockers (eg, diltiazem) acts on vascular smooth muscle ion channels
7. RECEPTORS
   • Definition:
     • A protein, often integral to a membrane, containing a region to which a ligand binds specifically to elicit a response.
     • Binding may be allosteric (at a site distant to the receptor).
     • Grouped into three classes based on mechanism of action:
       1. Altered ion permeability (ion channels / ionotropic)
          • Membrane spanning complexes with the potential to form a channel through the membrane
          • Three families:
            1. Pentameric
               • nicotinic Ach receptor at the NMJ → allows an Na channel to form
               • GABA A receptor which allows a Cl channel to form,
               • 5HT3 receptor
            2. Ionotropic glutamate – NMDA ligand gated ion channels.
               • They form Na, K and (NMDA only) Ca channels when glutamate binds
            3. Purinergic receptors activated by ATP, permeable to Na, K and Ca, and are associated with mechanosensation and pain.
       2. Production of intermediate messengers
          • Membrane bound systems that transduce a ligand gated signal presented on one side of the cell membrane into an intracellular signal transmitted by intermediate messengers. These messengers:
            • G proteins (most common) – eg, Nad and Adr
            • Tyrosine kinase – eg, insulin
            • Guanylyl cyclase – eg, NO, atrial natriuretic peptide
       3. Regulation of gene transcription
          • Steroids and thyroid hormones act through intracellular receptors to alter the expression of DNA and RNA, and indirectly alter the production of intracellular proteins.

Gladwin 2016

2013A 01: 23% pass.

A good answer to this question required candidates to think broadly about how drugs act and have a system for classifying their actions. One possible classification is action via receptors or non-receptor actions. Many candidates used categories such as physiochemical, receptor and enzymes. Common problems were failure to mention a whole class of drug actions e.g. drugs acting via voltage-gated ion channels or gene transcription regulation. Candidates also gave far too much detail in some sections e.g. a description of zero order and first order kinetics is not required. Candidates often did not give examples of the drug action they described.

Second Messenger

• A downstream intracellular signalling molecule that is either released or inhibited by drug binding to its receptor.
• Examples include cAMP and cGMP.

Steps in activation of second messenger

1. Ligand binding to extracellular site
2. Conformation change in receptor
3. Activation of intracellular domain of receptor
4. Activation of enzymatic process at intracellular site effector site
5. Change in second messenger concentration
6. Action of second messenger on substrate
7. Response

Examples:

• cAMP
  • NA, ACh (M2), ACTH, ANP, Glucagon, PTH, TSH.
  • Primary effector adenylyl cyclase.
  • Secondary messenger cAMP.
  • Secondary effector PKA
• cGMP
  • ANP and NO.
  • Primary effector guanylyl cyclase.
  • Secondary messenger cGMP.
  • Secondary effector Protein kinase G (cGMP-dependent protein kinase)
• Phosphoinositol signalling
  • Noradrenaline, ACh (M1/M3). Signal transducer Gq,
  • Primary effector PLC,
  • Secondary messenger IP3/DAG,
  • secondary effector PKC
• Arachidonic acid pathway
  • Histamine.
  • Primary effector phospholipase A.
  • Second messenger arachidonic acid.
  • Secondary effector COX/lipoxygenase.
• Tyrosine kinase pathway
  • Insulin, IGF, PDGF.
  • Primary effector Ras.
  • Second messenger Ras-GTP.
  • Secondary effector MAP3K (raf)

Gladwin 2016

2007B 14: 3 candidates (43%) passed this question.

Second messenger – Hormone/drug – receptor binding is coupled to a subsequent series of intracellular biochemical events that precipitate the ultimate hormone/drug effect.

Examples are G proteins an energy dependent process by which there is hydrolysis of G
protein-associated GTP to GDP. There are both stimulating and inhibitory proteins
which subsequently act to increase or decrease activity of the enzyme adenylyl cyclase,
resulting in increased levels of cyclic adenosine 3 ‘,5’-monophosphate ( cAMP) in the cell
which in turn activates protein kinases that phosphorylate various proteins, ion channels, and second messenger enzymes. 

Also G proteins stimulate hydrolysis of phosphatidyl-inositol-4,5-bisphosphate (PIP2) generating inositol-1,4,5-trisphosphate (IP3) and 1,2-diacylglycerol (DAG). Both systems increase intracellular calcium.

1. The ratio of the k_off / k_on
   • k_off describes the rate constant for the reaction DR → D + R
   • k_on describes the rate constant for the reaction D + R → DR
   • Rate constant is the rate at which a reaction will proceed in one direction if all other parameters are equal
   • The ratio of the two rate constants is the dissociation constant or k_d,
     • The likelihood of the drug-receptor complex to dissociate into drug and receptor
2. The implications for a low value for the ratio
   • A low value of k_d implies that the forward reaction of the drug binding to the receptor (k_on) is favoured over the reverse reaction of the drug and receptors disassociating (k_off)
   • This may be due to high affinity between the drug and receptor
   • Few molecules of drug are required to achieve a given level of receptor occupation than a drug affecting the same receptor with a high k_d
     • This drug is more potent
3. Affinity
   • Affinity is the attraction between 2 molecules in forming a complex
   • High affinity bonds have stronger inter-molecular bond
   • It is inversely related to the k_d
     • The higher the affinity, the lower the k_d and vice versa
4. The clinical implications for a high value of affinity
   • If a drug has high affinity for a receptor, it has a lower k_d than a drug that has low affinity for the same receptor
   • Fewer molecules of the drug are required to achieve receptor occupancy of a certain level
   • The drug is more potent
5. Two physiological factors that affect the rate constant
   • The rate constant is described by the Arrhenius equation

Where
k = the rate constant
A = the pre-exponential factor for the specific reaction describing the frequency of collisions in the correct orientation
Ea = the activation energy, the minimum energy required for the reaction to occur
R = Universal gas constant
T = Temperature in kelvin

As temperature increases the rate constant approaches A,
As the temperature decreases the rate constant approaches 0

Modifying the Arhenius Equation gives

Where ΔG⁰ = Gibb’s Free Energy, the change in enthalpy and entropy at a given temperature for the reaction

Gladwin 2016

2007B 12: 1 candidate (14%) passed this question.

The main points expected for a pass were:

• The ratio of k off/k on is the dissociation constant
• A low value indicates that less drug is required to bind to the receptors
• affinity is the reverse of dissociation constant
• Clinical application of high affinity include large effect at lower concentrations
• Physiological factors could include temperature

The main problem with this question was lack of knowlegde.