Theories of Drug Action at Receptors

## Theories of Drug Action at Receptors ### Learning Outcomes - Explain the concept of drug receptors, identifying the four main receptor superfamilies, including their locations, structures, and signaling mechanisms. - Describe affinity and efficacy in terms of drug-receptor interactions, detailing the molecular forces involved in drug binding and receptor activation. - Outline the relationships between drug concentration, receptor occupancy ($K*d$) and response ($EC*{50}$, $R {max}$), discussing the factors that influence these relationships. - Define ‘competitive’ and ‘non-competitive’ antagonism, and how to distinguish them graphically, also including the mechanisms of action. ### Receptors: What Makes Them Special? - Receptors are proteins that respond to an endogenous messenger by initiating a signal, including signal transduction pathways. - They possess a selective binding site for endogenous messengers, characterized by specific amino acid residues. - A wide structural range of physiological messengers (local mediators, hormones, neurotransmitters, growth factors) use receptors for signaling, which can be paracrine, endocrine, or autocrine. ### Four Main Receptor Superfamilies - **Ligand-Gated Ion Channels:** operate on the millisecond timescale (ms), allowing rapid ion flux across cell membranes. - Examples include nicotinic acetylcholine receptors and GABA receptors. - **G Protein-Coupled Receptors (GPCRs):** operate on the second to minute timescale (sec - min), activating intracellular signaling cascades via G proteins. - Examples include adrenergic receptors and muscarinic acetylcholine receptors. - **Catalytic Receptors:** operate on the minute to hour timescale (min - hr), often possessing intrinsic kinase activity. - Examples include receptor tyrosine kinases (RTKs) such as EGFR and insulin receptors. - **Nuclear Receptors:** operate on the hour to day timescale (hr - day), regulating gene transcription. - Examples include estrogen receptors and glucocorticoid receptors. - The type of receptor is related to the messenger, e.g., neurotransmitters (ion channel / GPCR) vs. sex hormones (nuclear receptor). ### Common Characteristics of Receptors - Selective binding site for endogenous hormone / transmitter, ensuring specificity of signaling. - Act as molecular switches, existing in inactive (R) and active states (R*), which undergo conformational changes upon ligand binding. - Signal amplification occurs, allowing a small number of ligand-receptor interactions to produce a large cellular response. ### Pharmacology at Drug Targets - **Enzymes:** Predominantly targeted by inhibitors. Example: aspirin inhibits cyclooxygenase (COX) enzymes. - **Transporters:** Modulate the movement of ions and small molecules across cell membranes. - **Ion Channels:** Can be blocked or modulated by drugs, affecting ion flow. - **Receptors:** Have at least two types of drugs with opposing actions: agonists and antagonists. ### Drug-Receptor Interactions: Receptor as a Molecular Switch - The receptor (R) exists in inactive and active (R*) states, leading to a response, with equilibrium between the two states. - Drugs (D) interact with receptors. - $D + R <br>ightharpoons DR$ where DR can be in inactive or active form. ### Affinity - Affinity: The ability of a drug to bind to its receptor, which is determined by the chemical structure of the drug and the receptor binding site. ### Efficacy - Efficacy: The ability of a drug, once bound, to activate the receptor by a conformational change, leading to a biological response. ### Agonists - Agonists bind and activate the target, possessing both binding affinity and activation efficacy, resulting in a cellular response. ### Antagonists - Antagonists bind to the target and block messenger (agonist) responses; they only have affinity but no activation efficacy, preventing receptor activation. ### Quantifying Drug Action: Binding - Drug binding obeys the Law of Mass Action. - $D + R <br>ightharpoonup DR$ - Forward Rate = $k {ON} [D][R]$ where $k {ON}$ is the association rate constant. - Reverse Rate = $k {OFF} [DR]$ where $k {OFF}$ is the dissociation rate constant. - At dynamic equilibrium: Forward Rate = Reverse Rate. - $k {ON} [D][R] = k {OFF} [DR]$ - Equilibrium dissociation constant: $K*D = \frac{[D][R]}{[DR]} = \frac{k {OFF}}{k {ON}}$ ### Quantifying Receptor Occupancy - $K_D = \frac{[D][R]}{[DR]}$ - Receptor occupancy (a): $a = \frac{[D]}{[D] + K_D}$ - If [D] = $K*D$, then $a = \frac{K_D}{K_D + K_D} = 0.5$ - $K*D$ is the concentration of drug needed to occupy 50% of receptors, indicating binding affinity. A lower $K*D$ indicates higher affinity. ### Why is $K*D$ Useful? - Structure Activity Relationships (SAR) in drug discovery relate changes in compound structure to their effects on activity at the molecular target. - Measuring affinity ($K*D$, e.g., radioligand binding) is an easy way of doing this. - Example: $b_2$-adrenoceptor agonists for asthma- Salbutamol and Formoterol - Formoterol is more selective than salbutamol for airway vs. heart adrenoceptors. - Formoterol has 100-fold higher affinity for airway adrenoceptors than salbutamol. ### Concentration-Response Curves - Drug responses change over a wide concentration range (typically 1000-fold or more), represented by a log scale, allowing visualization of the full range of drug effects. ### Real Concentration-Response Curve - Shows the effect of adrenaline on airway smooth muscle. - X-axis: [Concentration] on a log scale. - Y-axis: “Response”. ### Two Measurements from Concentration-Response Curves - Rmax: Maximum response, reflecting the maximal effect the drug can produce. - $EC*{50}$: Effective Concentration of agonist for 50% of its maximal response; a measure of agonist potency. A lower $EC*{50}$ indicates higher potency. ### Comparing Agonists - Example: Adrenaline vs. Salmeterol for airway relaxation.- Salmeterol is a partial agonist compared to adrenaline (lower $R*{max}$) – it has lower efficacy (ability to activate the receptor). ### Full and Partial Agonists - Adrenaline – Full agonist- Binds with both AFFINITY and EFFICACY leading to 100% activation - Salmeterol – Partial agonist- Binds with AFFINITY and some EFFICACY leading to 50% activation ### What Do CRCs Depend On? - $EC*{50}$ and $R*{max}$ values are determined by- (i) Drug-receptor interaction: agonist affinity and efficacy - (ii) Properties of the functional response – e.g. amplification - Concentration response curves provide useful parameters to compare agonists, but these are not direct measures of drug-receptor interaction ### Effects of Antagonists - Many receptor antagonists are competitive and reversible. - The antagonist binds non-covalently and can dissociate from the receptor. - Inhibition by the antagonist is surmountable with increasing agonist concentration - Agonist potency ($EC*{50}$) is reduced in the presence of antagonist, but not its maximum response ($R*{max}$). ### Reversible Competitive Antagonists - Rightward shift in agonist potency ($EC*{50}$). - NO change in $R*{max}$. - Competitive antagonists bind to the same 3D binding site on the target receptor as the agonist, so often share structural similarities. ### Non-Competitive Antagonism - Non-competitive antagonists bind at a different (allosteric) site from the agonist on the receptor. - Effects on responses are non-surmountable, decreased glutamate maximum response $R*{max}$. ### Key points - The four main classes of receptor are ligand-gated ion channels, G-protein- coupled receptors, receptor tyrosine kinases, and nuclear receptors. - Affinity is the ability of a drug to bind its receptor; efficacy is the intrinsic ability of a drug to activate its receptor once bound; whilst potency is an empirical measure of the concentration-dependence of drug response. - The affinity of a drug for its receptor may be defined by the equilibrium dissociation constant ($K*D$). - Because of signal amplification, maximum agonist response may occur without full occupancy of receptors. - Partial agonists cannot elicit the maximal possible receptor response, even when all receptors are occupied, because they have lower efficacy. - The effect of a competitive antagonist is surmountable by increasing the concentration of agonist. - Non-competitive antagonists bind at a site different to that for the agonist, and so such antagonism is non-surmountable by addition of excess agonist.