Mechanisms of action are the means through which medicines work. All medicines produce a desired therapeutic effect. Mechanisms of action explain how drugs achieve these desired effects.
Some drugs have more than one mechanism of action. Instead, they may achieve their therapeutic effects by multiple complementary mechanisms. However, most drugs have a single mechanism of action – many of which we have tabulated below.
As part of the PTCB exam, candidates will be asked test questions on some of the most widespread mechanisms of action. For the 2020 syllabus, there is now a 40% weighting to the subject of pharmacology – and MOAs play a sizeable part in this.
It takes time to learn about the many different drug classes.
Some drug classes state the mechanism of action itself. For example:
And there are many other drug classes whose mechanism of action is not as clear.
Statins, for example, work by inhibiting the HMG-CoA reductase enzyme; the rate-limiting enzyme in the mevalonate pathway of cholesterol production.
Loop diuretics work by inhibiting the Na+/K+/2Cl– cotransporter protein in the ascending limb of the loop of Henle. The loop of Henle is a U-shaped tubule within the kidney involved in the process by which urine is eliminated from the body.
Below, we review the top 20 drug classes and briefly summarize their mechanism of action.
|Drug / Drug Class||Mechanism of Action|
|Statins Atorvastatin Pravastatin||Statins work by inhibiting the HMG-CoA reductase enzyme; the rate-limiting enzyme in the mevalonate pathway of cholesterol production.|
|Beta-blockers Metoprolol Bisoprolol|
|Beta-1 adrenoceptors are found on the heart. Beta-2 adrenoceptors are found on the lungs. Memory tool: beta-1, 1 heart; beta-2, 2 lungs! Some beta-blockers are selective for the heart and work to reduce the force of contraction and speed of conduction of the heart. This relieves the heart from work and oxygen demand. Beta-blockers also prolong the refractory period of the AV node, making these drugs effective in the treatment of cardiac arrhythmias.|
|Beta-2 agonists Albuterol Salmeterol||Beta-2 agonists are used in the treatment of asthma and COPD. By stimulating (or “agonizing”) the beta-2 receptor, which is found in the lungs, beta-2 agonists work to cause smooth muscle relaxation – making it easier to improve air flow in the lungs.|
|Corticosteroids Dexamethasone Prednisolone||Corticosteroids bind to surface cell glucocorticoid receptors, which then navigate their way into the cell nucleus to alter gene expression. Corticosteroids enhance anti-inflammatory genes and downregulate pro-inflammatory genes.|
|Tetracyclines Minocycline Doxycycline||Proteins are necessary for cells, including bacterial cells, to survive. Tetracyclines work by inhibiting protein synthesis inside bacterial cells.|
|ACE inhibitors Captopril Lisinopril|
|ACE inhibitors block the angiotensin-converting enzyme – which normally converts angiotensin I into angiotensin II. Angiotensin II is responsible for effects such as vasoconstriction (which constricts blood vessels and increases blood pressure) and releasing the hormone, aldosterone, which works to increase blood pressure further. ACE inhibitors prevent these actions from angiotensin II and therefore they are used as antihypertensive drugs.|
Naproxen Ibuprofen Etoricoxib
|NSAIDs are non-steroidal anti-inflammatory drugs. They work by inhibiting the cyclooxygenase enzyme, or COX. There are 2 kinds of COX: COX-1 and COX-2. The therapeutic effects of NSAIDs come from COX-2 inhibition – reducing inflammation. Therefore, NSAIDs are used to treat mild-to-moderate pain and pain related to inflammation. Aspirin also works by inhibiting COX.|
Codeine Dihydrocodeine Tramadol Morphine
|Opioids are used in the treatment of pain. They work by acting as agonists of the mu opioid receptor.|
|Antifungal Drugs Ketoconazole Nystatin Clotrimazole||Azole antifungal drugs work by targeting ergosterol in fungal cell membranes. By targeting ergosterol, it impairs cell membrane synthesis, cell growth, and replication – damaging fungal cells.|
|Penicillins Benzylpenicillin Flucloxacillin Ampicillin Amoxicillin||Penicillins work by inhibiting enzymes responsible for linking up key elements in bacterial cell walls. By weakening bacterial cell walls, penicillins cause these cells to swell up, break, and ultimately die. The antimicrobial activity of penicillins comes from the fact that they contain a beta-lactam ring; a 4-sided square ring in their chemical structure. Another drug class – called cephalosporins – also contain a beta-lactam ring and work in much the same way. Examples include cefazolin, ceftriaxone, cefdinir, and cefoperazone.|
|Proton-pump inhibitors Lansoprazole Omeprazole Pantoprazole||PPIs are used to treat conditions that arise from excess gastric acid production. PPIs work to inhibit gastric acid production by blocking the “proton-pump” that feeds hydrogen ions into the stomach. PPIs irreversibly bind to the H+/K+-ATPase (aka. the proton pump) in gastric parietal cells.|
|Fluoroquinolones Ciprofloxacin Moxifloxacin||Fluoroquinolones are antibacterial drugs that work by inhibiting DNA synthesis. As cells cannot replicate, the rate of production is reduced, and this allows the body to fight off the infection.|
|Benzodiazepines Diazepam Midazolam Nitrazepam||Benzodiazepines are used to treat anxiety, seizures, to induce anesthesia, and insomnia. They work by enhancing the binding of the neurotransmitter GABA to the GABA A receptor. Once bound, it causes a “depressive” effect on neuronal synaptic transmission that lead to reduced anxiety, sleepiness, sedation, and an anticonvulsive effect.|
|Antipsychotics Haloperidol Chlorpromazine Risperidone Clozapine||Antipsychotics work by a complex range of methods, but one of the most common is blocking post-synaptic D2 receptors. D2 receptors are “dopaminergic” receptors that impact dopamine levels. D2 blockade is one of the primary ways that antipsychotic drugs reduce psychotic symptoms in affected patients.|
Fluoxetine Paroxetine Sertraline
|SSRIs are “selective serotonin reuptake inhibitors”. Whereas antipsychotics act on dopamine receptors, SSRIs act on serotonin levels. Specifically, SSRIs work to inhibit neuronal reuptake into neuronal cells. This means more serotonin is available between neurons to increase neurotransmission.|
|Antihistamines Cetirizine Loratadine Fexofenadine Chlorpheniramine||These 4 drugs are antagonists of the histamine, H1 type. H1 antagonism prevents the release of histamine from granules found in mast cells. Histamine is responsible for pro-allergy effects. Hence, these drugs are used to treat allergies, hay fever, itch, and hives.|
|Antihistamines Ranitidine||There is also a H2 receptor and, once this is blocked (or “antagonized”), it reduces gastric acid production. That’s why ranitidine is used in the treatment of peptic ulcer disease, dyspepsia, and GERD.|
|Heparin||To make clots, you need thrombin and factor Xa; two key elements in the clot forming pathway. Heparin works to inactivate factor Xa and thrombin. There are low-molecular weight versions of heparin, too, and these drugs preferentially inhibitor factor Xa. Examples include enoxaparin and dalteparin.|
|Warfarin||Warfarin is also used as an anticoagulant drug. However, warfarin works by inhibiting the enzyme vitamin K epoxide reductase – preventing the reactivation of vitamin K and the synthesis of pro-clotting factors.|
|Metformin||Metformin is used in the treatment of type 2 diabetes. It works by increasing the sensitivity (or “response”) to insulin. For example, this means it suppresses glucose production by the liver, increases glucose uptake into skeletal muscle, and it suppresses glucose absorption by the intestines. This collectively works to reduce blood sugar levels.|
If you would like to test your knowledge of pharmacology for technicians, we have a module (out of 10 Modules) on this subject and hundreds of PTCB practice tests to try. Check back to our PTCB Test Prep blog soon for more great content on how you can master your next pharmacy exam!
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