Pharmacodynamics: MRCOG Part 1 questions and key facts

A drug's therapeutic window (or therapeutic index) refers to the range of doses at which it is effective without causing unacceptable toxicity. A narrow therapeutic window means there is a small difference between the minimum effective concentration (MEC) and the minimum toxic concentration (MTC). Therefore, the drug must be administered with care and control, often requiring therapeutic drug monitoring (TDM) to ensure plasma concentrations remain within the desired range, preventing both sub-therapeutic effects and toxicity.

Drugs with a narrow therapeutic window inherently require close monitoring because even small variations in dosage or patient factors (like metabolism or excretion) can push the drug concentration outside the safe and effective range. This contradicts the idea that the drug can be administered without close monitoring or that the dosage range is wide. Impaired renal function or pregnancy can significantly alter drug pharmacokinetics, potentially increasing the risk of toxicity with narrow therapeutic window drugs, making these conditions reasons for more caution, not less.

Aspirin exerts its effects by irreversibly inhibiting cyclooxygenase (COX) enzymes through acetylation. While it inhibits both major isoforms, COX-1 and COX-2, aspirin demonstrates significantly greater selectivity for COX-1. This preferential inhibition of COX-1, particularly at lower doses used for antiplatelet effects, is central to both its therapeutic actions and its side effect profile.

The COX-1 isoform is constitutively expressed in many tissues, including gastric mucosa, platelets, and kidneys. It plays a crucial role in producing prostaglandins that protect the gastric lining and thromboxane A2, which promotes platelet aggregation. Aspirin's inhibition of platelet COX-1 underlies its anti-thrombotic effect, while its inhibition of gastric mucosal COX-1 contributes to its gastrointestinal side effects like gastritis and ulceration.

COX-2 is primarily an inducible enzyme, upregulated at sites of inflammation, and is responsible for producing prostaglandins involved in pain, fever, and inflammation. While aspirin also inhibits COX-2, higher doses are typically required for significant anti-inflammatory effects compared to the doses needed for antiplatelet action via COX-1 inhibition. COX-3 is a splice variant of COX-1, primarily found in the central nervous system, and its physiological role and interaction with aspirin are less well-characterised compared to COX-1 and COX-2. Therefore, stating aspirin is not selective or that its selectivity hasn't been confirmed is incorrect; its preference for COX-1 is well-established.

Diclofenac is a widely used Non-Steroidal Anti-Inflammatory Drug (NSAID). The primary mechanism of action for NSAIDs, including diclofenac, is the inhibition of Cyclo-oxygenase (COX) enzymes.

Diclofenac inhibits both COX-1 and COX-2 isoforms. These enzymes are crucial for the synthesis of prostaglandins and thromboxanes from arachidonic acid. By blocking COX enzymes, diclofenac reduces the production of these inflammatory mediators, leading to its anti-inflammatory, analgesic (pain-relieving), and antipyretic (fever-reducing) effects.

Other enzymes listed are targets for different drug classes: Angiotensin-converting enzyme (ACE) is inhibited by ACE inhibitors (e.g., ramipril) used in hypertension. Acetylcholinesterase is inhibited by the anticholinesterase drugs (e.g., neostigmine) used for myasthenia gravis and to reverse neuromuscular blockade. Reverse transcriptase is an enzyme found in retroviruses like HIV and is inhibited by antiretroviral drugs. Vitamin K epoxide reductase is inhibited by warfarin, an anticoagulant.