The answer so far
A medicine’s target is a biological structure or process that the medicine interacts with to change cell or organism behaviour. Targets are often proteins: receptors, enzymes, ion channels, or transporters. The word describes a level of explanation, not a guarantee that the medicine will help a person. A molecule can bind a target in a test tube without reaching it in the body, changing it enough to matter, or producing a useful clinical outcome.12
On this page
- Target, tissue and outcome are different
- Receptors are not the whole story
- Target engagement answers a narrower question
- Ask what “selective” means
- Enzymes change reaction rates
- Channels control flow and electrical state
- Transporters affect both exposure and effect
- Where the target sits changes the question
- Mechanism evidence and clinical evidence answer different questions
- A target is not an outcome
- Sources
From left to right: a target, a tissue and an outcome are different levels of explanation.
Target, tissue and outcome are different
The target might be a protein. The tissue is where it operates. The outcome is what changes for a person. Keeping these levels separate prevents a molecular observation from being presented as a demonstrated treatment benefit.1
Receptors are not the whole story
Some medicines alter signalling through receptors. Others affect enzymes, which help chemical reactions occur, or different cellular structures. A receptor-based animation is appropriate only when that mechanism fits the substance being explained.1
Target engagement answers a narrower question
Evidence that a medicine reaches and interacts with a target helps establish whether part of its proposed mechanism occurs. It does not establish the size of the clinical benefit, whether harms outweigh it, or whether it works for another condition. Clinical research addresses those broader questions.2
Ask what “selective” means
A medicine may interact more strongly with one target than another without acting exclusively on one target. Selectivity is therefore a preference measured under specified assay and exposure conditions, not a promise that other targets are never engaged. “Targeted” is not a synonym for “without side effects”.31
Enzymes change reaction rates
Enzymes are biological catalysts. A medicine can inhibit an enzyme, activate it, replace a missing enzyme, or alter the conditions in which it operates. Inhibiting an enzyme may be the intended treatment mechanism, but the same enzyme may be needed in another tissue or pathway. The result depends on substrate availability, exposure, reversibility, duration, and whether cells can compensate. An enzyme assay can show a direct molecular effect; a clinical study must establish whether that change improves a patient-important outcome and what other effects accompany it.14
Channels control flow and electrical state
Ion channels form pores through membranes. When they open or close, selected ions move across the membrane, changing electrical potential or intracellular signalling. Some channels respond to ligands, some to voltage, and others to mechanical or chemical conditions. Medicines may block a channel, stabilise a state, or alter the probability that it opens. Because channels contribute to nerve, muscle, heart, and secretory-cell function, a target can have effects in more than one organ. Selectivity for a subtype or state is useful, but it is not a promise of absolute specificity. For example, lidocaine used as a local anaesthetic blocks voltage-sensitive sodium channels in nerves, interrupting the electrical impulses that carry sensation. This illustrates a channel target in a specific preparation and setting; the same molecule can have different effects in other tissues.548
Transporters affect both exposure and effect
Transporter proteins move ions, nutrients, neurotransmitters, medicines, or metabolites across membranes. A medicine can inhibit a transporter, compete with normal cargo, or use a transporter to enter or leave a cell. Transporters therefore affect absorption, tissue distribution, passage across barriers such as the blood–brain barrier, and elimination by the liver or kidneys. A second medicine can change how much of a treatment reaches its proposed target by changing exposure, even when it does not bind that target. This is why the target and where the medicine goes cannot always be separated. For example, EMA explains that empagliflozin inhibits the kidney transporter SGLT2, reducing glucose reabsorption and increasing glucose in urine. That mechanism helps explain an effect on blood glucose; its clinical benefits and risks still require product- and indication-specific evidence.467
Where the target sits changes the question
A target with an accessible binding site on the outside of a cell may be reached by a molecule that cannot cross the membrane, while a target inside the cell may require the medicine to cross a membrane and remain unbound long enough to interact. Apparent concentration in blood is therefore not the same as concentration at the target site. Whether a proposed target is engaged in the relevant tissue depends on many factors. These include protein binding, metabolism and active metabolites, local pH, tissue barriers, receptor density and competing molecules. These variables can make two people respond differently even when they receive the same nominal dose.64
Mechanism evidence and clinical evidence answer different questions
A binding assay asks whether a molecule interacts with a target. A cell experiment asks what happens in a particular cellular setting. An animal study adds whole-organism biology, but its exposure, metabolism, and disease model may differ from human conditions. Clinical research asks whether the intervention changes outcomes in people compared with a specified comparator, and whether benefits remain acceptable beside harms. Evidence at one level can support a hypothesis for the next level without proving it. A target claim should therefore state the system, concentration, timing, and outcome that were actually measured.21
A target is not an outcome
The chain from target to outcome has several links: molecular interaction, cellular response, tissue physiology, symptoms or function, and longer-term clinical outcome. A medicine can change a target without improving how a person feels or functions. For the empagliflozin example, increased urinary glucose is close to the transporter mechanism; that observation alone cannot establish a patient-important benefit or show that harms are acceptable. A proposed mechanism can fail because the target was not sufficiently engaged, the disease biology was misidentified, the effect was too small, the comparator performed differently, or harms offset the benefit. When a headline says that a medicine targets a protein, ask which target or isoform was measured, what kind of evidence was used, whether the medicine reached the relevant tissue at an effective unbound concentration, what other targets were tested, and whether patient-important outcomes and harms were measured.127
Sources, kept close.
Selected supporting sources; this is not a complete literature review.
- How Do Medicines Work?NIH / NIGMS · Official scientific explainer
- Step 3: Clinical ResearchUS Food and Drug Administration · Regulator explanation
- Clinical pharmacodynamicsIUPHAR Pharmacology Education Project · Professional educational reference
- Drugs and their molecular targets: an updated overviewOverington et al. / Nature Reviews Drug Discovery · Peer-reviewed pharmacology overview
- Ion channelsIUPHAR Pharmacology Education Project · Professional educational reference
- What Happens to Medicine in Your Body?NIH / NIGMS · Official scientific explainer
- Jardiance: European public assessment report overviewEuropean Medicines Agency · Regulator product overview and mechanism
- Lidocaine: FDA clinical pharmacology reviewU.S. Food and Drug Administration · Regulatory clinical pharmacology review