For a drug to exert its effect, it must bind to the corresponding target. Drug targets mainly include enzymes, receptors, ion channels, hormones, and cytokines. Small molecules that bind to different targets to exert various effects are referred to by different names.
Enzymes—Inhibitors and Activators
Enzymes are proteins or RNA produced by cells in living organisms, characterized by their high specificity and catalytic efficiency. It is due to the presence of enzymes that chemical reactions within living organisms can occur efficiently and specifically under very mild conditions. Enzymatic reactions are an inherent essence of life.
Some small molecules can bind to enzymes and reduce their catalytic activity; these small molecules are known as enzyme inhibitors. Based on the mode of binding to the enzyme, inhibitors are classified into reversible and irreversible inhibitors. Reversible inhibitors bind to enzymes through non-covalent bonds (such as hydrogen bonds, ionic bonds, van der Waals forces, and hydrophobic interactions), leading to a decrease or loss of enzyme activity. These inhibitors can be removed using physical methods, allowing the enzyme to be reactivated. In contrast, irreversible inhibitors bind to enzymes via covalent bonds, resulting in a decrease or loss of enzyme activity that cannot be reversed by physical means. Aspirin is a typical example of an irreversible inhibitor, as it forms an irreversible covalent bond with the amino acid residues at the active site of cyclooxygenase (COX), leading to the acetylation and loss of COX activity.
Common cyclooxygenase (COX) inhibitors at BOC Sciences
| CAT | CAS | Name |
| B0084-451901 | 169590-42-5 | Celecoxib |
| B2693-120682 | 51146-57-7 | R-Ibuprofen |
| B2693-084941 | 15687-27-1 | Ibuprofen |
| B0084-315840 | 22204-53-1 | Naproxen |
| 50-78-2 | 50-78-2 | Aspirin |
| 15307-86-5 | 15307-86-5 | Diclofenac |
| B0084-051361 | 501-36-0 | Resveratrol |
| B2693-185746 | 70374-39-9 | Lornoxicam |
Reversible inhibitors can further be divided into competitive, non-competitive, and uncompetitive inhibitors based on their binding sites on the enzyme. Competitive inhibition refers to a situation where the inhibitor resembles the substrate’s structure and binds to the same site, competing with the substrate for binding to the enzyme. When the concentration of the inhibitor is much greater than that of the substrate, the enzyme-inhibitor complex (EI) is primarily formed, preventing product formation and inhibiting the enzymatic reaction. Sulfanilamide antibiotics (SAs) are a classic example of competitive inhibitors; SAs have a structure similar to para-aminobenzoic acid (PABA), the substrate for dihydropteroate synthase. When the concentration of SAs in the body is much higher than that of PABA, the binding site of dihydropteroate synthase is occupied by SAs, preventing PABA from being utilized by the enzyme to synthesize dihydrofolate, thereby obstructing bacterial folate synthesis and inhibiting their growth and reproduction.
Noncompetitive inhibitionrefers to a scenario where the inhibitor binds to the non-active site of the enzyme, forming an enzyme-inhibitor complex (EI), which then can bind to the substrate to form an enzyme-substrate-inhibitor complex (ESI). Alternatively, the enzyme can first bind to the substrate to form an enzyme-substrate complex (ES), which then binds to the inhibitor to form the ESI complex. Regardless of the formation pathway, the resulting intermediate ESI cannot decompose to produce the product, thus inhibiting the enzymatic reaction. The world’s first drug for treating neurofibromatosis, simitinib, is a non-ATP competitive inhibitor of MEK1/2. Simitinib and ATP can bind to MEK kinase without affecting each other, but the formed ESI intermediate cannot decompose to generate the product, ultimately leading to inhibited enzyme activity.
Uncompetitive inhibition refers to a type of inhibition where the inhibitor can only bind to the enzyme after the substrate has formed a substrate-enzyme complex (ES). This binding results in the formation of an enzyme-substrate-inhibitor complex (ESI), which cannot decompose to produce the product, thereby inhibiting the enzymatic reaction. Examples of uncompetitive inhibition include the inhibitory effects of hydrazine compounds on pepsin, cyanides on aromatic sulfatases, and various amino acids such as L-phenylalanine and L-arginine on alkaline phosphatase.
In addition, there is allosteric inhibition, where the inhibitor specifically binds to a site outside the enzyme’s active site, causing a conformational change in the enzyme protein molecule. This change leads to structural alterations at the active site, preventing substrate binding and catalytic activity. The world’s first approved TYK2 inhibitor, deucravacitinib, used for treating plaque psoriasis, is an example of an allosteric inhibitor.
In living organisms, there is also a class of enzymes that exist as inactive precursors, known as zymogens, synthesized or initially secreted within cells. Small molecules that convert inactive zymogens into active enzymes are referred to as enzyme activators. Dapagliflozin, an innovative glucose-lowering drug from Huailing Pharmaceuticals, is the world’s first glucose kinase activator.
Receptors—Agonists, Antagonists, Reverse Agonists
Receptors are biomolecules composed of glycoproteins or lipoproteins located on the cell membrane or within the cell that recognize and bind to bioactive molecules, leading to changes in cellular function. Receptors can identify and receive specific signals through ligand binding, accurately amplifying and transmitting these signals inside the cell, thereby initiating a series of intracellular signaling cascades that produce specific biological effects.
Some small molecules can bind to receptors and activate them, producing corresponding physiological effects or drug actions; these small molecules are known as receptor agonists. Examples include albuterol and clenbuterol, which activate β₂ receptors distributed on airway smooth muscle to induce bronchodilation, used for treating asthma.
Other small molecules bind to receptors but do not elicit biological effects; instead, they block the effects mediated by receptor agonists and are known as receptor antagonists. Antagonists require the presence of an agonist to exert their effects and can be classified into competitive and non-competitive antagonists. Competitive antagonism occurs when the antagonist binds to the same site on the receptor as the agonist, preventing the agonist from binding. Increasing the concentration of the antagonist can gradually inhibit the effects produced by the agonist until complete blockade occurs. Conversely, a sufficiently high concentration of the agonist can displace a certain concentration of the antagonist, still achieving the maximum effect as if the agonist were used alone. Naltrexone, an anti-addiction medication, is a competitive receptor antagonist that blocks the effects of endogenous opioids. Non-competitive antagonism occurs when the antagonist binds to a different site on the receptor, hindering the agonist’s ability to bind even if the concentration of the agonist is increased. Memantine, a non-competitive NMDA receptor antagonist, is used for treating moderate to severe Alzheimer’s disease.
Some receptors can self-activate and produce effects without an agonist, which is referred to as the receptor’s intrinsic activity. Small molecules that inhibit the intrinsic activity of a receptor are called reverse agonists. Reverse agonists have affinity but no intrinsic activity; when they bind to the receptor, they do not activate it but can antagonize the receptor’s intrinsic activity, resulting in effects opposite to those of the agonists. Pimozide, an antipsychotic drug, is a selective serotonin receptor reverse agonist that targets the 5-HT2A receptor, reducing its baseline activity to decrease the risk of hallucinations or delusions.
Ion Channels—Blockers
Ion channels are transmembrane protein molecules in the cell membrane that form highly selective hydrophilic pores, allowing certain ions to pass through selectively. Based on their gating mechanisms, ion channels are classified as voltage-gated, ligand-gated, and mechanically gated. Some small molecules can prevent ion channels from opening; these small molecules are known as ion channel blockers. Ion channel blockers are widely used in clinical applications, such as amlodipine, a calcium channel blocker for treating hypertension, dofetilide, a potassium channel blocker for antiarrhythmia, and benzocaine, a sodium channel blocker with local anesthetic properties.
Hormones and Cytokines—Agonists and Inhibitors
Hormones are a class of chemical substances produced by endocrine cells that play a crucial role in information transfer. Cytokines are small protein molecules with broad biological activity synthesized and secreted by immune and some non-immune cells in response to stimulation. Both hormones and cytokines typically initiate a series of signal transduction pathways by binding to their corresponding receptors, producing specific biological effects. Small molecules that enhance the effects of hormones and cytokines are called agonists, while those that diminish their effects are referred to as inhibitors; this classification for small molecules is relatively broad.
Summary
Although the types of action targets vary and the classifications of small molecule compounds differ, these classifications are not absolute and may overlap. This is due to the complexity of life, where some targets can perform multiple functions. For instance, receptor tyrosine kinases serve as both receptors and kinases; they can bind to ligands to perform receptor functions while also phosphorylating the tyrosine residues of target proteins to exert kinase activity. The N-acetylcholine receptor acts as both a receptor and an ion channel, recognizing signaling molecules to perform receptor functions while controlling ion flow in and out of the cell. This complexity may also contribute to the frequent confusion among researchers regarding inhibitors, activators, agonists, antagonists, reverse agonists, and blockers. It is hoped that this article can assist researchers in understanding these concepts better.