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In the field of pharmacology, potency is a measure of drug activity expressed in terms of the amount required to produce an effect of given intensity. A highly potent drug (e.g., morphine, alprazolam, chlorpromazine) evokes a larger response at low concentrations, while a drug of lower potency (ibuprofen, acetylsalicylic acid) evokes a small response at low concentrations. It is proportional to affinity and efficacy.
Affinity is the ability of the drug to bind to a receptor. Efficacy is the relationship between receptor occupancy and the ability to initiate a response at the molecular, cellular, tissue or system level. The response is equal to the effect, or (E), and depends on both the drug binding and the drug-bound receptor then producing a response; thus, potency depends on both affinity and efficacy. The agonist, the ligand, drug or hormone that binds to the receptor and initiates the response is usually abbreviated A or D. Below a certain concentration of agonist ([A]), E is too low to measure but at higher concentrations it becomes appreciable and rises with increasing agonist concentration [A] until at sufficiently high concentrations it can no longer be increased by raising [A] and asymptotes to a maximum Emax.
The Emax is the maximum possible effect for the agonist. The concentration of A at which E is 50% of Emax is termed the half maximal effective concentration and is abbreviated [A]50, or more commonly EC50. The term "potency" refers to the [A]50 value. The lower the [A]50, the less the concentration of a drug is required to produce 50% of maximum effect and the higher the potency.
Higher potency does not necessarily mean more side effects.
Many drugs that are highly potent, can also be highly toxic (ex: cancer drugs). For this reason, extreme care is used in developing potent drugs. Drug substances can be categorized by their potency by using occupational exposure limits (OELs). The system most used is a four-category system from SafeBridge. However, there are many other categorization systems including the Merck system, the Roche system, and the Affygility system.
A "potent" active pharmaceutical ingredient (API) is considered:
1. Activity at less than or equal to 150 μg/kg body mass 2. OEL less than or equal to 10 μg/m3 of air, as an 8-hour time-weighted average 3. Known carcinogen, mutagen, or teratogen 4. Recently developed compound with no potency/toxicological studies
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