50%阻害濃度（50%そがいのうど、IC50）または半数阻害濃度とは、化合物の生物学的または生化学的阻害作用の有効度を示す値である。
IC50は元来英語の"half maximal (50%) inhibitory concentration"の略語であるが、現在ではIC50の表記の方が一般的である。しばしば対象にされる化合物とは医薬品候補化合物である。この定量的値は、特定の薬物もしくはその他の物質（阻害剤）が注目する生物学的プロセス（もしくはプロセスの要素、例えば酵素、細胞、受容体、微生物）の半数を阻害するにはどれだけの濃度が必要かを示し、より低い値を示す化合物は阻害剤としての活性がより高いと言える。IC50は薬学研究において阻害剤の有効性を示す値として広く用いられている。便宜上-log IC50で算出されるpIC50が代わりに用いられることもある。この場合より大きな値は指数関数的により大きな有効性を示している。FDA（アメリカ食品医薬品局）によればIC50はin vitroにおける50%阻害のために必要な薬物の濃度を表している[1]。 このことはEC50の場合と対照的である。EC50は50%の効果を示す濃度のことであるが、この場合はin vivoでの血漿中濃度を意味している[1]。
目次
- 1 IC50の決定法
- 1.1 機能的アンタゴニスト試験（Functional antagonist assay）
- 1.2 競合結合試験（Competition binding assays）
- 2 脚注
- 3 関連項目
- 4 外部リンク
IC50の決定法[編集]
機能的アンタゴニスト試験（Functional antagonist assay）[編集]
ある薬物のIC50を決定するには、用量反応関係を明らかにし、異なった濃度のアンタゴニスト（薬物）がどのようにアゴニスト活性を抑制するかを調べる必要がある。注目するアンタゴニストのIC50値は、アゴニストの生物学的作用の最大値の半数が阻害されるために必要なアンタゴニストの濃度を決定することで算出される[2]。 IC50値は測定条件に依存する。一般的に、より高い濃度の阻害剤はアゴニスト活性をより低く抑える。IC50値は酵素の濃度が増加すると共に増加する。さらに、阻害の種類によりその他の因子がIC50値に影響することがある。ATP依存の酵素の場合、IC50値は（拮抗阻害の場合は特に）ATPの濃度と相互依存性を持つ。IC50値は２種類の阻害剤の有効性を比較するために用いられる。
競合結合試験（Competition binding assays）[編集]
この試験においては、単一濃度の放射性リガンド（通常アゴニスト）がそれぞれの試験管に使用される。通常、その解離定数値（Kd）と同じかより低濃度のリガンドが試験に用いられる。一定範囲の様々な濃度のその他の競合する非放射性化合物（通常拮抗剤）をそれぞれの試験管に加えた後、放射性リガンドの有効性を測定することで、特定の放射性リガンドの結合のレベルが決定される。競合曲線は計算によりロジスティック関数に回帰される。 この場合IC50は、放射性リガンドの全結合のうち50%が置換された時の競合するリガンドの濃度である。IC50値はチェン－プルソフ（Cheng-Prusoff）式に従い絶対阻害定数（Ki）に変換される[2][3]。
IC50と親和性[編集]
チェン－プルソフ式によればIC50と親和性は少なくとも競争するアゴニストと拮抗剤の間では関連があると言えるが、IC50は親和性を示す直接的な指標ではない[4]。
ここで、Ki は阻害物質の結合親和性、 IC50は阻害物質の機能的強度、[S]は基質の濃度、Kmは基質の酵素に対する親和性を表す。ある化合物のIC50値は放射性リガンドの濃度に依存して実験毎に変化しうるが、Ki は絶対的な値である。Kiは薬物の阻害定数であり、放射性リガンドが存在しないならば、競合試験において受容体の50％を占める競合リガンドの濃度となる[3]。
脚注[編集]
- ^ a b アメリカ食品医薬品局. “IC50 versus EC50”. 2011年8月23日閲覧。
- ^ a b NIH Chemical Genomics Center (2008年). “Assay Guidance // Assay Guidance Manual // Assay Operations for SAR Support”. 2011年8月23日閲覧。
- ^ a b Glaxo Wellcome and Science - Global. “Pharmacology Guide - Receptor binding techniques: competition (inhibition or displacement) assays”. 2011年8月23日閲覧。
- ^ Cheng Y, Prusoff WH (December 1973). “Relationship between the inhibition constant (K1) and the concentration of inhibitor which causes 50 per cent inhibition (I50) of an enzymatic reaction”. Biochem. Pharmacol. 22 (23): 3099–108. PMID 4202581.
関連項目[編集]
外部リンク[編集]
Visual demonstration of how to derive IC50 value: Arrange data with inhibition on vertical axis and log(concentration) on horizontal axis; then identify max and min inhibition; then the IC50 is the concentration at which the curve passes through the 50% inhibition level.
The half maximal inhibitory concentration (IC50) is a measure of the effectiveness of a compound in inhibiting biological or biochemical function. This quantitative measure indicates how much of a particular drug or other substance (inhibitor) is needed to inhibit a given biological process (or component of a process, i.e. an enzyme, cell, cell receptor or microorganism) by half. In other words, it is the half maximal (50%) inhibitory concentration (IC) of a substance (50% IC, or IC50). It is commonly used as a measure of antagonist drug potency in pharmacological research. According to the FDA, IC50 represents the concentration of a drug that is required for 50% inhibition in vitro.[1] It is comparable to an EC50 for agonist drugs. EC50 also represents the plasma concentration required for obtaining 50% of a maximum effect in vivo.[1]
Contents
- 1 Determining the IC50 of a drug
- 1.1 Functional antagonist assay
- 1.2 Competition binding assays
- 1.3 pIC50
- 1.4 Behavioral Assays
- 2 See also
- 3 References
- 4 External links
Determining the IC50 of a drug[edit]
Functional antagonist assay[edit]
The IC50 of a drug can be determined by constructing a dose-response curve and examining the effect of different concentrations of antagonist on reversing agonist activity. IC50 values can be calculated for a given antagonist by determining the concentration needed to inhibit half of the maximum biological response of the agonist.[2]
IC50 values are very dependent on conditions under which they are measured. In general, the higher the concentration of inhibitor, the more agonist activity will be lowered. IC50 value increases as agonist concentration increases. Furthermore depending on the type of inhibition other factors may influence IC50 value; for ATP dependent enzymes IC50 value has an interdependency with concentration of ATP, especially so if inhibition is all of it competitive. IC50 values can be used to compare the potency of two antagonists.
Competition binding assays[edit]
In this type of assay, a single concentration of radioligand (usually an agonist) is used in every assay tube. The ligand is used at a low concentration, usually at or below its Kd value. The level of specific binding of the radioligand is then determined in the presence of a range of concentrations of other competing non-radioactive compounds (usually antagonists), in order to measure the potency with which they compete for the binding of the radioligand. Competition curves may also be computer-fitted to a logistic function as described under direct fit.
In this situation the IC50 is the concentration of competing ligand which displaces 50% of the specific binding of the radioligand. The IC50 value is converted to an absolute inhibition constant Ki using the Cheng-Prusoff equation (see Ki).[2][3]
IC50 and affinity[edit]
IC50 is not a direct indicator of affinity although the two can be related at least for competitive agonists and antagonists by the Cheng-Prusoff equation.[4] For enzymatic reactions, this equation is:
where Ki is the binding affinity of the inhibitor, IC50 is the functional strength of the inhibitor, [S] is fixed substrate concentration and Km is the concentration of substrate at which enzyme activity is at half maximal (but is frequently confused with substrate affinity for the enzyme, which it is not).
Alternatively, for inhibition constants at cellular receptors:[5]
where [A] is the fixed concentration of agonist and EC50 is the concentration of agonist that results in half maximal activation of the receptor. Whereas the IC50 value for a compound may vary between experiments depending on experimental conditions, the Ki is an absolute value. Ki is the inhibition constant for a drug; the concentration of competing ligand in a competition assay which would occupy 50% of the receptors if no ligand were present.[3]
The Cheng-Prusoff equation produces good estimates at high agonist concentrations, but over- or under-estimates Ki at low agonist concentrations. In these conditions, other analyses have been recommended.[5]
pIC50[edit]
Sometimes, IC50 values are converted to the pIC50 scale.
Note the minus sign, which means that higher values of pIC50 indicate exponentially greater potency.
pIC50 is usually given in terms of molar concentration (mol/L, or M). Therefore to obtain a pIC50, an IC50 should be specified in units of M. When IC50 is expressed in μM or nM, it will need to be converted to M before conversion to pIC50.[6]
Behavioral Assays[edit]
The IC50 terminology is also used for some behavioral measures in vivo, such as a two bottle fluid consumption test. When animals decrease consumption from the drug-laced water bottle, the concentration of the drug that results in a 50% decrease in consumption is considered the IC50 for fluid consumption of that drug.[7]
See also[edit]
- Certain safety factor
- Dose-response relationship
- EC50 (half maximal effective concentration)
- LD50 (median lethal dose)
References[edit]
- ^ a b IC50 versus EC50
- ^ a b NIH Chemical Genomics Center // Assay Guidance // Assay Guidance Manual // Assay Operations for SAR Support
- ^ a b Glaxo Wellcome and Science - Global
- ^ Cheng Y, Prusoff WH (December 1973). "Relationship between the inhibition constant (KI) and the concentration of inhibitor which causes 50 per cent inhibition (I50) of an enzymatic reaction". Biochem Pharmacol 22 (23): 3099–108. doi:10.1016/0006-2952(73)90196-2. PMID 4202581.
- ^ a b Lazareno, S.; Birdsall, N. J. (1993). "Estimation of competitive antagonist affinity from functional inhibition curves using the Gaddum, Schild and Cheng-Prusoff equations". British journal of pharmacology 109 (4): 1110–1119. PMC 2175764. PMID 8401922. edit
- ^ Stewart MJ, Watson ID (1983). "Standard units for expressing drug concentrations in biological fluids". British Journal of Clinical Pharmacology 16 (1): 3–7. doi:10.1111/j.1365-2125.1983.tb02136.x.
- ^ Robinson, SF; Marks, MJ; Collins, AC (1996). "Inbred mouse strains vary in oral self-selection of nicotine". Psychopharmacology 124 (4): 332–9. PMID 8739548.
External links[edit]
- ELISA IC50/EC50 Online Tool
- IC50 to pIC50 calculator
- Online tool for analysis of in vitro resistance to antimalarial drugs
- IC50-to-Ki converter
