{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,10,15]],"date-time":"2026-10-15T01:41:47Z","timestamp":1792028507958,"version":"4.4.0"},"reference-count":21,"publisher":"Wiley","issue":"5","license":[{"start":{"date-parts":[[2019,4,12]],"date-time":"2019-04-12T00:00:00Z","timestamp":1555027200000},"content-version":"vor","delay-in-days":0,"URL":"http:\/\/onlinelibrary.wiley.com\/termsAndConditions#vor"}],"funder":[{"name":"ArronaxPlus Equipex","award":["ANR-11-EQPX-0004"],"award-info":[{"award-number":["ANR-11-EQPX-0004"]}]},{"name":"French National Agency for Research","award":["ANR-11-LABX-0018-01"],"award-info":[{"award-number":["ANR-11-LABX-0018-01"]}]}],"content-domain":{"domain":["onlinelibrary.wiley.com"],"crossmark-restriction":true},"short-container-title":["Pharmaceutical Statistics"],"published-print":{"date-parts":[[2019,10]]},"abstract":"<jats:title>SUMMARY<\/jats:title><jats:p>The interaction between ligands and receptors is often described in terms of 50% inhibitory concentrations (<jats:italic>IC50<\/jats:italic>). However, <jats:italic>IC50<\/jats:italic> values do not accurately reflect the dissociation constants (<jats:italic>Kd<\/jats:italic>), and the domain of application and precision of proposed approximations for <jats:italic>Kd<\/jats:italic> estimation are unclear. The effect of affinity and of experimental conditions on the differences between <jats:italic>IC50<\/jats:italic> and <jats:italic>Kd<\/jats:italic> has been assessed from exact mass action law calculations and from computer simulations. Competitions between [<jats:sup>111<\/jats:sup>In]DTPA\u2010indium and a few metal\u2010DTPA complexes for binding to a specific antibody are discussed as a practical example. Exact calculations of competition assays have been implemented in Microsoft Excel and performed for a variety of concentrations of receptor, tracer, and competitor. The results are identical to those of software packages. <jats:italic>IC50<\/jats:italic> is found larger than <jats:italic>Kd<\/jats:italic> by less than 20% only when tracer concentration is small compared with <jats:italic>Kd<\/jats:italic> and to the receptor concentration and when this receptor concentration is small compared with <jats:italic>Kd<\/jats:italic>. Otherwise, <jats:italic>Kd<\/jats:italic> and <jats:italic>IC50<\/jats:italic> may be very different and approximations proposed in the literature to obtain <jats:italic>Kd<\/jats:italic> values from graphically derived <jats:italic>IC50<\/jats:italic> are not acceptable as soon as the concentrations of tracer or of receptor approach <jats:italic>Kd<\/jats:italic>. Under most experimental conditions, <jats:italic>IC50<\/jats:italic> values do not reflect <jats:italic>Kd<\/jats:italic> values. Using available software packages to determine and report <jats:italic>Kd<\/jats:italic> values would allow for more meaningful comparisons of results obtained under different experimental conditions.<\/jats:p>","DOI":"10.1002\/pst.1943","type":"journal-article","created":{"date-parts":[[2019,4,12]],"date-time":"2019-04-12T12:07:05Z","timestamp":1555070825000},"page":"513-525","update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":27,"title":["Equilibrium, affinity, dissociation constants, IC5O: Facts and fantasies"],"prefix":"10.1002","volume":"18","author":[{"given":"Jacques","family":"Barbet","sequence":"first","affiliation":[{"name":"GIP Arronax  Saint\u2010Herblain Cedex France"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-1948-6701","authenticated-orcid":false,"given":"Sandrine","family":"Huclier\u2010Markai","sequence":"additional","affiliation":[{"name":"GIP Arronax  Saint\u2010Herblain Cedex France"},{"name":"Laboratoire Subatech, UMR 6457 Ecole des Mines de Nantes\/CNRS\/IN2P3\/Universit\u00e9 de Nantes  Nantes Cedex 3 France"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"311","published-online":{"date-parts":[[2019,4,12]]},"reference":[{"key":"e_1_2_7_2_1","doi-asserted-by":"publisher","DOI":"10.1002\/pst.426"},{"key":"e_1_2_7_3_1","doi-asserted-by":"publisher","DOI":"10.1093\/nar\/gkp253"},{"key":"e_1_2_7_4_1","doi-asserted-by":"publisher","DOI":"10.1371\/journal.pcbi.1004012"},{"key":"e_1_2_7_5_1","doi-asserted-by":"publisher","DOI":"10.1016\/j.tips.2006.01.008"},{"key":"e_1_2_7_6_1","doi-asserted-by":"publisher","DOI":"10.1111\/j.1749-6632.1949.tb27297.x"},{"key":"e_1_2_7_7_1","doi-asserted-by":"publisher","DOI":"10.1111\/j.1476-5381.1959.tb00928.x"},{"key":"e_1_2_7_8_1","doi-asserted-by":"publisher","DOI":"10.1016\/0006-2952(73)90196-2"},{"issue":"6","key":"e_1_2_7_9_1","first-page":"603","article-title":"Ligand dissociation constants from competition binding assays: errors associated with ligand depletion","volume":"31","author":"Goldstein A","year":"1987","journal-title":"Mol Pharmacol"},{"key":"e_1_2_7_10_1","doi-asserted-by":"publisher","DOI":"10.1111\/j.1476-5381.2009.00604.x"},{"issue":"1","key":"e_1_2_7_11_1","first-page":"1","article-title":"The kinetics of competitive radioligand binding predicted by the law of mass action","volume":"25","author":"Motulsky HJ","year":"1984","journal-title":"Mol Pharmacol"},{"key":"e_1_2_7_12_1","doi-asserted-by":"publisher","DOI":"10.1016\/0003-2697(80)90515-1"},{"key":"e_1_2_7_13_1","doi-asserted-by":"publisher","DOI":"10.1016\/S0003-2697(02)00289-0"},{"key":"e_1_2_7_14_1","doi-asserted-by":"publisher","DOI":"10.1016\/S0300-9084(73)80455-9"},{"key":"e_1_2_7_15_1","doi-asserted-by":"publisher","DOI":"10.1006\/jtbi.1993.1192"},{"key":"e_1_2_7_16_1","unstructured":"DelaageM DrocourtJL Princ\u00e9P.Proc\u00e9d\u00e9 de fixation de macromol\u00e9cules biologiques sur des supports. 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