{"id":1802,"date":"2024-03-29T03:22:37","date_gmt":"2024-03-29T03:22:37","guid":{"rendered":"https:\/\/medreasons.com\/?page_id=1802"},"modified":"2024-04-08T04:35:59","modified_gmt":"2024-04-08T03:35:59","slug":"pharmacokinetics","status":"publish","type":"page","link":"https:\/\/medreasons.com\/index.php\/posts\/pharmacology\/general-pharmacology\/pharmacokinetics\/","title":{"rendered":"Pharmacokinetics"},"content":{"rendered":"\t\t
\n\t\t\t\t
\n\t\t\t\t\t
\n\t\t
\n\t\t\t\t\t
\n\t\t\t\t
\n\t\t\t\t
\n\t\t\t\t\t\t\t\t\t

Does Phenytoin follow Pseudo Zero Order Elimination kinetics ?<\/h1>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
\n\t\t\t\t
\n\t\t\t\t\t\t\t\t\t

The straightforward answer to the above question is NO<\/span> . This misconception is prevalent because Phenytoin follows Zero order kinetics at toxic doses and first order kinetics at therapeutic doses.<\/strong><\/p>

To gain a better insight into this we need to understand what is Pseudo Zero order kinetics and elimination kinetics of Phenytoin separately.\u00a0<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t

\n\t\t\t\t\t
\n\t\t\t\t
\n\t\t\t\t
\n\t\t\t\t\t\t\t\t\t

Pseudo Zero Order Kinetics<\/h2>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
\n\t\t\t\t
\n\t\t\t\t\t\t\t\t\t

In pharmacology, we have two major types of elimination –<\/p>

  1. Zero order Kinetics<\/strong><\/span>:<\/strong> In which the amount of drug excretion is constant and it doesn’t depend upon the concentration of the drug.<\/li>
  2. First order Kinetics<\/strong><\/span>:<\/strong> Rate of drug elimination is directly proportional to the drug concentration.<\/li><\/ol>

    As per the rate law for the elimination of drug A :\u00a0 \u00a0 \u00a0\u00a0<\/p>

    R = K [ A ]x<\/sup><\/strong><\/p>

    where, R = Rate of elimination of drug A<\/p>

    \u00a0 K = Elimination constant\u00a0<\/p>

    \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 [A] = Plasma Concentration of drug A<\/p>

    \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0x = Order of elimination<\/p>

    Zero Order Kinetics: x = 0<\/p>

    First Order Kinetics: x = 1<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t

    \n\t\t\t\t\t
    \n\t\t
    \n\t\t\t\t
    \n\t\t\t\t
    \n\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\"Zero\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t
    \n\t\t\t\t
    \n\t\t\t\t
    \n\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\"First\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t
    \n\t\t\t\t\t
    \n\t\t\t\t
    \n\t\t\t\t
    \n\t\t\t\t\t\t\t\t\t

    Now lets think of a scenario where a patient swallows a tablet. As soon as the tablet enters into the mucosa of gastrointestinal tract the tablet encounters moisture. Now at the instant of this reaction, there is a large amount of drug (in tablet form) relative to the amount of moisture. It is at this instant where the drug’s dissociation is limited by the amount of moisture. So a large amount of it is dissociated and forms suspension. It appears to us that the dissociation of the drug is independent of the its own concentration (which is a characteristic of zero order kinetics) but actually the whole reaction is a first order kinetic reaction(more the amount of drug more is its dissociation).\u00a0\u00a0<\/p>

    Hence a pseudo zero order kinetics is a type of elimination kinetics that appears to follow zero order kinetics but is actually a result of first order elimination with a large amount of the drug . In a zero order reaction the rate of the reaction is independent of the amount of the reactant whereas in a pseudo order reaction the rate of the reaction appears to be\u00a0 independent<\/strong><\/span> of the amount of the reactant(drug) but is actually dependent on the concentration of other reactant(moisture).<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

    \n\t\t\t\t
    \n\t\t\t\t\t\t\t\t\t

    Phenytoin – Elimination Kinetics<\/h2>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
    \n\t\t\t\t
    \n\t\t\t\t\t\t\t\t\t

    Phenytoin follows Zero order kinetics at toxic doses and first order kinetics at therapeutic doses.<\/p>

    Phenytoin undergoes hydroxylation in liver to form hydroxy-phenytoin which is further converted to phenytoin catechol and then phenytoin-quinone which is eliminated into the urine. At therapeutic doses phenytoin is excreted via first order kinetics and at higher\/toxic doses it undergoes zero order elimination due to the saturation of hepatic hydroxylation system.\u00a0<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

    \n\t\t\t\t
    \n\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\"Phenytoin\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
    \n\t\t\t\t
    \n\t\t\t\t\t\t\t
    \n\t\t\t\n\t\t\t\t\t\t<\/span>\n\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t
    \n\t\t\t\t\t
    \n\t\t\t\t
    \n\t\t\t\t
    \n\t\t\t\t\t\t\t
    \n\t\t\t
    \n\t\t\t\t<\/i>Prev<\/span><\/span>Previous<\/span>Mucosal Protective Agents<\/span><\/span><\/a>\t\t\t<\/div>\n\t\t\t\t\t\t\t
    \n\t\t\t\t\t
    <\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t\t
    \n\t\t\t\tNext<\/span>Neuroanatomy<\/span><\/span><\/i>Next<\/span><\/span><\/a>\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t","protected":false},"excerpt":{"rendered":"

    Does Phenytoin follow Pseudo Zero Order Elimination kinetics ? The straightforward answer to the above question is NO . This misconception is prevalent because Phenytoin follows Zero order kinetics at toxic doses and first order kinetics at therapeutic doses. To gain a better insight into this we need to understand what is Pseudo Zero order […]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":536,"menu_order":1,"comment_status":"closed","ping_status":"closed","template":"","meta":{"_acf_changed":false,"site-sidebar-layout":"no-sidebar","site-content-layout":"","ast-site-content-layout":"full-width-container","site-content-style":"default","site-sidebar-style":"default","ast-global-header-display":"","ast-banner-title-visibility":"","ast-main-header-display":"","ast-hfb-above-header-display":"","ast-hfb-below-header-display":"","ast-hfb-mobile-header-display":"","site-post-title":"disabled","ast-breadcrumbs-content":"","ast-featured-img":"disabled","footer-sml-layout":"","ast-disable-related-posts":"","theme-transparent-header-meta":"default","adv-header-id-meta":"","stick-header-meta":"","header-above-stick-meta":"","header-main-stick-meta":"","header-below-stick-meta":"","astra-migrate-meta-layouts":"set","ast-page-background-enabled":"default","ast-page-background-meta":{"desktop":{"background-color":"var(--ast-global-color-4)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"ast-content-background-meta":{"desktop":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"footnotes":""},"class_list":["post-1802","page","type-page","status-publish","hentry"],"acf":[],"_links":{"self":[{"href":"https:\/\/medreasons.com\/index.php\/wp-json\/wp\/v2\/pages\/1802","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/medreasons.com\/index.php\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/medreasons.com\/index.php\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/medreasons.com\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/medreasons.com\/index.php\/wp-json\/wp\/v2\/comments?post=1802"}],"version-history":[{"count":112,"href":"https:\/\/medreasons.com\/index.php\/wp-json\/wp\/v2\/pages\/1802\/revisions"}],"predecessor-version":[{"id":1928,"href":"https:\/\/medreasons.com\/index.php\/wp-json\/wp\/v2\/pages\/1802\/revisions\/1928"}],"up":[{"embeddable":true,"href":"https:\/\/medreasons.com\/index.php\/wp-json\/wp\/v2\/pages\/536"}],"wp:attachment":[{"href":"https:\/\/medreasons.com\/index.php\/wp-json\/wp\/v2\/media?parent=1802"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}