Pharmacology
Summary
Pharmacodynamics pertains to the study of a drug's activity on the body, examining the effects and mechanisms of drug action. For instance, NSAIDs operate by inhibiting COX, which results in decreased prostaglandin synthesis leading to analgesia and reduced inflammation. Interactions between drugs at their sites of action, known as pharmacodynamic drug interactions, can lead to various effects. When multiple agonists, like terbutaline and albuterol (beta-2 agonists), are combined, it can result in overactivation of the sympathetic nervous system. Another example involves the combination of zolpidem, a GABA-A receptor allosteric modulator, and opioids, which are mu-receptor agonists. This duo can cause additive sedative outcomes and respiratory depression. Moreover, combining a beta agonist with a beta blocker may reduce beta agonist activity, potentially triggering an asthma attack.
Pharmacokinetics explores how the body processes drugs, primarily focusing on ADME—absorption, distribution, metabolism, and elimination. After oral administration, drugs are typically absorbed in the GI tract, though other routes exist such as sublingual, buccal, intranasal, and intravaginal. Post absorption, drug molecules distribute to various tissues with the extent being influenced by factors like physicochemical properties and plasma protein binding. Most drugs undergo metabolism in the liver, being transformed by enzymes like CYP450 into active or polar metabolites suitable for elimination. Elimination processes include excretion via the kidneys, fecal excretion, or expiration. Drug absorption can be affected by chelation, especially with polyvalent cations like Fe2+, Ca2+, and Mg2+ found in antacids, iron supplements, and dairy. It's advisable to separate administration by over 2 hours for drugs such as tetracyclines, fluoroquinolones, and bisphosphonates which are susceptible to chelation. Another key point is that drugs modifying stomach pH, like PPIs, can influence the absorption of other drugs. Combining these drugs with cola can elevate stomach acidity, leading to increased absorption. Lastly, the induction or inhibition of metabolizing enzymes, especially CYP450, can affect drug serum levels, and some medications can alter the renal excretion of other drugs. For instance, probeneCID restricts the renal excretion of penicillin by inhibiting OAT1 and OAT3 in the proximal tubule, resulting in raised plasma concentrations of penicillin.
Lesson Outline
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FAQs
Pharmacodynamics refers to the effect that a drug has on the body. This encompasses how the drug achieves its effects, for example through inhibitory or stimulatory actions at specific receptors. Pharmacokinetics relates to what the body does to a drug. This specifically involves the processes of absorption, distribution, metabolism, and elimination (ADME) that dictate how the drug enters, moves within, is metabolized by, and eventually leaves the body.
Multiple agonists refer to several drugs that all act on the same target to produce a similar effect. An example of this is terbutaline and albuterol, both of which are beta-2 agonists. When used together, these drugs can cause overactivation of the sympathetic nervous system. This is an example of an additive effect since the simultaneous use of the drugs results in a greater total effect than the use of either drug individually.
Absorption is a crucial part of pharmacokinetics; it defines how a drug enters the body and begins its journey through the system. The efficiency of absorption can be influenced by several factors including the route of administration (oral, sublingual, buccal, intranasal, intravaginally, etc.) and the individual's physiological state. Certain substances can interfere with absorption; for example, the absorption of some drugs can be inhibited by chelation or binding to polyvalent cations (Fe2+, Ca2+, Mg2+) from antacids, iron supplements, or dairy products. Hence, to avoid inhibition of absorption, administration of such substances should be separated by more than 2 hours from drug intake.
Distribution refers to the process by which the drug spreads from the point of absorption to different tissues across the body. This process is affected by a variety of factors including the physical and chemical properties of the drug, and plasma protein binding. The overall distribution profile of a drug can significantly impact its action, as it determines the concentration of the drug at the site/s of action and other tissues.
Drug metabolism, one of the key components of pharmacokinetics, predominantly occurs in the liver with enzymes such as CYP450 playing a major role. Certain drugs can induce or inhibit these enzymes, which can consequently affect the serum concentration of the drug or other drugs. If metabolism is induced, the drug may be broken down and eliminated more rapidly than expected, potentially decreasing its effectiveness. If metabolism is inhibited, there may be an unusually high level of the drug in the bloodstream, which could cause adverse effects.
