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Lecture 3

Lecture 3- Pharmacokinetics.docx

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NURS 2050
Cynthia Barkhouse Mckeen

Pharmacokinetics Lecture 3 Pharmacokinetics: the study of how medications enter the body, reach the site of action, metabolize and exit the body. Case examples: 1. An elderly male admitted with CAP. He’s getting levofloxacin 500 mg IV once daily and is switched to 500 mg po after he has improved clinically. Why is the oral and IV dose the same? Answer: There is no first pass. Bioavailability is the same (PO/IV= 100) 2. A 56 year old male with angina receiving sublingual nitroglycerin.Why do we give nitro sublingually? Answer: bi pass first pass effect 3.A 42 year old female with nephrolithiasis who received morphine 2.5 mg IV q4h and is discharged from the ER with a prescription for 10 mg po q4h of morphine IR. Why is the po dose of oral morphine higher than the IV? Answer: It has a lower bioavailability via mouth 4. A 72 year old male admitted with rhabdomyolysis. The physician reviews his usual medications and writes an order to “hold simvastatin & erythromycin”.Why did the physician specifically hold these two drugs? 1. Absorption: - Movement of a drug from its site of administration to the blood - the fraction of unchanged drug reaching the systemic circulation after administration by any route - Rate (time to onset) and extent/amount (intensity of effect) are different for various routes of administration. *** Barriers to absorption with each route are different. 1. Rate of dissolution: tablets could yield more rapid onset - ability to dissolve depends on its form or preparation. Solutions/suspensions are in a liquid state and are absporbed more readily. - Acidic meds pass through GI mucosa rapidly. - Meds with a basic pH do not get absorbed before reaching the small intestine 2. Surface area: Larger surface area, more likely to be absorbed (intestine) 3. Blood flow: More absorption when blood flow is high (creates a conc. Gradient) (e.g. with insulin- don’t massage the injection site) 4. Lipid solubility: More lipid soluble more readily absorbed (charge v uncharged, lipid bilayer) - Highly lipid soluble medications easily cross the cell membrane and are absorbed quickly. - Absorption of medication is impacted by food in the stomach. 5. pH partitioning: ASA Ion Trapping: ASA is nonionized in the stomach; therefore it is absorbed in the stomach. However, it becomes ionized in the intestines, which is why it is not absorbed. Ions cannot cross the membrane. Drugs will accumulate on the side that pH favors ionization. 6. Route of administration: Each route has a different rate of absorption - ie. When applied to the skin/oral, absorption is slower. Mucous membranes and respiratory airways are quicker because the these tissues are highly vascular. IV is the fastest Polar + ions cannot cross the cytoplasmic membrane. Lipid Soluble can! Bioavailability (F value) : the ability of a drug to be released from its dosage form and to be dissolved, absorbed, and transported by the body to the drug’s site of action. - For an IV drug, F generally 1= 100% - IV formulation: No barriers to absorption (virtually all drug in blood instantaneously) - For oral drugs – bioavailability is often less than 100% due to incomplete absorption at the site - 100% means that it all reaches circulation/unchanged, 0% means none of it does Examples (oral): Barriers to oral bioavailability 1. Inactivation or destruction in stomach (e.g insulin) 2. binding interactions with drugs (iron and dairy) 3. metabolism in lining of gut, first pass effect (ETOH) 4. malabsorptive syndromes 5. pH, gastric emptying, GI mobility, vomiting, etc. Concentration/Time Curves: AUC: Area under the curve (total exposure of the body to a drug) Cmax: maximal concentration achieved with dosage form (peak) Tmax: time at which it happens 2. Distribution - Movement of drugs throughout the body. - Affected by protein binding (e.g. Phenytoin (Dilantin) free drug is active (able to exert effects) Major Factors: 1. Blood flow to tissues- Problem conditions – abscesses, tumours do not have a blood supply, which can effect drug therapy. 2. Exiting the vascular system– Since drug do not produce their effects in the blood- they need to leave the vascular system. The drugs leave through capilliary beds (between the cells), which is vital for metabolism and excretion. - BBB – allows only fat soluble medications to pass into the brain and cerebral spinal fluid. Due to tight junctions in capillary beds drugs – pass through cells of the capillary wall. They need lipid solubility or transport pump - protein binding - bound and free drug. Albumin has no way to leave the bloodstream. 3. Entering cells- Enter cells for metabolism and excretion & for some, in order to have action - Most meds bind to a protein to some extent. - Meds bound to albumin cant have activity. Only unbound meds are active. - Older adults have a decrease in albumin, which is a result of a change in liver fx. This is why they are more at risk f
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