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Pharmacology Ch 4
Pharmacology
| Question | Answer |
|---|---|
| Pharmacokinetics | "Motion of Drugs" - pharmacokinetic processes determine the [drug] at its sites of action & thereby determine intensity & time course of responses. |
| 4 Basic Pharmacokinetic Processes | (Determine the [drug] at its sites of action. 1) absorption 2) distribution 3) metabolism 4) excretion |
| Absorption | Movement of drug from site of administration into the blood |
| Distribution | Drug movement from the blood to the interstitial space of tissues then into cells |
| Metabolism | (Biotransformation) enzymatically mediated alteration of drug structure |
| Excretion | Movement of drugs & their metabolites outside of the body |
| Elimination | Metabolism & Excretion |
| What is the intensity of the drug response related to? | It is directly related to the [drug] at site of action. |
| How do you maximize the drug response? | Use a strong enough [drug] to elicit the desired response. |
| How do you minimize harm in the drug response? | Avoid a [drug] that's too high - achieve balance by selecting the best route, dosage, & dosing schedule. |
| 3 Ways to Cross a Cell Membrane | 1) Channels & pores 2) Transport systems 3) Direct penetration of membrane (most common) - must be lipid soluble |
| Polar Molecules | Uneven distribution of charge - no net charge - equal # of protons & electrons |
| Ions | Molecules that have a net electrical charge |
| Quaternary Ammonium Compounds | Molecules that contain at least one atom of nitrogen that has bonds to 4 organic radicals, so they always carry a net positive charge & can't cross membrane. |
| pH Dependent Ionization | Certain drugs can exist in either charged or uncharged form - weak acids or bases can carry a charge depending on the pH of the surrounding medium. Acid becomes (-) in alkaline; Base becomes (+) in acid. |
| Ion Trapping (pH Partitioning) | Because ionization of drugs is pH-dependent, when pH on one side of membrane differs from the other side, drug molecules accumulate on the side that favors their ionization. |
| Aspirin & Ion Trapping | Orally administered aspirin moves from acidic stomach to alkaline plasma and accumulates in blood. |
| Rate of Absorption | Determines how soon effects will begin |
| Amount of Absorption | Helps determine how intense the effects will be |
| Factors Affecting Drug Absorption (5) | 1) Rate of dissolution 2) Surface area available for absorption 3) Blood flow 4) Lipid solubility 5) pH partitioning |
| Commonly Used Routes of Administration | Enteral (GI Administration) 1) Oral Parenteral 2) Intravenous 3) Intramuscular 4) Subcutaneous |
| Intravenous Administration Advantages | 1) Rapid onset 2) Precise control over blood levels 3) Can use large volumes of fluid 4) Can use irritant drugs b/c they're rapidly diluted in the blood |
| Intravenous Administration Disadvantages | 1) Irreversible 2) Expensive 3) Inconvenient 4) Poor self-administration 5) Risk fluid overload, infection, embolism 6) Drug must be water-soluble |
| Intravenous Administration Absorption | Barriers to Absorption: none (absorption bypassed) Absorption Pattern: Instantaneous & Complete |
| Intramuscular Administration Absorption | Barriers to Absorption: capillary wall (easy to pass Absorption Pattern: rapid with water soluble drugs, slow with poorly soluble drugs |
| Intramuscular Administration Advantages | 1) Permits use of poorly soluble drugs 2) Permits use of depot preparations |
| Intramuscular Administration Disadvantages | 1) Possible discomfort 2) Inconvenient 3) Potential for injury (bleeding in hemophiliacs) |
| Subcutaneous Administration Absorption | Barriers to Absorption: capillary wall (easy to pass Absorption Pattern: rapid with water soluble drugs, slow with poorly soluble drugs |
| Subcutaneous Administration Advantages | 1) Permits use of poorly soluble drugs 2) Permits use of depot preparations |
| Subcutaneous Administration Disadvantages | 1) Possible discomfort 2) Inconvenient 3) Potential for injury |
| Enteral Administration Absorption | Barriers: GI lining (through cells) & capillary wall Absorption Pattern: Slow & Variable |
| P-Glycoprotein | Found in the liver, kidney, placenta, intestine, & brain capillaries - can transport a variety of drugs OUT of cells |
| Enteral Administration Advantages | 1) Safer than injection (b/c potentially reversible) 2) Easy & Convenient (ideal for self admin) 3) Inexpensive |
| Enteral Administration Disadvantages | 1) Variability between patients 2) Inactivation of some drugs by gastric acid/digestive enzymes 3) Possible nausea & vomiting 4) Patient must be conscious & cooperative |
| To minimize risk of IV injections: | Inject slowly (over 1 min or more) it takes a minute to circulate blood & dilute the blood |
| CNS Advantages to Slow Injection | From antecubital vein in arm to brain takes ~15 sec - signs of toxicity evident in ~15 sec - if only 25% administered, you can stop & avoid more toxicity |
| Embolism | Vessel blockage at site distant from point of admin |
| Pharmaceutical Preparations for Oral Administration | Oral preparations can differ in properties despite having the same drug dosage. 1) Tablets 2) Enteric-Coated Preparations 3) Sustained-Release Preparations |
| Enteric-Coated Oral Formulations | Designed to release their contents in the small intestine - not the stomach |
| Sustained-Release oral Formulations | Designed to release their contents slowly, thereby permitting longer interval between doses |
| Additional Routes of Administration | 1) Topical 2) Transdermal 3) Inhaled 4) Rectal 5) Vaginal 7) Direct injection into specific site (heart, joints, nerves, CNS) |
| 3 Factors that Determine Distribution | 1) Blood flow to tissues 2) Exiting the vascular system 3) Entering the cells |
| Blood Flow to Tissues | Determines the rate at which drugs are delivered to a tissue - most tissues are well-perfused, so rarely an issue. |
| Abscesses & Tumors | Limit regional blood flow to these areas because these structures aren't well-vascularized |
| Exiting the Vascular System | Drugs in the CV system leave the blood at capillary beds by passing BETWEEN cells not THRU them |
| Blood-Brain Barrier | Presence of tight junctions between the cells in capillary walls of CNS means drugs must pass capillary cells not between them to get into CNS. (Also p-glycoprotein) |
| Placental Membranes | Do not constitute an absolute barrier to the passage of drugs. The same factors that determine drug movements across all other membranes determine movement across the placenta. |
| Albumin | Many drugs reversibly bind plasma albumin. When bound they can't leave CV system. Albumin-deficiency may cause toxicity in highly ptn bound drugs b/c more drug available. |
| BBB & Newborns | BBB not fully developed at birth, so newborns are more susceptible to brain medications & poisons. |
| Hepatic Drug-Metabolizing Enzymes | Most drug metabolism that takes place in the liver is catalyzed by the cytochrome p450 system of enzymes - metabolism doesn't always result in a smaller molecule |
| What is the most important consequence of drug metabolism? | Promotion of renal drug excretion (by converting lipid-soluble drugs into more polar forms) |
| Therapeutic Consequences of Drug Metabolism | 1) Accelerated renal drug excretion 2) Drug inactivation 3) Increased therapeutic action 4) Activation of prodrugs 5) Increased or decreased toxicity |
| Prodrug | A compound that is pharmacalogically inactive as administered then converted to active form in the body |
| Acetaminophen Metabolism | Acetaminophen is metabolized into N-acetyl-p-benzoquinone, which is a hepatotoxin |
| Special Considerations in Drug Metabolism | 1) Age 2) Induction of Drug-Metabolizing Enzymes 3) First-pass Effect 4) Nutritional Status 5) Competition between drugs |
| Age & Drug Metabolism | The liver doesn't develop its full capacity to metabolize drugs until about 1y after birth |
| Induction of Drug Metabolizing Enzymes | Some drugs can induce synthesis of hepatic drug-metabolizing enzymes & thereby accelerate their own metabolism & the metabolism of other drugs |
| First-Pass Effect | Rapid inactivation of some oral drugs as they pass through the liver after being absorbed |
| Nutritional Status & Drug Metabolism | Hepatic drug metabolizing enzymes require many cofactors to function. Malnutrition -> cofactor deficiency -> poor metabolism |
| Competition Between Drugs | 2 drugs metabolized by the same metabolic pathway can compete -> 1 accumulates to dangerous levels |
| Kidney & Drug Excretion | Most drugs are excreted through the kidney - renal failure may increase the duration & intensity of responses |
| Steps in Renal Drug Excretion | 1) Glomerular Filtration 2) Passive Tubular Reabsorption 3) Active Tubular Reabsorption |
| Factors that Modify Renal Drug Excretion | 1) pH-Dependent Ionization 2) Competition for Active Tubular Transport 3) Age |
| Age & Renal Drug Excretion | Infants kidneys are not fully developed for a few months - they can't excrete drugs well |
| Renal Excretion & Lipid Soluble Drugs | Drugs that are highly lipid soluble undergo extensive passive reabsorption back into the blood & therefore cannot be excreted by the kidney (until liver converts them into polar forms) |
| Nonrenal Routes of Drug Excretion | 1) Breast Milk 2) Bile 3) Lungs 4) Sweat/Saliva |
| Enterohepatic Recirculation | Drugs excreted into the bile that then enter the intestine may undergo reabsorption back into the portal blood. This can prolong a drugs sojourn in the body. |
| Time Course of Drug Responses | We need to regulate time when drug responses start, are most intense, & when they stop. 1) Plasma drug levels 2) Single-dose time course 3) Drug half-life 4) Drug levels produced with repeated doses |
| Plasma Drug Levels | For most drugs, there is a direct correlation between the level of drug in plasma & the intensity of therapeutic & toxic effects |
| Minimum Effective Concentration | The plasma drug level below which therapeutic effects will not occur |
| Toxic Concentration | The plasma drug level at which toxic effects begin |
| Therapeutic Range | Lies between the MEC and the toxic concentration - Drugs with a wide therapeutic range are relatively easy to use safely. |
| Half-Life | The time required for the amount of drug in the body to decline by 50%. Drugs with short 1/2-lives have short dosing intervals. |
| Plateau | Levels of drugs administered repeatedly will gradually rise & then plateau within about 4 1/2-lives. |
| Loading Dose | When a plateau must be achieved quickly, a large initial dose is administered. May be necessary for a drug w/ a long 1/2-life. |
| Maintenance Dose | A plateau is maintained by administering smaller doses. |
| Reducing Drug Level Fluctuations | 1) Give smaller doses at shorter intervals 2) Use continuous infusion 3) Use depot preparation |
| Decline from Plateau | When drug administration is discontinued, most (94%) of the drug in the body will be eliminated over 4 half-lives |
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