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What is Pharmacokinetics?
Study of drug movement throughout the body
- Absorption: movement of drug from site of admin --> blood.
- Distribution: movement from blood --> interstitial space of tissues and from there to cells.
- Metabolism: Enzymatically mediated alteration of drug structure
- Excretion: Movement of drugs and their metabolites out of the body. Drugs can be broken down into toxic and non-toxic metabolites. Both will be excreted through liver/kidneys which is why liver/kidney is vital to assess before Rx administration.
T/F: Oral administration of an Rx means taking a drug in pill or liquid form by mouth.
F: Oral means any means of getting a drug through stomach into GI.
3 ways to cross a cell membrane
Passage through channels or pores - only for smallest compounds. Most drugs will not pass this way.
Passage with the aid of a transport system - Very selective and may use energy. ie glucose. Direct penetration of the membrane itself (most common) ---Must be lipid soluble (lipophilic).
- *drugs must be soluble in a lipid solution
- *Think sugar in water vs. sugar in oil
- *Drugs move from areas of high to low concentration. Drugs that state “do not take with food” do so because the drug may bind to food and become inactive.
What are Polar Molecules
- Uneven electrical charges (i.e., water with no net charge); “like dissolves like” meaning alkaline drugs will dissolve best in an alkaline medium.
- Drugs that are polar have difficulty dissolving in the lip layers.
Ions: Molecules with a net electrical charge (positive or negative) don’t usually cross membranesQuaternary
Ammonium Compounds Nitrogen and those with a positive charge
Things you should know about the passage of drugs across membranes:
Passage of Drugs Across Membranes
Ions: drug must be non-ionized to be absorbed.
- pH-dependent Ionization:
- Acids ionize in alkaline media
- Bases ionize in acidic media
What is ion trapping?
Also called pH partitioning.
Drugs accumulate on side of membrane where pH favors ionization (i.e., base to acid side)
Blood/brain barrier, placental barrier, etc. Again, must be non- ionized in area to be transported. Blood/brain barrier is much more selective than placental. Especially important in fetuses and for poisonings
Factors affecting absorption...
Rate of dissolution: how quickly does the drug dissolve? Liquid drugs absorb more quickly because they readily dissolve in a liquid medium. Therefore encapsulated take longer to be dissolved.
Surface area: Area over which absorption will occur (not the surface area of the drug). Bariatric surgery will have diminished ability to absorb.
Blood flow: how vascularized is the area of absorption? What sort of circulation is available. Stomach cancers will increase vasculature in the stomach and absorption will increase.
What is a Parenteral route of Rx administration?
Anything that penetrates through skin to be absorbed.
What are the advantages of IV Rx admin?
Instant Absorption because it bypasses absorption altogether-->directly to distribution. (Must be sure of drug (patient, dose, administration, etc.) before you administer. Once it’s in the blood stream, it cannot be extracted. Be sure to have enough space and freedom of thought to get the dose/Rx correctly.)
Precise control over levels
- Can use large volume of fluids: Most Rx is acidic. With IV, must be diluted or they will irritate the vein. Too quickly or not enough dilution will burn the inside of the vein (intima).
- used for irritating drugs
What are some disadvantages of IV Rx admin?
Cannot self-administer: in some cases, pt can be taught how to self-administer.
Cannot reverse injection
Could cause fluid overload: Given through a system called piggy back. Fluid to dilute Rx piggy backs off of Rx IV. Fluid levels therefore must be monitored when administering IV drugs.
Also a PUSH. Lowest amount of fluid administered with Rx. Good idea with pts with heart problems because excess fluids can cause cardiac stress. Pulmonary edema can also be a problem when edema results.
Increased risk of infection: Sterile technique mandatory because you’re putting objects directly into BS.
Risk of embolism: Prime all lines to clear air so that only fluid enters BS.
Chance of error in administration
What are some advantages of intramuscular admin?
- Only barrier is capillary wall: fairly rapid absorption.
- Used for poorly soluble drugs
- Used primarily for vaccinations.
- Depot preparations to decrease number of injections: Depot--mixed with oil to be absorbed slowly over time.
What are some disadavantages of intramuscular Rx admin?
- Discomfort with administration: can overstimulate the nerve by rapid slapping or stimulating the area immediately before injection.
Very difficult to reverse/extract dose.
Subcutaneous route or Rx admin...
Most common are insulin and heprin (anticoagulant). Both are proteins which will be broken down in stomach.
Done to slow absorption, typically by 20-30 min. Should probably be done in area of the body that is not going to be active, unless soreness is the primary concern. Then should be given in an area of high activity to encourage rapid dispersion.
Same advantages and disadvantages as IM
Enteral (bum) Rx Admin
- Administration is easy
- High variability
- Inactivated by pH and first pass effect
- Requires compliance: Pt must be trustworthy to continue/complete treatment
- Local irritation: typically upset stomach.
Characteristics of Oral Rx admin?
- Includes tube down nose/throat and GI tube. (still GI)
- Preferred over parenteral except:
- When drugs are destroyed by gastric acidity
- When drug levels need to be tightly controlled
- Severe irritation would occur
- For depot (oil) preparations
- When patients cannot take oral preparations
T/F: Just about any Rx can be taken by inhaler.
Characteristics of Suppositories
Rectal: Absorbed through vasculature of GI. Must be sure it gets past internal sphincter. Double glove and use lube, but not too much or you’ll lose the suppository. Lay flat for 15-20 min. ie tylenol, especially with kids or adults who cannot keep in down.
Vaginal: usually used for localized effect. Absorption is poor systemically.
Where do Rx's typically leave the vascular system and why is it important they do?
- Capillary beds: Most common
- Blood-brain barrier
- Placental drug transfer
It's important they do because otherwise they cannot have their intended effect.
What is a Pro-Drug?
A drug that is activated a little at a time when it passes through the liver. This allows it to remain in the system over an extended period of time.
What does it mean when a drug is said to be teterogenic?
It is toxic to developing cells.
Although most Rx does not pass the placental barrier, this is why it is strongly recommended that pregant mothers take no drugs what-so-ever during their first trimester.
What must a Rx be in order to pass through the blood/brain barrier?
What kind of Rx can pass through the placentcial barrier?
Lipid soluble and non-ionized
What is protein binding?
- Plasma albumin most important
- Because albumin is too large a protein to leave the bloodstream…drug molecules that are bound to albumin cannot leave either.
- Drugs will compete for binding sites on albumin
Also called Biotransformation: Enzymatic alteration of drug’s chemical structure
Can either be metabolized into its active state or into its metabolites.
Is key component of the microsomal enzyme system because it metabolizes drugs within cells.
Within family is: CYP1, CYP2, CYP3, et al
Each enzyme is termed an isoform, since each is derived from a different gene...SO…. Genetic alterations in drug metabolism correlates to these enzymes
- Inhibitors: (Inhibits CYP450, and therefore increases drug effect)
- Decreases metabolism of substrates which leads to increased drug effect unless drug is a prodrug
Inductors: Increases drug metabolism of substrates which leads to decreased drug effect unless drug is a prodrug
Therapeutic Consequences of Drug Metabolism
- Accelerated renal drug excretion
- MOST IMPORTANT: Can totally inactive the drug
- Increase therapeutic action
- Activation of Pro-Drugs
- Increase or Decrease toxicity.
- Decreasing lipid solubility
- Drug Inactivation
- Conversion to inactive forms
Special Considerations in Drug Metabolism
- Age: Young-May not have produced enzymes for metabolism yet
- Elderly: May have exhausted supply of enzymes.
- Induction of drug metabolizing enzymes
- First pass effect
- Nutritional status
- Competition between drugs: Limited availability of enzymes to metabolize drug.
Via which route are most drugs excreted?
Through the kidneys.
What are the steps of renal excretion?
Glomerular filtration: Filters granular sediment, ie Magnesium, calcium, phosphorus, etc. Medications can bind to stones and be excreted.
Passive tubular reabsorption
Active tubular secretion
Mucous can be produced when epithelial cells slough off to excrete foreign bodies. Can occur with Foley catheters.
What are some factors that can affect renal excretion of drugs?
Urine should be slightly acidic to discourage UTI.
- pH dependent ionization: More acid can cause Rx to be reabsorbed if it’s an acid Rx.
- Competition for active tubular transport
What are some nonrenal routes of drug excretion?
Breast Milk: Can expose infant to drugs. Discuss with Health care provider
Bile: Will end up in stool.
Lungs: Might be able to smell it on breath.
Be mindful of contact with pts who are on excessive amounts of Rx. They will have some of those Rx on their skin, breath, saliva, etc.
What are 3 important components to the Time Course of Drug Response?
- Time drug response starts
- When drug response is most intense
- Time when drug response ceases
Why is it important to know blood plasma levels before administering a drug?
Usually a direct correlation between therapeutic and toxic responses and plasma drug levels.
Amount of prescribed Rx will often depend on plasma levels. ie vancomycin. Often a very narrow range between theraputic level and toxic level. Want to check plasma levels 30 minutes before drug is given.
What are three types of drug levels and how are they related?
- Minimum effective concentration (MEC)
- Toxic concentration
- Therapeutic range: Between MEC and Toxic.
Objective of drug dosing is to between MEC and Toxic.
What are some techniques for reducing fluxuations in drug levels?
Plateau Drug levels: When drug is administered repeatedly in the same dose…plateau will be reached in approximately 4 half lives.
Reduce the dosage size and interval
Loading dose vs. Maintenance Doses: If given loading dose, will reach plateau much sooner.
When a drug is d/c'd, how long will it take for it to be out of the pt's system?
When drug is D/C’d (discontinued) 94% of drug will be eliminated in 4 half lives
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