Name four relevant drug reservoirs.
3. Tissue (bone, liver, thyroid)
How does grapefruit juice affect CYP 450 metabolism?
Inhibits CYP3A4, increasing bioavailability of many orally administered drugs
What is pharmacogenomics?
- Application of genomic information towards the discovery and devo of novel specific drugs
- Such drugs may targeted for selective use among specific patient populations
NOTE: pharmacogenomics/genetics also aim to clarify the underlying basis for idiosyncratic drug responses -> physicians will be increasingly responsible for managing appropriate therapeutic strategies by tailoring drug dosage regimens correlating to patient's genetic profile
What is the one compartment open pharmacokinetic model (single IV dose)?
- Assumes the entire human body is one compartment.
- Works for drugs distributed fairly uniformly throughout the body.
- Assumes an open system (excretion)
- An adequate representation for many, but not all, drugs
What is the minimum effective concentration?
No responses will be observed below this concentration.
Describes the rate at which a drug leaves its site of administration and the extent to which that occurs
Briefly describe carrier-mediated transport.
- Some drugs are too large and/or too polar to diffuse across lipid membrane. These molecules enter cells via carrier that facilitates their mvmt down concentration gradient across membrane
- This transport cycle is a reversible process dictated by electrochemical gradients and concentrations of substrates and any co- or counter-transported ions
What is P-glycoprotein (MDR1)?
An efflux pump that limits the utility of chemotherapeutic agents
How does gender impact CYP 450 metabolism?
- There are certain CYP 450 enzymes known to be stimulated by androgens, and are therefore of higher abundance in males
- There are also certain CYP 450 enzymes known to be under control of estrogen, and are thus in higher abundance in females
What is systemic clearance?
- The sum total of clearance by the various organs
- CL = CL(renal) + CL(hepatic) + CL(other)
What is a half-life (T1/2)?
The time it takes for the plasma concentration or amount of drug in the body to be reduced by 50%
Can a slight change in the binding of highly bound drugs can result in significant changes in clinical response or cause a toxic response? Why or why not?
- Because it is the free drug in plasma that equilibrates with the site of pharmacologic or toxic response, i.e., a change from 99% bound to 98% bound can result in almost a 100% change in free concentration, which can cause a significant alteration in response
NOTE: Greatest concern for drugs w/narrow therapeutic window and when dosing regimens or elimination is altered
What are the 5 properties of biotransformation (enzymatic) reactions?
1. Obey Michaelis-Menton kinetics:
-> V = V(max) [S] / (Km + [S])
-> S = substrate concentration, Km = Michaelis constant
2. Rxn rate is proportional to level of enzyme at saturating substrate concentrations
3. Rxn rate is proportional to substrate when substrate is limiting
4. Max rate achieved when enzyme saturated
5. They may competitively or noncompetitively inhibited by other substrates
What are the two phases of drug biotransformation reactions?
- Phase I: functionalization
- Phase II: biosynthetic (or conjugation)
Which will provide tighter control of drug administration, increasing the dose, or decreasing the dose intervals?
- You will have tighter control if you shorten the dose intervals than if you increase the dose amount
- The more dose you give, the bigger the fluctuations
- In the same way, a longer interval will lead to more decay time, and more fluctuation
What is zero order kinetics?
- If the enzymes that metabolize the drug are rate-limiting, i.e., the enzyme is saturated at usual levels of drug in the body, then the same amount of drug (ug/hr) metabolized regardless of level of drug
- No plateau is observed
- EX: ethanol, heparin, phenytoin, aspirin, amobarbital, tetracycline
NOTE: same LD equation applies, LD = (Vd x Css)/F; Km is dose producing 50% of max elimination rate; Vm is max rate of the process
Describe sublingual drug administration, including its advantages and disadvantages.
Formulated in rapidly dissolving tablet, and may be placed under tongue where rich blood supply promotes rapid absorption
1. First pass (portal vein) bypassed, so > bioavailability (ex: Nitroglycerin)
2. Rapid absorption b/c good blood supply in mouth
3. Drug stability -> mouth pH relatively neutral
1. Holding dose in mouth inconvenient
2. Useful when drug dosage is small
What are the four processes that govern the onset, intensity, and duration of drug action?
1. Absorption 2. Distribution (only "free" drug accessible to tissues and locus of action) 3. Metabolism 4. Excretion (metabolism + excretion = elimination)
Why is the volume of distribution important?
If a drug distributes throughout total body water (around 40L), its concentration in all of these areas will be different than if it distributes only to the vasculature -> critical for dosing
What are microsomal enzymes?
Drug metabolizing enzymes associated with ER fragments called microvesicles, or microsomes (following homogenization and centrifugation)
What is the volume of distribution (Vd)?
- The fluid volume that would be required to contain all of the drug dose at the same concentration as exists in the blood or plasma
- V(d) = amount of drug in the body/C, where C = blood concentration
- For some drugs, the V(d) describes the primary fluid compartments in which a drug is distributed. For others, the V(d) has no relationship
1. EX: Imipramine has an apparent V(d) of 1,000 L, so the drug must concentrate in some organ or tissue because as it does so, its concentration in blood becomes increasingly small, making V(d) increasingly large
Describe percutaneous drug admin. What are some examples of patches?
- Creams or ointments applied to skin for local effect
- Also may be useful in patches for drugs that are highly lipid soluble and can pass through epidermis
- Examples of patches include: fentanyl (narcotic), nicotine, nitroglycerine, hormone (birth control) patches
How do drugs themselves affect drug metabolism?
- Large # of drugs can cause an increase over time in liver enzyme activity -> can increase metabolic rate in same or other drugs
- Onset 3-12 hours, maximal 1-5 days, persistence 5-12 cays
- Chronic administration can lead to a situation where there is a viscious cycle of dose escalation -> accumulation of toxic metabolites can also occur
- Broad: induce metabolism of themselves and a number of other drugs (ex: phenobarbitol)
- Narrow: induce metabolism of themselves and smaller # (narrower spectrum) of other drugs (ex: 3-MC)
NOTE: induction is generally a reversible process
Drug A has a 20% bioavailability and Drug B has 80% bioavailability. If 400mg of Drug A is given every 6 hours, how much of Drug B would you give in this same time interval?
- 100mg because Drug B has a higher bioavailability
- If the bioavailability were lower, you would have to give more of the drug to get to the same steady state
What are two major factors controlling transcapillary mvmt of drugs?
1. Lipid solubility
2. Molecular size
How are volume of distribution and the idea of drug reservoirs related?
Example: if 50% of drug accumulated in thumb, only the other 50% would distribute to the tissue(s) you are targeting -> this will TAKE AWAY drug that would normally be expected to distribute to other tissues, and alter the time course of the drug, maintaining a long descending phase of drug concentration (i.e., if drug later redistributes to the desired tissue)
How do intestinal motility and transit time affect drug absorption from the GI tract?
Contractions of small intestine regulate time the food is in contact with reabsorptive epithelium -> faster contraction = faster transit time
What are genotypic polymorphisms?
- Differences in DNA sequences encoding specific proteins
- Can be readily observed by molecular bio techniques like PCR and RFLP
- Can be mediated by changes as small as SNP's
- Unless screened, these differences can remain silent until individual is "challenged" by a drug -> resulting response is characteristic of the person's phenotype
What is the equilibrium and Henderson-Hasselbach equation for acids and bases?
1. HA A- + H+
2. pH = pKa + log(A-/HA)
1. BH+ B + H+
2. pH = pKa + log(B/BH+)
Briefly describe intraarterial drug administration.
- Can be used for delivery to specific target organs
- Requires great care, and is reserved for experts
What is TPMT?
- Thiopurine methyltransferase enzyme
- Handles a small fraction of Phase II drugs, but is very important in metabolizing many DNA-modifying drugs used in anti-cancer therapy
Name 3 reasons to study pharmacokinetics.
1. There is a relationship b/t the concentration of a drug (from its accessible compartment) and its effects
2. Develop a rational framework for dosing
3. Improve therapeutic effects by selecting dosing regimens to match patient parameters
Why do many drugs accumulate in tissues at higher concentration than those found in EC fluids and blood? Provide an example.
- Often arises from active transport or binding -> tissue binding typically arises through interactions with proteins and/or phospholipids (EC or IC)
- Binding to cellular constituents is usually saturable and reversible -> dependent on concentration, affinity, and binding capacity of tissue and physicochemical properties of the drug
How do you calculate the clearance for a particular organ?
- CL = Q [(Ca - Cv)/Ca] = Q x E
1. Q = blood flow to the organ
2. E = extraction ratio
3. Ca = arterial drug conc; Cv = venous drug conc
- For a drug cleared by a particular organ, clearance by that organ is most critical (DUH!)
- Blood flow (i.e., presentation of the drug) to the organ is the limiting variable
How does the blood perfusion rate affect drug distribution? Where is this rate the highest?
- Greater blood perfusion rate = greater drug distribution to an organ
- Total blood flow greatest to: brain, kidneys, liver, and muscle
- Highest perfusion rates to: brain, kidneys, liver, and heart -> expected that total drug concentration would rise most rapidly in these organs
How is the thyroid a drug reservoir?
Does the time point at which drugs enter their therapeutic window vary by route of administration?
Yes. Oral drugs take longer to have an effect (vs. IV) because they must reach the minimum effective concentration at their sites of action.
What are the 5 mechanisms of metabolic inhibition?
1. Competition among substrates for enzyme (ex: cimetidine)
2. Inactivation by formation of tight complex with the heme (ex: cobalt)
3. Depletion of cofactors; generally more commone with Phase II (ex: GSH depletion due to oxidative stress)
4. Enzyme inhibitors (ex: MAOI)
5. Increased degradation of enzyme (ex: CCl4)
What two processes determine the duration of drug action and control the rate of termination?
Elimination by the processes of metabolism and excretion
Why are some drugs administered directly to the cornea?
For local effect
Which CYP enzymes encode the enzymes involved in the majority of biotransformations?
CYP1, CYP2, and CYP 3
What happens when structural changes occur in proteins important for a drug's effect?
Differences between individuals will be expected in responses to this drug -> genetic polymorphism may dictate changes in structure and amount of protein
What is hydrolysis in Phase I rxns?
- Addition of water with breakdown of molecule
- Performed in blood plasma or and liver by esterases
- Esters (R-O-C=O-CH3) -> Alcohol and acid (ROH + CH3-COOH)
Provide an example of drug "redistibution."
- Thiopental: rapid acting anesthetic delivered to brain
following IV admin b/c a highly perfused tissue
- But, distribution to brain not exclusive, and plasma
concentration falls rapidly as it is delivered to other tissues -> concentration in the brain will mirror that in the plasma and fall, as well, thus terminating its action
- In contrast, fat tissue will gradually accumulate the drug because of its high lipid solubility
What are the two mechanisms of biotransport?
1. Passive diffusion 2. Carrier-mediated biotransport: A) Facilitated diffusion, and B) Active transport
What factors determine if a drug is uptaken by pinocytosis?
- Often depends on if there is a carrier mechanism
- Very large drugs may be too big for facilitated transport b/c you can't have too large of a hole in the cell
- Tends to be slower, but all carrier processes are limited by their turnover rate
How is the stomach a drug reservoir?
Traps basic drugs due to ionization (ex: codeine)
What is Cinetidine?
- An acid-reflux reducer that is an inhibitor of multiple classes of CYP 450
- Will lead to decrease in other drug elimination, and increase in their plasma concentrations
NOTE: It is important to understand what drugs a patient is taking concurrently
What is phenotypic variation in pharmacogenomics? How does it vary in a monogenic vs. polygenic trait?
- Response to a drug for which a pt has a polymorphic enzymatic process
- Can be characterized by frequency distribution patterns, and be indicative of number and extent of genotypic differences
- Monogenic trait: 2 or 3 distinct population distributions
- Polygenic trait: only 1 population distribution
What is oxidation in Phase I rxns?
- Addition of oxygen and/or removal of H -> most oxidation steps occur in ER
- Common examples include:
1. Alkyl group (CH3-CH3) to an alcohol (CH2OH-CH3)
2. Aromatic ring to phenol (hydroxyl on ring)
3. Oxidation at S or N to generate sulfoxide, or nitroxide derivative
NOTE: Multiple modifications may occur at different sites within the parent compound
How do most drugs cross biological membranes?
Describe absorption, metabolism, and excretion in the rising phase vs. the falling phase.
- Rising phase: Abs > Met + Exc - Falling phase: Abs < Met + Exc
What are the two primary ways in which a drug's effect "terminate?"
- Usually occurs by metabolism and excretion
- May also occur by redistribution from site of action to other tissues or to bound proteins -> one reason for this is blood flow differences b/t tissues or organs that see the drug initially
What is the therapeutic index?
- Therapeutic index = MTC/MEC - MTC = minimum toxic concentration - MEC = minimum effective concentration - Drugs with HIGH therapeutic index are relatively safe
Where in the cell do most biotransformation reactions take place? Are these locations different for Phase I and Phase II reactions?
- Most drug metabolizing enzymes are found in the ER and cytosol, but additional activity is found in mitochondria, nuclear envelope, and PM
- Phase I: most take place in ER
- Phase II: enzymes primarily localized in cytosol
What is the major catalyst of Phase I drug biotransformation?
CYP 450 monooxygenase enzyme family
What is a loading dose? When do you use it? How do you calculate it?
- Desired steady state of a drug times the volume of distribution adjusted for bioavailability
- Used in situations when you can't wait 5 half-lives to achieve therapeutic range (i.e., heart attacks, seriuous heart failure, overwhelming bacterial infections, etc.)
- LD = (Css x Vd)/ F
- Can be esp. dangerous due to high concentrations achieved -> followed-up with maintenance dose to keep Css in desired therapeutic window:
1. Dosing rate = (target Css x CL)/F
What is clearance?
- The theoretical volume of fluid (i.e. blood, or plasma) from which a drug is removed per unit time
- CL = rate of elimination/concentration
Does the decline of drug concentration vary by route of administration?
No. decline of drug concentration over time follows a characteristic time course, regardless of route of administration.
What is the most important concept to consider when designing a rational regimen for long-term drug administration?
- Clinicians usually want a steady-state concentration of drug within the therapeutic window -> to achieve this, they must administer the drug at the same rate it is eliminated: dosing rate = CL x Css
- When administered at this rate, the drug will eventually reach a plateau
What are the differences and similarities between facilitated diffusion and active transport? Provide an example of each.
- Facilitated diffusion: mvmt with the concentration gradient and without energy; exhibits saturation
1. Example: Riboflavin, Vit. B12
- Active transport: mvmt against the concentration gradient with energy use; exhibits saturation
1. Example: 5-fluorouracil
Describe the cytochrome P450 enzyme complex.
1. Drug binds to oxidized form of P450 enzyme, forming a complex
2. Reduced through P450 reductase (which is oxidized) to make reduced form of enzyme combined with drug
3. Molecular O2 added, oxidizing drug, eliminating H2O, and releasing oxidized P450 to begin a new cycle
4. Meanwhile, P450 reductase, which was oxidized while reducing P450 enzyme, is restored to reduced form via NADPH (and also ready for the next cycle)
NOTE: one O added to drug, and the other reduced to form H2O; NO ATP required
Describe the solubility requirements for a drug to get INTO and OUT OF a membrane.
Must be soluble in the lipid material of the membrane to cross the membrane barrier, and has to be soluble in the aqeous phase to get out of the membrane
What are 3 examples of pharmacogenomic markers of drug response?
a. 6-MP -> myelosuppression in poor metabolizers
b. Azathiopurine -> nonresponder in fast metabolizers
2. NAT2: Isoniazide -> slow acetylators show toxicity
3. CYP2C9: Warfarin -> reduced activity increases bleeding problems
What is the major determinant of the total fraction of drug bound to plasma protein at high drug concentrations?
# of binding sites
What are the volumes of the various fluid compartments (5)?
1. EC fluid: 13-16
2. Plasma: about 4
3. Interstitial fluids: 10-13
4. IC fluids: 25-28
5. Total body water: about 40
How is bone a drug reservoir?
Tetracycline and other divalent, chelating compounds can accumulate here (so can heavy metals)
What is Carbamezepine?
- Carbamezepine is an anti-seizure med, and a broad spectrum inducer of CYP 450 enzymes
- Will lead to increased metabolism of those agents metabolized by these CYP 450s, and need to adjust doses upward for those agents to compensate
What equation can you use to predict steady state concentration after various dosing schedules?
- Css = (F x dose)/ CL x T
- Dose = (CL x T x Css)/F
What is competitive drug inhibition? Provide an example.
- Some drugs can inhibit the metabolism of other drugs via competitive inhibition
- This may result in accumulation of that, or other drugs metabolized by those enzymes, which is particularly important in drugs with narrow therapeutic windows
- Warfarin inhibits tolbutamide elimination, which can lead to accumulation of drug and may require a downward adjustment of dose
How many half-lives does it take to achieve steady state?
- About 5
- Plateau principle: time to steady state independent of dose or dose interval, but strictly dependent on T(1/2)
- With time, a steady state max and min are achieved
1. In general, physician can only control dosing interval and dose (w/in limits of dosage form available), controlling the peak, trough, and magnitude of Css
2. Below therapeutic range, no effect achieved; above therapeutic range, toxicity may be seen
What is excretion?
- Removal of drug from the body
- A component of drug elimination that is combined action of metabolism and excretion
- Drugs may be excreted as parent compound or as a
What is intrathecal drug admin used for?
- Injection into subarachnoid space to rapidly bypass BBB and blood-cerebrospinal fluid barrier
- Can be used for local effects, i.e., spinal anesthesia
- Also used for tx of acute CNS infections, e.g., meningitis
What are 3 common mechanism by which genetics variations can control metabolism?
1. Complete loss of activity
2. Reduced catalytic activity
3. Enhanced catalytic activity
What two areas of the GI tract bypass first pass metabolism? Why?
- Large intestine
- They do not direct drugs to the hepatic portal metabolism
What is the formula for the elimination half-life? What is its clinical importance (4)?
- T(1/2) = 0.7 x (Vd/CL)
- Clinical importance:
1. Determine dosing interval. A drug is often given at half-life intervals.
2. A factor in determining dose.
3. May determine route. Drugs with very short half-lives are preferably given IV or by sustained-release tablets
4. Provides a good indication of the time required to reach steady-state after a dosage regimen is initiated.
Describe Fick's Law, and each of its components.
Diffusion rate = -DAK (Cout - Cin)/deltaX
1. Diffusion coefficient (D): inversely related to size of drug, i.e., larger drugs have smaller diffusion coefficients
2. Partition coefficient (K): reflects lipid solubility of the drug- drugs w/high K transported across membrane to greater extent b/c soluble in membrane
3. Surface area (A): larger SA promotes diffusion
4. Membrane thickness (deltaX): thinner the mem, quicker the diffusion
5. Concentration gradient (Cout-Cin): larger gradients promote drug diffusion
6. Ionization (affects K): major determinant of ability of drug to cross mem -> dictated by environemnt (pH) of body fluid drug is dissolved in
What are the major routes of drug administration?
- Enteral (via GI tract): 1) oral ingestion (most common), 2) sublingual, 3) rectal
- Parenteral (not via GI tract): 1) IV, 2) subcutaneous, 3) intramuscular, 4) intraarterial, 5) inhalation, 6) topical
What is bioavailability?
- Refers to fractional extent to which a given dose of drug reaches either its site of action or a biological compartment the drug has free access to its site of action
- Values range from 0 to 1 (IV = 1)
What are three factors that determine the total fraction of drug bound to plasma protein?
1. Drug concentration
2. Affinity for binding sites (major determinant at low concentrations)
3. Number of binding sites (limiting variable at high concentrations)
NOTE: this means that durg binding to plasma proteins is a saturable and nonlinear process.
What are the 2 types of elimination kinetics?
- First order: most drugs
1. With linear scale of drug concentration and time on the X and Y axis, exponential decay function
- Zero order (saturation kinetics): few, but important drugs
1. With linear scale of drug concentration and time on the X and Y axis, expressed as a straight line
Describe rectal drug administration, including its advantages and disadvantages.
Useful when drugs are not well tolerated via oral administration, and can be formulated in suppository form
1. Useful in children, unconscious, or vomiting pts.
2. No first pass metabolism
1. Erratic absorption
2. Not well accepted
What are 5 examples of Phase II conjugation?
1. Glucuronidation: main conjugation rxn in body; occurs in liver, and is catalyzed by UDP-glucoronsyl transferases
a. Aliphatic alcohols, phenols, and hydroxylated metabolites (i.e., morphine) commonly conjugated
2. Acylation: especially acetylation, e.g., sulfonamides
3. Glycine: add NH2CH2COOH, e.g., nicotinic acid
4. Sulfoxidation: sulfate (SO4), e.g., morphine and paracetamol
5. Glutathione (GSS): tripeptide of (glutamate-cysteine-glycine) leads to mercapturic acid metabolite
What are the disadvantages of IV drug administration (5)?
1. Expensive: sterility, pyrogen testing, and larger volume of solvent means greater cost of prep, transport, and storage
2. Requires trained personnel
3. No recall: drug can't be removed once administered
4. Risk of infection
5. Dangerous: toxicity can be a problem w/rapid drug administration (esp. if narrow therapeutic window)
How is the liver a drug reservoir?
Concentrates drugs like quinacrine (active transport)
What are the advantages and disadvantages of subcutaneous drug administration?
Delivery just under the skin
1. Bypass first metabolism
2. Varied absorption (rapid from aqueous solution vs. slow/sustained from repository or insoluble preps)
3. Can be given by patient, e.g., insulin
1. Can be painful
2. Irritant drugs can cause local tissue damage
3. Not suitable for large volumes - max 2 mL injection (small doses limit use)
How do you calculate the elimination rate constant (Ke) for a first order drug? How is the related to the elimination half-life?
- Ke = slope of the straight line obtained plotting log of the blood concentration (Y axis) vs. time (X axis)
1. Ke = CL/V(d), or 0.7/T(1/2)
2. So, T(1/2) = 0.7/Ke = 0.7 x (Vd/CL)
- This means that a constant fraction of drug is eliminated per unit time
1. The absolute amount of drug eliminated per unit time, however, will be concentration-dependent
Drug Y reduces the plasma concentration of Drug X regardless of route of administration of Drug X. This is most likely b/c Drug Y effectively reduces what pharmacokinetic parameter for Drug X?
- Half-life (i.e., by affecting the CL or Vd)
- Not bioavailability b/c always 1 when IV administration
- Not metabolism b/c this would increase the concentration of Drug X
What are their 4 common characteristics of cell membranes relative to pharmacokinetics?
1. Water permeable 2. Low MW (100-200 Da) drugs may pass through pores 3. Relatively impermeable to proteins and peptides 4. Lipophilic and non-polar, non-ionized compounds may pass through membrane
Where are weak acids and bases best absorbed in the GI tract? Where are most drugs absorbed?
- Weak acids well absorbed in stomach (pH = 1-3)
- Weak bases better absorbed in intestine (pH = 8)
- Most drugs absorbed in small intestine b/c exposed to larger surface area (also only 30 minutes in stomach, but 3 hours in small intestine)
What are 4 things that affect extent of drug distribution?
1. Physicochemical factors affecting transport across membranes
2. Plasma protein binding
3. IC binding
4. Permeability and/or transport characteristics of specific tissue membranes
Briefly describe cell membranes.
- Common barrier to drug movement - PM consists of phospholipid bilayer, with hydrocarbon chains oriented inward to form hydrophobic phase and hydrophilic heads oriented outward - Lipid phase has small aqueous channels, or pores - Membrane proteins embedded in bilayer serve as receptors, ion channels, or transporters to elicit electrical and chemical signaling pathways, and are often drug targets
If a patient's blood concentration of a drug is 80ug/mL, and the drug's half-life is 12 hours, in how many hours will the concentration be 5ug/mL?
4 half-lives x 12 hours = 48 hours
Describe drug protonation in the gastric compartment. How does this affect drug absorption?
- EVERYTHING is going to be protonated in the gastric compartment, but because weak bases have positive charge when protonated, they will NOT be absorbed here
- In the case of weak acids, they will be readily absorbed in the stomach (vast majority of it) because when protonated, they are uncharged
Name 1 advantage and 1 disadvantage of mucous membrane drug admin.
- Advantage: rapid absorption
- Disadvantage: can lead to systemic toxicity
What are Phase I (functionalization) drug reactions?
- Usually convert parent drug to an inactive metabolite by introducing or unmasking a functional group (-OH, -NH2, -SH)
- In some instances, activity is only modified, or increased (e.g., prodrug)
- If Phase I metabolites are sufficiently polar, they may be readily secreted in urine
NOTE: resulting products often highly reactive (free radicals) and potentially toxic -> resulting metabolite may then productively take part in Phase II rxns
Briefly describe passive diffusion.
- Occurs when drug concentration on one side of membrane is higher than that on other side - Drug diffuses across the membrane in an attempt to equalize drug concentration on both sides of the membrane
What are CYP450 enzymes?
- Heme-containing membrane proteins localized in smooth ER of numerous tissues
- Highest concentrations in the liver
- Multiple members (17 families)
- Broad and overlapping substrate specificity (i.e., non-specific), so often referred to as mixed function oxidase system
Drug A has a half-life of 24 hours. If it is administered every 12 hours, when will it reach steady state?
- Half-life (24 hours) x 5 = 120 hours
- The interval of administration does not matter, only the half-life
What determines whether drugs are acids or bases?
The presence of free functional groups such as carboxyl and amino groups
What are the advantages and disadvantages of IM drug administration?
1. Generally absorbed well due to high blood flow in muscle and lateral diffusion from site of injection
2. Avoid first pass metabolism
3. Rapid absorption by simple diffusion through capillary membranes
1. Trained personnel required
2. Site of injection will influence absorption (generally, deltoid the best)
3. Gender differences (gluteus maximus)
4. Pain and tissue damage possible
5. Volume limited to 4-5 mL
What is the two compartment open pharmacokinetic model?
- Assumes that much of a drug is in a particular compartment, and that an equilibrium exists between the blood and other areas (single IV dose)
- Distribution phase: drug concentration in blood drops rapidly as it equilibrates
- Elimination phase: drug follows standard first order kinetics model
NOTE: the graphical representation of this would begin with a curved (hyperbolic) drop, followed by a linear descent
What two processes generally dictate the onset of drug action and the peak intensity of response?
Absorption and distribution, which are concerned with the accumulation of drug within the body and its movement to and from its site of action
Describe the distribution of Phase I and Phase II rxns in regards to microsomal vs. non-microsomal.
- Majority of Phase I reactions in microsomal fraction
- Majority of Phase II reactions in non-microsomal fraction
Generally speaking, how do drugs produce their desired therapeutic effects?
As a result of their presence at appropriate concentration at their sites of action (this is also how/when they produce their unwanted side effects).
How is fat a drug reservoir?
- Lipid soluble compounds can readily partition into fat tissue, which then acts as a storage depot for the compound, prolonging its action
- Important source of interpatient variability
- Ex: thiopental
What is the pH-partition theory?
- A means to explain the influence of pH and pKa on the extent of the ability for a drug to cross the PM
- Assumes that drugs are absorbed only when they are non-ionized, and therefore have a higher lipid solubility
Why are capillaries good for drugs to move across?
Lined by single layer of endothelial cells that have fenestration (spaces) between them, making them permeable and providing large SA for drugs to penetrate
How might liver disease affect CYP 450 metabolism?
Liver disease will reduce metabolic capacity, changing pharmacokinetic responses to drugs
What are the 4 important pharmacokinetic parameters?
1. Clearance: body's efficiency of drug removal
2. V(d): apparent space the drug resides in
3. Elimination half-life: rate of drug removal
4. Bioavailability (F): fraction of drug absorbed
How do drugs typically cross capillaries?
May cross via filtration, passive diffusion, or pinocytosis (if less than 25 kD)
What are the 2 variables the physician can control to keep the drug in its therapeutic window?
1. Dose 2. Frequency of administration
What are 3 factors that affect rate of drug distribution?
1. Cardiac output and regional blood flow
2. Tissue volume
3. Capillary permeability
Is drug binding to plasma proteins specific or non-specific? Reversible or irreversible?
- Reversible -> dynamic equilibrium b/t bound and unbound drug
Provide 4 examples of drug formulation being manipulated to modify absorption for therapeutic advantage.
1. Delayed or sustained release tablets (enteric coatings)
2. Depot preparation (vehicle, i.e., lipid/oil suspension, that keeps it at site of injection to prolong absorption)
3. Rapid release formations (gel caps)
4. Transdermal patches
These different preparations affect the rise of drug concentration and its access to systemic circulation
What are the four types of drug distribution patterns? Provide some general examples of each.
1. Remains largely in vasculature
EX: plasma substitutes (dextran), drugs bound strongly to plasma protein
2. Uniformly distributed throughout body water
EX: some low molecular weight compounds (ethanol)
3. Concentrated specifically in one or more tissues
EX: iodine in thyroid, tetracycline in bone/teeth, lipid-soluble in fat
4. Non-uniform distribution in body (MOST COMMON): largely determined by ability to pass through membranes and lipid/water solubility
EX: highest concentrations often present in kidney, liver, and intestine, usually reflecting amt. of drug being excreted
What is CYP3A?
- CYP subfamily that metabolizes drugs during absorption in the GI tract, where it decreases the bioavailability of many oral drugs
- CYP3A4 and CYP3A5 isoforms are involved in metabolism of 50% of drugs
a. CYP2C and CYP2D6 also involved in the metabolism of many drugs
What is a drug reservoir? How do these typically arise?
- Bound and ion-trapped drugs (relatively inaccessible to systemic circulation or site of action) can serve to prolong the duration of response
- There is an equilibrium b/t free and bound forms of drug, and b/t ionized and non-ionized forms
- Result of factors that affect membrane transport (i.e., lipid solubility), and distribution of drugs (binding)
How is albumin a drug reservoir?
Limits the availability of free drug, altering its kinetics (ex: Warfarin around 99% bound)
List 8 factors that modify absorption.
1. Route of administration
2. Circulation to site of absorption
3. Formulation factors that affect drug solubility
4. Area of absorbing surface
5. Membrane thickness
6. Drug concentration
7. Physiochemical factors affecting membrane transport
8. Transport mechanisms
What is pinocytosis? Provide some examples.
- When a drug is taken up into the cell via membrane invaginations; often referred to as endocytosis
- Examples: Vitamin A, D, E, and K
- Often described as clathrin-dependent or clathrin-independent
Drug X exhibits an area under the curve (AUC) of 10mg/L/hr. When 350mg is administered as an IV bolus, and distributes to 100L, what is the half-life of the drug?
- CL = dose/AUC = 350/10 = 35
- T(1/2) = 0.7 x (Vd/CL) = 0.7 x (100/35) = 2hrs
In which part of the GI tract does no drug absorption occur?
Do slow acetylators or fast acylators (genetic polymorphisms) have higher plasma concentrations of drug?
- Fast acetylators have lower plasma concentration
- Slow acetylators have higher plasma concentration
Broadly speaking, how does metabolism change drugs? Why is this?
- Tends to convert active compounds into less active (or less toxic) compounds (inactivation or detoxification), and/or make them more polar and less lipid soluble to favor excretion
- Metabolism does this b/c the properties of drugs (i.e., nonpolar, lipid soluble) that allow them to achieve therapeutic (or toxic) concentrations in the body also hinder their removal
NOTE: these reactions may also catalyze conversion of inactive parent compounds (prodrugs) to their active forms, and/or lead to the generation of toxic metabolites
How does the acidic environment of the stomach affect absorption from the GI tract?
- Can cause inactivation/degradation of drug (Ex: Penicillin G)
- Most acidic drugs will be non-ionized in the stomach and will readily enter the systemic circulation, while basic drugs will be ionized and not absorbed until they reach the small intestine
Why might you test for CYPs in certain individuals?
- CYP2C19 absent in 15-30% of Asians
- CYP2C9 absent in about 1% of Caucasians
- CYP2D6 absent in 7% of Caucasians
These are all metabolizers of important drugs, including
Briefly describe how drug binding to plasma proteins can affect drug distribution. How is this different for acids vs. bases?
- Extensive binding to plasma protein (or macromolecules) causes drugs to stay in central drug compartment, which can limit distribution of drug to its site(s) of action
- Albumin binds to widest range of drugs (50% of plasma protein) -> esp. acidic drugs
- Basic drugs often bind globulins, e.g., alpha-acid glycoprotein (many endogenous substances, i.e., steroids, vitamins, and metal ions also bind to globulins)
What are 3 examples of Phase I reactions?
What is metabolism?
Often used to refer to normal anabolic and catabolic reactions (protein, fat, carbs, nucleic acids, hormones, etc.), but also used to refer to the chemical transformation of both endogenous and exogenous agents
What is the driving force for drug mvmt across a membrane?
- The concentration gradient that exists for the non-ionized form of the drug
- The drug will move to diminish the concentration gradient until the non-ionized drug concentrations on the two sides are equal
Briefly describe inhalation admin of drugs.
- For admin of gaseous and volatile compounds
- Local effect: bronchodilators; Systemic affect: general anesthesia
- Rapid absorption; relatively efficient absorption of gases b/c large alveolar SA and extensive pulmonary circulation
- Important route for some drugs of abuse
- Allergic rxn a concern
What does it mean that most drugs are weak electrolytes?
They can exist in charged and uncharged forms in solution
What is pharmacogenetics?
The genetic basis for differences among the population in drug responses (therapeutic or toxic)
What general affect do Phase II reactions have on drugs?
Make them more water soluble + less lipid soluble = more polar -> less re-absorption in kidney
NOTE: acetylated metabolites favor re-absorption in the kidney b/c often less water soluble
How does gastric emptying time affect drug absorption in the GI tract? Name 4 factors that affect gastric emptying.
- Generally drugs are better absorbed in the S. intestine b/c of the larger SA, so increasing gastric emptying (GE) will increase drug absorption. Affected by:
1. Volume of ingested material: bulky materials tend to empty more slowly than liquids
2. Type of meal: fatty foods decrease GE
3. Body position: lying on L side decreases GE
4. Drugs: anticholinergics, analgesics, and narcotics reduce GE
Why would you administer a drug topically?
To produce local effect (can also be used to produce systemic effect)
What is the general conern about drugs given in a solid form?
- They must be dissolved before absorption takes place (solid dissolution, then blood absorption)
- If absorption is slow relative to dissolution, the absorption is the primary concern
- However, if dissolution is the slow, rate determining step, then factors affecting dissolution will control the whole process
- More common problem with drugs that have low solubility (below 1g/100ml) or that are given in high doses, i.e., Griseofulvin
How does species affect drug metabolism?
Sleeping time varies inversely to proportion of relative enzyme activity when animals are given Barbiturates, i.e., lower sleep time = more enzyme activity -> Mice lowest, then rabbit, rat, dog
In what organs does biotransformation most often take place?
- Predominantly the liver, but every tissue exhibits some level of metabolic activity
- Significant capacity for metabolism in GI, kidneys, and lung
- Brain also has metabolic enzymes thought to play a role in the etiology of several neurodegenerative disorders and responses to environmental toxins
What equation would you use to determine the IV infusion rate of a drug?
- Css = infusion rate/ total body clearance
- Time required to achieve steady state is still 5 half-lives
- A frequent route of administation
What is the multicompartment pharmacokinetic model?
- The most realistic model, but requires computer assistance (more widely used than the other 2 models)
- Measures the area under the curve
- CL = dose/AUC
What is first-pass metabolism? What is its effect on bioavailability?
- Drugs administered orally pass through the GI tract and are directed to the liver via portal circulation
- Because most drugs are metabolized (and inactivated) in the liver, this tends to reduce the free concentration of drug available to systemic circulation
- Severely limits bioavailability
List 6 epidemiological factors that can impact CYP expression.
1. Species differences
2. Genetic differences/polymorphisms
3. Age differences
What is reduction in Phase I rxns?
Addition of hydrogen or removal of oxygen, i.e., azo (N=N) or nitro (NO2) groups -> amines (NH2)
What is the major organ of drug excretion? What is its functional unit, and what 3 major properties relevant to drug elimination occur there?
1. Glomerular filtration
2. Tubular secretion: active secretion of weak electrolytes (esp. acids) and reabsorption of H2O
3. Tubular absorption: passive transfer of lipid soluble drugs, and reabsorption of water
NOTE: Whatever does not get reabsorbed is put out in the urine
What is the major determinant of the total fraction of drug bound to plasma protein at low drug concentrations?
Affinity of the drug for its binding site
What is biotransformation?
The chemical modification of xenobiotics by endogenous enzymes
What are the implications of the Henderson-Hasselbach equation? What is this called?
At steady state:
1. Acidic drugs will accumulate on basic side of the membrane (gastric -> plasma)
2. Basic drugs will accumulate on acidic side of the membrane (plasma -> gastric)
This is called ion trapping: an important determinant for where, when, and how much absorption, distribution, and excretion takes place for a given drug
What is the Michaelis concentration?
The concentration that generates a half maximal velocity
What is the volume of distribution?
- V(d) = amt. of drug administered/plasma drug conc.
- Clinically relevant parameter that indivates apparent body volume a given drug is located in -> generally indicative of compartment to which it distributes
- Often used as a diagnostic tool
NOTE: if you knew what you wanted the plasma drug concentration to be, you could multiply that by the volume of distribution to get the amount of drug that you need to administer
What are the 6 routes of drug excretion?
1. Renal: urine
2. Liver/intestines: feces
3. Lungs: major route for inhaled agents
4. Sweat (minor)
5. Saliva (minor)
6. Breast milk
What are Phase II (biosynthetic) reactions?
- Lead to covalent addition of a functional group (glucuronic acid, glutathione, amino acids, or acetate) onto the parent compound, or reactive product of a Phase I reaction
- These covalent modifications of parent compound are generally inactive and readily excreted
NOTE: the 6-glucuronide metabolite of morphine is a more potent analgesic than morphine itself
What are the 2 variations to the typical capillary structure that result in variation from normal drug tissue permeability?
1. Permeability is greatly increased in renal capillaries by large spaces b/t endothelial cells -> more extensive distribution of many drugs out of capillary bed by filtration
2. Brain capillaries have tight junctions b/t cells that are relatively impermeable, and restrict transfer of molecules from blood to brain tissue (BBB); lipid soluble compounds can be readily transferred, but transfer of polar substances severely restricted
What are the advantages and disadvantages of oral ingestion (enteral - most common route)?
1. Safe- painless, easy to take
2. Cheap- no need to sterilize, compact, multi-dose bottles
1. Slow onset (generally >1 hr); not useful for emergencies
2. Patient non-compliance
3. Low bioavailability (first pass metabolism)
What enzymes harbor the largest portion of Phase II drugs?
- Followed by: STs, GSTs, and NATs
What are the advantages of IV drug administration (5)?
1. If speed of drug delivery is an issue (i.e., emergency medicine), this is most rapid delivery method (rapid and complete delivery -> high bioavailability)
2. Patient compliance
3. No first pass effect
4. Flexible rate of administration
5. Veins relatively insensitive to irritation by drugs
What is the minimum toxic concentration (maximum tolerable concentration)?
This is the minimum concentration above which you will observe adverse responses.
How does age impact drug metabolizing activity?
Increase in drug metabolizing activity until you reach adulthood, then it seems to steady out, and finally decrease in the elderly (>65)