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Ocular Pharmacology > Week 1 > Flashcards

Week 1 Flashcards

1
Q

What is pharmacokinetics?

A

Its the study of how the drug enters the body.

E.g. oral, injection, inhalation, topical

ADME

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2
Q

What is pharmacodynamics?

A

The effects the drug exerts onto the body

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3
Q

What are the step to take for safe drug administration?

A
  1. Select management options
    a. Establish a diagnosis or diagnoses
    b. Evaluate the expected prognosis of the condition (how long will it take to treat? What would happen if we choose no medical intervention)
    1. Choosing suitable medicines so that the best available option is selected:
      a. Designs a management plan in consultation with the patient (take into account patient needs, systemic conditions, lifestyle)
    2. Using medicines safely and effectively to get the best possible results
      a. Implement the agreed management plan
      Prescribe pharmacological and other regimens to treat ocular disease injury
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4
Q

List the 10 question to ask for drug therapy (hint think of the picture)

A
  1. What is the problem?
  2. Is there a solution?
  3. What sort of therapy would be appropriate?
  4. How would your drug act?
  5. For how long will you treat?
  6. How will you monitor drug action?
  7. How much drug will you give?
  8. What’s special about your patient?
  9. Can you write the prescription?
  10. Any warning for patient or staff?
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5
Q

What 3 thing must you consider before prescribing ocular medications?

A

Eye conditions - signs and symptoms
Action of treatment
Complaince

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6
Q

What considerations about eye conditions?

A
  • Does the px’s complaint correlate with what is expected from a specific eye condition or their ocular signs?
    • Does the information provided by the px support the diagnosis of the specific eye condition?
    • Are the ocular signs consistent with each other
    • Would prescribing a therapeutic agent eliminate the px’s problem?
      Are there any other non-therapeutic treatments that could be used to eliminate the px’s problem?
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7
Q

What considerations about drug action?

Hints

(1. Drug and dose
2. Side effects
3. other meds or this is before
4. duration of use
5. drug interactions)

A
  • What are the therapeutic agents that could be used and what dosage would need to be used?
    • Is the patient likely to experience any symptoms/side effects with the therapeutic agent?
    • Is the px currently using any therapeutic agent or used a therapeutic agent in the past for the same condition?
    • How long is the condition likely to last, and how long is the patient likely to need to be using the therapeutic agent?
      Will there be any interaction of a chosen therapeutic agent with other medication the px is taking?
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8
Q

Considerations about Patient compliance

A
  • Is the px likely to be compliant with the regiment that they are being prescribed with?
    • Will the cost of the therapeutic agent affect compliance?
      Are there any other considerations that need to be taken?
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9
Q

What effects pharmacokinetics?

A
  • ADME
    • Diseases
    • Drug interactions
      Physiological and genetic variations between people
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10
Q

What factors effect drug absorption?

A

○ Molecular characteristics and viscosity of the drug (formulation and physiochemical properties of the drug)
○ Properties of any tissues that make a barrier (tight junction, tissue thickness)
Environment next to the barrier (pH, enzymes)

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11
Q

What are the sources of drug entry into the eye?

A
  • Eyelids
    • Tear films
    • Cornea
    • Conjunctiva/sclera
    • Blood supply (retina, ciliary body)
      Aqueous or vitreous humor
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12
Q

What Properties affect absorption of drugs in the eye ? (9)

A
  • The type of tissue
    • Blood vessels
    • Puncta
    • Tear volume, eye drop volume
    • Time in between eye drops to prevent dilution of first drop (10 min)
    • Lower tear volume, less dilute, more drug
    • Reflex tearing
    • Inflammation, breaks in gap junction of corneal epithelium
    • Stinging of drops, preservatives
      Very dry eyes may not absorb well depending on type of drug
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13
Q

What are the 3 ways for absorption to occur in the anterior eye?

A

Throught the conjunctiva, cornea and the lacrimal fluid

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14
Q

What are the advantages and disadvantages of absorption throught the conjunctiva?

A

A: - large surface area

  • permeability is greater than the cornea
  • vascular

D: - only 20% of the drug reaches the iris or ciliary body because most is removed via the opthalmic vein

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15
Q

What are the advantages and disadvantages of absorption throught the cornea?

A

A: Controls the diffusion of drugs into the inner chambers of the eye

D: - Non-vascular
- Drugs must have intermediate solubility in both lipid and aqueous phases
Drugs must be of low molecular phases

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16
Q

What are the advantages and disadvantages of absorption throught the lacrimal fluid?

A

A: - pH 7.4 (neutral)
- Good buffering capability of unbuffered solutions with pH from 3.5 - 10 (e.g. carbonic acid, weak organic acids and protein)

D: - Aims to remove drug from the surface of the eye

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17
Q

How do you reduce loss of drug when doing eye drop instillation?

A

reduce number of drops, waiting between drops (5 -10 min), occluding puncta after drop instillation

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18
Q

What are the tear properties of the tear film?

A

○ Tears consists most of aqueous fluid and is approximately 40 nm thickness
○ Contains oxygen, metabolites, electrolytes, antimicrobial peptides, proteins and soluble immunoglobulins
○ pH ~ 7.4
Typical tear volume ~ 8 - 10 micron litres expands to ~ 30 micron Litre

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19
Q

How does adding an eye drop effect the properties of the tear film?

A

§ Adding an eye drop = adding ~30-60 micron Litres

Extra fluid results in loss of therapeutic agent via nasolacrimal drainage or overflow

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20
Q

What are the effects of drug with the tears?

A

○ Dilution
§ Can be unknown depending on tear production
§ Dry eyes (less aqueous humor, more drug)
§ Some medications may decrease tear production
§ Tear pH can change drug pH
○ Protein concentration
§ Proteins can interact with drugs
§ Proteins can affect tear production
○ Solubility of drug
Lipid vs aqueous

21
Q

What two other factors effect drug absorption?

A

○ Level of irritation
§ Tear pH: acidity or alkalinity (can alter pH of drug, can be both good and bad)
§ Drug washed away with reflex tearing
§ Higher concentration of drug needed, also can result in reflex tearing
○ Patient status
Children (trouble administering the drug, blinking the drug out)

22
Q

What does rate of diffusion depend on?

A

Rate of diffusion depends on molecular size (larger size takes longer to diffuse) and concentration gradient (travel from higher concentration gradient to a lower one, until they equalise)

23
Q

What is Fick’s First Law?

A

§ Rate of diffusion = concentration gradient between compartments (from high to low concentration)
§ Example above
§ It’s a linear relationship and only true for passive diffusion not for active transportation
§ Ocular absorption (depends on drug concentration in the tear)
□ Cornea - 10 - 20 minutes after instillation
□ Aqueous humor (depends on drug elimination) - 3 hours maximal concentration
This is important for drug administration in terms of how much drug to give and how often

24
Q

What is the partition co - efficient?

A
  • Partition co-efficient and ionisation
    ○ Partition coefficient is a ratio describing how well a compound dissolves in lipid compared to water
    § Higher number, more lipid soluble, higher permeability through the cornea
    Needs to be water soluble enough not to get stuck in the cornea, partition co-efficient needs to be in the middle (not too high, not too low
25
Q

What is Ionisation?

A

○ Ionisation describes a drug changing “charge” by gaining or losing electrons in different solutions
§ Ionised particles have low lipid solubility and unable to passively permeate a membrane
§ Most drugs are weak acids or bases so they exists in two forms: un-ionised (alkaline) and ionised (acid), which is dependent on pH

26
Q

What is pH and why is it important?

A
  • pH is used to measure how acidic or basic an aqueous solution is, and it depends on the concentration of hydrogen (H*) ion in the solution
    Lower pH, more free H+, more acidic; higher pH, more hydroxide (OH-), more basic (unionised)
27
Q

What is pKa?

A

○ The pH where concentration of drug ionised and unionised are equal is called the dissociation constant (pKa)
○ Each drug has a pKa, which determines solubility of the drug
§ Lower pKa, more acidic, drug will dissolve more readily in hydrophilic compartments (will stay in the watery aqueous areas)
§ Ionised ion stay in the aqueous areas and unionised goes to the lipid areas

28
Q

Describe the cornea sandwich and how it work?

A

Cornea: Sandwich
- Tear film - wants ionised drug
- Epithelium - wants non - ionised drug
○ Lipid rich - barrier to hydrophilic drugs
○ Has tight junctions
- Stroma - wants ionised drug
○ Easy passage from hydrophilic drugs (acts as a depot)
- Endothelium - wants non - ionized drug
○ Lipid rich but small volume
Maintains corneal hydration; has leaky junctions

29
Q

Describe Drug Distribution and how it is measured?

A
  • How much drug is available for use in the tissue and where is it?
    • Reversible transfer of drugs from the bloodstream to various other tissues of the body
    • Measured by bioavailability
    • In the eye, distribution depends on:
      ○ Properties of the drug
      ○ Perfusion of the tissue
      ○ Special physiological barriers
      Passive diffusion/active transport
30
Q

What is volume of distribution?

A

Theoretical volume that would contain the total body content of the drug (Q) at a concentration equal to that present in the plasma (Cp)
Changes with age, sex, body composition, disease states, tissue components

31
Q

What is the loading dose?

A

The dose required initially to reach the required therapetic levels.

= (desired C(p) x V (d)) /F

32
Q

What can drugs bind to in the eye?

A
  • Drug can bind to melanin (pigment), which can affect drug duration and potency
  • Binding of lipid soluble drugs can also induce localised toxic effects
33
Q

What does the crystalline lens block?

A
  • Crystalline lens prevents entry of large protein and hydrophilic drugs

Some lipid soluble drugs can result in cataract formation

34
Q

What is a major reservior for hydrophilic drugs?

A
  • Vitreous is a major reservoir for hydrophilic drugs

Diffusion can be affect by vitreal structural changes with age

35
Q

Tips for crossing blood retina barrier?

A
  • Retina has tight junctions forming the blood retina - barrier
    ○ Lipophilic drugs can cross over this barrier
    Usually need non-topical to help the drug distribute to the retina
36
Q

What is drug metabolism?

A
  • Metabolism
    ○ Relates to the elimination of drugs and any toxic by-products involving enzymes
    ○ Main area of metabolism is the liver - can affect bioavailability of oral drugs
    ○ Involved in activating drugs (prodrugs)
    ○ Can produce toxic by-products
    In the eye, most metabolism takes place in the ciliary body
37
Q

What is drug elimination or excretion?

A
  • How is the drug removed from the tissue?
    • Main measurement is clearance
    • Main area of clearance are kidneys and liver
      In the eye, most clearance takes place in the ciliary body and retina
38
Q

What is drug clearance?

A
  • The volume of blood completely cleared (irreversibly) of drug per unit of time (volume/time)
    • In the eye, drug and its metabolites are usually excreted though the systemic bloodstream
    • Calculation of maintenance dose to keep target plasma concentration at steady state
      (steady state is the rate of drug administration equals the rate of drug elimination so that plasma drug concentration stays constant)
39
Q

what is the formula for Maintenace dose?

A

Maintenance dose = (desired C(p) x CL)/F

40
Q

What is the formula for Half life?

A

Half life = In 2 x Vd/CL

41
Q

What is Half life

A
  • Half - life is the time taken for the plasma drug concentration to fall by one - half (50%)
    Dependent on distribution and clearance
    • Determines
      ○ Duration of action of a drug after a single dose
      ○ Dosing frequency
      Time taken to reach steady state
42
Q

What is first order elimination?

A

as the drug concentration is higher, more elimination of the drug; half-life is constant

43
Q

What is zero order elimination?

A

changing the drug concentration does not affect elimination

44
Q

What is Michaelis - Menten elimination?

A

when enzymes are involved, part A similar to first-order elimination; part B happens in higher drug concentrations elimination rate levels off due to all the enzymes becoming saturated

45
Q

Iris tips

A

○ Adjusts amount of light reaching the retina
○ Contains pigment, reversible binding of drug
Multiple doses of drug can reduce bound drug to be released

46
Q

Ciliary Body tips

A

○ Produces aqueous humour and controls accommodation

○ Main area of metabolism and elimination of drugs from the eye

47
Q

Metabolism Tips

A
  • Mostly in the liver; majority of drugs become more water-soluble metabolites following metabolism to be more readily excreted
    - Enzymes have been found in the cornea, lens, iris - ciliary body, retina, choroid, retinal pigmented epithelium, optic nerve and sclera
    - Prodrugs - need to be metabolised to become active e.g. cleaving or conjugating
    Metabolites
48
Q

Elimination Tips

A
  • Anterior eye - aqueous humour (anterior chamber angle)

- Posterior eye - blood retinal barrier, blood - aqueous barrier

Ocular Pharmacology (5 decks)

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