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a. SL12102: The science of medicines > L7 - Physiciochemical Properties > Flashcards

L7 - Physiciochemical Properties Flashcards

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Biology 101 flashcards
1
Q

What must a drug do for it to have a biological effect? And why?

A

It must be released from its dosage form
- the body doesn’t absorb solids

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2
Q

What is the drug’s bioavailability influenced by?

A
  • the dosage form, including excipients
  • the physiology of the site of administration
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3
Q

What 2 steps does drug dissolution involve?

A
  • solvation of drug molecules at the crystal surface
  • drug molecules diffuse across diffusion layer to bulk dissolution medium
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4
Q

What is the Noyes-Whitney eqn?

A

dm/dt = DA(Cs-C)/h

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5
Q

What do the letters stand for in the Noyes-Whitney eqn?

A

m - mass of drugd released into solution (kg)
t - time (s)
dm/dt - dissolution rate of the drug (kg s-1)
D - diffusion coefficient into dissolution medium (m s-1)
A - SA of drug particle (m2)
Cs - solubility of drug (kg m-3)
C - conc of drug in bulk solution at time (kg m-3)
h - thicknedd of boundary layer (m)

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6
Q

What are the key parameters for the Noyes-Whitney eqn?

A
  • effective SA
  • molecular weight (MW)
  • solubility (hydro-/lipohilicity, crystal structure)
  • GI tract (viscosity, vol of liq, movement, etc)
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7
Q

What are the 2 drug absorption pathways?

A
  • transcellular pathway (across cells)
  • paracellular pathway (alonside cells)
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8
Q

What are the 3 different types of junctions?

A
  • tight junctions
  • adherens junctions
  • desmosomes
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9
Q

What are tight junctions?

A

Membranes of neighbouring cells fused by intimate connections between cell surface proteins

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10
Q

What are adherens junctions?

A

They connect actin filaments in the cytoskeletons of neighbouring cells together

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11
Q

What are desmosomes?

A

Fibrous proteins cross the gap between cells and anchor keratin filaments in the cytoskeletons together

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12
Q

What is the rate limiting for the paracellular pathway?

A

The passing through tight junctions

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13
Q

What are the 3 ways that drugs get absorbed through transcellular absorption?

A
  • passive diffusion
  • facilitated diffusion
  • active transport
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14
Q

What is passive diffusion?

A

(Main pathway of drug absorption)
Solutes diffuse into cells DOWN a conc grad
- must partition into the lipib bilayer and out again into cytoplasm

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15
Q

What is facilitated diffusion?

A

Selective, carrier-mediated transport of a drug DOWN a conc grad

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16
Q

What is active transport?

A

Selective, carrier-mediated transport of a drug DOWN or AGAINST a conc grad
- requires energy input

17
Q

What is endocytosis?

A

(Very minor pathway)
Internalisation of the plasma membrane, engulfing extracelular fluid
- limited relevance to absorption of small drugs in GI tract

18
Q

What is passive diffusion in terms of equilibrium?

A

A system not in equilibrium moves towards equilibrium, which means the flor (flux) must occur

  • low entropy (ordered) to high entropy (disordered)/homogenous as they diffuse
19
Q

What is Fick’s First Law?

A

J = -D dc/dx

J - flux
D - diffusion coefficient
dc/dx - conc grad

20
Q

What are details about Fick’s First law?

A
  • flux is proportional to potential energy grad
  • the further a system is from eq (high dc/dx) the faster it moves towards it
  • -ve sign - flow from high to low conc
  • free energy minimised, entropy maximised when mixing of solutes is uniform
21
Q

What is the Stokes-Einstein eqn?

A

D = kb x T/ 6pi x n x r

D - diffusion coefficient
Kb - Boltzmann constant
T - temp
n - viscosity of solvent
r - particle radius

22
Q

What does the stokes-einstein eqn relate diffusivity to?

A
  • local environmental conditons
  • properties of solvent which molecules is diffusing
  • properties of the diffusing molecule
23
Q

What is the speed of diffusion like?

A

Molecules move really quickly, moving randomly and colliding with other molecules

24
Q

What is mean distance moved propertional to in a collection of diffusin molecules?

A

Mean distance moved (x) in a specific direction is proportional to the square root of time

x = sqrt(2D t)

25
Q

What is passive diffusion like across epithelia?

A

Aq -> lipid -> aq

Dosage form released, drug needs to diffuse to the epithilium and then through it

26
Q

What is Fick’s first law on passive diffusion?

A

J = ADK(Co-Ci)/h

K - partition coefficient
Co-Ci = conc grad

27
Q

What are the membrane properties regarding fick’s first law in passive diffusion?

A
  • flux is inversely proportional to membrane thickness
  • flux is proportional to membrane area
28
Q

What are drug properties like regarding fick’s first law in passive diffusion?

A

Flux is proportional to the
- conc grad
- partition coefficient

29
Q

What are the drug and membrane properties like regarding fick’s first law in passive diffusion?

A

Flux is proportional to the diffusivity of drug in the membrane
- influenced by drug size and nature of the membrane

30
Q

What does K quantify?

A

The distribution of a drug between the aq phase and the lipid membrane

31
Q

What does lipholicity measure in practice?

A

The partitioning of a drug between oil and water
- indication of how lipophilic a drug is

32
Q

What is P?

A

The oil/water partition coefficient
P = Coil/Cwater
Determined using octanol and water, expressed as LogP

33
Q

What are the methods for determining P?

A
  • solubility
  • shake/flask method
34
Q

What is the solubility method like for determining P?

A

Determine drug solubility in oil and its solubility in water
P = Soil/Swater

35
Q

What is the shake/flask method like for determining P?

A
  1. Add drug to equal vols of water and oil
  2. Shake for extended period of time
  3. Allow phases to separate
  4. Determine conc of drug in each phase
36
Q

What is LogP when the drug is more soluble in octanol than water?

A

P > 1
Log P > 0

37
Q

What is LogP when the drug is less soluble in octanol than water?

A

P < 1
LogP < 0

38
Q

What LogP do approximately 90% of small molecule drugs on the market have?

A

LogP between 0 and 5

39
Q

What is the optimum Logp range for oral delivery

A

LogP 1 to 3

a. SL12102: The science of medicines (25 decks)

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