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PH3503- Formulations 3 > Lec 15- Polymers in drug delivery > Flashcards

Lec 15- Polymers in drug delivery Flashcards

1
Q

Polymer

A
  • Substances of high molecular weight made up of repeating monomer units
  • Can be used to form a diffusion barrier, packaging
  • Biodegradable polymers break down in to monomers and then solubles into the monomers
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2
Q

Monomer units

A
  • Ethylene (CH2=CH2)
  • Styrene
  • Methylcellulose
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3
Q

Polyethylene

A
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4
Q

Macro-structure of polymer

A
  • Linear
  • Branched
  • Cross-linked- cross carmellose sodium- can be used as part of the excipients such as this is a disintegrate
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5
Q

More structural

A
  • Star
  • Comb
  • Ladder
  • Semi-ladder
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6
Q

Dendrimer

A
  • Highly branched constructs formed from a central core which define their initial geometry
  • Tend to be spherical
  • NB- small molecule can be trapped in pores within the dendrimer
  • Encapsulation of drugs
  • Covalently attached targeting moieties
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7
Q

Polymer Molecular Architecture

A
  • Homopolymer (Single monomer)- A-A-A-A-A-A-A-A
  • The copolymer (more than one monomer)
    • Alternating copolymer- A-B-A-B-A-B-A-B
    • Random co-polymer- AAABBABABBBBAAA
    • Block copolymer- AAAAABBBAAAAABBB-
  • Graft co-polymer
    • Non-linear block- one polymer with another branching from it
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8
Q

Combination of arrangements

A
  • Star homopolymer
  • Star block co-polymer
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9
Q

Polydispersity

A
  • Small molecules of a given pure compound all have the same molecular weight- Monodisperse
  • During synthesis, polymer chain grow at different rates
  • The resulting polymer has a range of molecular weights
    • Polydisperse or heterodisperse
  • Described by an average molecular weight and an molecular weight distribution
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10
Q

Two types of molecular weight

A
  • Number average molecular weight
    • Determined by
      • Chemical analysis
      • Osmotic pressure
  • Weight average molecular weight (Mw)-
    • Determined by
      • Light scattering technique
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11
Q

Number average molecular weight (Mn)

A
  • The statistical average molecular weight of all the polymer chains in the sample
    *
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12
Q

Example question

A
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13
Q

Determination of Mn

Number average molecular weight

A
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14
Q

Weight average molecular weight

A
  • Mw takes into account the molecular weight of a chain in determining contributions to the molecular weight average
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15
Q

Weight average molecular weight

Mw

A
  • However- Mass= number of moles x MW
  • learn the red box
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16
Q

Example question

A
17
Q

Degree of Polydispersity

A
  • Mw > Mn if the mixture is polydisperse
  • If the average molecular weight measured by light scattering techniques is greater than that obtained by osmotic pressure => the mixture is polydisperse
  • The ratio Mw/Mn measures the degree of polydispersity
  • In our example 1750/1600 = 1.09
  • The closer the value is to 1 the higher the mono-dispersity of that polymer
18
Q

Short answer question

A
19
Q

Applications

A
  • Packaging
    • Tops, Bungs, Containers
  • Viscosity modifiers, suspending and emulsifying agents
  • Disintergrants- especially cross-linked polymers
  • Coating material
  • Gels, wound-dressing
20
Q

Applications

Polymeric delivery systems

A
  • Membranes and matrices
  • Adhesives
  • Nano and micro-particles- spray drying can achieve drug encapsulated within the polymer (oil in water- with the drug in the oil phase and polymer in the water)
  • Hydrogels- transdermal (often gel structured around the gel- often release can be modified due to the response from the body (such as body temperature))
  • Ion exchange resins
21
Q

Synthetic polymer

(Eudragit)

A
  • Polymers can be synthesised with pH-dependent solubility (Acidic moieties) or viscosity, biodegradability or membrane-forming characteristics
  • If biodegradable
    • Need to consider the safety of degradation products- often use lactic acid (lactic acid is a natural product from the body)
    • Kinetics of degradation may need careful control
22
Q

Natural polymers

A
  • Vary in purity
  • Often require cross-linking to control the release
    • Polypeptides and proteins
      • Albumin, gelatin
    • Polysaccharides
      • Starch, Chitosan
  • Purity can be a problem
  • Natural often have microbiological contamination
23
Q

Controlled-release mechanisms (systems)

A
  1. Diffusion
    • Reservoir systems
    • Monolithic systems
    • Swelling systems
  2. Osmotic systems
  3. Biodegradable systems
  4. Mechanical pumps
24
Q

Controlled release mechanisms

A
  • Any or all of these mechanisms can occur in a given release system
    • Diffusion
    • Swelling followed by diffusion
    • Degradation
  • For controlled drug delivery formulations, a material must be chemically inert and free of leachable impurities
  • Must have an appropriate physical structure with minimal undesired ageing and be readily processable
25
Q

Biodegradable polymers

A
  • These materials degrade by natural biological processes, eliminating the need to remove the finished delivery system
    • (A) Represent BULK-ERODING- sponge
      • Parts of the polymer are biodegraded, leaving gaps for the drug to be released
    • (B) Represent SURFACE-ERODING- onion
      • Layer by layer lost from the surface
      • This will achieve a more constant release profile- as the layers will degrade at the same rate allowing a consistent drug release
26
Q

Biodegradable polymers

Different polymers degrade at different rates within the body and therefore polymer selection can be tailored to achieve desired release rate

Factors effecting degradation

A
  • Degradation can be affected by a wide range of factors
  • Structure and composition
  • Physical and physicochemical factors-
  • Purity
    • Presence of ionic groups
    • Presence of low MW compounds
  • MW and distribution
  • Morphology
  • Processing conditions
  • Sterilisation process and storage history
  • Shape
  • Site of implantation
27
Q

Polylactide and polylactide-co-glycolide

A
  • Polymers of lactic acids and glycolic acid
    • Homopolymer (usually lactide PLA)
    • Co-polymer (PLGA)
28
Q

Current market for PLGA formulations

A
  • PLA and PLGA
    • Undergo hydrolysis from biologically compatible and metabolisable moieties
    • Currently used as suture, bone implants (as bone regenerates the polymer disintegrates) and screws and depot formulations
  • Marketed depot formulations include microparticles containing octreodide (sandostatin LAR) and Risperdal Consta
29
Q

PLGA as a polymer for drug delivery

A
  • Advantages
    • Long safe history
    • Biodegrades to natural metabolites
  • Control of degradation
    • MW- change MW with different grades
    • Polymer composition (copolymer ratio)
      • Hydrophobicity, crystallinity, glass transition as well as release profile
    • Particle size, shape, morphology and loading
30
Q

PLGA degradation

A
  • Bulk erosion is the main degradation pathway for PLGA
    • Random scission of ester bonds proceeding homogeneously through polymer backbone
  • Three phase mechanism proposed
    1. Initial rapid decrease in MW with no soluble monomer formation
    2. Decrease in MW with rapid mass loss and formation of soluble products
    3. Formation of soluble monomers resulting in complete polymer degradation
  • Incorporated drugs have the potential to modify degradation rates
31
Q

Surface-Eroding systems

A
  • Polyanhydrides as biodegradable polymers for drug delivery (surface eroding)
    • Matrices can degrade over periods ranging from 1 day to many months and any times in between
    • Degradation is through surfaceErosion, leading to ZERO ORDER release
    • Marketed product- Gliadel wafers
32
Q

Cellulose derivatives

A
  • Polysaccharides formulated into hydrophilic matrices
    • Popular biomaterial
    • MW and particle size can influence release from these matrices along with polymer relaxation
      • HPMC
      • HEC
      • HPC
      • MCC
33
Q
A

PH3503- Formulations 3 (21 decks)

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