STEVE Flashcards
Which nanocarrier types rely on “attachment of drug
-Polymer therapeutics
(including denrimers, micelles, pegylation)
-Inorganics
Which nanocarrier types typically rely on matching drug ‘solubility’ within nanocarrier?
- Micelles
- Dendrimers
- Nanoemulsions
- Solid lipid nanoparticles
- Liposomes
- Polymer nanoprecipitates / particles
How have polymer therapeutics been used for aspririn? + What are the benefits?
Usin aspirin as a monomer - polymerising with diacid chloride - to form polyaspirin.
- Polyaspirin eilminates the stomach irritation that is experienced with aspirin (gastric ulcers & bleeding)
- Delayed release over long periods
- Potential for better moulding and processing.
How have polymer therapeutics been used for morphine? + What are the benefits?
Morphine and glutaric anhydride used to synthesis polymorphine
- polymorphine has ester and anyhdride weak links - good for biodegradation and drug release
- Slow release of pain killers
What are the types of polymer therapeutics?
- Polymers made from drugs (>50 wt% of the polymer is drug)
- Polymer is the drug
- Polymer drug conjugates
- Polymer-drug micelles
Why is it an advantage to increase time to clear drug from the body?
Drugs that are rapidly eliminated have limited therapeutic value and require repeated dosing
What are some of the limitations of PEG conjugation?
- PEG can accumulate and cause problems
- No added stability to the protein outside the body
- PEG is a ‘one-shot’ approach - not tailored to different proteins
What does PEG stand for
Polythylene glycol
What are the principle of PEG conjugation strategies?
- Requires modification of the PEG-OH chain end
- Required a complementary functionality on the drug or protein
What effect do PEG groups have on proteins?
PEG groups are often degradable. - so don’t impact the behaviour of the protein, but impact the solubility.
Explain polymer-drug conjugation of the anti-cancer drug - doxorubicin?
Doxorubicin is highly toxic cytostatic drug with a large number of side effects.
Conjugation of polymers may leads to reduction in non-specific toxicity and better circulation lifetimes.
Conjugated doxorubicin - Passive targeting of tumours through enhanced permeation and retention effect.
What are the principles of polymer-drug micelles?
PEGylation of drug to make “drug surfactants”
How does PEGylation to form ‘drug surfactants’ then form polymer-drug micelles?
Can undergo self-assembly into micelles when
conc. > CMC (critical micelle conc.)
What are the advantages of drug conjugation?
- Enhanced biostability
- Increased drug loading
- Extended circulation time
- Selective recognition
- Triggered drug release
- Combination therapy
Why does polymer conjugation get more challenging to chaieve as the molecular weight of the polymer used increases?
- Chain ends decrease in concentration as the molecular weight (chain length) increases.
- Polymer chains coil and steric issues affect the backbone - also the chain ends may be ‘lost’ within the coils
How are gold nanoparticles used in nanomedicne?
Drug compounds e.g. doxorubicin - bind to gold surface
Gold coated silica particles are stabilised using thiol terminated PEG (stabiliser)
How are gold nano particles used for cancer treatment?
Localised heating via Au-nanoshells kills cancer selectively
Accumulation of Au-nanoshells at tumour, Near-infrared laser heats Au-nanoshells, kills cancer selectively
How are passive micelles used for nanomedicine?
Large polymeric micelle - inner core (hydrophobic) could be used to encapsulate poorly soluble hydrophobic drugs.
-Drug Stabilised within core - used for a nanocarrier
How are targeting MIXED micelles used for nanomedicine?
Combinations of 2 different types of A-B block copolymers - control number of targeting ligands by ration of two block copolymers
e.g, for Antibody targeting
How are polymer micelles self-assembled by dialysis?
Via solvent exchange:
Solvent environment within dialysis bag changes dramatically
- Switch from good solvent environment to not-so-good, to then really bad solvent environment.
-Changing solvent conditions force drug to interact with hydrophobic chains as they assemble
- Hydrophobic chains start to aggregate together
How do dendrimers form unimolecular micelles?
If hydrophobic core & hydrophilic (charge stabilised) surface - acts as a unimolecular micelle
- internal hydrophobic cavities
- External hydrophilic functional groups
Drug loading by dissolving drug & dendrimer in MeOH, addition of water & slow evaporation of MeOH
What is a nanoemulsion?
Submicron stabilised emulsion droplets
-Surfactant stabilised
What are solid lipid nanoemulsions?
Submicron stabilised solid-oil droplets
-Substitution of the liquid oil phase within an emulsion, with a fatty acid (or derivative) that is solid at elevated temperatures
What are the advantages/disadvantages of solid lipid nanoparticles
Advantages:
• Improved drug stability
• Control over drug release
• Lipids are generally biodegradable
• Possibility to avoid organic solvents during preparation
• Relatively easy to scale-up and sterilize
Issues:
• Lipids crystallise and exclude “dissolved” drug
• Crystallised lipid leads to low loading of drug
How are solid lipid nanoparticles formed by emulsification/evaporation?
(Solid lipid + solvent + drug)
creates 2 phase system with water, add surfactant to create system which is emulsifiable
- solvent evaporation - to give nano-scale emulsion
-removal of solvent decreases the droplet size
What are the methods of forming solid lipid nanoparticles?
- High shear homogenization
- Ultrasonication/ high speed homogenization
- Solvent emulsification/evaporation
How are polymer nanoprecipitates formed?
- Organic solution (water miscible) of polymer and drug is rapidly added to water (anti-solvent)
- Good solvent diffuses into poor solvent environment (water)
- Decreasing solvent quality leads to aggregation (assembly into small structures)
- Small structures aggregate
- Aggregation is arrested by colloidal stability (steric/charge)
- Final stable nanoprecipitate - Hydrophobic drug is encapsulated in core if initially present in good solvent.
What factors control polymer nanoprecipitation?
- Good solvent choice
- Concentration in the good solvent
- Dilution range
- Drug/polymer miscibility
- Viscosity
- Temperature
- Rate of addition to antisolvent
- Possible by microfluidic approaches
Define liposomes?
• Comprise assembled amphiphilic molecules
• Bilayer of surface active material
• Clear internal hydrophilic cavity surrounded by
hydrophilic stabilising groups (charge or steric) and
sandwiching a hydrophobic bilayer
• For small molecule liposomes, usually made from double
chain surfactants (or lipids)
• Hydrophilic drug encapsulation in the core plus
optional hydrophobic drug encapsulation in the bilayer
What are the advantages of using liposomes as nanocarriers?
- Very low toxicity
- Often biodegradable
- Low immunogenicity
- Potential for targeted delivery
- Protection of sensitive water soluble drugs
- Enhanced drug solubility
- Improvement of pharmacokinetics
What are some disadvantages of using liposomes as nanocarriers?
- Encapsulation into core often has low efficiency
- Leakage of drug from bilayer and/or core during storage (Bilayer approx 5-6 nm thick)
- Difficult to scale to large volumes
- High batch to batch variation
- High cost
What is the differentce between unilamellar and multilamellar vesicales?
Unilamellar Vesicles - contain 1 bilayer
Miltilamellar vesicles - contain miltiple bilayers
Describe the general production of liposomes
Lipids are generally used to produce liposomes. The following process is used to encapsulate a hydrophilic drug in a liposome:
• First the lipids are dissolved in an organic solvent.
• Rotary evaporation is then used to produce a dry lipid film.
• Water containing the hydrophilic drug is added to rehydrate the lipid film with gentle stirring to produce multilamellar vesicles.
• Processing techniques are required to achieve the desired liposome structure – i.e.
- Extrusion is used to produce large unilamellar vesicles
- Sonification /Homogenisation used to produce small unilamellar vesicles
• Purification techniques such as ultrafiltration or column chromatography are then completed to produce the final liposomal product
What is the difference between liposomes produced by extrusion or sonification/ Homogenisation?
Extrusion - Large unilamellar vesicles
Sonification/ Homogenisation - Small unilamellar vesicles
What are the issues with standard Doxorubicin drug?
Doxorubicin is highly toxic and targets DNA with proliferating cells.
Causes cardiomyopathy (heart disease and scarring) which may lead to congestive heart failure and death .
What are the structural properties of doxorubicin?
- Amphiphilic molecule
- Well known to self-assemble in water at very low concentration (hydrophobic interaction and π-π stacking or aromatic groups)
What are the benefits of putting doxorubicin in liposomes?
– Avoid non-specific organ toxicity
– Extend the circulation time after IV injection
– Targeting of solid tumour sites by the Enhanced Permeation and Retention (EPR) effect
What are the differences between healthy tissue and tissue with tumour?
Healthy tissue: Endothelial cells packed close together, dissusion can happen through this into healthy tissue - active/highly controlled process.
-Also have a lymph system - removal of toxic material
Tumour :
Random/ Rapid growth
-Pulls open vascular system - causes stress/ strain on the blood vessel - leads to gaps in endothelial - nm size gaps
How do nanocarriers target tumours?
Cytotoxic drug thats small enough to fit between the gaps between endothelial cells in blood vessels, by passive targeting - drug will filter into tumour tissue. Tumours don’t have a very strong lymphatic system - so anything that arrives at tumour stays there
Explain the enhanced permeation retention (EPR) effect
- Nanoparticles direct drug away from sites with tight epithelial junctions in the vasculature (blood vessels) such heart and muscle.
- Accumulation in areas where fenestrations (gaps) exist eg liver, spleen, bone marrow, areas of inflammation, and neoplasms (new/abnormal tissue growth) – ENHANCED PERMEATION
- Lack of lymphatic drainage from tumours also leads to poor removal of nanoparticles – ENHANCED RETENTION
What issues with doxorubicin liposomes?
– Phagocytic cells (cells that “eat” foreign objects)
• The mononuclear phagocyte system (MPS) recognises particles as ‘‘foreign’’
• Leads to removal from the bloodstream
• Phagocytosis of Dox-liposomes releases doxorubicin and causes cell death which reduces the MPS capacity
What are some requirements for Dox-liposome manufacture?
– Methodology must be straightforward, simple and use cheapest available materials.
– Liposome generation must be as uniform as possible (reproducible size distributions)
– Optimal loadings are 100% trapping efficiency at the desired drug-to-lipid ratio
• No need to remove un-encapsulated doxorubicin from the sample.
– Drug release rates need to be improve drug activity
• Decrease toxicity or increase efficacy
•Rapid (instantaneous) doxorubicin release after iv injection results in no benefit over free drug
• Complete drug retention (no drug release) may result in a therapy that is neither toxic nor efficacious. Possible substantial drug delivery to tumours and substantial reductions of delivery to cardiac tissue, BUT no therapeutic value
How are Doxorubicin liposomes formed by passive encapsulation?
– Hydration of the dried lipid film with an aqueous solution of doxorubicin.
– Targets the aqueous volume trapped inside the liposome during formation.
– After rehydration drug/liposomes are co-dispersed and a fraction is entrapped directly
– Driven by a combination of hydrophilic, hydrophobic, and ionic interactions.
– Doxorubicin is amphiphilic so, depending on pH, may reside in the aqueous core and may partition into the lipid bilayer.
– Typically maximum efficiencies = 80%
– Drug-to-lipid ratio is low and dependent doxorubicin water solubility (<10 mM).
– Removal of unencapsulated drug is critical and difficult at production scale
How are Doxorubicin liposomes formed by active encapsulation?
– Addition of doxorubicin to preformed liposomes
– Generation of a trans-bilayer ion (including pH) gradient needed
– Leads to a redistribution of drug occurs across the liposomal bilayer and trapping in the core: Mechanism of trapping - internal aqueous pH and induced drug precipitation.
– Drug-to-lipid ratios as high as 0.3:1 (wt:wt) = approx. 48,000 doxorubicin molecules per 100 nm diameter liposome.
Explain the steps of active encapsulation dox-liposome manufacture using citrate
1- Rehydrate the liposome layer with an aqueous solution of citrate (acid buffer)
- pH gradient generated.
2- Base added or dialysis, column chromatography to modify external pH (increase)
- Dox-HCl (Dox-hydrochloride salt) in equilibrium with Dox-NH2
- Amine becomes deprotonated and is hydrophobic - so works way into core - where its protonated (core pH=3.5)
3- Doxorubicin citrate salt is formed
Explain the steps of active encapsulation dox-liposome manufacture using ammonium sulphate
1- Add ammonium sulphate directly to generate an ammonium sulphate gradient - by hydration of dried lipid with (NH4)2SO4 solution.
2- External environment changed to NaCl at same pH (no base washing)
3- Ammonium counter ion within core of liposome will dissociate
- Ammonia (NH3) will permeate out of the liposome - rapid permeation
4- pH gradient established
Doxorubicin (Dox-NH2). will permeate in also.
5- Doxorubicin & acid environment within the core forms a sulphate salt
What is the clinical benefit of Dox-liposome?
prevent MPS (mononuclear phagocyte system) activation and prolong circulation times
What is the main requirement for drug delivery of doxorubicin by liposomes?
The doxorubicin must get out of the liposome
– move from the inside of the liposome to the outside
– must pass through the lipid bilayer
– drug must interact with the membrane interface (inside of the liposome), the lipid head-groups, the lipid acyl chains, and the membrane interface on the outside of the liposome.
– aqueous core pH and interfacial dictate the proportion of drug in the neutral and in the charged form.
– both neutral and charged drugs are believed to permeate the bilayer (neutral > charged)
– anionic lipids increases charged doxorubicin concentration at the interface resulting in an increased rate of drug release
– inclusion of PEG-modified phosphatidylethanolamine (anionic) increases doxorubicin release – potential increased hydration at the interface and anionic nature.
What problems are addressed by solid drug nanoparticles
- Oral dosing is the MAIN patient-acceptable route of drug administration
- Many drug compounds have low water- solubility
- Low bioavailability (% of drug that enters the systemic circulation after oral dose)
- Low permeation across key barriers, eg: • Gut-blood
- Blood-brain
- Difficult to formulate
- Difficult to dose
- Poor distribution in target disease areas
What is meant by intestinal permeability?
Intestinal permeability = how much drug goes through the gut after an oral dose
How do drugs. permeate gut to absorb into the blood
Through the small intestine
- has mucosa layer
- Sub mucosa layer
Inside small intestine is actually a huge surface area - due to villi - direct route for exchange with the blood system
Structure of villus:
- cells control what goes from gut into the blood
- what comes from the blood to the gut
-Active transfort mechanism - taking nutrients from the gut & moving into the blood stream & also identifying species not beneficial to the body
What are the routes of drug uptake?
Gut to blood: Passive and active process
• Absorption/permeation
Blood to gut: Active process
• Against concentration gradient
• “Efflux”
What are the processes for making solid drug nanoparticles?
– Liquid processes (‘bottom-up’ techniques)
• Nanoprecipitation
• Emulsion manipulation
– Solid processing (‘top-down’ techniques; attrition)
• Homogenisation
• Nanomilling
How are SDNs made by nanomilling?
A) Solid technique - take particle dispersion & smash it up - milling
Nanomilling - milling beads break particles into smaller pieces - must have stabilisers (polymers/surfactants)
- Grinding of large particles using solid (metal or ceramic) beads and mechanical agitation
- Plates and beads lead to impact on large drug particles and breaking up into smaller particles
- Liquid MUST have stabilisers (polymers/surfactants) present to stabilise new active surfaces
How are SDNs made by high pressure homogenisation?
- Particle dispersion of bigger particles
- move valve up - force liquid under high pressure though a tiny gap - smashes particles into smaller ones
- Caviation forces are dramatic
- Stabilisers required to protect particle
- As previous for solid lipid nanoparticles
- Solid slurry is forced through small gap between valve and seat
- High pressure generated
- Cavitation forces break apart large particles
- Stabilisers required
How are SDNs made by emulsion processing?
Emulsion processing:
- Dissolved drug into an oil droplet & then evaporate solvent ot of emulsion after its made
- when theres not solvent left - only drug - drug particle is made
- As previous for solid lipid nanoparticles
- Solution of drug in water immiscible solvent
- High shear mixing and evaporation of solvent to precipitate or crystallise drug compound.
- Stabilisers for emulsion used to stabilise final dispersion
How are SDNs made by Nanoprecipitation?
Nanoprecipitation of Polymers/Drug
- into aqueous solution of stabilisers
- Ends p with particle dispersion
- As previous for polymer nanoprecipitation
- Drug dissolved in water miscible solvent
- Added to water (anti solvent)
- Stabilisers in water to prevent macrophase separation
What is attrition?
Breaking material into pieces
What are the problems with breaking large solid particles into smaller ones?
Surface tension between air & water - air is hydrophobic - surfaces interacting cohesive
- Hydrophobic drug particles in water - similar solid-liquid interfacial tension - same tension observed
- Particles of hydrophobic drug - with interfacial energy
What happens during attrition methods if surfactants aren’t present?
- when smashed into 2 bits - create more surface energy - put energy into the system
- Increase in interfacial energy of system similar energy input to create new surface
- Break again: generated even more surface, created more interfacial energy
- If don’t stabilise the surfaces - the system will try and minimise the interfacial energy by reforming the starting materials - will aggregate
What is the surface energy of a material?
Energy required to create a unit area of new surface
What happens when surfactants are used in attrition methods?
The presence of surfactants and polymers in the aqueous phase minimises the interfacial energy to prevent aggregation
What factors affect the selection of surfactants for attrition?
Stabiliser: needs to interact with particle and bind efficiently to prevent aggregation
- Drug chemistry (surface interactions)
- Concentrations of polymer and surfactant
- Balance of entropy loss and total thermodynamic gain
What components are in the final total drug formulation?
- Drug
- Surfactant
- Polymer
- Inorganic filler
- Drying agent
- Antimicrobial
What are the advantages/ limitations of nanomilling?
• Milling low melting point materials not possible (energy and forces involved) - because milling generates a lot of heat -Need to have melting point >80-90 deg • Must be very poorly soluble drug • Many cycles may be needed • Possible to mill the nano-mill so potential for contamination from metal or ceramic • Drug cannot be water or heat sensitive (hydrolytic stability) (if compound hydrolytically unstable - heat and presence of water leads to degradation of drug compound)
