liposomal drug delivery

what is interfacial tension?

• the force per unit length existing at an interface between 2 immiscible liquids

• if one liquid with air on top, there is surface tension

• to move a molecule from the inner layers to the surface, work needs to be done against the force of surface tension

• in other words, each molecule near the surface of a liquid possesses a certain excess of potential energy as compared to the molecules in the bulk of the liquid

• to increase the surface of the liquid without any additional changes in the liquid state, in particular without changes in liquid temperature, work must be done against surface tension

what is a liposome?

• structure composed of lipid bilayers that encloses a volume of aqueous liquid (buffer)
• used as a drug carrier as well as a model for biological membranes

what are the types and sizes of liposomes?

• small unilamellar vesicles (SUVs): 0.025-0.05µm

• large unilamellar vesicles (LUVs): 0.1µm

• multilamellar vesicles (MLVs): 0.05-10µm

• sizes vary with components used, storage, method of preparation and type of liposome preparation

what are materials used for liposomes?

• phospholipids: glycerol-containing often used, zwitterionic @ pH 7 (phosphatidylcholine/lecithin, phosphatidylinositol, phosphatidyl serine)
• steroids (cholesterol)
• imparting charge to liposomes: stearylamine for +ve, dicetyl phosphate for -ve

what are the important parameters of liposomal drug delivery?

• captured volume (internal trapped volume): volume enclosed by given amount of lipid with units of L per mol of total lipid

• encapsulation efficiency: fraction of drug sequestered by liposomes (e.g. MRVs better than SUVs)

what are the advantages of liposomes?

• drug carrier made of natural materials: do not elicit immune response in body
• carrier that can encapsulate many drugs (hydrophilic and lipophilic)
• can vary by seize by method of preparation
• can target to RES as well as other areas of body (monclonals)
• carrier protects drug against degradation
• the liposomes are degraded to non-toxic by-products
• small enough to deliver IV

what are the disadvantages of liposomes?

• destroyed by RES and targeting sometimes difficult to other areas of the body
• preparation and reproducibility are a challenge
• liposomes tend to be "leaky" with small MW hydrophilic drugs
• components of liposome liable to peroxidation (place under N), hydrolysis, and oxidation
• can add antioxidants (⍺-tocopherol) and freeze dry for reconstitution

what are the methods of liposome preparation?

• multilamellar vesicle formation
• sonication for small unilamellar vesicles
• SUVs by extrusion
• SUVs using detergent dialysis

what is the preparation of MLVs?

• rotary evaporation: phospholipid dissolved in chloroform and EtOH
• white film of large surface area
• hydration stage: aqueous buffer added
• MLVs form spontaneously: large range of sizes
• sonicate/extrude to get SUVs

what are SUVs using dialysis?

• gives more control over size
• detergents such as bile salts, used and triton X-100 to solubilize
• after sonication with detergent, place in equipment and dialyze
• suspension goes from milky to clear as micelles turn to liposomes

what are the techniques for physical characterization?

• size: electron microscopy, light scattering, ultracentrifugation

• # of lamellae: NMR, small angle X-ray scattering

• bilayer fluidity: fluorescent probes

• charge: microelectrophoresis

• encapsulated volume:: encapsulation of water-soluble markers

what are the types of electron micrograph of LUVs?

negative staining, thin section, freeze fracture

what is amphotericin?

• often higher dose is needed to control therapy of immunocompromised patients (e.g. AIDs and transplantation)
• issues of toxicity with these patients, but drug is still best one in the clinic
• amphipathic molecule that has higher preference for ergosterol of fungal membranes than for cholesterol of human membranes

what is amphotericin B?

• polyene macrolife
• insoluble in water and solublizied by sodium deoxycholate

what are the formulations available for amphotericin B?

• fungizone: ampho as deoxycholate; final product is colloidal dispersion of micelles; all (-)-charged
• amphotec/ABCD: ampho + Na cholesteryl sulfate; (-)-charged
• ambisome: all SUVs (-)-charged, lyophilized product; need water/dextrose to reconstitute
• abelcet/ABLC: not liposomes; lowest toxicity and very expensive

what are stealth liposomes?

• special formulation to increase the half-life of liposomes
• doxorubicin HCl (doxil) for IV formulated this way
• "pegylated" liposomes: binding of methoxy polyethylene glycol on surface of liposomes
• circulate "undetected" by mononuclear phagocyte system in body

what is nystatin?

• topical antifungal agent
• toxic when administered systemically
• systemic formulation
• may be used for salvage therapy: potential agent in cases which do not respond to currently available antifungal drugs
• in vivo efficacy: candidiasis, aspergillosis

what are lipid cochleates?

• obtained as a precipitate after the addition of Calcium cations to (-)-charged performed liposomes
• formed mainly out of phosphatidylserine and calcium: calcium causes liposome made from PS to aggregate and fuse
• Ca brings the bilayers together through partial dehydration and cross-linking of opposing molecules of PS
• success in entrapping amphotericin B
• stable for oral delivery of drugs

what are cochleate delivery vehicles?

• stable-phospholipid-calcium precipitates

• composed of naturally occurring materials

• multilayered structure, containing little or no internal aqueous space

• resistance to degradation in GI tract

• increases shelf life stability

• very inexpensive

what is the process for making cochleates?

1. calcium interaction with (-)-charged lipid displaces H2O and condenses lipid
2. calcium lipid sheets "roll up" into cochleate strucutres, to minimize contact with water, excluding H2O and O2

what is the mechanism of cochleate formation?

• calcium ion maintain the cochleate in its rolled form, bridging each successive layer through ionic interactions
• hydrophobic and amphipathic drug molecules can easily insert themselves into the lipid bilayer membranes
• (+) and (-) charged molecules can be induced into the calcium residing polar head region of the lipi bilayers

what is the membrane fusion of nanocochleates?

• perturbation and reordering of the cell membrane is induced, resulting in a fusion event between the outer layer of the cochleate and the cell membrane
• the fusion is induced by the formation of Ca(PS)2 complex and results in the delivery of a small amount of siRNA into the cytoplasm of the target cell
• the cochleate could then break free of the cell and be available for another fusion event, either with this or another cell
• alternatively, with phagocytic cells, the cochleate may be taken up by endocytosis, and the siRNA leaks out from it within the endocytic vesicle

what is the stability of cochleates?

• free of water and resistant to the oxygen penetration internally, thus imparting shelf-life stability to the drugs by protecting them from the 2 most important factors affecting their physical and chemical stability
• can be stored in buffer at 4℃ or lyophilized to a powder and stored at room temperature without altering the structure and the intactness of the encochleated molecules

what is CAMB?

compared to 2 commercially available amphotericin B preparations, DAMB and LAM

what are nanoparticles in drug delivery?

• metal-based: metal used to anchor the drugs
• lipid-based: polar groups attached to a hydrophobic tail
• polymer-based: amphiphilic copolymers
• biological: viruses
• drugs and the molecules self-assemble into a nanoparticle under proper conditions in a liquid medium

what are the challenges to nanoparticles in drug delivery?

stabilization, extended circulation, targeting

what is micellar delivery?

• micelle carrying drug molecules in its core

• polycation-DNA particle in the morphology of a micelle

• endocytosis and transduction deliver the micelle-carried drug into the cell

what is the solubility of paclitaxel PTX?

• by incorporation into tyrospheres, solubility is increase ~5000-fold
• increased solubility creates a larger driving force for skin permeation

what is paclitaxel?

• for treatment of psoriasis

• topical gel was demonstrated to be safe and well-tolerated when applied once/twice daily to either normal/psoriatic skin

• problem: toxicity and very low aqueous solubility

what is skin distribution analysis?

• tyrospheres delivery payload to the basal layer of the epidermis but not into the dermis
• disease-state biopsy skin may be a better model for delivery of actives
• skin sectioning and fluorescence visualization

what is the quantification of delivered paclitaxel by LC-MS?

• skin distribution analysis utilizing cadaver skin shows that tyrospheres deliver significant amounts of drug into the epidermis
• minimal drug found in dermis and receptor fluid → topical delivery
• problems of low solubility and toxicity solved !!