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MICROSPHERES

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Introduction

Advantages

Polymers used for preparation

General Method of Preparation

Characterization of Microspheres.

Application of Microspheres

• DEFINITION :

• Microspheres can be defined as solid, approximately spherical particles ranging in size from 1 to 1000 µm.

• They are made up of polymeric substance in which the drug is dispersed through out the microspheres matrix.

• They are made from polymeric, waxy, or other protective materials such as starches, gums, proteins, fats and waxes and used as drug carrier matrices for drug delivery.

• related terms - “microbeads” and “beads” are used alternatively

• Rationale for Drug Encapsulation into Microparticles

iProduction of novel product

• Protection of the product from the surrounding environment, hence improving the shelf life of the product and stability of the system

• Protection of environment from product, where active core material is hazardous or toxic

• Separation of components, allowing control of incompatibility of components

• Control rate of release of core material, by rupture of polymer wall e.g. by impact or long acting sustained release

• Masking undesired properties of active component e.g. odour, taste

• Formation of solid systems e.g. conversion of liquid components to free flowing powders

• Targeting of site of release of active material

• Examples on polymers used in microspheres CDDS

• 1- Chitosan.

• 2-Gelatine .

• 3-Polyadipic anhydride

• 4-Gellan- gum .

• 5-Polypeptide .

• 6-Albumin .

• 7-Poly lactic acid (PLA) .

• 8-Poly lactic - co- glycolic acid (PLGA) .

• ADVANTAGES OF MICROSPHERES IN DRUG DELIVERY

• 1. Controlled release delivery:

• microspheres are used to control drug release rates thereby decreasing toxic side effects, and eliminating the inconvenience of repeated injections.

• Products in the market for control drug delivery:

• Luproline (Lupron Depot) (depote susp.7.5 mg, 4 months, prostate cancer)

• Goseriline acetate (Zoladex) in the United States (prostate cancer and breast Cancer)

• 2.Protein/Peptide Stability :

• The success of proteins to be prepared as pharmaceutical products is to have an efficient drug delivery system that allows the protein drugs to gain access to their target sites at the right time and for the proper duration.

• Microspheres help to protect proteins because they are not allowed to react with anything until the polymer is degraded, thus minimizing the contact with solutions that could cause the proteins to react.

• Examples:

• 1- Albumine .

• 2- Vaccines (HI), diphtheria toxoid (DT), tetanus toxoid (TT), and pertussis toxin (PT) in poly (lactic acid) and PLGA microspheres were prepared by spray drying, all antigens were found to maintain their immunogenicity.

• 3- lyzozymes

• 3.Drug targeting :

• Drug targeting could be the greatest advantage of microspheres.

• Most drugs are targeted in the body to give desired results either in specific tissues or organs.

• A-Passive Targeting: :

• Passive targeting depends on the size of microspheres.

• Eg: lung’s capillaries will let the passage of particles less than seven microns (micrometers) through.

• If one wants the drug to be released into the lungs then the correct size would be around ten microns since they would then be captured in the capillaries . Eg. Carboplatine microspheres .

• B- Active targeting: :

• Intigrin, Lectin, immunoglobulins, lipoproteins, monoclonal antibodies, specific peptides and receptor antagonists were all used as ligands conjugated with microsperes as leading molecules for precise targeting

• 4. Gene delivery:

• Encapsulation of therapeutic agents such as DNA in microspheres protects the agent from enzymatic degradation, enhances tissue specificity due to localized delivery, eliminates the need for multiple administration, and allows for controlled and sustained delivery

• 5. Microspheres in diagnostic materials :

• Gamma emitters such as 99Tc and 131I have been incorporated with microspheres for diagnostic purposes.

• Radio labeling of microspheres is usually achieved either during or after their preparations.

• RELEASE OF DRUG FROM MICROSPHERES :

• Microspheres belong to the monolithic system which refers to a rate controlling polymer matrix through which the drug is dissolved or dispersed.

• a reservoir device consists of shell like dosage form with the drug contained within a rate controlling membrane.

• In non biodegradable polymer: the drug is released by dissolution into the polymer and then diffusion through the polymer wall .

• Eg: levonorgestrel (Norplant®) 5-year contraceptive delivery system

• Biodegradable polymers: The in vivo elimination time is determined by, the nature of the polymer chemical linkage, the solubility of the degradation products, the size, shape and density of the device, the drug and additive content, the molecular weight of the polymer, and the implantation site.

• Eg: DURIN It has successfully achieved controlled, zero-order drug release for up to 6 months in vivo. Because of the broad range of physical properties and degradation times that can be designed into biodegradable polyesters

• DURIN implants can deliver a wide variety of drugs including both hydrophobic and hydrophilic compounds as well as small and large molecules.

• 3- Different patterns of release could be achieved through other modifications as: coating of microspheres with other polymer for further prolongation of release time.

• Eg:algenate-polyethylenimine coated alginate microspheres loaded with furosemide.

• The membrane acted as a physical barrier to drug release from the beads. Alginate coating of algenate-polyethylenimine beads further prolonged the release of the drug by increasing membrane thickness and reducing swelling of the beads possibly by blocking the surface pores.

• 4- Another approach of modifying the release time of drug from microspheres is by using blend of polymers with different properties like the use of blend of PLGA and polyoxyethylene .

• PLGA microspheres made of different molecular weights of the polymer have been prepared.

• Three molecular weights (6,000, 30,000 and 41,000) were blended to achieve long term release where the higher molecular weights degrade more slowly.

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POLYMER Used for Microspheres preparation: Biodegradable Polymers:

• Lactides&Glycolides and their copolymer

• Polyanhydrides

• Polycynoacrylates

• NON Biodegradable Polymers:

• Poly methyl methacrylate

• Acrolein

• Epoxy Polymer

• Glycidyl methacrylate

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• Preparation of microspheres should satisfy certain criteria

• Ability to incorporate high concs of the drug

• Stability of the preparation after synthesis with a clinically acceptable shelf life

• Controllable particle size and dispersibility in aq vehicles for injection

• Release of active agent with good control over a wide time scale

• Biocompatibility with a controllable biodegradability

• Susceptibility to chemical modification

• The administration parameters that can be satisfactorily controlled are as follow:

• Taste and odour masking

• Conversion of oil and other liquids, facilitating ease of handling

• Protection of the drug from the environment

• Delay of volatilisation

• Freedom from incompatibilities between drug and excipients, especially the buffers

• Improvement of flow properties

• Dispersion of water insoluble substance in aqueous media

• Production of sustained release, controlled release and targeted medication

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General Method of Preparation:

a) Single Emulsion Technique

b) Double Emulsion Technique

c) Polymerization Technique

d) Phase Separation Coacervation

e) Spray drying &spray coating

f) Solvent Extraction

• Single Emulsion Technique

• natural polymers i.e. those of proteins and carbohydrates are prepared by single emulsion technique.

• The natural polymers are dissolved or dispersed in aqueous medium followed by dispersion in non-aqueous medium like oil.

• Next cross linking of the dispersed globule is carried out.

• The cross linking can be achieved either by means of heat or by using the chemical cross linkers.

• A) Single Emulsion Technique

Aqueous solution /suspension of polymer(natural polymer)

stirring / sonication

dispersion in organic phase oil/chcl3

cross linking

Heat denaturation chemical crosslinking

(by adding dispersion (butanol,HCHO,Glutaraldehyde)

To heated oil)

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Microspheres in org.phase Microspheres in org.phase

Centrifugation,washing,separation

Microspheres

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Double Emulsion TechniqueInvolves the formation of the multipleemulsions or the double emulsion of type w/o/w and is best suited to water soluble drugs, peptides, proteins and the vaccines.

• This method can be used with both the natural as well as synthetic polymers.

• The aqueous protein solution is dispersed in a lipophilic organic continuous phase. This protein solution may contain the active constituents.

• The primary emulsion is subjected then to the homogenization or the sonication before addition to the aqueous solution of the poly vinyl alcohol (PVA).

• This results in the formation of a double emulsion.

• The emulsion is then subjected to solvent removal either by solvent evaporation or by solvent extraction.

• a number of hydrophilic drugs like leutinizing hormone releasing hormone (LH-RH) agonist, vaccines, proteins/peptides and conventional molecules are successfully incorporated into the microspheres using the method of double emulsion solvent evaporation/ extraction.

• B) Double Emulsion Technique

Aqueous solution of polymer

dispersion in oil/organic phase, vigorous homogenisation(sonication)

Primary emulsion(w/o)

addition of aqueous solution of PVA

W/O/W multiple emulsion

addition of large aq. Phase

MICROSPHERES in solution

• Separation,washing, drying

MICROSPHERES

• C)Polymerization

A) Normal Polymerization:

Normal Polymerization is done by bulk, suspension, pption,emulsion and polymerization process.

• C)Polymerization

1. Bulk polymerization:

Monomer Bioactive material Initiator

Heated to initiate polymerization

Initiator accelerate rate of reaction

Polymer(Block)

Moulded/fragmented

MICROSPHERES

• B)Suspension polymerization

• Suspension polymerization also referred as bead or pearl polymerization.

• Here it is carried out by heating the monomer or mixture of monomers as droplets dispersion in a continuous aqueous phase. The droplets may also contain an initiator and other additives.

• 2)Suspension polymerization

Monomer Bioactive material Initiator

Dispersion in water and stabilizer

Droplets

Vigorous ,Agitation Polymerization by Heat

Hardened microspheres

Separation&Drying

MICROSP HERES

• 3) Emulsion Polymerisation

Monomer/ Aq.Solution of NaOH with

Bioactive material Initiator,Surfactant abovecmcStabiliser

Dispersion with vigorous stirring

Micellar sol. Of Polymer in aqueous medium

Polymerization

Microspheres formation

MICROSPHERES

• B u lk polymerization has an advantage of formation of pure polymers.

• Interfacial polymerization:

• It involves the reaction of various monomers at the interface between the two immiscible liquid phases to form a film of polymer that essentially envelops the dispersed phase.

• Phase separation coacervation

• This process is based on the principle of decreasing the solubility of the polymer in organic phase to affect the formation of polymer rich phase called the coacervates.

• In this method, the drug particles are dispersed in a solution of the polymer and an incompatible polymer is added to the system which makes first polymer to phase separate and engulf the drug particles.

• Addition of non-solvent results in the solidification of polymer. Poly lactic acid (PLA) microspheres have been prepared by this method by using butadiene as incompatible polymer.

• The process variables are very important since the rate of achieving the coacervates determines the distribution of the polymer film, the particle size and agglomeration of the formed particles.

• The agglomeration must be avoided by stirring the suspension using a suitable speed stirrer since as the process of microspheres formation begins the formed polymerize globules start to stick and form the agglomerates.

• D)Phase Sepration Coacervation

Aq./organic solution of polymer

Drug dispersed or dissolved in the polymer solution

Phase sepration by salt addition, non solvent addition

add. Incompatible polymer,etc

Polymer rich globules

Hardening

microspheres in aqu./organic phase

sepration/drying

MICROSPHERES

• Spray drying

• drying of the mist of the polymer and drug in the air. Depending upon the removal of the solvent or cooling of the solution, the two processes are named spray drying and spray congealing respectively.

• The polymer is first dissolved in a suitable volatile organic solvent such as dichloromethane, acetone, etc.

• The drug in the solid form is then dispersed in the polymer solution under high speed homogenization.

• This dispersion is then atomized in a stream of hot air. The atomization leads to the formation of the small droplets or the fine mist from which the solvent evaporates instantaneously leading the formation of the microspheres in a size range 1-100 μm.

• Microparticles are separated from the hot air by means of the cyclone separator while the traces of solvent are removed by vacuum drying.

• One of the major advantages of the process is feasibility of operation under aseptic conditions.

• The spray drying process is used to encapsulate various penicillins

• Very rapid solvent evaporation, however leads to the formation of porous Microparticles.

• E)Spray Drying

Polymer dissolve in volatile organic solvent(acetone,dichloromethane)

Drug dispersed in polymer solution under

high speed homogenization

Atomized in a stream of hot air

Due to solvent evaporation small droplet or fine mist form

Leads to formation of Microspheres

Microspheres separated from hot air by cyclone separator,Trace of solvent are removed by vacuum drying

• F)Solvent Extraction

Drug is dispersed in organic solvent (water miscible organic solvent such as Isopropanol)

Polymer in organic solvent

Organic phase is removed by extraction with water (This process decreasing hardening time for microspheres)

Hardened microspheres

• Characterization of Microspheres

Partical size & shape – LM, SEM, CLSM

• Both can be used to determine the shape and outer structure of microparticles.

• LM provides a control over coating parameters in case of double walled microspheres.

• The microspheres structures can be visualized before and after coating and the change can be measured microscopically.

• SEM provides higher resolution in contrast to the LM.

• SEM allows investigations of the microspheres surfaces and after particles are cross-sectioned, it can also be used for the investigation of double walled systems.

• Confocal fluorescence microscopy is used for the structure characterization of multiple walled microspheres – not only on surface also inside the particles, if material is sufficiently transparent

Release study

Usually carried out in phosphate saline buffer ph 7.4.

Two method 1) Rotating paddle dissolution

apparatus.

2) Dialysis method

Iso electric point

Microelectrophoresis apparatus is used to measure elecrophoretic mobility of microspheres from which isoelectric point can be determine.

It can be correlated to surface charge or ion adsorption of microspheres.

Density Determination

Density measured by using a multivolume pychnometer..

• Capture efficiency:

- Percent entrapment can be determined by allowing washed microspheres to lyse

- % entrapment = actual content/theoretical content X100

• Angle of contact:

• The angle of contact is measured to determine the wetting property of a micro particulate carrier. It determines the nature of microspheres in terms of hydrophilicity or hydrophobicity.

• specific to solid and affected by the presence of the adsorbed component.

• The angle of contact is measured at the solid/air/water interface.

• Infrared Spectroscopy:

• FT-IR is used to determine the degradation of the polymeric matrix of the carrier system.

• The IR beam passing through the ATR cell reflected many times through the sample to provide IR spectra mainly of surface material.

• The ATRFTIRprovides information about the surface composition of the microspheres depending upon manufacturing procedures and conditions.

• Surface carboxylic acid residue:

• The surface carboxylic acid residue is measured by using radioactive glycine.

• Surface amino acid residue:

• Surface associated amino acid residue is determined by the radioactive c14-acetic acid conjugate.

• Application of Microspheres

Microspheres in Vaccine delivery

• vaccine is protection against the micro organism or its toxic product.

• An ideal vaccine must fulfil the requirement of efficacy, safety, convenience in application and cost.

• Biodegradable delivery systems for vaccines that are given by parenteral route may overcome the shortcoming of the conventional vaccines

• Targeting of Drug

• The therapeutic efficacy of the drug relies on its access and specific interaction with its receptors.

• The ability to leave the pool in reproducible, efficient and specific manner is center to drug action mediated by use of a carrier system.

• Ocular

• Intranasal – bioadhesive microspheres

• Oral

• Monoclonal antibodies mediatedmicrospheres targeting

• Immunomicrospheres.

• This targeting is a method used to achieve selective targeting to the specific sites.

• Monoclonal antibodies are extremely specific molecules. This extreme specificity of monoclonal antibodies (Mabs) can be utilized to target microspheres loaded bioactive molecules to selected sites.

• Mabs can be directly attached to the microspheres by means of covalent coupling.

• Imaging

• Various cells, cell lines, tissues and organs can be imaged using radio labelled microspheres.

• The particle size range of microspheres is an important factor in determining the imaging of particular sites.

• The particles injected intravenously apart from the portal vein will become entrapped in the capillary bed of the lungs.

• This phenomenon is exploited for the scintiographic imaging of the tumour masses in lungs using labelled human serum albumin microspheres.

• Topical porous microspheres

• Microsponges are porous microspheres having interconnected voids of particle size range 5-300 μm.

• These microsponges having capacity to entrap wide range of active ingredients

• such as emollients, fragrances, essential oils etc., are used as topical carries system

• further, these porous microspheres with active ingredients can be incorporated into formulations such as creams, lotions and powders.

• Microsponges consist of non collapsible structures with porous surface through which active ingredients are released in a controlled manner

• Chemoembolisation

• Chemoembolisation is an endovascular therapy, which involves the selective arterial embolisation of a tumour together with simultaneous or subsequent local delivery the chemotherapeutic agent.

• The theoretical advantage is that such embolisations will not only provide vascular occlusion but will bring about sustained therapeutic levels of chemotherapeutics in the areas of the tumour.

• Surface modified microspheres

• Different approaches have been utilized to change the surface properties of carriers to protect them against phagocytic clearance and to alter their body distribution patterns .

• The adsorption of the poloxamer on the surface of the polystyrene, polyester or poly methyl methacrylate microspheres renders them more hydrophilic and hence decrease their MPS uptake.

• Protein microspheres covalently modified by PEG derivatives show decreased immunogenicity and clearance.

Magnetic Microspheres – Magnetite containing matrices

allows high local conc of therapeutic agents