Microsphers For Colonic Drug Delivery

Published Date: 02 Nov 2017

Conventional medication systems that require multi-dose therapy are not without problems. With a view to overcome these problems, the current trend in pharmaceutical research is to design and develop new formulations, thereby enhancing the therapeutic efficacy of existing drugs. Moreover, the impetus for research into drug delivery can be attributed to the exorbitant cost and large development period involved in new drug development with concomitant recognition of the therapeutic advantages of controlled and site specific delivery.

The goal in drug delivery research is to develop formulations to meet therapeutic needs relating to particular pathological conditions. One such is targeting to the colon. Colon as a site offers distinct advantages on account of a near neutral pH, a much longer transit time, reduced digestive enzymatic activity, and a much greater responsiveness to absorption enhancers1.

Colon also offers site-specific delivery of drugs to lower parts of the GI tract is advantageous for localized treatment of several colonic diseases, mainly inflammatory bowel diseases (Crohn’s disease and ulcerative colitis), irritable bowel syndrome, and colon cancer. Other potential applications of colonic delivery include chronotherapy, prophylaxis of colon cancer and treatment of nicotine addiction. Targeting of drugs to the colon by the oral route could be achieved by different approaches including matrix and coated systems, for which the drug release is controlled by the gastrointestinal pH, transit times or intestinal flora. The method by which the drug release will be triggered by the colonic flora appears to be more interesting with regard to the selectivity. A number of synthetic azo polymers and natural or modified polysaccharides (chondroitin sulphate, guar gum, xanthan gum, locust gum, inulin, dextrans, starch, amylose,pectins) degraded by the human colonic flora, have thus been investigated as colonic drug delivery carriers.2

1.1 COLON SPECIFIC DRUG DELIVERY: 3

During the last decade there has been interest in developing site specific formulations for targeting drug delivery to the colon. The colon is a site where both local and systemic drug delivery can take place. A local means of drug delivery could allow topical treatment of amoebiasis, irritable bowel disease, inflammatory bowel disease e.g. ulcerative colitis or crohn"s disease. Such conditions are usually treated with glucocorticoids, salphasalazine and Mebeverine.

Treatment might me more effective if the drug substances are were targeted directly on the site of action in the colon. Lower doses might be adequate and, if so, systemic side effects might be reduced. A number of other serious diseases of the colon, e.g. colorectal cancer, might also be capable of being treated more effectively if drugs were targeted on the colon. Site specific means of drug delivery could also allow oral administration of peptide and protein drugs, which normally become inactivated in the upper parts of the gastrointestinal tract. Vaccines, insulin and growth hormone are example of such candidates. However, the permeability of the epithelium of the colon to peptide and protein drugs is fairly poor, and bioavailability is usually very slow. Colon specific systems could also be used in conditions in which a diurnal rhythm is evident. E.g. asthma, rheumatoid arthritis, ulcer disease and ischemic heart disease. The incidence of asthma attacks is, for example greatest during the early hours of the morning because dosage forms remain longer in the large intestine than in the small intestine , colon specific formulations could be used to prolong drug delivery.

Rectal administration offers the shortest route to targeting drugs on the colon. However, reaching the proximal part of the colon via rectal administration is difficult. Rectal administration can also be uncomfortable for the patient, and compliance may be less than optimal. There are several ways, in which drugs can be targeted on the colon when they are given by mouth. In the time dependent formulations the drug concerned is released during the period of gastrointestinal transit time. Release from formulations that contain pH dependent polymers takes place on the basis that pH is higher in the terminal ileum and colon than in the upper parts of the gastrointestinal tract. The colon is also home to large numbers of bacteria of many kinds. Prodrugs and dosage forms from which drug release is triggered by the action of colonic bacterial enzymes have therefore been devised.

1.2 STRUCTURE AND FUNCTION OF COLON:4

The colon forms the lower part of the GIT and extends from the ileo-caecaal junction to the anus. The colon is upper five feet of the large intestine and the rectum is the lower six inches. While the colon is mainly situated in the abdomen, the rectum is primarily a pelvic organ. The colon is cylindrical tube, which is lined by a moist, soft pink lining, called the mucosa; the pathway is called the lumen and is approximately 2-3inches in diameter. The junction of small intestine (ileum) and the colon is in the lower right abdomen. The first portion of the colon is spherical and is called caecum. The appendix hangs off the caecum. The next portion of the colon, in the order in which the contents flow, is the ascending (proximal) colon. The colon then turns to a long horizontal segment, the transverse colon. Beneath the rib cage, the colon turns downward at the splenic flexure, to become the descending (distal) colon.

In the left lower portion of the abdomen, the colon makes an S shaped curve from the hip over the midline known as the sigmoid colon.

The major function of the colon is the consolidation of the intestinal contents into faeces by the absorption of water and electrolytes and to store the faeces until excretion. The absorptive capacity is very high; each day about 2000ml of fluid enters the colon through the ileo-caecal valve from which more than 90% of the fluid is absorbed.

Figure 1 Anatomy of the Colon.

1.2.1. pH in the colon:

The pH is 7.5±0.5 in the terminal ileum. On entry into the colon, the pH decreases to 6.4±0.5. The pH in the mid colon is 6.6±0.8 and in the left colon 7.0±0.7. There is a fall in pH on entry into the colon due to the presence of short chain fatty acids arising from bacterial fermentation of polysaccharides.

E.g. lactose is fermented by the colonic bacteria to produce large amounts of lactic acid resulting in the decrease of pH to 5.0.

S.no

Region

Length(m)

Surface area(m2)

pH

Residence Time

Micro-organisms

(CFU/mL)

1

Oesophagus

0.3

02

6.8

>30sec

Unknown

2

Stomach

0.2

0.2

1.8-2.5

1-5 hr

≤102

3

Duodenum

0.3

0.02

5-6.5

>5min

≤102

4

Jejunum

3

100

6.9

1-2 hr

≤102

5

Ileum

4

100

7.6

2-3 hr

≤102

6

Colon

1.5

3

5.5-7.8

15-48 hr

≤1011

Table No: 1 Anatomical and physiological characteristics of GIT.

1.2.2. Transit of Material in the colon:

The average gastric emptying time is 2 hrs. the presence of food generally increases gastric residence and in some cases with regular feeding, dosage forms have been shown to reside in the stomach for more than 12hrs.

Small intestinal transit time is constant at 3-4hrs.

Colonic transit time is highly variable with a median transit time of 20.9hrs. Smaller units travel through the colon more slowly than larger ones. Hence, additional retention of a dosage form within the colon could perhaps be achieved by the use of a multiparticulate formulation, rather than a large single unit, thus ensuring that it does not pass too rapidly through the colon and be excreted before the entire drug has been released.

1.3. APPROACHES FOR COLON SPECIFIC DRUG DELIVERY:1,5

The different approaches for colon specific drug delivery are

1.3.1. Drug release based on variation of pH:

In the stomach the pH ranges between 1and 2 during fasting but increases in postprandial state. The pH is about 6.5 in the proximal small intestine and about 7.5 in the distal small intestine. From the ileum to the colon pH declines significantly. It is about 6.4 in the caecum. However, pH values as low as 5.7 have been measured in the ascending colon in healthy volunteers. The pH in the transverse colon is 6.6, in the descending colon 7.0. Use of pH dependent polymers is based on the difference in pH levels. The polymers described as pH dependent in colon specific drug delivery are insoluble at low pH levels but become increasingly soluble at pH rises. There are various problems with this approach, however. The pH in the gastrointestinal tract varies between and within individuals. It is affected by diet and disease, for example during acute stage of inflammatory bowel disease the colonic pH has been found to be significantly lower than normal. In ulcerative colitis pH values between 2.3 and 4.7 have been measured in the proximal parts of the colon. Although a pH dependent polymer can protect a formulation in the stomach and proximal small intestine, it may start to dissolve even in the lower small intestine, and the site-specifity of formulations can be poor. The decline in pH from the end of the small intestine to the colon can also result in problems. Lengthy lag times at the ileo-caecal junction or rapid transit through the ascending colon can also result in poor site-specifity of enteric coated single unit formulations.

EudragitTM products are pH-dependent methacrylic acid polymers containing carboxyl groups. The number of esterified carboxyl groups affects the pH level at which dissolution takes place. EudragitTM S is soluble in pH 7 and above and EudragitTM L in pH 6 and above.

EudragitTM S has also been used in combination with another methacrylic acid copolymer, EudragitTM L 100-55, in colon targeted systems to regulate drug delivery. Dissolution studies showed that drug release profiles from enteric coated single unit tablets could be altered in vitro by changing the ratios of the polymers, in the pH range 5.5 to 7.0.

Hydroxypropyl Methylcellulose acetate succinate has been included in outer layers of single unit press coated tablets with a view to preventing drug release in the stomach and small intestine.

Polymer

Threshold pH

Eudragit L-100

6.0

Eudragit S-100

7.0

Eudragit L-30D

5.6

Inulin

6.8

Shellac

5.5

Pectin

8.2

Hydroxy propyl methylcellulose phthalate

4.5-4.8

Hydroxy propyl methylcellulose phthalate 50

5.2

Hydroxy propyl methylcellulose phthalate 55

5.4

Cellulose acetate trimelliate

4.8

Cellulose acetate phthalate

5.0

Table no: 2 Threshold pH of commonly used polymers

1.3.2. Drug release based on gastrointestinal transit time:

The time of transit through the small intestine is independent of formulation. It has been found that both large single-unit formulations and small multi-unit formulations take three to four hours to pass through the small intestine. Transit time through the small intestine is unaffected by particle size or density, or by the composition of meals. Because the time taken by formulations to leave the stomach varies greatly the time of arrival of a formulation in the colon cannot be accurately predicted. However, using systems that are protected in the stomach can minimize the effects of variation in gastric residence time and the drug release can be targeted on the colon by means of formulations that release the drug they contain a certain time after gastric emptying. Such formulations pass through the stomach and small intestine and the drug is then released at the end of the small intestine and the beginning of the colon. Accordingly, formulations that dependent for drug release on time of transit through the small intestine also usually that depend for the drug release on changes in the pH in the gastrointestinal tract. Transit times through the colon that are faster than normal have been observed in patients with irritable bowel syndrome, diarrhea and ulcerative colitis. Systems that depend on gastrointestinal transit time for drug release are therefore not ideal for drug delivery in the colon for treatment of colon related diseases.

A drug delivery system (pulsicapTM), from which there is rapid drug release after a lag time, has been developed to allow release of drug in the large intestine. The system involves an insoluble capsule body with a hydro gel plug. The plug is ejected from the capsule when it has swelled after a particular lag-time. A release profile is characterized by a period during which there is no release followed by rapid and complete drug release. Releasing using this system was found to be reproducible in vitro and in vivo. When gastrointestinal transit of the formulations were observed by means of gammascintigraphy it was found in six of eight subjects that the devised reached the colon before the drug was released. The formulation had been administered with the subjects in a fasting state.

1.3.3. Drug release based on the presence of colonic micro flora:

Both anaerobic and aerobic microorganisms inhabit the human gastrointestinal tract. In the small intestine the micro flora is mainly aerobic, but in the large intestine it is anaerobic. About 400 bacterial species and some fungi have been found in the colon. Most bacteria inhabit in the proximal areas of the large intestine, where energy sources are greatest. Carbohydrates arriving from the small intestine from the main source of nourishment for bacteria in the colon. The carbohydrates are split into short-chain fatty acids, carbon dioxide and other products by the enzymes glycosidase and polysaccharides. Protease activity in the colon can result in cleavage of proteins and peptides. In the proximal colon the pH is lower than at the end of the small bowel because of the presence of short-chain fatty acids and other fermentation products. The presence of colonic micro flora has formed a basis for development of colon-specific drug delivery systems. Interest has focused primarily on azo reduction and hydrolysis of glycoside bonds. However, the colonic micro flora varies substantially between and within individuals, reflecting diet, age and disease. Such variations need to be taken into account in developing colon-specific formulations depending on the presence of colonic micro flora.

Sulphasalazine, used in the treatment of ulcerative colitis and Crohn’s disease, is a colon-specific prodrug. In the colon sulphasalzine is split by bacterial azo reduction into 5-aminosalicylic acid sulphapyridine. Polymers and polyamides containing azo groups have been used to convey 5-aminosalicylic acid to the large intestine. Colon targeting by means of azo polymers is associated with many problems. Microbial degradation of azo polymers is usually slow, and drug delivery can be incomplete and irregular.

The colonic micro flora produces a wide range of glycosidases capable of hydrolyzing glycosides and polysaccharides. Glucoronides which are less subject to hydrolysis in the small intestine than glycosides have also been used as drug carriers. The advantage of polysaccharides is that they are easily available. Disadvantages are that most of polysaccharides it is necessary to ensure that no drug is released until it reaches the colon. Amylose has been used in coatings of colon-specific formulations. Amylose, a major component of starch, swells too much on its own, but amylase-ethyl cellulose coatings have been investigated in connection with targeting of drug release in the colon. Pectin is a polysaccharide, found in the cell wall of plants. It is totally degraded by colonic bacteria but is not digested in the upper gastrointestinal tract. One disadvantage of pectin is its solubility. This can however be adjusted by changing its degree of methoxylation, or preparing calciumpectinate. The film-coating properties of pectin have been improved through use of ethyl cellulose.

Cross-linked guar gum has also been used as drug carrier in matrix tablets. Dextran ester pro drugs have also been investigated as means of transporting drugs to the colon. Chitosan is a high molecular weight polysaccharide that is degraded by colonic micro flora. Insulin and 5-ASA have been administered to rats in enteric-coated chitosan capsules.

1.3.4. Pressure-controlled drug delivery systems:

As a result of peristalsis, higher pressures are encountered in the colon than in the small intestine. Pressure controlled colon delivery capsules have been prepared using ethyl cellulose, which is insoluble in water. In such systems drug release occurs following disintegration of a water-insoluble polymer capsule as a result of pressure in the lumen of the colon. The thickness of the ethyl cellulose membrane is the most important factor for disintegration of the formulation. E.g. pulsicap.

1.4. CAPSULES 6

The Hide

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Enclose medicines in a relatively stable shell known as a capsule. The word capsule is dervied from latin word "capsula" which means small box or container. In more recent times the capsule has been used primarily to describe a solid dosage form which consits of a container usally made of gellatin filled with a medicinal substance.

Capsules are easier to swallow and are used by manufacturers when the drug can’t be compacted into a solid tablet. They are also useful when the drug needs to be mixed with oil or other liquid to aid absorption in the body. It is normally a shell or container made of gelatin that contains the drug. Capsules afford a tasteless, odorless enclosure, convenient for administration of variety of medicaments, which are otherwise difficult to administer. However, aqueous or hydro alcoholic liquids cannot be enclosed in capsules because they dissolve gelatin.

The two main types of capsule are,

Soft gelatin capsule.

Hard gelatin capsule

1.4.1SOFT GELATIN CAPSULES

Soft capsules are a single-unit solid dosage form, consisting of a liquid or semi-solid enveloped by a one-piece hermetically sealed elastic outer shell. They are formed, filled and sealed in one continuous operation, preferably by the rotary die process. Depending on the polymer forming the shell, they can be subdivided into two categories, namely soft gelatin capsules or ‘softgels’ and non-gelatin soft capsules. The majority of soft capsules are made from gelatin owing to its unique physical properties that make it an ideal excipient for the rotary die process.

Composition of Soft gelatin Capsules

The shell of a soft gelatin capsule is composed of gelatin, a plasticizer or a combination of plasticizers and water. In addition, it may contain preservatives, colouring and opacifying agents, flavourings and sweeteners, possibly sugars to impart chewable characteristics to the shell, gastro resistant substances and in special cases even active compounds. The water serves as a solvent to make a molten gelatin mass with a pourable viscosity at 60–70°C. The ratio by weight of water to dry gelatin (W/G) can vary from 0.7 to 1.3, depending on the viscosity of the gelatin being used. After capsule formation, most of the water is removed by drying, leading to finished capsules with a moisture content of 4–10%.

1.4.2. HARD-GELATIN CAPSULES

The hard capsule consists of two separate parts each a semi closed cylinder one part of the cap is slightly larger diameter tan the other which is called the body and is longer the cap fits closely over the body to form a sealed unit The powder inside the capsule contains the active ingredient(s) mixed to different excipients like binders, disintegrant, fillers, glidant and preservatives.

ADVANTAGES:

Capsules offer the following unique advantages as a dosage form:

Better Suited for Cytotoxic /High Potency Drugs than Tablets

Improve Stability with Sensitive Drug Compounds

Enable Combinations of Drugs in One Capsule

Help Address the Issue of Poorly Soluble Compounds

Capsules have popular dosage form because they provide a smooth, slippery,

Easily swallowed and tasteless shell for drug.

Ease to use and portability.

Beneficial for drug having an unpleasant taste and odour

Different Sizes of Capsules

The largest size of capsule is ‘000’ and the smallest ‘5’

Figure No 2 : Different sizes of capsules

Size

Volume(ml)

Locked length(mm)

External diameter(mm)

5

0.13

11.1

4.91

4

0.21

14.3

5.31

3

0.3

15.9

5.82

2

0.37

18

6.35

1

0.5

19.4

6.91

0

0.68

21.7

7.65

00

0.95

23.3

8.53

000

1.37

26.14

9.91

Table No 3: Parameters of empty capsules shells

1.5. Controlled release drug delivery system 7

Oral route still remains the most popular for drug administration by virtue of its convenience to the patient. A sizable portion of orally administered dosage forms, so called conventional, are designed to achieve maximal drug bioavailability by maximizing the rate and extent of absorption. While such dosage forms have been useful, frequent daily administration is necessary, particularly when the drug has a short biological half life. This may result in wide fluctuation in peak and trough steady-state drug levels, which is undesirable for drugs with marginal therapeutic indices. Moreover, patient compliance is likely to be poor when patients need to take their medication three to four times daily on chronic basis.

Fortunately, these short comings have been circumvented with the introduction of controlled release dosage forms. These dosage forms are capable of controlling the rate of drug delivery, leading to more sustained drug levels and hence therapeutic action. During past few decades, significant advance have been made in the area of controlled release as evidenced by an increasing number of patents, publication, as well as commercial controlled release products for the delivery of variety of pharmaceutical compounds. With a controlled release formulation a predictable and reproducible release rate can be achieved, at the target site for desired duration. This results in optimum biological response, prolonged efficacy, decreased toxicity as well as reduction in required dose levels as compared to the conventional mode of delivery.

Figure no 3: Plasma drug concentration profiles for conventional tablet or capsule, a sustained release and a zero order controlled release formulation.

Advantages of controlled release delivery system:8,9,10

Controlled release technology may provide increased clinical value as well as

extended product life.

The advantages of an ideal controlled release dosage form over an immediate release product include

Improved therapeutic efficacy - Reduction in drug plasma level fluctuations; maintenance of steady plasma level of the drug over a prolonged time period, ideally simulating an intravenous infusion of a drug.

Reduced side effects – Drug plasma levels are maintained and improve the tolerability within a narrow range without any fluctuations. This greatly reduces the possibility of side effects.

Patient compliance – Oral drug delivery is the most common and convenient for patients and a reduction in dosing frequency enhances compliance.

Reduced health care cost – The total cost of therapy of the controlled release product could be comparable or lower than the immediate release product.

Limitations of controlled release delivery system

Potential disadvantages of controlled release dosage form include

The possibility of dose dumping,

Less facile dose adjustment,

Increased potential for hepatic first-pass metabolism,

Possible delay in onset of action and

Possibly poor system availability.

Criteria for selection of drug suitable for controlled release:-11

Therapeutic index of drug should not be narrow.

The drug should not have narrow absorption window.

Biological half-life of drug should neither too short nor too long [4-8hours].

Drug should be stable in GIT.

It should not cause in irritation in GIT.

It should be soluble in biological fluids.

Dose should not be larger [<1000mg].

Drug should not show cumulative action and undesirable side effects.

The drug should possess a clear advantage when formulated as controlled release dosage form.

1.6. Modified release through microencapsulation:-

Microencapsulation12 has been widely employed in the design of controlled release dosage forms. Usually the drug substance is encapsulated into biocompatible or biodegradable polymer matrix forming particles with diameter in the range of 1-1000μm.13 Microencapsulation is perhaps the most widely accepted technique for oral and parenteral controlled release. The structure of microsphere varies according to the process of preparation, but generally, there are two distinctive types.

The reservoir type (or microcapsule)

The monolithic type (or microsphere)

The mechanism by which the active ingredient is released from the microsphere includes diffusion of the drug through the polymer matrix or as the polymeric material erodes. In the case of reservoir microsphere, the release rate is constant (i.e zero order rate) where as in monolithic matrices the release rate generally decreases with time.

Benefits of microencapsulation:-

Safe handling of toxic substances

Production of sustained, controlled release and targeted medication or delivery systems

Reduced dose dumping potential compared to large implantable devices

Improvement of flow of powders.

1.7. Introduction to Microspheres14

The process of targeting and site specific delivery with absolute accuracy can be achieved by attaching bioactive molecule to liposomes, biodegradable polymer, implants, monoclonal antibodies and various particulate carriers. The microparticulate delivery systems are considered and accepted as a reliable means to deliver the drug to the target site with specificity, if modified, and to maintain the desired concentration at the site of interest without untoward effect. The term microcapsule is defined as a spherical particle with size varying from 50 nm to 2 mm, containing a core substance. Microspheres are, in strict sence, spherical empty particles. However, the terms microcapsules and microspheres are often used synonymously. In addition, some related terms are used as well. For example, essentially "micro beads" and "beads "are used alternatively. Spheres and spherical particles are also used for a large size and rigid morphology. The microspheres are characteristically free flowing powders consisting of proteins or synthetic polymers, which are biodegradable in nature, and ideally having a particle size less than 200 µm. Solid biodegradable microspheres incorporating a drug dispersed or dissolved throughout particle matrix have the potential for the controlled release of drug. These carriers received much attention not only for prolonged release but also for the targeting of the anticancer drugs to the tumour.

1.7.1. Materials used in microspheres15,16

A number of different substances both biodegradable as well as non biodegradable have been investigated for the preparation of microspheres. These materials include the polymers of natural and synthetic origin and also modified natural substances. Synthetic polymers employed as carrier materials are methyl methacrylate, acrolein, lactide, glycolide and their copolymers, ethylene vinyl acetate copolymer , polyanhydrides etc. The natural polymers used for the purpose are albumin, gelatin, starch, collagen and carrageenan etc. Some of the polymers used in the preparation of the microspheres are classified as,

1. Synthetic Polymers

1.1 Non biodegradable polymers

PMMA (Poly Methyl Methacrylate)

Acrolein

Glycidyl Methacrylate

Epoxy polymers

1.2 Biodegradable Polymers

Lactides and Glycolides and their copolymers

Poly alkyl cyano acrylate

Polyanhydrides

2. Natural Polymers

Protein

Albumins

Gelatin

Collagen

2.2 Carbohydrates

Starch

Agarose

Carrageenan

Chitosan

2.3 Chemically modified carbohydrates

DEAE Cellulose

Poly (acryl) dextran

Poly (acryl) Starch

1.7.2. Prerequisites for Ideal Micro particulate Carriers

The material used for the preparation of micro particulate should ideally fulfill the fallowing prerequisites :

Longer duration of action

Control of content release

Increase of therapeutic efficacy

Protection of drug

Reduction of Toxicity

Biocompatibility

Sterilizability

Relative stability

Water solubility and dispersability

Bioresorbability

Target ability

Polyvalent

1.8. General Methods of preparation of Microspheres14-17

The microspheres can be prepared by using any of the several techniques discussed below. The choice of the technique mainly depends on the nature of the polymer used, the drug, the intended use and the duration of therapy. Moreover, the method of preparation and it’s choice are equivocally determined by some formulation and technology related factors as mentioned below:

The particle size requirement:

The drug or the protein should not be adversely affected by the process.

Reproducibility of the release profile and the method.

No stability problem.

There should no toxic product associated with the final product.

Synthetic polymers are e now materials of choice for the controlled release as well as targeted micro particulate carriers. The initial work was carried out on the non biodegradable polymers but later on, the interest has been shifted to the biodegradable polymers. Different types of methods are employed for the preparation of the microspheres. These include in situ polymerization, Solvent evaporation, coacervation phase separation, spray drying and spray congealing.

1.8.1. Single emulsion technique15

The micro particulate carriers of 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 fallowed by dispersion in the non aqueous medium e.g., oil. In the second step of preparation, 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. The chemical cross linking agents used include glutaraldehyde, formaldehyde, terephthaloyl chloride, diacid chloride, etc. Cross linking by heat is affected by adding the dispersion to previously heated oil. Heat denaturation is however, not suitable for the thermo labile drugs while the chemical cross linking suffers disadvantages of excessive exposure of active ingredient to chemicals if added at the time of preparation.

1.8.2. Double emulsion technique17

Double emulsion method of microspheres preparation involves the formation of the multiple emulsions or the double emulsion of type w/o/w and is best suited for the water soluble drugs, peptides, proteins, vaccines. This method can be used with both the natural as well as the synthetic polymers. The aqueous protein solution is dispersed in a lipophillic organic continuous phase. This protein solution may contain the active constituents. The continuous phase is generally consisted of the polymer solution that eventually encapsulates of the protein contained in dispersed aqueous phase. The primary emulsion is then subjected 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 process. The solvent evaporation is carried out by maintaining emulsion at reduced pressure or by stirring the emulsion so that the organic phase evaporates out. In the latter case, the emulsion is added to the large quantity of water into which organic phase diffuses out. The solid microspheres are subsequently obtained by filtration and washing. A number of hydrophilic drugs like luteinizing hormone releasing hormone agonist, Vaccines, protein /peptides and conventional molecules are successfully incorporated in to the microspheres using the method of double emulsion solvent evaporation/extraction.

1.8.3. Polymerization Techniques

The polymerization techniques conventionally used for the preparation of the microspheres are mainly classified as :

1.8.3.1 Normal Polymerization

The two processes are carried out in a liquid phase. Normal polymerization proceeds and carried out using different techniques a s bulk, suspension precipitation, emulsion and micellar polymerization process. In bulk polymerization, a monomer or a mixture of monomers along with the initiators is usually heated to initiate the polymerization and carry out the process. The catalyst or the initiator is added to the reaction mixture to facilitate or accelerate the rate of the reaction. The polymer so obtained may be moulded or fragmented as microspheres. For loading of drug, absorptive drug loading or adding drug during the process of polymerization may be opted. The suspension polymerization, which is also referred to as the bead or pearl polymerization is carried out by heating the monomer or mixture of monomers with active principles as droplets dispersion in a continuous aqueous phase. The droplets may also contain an initiator and other additives. The emulsion polymerization, however differs from the suspension polymerization as due to presence of the initiator in the aqueous phase, which later on diffuses to the surface of the micelles or the emulsion globules. On the other hand the suspension and emulsion polymerization can be carried out at lower temperature, since continuous external phase is normally water through which heat can easily dissipate. The two processed also lead to the formation of the higher molecular weight polymer at relatively faster rate. The major disadvantage of suspension and emulsion polymerization is, association of polymer with the un reacted monomer and other additives.

1.8.3.2. Interfacial Polymerization

Interfacial polymerization essentially proceeds involving 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. In this technique two reacting monomers are employed; one of which is dissolved in the continuous phase while the other being dispersed in the continuous phase. The continuous phase is generally aqueous in nature throughout which the second monomer is emulsified. The monomers present in either phases diffuse rapidly and polymerize rapidly at the interface. Two conditions arise depending upon the solubility of formed polymer in the emulsion droplet. If the polymer is soluble in the droplet it will leads to the formation of the monolithic type of the carrier on the other hand if the polymer is insoluble in the monomer droplet, the forward carrier is of capsular type8. The degree of polymerization can be controlled concentration, the composition of the vehicle of either phases and by the temperature of the system. The particle size can be controlled by controlling the droplets or globule size of the disperse phase. The polymerization reaction can be controlled by maintaining the concentration of the monomers, which can be achieved by addition of an excess of the continuous phase. The interfacial polymerization is not widely used in the preparation of the micro particles because of certain drawbacks, which are associated with the process such as :

Toxicity associated with the unreacted monomer

High permeability of the film

High degradation of the drug during the polymerization

Fragility of microcapsules

Non biodegradability of the microspheres

1.8.4. Phase separation Coacervation Technique

Phase separation method is specifically designed for preparing the reservoir type of the system, i.e. to encapsulate water soluble drugs e.g. peptides, proteins, however, some of the preparations are of matrix type particularly, when the drug is hydrophobic in nature e.g. Steroids. In matrix type device the drug or the protein is soluble in the polymer phase. The process is based on the principle of decreasing the solubility of the polymer in the organic phase to affect the formation of the polymer rich phase called coacervates. The coacervation can be brought about by addition of the third component to the system which results in the formation of the two phases, one rich in the polymer, while the other one , i.e. supernatant, depleted of the polymer. The method of choice is largely dependent upon the polymer and set of conditions. The method are based on the salt addition non solvent addition, addition of the incompatible polymer or change in PH . In this technique the polymer is first dissolved in a suitable solvent and then drug is dispersed by making its aqueous solution, if hydrophilic or dissolved in the polymer solution itself, if hydrophobic. Phase separation is then accomplished by changing the solution conditions by using any of the method mentioned above. The process is carried out under continuous stirring to control the size of the micro particles. The process variables are very important since the rate of achieving the coacervate determines the distribution of the polymer film, the particle size and agglomeration must be avoided by stirring the suspension using a suitable speed stirrer since as 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. Therefore, the process variable are critical as they control the kinetic of the formed particles since there is no defined state of equilibrium attainment.

1.8.5. Spray Drying and Spray Congealing

Spray drying and spray congealing methods are based on the drying and spray congealing methods are based on the drying of the mist of the polymer and drug in the air. Depending upon the removal of the solvent or the cooling of the solution, the two processes are named spray drying and the spray congealing respectively. The polymer is first dissolved in a suitable volatile organic solvent such as dichloromethane, acetone, etc18. 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 automization 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. Micro particles are separated from the hot air by means of the cyclone separator while the traces of the solvent are removed by vacuum drying one of the major advantages of the process is feasibility of operation under aseptic conditions. The two processes are rapid, requiring single stage operation, suitable for both batch and bulk manufacturing. These techniques have been used to encapsulate a large number of the drugs.

1.8.6. Solvent extraction method19

Solvent extraction method used for the preparation of micro particles, involves removal of the organic phase by extraction of the organic solvent. The method involves water miscible organic solvents such as isopropanol. Organic phase is removed by extraction with water. This process decreases the hardening time for the microspheres. One variation of the process involves direct addition of the drug or protein to polymer organic solution. The rate of solvent removal by extraction method depends on the temperature of water, ratio of emulsion volume to the water and the solubility profile of the polymer.