INTRODUCTION:

In-Situ Gel Delivery System In situ gelation is a process of gel formation at the site of action after the formulation has been applied at the site. Insitu gel henomenon based upon liquid solution of drug formulation and converted into semi-solid mucoadhesive key depot. It permits the drug must be delivered in a liquid form or solution form¹.

Advantages of In-Situ Gel Nasal Formulation¹:

· Increased residence time of drug in nasal cavity.

· Decreased frequency of drug administration.

· Results in rapid absorption and onset of effect.

· Avoids degradation of drug in gastrointestinal tract resulting from acidic or enzymatic degradation.

· Low dose required.

· Minimized local and systemic side effects.

· Improved bio-ability of drug.

· Direct transport into systemic circulation and CNS, is possible.,

· Offers lower risk of overdose of CNS acting drug

· Improved patient compliance.

Various Approaches of In-Situ Gelation¹:

To cause sol to gel phase transition on the nasal surface the following type of systems are recognized:

· pH Triggered system

· Temperature dependent system

· Ion activated system

· Induced photo polymerization gelation (UV Induced gelation)

· Solvent exchange induced gelation.

Polymers used in thermoreversible in situ formulations²:

1. Pluronics or Poloxamers:

These are a class of thermoreversible gels that have the capacity to make, break and modify the bonds responsible for holding the network together. There are different classes of Pluronics (pluronic F-127, F-188 etc). Their hermoreversible property make them useful as a carrier for most routes of administration including oral, topical, intranasal, vaginal, rectal, ocular and parenteral routes. The potential use of PF-127 as an artificial skin has also been reported. Poloxamer 407 (PF-127) is a nonionic surfactant composed of polyoxyethylene polyoxypropylene copolymers in a concentration ranging from 20-30%. These polymers are produced by condensation of ethylene oxide and propylene oxide. These are white, waxy, free flowing granules that are practically odorless and tasteless. Reverse thermal gelation and low toxicity have been the basis of research into the use of PF-127 as a possible drug delivery system in man. It has been considered for topical delivery of lidocaine, anti cancer agents and for the covering of burnt wounds. Its use in ophthalmic purpose was also studied using pilocarpine as model drug and PF-127 as vehicle. Finally it is also studied as a potential vehicle for injectables by both the intramuscular and subcutaneous routes. The aqueous solutions of Poloxamer are stable in the presence of acids, alkalis and metal ions. Commonly used Poloxamers include the 188(F-68 grade), 237(F-87grade), 338(F-108 grade) and 407(F-127grade) which are freely soluble in water. The flake form is designated as “F”. Of all these PF-127 has a good solubilizing capacity, low toxicity and is considered as a good carrier for drug delivery systems. PF-127 is more soluble in cold water than in hot water as a result of increased salvation and hydrogen bonding at low temperatures. These Poloxamers have the reversible property of being gel upon warming to room temperature and convert back to liquid when refrigerated (4-50C).

Figure 1: Chemical structure of Pluronic F-127 (a) ethylene oxide portion (b) propylene oxide portion.

Hydroxy propyl methyl cellulose (HPMC), Methyl cellulose, Poly-(N-isopropylacrylamide) are the other thermoreversible polymers which can be used as a carrier in the delivery of various drugs. Following considerations must be kept in mind while selecting a thermoreversible polymer for nasal administration:

· Quick transition from liquid to solid upon temperature change: this keeps the gel to stay at the site.

· Prevent the wastage of dosage form from the applied site.

· Solid- to- gel state reversible property of polymer may be adjusted from temporary to permanent by changing its chemical composition.

· Increase drug concentration at the site of deposition.

2. Carbopol:

They are very high molecular weight polymers of acrylic acid and are used mainly in liquid or semisolid pharmaceutical formulations such as gels, suspensions and emulsions, as a thickening and viscosity agent in order to modify the flow characteristics. 46 They are also used for mucoadhesive properties and a relevant amount of work has been done on the bioadhesive potential of carbopol polymers. Carbopol are used in formulations for ophthalmic, rectal, buccal, nasal, intestinal, vaginal and topical preparations. Carbopol gels are prepared by the dispersion of polymers in water. In which it swells upto1000 times the original volume (BF Goodrich handbook) and neutralizes the system. It permits the ionization of the carboxylic groups and as a result strong gel forms.

3. Chitosan:

Chitosan is a biodegradable, thermosensitive, polycationic polymer obtained by alkaline deacetylation of chitin, a

natural component of shrimp and crab shell. Chitosan is a biocompatible Ph dependent cationic polymer, which remains dissolved in aqueous solutions up to a pH of 6.2. Neutralization of chitosan aqueous solution to a pH exceeding 6.2 leads to the formation of a hydrated gel like precipitate. The pH gelling cationic polysaccharides solution are transformed into thermally sensitive pH dependent gel forming aqueous solutions, without any chemical modification or cross linking by addition of polyol salts bearing a single anionic head such as glycerol, sorbitol, fructose or glucose phosphate salts to chitosan aqueous solution.

4. Gellan gum:

Gellan gum (commercially available as Gelrite TM or Kelcogel TM) is an anionic deacetylated exocellular polysaccharide secreted by Pseudomonas elodea with a tetrasaccharide repeating unit of one α-L-rhamnose, one β-D-glucuronic acid and two β-D-glucuronic acid residues . It has the tendency of gelation which is temperature dependent or cations induced. This gelation involves the formation of double helical junction zones followed by aggregation of the double helical segments to form a three-dimensional network by complexation with cations and hydrogen bonding with water. The formulation consisted of gellan solution with calcium chloride and sodium citrate complex. When administered orally,the calcium ions are released in acidic environment of stomach leading to gelation of gellan thus forming a gel in situ. In situ gelling gellan formulation as vehicle for oral delivery of theophylline is reported.

5. Xanthan gum:

Xanthan gum is a high molecular weight extra cellular polysaccharide produced by the fermentation of the gram-negative bacterium Xanthomonas campestris. The primary structure of this naturally produced cellulose derivative contains a cellulosic backbone (β- D-glucose residues) and a trisaccharide side chain of β-D-mannose-β-D-glucuronicacid-α-D-mannose attached with alternate glucose residues of the main chain. The anionic character of this polymer is due to the presence of both glucuronicacid and pyruvic acid groups in the side chain.

6. Alginic acid:

It is a linear block copolymer polysaccharide consisting of β-D-mannuronic acid and α-L-glucuronic acid residues joined by 1, 4-glycosidic linkages. The proportion of each block and the arrangement of blocks along the molecule vary depending on the algal source. Dilute aqueous solutions of alginates form firm gels on addition of di and trivalent metal ions by a cooperative process involving consecutive glucuronic residues in the α-Lglucuronic acid blocks of the alginate chain. Alginic acid can be chosen as a vehicle for ophthalmic formulations, since it exhibits favorable biological properties such as biodegradability and nontoxicity. A prolonged precorneal residence of formulations containing alginic acid was looked for, not only based on its ability to gel in the eye, but also because of its mucoadhesive properties.

APPROACHES OF IN SITU GELLING SYSTEM³:

The various approaches for in situ gelling system

1. STIMULI RESPONSIVE IN SITU GELLING SYSTEM

· Temperature induced in situgel systems

· pH induced in situgel systems

2. OSMOTICALLY INDUCED IN SITU GELLING SYSTEM:

3.CHEMICALLY INDUCED IN SITU GEL SYSTEM:

· Ionic cross linking

· Enzymatic cross linking

· Photo-polymerization

1. STIMULI RESPONSIVE IN SITU GELLING SYSTEM:

Physical or chemical changes in response to small external changes in the environmentalcondition.

Temperature induced in situ gel system:

Temperature is the most widely used stimulus in environmentally responsive polymer systems. The change of temperature is not only relatively easy to control, but also easily applicable both in vitro and in vivo. In this system, gelling of the solution is triggered by change in temperature, thus sustaining the drug release. These hydrogels are liquid at room temperature (20–25 °C) and undergo gelation when in contact with body fluids (35– 37 °C), due to anincrease in temperature. The polymers which show temperature induced gelation are poloxamers or pluronics, cellulose derivatives (methyl cellulose, HPMC, ethyl (hydroxyl ethyl) cellulose (EHEC) and xyloglucan etc.

PH induced in situgel systems:

Polymers containing acidic or alkaline functional groups that respond to changes in pH are called pH sensitive polymers. The pH is an important signal, which can be addressed through pH-responsive materials. Gelling of the solution is triggered by a change in pH. At pH 4.4 the formulation is a free-running solution which undergoes coagulation when the pH is raised by the body fluid to pH 7.4. The polymers which shows pH induced gelation are cellulose acetate phthalate (CAP)Latex, Carbomer and its derivatives polyvinylacetyldiethyl aminoacetate (AEA), Polymethacrilic acid (PMMA), polyethylene glycol (PEG), pseudo latexes etc.

2. OSMOTICALLY INDUCED IN SITU GELLING SYSTEM:

In this method, gelling of the solution instilled is triggered by change in the ionic strength. It is assumed that the rate of gelation depend on the osmotic gradient across the surface of the gel. The aqueous polymer solution forms a clear gel in the presence of the mono or divalent cations. The polymer which showsosmotically induced gelation are gellan gum, hyaluronic acid and alginates etc.

3.CHEMICALLY INDUCED IN SITU GEL SYSTEM:

The chemical reaction which forms in situ gel systems are Ionic crosslinking, enzymatic crosslinking and Photo-polymerization

Ionic cross linking:

Certain ion sensitive polysaccharides such as carragenan, Gellan gum (Gelrite), Pectin, Sodium Alginate undergo phase transition in presence of various ions such as K+ , Ca2+, Mg2+,Na+. These polysaccharides fall into the class of ion-sensitive ones. For example, Alginic acid undergoes gelation in presence of divalent/polyvalent cations e. g. Ca2+ due to the interaction With guluronic acid block in alginate chains.

Enzymatic cross linking:

In situ formationcatalyzedby natural enzymes has not been investigated widely butseems to have some advantages over chemical and photochemical approaches. For example, anenzymatic process operates efficiently under physiologic conditions without need for potentially harmful chemicals such as monomers and initiators.

Photo-polymerization:

In situphoto-polymerization has been used in biomedical applications for over more than decade. A solution of monomers or reactive macromere and initiator can be injected into a tissues site and the application of electromagnetic radiation used to form gel. Acrylate or similar polymerizable functional groups are typically used as the polymerizable groups on the individual monomers and macromere because they rapidly undergo photo-polymerization in the presence of suitable photo initiator. Photopolymerizable systems when introduced to the desiredsite via injection get photocured in situwith the help of fiber optic cables and then release thedrug for prolonged period of time. A photo-polymerizable, biodegradable hydrogel as a tissuecontacting material and controlled release carrier is reported by Sawhney et al.

Evaluation of formulation⁴:

Clarity:

The clarity of in situ gel was examined by visually under dark background.

pH of the gel:

The normal range of nasal mucosal pH is 6.2 to 7.0 pH. The advisable pH of the nasal formulation is in the range of 5.5 to 7. For determining the pH of the formulation of nasal insitu gel, taken 1 ml quantity of each formulation transferred into a different beaker and diluted it with distilled water up to 25 ml and then pH of each formulation was determined by using pH meter (model no CL 54 ).

Drug content:

1 ml of formulation was taken in 10 ml volumetric flask and then it was diluted with 10 ml of distilled water then volume adjusted to 10 ml, 1 ml from this solution again diluted with distilled water up to 10 ml. After this absorbance of prepared solution was measured at particular wavelength of the drug by using U.V visible spectrophotometer.

Viscosity measurement:

Viscosity of nasal insitu gel was measured by using (cone and plate viscometer) programmable Brookfield dv2nd model viscometer .The viscometer was equipped with the temperature control unit and the sample were equilibrated for 10 min before the measurement .The viscosity of nasal insitu gel were recorded at various temperature from 4°c to 40 °c respectively against increasing the shear rate.

Measurement of gelation temperature:

The gelation temperature was described by miller & Donovan technique. In this phase transition occurred from liquid phase to a gel phase. In this 2 ml insitu gel transferred to test tube and placed into water bath then the temperature of water bath increased slowly and constantly. Gel was allowed to equilibrate for 5 minute at each setting, then formulation was examined for gelation. When the meniscus would no longer move upon tilting to 90°, this is known as a gelation temperature.

Determination Of Mucoadhesive Strength:

Mucoadhesive strength is known as the force to detach the insitu gel formulation from nasal mucosal tissue, for determining the mucoadhesive strength we use modified special chemical balance .A small section of nasal mucosa of goat was cut & tied or fixed on 2 glass vial with the help of rubber band or thread and stored it at 37°c ±2°c for 10 minute and then 50mg of gel was placed on first vial and it placed below the height adjustable balance, while on another hand second vial was fixed in inverted position to the underside of the same balance after this height both vial were adjusted and come in intimate contact for 5 minute to ensure the contact between nasal mucosal tissue and the insitu gel formulation. Then weight was put off on the other side of balance, until vials got detached, it expressed as the strength or stress in dyne/cm².

A. Stress is calculated by the formula:

Detachment Stress (dyne/cm²) = M × G ÷ A

Where

M = wt required for detachment of two vials in gm

G = acceleration due to gravity

A = Area of tissue exposed.

Figure 2 : Modified Balance, B Weights, C Glass Vial, E, F Membrane, G Height Adjustable Pan.

In vitro Diffusion Study of In situ Gel:

Franz having capacity 2.4 diameter and 15 ml diffusion cell was used for in vitro diffusion study of insitu gel. Dialysis (.22μm pore size) or cellophane membrane (12000-18000 mol wt) with diffusion area .8cm² used.60 ml of phosphate buffer (6.4-6.6pH) was prepared and membrane was soaked with phosphate buffer (6.4- 6.6 pH), after this temperature was maintained at 37°C±0.5°C, after this phosphate buffer placed into the acceptor chamber and gel containing drug equivalent to 10 mg was placed in donor chamber, at predetermined time point, 1ml sample was withdrawn from acceptor chamber and then replaced the sample volume with equal amount of phosphate buffer after each sampling process, for a period of 300 minute, after each sampling, the samples were suitably diluted and measured spectrophotometrically at specific wavelength of drug. The concentration of drug was determined with the help of previous calibration curve.

Invitro Permeation Study Of Insitu Gel:

To check permeation of drug and capacity of permeation enhancer which was added in formulation. Fresh nasal tissue section of goat obtains from slaughter house. Tissue was inserted in the diffusion cell. Gel containing drug equivalent to 10 mg was placed in donor chamber, at predetermined time point, 1ml sample was withdrawn from acceptor chamber and replacing the sampled volume with same amount of phosphate buffer, for a period of 300 minute, after each sampling, the sample were suitably diluted and measured spectrophotometrically at specific wavelength of drug.

B. Permeability coefficient calculated from the slope of the graph:

P = Slope × Vd ÷ s

Vd = volume of the donor solution

S = surface area of tissue

P = permeability coefficient.

D.S.C (Differential Scanning Calorimetry), X Ray Diffraction and FTIT (Fourier Transform Infra –Red Spectroscopy) Studies: used for drug and polymer interaction, compatibility and to check matrix formation.

CONCLUSION:

Used of biodegradable, water soluble, thermo sensitive, pH sensitive polymer for the nasal in situ gel formulations can make them more acceptable and excellent drug delivery system. Exploitation of polymeric in situ gels for controlled release of various drugs, good stability and biocompatibility, bioavailability of drug characteristics make the nasal in situ gel dosage forms very reliable. Nasal in situ gel enhanced the nasal residence time due to its viscosity and mucoadhesive strength. For optimum formulation can be achieved with better rheological properties gelation time, gelation temperature, pH, mucoadhesive strength, and in vitro release and permeation studies.

REFERENCE:

1. Bajpai Vibha, “In Situ Gel Nasal Drug Delivery System – A Review”, International Journal of Pharma Sciences, Vol. 4, No. 3 (2014): 577-580.

2. Sreeja C Nair, Mable Sheeba John, Anoop K R, “In Situ Gel: An Innovative Approach for Safe and Sustained Nasal Drug Delivery”, International Journal of Pharmaceutical Sciences Review and Research., 24(1), Jan – Feb 2014; no. 01, 1-7.

3. J.U.Kute*, A. B. Darekar, R.B. Saudagar, “in situ gel-novel approach for nasal delivery”, world journal of pharmacy and pharmaceutical sciences, Volume 3, Issue 1, 187-203.

4. Bajpai Vibha, “In Situ Gel Nasal Drug Delivery System – A Review”, International Journal of Pharma Sciences, Vol. 4, No. 3 (2014): 577-580.

Received on 29.09.2015 Modified on 16.10.2015

Res. J. Pharm. Dosage Form. and Tech. 7(4): Oct.-Dec., 2015; Page 261-265

DOI: 10.5958/0975-4377.2015.00037.3