“In-Situ Gelling System for Drug Delivery”

Represented by Ahmed Mohamed Saad Ibrahim Shaddad

Introduction In situ is Latin phrase which is translated literally

as in position. In situ gel is drug delivery systems that are in sol.

Form before administration in the body, but once administrered, undergo gelation in situ, to form a gel.

The formation of gels depends on factors like temperature modulation, pH change, presence of ions and ultra violet irradiation, from which the drug gets released in a sustained and controlled manner

Advantages of in situ gelling system

1-Ease of administration. 2-Imporoved local bioavailability. 3-Reduced dose concentration. 4-Reduced dosing frequency. 5-Improved patient compliance and comfort. 6-Simple formulation and manufacturing so less

investment and cost.

Approaches for in situ gelling system

1. Stimuli-responsive in situ gel system (Temperature , pH) 2. Osmotically induced in situ gel systems (Ion‐activated systems) 3. Chemically induced in situ gel systems (Ionic, Enzymatic cross linking, Photo-polymerization.)

Stimuli-responsive in situ gel system

A. Temperature induced in situ gel systems Transitions from sol-gel is triggered by increase or decrease in temperature3 strategies I. Positively thermosensitive A positive temperature sensitive hydrogel has an upper critical solution temperature (USCT), such hydrogel contracts upon cooling below the UCST e.g (poly acrylic acid(PAA))

II. Negatively thermosensitive Negatively temperature sensitive hydrogels have a lower critical solution temperature (LCST), and contract upon heating above LCST.E.G poly (N-isoprpylacrylamide) (PNIPAAm)

III. Thermally reversible (pluronic, tetronics)Pluronics are poly (ethylene oxide)-poly (propylene oxide) poly (ethylene oxide) (PEO-PPOPEO) triblock co-polymer that are fluid at low temperature but forms thermos responsible gel when heated as a consequences of disorder-order transition in micelle packing which makes thes polymers suitable for in situ gelation.

Below this temp. sol convert to gel

Above this temp. sol convert to gel

B. pH induced in situ gel systems

All the pH sensitive polymers contain pendant acidic or basic groups that either accept or release protons in response to changes in environmental pH.

Increases if polymer contains weakly acidic (anionic) groups.

Decreses if polymer contains weakly basic (cationic) groups. E.g: Poly acrylic acid (PAA), poly vinylacetate

diethylaminoacetate , polyethylene glycol (PEG)

Osmotically induced in situ gel systems (Ion‐activated systems):

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 typically found in the tear fluids. The electrolyte of the tear fluid and especially Na+, Ca2+ and Mg2+ cations are particularly suited to initiate gelation of the polymer when instilled as a liquid solution in the conjunctival cul-de-sac.

Chemically induced in situ gel systems

A. Ionic cross linking Polymers may undergo phase transition in presence of various

ions K- carrageenan forms rigid, brittle gels in presence of K+. I- carrageenan forms soft elastic gels in presence of Ca2+. Gellan gum (Gelrite), Pectin, Sodium Alginate

undergo phase transition in presence of various ions such as K+ , Ca2+, Mg2+ Na+.

B.Enzymatic crosslinking enzymatic process operates efficiently under

physiologic conditions without need for potentially harmful chemicals such as monomers and initiators.

Cationic pH-sensitive polymers containing immobilized insulin and glucose oxidase can swell in response to blood glucose level releasing the entrapped insulin in a pulsatile fashion

Adjusting the amount of enzyme also provides a convenient mechanism for controlling the rate of gel formation, which allows the mixtures to be injected before gel formation

C. Photo-polymerization A solution of monomers or reactive macromer 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 macromers because they rapidly undergo photo-polymerisation in the presence of suitable photo initiator.

Long wavelength ultraviolet and visible wavelength are used but short wavelength ultraviolet is not because it has limited penetration of tissue and biological harmful.

Polymers used as in situ gelling agents:- Ideal Characteristics (of polymer): 1. It should be biocompatible. 2. It should have pseudo plastic behavior. 3. It should have good tolerance. 4. It should be capable of adherence to mucus. 5. Polymer should be capable of decreasing

viscosity with increasing shear rate there by lowering viscosity during blinking

Polymers used in in situ gel

&

Natural

synthetic

Natural polymer Pectin is used for orally administered in situ gelling as in

paracetamol administration. Xyloglucan gels have also been used as vehicles for a sustained

release of percutaneous formulations of non-steroidal anti-inflammatory drugs.

Gellan gum is used for orally administered in situ gelling as in theophylline as it increase bioavailability four or five time.

Alginic acid has been proposed in the field of pharmaceutics for its in situ gelation properties, particularly for the application of alginate gels for ocular drug delivery.

Xanthan gum has been tested for the preparation of sponge like in situ gelling inserts for the delivery of proteins and peptides in the nasal cavity.

Chitosan based gels may be broadly divided into thermally non-reversible gels and the far smaller group of thermally reversible gels.

Natural polymer

Carbomer used in timolol maleate for ocular delivery that showed therapeutically efficacious and a diffusion controlled tybe of release behavior over 24 h periods.

Pluronic F127 used in sumatriptan drug for nasal administration it is prolonging nasal residence time and increase nasal absorption.

Synthetic polymers: Synthetic polymers are popular choice mainly for

parenteral preparations. Aliphatic polyesters such as poly (lactic acid), poly (glycolic acid), poly (lactide- coglycolide), poly (decalactone), poly ε-caprolactone have been the subject of the most extensive recent investigations.

Various other polymers like triblock polymer systems composed of poly (D, L-lactide)-block poly (ethylene glycol)-block-poly (DL-lactide), blends of low molecular weight poly (D, L-lactide) and poly (ε-caprolactone) are also in use.

These polymers are mainly used for the injectable in situ formulations. The feasibility of lactide/glycolide polymers as excipients for the controlled release of bioactive agents is well proven.

Applications of in situ gelling system in drug delivery:

ORAL

NASAL

OCULAR

RECTAL AND VAGINAL

PARENTRAL

1- Oral

Pectin , xyloglucan and gellan gum are the natural polymers used for in situ oral drug delivery system.

The main advantage of using pectin for those formulation is that it is water soluble so organic solvent are not necessary in the formulation.

An increased bioavailability with sustained drug release profile of theophylline in rats and rabbits was observed from gellan formulations as compared to the sustained release dosage form.

Xyloglucan forms thermally reversible gel on warming to body temperature

2- Nasal Gellan gum and xanthan gum were used as in situ gel

forming polymers In situ gel was found to inhibit the increase in nasal

symptoms as compared to marketed formulation nasonex Animal experiments demonstrated hydrogel formulation to

decrease the blood-glucose concentration by 40-50% of the initial values for 4-5 h after administration with no apparent cytotoxicity.

Therefore, these types of systems are suitable for protein and peptide drug delivery through nasal route

Advantages of In-Situ Gel Nasal Formulation 1- Increased residence time of drug in nasal cavity. 2- Decreased frequency of drug administration. 3- Results in rapid absorption and onset of effect. 4- Avoids degradation of drug in gastrointestinal tract

resulting from acidic or enzymatic degradation. 5- Low dose required. 6- Minimized local and systemic side effects. 7- Improved bio-ability of drug. 8-Direct transport into systemic circulation and CNS, is

possible. 9- Offers lower risk of overdose of CNS acting drug. 10- Improved patient compliance.

3- Ocular For in situ gels based ocular delivery, natural

polymers such as gellan gum, alginic acid and xyloglucan are most commonly used polymers

Local ophthalmic drug delivery has been used for various compounds such as antimicrobial agents, anti-inflammatory agents and autonomic drugs used to relieve intraocular tension in glaucoma.

Much of the interest in the pharmaceutical application of gellan gum has concentrated on its application for ophthalmic drug delivery. Drug release from these in situ gels is prolonged due to longer precorneal contact times of the viscous gels compared with conventional eye drops.

Advantages of in situ gelling in ocular Formulation 1. Generally more comfortable than insoluble or

soluble insertion. Less blurred vision as compared to ointment.

2. Increased bioavailability due to increased precorneal residence time.

3. Decreased naso-lacrimal drainage of the drug which causes undesirable side effects.

4. Drug effect is prolonged hence frequent instillation of drug is not required.

5. The principle advantage of this formulation is the possibility of administering accurate and reproducible quantities, in contrast to already gelled formulations and moreover promoting precorneal retention.

4- Rectal and Vaginal: Miyazaki et al, 2001.investigated the use of

xyloglucan based thermoreversible gels for rectal drug delivery of indomethacin

Administration of indomethacin loaded xyloglucan based systems to rabbits indicated broad drug absorption peak and a longer drug residence time as compared to that resulting after the administration of commercial suppository

For a better therapeutic efficacy and patient compliance, a mucoadhesive, thermosensitive, prolonged release vaginal gel incorporating clotrimazole-β-cyclodextrin complex was formulated for the treatment of vaginitis.

5- Parenteral A novel, injectable, thermosensitive in situ gelling

hydrogel was developed for tumor treatment This hydrogel consisted of drug loaded chitosan solution

neutralized with β-glycerophosphate. Local delivery of paclitaxel from the formulation

injected intratumorally was investigated using EMT-6 tumors implanted subcutaneously on albino mice

Ito et al, 2007. designed and synthesized injectable hydrogels that are formed in situ by cross-linking of hydrazide modified hyaluronic acid with aldehyde modified versions of cellulose derivatives such as carboxymethylcellulose, hydroxypropylmethylcellulose and methylcellulose

These in situ forming gels were used for preventing postoperative peritoneal adhesions thus avoiding pelvic pain, bowel obstructions and infertility

Synthetic polymers are popular choice mainly for parenteral preparations. The trend in drug delivery technology has been towards biodegradable polymers, requiring no follow up surgical removal, once the drug supply is depleted. These polymers are mainly used for the injectable in situ formulations.

The feasibility of lactide/glycolide polymers as excipients for the controlled release of bioactive agents is well proven.

These materials have been subjected to extensive animal and human trials without evidence of any harmful side effects. When properly prepared under GMP conditions from purified monomers, the polymers exhibit no evidence of inflammatory response or other adverse effects upon implantation.

Drug release from hydrogels:

1-Diffusion-Controlled Release Systems

a) Reservoir Systems:

A reservoir delivery system consists of a drug core enclosed in a hydrogel membrane, usually in the form of capsules, cylinders, spheres or slabs. In order to maintain a constant release rate the drug concentration difference must remain constant. This is achieved by concentrating the drug in the centre of the device

Drug release from a reservoir system by diffusion through the hydrogel membrane

b) Matrix Systems:

In matrix systems the drug is dispersed throughout the hydrogel lying within the three-dimensional structure of the polymer. Matrix tablets are constructed through a compression of a mixture of drug and polymer powders. Drug release occurs through the macromolecular mesh or water- filled pores. Note that the release rate is here proportional to the square root of time initially rather than the constant time-independent rate available with reservoir systems.

Drug release from a matrix system by diffusion through the entire hydrogel.

2- Swelling-Controlled Release Systems

In swelling-controlled release systems the drug is dispersed within a glassy polymer as in a matrix device. Once the polymer comes into contact with water or another biofluid it begins to swell. The glass transition temperature of the polymer is lowered allowing a relaxation of molecular chains so that the drug can now diffuse out of the swollen rubbery area of the polymer. Figure 12 shows how the swelling edge of the tablet expands beyond its original boundary. This is also known as Case II transport and is characterised by constant, i.e. time-independent, release kinetics.

In some cases a combination of swelling controlled release as well as diffusion occurs, this is known as anomalous transport

The swelling edge of the tablet expands beyond its original boundary.

3-Chemically-controlled delivery systems In addition to diffusion and swelling-controlled

delivery systems discussed previously, a third type of molecule release mechanism is chemically-controlled delivery. That classified as:

(A) Purely kinetic-controlled release where polymer degradation (bond-cleavage) is the rate-determining step and diffusion term is assumed to be negligible.

(B) reaction-diffusion-controlled release in which both reaction (e.g. polymer degradation, protein–drug interaction) and diffusion terms must be included in the model to accurately predict drug release. The reaction-diffusion-controlled release is particularly intriguing as more synthetic hydrogel systems designed with drug-binding capacity are utilized in drug delivery and tissue engineering.

Evaluation and characterization of in situ gels systems

1. Viscosity and rheology:

This is an important parameter for the in situ gels, to be evaluated. Viscosity and rheological properties of in situ forming drug delivery systems may be assessed using Brookfield rheometer or some other type of viscometers such as Ostwald's viscometer. The viscosity of these formulations should be such that no difficulties are envisaged during their administration by the patient, especially during parenteral and ocular administration

2- Sol-Gel transition temperature and gelling time:

For in situ gel forming systems incorporating thermoreversible polymers, the sol-gel transition temperature may be defined as that temperature at which the phase transition of sol meniscus is first noted when kept in a sample tube at a specific temperature and then heated at a specified rate. Gel formation is indicated by a lack of movement of meniscus on tilting the tube. Gelling time is the time for first detection of gelation as defined above

Sol-Gel transition temperature and gelling time.

3. Gel Strength:

This parameter can be evaluated using a rheometer. Depending on the mechanism of the gelling of gelling agent used, a specified amount of gel is prepared in a beaker, from the sol form. This gel containing beaker is raised at a certain rate, so pushing a probe slowly through the gel. The changes in the load on the probe can be measured as a function of depth of immersion of the probe below the gel surface

4- In vitro drug release studies For the in situ gel formulations to be administered by

oral, ocular or rectal routes, the drug release studies are carried out by using the plastic dialysis cell. The cell is made up of two half cells, donor compartment and a receptor compartment. Both half cells are separated with the help of cellulose membrane. The sol form of the formulation is placed in the donor compartment. The assembled cell is then shaken horizontally in an incubator. The total volume of the receptor solution can be removed at intervals and replaced with the fresh media. This receptor solution is analyzed for the drug release using analytical technique. For injectable in situ gels, the formulation is placed into vials containing receptor media and placed on a shaker water bath at required temperature and oscillations rate. Samples are withdrawn periodically and analyzed.

5- Texture analysis:

The firmness, consistency and cohesiveness of hydrogels are assessed using texture analyzer which mainly indicates the syringeability of sol so the formulation can be easily administered in vivo. Higher values of adhesiveness of gels are needed to maintain an intimate contact with surfaces like tissues.

6- Clarity:

The clarity of formulated solutions can be determined by visual inspection against black and white background.

8. Antibacterial activity:

The microbiological growth of bacteria is measured by concentration of antibiotics and this has to be compared with that produced by known concentration of standard preparation of antibiotic. To carryout microbiological assay serial dilution method is employed

7- Accelerated stability studies:

Formulations are placed in ambient colour vials and sealed with aluminium foil for a short term accelerated stability study at 40±2oC and 75±5% RH as per International Conference on Harmonization (ICH) Guidelines. Samples are analyzed every month for clarity, pH, gelling capacity, drug content, rheological evaluation, and in vitro dissolution

8- Ocular irritancy test:

The Draize irritancy test was designed for the ocular irritation potential of the ophthalmic product prior to marketing. According to the Draize test, the amount of substance applied to the eye is normally 100μl placed into the lower cul-de-sac with observation of the various criteria made at a designed required time interval of 1hr, 24hrs, 48 hrs, 72hrs, and 1week after administration. Three rabbits (male) weighing 1.5 to 2kg are used for the study. The sterile formulation is instilled twice a day for a period of 7 days, and a cross over study is carried out (a 3 day washing period with saline was carried out before the crossover study). Rabbits are observed periodically for redness, swelling, watering of the eye

Commercial formulations of in situ polymeric systems Timoptic-XE: sterile, isotonic, buffered and

aqueous gel forming solutions of timolo maleate.

Regel:depot-technology: Oncogel® is a frozen formulation of paclitaxel in Regel. It is a free flowing liquid below room temperature which upon injection forms a gel in situ in response to body temperature

Cytoryn: This is one of the Macromed's products, which is a novel, peritumoral, injectable depot formulation of interleukin-2 (IL-2) for cancer immunotherapy using Regel drug delivery system.

Conclusion In conclusion, the primary requirement of a successful controlled

release product focuses on increasing patient compliance which the in situ gels offer. Exploitation of polymeric in situ gels for controlled release of various drugs provides a number of advantages over conventional dosage forms. Sustained and prolonged release of the drug, good stability and biocompatibility characteristics make the in situ gel dosage forms very reliable. Use of biodegradable and water soluble polymers for the in situ gel formulations can make them more acceptable and excellent drug delivery systems.