INTRODUCTION:

The vaginal cavity is an important area of female reproductive tract. Now a day’s vagina consider as a well established route for drug administration due to some of its unique features like avoidance of first pass effect, large permeation area, permeability to large molecular weight drugs, rich vascularization and relatively low enzymatic activity^1,2. The mucous permeability and dense network of blood vessels of vagina made this anatomic region as an excellent route for systemic drug delivery. In recent research shows that a wide range of compounds including steroids, proteins and peptides successfully delivered through this route^2,3.Vaginal cavity is also an effective site for the uterine targeting of various therapeutic agents such as terbutaline, progesterone and danazol^1,3,4. Vagina has been studied as a novel route for the delivery of chemotherapeutic agents for treatment of uterine cancers. Ideal vaginal drug delivery systems should be easy to use, discreet, of reversible application, painless to the patient, cost effective, widely available, and safe for continuous administration. It should also allow self-administration, with minimal interference with body functioning and daily life.^5,6

Despite of the above unique features vagina is also considered as highly sensitive area due to its tendency to under goes different kind of infections. Most common vaginal infection are Bacterial vaginosis, Vulvovaginal candidiasis, Trichomonas.^5,6

Most common causes of vaginal infections are due to the activity of different species of bacteria, fungi, and yeasts. Other than that improper and abundant use creams, sprays and douching, unhygienic and uncomfortable use of tight clothing, unsafe sexual activity and all other activities that may disrupt the normal microenvironment of the vagina are consider as causes of infections.^6,7

Some most commonly introduced delivery systems via vaginal route includes solution (foam, douches), aerosols, semisolids, (creams, ointments, gels), tampons, tablets, capsules, peccaries, suppositories, particulate systems, intravaginal rings, inserts, sponges and powders.^8,9,10 The available vaginal dosage forms have certain limitations such as leakage and low residence time, messiness,leading to poor patient compliance and loss of therapeutic efficacy¹⁰. Therefore, novel concepts and dosage forms are needed. Extensive research is ongoing to develop better vaginal drug delivery systems that can fulfill the user’s requirements.

Advantage of vaginal delivery system

Disadvantage of vaginal delivery system

• Ease of access

• Reduced side effects

• Great permeation area

• High vascularization

• Relative low enzymatic activity

• Avoidance of first pass metabolism

• Unawareness and gender specificity

• Genital hygiene issues

• Menstrual cycle-associated vaginal changes

• Coitus interference

• Local side effects

• Variable drug permeability

Vaginal Anatomy and Physiology with Respect to Drug Delivery

The human vagina is a fibro muscular tube approximately 6-10 cm in length comprised of three principal layers: an outer fibrous layer, a middle muscular layer and an innermost mucosal layer called epithelium layer^1,5. The epithelial layer consists of lamina propria and a surface epithelium, which is composed of non cornified, stratified squamous cells. The vaginal epithelium is essentially devoid of glands, but its surface is kept moist by a cervical secretion, whose composition and volume varies with age, stage of menstrual cycle, and degree of sexual excitement⁶. In the pelvis region of vagina have two diaphragms one is urogenital diaphragms and another pubococcygeus from pelvic diaphragms, act as sphincters to the vaginal introitus¹. The women of reproductive age having numerous folds on vagina named “rugae”, which provide distensibility, support as well as increase surface area of vaginal wall. The vaginal rugae and microridges on the epithelial cell surface permit the vagina to expand, allow the placement of vaginal formulations and increase the surface area of the vagina thus enhancing drug absorption.^5,6 Vagina consist two type of nerve supply, one is peripheral and second is an autonomic nerve¹. The vagina has remarkable features in terms of vaginal secretion, pH, enzyme activity and microflora. These factors affect formulation spreading and retention as well as absorption and drug release in vagina. The vaginal physiology is mainly influence by age, hormonal balance, pregnancy, pH changes and concentration of microflora. Human vaginal fluid mostly contain enzymes, enzyme inhibitors, proteins, carbohydrate, amino acids, alcohols, hydroxyl-ketones and aromatic compounds^10,12,13. The composition of fluids is effected by cyclic changes caused by hormonal influence and state of arousal, which can alter the drug release pattern from vaginal drug delivery system. The thickness of vaginal epithelium, amount and composition of vaginal fluid also changes throughout the menstrual cycle¹⁴. In general Vagina maintains a pH between 3.8-4.8, which influence by frequency of coitus, presence of cervical mucus and the amount of vaginal transudate. The lactic acid produced from glycogen by lactobacillus present in vagina plays an important role in maintains acidic pH environment.

Absorption of drug by vaginal route^7,26:- Absorption of drugs mainly takes place in the vaginal cavity as follows-

• Transcellular: - Concentration dependent transport following diffusion mechanism.

• Paracellular:- Tight junctions mediated transport of drug molecules

• Vesicular:- Receptor mediated transport for specific chemical entity.

Factors affecting absorption of drug by vaginal route:^7,10,17,26

Physiological Factors: - The physiological factors mainly affects the drug absorption are discussed as follows-

A. Vaginal secretions: The vaginal discharge is a mixture of multiple secretions that stored in the vagina from peritoneal, follicular tubal, uterine, Bartholin's and Skene's glands. In presence of moisture, solid dosage forms should ideally disperse in the vaginal canal immediately after insertion to avoid inconvenience to the users. The rate and extent of drug release from a dosages form is mainly depends on amount of vaginal secretion.

B. Enzyme activity: The specific enzymatic activity of four different amino peptidases in vaginal homogenates decreases in the order: sheep > guinea pig > rabbit ≥ human ≥ rat. The human genital tract has lower enzymatic activity leading to less degradation of protein and peptide drugs in the vagina than the gastrointestinal tract.

C. Vaginal pH: The pH of the healthy female genital tract is acidic (pH 3.8–4.8) and is maintained within that range by bacterial conversion of glycogen from exfoliated epithelial cells to lactic acid.

D. Mechanism of drug absorption: Drugs are transported across the vaginal membrane by the transcellular route, intracellular route or vesicular and receptor mediated transport mechanisms. A physical model of the vaginal membrane as a transport barrier has been described. The physiological factors (e.g. cyclic changes in the thickness and porosity of the epithelium, volume, viscosity and pH of the vaginal fluid) and physicochemical properties of drugs (e.g. molecular weight, lipophilicity and ionization) affect absorption across the vaginal epithelium. The absorption of drugs, targeted for local action in the vagina, is not desirable.

Formulation related Factors ^22,26,27:- Following formulation factors play major role in terms of drug relaease and absorption through vaginal cavity.

A. Physicochemical properties of the drug: Properties such as solubility, dissolution rate, pKa, presence of functional group, chemical stability, molecular weight, shape, size and charge on the surface plays important role in repect of drug absorption.

B. Drug release: The small volume of vaginal fluid makes dissolution as rate limiting step for systemic absorption of drugs from vaginal formulations.

C. The effective area of contact: Area of vaginal cavity is approximately 60 cm². Drug release and absorption of drugs may influenced with the size of effective area. Its an important parameter for semi solid formulations.

D. The hydrophilicity and viscosity: The extent of spreading and distribution of semi solid formulations intended for vaginal infections shall be influenced by the viscocity and hydrophilicity of the formulation.

E. Contact time: The extent of flow and retention of the medicament within the vaginal cavity depends on the type of formulation and vaginal secration prolong the contact time better will be the amount of absorption.

F. Concentration: Rate of absorption via passive diffusion can be increased by increasing drug concentration in vaginal fluid. However, high drug concentration may cause severe localized irritation or other adverse tissue reactions.

Following types of vaginal drug delivery systems are available^16-25

In the vaginal drug delivery system the following types of dosage forms are available.

Tablet:-^16-18

Vaginal tablets may contain binders, disintegrates and other excipients that are used to prepare conventional oral tablets. It has the advantage of ease of manufacture and insertion. Mucoadhesive polymers are sometimes used in vaginal tablet formulation to increase vaginal residence time. Drugs that are administered as vaginal tablets include itraconazole, clotrimazole and prostaglandins. Presence of hydrophobic or release retarding materials may decrease the absorption of a drug from a vaginal formulation. Highly hydrophobic drugs may not be suitable for vaginal tablets. Presence of penetration enhancers such as surfactants, bile salts can significantly enhance absorption at the same time the irritation properties of the materials should be taken under consideration.

Semisolids:-^19-24

Vaginal Creams, Gels and Ointment:- Creams, gels and ointment are usually comes under semisolid vaginal formulations used for delivery of contraceptives and antibacterial drugs for local infections. These delivery systems used very frequently. Basic qualities of vaginal semisolid formulations should have the properties like easy to use, non-toxic and nonirritating to the mucus membrane. Creams and gels may not deliver an exact dose because of nonuniform distribution and leakage. It is reported that in the treatment of bacterial vaginosis, metronidazole and clindamycin vaginal cream are found as effective as orally administered drugs. Chemical entities like anti-infectives (e.g. Nystatin, clotrimazole, miconazole, clindamycin, and sulfonamides); hormones (e.g. progesterone, dinesetrol) and contraceptives etc. applied through semisolid dosage form. The anti-HIV agents or microbicides currently aimed through topical intravaginal formulations to reduce the mucosal and prenatal virus transmission.

During the past few years, considerable work has been done on the development of hydrogel drug delivery systems. These hydrogels, when placed in an aqueous environment, swell and retain large volumes of water in their swollen structure and release drug in a control release fashion. A swelling controlled miconazole hydrogel delivery system for intravaginal administration has been reported. A 3% alginate gel of nonoxynol-9 has been investigated for intravaginal spermicidal delivery. Recently, gel microemulsions have been proposed as a nontoxic vaginal formulation. A gel microemulsion based formulation of a spermicide with anti-HIV effect, has been developed.¹² Antibacterial agents and drugs for cervical ripening and induction of labor are also available as a vaginal gel. ^9,20,21.

Suppositories:²⁵

Solid suppositories are the most common dosage forms. Typically, these are of torpedo-shaped dosage forms but in case of vagina oval shape is more preferred. The composition is largely dictated by the physicochemical properties of the drug and the desired drug release profile. The most commonly used base for vaginal suppositories consist of combination of the various molecular weight polyethylene glycols, surfactants and preservatives. They are buffered to acidic pH about 4.5 and are designed to melt in vaginal cavity to release active constituent over prolong period of time. It is reported that suppository systems are well used to administer drugs for cervical ripening prior to childbirth, local delivery of anti fungal drugs for vaginal candiasis and progesterone for hormonal replacement therapy.

Liquids:²⁶

Solution, suspension and emulsion

The vaginal douches and solutions are available in market. They are used for irrigation cleansing of vagina. The unit dose douches are prepared which are mixed with warm water and applied as inserters in vagina. Vaginal solutions, emulsions and suspensions are liquid preparations intended for a local effect, for irrigation and diagnostic purposes. They may contain excipients, for example to adjust the viscosity of the preparation, to adjust the pH of formulation, to increase the solubility of the active substance(s) or to stabilize the preparation. The excipients do not adversely affect the intended medical action or, cause undue local irritation at the concentrations used. Vaginal emulsions may show evidence of phase separation but are readily re dispersed on shaking. Vaginal suspensions may show sediment that is readily dispersed on shaking to give a suspension which remains sufficiently stable to enable a homogeneous preparation to be delivered. They are supplied in single-dose containers. The container is adapted to deliver the preparation to the vagina or it is accompanied by a suitable applicator.

Aerosol:²⁶

Aerosols preparations are very common among different vaginal dosage forms. It is popular to deliver estrogenic substances and contraceptive agents. Marketed preparations like povidone-iodine vaginal foam, and other contraceptive foams are available in the market. The vaginal aerosol is used as novel approaches. The foams are used intravaginally in the same manner as creams. The aerosol package contains an inserter that is filled with foam and the contents placed in the vagina through activation of the plunger. The foams are generally o/w emulsions resembling light creams. They are water miscible and non greasy. Some commercial rectal foams use inserters. One such product, Proctofoam (Schwartz), contains pramoxine hydrochloride to relieve inflammatory anorectal disorders

Vaginal route for sustained/controlled-release drug delivery^7,18

Sustained and controlled-release devices for drug delivery in the vaginal and uterine areas are most often for the delivery of contraceptive steroid hormones.

1) Rings:^1,2,5

Vaginal rings are circular type drug delivery devices designed to release the drug in a controlled fashion after insertion into the vagina. Vaginal rings comprise a potential revolution in contraceptive technology and hormone replacement therapy. Vaginal ring became popular due to their ease of administration, lack of gastrointestinal related symptoms, high efficacy, no interfere with caution, does not require a daily intake of pills and allows continuous delivery of low dose Steroids. They are approximately 5.5 cm diameter with a circular cross section diameter of 4–9 mm. The rate of drug release can be modified by changing the core diameter or thickness of the non medicated coating. The material for making vaginal ring is usually polymeric in nature. Ethylene vinyl acetate polymers are classified by the content of vinyl acetate. The addition of vinyl acetate units in the polyethylene provides the following advantages: increased flexibility, improved optical properties, greater adhesion, and increased impact and puncture resistance. Most of the women judged ring easier application. No adverse effects were experienced among the test group during the study period. For most contraceptive rings may placed in the vagina for 21 days followed by a week of ring free period. NuvaRing^R is a example of combined contraceptive vaginal ring available in the US market.

2) Inserts:^1,2,5

These types of systems contains flat rectangular polymeric slab enclosed in a pouch of knitted polyester removal system. The buff colored semi transparent hydrogel slab contains drug. The retrieval system is in the shape of long knitted tape that is used to retrieve the slab.

A more common contraceptive device is the intrauterine device (IUD) popularly used to deliver progesterone for cervical ripening and induction of labor. Two types of medicated IUD are generally used; contraceptive metals and steroid hormones. The metal device is exemplified by the CU-7, a polypropylene plastic device in shape of number seven. Copper is released by a combination of ionization and chelation from a copper wire wrapped around the vertical limb. Progestasert, a reservoir system, is diagrammed in

3) In SITU gels:

Mucoadhesive formulations prepared using temperature-sensitive and mucoadhesive polymers, poloxamer and polycarbophil. These are basically semisolid formulations that may introduce in the vaginal cavity by means of suitable inserting device. The water insoluble/ soluble polymers swell in vagina and form bioadhesive gels that adhere on vaginal layer. This allows continuous release up to 25 to 50 hrs. e.g. CRIONE GEL (Wyeth-Ayrest pharma)

Table 1- Some marketed vaginal products:^1,3,20

Dosage form	Brand name	Active agent	Intended use
Ring	Etonogestrel, ethinyl estradiol (NuvaRing ®)	Etonogestrel, ethinyl estradiol	Contraceptive
Cream	Clotrimazole Trivagizole®)	Clotrimazole	Anti-fungal
Gel	Acid Jelly®a	Oxyquinoline sulphate, ricinoleic acid, acetic acid	Maintenance of vaginal acidity, antiseptic
Tablet	Estradiol, (Vagifem®)	Estradiol and Estradiol hemihydrates	Atropic vaginitis
Ointment	Tioconazole(Trivagizole®)	Ticonazole	Anti-fungal, vaginal, Candida infection
Ring	Estradiol (Estring®)	Estrogen	Hormone therapy
Suppositories	Dinoprostone (Cervidil®)	Dinoprostone	Induction of labor
Vaginal Gel	Advantage- S®	Nonoxynol-9	Contraceptive
Vaginal Gel	Conceptrol®	Nonoxynol-9	Contraceptive
Vaginal Gel	Gynol II®	Nonoxynol-9	Contraceptive
Vaginal Gel	Metrogel Vaginal®c	Metronidazole	Bacterial vaginosis
Vaginal Gel	Crinone®d	Progesterone	Infertility, secondary, Amenorrhea
Vaginal Gel	Prostin E2®e	Dinoprostone	Labour inducer

Other novel approaches:^2,5,8

1) Medicated Vaginal Tampons- A medicated vaginal tampon, approved as a medical device by the Food and Drug Administration (FDA) is available in the market (Ela Tampon, Rostam, Israel). This bifunctional tampon contains a polymeric delivery system (strips) that absorb menstrual fluid while gradually releasing lactic acid and citric acid.

2) Films-

Vaginal films are polymeric drug delivery systems shaped as thin sheets, usually ranging from 220 to 240 μm in thickness. These systems are often square (approximately 5 cm × 5 cm), colorless, and soft, presenting a homogenous surface. Vaginal films are produced with polymers such as polyacrylates, polyethylene glycol, polyvinyl alcohol, and cellulose derivatives.

Evaluation techniques:

In-vitro and In-vivo evaluation of vaginal formulations:²⁷^-31

These studies include the determination of drug release and bioadhesive / mucoadhesive characteristics in addition to various physical and chemical properties of different formulations. The release characteristics of a drug from a vaginal formulation can be determined in simulated vaginal fluid (pH 4.2) and in various dissolution media (pH range 2–12) by different types of diffusion cells with certain modifications and a vaginal dissolution tester. The bioadhesive strength of the vaginal formulation can be measured by various techniques. In vivo studies are conducted in different animal models to assess efficacy, distribution, spreading and retention of formulations in the vagina. Gamma scintigraphy and colposcopy are desirable techniques for assessing the distribution, spreading and retention of vaginal formulations in sheep and humans. However, the significance of these findings is debatable. Two imaging techniques are being developed to measure the degree of coverage in the vaginal vault: are magnetic resonance imaging (MRI) and by intravaginal optic probe. Several animal models such as sheep, rats, rabbits, rhesus monkeys, macaque monkeys, dogs and mice have been used in different studies in the development of vaginal formulations. White rabbits are used for primary irritation and sub chronic toxicity testing. Recently developed vaginal-Ectocervical (VEC) tissue models will serve as useful, highly reproducible, non-animal tools to assess the irritation due to vaginal care product.

The In vitro test system:^10,27,28

(a) Cell culture systems

· Human vaginal epithelial cells

· Primary human vaginal keratinocytes

· CRL-2614(Ect/E6E7)- Immortalised epithelial cell line from human endocervix

· CRL-2615(End1/E6E7)- immortalized epithelial cell line from human endocervix

· CRL-2614(Ect1/E6E7)- immortalized epithelial cell line from human vagina

· Immortalised human vaginal epithelial cell line

(b) Reconstructed tissues

· Human reconstructed vaginal mucosa integrating LCs

· Human reconstructed vaginal mucosa based on LCs, SiHa and NHVC

· Human reconstructed vaginal mucosa based on LC integrated within a pluristratified epithelium

· CellEstrous

· VEC 100

· VLC 100

· VEC 100 FT

· VLC 100 FT

· RHVE

(c) Explants

· Human vaginal tissue

· Procine vaginal epithelium

(FT- full thickness, RHVE- reconstructed human vaginal mucosa, VEC- vaginal ectocervical cells, VLC- vaginal ectocervical)

Table 2- Models used for In-vivo test^27-35

Animal	Purpose and methods	Scientist or author	Dosage form
Rabbit	Bacterial vaginosis, HIV microbicides and end point method	1.Gertrude-E. C. et al. 2. Bansal K. et al. 3. Chien Y. W et al	gels, creams, ointments, tablets, suppositories, inserts, films, sponges etc.
Rhesus monkey	Yeast infection, bacterial infection, HIV microbicides and end point method	1.Gertrude-E. C. et al. 2. Chien Y. W et al	filled capsules, gels, creams, tablets, suppository etc.
Bonnet monkey	Bacterial vaginosis, viral vaginitis, HIV microbicides and end point method	1.Gertrude-E. C. et al. 2. Bansal K. et al.	gels, creams, ointments, tablets, suppositories, inserts, films, sponges etc.
Chinese rhesus macaque	Bacterial vaginosis, viral vaginitis, HIV microbicides and end point method	1.Gertrude-E. C. et al. 2. Bansal K. et al.	gels, creams, ointments, tablets, suppositories, inserts, films etc.
Pig tailed macaque	Bacterial vaginosis, viral vaginitis, HIV microbicides and end point method	1.Gertrude-E. C. et al. 2. Pullium j. k. et al	gels, creams, ointments, tablets, suppositories, inserts, films, sponges etc.
Pig	Bacterial vaginosis, viral vaginitis, yeast infection, HIV microbicides and end point method	1.Gertrude-E. C. et al. 2. Chien Y. W et al	gels, creams, ointments, tablets, suppositories, inserts etc.
Mouse	Bacterial vaginosis, viral vaginitis, HIV microbicides and end point method	1.Gertrude-E. C. et al. 2.Chien Y. W et al	gels, creams, ointments, tablets, suppositories, inserts, films, sponges etc.
Swiss Webster mouse	viral vaginitis, HIV microbicides and end point method	1.Gertrude-E. C. et al.	gels, creams, ointments, suppositories, inserts, films, sponges etc.
Rat	Bacterial vaginosis, viral vaginitis, HIV microbicides and end point method	1.Gertrude-E. C. et al. 2. Van C. J. et al.	gels, creams, ointments, suppositories, inserts, films, sponges etc.

In-vivo studies include:²⁷^-36

The bioavailability: can be determined by:

· Monitoring quantities of systemically absorbed materials, e.g., peptides and proteins.

· Measurement of pharmacological activity and analysis of vaginal cleavage.

· Gamma scintigraphy is a valuable method assessing the distribution, spreading and retention of vaginal formulations.

· Colposcopy has also been used for direct in vivo visualization and analysis.

CONCLUSION:

The vaginal route has been traditionally used for the local application of drugs, but is now becoming more popular route for noninvasive, transmucosal delivery of both local and systemic therapeutically active compounds and gaining importance as a possible site for systemic delivery. Vaginal route has proved its suitability as a site for prevention of STDs, AIDS and to control conception. At present various type of vaginal dosage forms are available in the market. Novel vaginal delivery systems overcome some of the key limitations associated with conventional vaginal dosage forms and became more reliable and patient friendly. Based on the literature and detail study it can be conclude that, a huge amount of research should be carried out on vaginal drug delivery systems to utilization of this anatomical area as a promising route for drug delivery in near future.

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Received on 27.12.2014 Accepted on 20.03.2014

Asian J. Res. Pharm. Sci. 4(2): April-June 2014; Page 86-92