But, if the physician in so doing has caused the seignior's death or has he destroyed the seignior's eye, they shall cut off his hand" the forgoing excerpts are from 282 laws of King Hammurabi's Code. The eye is unique in its therapeutic challenges. An efficient system, that of tears and tear drainage, which quickly eliminates drug solutions which makes topical delivery to the eye somewhat different from most other areas of the body. Preparations for the eye comprise a variety of different types of products; they may be solutions (eye drops or eyewashes), suspensions, or ointments.
Any modern text on drug product design and evaluation must place into perspective the unique nature of the ophthalmic dosage form in general more specifically. It must consider that the bodily organ which, probably better than any other, serves as a model structure for the evaluation of drug activity, the eye. In no other organ can the practitioner, without surgical or mechanical interaction, so well observe the activity of the drug being administered.
Most ocular structures can be readily viewed from cornea to retina and in doing so; any signs of ocular or systemic disease can be detected long before sight-threatening or certain health threatening disease states become intractable. Behind the relative straightforward composition nature of ophthalmic solutions and ointments, however, like many physicochemical parameters which affect drug stability, safety and efficacy as they do most other products.
Additionally, specialized dosage forms such as parenteral type ophthalmic solutions for intraocular, subtenons, and retrobulbar use; suspensions for insoluble substances such as hydrocortisone; and solids for reconstitution such as ecothiophate iodide and tetracycline, all present the drug product designer with composition and manufacturing procedure challenges in the development of pharmaceuticals. Opthalmic products, like most others in the medical armamentarium, are undergoing a process termed optimization.
New modes of delivering a drug to the eye are being actively explored ranging from a solid hydrophobic device which is inserted into the ophthalmic cul-de-sac, to conventionally applied dosage forms which, due to their formulation characteristics markedly increase the drug residence time in the orbit of the eye, thus providing drug for absorption for prolonged period of time and reducing the frequency with which a given drug product must be administered . Ocular diseases are mainly treated topically by application of drug solutions administered as eye drops.
These conventional dosage forms account for 90% of the available ophthalmic formulations. This can be due to the simplicity and convenience of such dosage forms . It is often assumed that drugs administered topically to the eye are rapidly and totally absorbed and are available to the desirable site in the globe of the eye to exert their therapeutic effect. Indeed, this is generally not the case. When a quantity of topical ophthalmic dosage form is applied to the eye, generally to the lower cul-de-sac, several factors immediately begin to affect the availability of the drug contained in that quantity of the dosage form.
Upon application of 1 to 2 drops of a sterile ophthalmic solution, there are many factors, which will participate in the removal of the applied drops from the lower cul-de-sac . The first factor effecting drug availability is that the loss of the drug from the palpebral fissure. This takes place by spillage of the drug from the eye and its removal via nasolacrimal apparatus. The normal volume of tears in human eye is estimated to be approximately 7 µl, and if blinking does not occur the human eye can accommodate a volume of 30 III without spillage from palpebral fissure.
With an estimated drop volume of µl, 70% of the administered volume of 2 drops can be seen to expel from the eye by overflow. If blinking occurs, the residual volume of lO µl indicates that 90% of the administer volume of two drops will be expelled. The second factor is the drainage of the administered drop via the nasolacrimal system into the gastrointestinal tract which begins immediately upon instillation. This takes place when reflex tearing causes the volume of the fluid in the palpebral fissure to exceed the normal lacrimal volume of 7 - 10 µl.
Fig (l) indicates the pathways for this drainage. A third mechanism of drug loss from the lacrimal fluid is systemic absorption through the conjunctiva of the eye. The conjunctiva is a thin, vascularized membrane that lines the inner surface of the eyelids and covers the anterior part of the sclera. Due to the relative leakiness of the membrane, rich blood flow and large surface area, conjunctival uptake of a topically applied drug from the tear fluids is typically an order of magnitude greater than corneal uptake . Figure (1): The pathways for drainage of drug from the eye 
In competition with the three foregoing drug removal from the palpebral fissure is the transcorneal absorption of drug, the cornea is an avascular body and, with the percorneal tear film first refracting mechanism operant in the physiological process of sight. It is composed of lipophilic epithelium, Bowman's membrane, hydrophilic stroma, Descement's membrane and lipophilic endothelium. Drugs penetrate across the corneal epithelium via the transcellular or paracellular pathway. Lipophilic drugs prefer the transcellular route.
Hydrophilic drugs penetrate primarily through the paracellular pathway which involves passive or altered diffusion through intercellular spaces, for most topically applied drugs, passive diffusion along their concentration gradient, either transcellularly or paracellularly, is the main permeation mechanism across the cornea . Physicochemical drug properties, such as lipophilicity, solubility, molecular size and shape and degree of ionization affect the route and rate of permeation in cornea . 1. 2 Microemulsions Oil and water are immiscible. They separate into two phases when mixed, each saturated with traces of the other component .
An attempt to combine the two phases requires energy input to establish water-oil contacts that would replace the water-water and oil-oil contacts. The interfacial tension between bulk oil and water can be as high as 30- dynes/cm . To overcome this, surfactants can be used. Surfactants are surface-active molecules. They contain water-loving (hydrophilic) and oil-loving (lipophilic) moieties . Because of this characteristic, they tend to adsorb at the water-oil interface. If enough surfactant molecules are present, they align and create an interface between the water and the oil by decreasing the interfacial tension .
An emulsion is formed, when a small amount of an appropriate surfactant is mechanically agitated with the oil and water. This results in a two-phase dispersion where one phase exists as droplets coated by surfactant that is dispersed throughout the continuous, other phase. These emulsions are milky or turbid in appearance due to the fact that the droplet sizes range from 0. 1 to 1 micron in diameter . As a general rule, the type of surfactant used in the system determines which phase is continuous. If the surfactant is hydrophilic, thenoil will be emulsified in droplets throughout a continuous water phase.
The opposite is true for more lipophilic surfactants. Water will be emulsified in droplets that are dispersed throughout a continuous oil phase in this case . Emulsions are kinetically stable, but are ultimately thermodynamically unstable. Over time, they will begin to separate back into their two phases. The droplets will merge together, and the dispersed phase will sediment (cream) . At this point, they degrade back into bulk phases of pure oil and pure water with some of the surfactant dissolved in preferentially in one of the two . 1. 2. Characteristics of Microemulsions If a surfactant that possesses balanced hydrophilic and lipophilic properties is used in the right concentration, a different oil and water system will be produced. The system is still an emulsion, but exhibits some characteristics that are different from the milky emulsions discussed previously. These new systems are called “microemulsions”. The interfacial tension between phases, amount of energy required for formation, droplet sizes and visual appearance are only a few of the differences seen when comparing emulsions to microemulsions.
Microemulsions are in many respects small-scale emulsions. They are fragile systems in the sense that certain surfactants in specific concentrations are needed for microemulsion formation . In simplest form, they are a mixture of oil, water and a surfactant. The surfactant, in this case, generates an ultra-low free energy per unit of interfacial area between the two phases (103mN/m) which results from a precise balance between thehydrophilic and lipophilic nature of the surfactant and large oil-to-water interfacial areas.
These ultra-low free energies allow thermodynamically stable equilibrium phases to exist, which require only gentle mixing to form . This increased surface area would ultimately influence the transport properties of a drug . The free energy of the system is minimized by the compensation of surface energy by dispersion entropy. The flexible interfacial film results in droplet sizes that fall in a range of 10-100 nm in diameter for microemulsion systems. Although these systems are formed spontaneously, the driving forces are small and may possibly take time to reach equilibrium .
This is a dynamic process. There is diffusion of molecules within the microstructures and there are fluctuations in the curvature of the surfactant film. These droplets diffuse through the continuous phase while kinetics of the collision, merging and separation of droplets occur [13, 10]. With droplet sizes in the nanometer range, microemulsions are optically transparent and are considered to be solutions. They are homogeneous on a macroscopic scale, but are heterogeneous on a molecular scale . Microemulsions usually exhibit low viscosities and Newtonian flow characteristics.
Their flow will remain constant when subjected to a variety of shear rates. Bicontinuous formulations may show some non-Newtonian flow and plasticity . Microemulsion viscosity is close to that of water, even at high droplet concentrations. The microstructure is constantly changing, making these very dynamic systems with reversible droplet coalescence . To study the different properties of microemulsions, a variety of techniques are usually employed. Light scattering, x-ray diffraction, ultracentrifugation, electrical conductivity, and viscosity measurements have been widely used .
These are only a few of themany techniques used to characterize microemulsions. Instrumentation and their application to microemulsions will be discussed in a later chapter. 1. 2. 2 Types of Microemulsions Microemulsions are thermodynamically stable, but are only found under carefully defined conditions . One way to characterize these systems is by whether the domains are in droplets or continuous . Characterizing the systems in this way results in three types of microemulsions: oil-in-water (o/w), water-in-oil (w/o), and bicontinuous.
Generally, one would assume that whichever phase was a larger volume would be the continuous phase, but this is not always the case. Figure (2): Possible nanostructures present within microemulsions: a) o/w; b) o/w, and c) Bicontinuous  Oil-in-water microemulsions are droplets of oil surrounded by a surfactant (and possibly co-surfactant) film that forms the internal phase distributed in water, which is the continuous phase. This type of microemulsion generally has a larger interaction volume than the w/o microemulsions .
The monolayer of surfactant forms the interfacial film that is oriented in a “positive” curve, where the polar head-groups face the continuous water phase and the lipophilic tails face into the oil droplets . The o/w systems are interesting because they enable a hydrophobic drug to be more soluble in an aqueous based system, by solubilizing it in the internal oil droplets. Most drugs tend to favor small/medium molecular volume oils as opposed to hydrocarbon oils due to the polarity of the poorly water-soluble drugs. An o/w drug delivery tends to be straightforward when compared to w/o microemulsions.
This is the result of the droplet structure of o/w microemulsions being retained on dilution with the biological aqueous phase . Water-in-oil microemulsions are made up of droplets of water surrounded by an oil continuous phase. These are generally known as “reverse-micelles”, where the polar headgroups of the surfactant are facing into the droplets of water with the fatty acid tails facing into the oil phase. This type of droplet is usually seen when the volume fraction of water is low, although the type of surfactant impacts this as well.
A w/o microemulsion used orally or parenterally may be destabilized by the aqueous biological system. The biological system increases the phase volume of the internal phase, eventually leading to a “percolation phenomenon” where phase separation or phase inversion occurs . Oral peptide delivery in w/o microemulsions is still used, however, The hydrophilic peptides can be easily incorporated into the water internal phase and are more protected from enzymatic proteolysis by the continuous oil phase than other oral dosage forms [17, 18].
A w/o microemulsion is best employed, though, in situations where dilution by the aqueous phase is unlikely, such as intramuscular injection or transdermal delivery [17, 19]. When the amount of water and oil present are similar, a bicontinuousmicroemulsion system may result. In this case, both water and oil exist as a continuous phase. Irregular channels of oil and water are intertwined, resulting in what looks like a “sponge-phase” [ 20, 21]. Transitions from o/w to w/o microemulsions may pass through this bicontinuous state.
Bicontinuousmicroemulsions, as mentioned before, may show non-Newtonian flow and plasticity. These properties make them especially useful for topical delivery of drugs or for intravenous administration, where upon dilution with aqueous biological fluids form an o/w microemulsion . 1. 2. 3 Preparation of Microemulsion The preparation of microemulsions requires the determination of the existence range of microemulsions, which can be determined by visual observation of various mixtures of surfactant, co-surfactant, oily phase, and aqueous phase reported in a phase diagram.
Two techniques are presented in the literature, each of them resulting in microemulsions: (1)"Exact" process by autoemulsification; (2) process based on supply of energy. 1. 2. 3. 1 Autoemulsification: Due to the spontaneous formation of the microemulsions, they can be prepared in one step by mixing the constituents with magnetic stirrer. The order of the addition of the constituents is not considered a critical factor for the preparation of micro emulsions, but it can influence the time required to obtain equilibrium.
This time will increase if the co-surfactant is added to the organic phase, because its greater solubility in this phase will prevent the diffusion in the aqueous phase. This method is easier and much simpler then "supply of energy" method . 1. 2. 3. 2 Process based on supply of energy: In this case, microemulsions are not obtained spontaneously. A decrease of the quantity of surfactants results in the use of high-pressure homogenizers in order to obtain the desired size of droplets that constitute the internal phase as opposed to the former technique .
Benita and Levy  have studied the efficacy of various equipment for obtaining particles of different sizes. Two steps are required: the first step produces a coarse emulsion (0. 65 mm) by using a high-speed mixer. The second step consists of using a high pressure homogenizer. The dispersion of the oily phase in the aqueous phase is also facilitated by heating the phases before mixing them, the choice of the temperature depending on the sensitivity of the drug to heat.
Cooling the preparation is required before its introduction in the high-pressure homogenizer, which can raise the temperature. A blue opalescent micro emulsion is obtained. 1. 2. 4 Review of literature: The microemulsion dosage form provided a delayed pharmacological action compared to the pharmacological action of regular eye drops. This observation led to the conclusion that the micro emulsion eye drops have a real advantage compared to regular eye drops which must be administered four times a day due to the short duration of the pharmacological action.
According to Naveh et al. , it appeared that the retention of pilocarpine content in the internal oil phase, and the oil-water interface of the emulsion are sufficient to concomitantly enhance the ocular absorption of the drug through the cornea, and also increasing the corneal concentration of pilocarpine. After comparing the diffusion profiles of two microemulsions preparations and an aqueous solution of pilocarpine, Hasse and Keipert  studied their pharmacological effect in vivo by using six rabbits for each group.
The obtained results were different from those observed in vitro. The two microemulsions provided a delayed release compared to the release of the drug incorporated in the aqueous solution. No experimental study has been conducted with microemulsions prepared by autoemulsification. However, several trials were conducted with microemulsions prepared by supply of energy. Melamed et al.  prepared micro emulsions containing adaprolol maleate. According to these authors, no ocular irritation was noticed in the group of forty healthy volunteers as opposed to regular eye droplets.
The depressor effect was delayed; the intra-ocular pressure was still high 6 and 12 h after the instillation of the micro emulsion. A single instillation of microemulsion or corresponding placebo, namely microemulsion without any drug, was administered to twenty healthy volunteers. The determined parameters were the pupillary diameter and variation of intra-ocular pressure. The effect of the micro emulsion which contains pilocarpine is obvious as compared to the placebo and was noticed within 1 h from instillation. The return to the initial values was noticed within 12 h [28,29]. Lv et al. 32] investigated micro emulsion systems composed of Span20/80, Tween20/80, n-butanol, H20, isopropyl palmitate (IPP)/isopropy lmyristate (IPM) as model systems of drug carriers for eye drops. The results showed that the stability of the chloramphenicol in the micro emulsion formulations was increased remarkably. Study of the effect of a single dose of atenolol 4% eye drops on 21 patients with primary open-angle glaucoma during a double-blind clinical trial. Monitoring of intraocular pressure (IOP), blood pressure, and pulse rate. At three and six h after medication, the average reduction of IOP was 7. and 4. 1 mm Hg respectively compared to the baseline readings without medication. The reduction of IOP at four h after medication was 6. 3 mm Hg compared to the pretreatment value. This corresponds to an average change from the pretreatment value of 22 percent. Blood pressure and pulse rate did not change significantly. We observed no subjective or objective ocular side effects. The duration of the effect of a single dose of Atenolol 4% eye drops is approximately six h. Atenolol 4% eye drops may become a useful agent in the medical treatment of glaucoma if a long-term effect and no ocular side effects . . 3 Atenolol Atenolol is a selective ? 1 receptorantagonist, a drug belonging to the group of beta blockers (sometimes written ? -blockers), a class of drugs used primarily in cardiovascular diseases. Introduced in 1976, atenolol was developed as a replacement for propranolol in the treatment of hypertension. The chemical works by slowing down the heart and reducing its workload. Unlike propranolol, atenolol does not pass through the blood-brain barrier thus avoiding various central nervous system side effects. 25] Atenolol is one of the most widely used ? -blockers in the United Kingdom and was once the first-line treatment for hypertension. The role for ? -blockers in hypertension was downgraded in June 2006 in the United Kingdom to fourth-line, as they perform less appropriately or effectively than newer drugs, particularly in the elderly. Some evidence suggests that even in normal doses the most frequently used ? -blockers carry an unacceptable risk of provoking type 2 diabetes. Figure (3): Chemical structure of Atenolol