Common topical eye preparations are in the form of drops (eye drop) and ointment (eye ointment). The selection of the right type of preparation can ensure that the active substance reaches the target tissue according to the desired duration and concentration. Topical eye drugs are more effective for the eye's anterior segment diseases. Conjunctivitis, keratitis, blepharitis, and uveitis are the eye's anterior segment diseases.





Factors Affecting Pharmacokinetics of Topical Eye Drugs

Topical eye drugs pharmacokinetics is influenced by:
  • the carrier types, 
  • the concentration of the preparation, 
  • the lipophilic or hydrophilic nature of the active substance contained. 

Lipophilic medicines are well absorbed through the cornea. Meanwhile, permeation through the conjunctiva and sclera allows for lipophilic and hydrophilic drugs. Topical eye medications' effectiveness is influenced by the patient's cornea conditions and eye barrier, the drug's physical form, and the drug's bioavailability.



Types of Eye Drops

Eye drops are available in solutions, emulsions, and suspensions and have advantages and disadvantages.


a. Solution Eye Drops
The solution eye drop is one of the most common of topical eye drops. Solution eye drops are easy to apply, quickly absorbed, and most do not interfere with the vision, so they are comfortable to use during the day. Solution eye drops can be found in bottles and mini doses, with a liquid pH ranging from 7.4 (pH 4-8).

Although they tend to be easier to use, the solution eye drops have a low bioavailability. Only about 5% of the dose given through solution eye drops can reach the deeper eye tissue—the tears wast the rest about 15-30 seconds after administering the drug. Solution eye drops are also easier to enter into the lacrimal duct and are more likely to cause systemic effects.

b. Emulsion Eye Drops
Oil in water (o / w) emulsion is an eye drop preparation mainly used as an artificial tear or lubricant.
There are also emulsion drops filled with antibiotics and anti-inflammatory. Several studies have shown emulsion eye drops increase precorneal drug duration, corneal permeation, and ocular bioavailability.

c. Suspension Eye Drops
Suspension eye drops also have a better drug contact duration than a solution because the suspended particles tend to stay in the precorneal sac. The duration of the drug's action depends on the active substance's particle size in the suspension. The small particle size facilitates permeation, whereas the larger particle size makes the permeation longer, and the drug dissolves slower.



Types of eye ointments

Unlike eye drops, eye ointments generally do not cause a stinging feeling when applied to the eyes, especially when they hit pain-sensitive areas such as the cornea. The eye ointment consists of an active substance mixed with a semisolid and solid (paraffin), which will melt at the eye's physiological temperature (34 C).

Eye ointments are usually packaged in a tube. The ointment preparation has better ocular bioavailability and facilitates more extended drug release. In contrast to eye drops, eye ointments have a longer duration of contact with the eye surface. The use of ointments can cause side effects such as irritation and blurred vision for a while after use.



Choosing an Eye ointment or Eye Drops

Choosing eye ointments or eye drops according to the therapeutic goals to be achieved. Eye drops are usually used in addition to being therapeutic (antibiotic, antifungal, anti-inflammatory), they are also available as topical drugs used before performing some procedures such as analgesics (pantocaine) and for diagnostics (e.g., fluorescein drops). Eye ointments are generally used for therapeutic purposes only.

Adjusting to Eye Diseases

Selecting drops or ointments should be adjusted to the patient's eye disease. For example, in the corneal abrasion, eye ointment is recommended because it has better lubrication properties and can provide a barrier effect to prevent further corneal damage.

Avoid applying Eye ointment in open globe injury or deep corneal ulcer cases. Because if oil-based eye ointments reach intraocularly, it can cause:
  • severe uveitis, 
  • corneal endothelial damage, 
  • vitreous opacities, 
  • and macular edema.

Eye ointments are also more widely used in cases of blepharitis. Whereas for cases of conjunctivitis, episcleritis, scleritis, keratitis, and uveitis, eye drops are preferred. 

For dry eye diseases, lubricants are needed to provide comfort. Eye drops are generally preferred because they do not interfere with the vision, so they can still be used during working hours. Eye ointment may be needed for adjunct in severe dry eye syndrome cases due to better lubrication properties and longer contact with the eye surface. For dry eyes due to lagophthalmos, eye ointments can also protect the eye's surface exposed to outside air while sleeping.



Frequency of Administration and Patient Compliance

Generally, giving eye drops is more frequent than eye ointments to achieve the desired target dose. Of course, this is taken into consideration regarding patient compliance with medication.
The administration of several eye drops can have a stinging effect on the eyes, so children refuse to apply them repeatedly. Meanwhile, elderly patients sometimes forget to put drops of medicine if the administration frequency is too frequent.

Adult patients are generally more familiar with and comfortable using eye drops than eye ointments because they do not give side effects to temporary visual disturbances. For infant patients, eye ointments are easier to apply than eye drops. The eye ointment should be given at night before bed to not interfere with the patient's activities.




Conclusion
There are various topical preparations for the eyes, including solution eye drops, emulsion eye drops, suspension eye drops, and eye ointments. Solution eye drops are easy to use, quickly absorbed, and most do not interfere with vision, so they are comfortable to use during the day. However, only a small part of the dose given can reach the deeper eye tissue, while the rest will be wasted with the tear film.

Emulsion eye drops are reported to increase precorneal drug duration, corneal permeation, and ocular bioavailability. Suspension eye drops also have a better drug contact duration than a solution because the suspended particles tend to stay in the precorneal sac. The duration of the drug's action depends on the particle size of the active substance in the suspension, where permeation occurs faster, the smaller the particle size.

The eye ointment is non-stinging when applied to the eye, has good ocular bioavailability, facilitates longer drug release, and a longer duration of contact with the eye surface.
Selecting topical eye drug preparation depends on the patient's eye disorder, the frequency of administration needed, and the patient's compliance level.