Physiological Pharmaceutics

Ocular drug delivery 265endothelial damage. Intraocular contamination with these vehicles should therefore beavoided. Formulations based on white petrolatum-mineral oil have a residence half-time ofover an hour in man 19 .Antibiotics such as tetracyclines are used in the form of an ointment, producingeffective antibacterial concentrations in the anterior chamber for several hours, whereas anaqueous solution of tetracycline is ineffective for intraocular infections. A problem withextremely lipophilic drugs, including corticosteroids, is that the therapeutic agent may notbe released as it partitions into the oil base. For these drugs, alternative systems such aswater-soluble inserts may be preferable.ParticulatesPoorly soluble drugs for ophthalmic administration are frequently formulated as micronisedsuspensions. Larger particles theoretically provide prolongation of effect due to theincreased size of the reservoir; however, an increase in particle size is associated withirritation giving rise to an increased rate of removal, assisted by agglomeration of particlesand ejection. The relationship between particle size and retention is poorly understood andsize is probably not the only determining factor, with parameters such as zeta potential andsurface chemistry also being important. Increasing the size of particles to 25 µm in diameterwill increase retention time to around 12 hours in a rabbit, compared to 3 µm particleswhich disappear almost as fast as aqueous solutions 20 . However, increasing the particle sizeof 0.1% 3H-dexamethasone suspension, from 5.75, to 22 µm demonstrated that clearanceof the larger particle size from the eye is faster than complete dissolution 21 . Similarly,increasing the concentration of fluorometholone solids in a suspension did not increase theaqueous humour drug concentration-time profile 22 . Hence, small particle sizes generallyimprove patient comfort and acceptability of suspension formulations.Interestingly, submicron or nanosphere formulations demonstrate therapeuticadvantages over aqueous solutions, although one would expect rapid clearance from theeye. For example, pilocarpine (2% w/v) adsorbed onto a biocompatible latex of average size0.3 µm will maintain a constant miosis in the rabbit for up to 10 hours compared to 4 hourswith pilocarpine eye drops 23 . Other nanoparticle systems have been investigated for theprolongation of contact time in order to increase the ocular absorption. Betaxolol-poly-ecaprolactonenanoparticles produce a significantly greater reduction in intra-ocular pressurecompared to the commercial eyedrops 24 . Similar improvements were obtained with carteolol(a ß-blocker) which caused a better penetration of the drug in the nanosphere formulation.Sustained release devicesAlthough some particulate systems could be classified as sustained release devices, the termis applied here only to macroscopic devices such as inserts. Provision of a matrix to sustaindrug release in the eye can be achieved in several ways. For example, a hydrophilic (soft)contact lens can serve as a drug reservoir. The drug is incorporated into the lens by eitherinstilling drops on the lens when in the eye, or pre-soaking the lens in a solution of the drug.Other systems are not placed on the cornea, but are inserted under the eyelid, such asinsoluble inserts of polyvinyl alcohol or soluble collagen which dissolve in lacrimal fluid ordisintegrate while releasing the drug. Soluble inserts are made of such substances as gelatin,alginates, agar and hydroxypropylmethylcellulose. These systems have been developed as amethod for delivering larger amounts of drugs to the eye over a long period.Ocuserts ® (Alza Corporation, U.S.A.) are insoluble inserts containing pilocarpine usedin the treatment of glaucoma, and have a one-week duration of action. The majoradvantages of this system include longer duration of drug action, avoidance of