Sustained Delivery of Timolol Maleate for Over 90 Days by Subconjunctival Injection

Abstract

Purpose: Medical treatment of glaucoma relies on intraocular pressure (IOP)-lowering medications, typically administered daily by the patient. While these medications are effective when applied correctly, patient adherence is a major obstacle in glaucoma treatment. We have developed a sustained-release formulation of timolol maleate that can be injected subconjunctivally to avoid patient noncompliance.

Methods: A biodegradable microsphere formulation for timolol maleate was injected subconjunctivally in normal rabbits. We measured timolol levels in tears, aqueous humor, vitreous humor, and serum of study rabbits. Furthermore, IOP profiles were recorded longitudinally. Tissue compatibility and side effects were evaluated using histochemistry.

Results: The microsphere formulation led to measureable amounts of timolol in the aqueous humor and the tear film for up to 90 days. Timolol was not detectable in the serum at any time. A significant reduction of IOP was observed in treated eyes. Clinically, the subconjunctival administration of the microspheres was well tolerated with no signs of inflammation or infection. The absence of local inflammation was confirmed by histology.

Conclusions: A single subconjunctival administration of timolol microspheres achieved delivery and IOP reduction in rabbits for up to 90 days without local or systemic inflammation or toxicity. This approach has the potential to improve the management of glaucoma in patient populations, who are challenged to adhere to a regimen of daily eye drops.

Keywords: glaucoma pharmacology; rabbit; sustained delivery; timolol maleate.

FIG. 1.

Release curve for timolol maleate microspheres PLGA 502H/P(dI)LA in vitro in an infinite sink system. PLGA, poly(D,L-lactic-co-glycolic acid).

FIG. 2.

Rabbit eye showing the bleb induced by microsphere administration immediately after injection. The bleb is resorbed over the first 24 h.

FIG. 3.

IOP profile of rabbits having received subconjunctival injections of timolol (white bars, N = 10) or empty (black bars, N = 5) microspheres. Bars indicate average IOP ± standard deviation. *P < 0.05; **P < 0.01. IOP, intraocular pressure.

FIG. 4.

Detection of timolol in the aqueous humor and tear film of rabbits after injection of timolol microspheres. The drug was detected in both aqueous humor and tears over 90 days after injection. Open bars: Concentration in aqueous humor samples, shaded bars: concentration in tear samples. Bars indicate average concentrations of all samples ± standard deviation.

FIG. 5.

Histology of the injection site. (A) Appearance of the sclera (SC) and conjunctiva (CN) at 100 × magnification. (B) Histology of conjunctival tissue at 400 × magnification. An infiltration of inflammatory cells is not observed. PLGA microspheres are lost during tissue sectioning and accordingly are not detected.