Clinical trials for testing ocular hypotensive agents in human subjects have all the attendant problems associated with any NCE, although the accessibility of the eye makes direct exposure to the target tissues much easier. Nevertheless, safety is of prime concern, since even topical ocular dosing can produce a high systemic exposure due to the ocular drainage from the tear ducts into the nasal passages, followed by exposure to the back of the throat, tongue, and ultimately the stomach. Since NCE absorption from the nasal epithelium, throat, and tongue is quite robust, topical ocular dosing needs to limit the amount that will ultimately enter the systemic circulation. Thus, for instance, topical ocular dosing with beta blockers, a2-adrenergic agonists, and other systemically active drugs can have profound effects on the blood pressure, heart rate, and pulmonary circulation, as well as respiratory activity and CNS function. Other side-effects of topical ocular dosing of drugs (see below), including burning, stinging, foreign body sensation, and hyperemia, can severely affect patient compliance and thus the net result of the trial.
From an efficacy perspective, clinical trials of ocular hypotensive agents can be relatively short with well-defined endpoints of IOP reduction following once or twice daily dosing. However, recruitment of glaucoma and ocular hypertensive patients and the provision of the NCE in the correct formulation with reasonable shelf-life and stability are formidable challenges fraught with logistics issues and, ultimately, patient compliance.
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