Page 102 - Pharmaceutics IV (02-06-01305)

Ophthalmic DD: Introduction

Anatomy & physiology

Nasolacrimal system: nasolacrimal drainage
• Most of the topical dose is lost through the nasolacrimal drainage

before it can reach the eye layers (low ocular bioavailability).
• The tear turnover rate (reflex tearing), and blinking rate (reflex

blinking) are all natural ways of the eye to protect itself and limit
penetration through its surface.

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Ophthalmic DD: Introduction

Anatomy & physiology

Nasolacrimal system: nasolacrimal drainage
• Both the volume and the composition of instilled eye drops are

important:
➢ An average drop volume is 39 or 50 µL. The rate of drainage has a

positive correlation with instilled volume (reflex blinking).
➢ An irritant formulation (pH, osmolarity, …) will induce more tear

secretion (reflex tearing) and increased blinking that will enhance the
drainage out of the eye surface and reduce amount of drug available
for absorption.

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Salma E. EL-Habashy 99