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Improving the ocular absorption of phenylephrine.

D S Chien, R D Schoenwald

Biopharmaceutics & Drug Disposition

|September 1, 1986

Summary

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A novel phenylephrine prodrug and its base form, when suspended in oil, show enhanced mydriatic activity in rabbit eyes compared to phenylephrine HCl. This suggests a pH-dependent absorption mechanism for improved ocular drug delivery.

Area of Science:

Ophthalmology
Pharmacology
Drug Delivery

Background:

Phenylephrine hydrochloride (HCl) is a common mydriatic agent.
Ocular drug delivery faces challenges due to tear film pH and rapid clearance.
Developing formulations that enhance drug absorption is crucial for effective ophthalmic treatments.

Purpose of the Study:

To compare the mydriatic activity of a phenylephrine prodrug and phenylephrine base in sesame oil versus phenylephrine HCl.
To investigate the effect of formulation vehicle and pH on phenylephrine's ocular efficacy.
To explore a novel approach for improving weak base/acid drug delivery in the eye.

Main Methods:

Synthesis of an oxazolidine prodrug and isolation of phenylephrine base.
Formulation of phenylephrine HCl as an aqueous solution and sesame oil suspension.

Related Experiment Videos

Intraocular instillation of 10 microliter volumes into rabbit eyes.

Measurement of pupillary diameter and tear fluid pH over time.

Main Results:

The phenylephrine prodrug suspension showed comparable mydriatic activity to a higher concentration of phenylephrine HCl solution, with an earlier peak effect.
Phenylephrine base suspensions in sesame oil exhibited greater mydriatic activity than equimolar phenylephrine HCl suspensions.
Instillation of phenylephrine base increased tear fluid pH, unlike phenylephrine HCl, favoring phenylephrine absorption.

Conclusions:

Phenylephrine prodrugs and the base form in non-aqueous vehicles can enhance mydriatic efficacy.
The increased efficacy is attributed to pH-mediated enhancement of drug absorption.
This formulation strategy is potentially applicable to other weak acids or bases with pKa values outside the physiological tear pH range.