薬 剤 学, 80 (4), 193-197 (2020)≪若手研究者紹介≫

Unlocking the Potential of Eyelid Skin-Based
Drug Delivery: A Filipino Pharmaceutical
Scientist’s Journey

Gerard Lee Lo See*
Graduate School of Pharmaceutical Sciences, Josai University

1.  Introduction
In 2015, Dr. See, a lecturer at the University of San
Carlos, led the Philippine delegation in a two-week
Sakura Science Exchange Program which was hosted
by the Laboratory of Drug Safety Management, Josai
University (Prof. Yutaka Inoue). It was his ﬁrst academic immersion in Japan. His brief encounter with
the Japanese culture and academic systems deepened
his interest for pharmaceutical science research andinspired him to pursue further studies. Leaving the
comfort of his home for the next three years was toughyet a fruitful decision.
In spring of the following year, he joined the Laboratory of the renowned dermal scientist Professor Kenji
Sugibayashi where they pioneered the technique ofdelivering ophthalmic drugs through the eyelid skin.
Having worked hard together, they produced 4 scientiﬁc papers on eyelid-skin based drug delivery, 3 international research presentations, 2 best oral presentation awards, and the first Filipino pharmaceuticalscientist to receive the post-doctoral award given by the*Gerard Lee Lo See is a Filipino pharmaceutical scientist
and currently a post-doctoral researcher in Josai University, Japan. He completed his Bachelorʼs and Masterʼs degrees
from the University of San Carlos, Philippines. He obtained
his Doctorate in Pharmaceutical Sciences from Josai University in 2019. The International Pharmaceutical Federation awarded him the Best Oral Presentation Award
consecutively for 2018 and 2019. The Academy of Pharmaceutical Sciences and Technology, Japan recognized Dr. See
as the ﬁrst Filipino pharmaceutical scientist to receive the
Post-Doctoral Presentation Award in 2019. In 2020, he wasrecognized by the Philippine Pharmacists Association as
Outstanding Pharmacist in Research.
E-mail: glsee@usc.edu.phAcademy of Pharmaceutical Sciences and Technology,
Japan.
What were the milestones towards these achievements? Letʼs get to know the details in the succeeding
sections.
2.  Eyelid Skin as Potential Site for Drug
Delivery to Ocular Tissues
For several decades, eye drops have remained as themethod of choice in delivering ophthalmic drugs andthe most extensively utilized pharmaceutical formulations for various ocular diseases (i.e., the eyeball andsurrounding tissues). Currently, eye drops account for
about 90% of ophthalmic medicines, primarily due totheir ease of administration and good patient compliance. However, several drawbacks are associated withthe utilization of eye drops. Eye medications cannot be
administered beyond the capacity of the conjunctival
sac due to its limited volume. Most eye drops exhibit
low bioavailability, poor targeting efﬁcacy, and are virtually impossible to administer during sleep. Anatomical and physiological constrains such as tear turnover,nasolacrimal drainage, reflex blinking, and ocularstatic and dynamic barriers impede the bioavailability
and controlled delivery of drugs administered as eye
drops. With these impediments, it is essential to search
for an alternative approach to deliver ophthalmic drugs
with high targeting ability while simultaneously improving drug absorption into the ocular tissues.
We paid attention to applying formulations and delivering ophthalmic drugs onto the (lower) eyelid skin.
The lower eyelid skin exhibits less movement due toblinks compared with the upper eyelid skin, making ita good site for administration. Moreover, it is very interesting for skin researchers to explore the eyelids,

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Fig. 1 Photomicrographs of rat eyelid skin under CLSM after in vivo absorption studies. Transmission
Images (A1–L1); Confocal laser scanning images (A2–L2) 1).

which is the thinnest skin layer on the human body(＜1 mm). Generally, the thinner the stratum corneum,
the greater the drug permeation. In addition, drug
administration through the skin provides several advantages because it generally facilitates avoidance ofpremature metabolism, decreased toxicity, fewer side
effects as well as greater patient compliance.
With the limited number of studies investigating thedelivery of drugs into the conjunctiva through eyelidskin and the paucity of data describing eyelid drug
permeation, we were prompted to study hydrophilic
and lipophilic model compounds and to compare permeability characteristics through the eyelid and theabdominal skin. Pilocarpine hydrochloride, tranilast,
antipyrine, diclofenac sodium, aminopyrine, and lidocaine were used to evaluate their skin permeation; theformer two were used to evaluate the skin concentration, and two ﬂuorescent dyes (ﬂuorescein sodium andrhodamine B) were used to determine the in vivo distribution of the drugs in the conjunctiva.
Drug permeation through the eyelid skin was higher
than through the abdominal skin independent of thelipophilicity of the applied drugs. Figure 1 shows the
drug distribution in skin after application of its solution in eye drops or by topical application onto thelower eyelid skin. The presence of rhodamine B andﬂuorescein sodium administered as eye drops as mark194

edly observed in the conjunctiva, as indicated by theﬂuorescence in the images 10 min after administration,
whereas these were absent at 2 and 8 h. For topicaladministration onto the eyelid skin, a stronger intensity of the ﬂuorescence given off by the model compounds was observed over time, which indicated greater amounts of dye compound in the conjunctiva.
The eyelid skin is known to be thinner than the conventional skin administration sites, and it offers higher drug permeation, as demonstrated by the results ofthis study. Developing formulations dedicated to eyelid
administration such as an eyelid patch is recommended. In fact, ointments are often used for night time
application but are associated with discomfort due togreasiness, tear ﬁlm instability, and uncertain drug
disposition after eye shutting while asleep. Thus, eyelidskin application is seen to be beneﬁcial even during
sleep when the use of liquid and semi-solid eye preparations is deemed impossible.
3.  Enhancement of Drug Delivery into
the Eyeball through Eyelid Skin
by Iontophoresis
In the previous section, we established that drugs
applied to the lower eyelid skin could migrate to the
conjunctiva, and the drug concentration was maintained in the conjunctival sac for a long duration com-

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Fig. 2

Photomicrographs of rat eyeballs after in vivo distribution studies. All observations were performed withoutchanging observation conditions for each drug 2).

pared with eye drop application. However, the migration of drugs to the eyeball after eyelid skin application
was considered to be lower than eye drops because
drugs, in the case of eye drops, were delivered directly
onto the eyeball. In this case, we considered the necessity of utilizing physical approaches, such as iontophoresis (IP), to further promote percutaneous absorption
and delivery to the eyeball via the lower eyelid skin.
IP is a non-invasive technique to increase the penetration of ionized compounds into the body across theepithelial surface, including the skin and ocular surface. The basic principle of IP is that ions with opposing
charges attract and ions with the same charge repel.
Moreover, ionized substances are driven into the tissue
by electrorepulsion and electroosmosis. This method
has the potential to control the penetration of drugs
depending upon the applied current density and application time. Because of its recognizable advantages,
such as the ease of application, non-invasiveness, andincreased drug penetration directly into the target
tissue, IP has been used to deliver drugs across various
barriers such as the skin and eyes. Moreover, topicalpreparations utilizing IP have already been put topractical use in ocular drug delivery by direct application of an IP device to the cornea or the eyelids but
none have been done on the lower eyelid skin.
In vivo study reveals that the concentration of pilocarpine in the eyeball is significantly increased by
3-fold with IP application over 0.5 h. For the 2-h application, however, it did not signiﬁcantly increase withIP, suggesting that the steady state of pilocarpine had

been reached within 0.5 h after IP application. In vivo
distribution studies of two fluorescent dyes further
supported the ﬁndings of our in vivo skin permeation
studies. Topical administration with IP showed that
corneal permeation of the both ﬂuorescein sodium andrhodamine B was observable 0.5 h after administration(Fig. 2).
The eyelid skin is known to be thinner than most
known application skin sites, and offers higher drug
permeation into target tissues. With an aid of physicalpermeation enhancement methods, it is expected thatpermeation rates will exceed conventional drug delivery methods. IP application on the eyelid skin allowed
the delivery of pilocarpine to the eyeballs. The concentration of pilocarpine after eye drop is important andthe measurement of its pharmacokinetic proﬁle as wellas the disposition in the eye will be considered in succeeding studies.
The use of IP enhanced the penetration and deliveryof drugs into the eyeball after topical application onto
the lower eyelid skin. Higher concentrations of pilocarpine were detected in the eyeball after IP application.
Histomorphological evidence conﬁrmed the migrationof dyes into the posterior chamber of the eyeball after
topical administration onto the lower eyelid skin with
IP.
4.  Pharmacokinetics and Tissue Distribution of
Pilocarpine Following Eyelid Skin Delivery
Extending the delivery of drugs into the eyes while
reducing systemic bioavailability is of utmost impor-

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195

Fig. 3

Disposition of pilocarplne after topical eyelid, eye drops, and intravenous administration. Eyelid skin(◆); Conjunctiva (●); Eyeball (▲); Plasma (■). Each value represents the mean ± S.D. (n＝4) (*p＜0.05) 3).

tance, especially for the management of chronic oculardiseases. The direct and sustained delivery of drugs
into the eyes is seen to be a practical strategy in the
management of these diseases. Furthermore, the eyelidskin is proximate to the conjunctiva, which could function as a reservoir for ocular drugs. In this case, thedrug may not be eliminated rapidly because it is localized in the vasculature, allowing sustained delivery ofthe drug into the eyeball from the eyelid skin. Of note,the conjunctiva is 2 to 30 times more permeable todrugs than the cornea, and it has a direct relation withthe eyeball, the target site in most chronic ocular diseases.
We selected pilocarpine as a model drug in this investigation. Pilocarpine hydrochloride, a nonselective
muscarinic receptor agonist, has long been used in thetreatment of glaucoma by increasing trabecular outﬂow
leading to reduced intraocular pressure. In ciliarysmooth muscle cells, pilocarpine binds to and activatesmuscarinic M 3 receptors and stimulates the contractionof longitudinal ciliary muscle manifested as miosis.
Current delivery systems, eye drops and ocular inserts,
are available for the transport of pilocarpine into theeyes, but they are linked with disadvantages. Pilocarpine drops deliver high dose of drugs, which initiallycause side effects of headache, dimmed vision due tomiosis, and myopia.
There have been no investigations whether topicalapplication onto the lower eyelid skin may be useful forthe treatment of chronic eye diseases. The only evidence is in our previous work, wherein the eyelid skin
was found to be more permeable to drugs. To further
elucidate the value of delivering drugs onto the eyeballthrough the eyelid skin, pharmacokinetic studies ofpilocarpine were conducted. Our pharmacokinetic results afﬁrmed our hypothesis, and these results wereveriﬁed using a direct pharmacodynamic study of pilo196

carpine in rats.
Among the three methods of application (Fig. 3),
topical eyelid application proved to be superior in delivering pilocarpine into the eyes for an extended period. It was evident that the mean residence time (MRT)of pilocarpine in the ocular system was signiﬁcantly
longer compared with eye drops and the intravenous
route. A longer residence time corresponded to a slower absorption rate and consequently a slower elimination process, which allowed the drug to accumulate in
the speciﬁc ocular tissues for a longer period of timeand exhibit its therapeutic efﬁcacy. Thus, high drugconcentrations were detected in the conjunctiva andeyeball after topical eyelid application. Notably, pilocarpine plasma concentrations were lower after topicalapplication onto eyelid skin, suggesting ocular absorption and speciﬁc accumulation of the drug in the conjunctiva driven by a high concentration gradient in theeyelid skin and then diffusing into the eyeball. Veryhigh concentrations of the drug in the application site,
eyelid skin, remained for the whole observation time.
However, the concentration started to decline in the
conjunctiva and eyeball from 4 h after drug instillation.
A possible depletion of drug from the application tube
may have occurred, since our previous in vitro study
showed that about 5–10% of the applied dose permeated over 4 h. Hence, a longer half-life time and MRT
after eyelid skin administration in comparison to eye
drop instillation are good indicators for a longer duration of therapeutic response, possibly resulting to alower dosing frequency.
When drugs were delivered as eye drops, a very different situation was observed. Drugs penetrate the eyeby absorption across the cornea from the precorneal
tear ﬁlm. The kinetic behavior of drugs in the tears has
a direct bearing on the efﬁciency of drug absorption by
the eye. Our study demonstrated that drugs following

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eye drops rapidly entered the eyeball and exerted apharmacologic effect, but this was short-lasting due tothe short MRT and particularly fast elimination ratefrom the application site. Eye drops typically remainon the ocular surface for a very short period of time
before being washed away by the tears into the nasolacrimal duct, a gateway for the systemic circulation.
Moreover, high plasma drug concentration, as shownin this study, after eye drops indicated non-specificaccumulation of the drug in the non-target organs. Thisphenomenon emphasizes the possible occurrence of
unwanted side effects associated with pilocarpine when
given as eye drops. Furthermore, in the case of intravenous route, although pilocarpine in the systemic
circulation reached the ocular tissues, high doses were
essential, which may lead to unwanted side effects.
Thus, the ocular kinetics of pilocarpine after eye drops
application is more like that after intravenous dose.
There is extensive evidence indicating differences in
the permeability of skin to chemical substances depending upon the application site. Based on the previous experiments, it was shown that the skin permeability of pilocarpine was 2.6-fold higher in the case ofeyelid skin than for the abdominal skin of hairless rats.
In order to prove the results obtained from in vitro
experiments in in vivo conditions, topical applicationon the abdominal skin was performed. The results ofthis experiments conﬁrmed the trend observed in vitro,where skin permeability for pilocarpine was low for
abdominal skin, whereas the eyelid skin was permeable
to a greater extent. The eyelid skin, through the conjunctiva, behaves like a reservoir wherein a drug absorbed after topical application remains in the speciﬁctarget organ and exerts a speciﬁc pharmacologic effect.
The higher tissue-to-plasma concentration indicatesthat drug accumulation takes place in the speciﬁc target organ following topical application.
Direct pharmacologic observation was conducted tofurther confirm the results of the pharmacokinetic
data. Pupil size reduction, miosis, is attributed to thepharmacologic effect of pilocarpine. Our findings
showed that pupil size reduction increases over timeafter topical eyelid application indicating a prolonged
pharmacologic effect of pilocarpine. This further confirmed localized drug action with sustained releasefeatures.

Topical eyelid skin application was shown to havethe highest mean residence time and a higher drugconcentration in the conjunctiva and eyeball, suggesting localized drug absorption and speciﬁc drug accumulation in the ocular tissues. Consequently, a longer
duration of therapeutic response may permit less-frequent dosing.
5.  Summary and Acknowledgements
In summary, the eyelid skin allows drugs to permeate at a higher rate regardless of drug lipophilicities
which is attributed to the thinner size of the stratum
corneum and lesser amount of neutral lipids. Topicalapplication onto the lower eyelid skin resulted to higher drug penetration into the eyeball with the application of iontophoresis. Topical application onto thelower eyelid skin allows extended delivery of drugs into
the ocular region compared to eye drops based on pharmacokinetics and pharmacodynamics data. Hence, it isrational to prepare the topical formulations directed tothe eyelid skin, which is suitable for drugs requiringlong-term treatment.
Dr. See would like to express his sincerest gratitude
to the following individuals who have significantly
contributed to the success of this work; Professor Kenji Sugibayashi for the scientiﬁc and technical guidance,
for being a constant source of encouragement and support. He was a source of inspiration in the work ofscience and a true leader; To Professor Hiroaki Todo
and Professor Konstanty Wierzba, for the technicalguidance, scientiﬁc inputs, research inspirations, andunending support in the implementation of the study;
Finally, Professor Yutaka Inoue for the support andencouragement.
References
1) G. L. See, A. Sagesaka, S. Sugasawa, H. Todo, K. Sugibayashi, Eyelid skin as a potential site for drug delivery
to conjunctiva and ocular tissues, Int. J. Pharm., 533,
198–205 (2017).
2) G. L. See, A. Sagesaka, H. Torizuka, H. Todo, K. Sugibayashi, Iontophoresis-aided drug delivery into theeyeball via eyelid skin, J. Drug Deliv. Sci. Technol., 47,
380–385 (2018).
3) G. L. See, A. Sagesaka, H. Todo, K. Wierzba, K. Sugibayashi, Pharmacokinetics and tissue distribution of
pilocarpine after application to eyelid skin of rats, J.
Pharm. Sci., 108, 2942–2948 (2019).

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