Advanced drug delivery prepared by yasser alyassery

1. Ocular Applications
of Nanoparticulate Drug-Delivery Systems

2. INTRODUCTION
• The absorption of topically applied ophthalmic drugs is very poor because of
efficient mechanisms protecting the eye from harmful materials and agents.
• These protective mechanisms, such as reflex blinking, lachrymation, tear
turnover, and drainage, result in the rapid removal of foreign substances
from the eye surface.
• Moreover, the very tight epithelium of the cornea also compromises the
permeation of drug molecules.
• the contact time of conventional eye drops is only about five minutes, and
less than 5% of the applied dose penetrates passively across the cornea and
reaches the intraocular tissues.

3. • Thus, frequent instillations of eye drops are necessary to
maintain therapeutic drug level in the tear film or at the site of
action.
• However, the frequent use of highly concentrated drug solutions
may induce toxic side effects after absorption via the blood
vessels in the conjunctival stroma or nasal mucosa into systemic
circulation.
• Moreover, cellular damage at the ocular surface could occur

4. • The rationale for the development of various particulate systems
for sustained drug delivery is based on
possible entrapment of the particles in the ocular mucus layer
covering the eye surface
and the interaction of bioadhesive polymer chains with mucins.

5. This will increase the precorneal residence time of the drug,
allowing for an extension of the absorption time.
Furthermore, by controlling drug release and enhancing drug
absorption, this could also improve corneal drug penetration.
 One attractive feature of utilizing nanoparticles for ocular drug delivery is
that it is in a liquid dosage form which can be easily administered by
patients

6. • Another significant challenge is to deliver therapeutic doses of
drugs to treat diseases affecting the posterior segment of the eye.
• It is difficult to deliver drugs to the posterior segment by topical
application because of the diffusional distance and the counter-
directional intraocular convection from the ciliary body to Schlemm’s
canal.

7. • The most logical way is to deliver drugs by intraocular injections
thus bypassing anatomical and physiological barriers.
• However, drugs of short half-lives (e.g., antibiotics, antiviral
drugs) would require repeated injections, which could increase
the risk of retinal detachment or hemorrhage.

8. Therefore, biodegradable nanoparticles were developed for
intraocular administration in order to obtain a controlled,
sustained drug release and thus reducing the number of injections
required

9. • The choice of the polymer for preparing particulate systems will
depend on the physicochemical properties of the drug.
• The goal is to achieve high drug loading in order to minimize the
required volume to be instilled into the eye.
• Corneal uptake and transport of nanoparticles should be
facilitated by small particle size (100–200 nm)

10. Drug-release kinetics are regulated by
1. the composition and preparation procedure of the particles
2. the molecular weight and degradation of the polymers
3. the physicochemical properties of the entrapped drug molecule

11. POLYMERS OF NATURAL ORIGIN
• As they are biocompatible, polymers of natural origin such as
proteins and polysaccharides have been investigated for use in
the production of micro- and nanoparticles.
• A denaturation process induced by either heating or cooling and
subsequent chemical cross-linking procedures to create a denser
particle matrix prepare protein nanoparticles.

12. • Another preparation method is based on desolvation of
the macromolecules, which leads to precipitation or the
formation of a coacervate phase.
A cross-linking agent (e.g., glutaraldehyde) is added to harden the
native particles

13. • Owing to the presence of charged groups, protein nanoparticles can
be used as a matrix in which the drug molecules may be physically
entrapped or covalently linked
Some examples of particles for ocular purpose reported in literature are summarized in Table 1
• Various in vivo studies in rabbits reported that a prolonged effect of drugs
(pilocarpine, piroxicam) incorporated in albumin particles was observed when
compared to commercial preparations or aqueous and viscous solutions

15. • However, in one study, topical application of hydrocortisone-loaded
albumin particles in rabbits resulted in a lower tissue concentration of the
drug as compared to the application of a simple drug solution.
 This result is possibly due to the strong binding of the drug to the particles.
 The retention of nanoparticles was found to be higher in inflamed tissue
as compared with the normal tissue

16. • Ganciclovir, the most widely used antiviral drug for the
treatment of cytomegalovirus retinitis, was formulated in
nanoparticles for intravitreal administration in rats.
 A prolonged residence in the vitreous cavity (two weeks) showed no evidence
of inflammatory reaction in the retinal tissue and also did not affect the vision.

17. The types of polysaccharides which have been investigated for the
production of ocular particulates are
Chitosan
Pectin
Carrageenan.

18. • Chitosan, a deacetylated chitin, is biodegradable,
biocompatible, and nontoxic.
The polycationic chitosan was investigated as an ophthalmic vehicle
because of its probable superior mucoadhesiveness due to its ability
to produce molecular attractive forces by electrostatic interactions
with the negative charges of the mucus.
 an increase in the precorneal retention time of chitosan solutions.
 chitosan appears to enhance the permeability of the cornea
transiently due to the interaction with tight junction structures.

19. • For the preparation of chitosan nanoparticles, a number
of techniques are employed include:
 covalent chemical cross-linking
 ionic cross-linking (ionotropic gelation with tripolyphosphate)
 desolvation.

20. • Chitosan microspheres have been prepared by water-in-
oil evaporation and spray-drying.
By adjusting the molecular weight and the deacetylation
degree of the polymer used, one could achieve the
degradation and release rate required for a specific drug

21. In one study, the animals treated with cyclosporine A-loaded
nanoparticles were found to show significantly higher corneal and
conjunctival drug levels than those treated with a suspension of
cyclosporin A in a chitosan aqueous solution or in water (2–6-fold
increase)

22. In a recent study in rabbits, amounts of fluorescent nanoparticles in
cornea and conjunctiva were significantly higher than those for a
control solution.
These amounts were fairly constant for up to 24 hours. A higher
retention of chitosan nanoparticles in the conjunctiva as compared
to that in the cornea was observed.

23. • Confocal microscopy studies suggest that nanoparticles
penetrate into the corneal and conjunctival epithelia by a
paracellular/transcellular pathway which is different from
the pathway used by the poly(alkylcyanoacrylate) (PACA) and
poly(epsilon-caprolacton) (PECL) nanoparticles

24. • To improve the release kinetics of ofloxacin
chitosan microspheres loaded with ofloxacin were also incorporated
in poly(ethylene oxide) (PEO) inserts.
The in vivo results in rabbits did not demonstrate a
biopharmaceutical improvement compared to plain PEO inserts.

25. However, a significant increase in the Cmax value in the aqueous
humor was observed which could partly be due to the
permeabilityenhancing effect of chitosan across the cornea

26. • Pectin nanoparticles loaded with fluorescein showed an increase
in the precorneal retention time from 0.5 to 2.5 hours, when
compared with a fluorescein solution.
• In vivo tests with piroxicam-loaded pectin particles indicated a
2.5-fold increase in the amount of drug concentration in the
aqueous humor as compared to a commercial eye drop solution.

27. • the first study on enzyme-sensitive microparticles made of the natural
biodegradable potato starch acetate for retinal targeting.
• After a three-hour incubation, about 8% of the cells of a retinal pigment
epithelium (RPE) culture took up microparticles. The viability of cultured RPE
cells was at least 82% after 24-hour incubation with the microparticles. The
degradation of potato starch acetate in RPE cells is catalyzed by esterases
and amylases. Considering the low toxicity, it seems that these
microparticles are suitable for drug delivery to the posterior segment of the
eye

28. • Carrageenans are a group of water-soluble sulfated galactans
extracted from brown seaweed.
• Microspheres containing gelatin, lambda carrageenan, and timolol
were prepared by spray-drying.
• The different ratios of carrageenan and gelatin proved to be
useful in modulating the drug-release profiles and mucoadhesive
properties.

29. • After administration of a microsphere (50/50 mixture)
suspension, the bioavailability (AUC values) of timolol in the
aqueous humor in rabbits were 5.6 and 2.5 times higher in
comparison with commercial eye drops and in situ gelling system,
respectively.

30. ACRYLATES
• Poly(alkylcyanoacrylates)
In the past, two biodegradable and biocompatible polymers, PACAs and
poly(alkylmethacrylates), were quite popular for use in the preparation of drug
carriers of particle size range from 200 to 500 nm.

31. The use of these polymers is based on their mucoadhesive or
bioadhesive properties.
The difference between the two polymers lies mainly in their
degradation rate:
polymers with a longer side chain are degraded more slowly, resulting
in a slower release of the drug incorporated in PACA particles

32. The drug is either
incorporated in PACA particles during the polymerization process
or
adsorbed to the nanoparticle surface after the particle is formed.
 The ultimate particle size of the particles and the degree of drug loading are
dependent on several preparation parameters such as
1. pH of the preparation medium
2. type of stabilizer or surfactant used

33. • PACA nanospheres were shown to **adhere to the eye surface,
and were **taken up by a transcellular pathway in the outer cell
layers of conjunctiva and cornea.
This could be explained by either endocytosis or lysis of the cell wall
induced by particle degradation products.

34. Although the concentration of the drug in ocular tissues was
shown to be higher in inflamed tissues, a condition where the
permeability of the cell membrane is increased, no full penetration
across the cornea into the anterior chamber was observed

35. Many studies confirmed that the use of nanoparticles with these
polymers can improve the clinical effects of a therapeutic agent
(i.e., in glaucoma therapy or antibiotic therapy) and also minimizes
its side effects

36. • Sustained drug release from the polymer matrix and prolonged
therapeutic effect were observed, except in the situation where
the drug had a high affinity for the polymer.
• The increased biological response was attributed to improved
ocular penetration and bioadhesion.

37. The precorneal residence time of PACA nanoparticles could
further be increased by
 incorporating the particles into a polyethylene glycol (PEG) gel, or
 coatingthe particles with PEG

38. Acyclovir-loaded, PEG-coated polyethyl-2-cyanoacrylate
showed a 25-fold increase in the drug level in aqueous humor when compared
with an aqueous solution of the free drug.
This result is probably due to a longer interaction of the nanoparticles with the
corneal epithelium and the penetration-enhancing effect of PEG

39. Acrylate Derivatives
Polyacrylic acid (PAA) and carbomers are used in the preparation of
viscous eye drops, artificial tears, hydrogels, and inserts.
 The mucoadhesive properties of these polymers are mainly due
to hydrogen bonding and interpenetration of the polymer chains
and the mucus layer at the eye surface.

40. • The hydration state and pH of microspheres in the lacrimal fluid
were shown to be a factor affecting the residence time of the
microspheres in the eye.
• In one study, radiolabeled PAA microspheres made with
Carbopol® 907 cross-linked with maltose was applied either in
dry form or in a prehydrated form.

41. • The clearance rate of the microspheres applied in the dry form
was found to be higher than the prehydrated form probably due
to incomplete hydration in the lachrymal fluid.
• Furthermore, the clearance of hydrated microspheres at pH 7.4 is
higher than at pH 5.0 because the presence of protonated
carboxyl groups at pH 5.0 in PAA permits enhanced bioadhesion
through hydrogen bond formation with mucins.

43. • In a recent study, De et al. (28) demonstrated the biocompatibility and
adhesive properties of PAA nanoparticles on human corneal epithelial cells.
• A controlled release of the partly ionically entrapped
brimonidine was obtained due to the cation-exchange properties
of the polyanionic PAA matrix of the nanoparticles.

44. A major problem of PACA nanoparticles is the low loading
capacity for hydrophilic drugs.
 To improve drug loading, attempts are made to increase the hydrophilicity of
the particle surface by copolymerization of methylmethacrylate (MMA) and
sulfopropylmethacrylate (SPM).

45. • Bound drug molecules are released from the carrier by competitive
replacement by other ions.
• These copolymer nanoparticles loaded with the muscarinic agonists
arecaidine propargyl ester (APE) and (S)-(+)aceclidine were evaluated in
rabbits.
• A twofold increase in drug absorption was obtained when the nanoparticles
were instilled together with bioadhesive hyaluronan

46. Various cationic acrylic copolymers were also examined to prepare
nanoparticles with a positive charge in order to facilitate an
effective adhesion of the delivery system to the ocular surface

47. • Pignatello et al. formulated nanoparticles with Eudragit® RL and RS using a
modified quasiemulsion solvent diffusion technique and solvent evaporation
method.
• Using these nano particles, they demonstrated that sustained release and
increased absorption of the incorporated nonsteroidal, antiinflammatory
drugs were achieved.
• Furthermore, no inflammation or discomfort was observed in the rabbit’s
eye, suggesting that the nanoparticles were well tolerated.

48. • Hsiue et al. investigated the use of a thermosensitive polymer,
poly-N isopropylacrylamide (PNIPAAm), in controlled-release
delivery systems for glaucomatherapy.
• Cross-linked PNIPAAm nanoparticles containing epinephrine
were administrated to rabbits, and its effect on the intraocular
pressure (IOP) was monitored.
• The results indicated that a decreased pressure response which
lasted eight times longer than that of conventional eye drops
was observed.

49. POLY(EPSILON-CAPROLACTON)
• biocompatible and biodegradable polymer
• It is slightly more hydrophobic when compared with PACA.
• PECL nanoparticles and nano capsules for ophthalmic use can be
prepared by solvent extraction or solvent evaporation method.
• Upon instillation, the PECL particles form aggregates and reside
in the cul-de-sac, gradually releasing the drug.

50. • This hypothesis was put forward to explain the much more
pronounced reduction in IOP in glaucomatous rabbits after
administration of PECL nanoparticles when compared to PACA and
PLGA microspheres.
• PECL nanoparticles also seem to be able to penetrate the outer
corneal cell layers, but contrary to PACA, no cellular damage was
observed.
• This suggests that an endocytic mechanism may be involved.
• The penetration of the particles is found to be size-dependent:
nanoparticles but not microspheres were found in the corneal cells.

51. • Numerous in vivo studies demonstrated that enhanced corneal
absorption and prolonged therapeutic effects of drugs are
possible when the drugs are incorporated in PECL nanoparticles.
• The drugs tested were: betaxolol, metipranolol, carteolol,
cyclosporin A, and indomethacin.
• Furthermore, nanocapsules seem to display a better effect than
nanospheres, probably because the drug entrapped was in the
unionized form in the oily core of the delivery system and could
diffuse more easily to the cornea.

52. A strategy designed to enhance the interaction with the
mucus layer in the eye is coat PECL particles with cationic
bioadhesive polymers.
Compared with noncoated particles, the corneal and aqueous
humor indomethacin concentrations were doubled for chitosan-
coated nanocapsules.
 However, coating with polylysine did not seem to improve the
bioavailability of the tested drug.

53. • The rise in bioavailability was attributed to the structural
similarity between chitosan and mucin, rather than to its positive
charge.
• Chitosan-coated PECL particles exhibit an important interaction
with the mucus layer, but the penetration in the corneal
epithelium is lower when compared with that of the uncoated
particles.

54. De Campos et al. compared the effect of coating PECL nanocapsules with
PEG versus chitosan on the interaction of PECL nanocapsules with the
ocular mucosa.
• The in vivo study showed that the nanoparticles entered the corneal epithelium by a
transcellular pathway.
• The penetration rate and depth were dependent on the coating composition.
• PEG coating enhanced the passage of the PECL particles.
• nanocapsules across the whole epithelium, whereas the chitosan coating favored
the retention in the superficial layers of the epithelium.
• Consequently, the design of colloidal carriers with different ocular distribution
seems to be possible.

55. POLY(D,L-LACTIC ACID) AND
POLY(D,L-LACTIDE-CO-GLYCOLIDE)
• FDA-approved products (additives) for parenteral use.
• microspheres and nanoparticles made of these polymers were
investigated primarily for the controlled release of drugs after
intravitreal or subconjunctival injection rather than for topical
application.

56. PLA is a synthetic, biocompatible, and biodegradable polymer and PLGA a
copolymer of poly(lactic acid) and poly(glycolic acid).
The degradation rate depends on the molecular weight, conformation, and
polymer composition.
The drug is released out of the spheres by diffusion and by hydrolysis of the
PLA/PLGA matrix.
In an attempt to optimize the mucoadhesive properties, PLA can be
copolymerized with other polymers (e.g., PECL or PEG), so that more
appropriate “tailor-made” polymers can be developed.

57. Several methods have been proposed for the preparation of the PLA/PLGA
particles. However, the most popular technique is the emulsification
solvent evaporation method.
The uptake of PLGA nanoparticles in conjunctival epithelium is dependent
on the particle size: 100 nm particles exhibited the highest uptake as
compared to larger particles (800 nm and 10 μm).
The saturable particle uptake occurs via adsorptive-mediated endocytosis
which is independent from clathrin- and caveolin-1-mediated pathways.

58. Some examples of PLA/PLGA particles for ocular purpose reported in literature are summarized
in Table 3.

59. • Giannavola et al. changed the surface properties of PLA nanoparticles
loaded with acyclovir by incorporating PEGylated 1,2-distearoyl-3-
phosphatidylethanolamine (DSPE-PEG) into the polymer instead of coating
the external surface as in the case of PACA nanoparticles.
• After administration of the nanoparticle suspension in the rabbit’s eye, the
AUC0–6 values in the aqueous humor were found to be significantly greater
for the PEG coated PLA nanospheres than for the uncoated particles.
• A drop of 48% in AUC value was observed when PEG-coated particles were
administrated to the eye after removal of the mucus layer.

60. • This decrease was attributed to the absence of PEG–mucin interactions.
Thus, differences in bioavailability of the drugs could be correlated with the
different interactions between the nanoparticle’s surface and the corneal
epithelium.

61. • Higher and prolonged vancomycin concentration was shown in
the aqueous humor by incorporating the drug in PLGA
microspheres.
• However, increasing the viscosity of the microsphere suspension
by adding a viscosifying agent (hydroxypropyl cellulose) did not
seem to improve any further drug absorption, probably because
of the rapid ocular clearance of the highly hydrophilic drug.
• No irritation or ocular discomfort was observed in rabbits.

62. • Microspheres have been investigated for use in delivering many
ophthalmic drugs for intravitreal or subconjunctival injections.
• This includes retinoic acid, adriamycin, 5-fluorouracil,
dexamethasone, cyclosporin A, and ganciclovir.

63. It has been shown that intracellular drug delivery to retinal
epithelial cells may be possible by coating the PLA microspheres
with gelatin, probably via phagocytosis.
 Recent studies demonstrated that PLA/PLGA microspheres may be useful in
sustained or even extended slow drug delivery targeted to the posterior
segment of the eye for the treatment of chronic diseases or gene therapy.

64. As the integrity and activity of proteins and oligonucleotides is preserved
when encapsulated within PLA, the nanoparticles were evaluated for
delivering these molecules to the retina.
After intravitreal injection in rats, transretinal movement of the
nanoparticles with a preferential localization in the retinal epithelial cells
was observed.
A mild transient inflammatory reaction after injection was reported. The
presence of nanoparticles within the retinal epithelium cells could be
detected even after four months of a single injection.
This suggests that a steady and continuous delivery of drugs could be
achieved.

65. However, interference with visual acuity due to floating of
nanoparticles in the vitreous cavity could be a drawback.
Before clinical implementation of the nanoparticles, the possible
effects of the nanoparticles on the retinal function and the vision
have to be investigated.

66. LIPIDS
• Instead of using macromolecules, several lipids were proposed for
use to prepare nanoparticles.
• Solid–lipid nanoparticles (SLNs) consist of a biocompatible lipid
core and an amphiphilic surfactant as an outer shell.

67. • The advantages of SLNs are:
1. scalable manufacturing process using hot or cold high-
pressure homogenization
2. easy modulation of the drug-release profile
3. no organic solvents
4. the wide range of lipid/surfactant combinations
5. the high drug payload

68. • Cavalli et al. (42) evaluated the use of SLNs as carriers for
tobramycin.
• Compared to commercial eye drops, the tobramycin-loaded SLNs
produced a significantly higher bioavailability: a 1.5-fold increase
in Cmax and fourfold increase in AUC.
• The SLN dispersion was well tolerated, with no evidence of ocular
irritation.

69. • The longer retention observed for SLNs on the corneal surface
and in the cul-de-sac is probably related to their relatively small
size.
• The nanoparticles are presumed to be entrapped and retained in
the mucus layer.

70. CONCLUSIONS
The in vivo studies that were used to evaluate the use of nanoparticles for
ocular drug delivery were primarily performed on animals.
From the results to date, one could conclude that the nanoparticles should
possess bio- or mucoadhesive properties in order to achieve a long precorneal
retention time and to improve drug absorption.
The intraocular use of biodegradable, slow-releasing nanoparticles looks very
promising for drug delivery targeted to the tissues of the posterior segment in
order to treat chronic diseases or for gene therapy.