SOLID LIPIDN A N O P A R T I C L E S

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Solid Lipid Nanoparticles for the Delivery ofPharmaceutical ActivesBy: Andrew Loxley, PhD

NANOPARTICLEFORMULATIONS

M any o f th e a fo rem en tio n ed

fo rm u la tio n ap p ro ach e s u tiliz e

n an o te ch no log y, th a t is , th e p rep a ra tio n o f

su b -m ic ro n s tru c tu re s co n ta in in g th e A P I.

Fo r B C S c la ss II an d IV A P Is , th e s im p le s t

n an op a r tic le is m ad e o f p u re A P I, fo rm ed

by to p -d ow n p ro ce sse s s ta r tin g w ith b u lk

A P I, su ch a s m illin g , g rin d in g ,

h om og en iz a tio n , u ltra so n ic a tio n , an d a re

s tab iliz ed in d isp e rs io n by th e p re sen ce o f a

su rfa c tan t.5A lte rn a tive ly, b o ttom -up “ se lf-

a ssem bly ” p ro ce sse s can b e u sed, su ch a s

an ti-so lven t p re c ip ita tio n an d m ice lla r

in co rp o ra tio n by d ilu tio n . Fo r ex am p le ,

in so lu b le A P Is m ay b e in co rp o ra ted in to

n an o -s iz ed ve s ic le s o r lip o som es , in th e

fo rm o f p a r tic le s d isp e rsed in th e aq u eou s

co re o f th e ve s ic le s , o r a s m o lecu la rly

d isso lved m a te ria l in th e lip id b ilaye r.6

B iod eg rad ab le p o lym e rs h ave a lso b een

u sed to fo rm A P I-lo ad ed n anop a r tic le s o r

b lo ck co po lym e r m ice lle s o r p o lym e rsom es ,

u su a lly by em u ls if ic a tio n /so lven t-

evap o ra tio n te ch n iq u e s .7 ,8

B io com pa tib le

an d b io d eg rad ab le in o rg an ic n an op a r tic le s

c an b e lo ad ed w ith A P I v ia a

m ic ro em u ls io n te ch n iq u e .9B io log ic s an d

o th e r w a te r-so lu b le d ru g s h ave b een

in co rp o ra ted in to th e aq u eou s co re o f

lip o som es , in to th e aq u eou s d om a in s o f

b io d eg rad ab le p o lym e r n an op a r tic le s

p rep a red by w a te r-in -o il- in -w a te r

em u ls io n /so lven t evap o ra tio n , an d ch a rg e -

n eu tra liz a tio n n an o -com p lexe s m ad e by

in te ra c tio n w ith o ppo s ite ly ch a rg ed

po lye le c tro lye s , o r by a tta chm en t to g o ld

n an op a r tic le s .10 -1 3

Issu e s w ith sh e lf- life s tab ility o f th e

f in ish ed p ro d u c t o r th e n eed fo r o rg an ic

so lven ts in p ro ce ss in g fo r m any o f th e se

ap p ro ach e s ren d e r th em le ss th an id ea l.

INTRODUCTION

An increasing number of active pharmaceutical ingredients (APIs) under development are poorly water

soluble and therefore have poor bioavailability. These are designated Biopharmaceutical Classification System

(BCS) class II and class IV APIs.1-3 Creative formulation efforts are required to produce a finished drug product

from these APIs that has acceptable pharmacokinetics. A common formulation approach with such

compounds is to focus on creating and stabilizing very small particles of the API in an attempt to increase

the surface area available for dissolution in vivo, and hence the rate of dissolution, and consequently plasma

or tissue levels of API. Another approach is to create so-called solid solutions of the API.4

Biologics (proteins, peptides, oligonucleotides, and SiRNAs) are water soluble but bring their own

formulation and delivery challenges. Shelf-life stability and enzymatic degradation are two main areas of

concern, and formulation design focuses on stabilizing the API in storage and protecting it from endogenous

enzymes until it reaches its therapeutic target. In more advanced formulations, the API is formulated into a

delivery vehicle that specifically targets tissue or cells to maximize the therapeutic index.

F I G U R E 1

SOLID LIPIDN A N O P A R T I C L E S

XX

SOLID LIPID NANOPARTICLES –MATERIALS & SYNTHESIS

M any b io com pa tib le /b io d eg rad ab le lip id s

a re so lid a t ro om tem pe ra tu re , c an b e o b ta in ed

in h ig h pu rity, a re g en e ra lly re cog n iz ed a s sa fe

(G R A S ), an d a re in ex p en s ive . S om e com m on

so lid lip id s u sed to m ake so lid lip id

n an op a r tic le s (S LN s) in c lu d e trig ly ce rid e s (eg ,

C om p rito l 8 8 8 ATO and D yn asan 112 ),

c a rn au b a w ax , b ee sw ax , c e ty l a lco h o l,

em u ls ify in g w ax , ch o le s te ro l, an d ch o le s te ro l

b u ty ra te .

N ano - an d m ic ro p a r tic le s m ad e o f th e se

lip id s an d su sp en d ed in w a te r o ffe r an o p tio n

fo r fo rm u la tin g b o th B C S c la ss II an d IV A P Is

a s w e ll a s b io log ic s th a t m ay ove rcom e th e

issu e s o f sh e lf-life s tab ility an d th e co s t an d

tox ic ity a sso c ia ted w ith th e u se o f o rg an ic

so lven ts . In e ffe c t, th e co n cep ts o f

n an op a r tic le s an d so lid so lu tio n s a re b e in g

com b in ed .

N anop a r tic le s o f th e se lip id s m ay b e m ad e

u s in g a tem p la ted sy n th e s is from a

m ic ro em u ls io n o f th e m o lten lip id in aq u eou s

su rfa c tan ts , by p rec ip ita tio n o f th e w ax from a

so lu tio n in a n on -io n ic su rfa c tan t o n ad d itio n o f

w a te r, o r by em u ls ify in g th e m o lten lip id in to a

h o t aq u eo u s su rfa c tan t so lu tio n w ith h ig h -sh ea r

m ix in g to o b ta in th e d e s ired su bm ic ro n p a r tic le

s iz e .14 -1 6

API ENCAPSULATION IN SLNs

Sm a ll m o lecu le s can b e en trap p ed w ith in

th e lip id m a trix o f th e n an op a r tic le s by

d isso lv in g o r d isp e rs in g th e m a te ria l in th e

m o lten lip id p rio r to p a r tic le fo rm a tio n . S o u to ’s

P hD th e s is o n th e d e live ry o f A P Is u s in g SLN s

lis ts m o re th an 100 A P Is th a t h ave b een

en cap su la ted in SLN s.17

A n SEM o f th e p a r tic le s o f a ty p ic a l S LN

d isp e rs io n (in th is c a se o f p a r tic le s co n ta in in g

th e su n sc re en o c ty lm e th oxy c in n am a te ) is

sh ow n in F ig u re 1 . P a r tic le s o f th is ty p e a re

m ad e a t com m erc ia l sc a le fo r fo rm u la tio n in to

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SOLID LIPIDN A N O P A R T I C L E S

to p ic a l p ro d u c ts to p rov id e U V-p ro te c tio n .

In som e ca se s , th e A P I is n o t com pa tib le

w ith th e lip id an d is ex p e lled from th e

n an op a r tic le , u su a lly d u rin g co o lin g an d

so lid if ic a tio n . T h is c an le ad to u nd e s irab le

m ac ro sco p ic c ry s ta ls o f A P I in th e f in a l

fo rm u la tio n o r p h a se sep a ra tio n o f th e

p a r tic le s to s tru c tu re s a s com p lex a s “n an o -

sp o on s .”18

B y m ix in g liq u id lip id s w ith th e

so lid lip id p rio r to p a r tic le fo rm u la tio n , lip id

c ry s ta lliz a tio n is h in d e red o r p reven ted, an d a

m o re am o rp hou s n an op a r tic le in te rn a l

s tru c tu re is a ch ieved . In th is w ay, A P Is a re le ss

lik e ly to b e ex p e lled from SLN s du rin g th e

co o lin g s tep o f th e ir p rep a ra tio n , an d s tab le

S LN s m ay b e fo rm u la ted w ith a w id e r ran g e

o f A P Is .19

A s on e ex am p le c re a ted a t P a r tic le

S c ien ce s , f lu o re scen t S LN s h ave b een

p rep a red by add in g a flu o re scen t d ye to th e

m o lten lip id p rio r to p a r tic le p rep a ra tio n .

G reen flu o re scen t S LN s w ere p rep a red w ith

py r rom e th en e 5 67A dye , an d red flu o re scen t

S LN s w ith 1 ,1 ’-d io c tad ecy l-3 ,3 ,3 ’,3 ’-

te tram e thy lin d o ca rb o cy an in e p e rch lo ra te (D iI)

by a m od if ied p rep a ra tio n te ch n iq u e to

accom m oda te th e low so lu b ility o f D iI in th e

m o lten lip id . F lu o re scen t S LN s a re u se fu l to

fo llow th e fa te o f p a r tic le s ap p lied m uco sa lly

in v ivo an d d e te rm in in g e ff ic ien cy o f u p take

by an tig en p re sen tin g ce lls in v itro in th e

d eve lo pm en t o f a n ove l H IV vacc in e .20

T issu e

sam p le s tak en from pen ile ep ith e lia l ex p lan ts

a fte r ap p lic a tio n o f flu o re scen t S LN s sh ow

pa r tic le s p en e tra ted w e ll in to th e tis su e

(F ig u re 2A ), an d d end ritic c e lls a re sh ow n to

in te rn a liz e g reen flu o re scen t S LN s fo llow in g

in cu b a tio n in v itro (F ig u re 2B ).

T h e g reen flu o re scen t p a r tic le s w e re a lso

u sed in a p ro o f-o f-co n cep t s tu d y fo r th e

d eve lo pm en t o f an in h a lab le A P I-lo ad ed SLN

d isp e rs io n . A flu o re scen t d ye -lo ad ed SLN

d isp e rs io n w as ae ro so liz ed u s in g an OTC

neb u liz e r, an d th e ae ro so l p lum e from th e

m ou th p ie ce w as illum in a ted by U V lig h t. T h e

g reen flu o re scen t g low o f th e p lum e sh ow ed

th a t th e SLN s w ere in d eed in th e ae ro so l

d ro p le ts , an d an a ly s is o f th e co nd en sed

ae ro so l sh ow ed th a t th e p a r tic le s iz e

d is tr ib u tio n o f th e SLN s in th e o rig in a l

d isp e rs io n w as m a in ta in ed in th e d ro p le ts . T h e

d ro p le t s iz e o f th e ae ro so l w as a lso fo u nd to

b e id ea l fo r d e live ry to th e d eep lu n g (a ro u nd

5 m ic ro n s). T h is w o rk co u ld le ad to im p roved

pu lm on a ry d e live ry o f w a te r-in so lu b le A P Is

fo r a cu te tre a tm en t in h o sp ita ls w h e re d o se s

m ay n eed to b e h ig h .

SURFACE ENTRAPMENTOF APIs WITH SLNs

In s te ad o f in co rp o ra tin g th e d ru g in to th e

p a r tic le , an ad d itio n a l w ay to ex p lo it S LN s is

to a tta ch th e A P I to th e su rfa ce o f th e p a r tic le .

T h e su rfa ce p ro p e r tie s o f S LN s can b e va ried

w id e ly an d ta ilo red to th e f in a l ap p lic a tio n .

Fo r ex am p le , th e ch o ic e o f em u ls if ie r

(c a tio n ic , n o n -io n ic , an io n ic , an d po lym e ric )

h a s a s tro n g in flu en ce o n th e su rfa ce e le c tr ic a l

ch a rg e o n th e n an op a r tic le s , m ea su red by th e

ze ta p o ten tia l o f th e p a r tic le s , a s sh ow n in

F ig u re 3 .

Fo r S LN s th a t co n ta in lo n g -ch a in fa tty

ac id s o r u se th em as th e em u ls if ie r, th e

ca rb oxy l g ro u p s p re sen t a t th e p a r tic le su rfa ce

can b e u sed to cova len tly a tta ch p ro te in s an d

am in e -te rm in a ted p ep tid e s u s in g s tan d a rd

co up lin g ch em is tr ie s (su ch a s ca rb o d iim id e

co up lin g ).

B io log ic s a re g en e ra lly ch a rg ed in

aq u eou s so lu tio n , an d a s su ch a re a ttra c ted

e le c tro s ta tic a lly to su rfa ce s o f o p po s ite ch a rg e ,

an d m ay b ecom e s tro n g ly a tta ch ed th e re a s a

re su lt. We h ave fo u nd th a t e le c tr ic a lly ch a rg ed

SLN s (ca tio n ic o r an io n ic ) s tro n g ly an d

ir reve rs ib ly b in d p ro te in s w ith a tta chm en t

e ff ic ien c ie s o f a ro u nd 90% (a ro u nd 650

m ic rog ram s p ro te in p e r m g o f SLN so lid s ) .

E v id en ce th a t th e p ro te in is a tta ch ed a t th e

p a r tic le su rfa ce is p rov id ed by th e o b se rva tio n

th a t a fte r m ix in g th e SLN and p ro te in an d

a llow in g en ough tim e fo r th e p ro te in to ad so rb

a t th e p a r tic le su rfa ce , th e pH -d ep en d en ce o f

th e SLN ’s ze ta -p o ten tia l g o e s from th a t o f th e

n aked SLN to th a t o f th e p u re p ro te in .

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SOLID LIPIDN A N O P A R T I C L E S

E ssen tia lly th e SLN s su rfa ce p ro p e r tie s

b ecom e dom in a ted by th e p ro te in a tta ch ed

th e re (F ig u re 4 ).

B a sed on th e en co u rag in g tissu e an d

ce llu la r u p take re su lts an d th e ab ility to

e ff ic ien tly an d s im p ly a tta ch p ro te in s to th e

SLN su rface , n an op a r tic le s m ad e o f ca rn au b a

w ax and fo rm u la ted to ca r ry g p140 (a m od e l

H IV an tig en ) w e re ap p lied to th e vag in a l

m u co sa o f m ice to eva lu a te th is ro u te o f

adm in is tra tio n a s a n ove l ap p ro ach to

vacc in a tio n ag a in s t H IV. A s co n tro ls in th is

ex p e rim en t, m ice w e re a lso vacc in a ted by

su b cu tan eo u s in je c tio n o f th e SLN -gp140

fo rm u la tio n a s w e ll a s a fo rm u la tio n u s in g

a lum , th e o n ly p a r tic le s u sed in g en e ra lly

ap p roved p a r tic le -co n ta in in g vacc in e s in th e

U S . T h e sy s tem ic ch a llen g e re su lts w ith th e

SLN s w ere eq u iva len t to th e a lum con tro l

(d a ta n o t sh ow n ), in d ic a tin g th a t th e se

p a r ticu la r S LN s a re p o ten tia lly p rom is in g

ad ju van ts fo r sy s tem ic vacc in a tio n .

STERILIZATION

Fo r p a ren te ra l adm in is tra tio n , S LN

d isp e rs io n s m u st b e s te rile . T h e m ean p a r tic le

d iam e te r o f S LN s is o ften m o re th an 200 nm ,

so s te rile f iltra tio n is n o t p o ss ib le in th e se

ca se s . A u to c lav in g th e f in ish ed d isp e rs io n is

n o t p ra c tic a l a s th e lip id s m e lt a t tem pe ra tu re s

u sed to te rm in a lly h ea t-s te riliz e

p h a rm aceu tic a l p ro d u c ts , an d th e m o lten lip id

d ro p le ts co a le sce a s th e re is n o ap p lied sh ea r

to p reven t th is . O p tio n s a re th e re fo re lim ited

to a sep tic m anu fac tu rin g p ro ce sse s fo llow in g

s te riliz a tio n o f th e s ta r tin g m a te ria ls (g am m a

o r e -b eam ir rad ia tio n o f th e f in a l d isp e rs io n )

o r ex p o su re to e thy len e ox id e g a s (E O ).

B ac te ria l en d o tox in s in raw m a te ria ls n eed to

b e m on ito red, e sp ec ia lly w h en raw m a te ria ls

a re o f n a tu ra l o rig in . I t m ay b e po ss ib le to

lyo p h iliz e th e SLN d isp e rs io n , an d th is

lyo p h ile c an b e ir rad ia ted o r ex p o sed to EO .

We h ave d em on s tra ted th a t lyo p h iliz ed SLN s

m ad e o f ca rn au b a w ax a re re ad ily red isp e rsed,

an d th e o rig in a l p a r tic le s iz e d is tr ib u tio n is

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re cove red . O f co u rse , S LN w ith ap p ro p ria te ly

sm a ll p a r tic le s iz e can b e s te riliz ed u s in g

f iltra tio n .

STABILITY

T he sh e lf- life s tab ility o f S LN s can b e

ve ry g ood . L ip id s can b e ch o sen th a t d o no t

hy d ro ly ze in aq u eou s su sp en s io n (an o th e r

ad van tag e ove r n an op a r tic le s m ad e from

po lym ers , su ch a s P LG A , w h ich hyd ro ly ze s

w ith a ra te th a t is d ep en d en t o n po lym e r

s tru c tu re , an d th e re fo re m u st b e lyo ph iliz ed fo r

p ra c tic a l u se ). T h e ve ry sm a ll p a r tic le s iz e an d

d en s ity c lo se to u n ity o f S LN s m ean s g rav ity

h a s little e ffe c t o n th e p a r tic le s in d isp e rs io n ,

an d B row n ian m o tio n is su ff ic ien t to m a in ta in

co llo id a l d isp e rs io n w ith o u t c re am in g o r

sed im en ta tio n . A ny su ch sep a ra tio n can u su a lly

b e com p le te ly reve rsed by g en tle ag ita tio n ,

even if it is o b se rved . T h e p a r tic le s iz e

d is tr ib u tio n an d ze ta p o ten tia l rem a in s s tab le

ove r tim e (F ig u re 5 ) a s n e ith e r O stw a ld

rip en in g no r p a r tic le d isso lu tio n o ccu r in th e se

sy s tem s , an d th e su rfa ce ch a rg e d e te rm in in g

m o ie tie s a re im m ob ile . Fo r S LN s m ad e w ith

n a tu ra l lip id s , an d no t m ad e by an a sep tic

p ro ce ss , th ey can b e p rep a red w ith lo n g -te rm

stab ility ag a in s t b io log ic a l g row th u s in g

s tan d a rd p re se rva tive s w h en to le rab le .

SUMMARY

SLN s a re ea s ily p rep a red n an op a r tic le s

m ad e from in ex p en s ive , sa fe , s tab le , an d

b io d eg rad ab le m a te ria ls an d can b e lo ad ed

in te rn a lly o r ex te rn a lly w ith A P Is fo r

co n tro lled d e live ry. A s su ch , th ey o ffe r a

h ig h ly ve rsa tile p la tfo rm and on e th a t sh o u ld

b e co n s id e red w hen wo rk in g w ith A P Is th a t

p re sen t so lu b ility an d /o r b io ava ilab ility

ch a llen g e s .

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8 7 3 .

1 3 . P ac io ti G F, e t a l. C o llo id a l g o ld : a n ove l n an op a r tic le vec to r fo r tum o r d ire c ted

d ru g d e live ry. D ru g D e live ry. 2 0 0 4 ;1 1 :1 6 9 -1 8 3 .

1 4 . M um pe r R J. M ic ro em u ls io n s a s p recu rso rs to so lid n an op a r tic le s . U S Pa ten t N o .

7 1 53525 . D ecem be r 2 6 , 2 0 06 .

1 5 . P a r tic le S c ien ce s (u n pub lish ed )

1 6 . M itch n ick M , e t a l. C om po s ite U V sunb lo ck com po s itio n s . U S Pa ten t N o .

5 7 33531 . M arch 31 , 1 9 98 .

1 7 . S o u to E B . S LN and N LC fo r to p ic a l d e live ry o f an tifu n g a ls . P hD T hes is , F re e

U n ive ris ty o f B e rlin , 2 0 0 5 .

1 8 . Jo re s K , e t a l. F rom so lid lip id n an op a r tic le s (S LN ) to n an o sp oon s . v is io n s an d

rea lity o f co llo id a l lip id d isp e rs io n s . P ro ceed in g s o f th e 3 0 th A nnu a l M ee tin g &

E xpo s itio n o f th e C on tro lled R e le a se S o c ie ty. A b s tra c t N o . 1 8 1 . Ju ly 1 9 -2 3 ,

2 0 03 . G la sg ow, S co tlan d, U K .

1 9 . K hu ran a S . N ano s tru c tu red lip id ca r rie rs an d th e ir ap p lic a tio n in d ru g d e live ry.

In t J B iom ed E ng Tech . 2 0 09 ;2 (2 ):1 5 2 -1 7 1 .

2 0 . A ria s M , e t a l. H IV-g p140 an tig en -ad so rb ed w ax n anop a r tic le s in d u ce s tro n g in

v ivo sy s tem ic an d m uco sa l h um o ra l im m une re sp o n se s . Po s te r p re sen ted a t

A ID S Vacc in e M ee tin g , C ap e Tow n ; 2 0 08 .

Dr. Andrew

Loxley is

Director of

New

Technologies

at Particles

Sciences Inc.,

a contract

research

organization

in Bethlehem, PA, specializing in

pharmaceutical formulation development.

He leads a variety of projects, many based

on novel and proprietary

nanotechnologies, in fields from HIV

vaccine and microbicide development to

gene-silencing SiRNA delivery. Prior to

joining Particles Sciences, he led the

development efforts in next-generation

lithium ion batteries at A123 Systems Inc,

electrophoretic displays at EINK Corp.,

and latex-based adhesives at Synthomer

Ltd. He earned his BSc in Chemistry from

the Univeristy of Sussex and his PhD in

Physical Chemistry focusing on

microencapsulation from the University of

Bristol.

B I O G R A P H Y