Polymer Chemistry and Liposome Technology - ACS Symposium

Jul 23, 2009 - Polymer Chemistry and Liposome Technology. David A. Tirrell ... The "Stealth" Liposome: A Prototypical Biomaterial. Chemical Reviews. L...
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Chapter 11

Polymer Chemistry and Liposome Technology David A. Tirrell

The Impact of Chemistry on Biotechnology Downloaded from pubs.acs.org by YORK UNIV on 12/02/18. For personal use only.

Polymer Science and Engineering Department, University of Massachusetts, Amherst, MA 01003

Polymer chemistry has a great deal to offer in the construction of synthetic liposomal membranes for use in biology and medicine. This chapter explores the preparation and properties of polymeric liposomes, with particular emphasis on the use of controlled polyelectrolyte adsorption to manipulate liposomal membrane properties. Nature has combined the d i s c i p l i n e s o f polymer c h e m i s t r y and c o l l o i d s c i e n c e to remarkable e f f e c t i n her d e s i g n o f the b i l a y e r membranes t h a t s u r r o u n d c e l l s and s u b c e l l u l a r o r g a n e l l e s . These membranes do many i n t r i g u i n g t h i n g s : t h e y r e c o g n i z e one a n o t h e r , t h e y respond to the b i n d i n g o f drugs and hormones, and they c o n t r o l the flow o f m a s s , i n f o r m a t i o n and energy w i t h i n and between c e l l s . I t would be n a t u r a l , t h e n , f o r us to s e l e c t such s t r u c t u r e s as we t r y to i n t e r vene i n b i o l o g i c a l p r o c e s s e s through the c o n t r o l l e d d e l i v e r y o f d r u g s , markers and g e n e t i c m a t e r i a l . F o l l o w i n g Bangham's d i s c o v e r y 0) o f the b a r r i e r p r o p e r t i e s o f l i p i d v e s i c l e s , o r l i p o s o m e s , t h e r e has grown an enormous l i t e r a t u r e devoted to l i p o s o m a l d e l i v e r y ( 2 - 6 ) . What has emerged more r e c e n t l y i s the r e a l i z a t i o n t h a t polymer chemi s t r y has much to o f f e r i n the t e c h n o l o g i c a l development o f l i p o s o m a l d e l i v e r y systems. Research on l i p o s o m a l d e l i v e r y systems has been m o t i v a t e d i n l a r g e p a r t by the n o t i o n t h a t the m a t e r i a l to be d e l i v e r e d can be e n t r a p p e d i n the l i p o s o m a l i n t e r i o r , and t h a t the e n t r a p p e d m a t e r i a l w i l l remain i n s i d e u n t i l the l i p o s o m e i s opened a t i t s t a r g e t . Implementation o f t h i s i d e a r e q u i r e s the f o r m u l a t i o n o f l i p o s o m a l systems t h a t a r e r e s i s t a n t to l e a k a g e o f entrapped s u b s t a n c e s , r e s i s t a n t to a t t a c k by f o r e i g n agents used i n the p r e p a r a t i o n o f the d e l i v e r y system ( e . g . s u r f a c t a n t s and o r g a n i c s o l v e n t s ) , s t a b l e toward a t t a c k by endogenous agents ( e . g . enzymes o r plasma p r o t e i n s ) and c a p a b l e o f t i s s u e l o c a l i z a t i o n o r s e l e c t i v e r u p t u r e a t the target. The f o l l o w i n g d i s c u s s i o n w i l l p r o v i d e an o v e r v i e w o f the p r e s e n t s t a t e o f the union between polymer c h e m i s t r y and l i p o s o m e technology. We w i l l a d d r e s s f i r s t the methods o f p r e p a r a t i o n o f

0097-6156/88/0362-0152$06.00/0 © 1988 American Chemical Society

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p o l y m e r i c l i p o s o m a l s y s t e m s , and then p r o c e e d to an e v a l u a t i o n o f the c o n t r i b u t i o n s t h a t polymer c h e m i s t r y can make to the s o l u t i o n o f the problems o u t l i n e d above. Preparative

Methods

The major approaches to the p r e p a r a t i o n o f p o l y m e r i c l i p o s o m a l s y s tems may be d i s t i n g u i s h e d on the b a s i s o f the l o c u s o f p o l y m e r i z a tion. Most e f f o r t to date has been d i r e c t e d toward the p r e p a r a t i o n o f p h o s p h o l i p i d analogues t h a t a r e i n t r i n s i c a l l y p o l y m e r i z a b l e through r e a c t i o n s o f a t t a c h e d v i n y l , b u t a d i e n y l , d i a c e t y l e n i c , i s o c y a n o , t h i o l or d i s u l f i d e f u n c t i o n s . T h i s approach produces l i p o s o m e s i n which the p r i m a r y p e r m e a b i l i t y b a r r i e r - the b i l a y e r i t s e l f - i s polymeric. The second approach i n v o k e s the attachment o f e x t r i n s i c macromolecules to the l i p i d b i l a y e r , i n a f a s h i o n analogous to the assembly o f b i o l o g i c a l membranes. Polymerizable L i p i d s . The p r e p a r a t i o n o f p o l y m e r i z a b l e l i p i d s has been e x t e n s i v e l y reviewed ( 7 - 1 0 ) , so the p r e s e n t d i s c u s s i o n w i l l be brief. I t i s u s e f u l to d i s t i n g u i s h two p o r t i o n s o f a m p h i p h i l i c , membrane-forming l i p i d s : a p o l a r headgroup t h a t forms the i n t e r f a c e between the membrane and i t s e n v i r o n m e n t , and the hydrocarbon c h a i n s t h a t p r o v i d e the p r i m a r y b a r r i e r to the escape o f e n t r a p p e d s o l u t e s . I t i s i m p o r t a n t i n the d e s i g n o f p o l y m e r i z e d l i p o s o m e s t h a t one keep i n mind t h e s e d i s t i n c t f u n c t i o n s , and t h a t the l o c u s o f p o l y m e r i z a t i o n be s e l e c t e d i n such a way t h a t the d e s i r e d f u n c t i o n a l properties are p r e s e r v e d o r enhanced. P o l y m e r i z a t i o n through f u n c t i o n a l groups appended to the l i p i d headgroup would be expected to p r e s e r v e the p a c k i n g p r o p e r t i e s o f the hydrocarbon c h a i n s (assuming t h a t the geom e t r i c r e q u i r e m e n t s o f the p o l y m e r i z a t i o n a r e c o n s i s t e n t w i t h t h o s e o f c h a i n p a c k i n g ) , but would a l t e r the s u r f a c e r e c o g n i t i o n b e h a v i o r o f the membrane. On the o t h e r h a n d , p o l y m e r i z a t i o n i n the h y d r o c a r bon c o r e - e i t h e r i n the m i d d l e o f the c h a i n o r a t i t s end - o f f e r s the p r o s p e c t o f an u n p e r t u r b e d l i p i d s u r f a c e , but compromises i n a p r o f o u n d way the p a c k i n g o f the hydrocarbon c h a i n s . It i s important to p o i n t o u t , however, t h a t such changes i n p a c k i n g may be a d v a n t a g e o u s , i n t h a t the b a r r i e r p r o p e r t i e s o f the b i l a y e r may be s i g n i f i c a n t l y enhanced by p o l y m e r i z a t i o n . An example o f reduced s o l u t e l e a k a g e from p o l y m e r i z e d l i p o s o m e s w i l l be d i s c u s s e d i n the f o l l o w i n g section. Attachment o f E x t r i n s i c M a c r o m o l e c u l e s . Attachment o f s y n t h e t i c polymers to the l i p o s o m a l s u r f a c e has been a c c o m p l i s h e d by a t l e a s t f o u r d i f f e r e n t r o u t e s (Scheme I ) . The s i m p l e s t r o u t e - but a l s o t h a t l e a s t amenable to c o n t r o l - i n v o l v e s the a d s o r p t i o n o f u n m o d i f i e d polymer c h a i n s (1J_). In a d o p t i n g t h i s r o u t e , one must walk a f i n e l i n e between systems i n which the p o l y m e r - b i l a y e r i n t e r a c t i o n s a r e too weak to m a i n t a i n a s u b s t a n t i a l s u r f a c e c o n c e n t r a t i o n o f c h a i n s , and systems i n which those i n t e r a c t i o n s a r e s t r o n g enough to cause v e s i c l e r e o r g a n i z a t i o n and l y s i s . A second r o u t e - w i t h d i s t i n c t advantages and w e l l e s t a b l i s h e d b i o l o g i c a l p r e c e d e n t - i s the use o f h y d r o p h o b i c " a n c h o r i n g groups" to h o l d i n p l a c e even weakly bound polymer c h a i n s . S u i t a b l e anchors a r e s i n g l e o r double c h a i n s u r f a c t a n t s (12,13) o r c h o l e s t e r o l 04), p r i n c i p l e such anchors a

n

d

l

n

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Scheme I .

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may be i n t r o d u c e d f i r s t e i t h e r to the polymer c h a i n (Scheme I, route 2) o r t o the v e s i c l e s u r f a c e (Scheme I, r o u t e 3 ) . The b i o l o g i c a l a n a l o g y o f c o u r s e i s the w i d e s p r e a d o c c u r r e n c e o f h y d r o p h o b i c p e p t i d e sequences i n membrane-bound p r o t e i n s , which s e r v e a s i m i l a r a n c h o r i n g function. F i n a l l y , both Regen (1J5) and R i n g s d o r f (1_6) have s u g g e s t e d t h a t the f o r m a t i o n o f l i p o s o m e s from s u r f a c t a n t s b e a r i n g p o l y m e r i z a ­ b l e c o u n t e r i o n s might be f o l l o w e d by p o l y m e r i z a t i o n w i t h i n the double l a y e r to p r o v i d e a s u r f a c e c o a t i n g o f p o l y e l e c t r o l y t e c h a i n s (Scheme I, r o u t e 4 ) . The r e a d e r s h o u l d a l s o be aware o f a v e r y s u b s t a n t i a l , r e l a t e d body o f work t h a t a d d r e s s e s the a n c h o r i n g o f e x t r i n s i c , n a t u r a l l y o c c u r r i n g macromolecules to l i p o s o m a l s u r f a c e s ( 2 - 6 , 1 7 ) . Because t h i s work has a l r e a d y been e x t e n s i v e l y r e v i e w e d , the p r e s e n t c h a p t e r w i l l not d i s c u s s t h i s v e r y i n t e r e s t i n g and i m p o r t a n t s u b j e c t . Properties

o f P o l y m e r i c Liposomes

Liposomes from P o l y m e r i z a b l e L i p i d s . A g a i n the d i s c u s s i o n w i l l be b r i e f , and the r e a d e r i s d i r e c t e d to the s e v e r a l e x c e l l e n t r e v i e w s of polymerized v e s i c l e s (3). We d i s c u s s here o n l y one v e r y r e c e n t example o f the p r e p a r a t i o n and p r o p e r t i e s o f such a s y s t e m . This example has much to recommend i t , and i s d e s c r i b e d by the o r i g i n a l a u t h o r s a s , "the p o l y m e r i z a b l e l i p i d o f c h o i c e f o r a wide v a r i e t y o f m e c h a n i s t i c and p r a c t i c a l a p p l i c a t i o n s . " Regen and coworkers d e s c r i b e d i n 1986 the p r e p a r a t i o n and polymerization of 1,2-bis[12(lipoyloxy)dodecanoyl]-sn-glycero3 - p h o s p h o c h o l i n e 1 (18). Aqueous d i s p e r s i o n s o f 1 produced by 0 0 S — S II II CH 0C(CH ) 0C(CH2)i CHCH CH2

I

2

2

ι

t

1 1

2

0

0

S —

S

ii

II

I

I

CH0C(CH ) 0C(CH )i CHCH CH2 2

2

1 1

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2

1

0

ι"

+

CH 0P0CH CH N(CH ) 2

2

2

3

3

0 i n j e c t i o n o f e t h a n o l i c s o l u t i o n s were shown to c o n s i s t o f v e s i c l e s o f a v e r a g e d i a m e t e r 270-400A. Treatment o f such d i s p e r s i o n s w i t h 10 m o U d i t h i o t h r e i t o l (DTT) f o r 4 hr a t 2 7 ° C produced complete p o l y ­ m e r i z a t i o n o f 1 , as shown by t h i n l a y e r c h r o m a t o g r a p h i c a n a l y s i s f o r u n r e a c t e d monomer and by the d i s a p p e a r a n c e o f t h e c h a r a c t e r i s t i c UV a b s o r p t i o n maximum o f the monomer a t 333 nm. P o l y m e r i z a t i o n was accompanied by a small d e c r e a s e i n p a r t i c l e s i z e as r e p o r t e d by dynamic l i g h t s c a t t e r i n g , but the p r e s e n c e o f c l o s e d v e s i c l e s was s u g g e s t e d by e l e c t r o n m i c r o s c o p y and c o n f i r m e d by s o l u t e e n t r a p m e n t . The p o l y m e r i z e d v e s i c l e s were s t a b l e to l y s i s even i n 1$ s o l u t i o n s o f sodium d o d e c y l s u l f a t e (SDS) a t 6 0 ° C ; i n c o n t r a s t , v e s i c l e s o f monomeric 1 were d e s t r o y e d i n 0.05% SDS a t room t e m p e r a t u r e . The

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s i z e d i s t r i b u t i o n o f the p o l y m e r i z e d v e s i c l e d i s p e r s i o n was shown to be s t a b l e f o r a t l e a s t 3 months a t room t e m p e r a t u r e , and the d i s p e r s i o n r e t a i n e d 61% o f i t s e n t r a p p e d s u c r o s e a f t e r 4 hr a t 2 3 ° C , as compared to 32% r e t e n t i o n by monomeric 1 under i d e n t i c a l c o n d i t i o n s . T h i s system indeed has much to recommend i t . The p o l y m e r i z a t i o n c o n d i t i o n s a r e r e m a r k a b l y m i l d , and appear to c o n f e r on the b i l a y e r i n c r e a s e d b a r r i e r p r o p e r t i e s and improved r e s i s t a n c e to d e t e r g e n t lysis. In a d d i t i o n , the c o n s t r u c t i o n o f the polymer c h a i n by the f o r m a t i o n o f d i s u l f i d e bonds l e a v e s open the p r o s p e c t o f depolymeri z a t i o n i n a r e d u c i n g b i o l o g i c a l environment and subsequent degradation. Liposomes s i d e r the a r e bound Ringsdorf surfactant

Bearing E x t r i n s i c Macromolecules. This section w i l l conp r o p e r t i e s o f l i p o s o m e s to which e x t r i n s i c macromolecules by a d s o r p t i o n o r by h y d r o p h o b i c a n c h o r s . Regen (1J5) and (1_6) have d i s c u s s e d the consequences o f p o l y m e r i z a t i o n o f counterions (9).

Liposomes B e a r i n g Adsorbed C h a i n s . Much o f the work to date on l i p o s o m e s b e a r i n g adsorbed c h a i n s has concerned the pH-dependent a d s o r p t i o n o f a c i d i c p o l y e l e c t r o l y t e s on l i p o s o m e s prepared from phosphatidylcholines (11,19-21). I n t e r e s t i n t h i s problem has i t s o r i g i n i n the pH-dependent c o n f o r m a t i o n a l and s o l u b i l i t y p r o p e r t i e s of a c i d i c polyelectrolytes. One might e x p e c t , f o r example, t h a t a p o 1 y ( c a r b o x y l i c a c i d ) would be c o n v e r t e d upon a c i d i f i c a t i o n from a c h a r g e d , h y d r o p h i l i c s t r u c t u r e to a g l o b u l a r , hydrophobic c o i l , p o o r l y s o l v a t e d by aqueous m e d i a . The l i p o s o m a l s u r f a c e p r o v i d e s a p l a c e o f r e f u g e f o r the h y d r o p h o b i c c h a i n ; the c h a i n i n t u r n a l t e r s i n a profound way the g e o m e t r i c and thermodynamic f a c t o r s t h a t comb i n e to determine b i l a y e r s t r u c t u r e . One can e a s i l y imagine t h a t the p o l y m e r - p h o s p h o l i p i d m i x t u r e might adopt a t o t a l l y d i f f e r e n t a g g r e gate m o r p h o l o g y , and t h a t i n the c o u r s e o f the s t r u c t u r a l r e o r g a n i z a t i o n the b a r r i e r p r o p e r t i e s o f the b i l a y e r would be l o s t . Scheme II shows a r e a s o n a b l e working h y p o t h e s i s , i n which p r o t o n - d r i v e n p o l y e l e c t r o l y t e a d s o r p t i o n i n d u c e s a v e s i c l e - t o - m i c e l l e t r a n s i t i o n upon acidification (22). Scheme II s u g g e s t s t h a t p o l y e l e c t r o l y t e a d s o r p t i o n might p r o v i d e a means o f p r e p a r i n g pH-dependent l i p o s o m a l d e l i v e r y s y s t e m s . Given the v a r i a t i o n s i n pH t h a t c h a r a c t e r i z e c e r t a i n p a t h o l o g i c a l s t a t e s (23) and c e r t a i n s u b c e l l u l a r o r g a n e l l e s ( 2 4 ) , the a b i l i t y to a l t e r membrane p r o p e r t i e s i n a c o n t r o l l e d , pH-dependent manner i s a powerf u l methodology. I n d e e d , i t has become c l e a r t h a t the c e l l uses s u b t l e changes i n pH to c o n t r o l i t s i n t r a c e l l u l a r p r o c e s s i n g o f l i g a n d s and r e c e p t o r s ( 2 4 ) , and t h a t i n f e c t i o u s m i c r o o r g a n i s m s have d e v e l o p e d mechanisms to e x p l o i t t h e s e pH changes to g a i n e n t r y to the cytoplasm (25). Most o f o u r work to date has concerned the i n t e r a c t i o n s o f p o 1 y ( a c r y l i c a c i d ) d e r i v a t i v e s w i t h v e s i c l e membranes p r e p a r e d from phosphatidylcholines. In p a r t i c u l a r , p o l y ( 2 - e t h y 1 a c r y l i c a c i d ) (PEAA, 2) i s a h y d r o p h o b i c p o l y ( c a r b o x y l i c a c i d ) t h a t undergoes a c o n f o r m a t i o n a l t r a n s i t i o n o f the k i n d d e s c r i b e d above ( 2 6 - 2 8 ) . That t h i s c o n f o r m a t i o n a l t r a n s i t i o n o c c u r s near n e u t r a l pH makes PEAA a c a n d i d a t e f o r use i n pH-dependent l i p o s o m a l d e l i v e r y systems i n b i o l o g y and m e d i c i n e .

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Scheme I I . (Reproduced with permission from Ref. 22. Copyright 1985 Huthig and Wepf Verlag.)

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C H 0 C H 3

I [CH C] 2

2

I

C0 H 2

F i g u r e 1 shows t h a t PEAA indeed f u n c t i o n s w e l l i n t h i s r o l e (19). The F i g u r e shows the e f f l u x o f the f l u o r e s c e n t dye c a r b o x y f l u o r e s c e i n from u n i l a m e l l a r v e s i c l e s o f egg y o l k p h o s p h a t i d y l c h o l i n e suspended i n an aqueous s o l u t i o n o f PEAA. Escape o f the dye i s v e r y slow a t pH 7 . 4 , but e s s e n t i a l l y i n s t a n t a n e o u s upon a c i d i f i c a t i o n o f the s u s p e n s i o n to pH 6 . 5 . We have s i n c e demonstrated the p H - t r i g g e r e d r e l e a s e o f o t h e r s u b s t a n c e s from p h o s p h a t i d y l c h o l i n e v e s i c l e s by t h i s method, and the t e c h n i q u e s h o u l d be c o m p l e t e l y g e n e r a l i n i t s a p p l i c a t i o n to the c o n t r o l l e d r e l e a s e o f water s o l u b l e compounds. A d d i t i o n a l uses o f these and r e l a t e d systems a r e suggested by the f a c t t h a t many enzymes a r e known to y i e l d a c i d i c p r o d u c t s v i a o x i d a t i v e or h y d r o l y t i c r e a c t i o n s . G l u c o s e o x i d a s e , f o r example, c a t a l y z e s the o x i d a t i o n o f g l u c o s e to g l u c o n i c a c i d , which i s a s u i t a b l e source o f H . A phosphatidylcholine suspension containing both g l u c o s e o x i d a s e and PEAA s h o u l d then be s e n s i t i v e to g l u c o s e c o n c e n t r a t i o n , in that i n c r e a s i n g glucose concentrations should lead to v e s i c l e r u p t u r e w i t h r e l e a s e o f c o n t e n t s . Such systems might prove to be u s e f u l i n s e l f - r e g u l a t e d i n s u l i n d e l i v e r y o r i n m o n i t o r * ing of glucose concentrations in physiologic f l u i d s . F i g u r e 2 demonstrates the v i a b i l i t y o f t h i s approach to the p r e p a r a t i o n o f g l u c o s e - s e n s i t i v e membranes ( 2 0 ) . The F i g u r e shows the r e s u l t s o f an experiment i n which d i l a u r o y l p h o s p h a t i d y l c h o l i n e was suspended i n an u n b u f f e r e d , aqueous s o l u t i o n o f PEAA and g l u c o s e o x i d a s e a t pH 7 . 4 . Upon a d d i t i o n o f g l u c o s e , the o p t i c a l d e n s i t y o f the s o l u t i o n was r a p i d l y r e d u c e d , and reached a v a l u e l e s s than 10% o f the o r i g i n a l a f t e r a p p r o x i m a t e l y 30 m i n . The r e d u c t i o n i n o p t i c a l d e n s i t y s i g n a l s a r e o r g a n i z a t i o n o f the b i l a y e r t h a t must be a n a l o gous to t h a t i n d u c e d by d i r e c t a d d i t i o n o f H* as i n F i g u r e 1 , so t h a t we would a n t i c i p a t e q u a n t i t a t i v e r e l e a s e o f v e s i c l e c o n t e n t s under the c o n d i t i o n s o f F i g u r e 2. T h i s system i s o f i n t e r e s t not o n l y from the p o i n t o f view o f i t s p o t e n t i a l a p p l i c a t i o n s i n d i a g n o s i s and t h e r a p e u t i c s , but a l s o by v i r t u e o f i t s a n a l o g y to the "second mess e n g e r " s i g n a l l i n g p r o c e s s e s so important i n c e l l b i o l o g y . That i s , the s i g n a l i s i n i t i a t e d by a r i s e i n the c o n c e n t r a t i o n o f g l u c o s e , but i t i s a second s u b s t a n c e ( H ) t h a t c a r r i e s the message to the e f f e c t o r m o l e c u l e (PEAA). The a n a l o g y to hormonal second messenger systems i s c r u d e , but q u i t e r e a l . +

+

^iBosojes Beairi^ We have v e r y r e c e n t l y e x ten d e 3 o i ï F ^ system by i m m o b i l i z i n g PEAA on the s u r f a c e o f egg l e c i t h i n v i a a h y d r o p h o b i c anchor (Maeda, M . ; Kumano, Α . ; T i r r e l l , D.A. P r e p r i n t s ACS Pi v . Polym. Chem., in p r e s s ) . The method i n v o l v e s the c o u p l i n g o f t h i o l a t e d PEAA to the maleimido p h o s p h o l i p i d 3 , which was i n c o r p o r a t e d a t a l e v e l o f c a . 10% i n t o preformed l e c i t h i n v e s i c l e s . Michael a d d i t i o n o f the polymer-bound t h i o l groups to the N - a l k y l m a l e i m i d e f u n c t i o n s on the v e s i c l e s u r f a c e r e s u l t s i n i m m o b i l i z a t i o n o f 50-60 ug o f PEAA per mg ôf l i p i d . F o l l o w i n g f r a c t i o n a t i o n o f the sample by s i z e e x c l u s i o n

11.

TIRRELL

Polymer Chemistry and Liposome Technology

159

Figure 1. E f f l u x of carboxyfluorescein from sonicated egg yolk phosphatidylcholine v e s i c l e s suspended i n 50 mM T r i s - H C l , 100 mM NaCl at indicated pH. (Reproduced with permission from Ref. 19. Copyright 1985 New York Academy of Sciences.)

TIME (min)

Figure 2. O p t i c a l density of a multilamellar suspension of DLPC i n an aqueous s o l u t i o n of PEAA and glucose oxidase, p r i o r and subsequent to addition of glucose.

160

T H E IMPACT OF CHEMISTRY ON BIOTECHNOLOGY

c h r o m a t o g r a p h y , a c i d i f i c a t i o n o f the v e s i c l e f r a c t i o n s causes r a p i d release of vesicle contents. These r e s u l t s demonstrate t h a t i t i s i n d e e d p o s s i b l e to anchor s u f f i c i e n t PEAA to e f f e c t u s e f u l changes i n membrane s t r u c t u r e and f u n c t i o n .

0

0"

CH 0P0CH CH NHCh^ 2

2

2

VCH -N

I

2

π

CH0C0R

I

0

CH 0C0R 2

R =

C

1 3

H

2 7

Conclusions Polymer c h e m i s t r y o f f e r s a number o f powerful s t r a t e g i e s f o r the c o n t r o l l e d m a n i p u l a t i o n o f the p r o p e r t i e s o f l i p o s o m a l membranes. P o l y m e r i z a t i o n w i t h i n the b i l a y e r can p r o v i d e membranes o f improved s t a b i l i t y and b a r r i e r p r o p e r t i e s . C o n t r o l l e d adsorption or anchoring o f p o l y e l e c t r o l y t e s can r e n d e r the l i p o s o m a l membrane s e n s i t i v e to chemical and p h y s i c a l s i g n a l s , and so can a f f o r d l i p o s o m a l d e l i v e r y systems s u s c e p t i b l e to s e l e c t i v e r u p t u r e a t p r e d e t e r m i n e d p h y s i o ­ l o g i c a l t a r g e t s o r under p r e d e t e r m i n e d p a t h o l o g i c a l c o n d i t i o n s .

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3. Ostro, M.J., ed. Liposomes; Marcel Dekker: New York, 1983. 4. Bangham, A.D., ed. Liposome Letters; Academic Press: New York, 1983.

5. 6. 7. 8. 9.

Knight, C.G., ed. Liposomes: From Physical Structure to Therapeutic Applications; Elsevier: Amsterdam, 1981. Gregoriadis, G.; Senior, J . ; Trouet, Α., eds. Targeting of Drugs; Plenum: New York, 1982. Fendler, J.H. Membrane Mimetic Chemistry; Wiley: New York, 1982. Tirrell, D.A.; Donaruma, L.G.; Turek, A.B., eds. Macromolecules as Drugs and as Carriers for Biologically Active Materials; N.Y. Acad. Sci.: New York, 1985. Bader, H.; Dorn, K.; Hupfer, B.; Ringsdorf, H. Adv. Polym. Sci. 1985,

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10. Fendler, J.H.; Tundo, P. Acc. Chem. Res. 1984, 17, 3. 11. Seki, K.; Tirrell, D.A. Macromolecules 1984, 17, 1692. 12. Sunamoto, J . ; Iwamoto, K.; Takada, M.; Yuzuriha, T.; Katayama, K. In Recent Advances in Drug Delivery Systems; Anderson, J.M.; Kim, S.W., eds. Plenum: New York, 1984, p. 153. 13. Martin, F.J.; Papahadjopoulos, D. J. Biol. Chem. 1982, 257, 286. 14. Ottenbrite, R.M.; Sunamoto, J . ; Sato, T.; Oka, M. Prepr. ACS Div. Polym. Chem. 1985, 26(1), 212. 15. Fukuda, H.; Diem, T.; Stefely, J . ; Kezdy, F.J.; Regen, S.L. J. Am. Chem. Soc. 1986, 108, 2321.

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Aliev, K.V.; Ringsdorf, H.; Schlarb, B.; Leister, K.H. Makromol. Chem. Rapid Commun. 1984, 5, 345. Sunamoto, J . ; Iwamoto, K. CRC Crit. Revs. Ther. Drug Carrier Systems 1986, 2, 117. Sadownik, Α.; Stefely, J . ; Regen, S.L. J. Am. Chem. Soc. 1986, 108, 7789. Tirrell, D.A.; Takigawa, D.Y.; Seki, K. Ann. N.Y. Acad. Sci. 1985, 446, 237. Devlin, B.P.; Tirrell, D.A. Macromolecules 1986, 19, 2465. Borden, K.A.; Eum, K.M.; Langley, K.H.; Tirrell, D.A. Macromolecules 1987, 20, 454. Takigawa, D.Y.; Tirrell, D.A. Makromol. Chem. Rapid Commun. 1985, 6, 653. Yatvin, M.B.; Kreutz, W.; Horwitz, B.A.; Shinitsky, M. Science 1986, 210, 1253. Yamashiro, D.J.; Tycko, B.; Fluss, S.R.; Maxfield, F.R. Cell, 1984, 37, 789. White, J.; Kielian, M.; Helenius, A. Quart. Revs. Biophys. 1983, 16, 151. Fichtner, F.; Schonert, H. Colloid Polym. Sci. 1977, 255, 230. Joyce, D.E.; Kurucsev, T. Polymer 1981, 22, 415. Sugai, S., Nitta, K.; Ohno, N.; Nakano, H. Colloid Polym. Sci. 1983, 261, 159.

RECEIVED August

14, 1987