Use of Bioerodible Polymers in Self-Regulated Drug Delivery Systems

Chapter 13

Use of Bioerodible Polymers in Self-Regulated Drug Delivery Systems J. Heller, S. H. Pangburn, and D. W. H. Penhale

Downloaded by UNIV LAVAL on April 27, 2016 | http://pubs.acs.org Publication Date: September 4, 1987 | doi: 10.1021/bk-1987-0348.ch013

Polymer Sciences Department, SRI International, Menlo Park, CA 94025

Two types of self-regulated drug delivery systems are described. One system is designed to deliver a therapeutic agent dispersed in a pH-sensitive polymer in response to the presence and concentration of a specific molecule external to the system. Control over rate of drug release is achieved by changes of erosion rate of the pH-sensitive polymer that occurs in response to pHchanges caused by an enzyme-substrate reaction. Delivery on insulin in response to glucose concentration is of particular interest. The other delivery system is passive until triggered by a specific external molecule. Triggering is achieved by activating an antibody deactivated enzyme which upon activation removes a protective hydrogel surrounding the delivery device. Although a number of applications are possible, delivery of naltrexone to rehabilitated opiate addicts is of particular interest. A self-regulated drug delivery system i s one that i s capable of receiving feedback and adjusting the drug output i n response to t h i s feedback (J_). Two fundamentally different approaches are possible in developing such delivery systems: i n one approach, the feedback signal modulates the rate of drug release from the delivery system, whereas i n the other approach the feedback signal triggers drug release from a passive device. In t h i s chapter we discuss our progress i n developing both modulated and triggered delivery systems. Modulated Drug Delivery Systems The p r i n c i p l e upon which t h i s approach i s based i s an enzymesubstrate reaction that produces a change i n pH and a h y d r o l y t i c a l l y l a b i l e , pH-sensitive polymer containing dispersed therapeutic agent that can vary the erosion rate and concomitant drug release i n response to that pH change. Figure 1 shows two types of drug

0097-6156/87/0348-0172S06.00/0 © 1987 American Chemical Society

Lee and Good; Controlled-Release Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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delivery devices that can respond to a pH-change caused by an enzyme-substrate reaction.


The device shown i n Figure 1(a) consists of a pH-sensitive polymer surrounded by a hydrogel containing immobilized enzyme. As the enzyme substrate diffuses from the external environment into the hydrogel and i s converted to either an a c i d i c or basic product, the pH within the hydrogel either increases or decreases and the polymer then responds to that pH change by a l t e r i n g i t s erosion rate. An alternative device i s shown i n Figure K b ) . In t h i s device the therapeutic agent and the enzyme are both dispersed i n the polymer, and the enzyme-substrate reaction occurs i n the outer layers of the device. The altered pH i n the outer layers then modifies erosion rate of the polymer. A key component of these delivery systems i s a polymer that i s capable of a l t e r i n g erosion rates rapidly and reproducibly i n response to small changes i n external pH. An additional requirement i s that drug delivery from the polymer must be an erosion-controlled process. In developing such systems, we have investigated two enzymesubstrate reactions as a means of modulating polymer erosion rates: urea

urease

^ H HC0 N

4

3

+ NH^OH

and glucose

glucose oxidase

^

g

l

u

c

o

n

i

c

a

c

i

d

Because one system leads to a pH increase and the other system leads to a pH decrease, two polymer systems are needed. Urea-Urease Modulated System. This system was developed several years ago as a model for t e s t i n g the f e a s i b i l i t y of t h i s concept. Because i t has been described previously (2), i t w i l l only be summarized here. Because the urea-urease i n t e r a c t i o n leads to a pH increase, a polymer that increases erosion rate with increasing pH i s needed. A useful polymer for t h i s application i s a p a r t i a l l y e s t e r i f i e d copolymer of methyl v i n y l ether and maleic anhydride. This copolymer undergoes surface erosion with an erosion rate that i s extraordinarily pH-dependent ( J ) . The polymer dissolves by ionization of the carboxylic acid groups as shown below: 0CH 2

OCH3

3

[-CH -CH-CH

- CH-]

COOR

COOH

insoluble

n

^[-CH -CH-ÇH 2

COOR

COO"

soluble


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CONTROLLED-RELEASE TECHNOLOGY


Figure 2 shows the results obtained when this polymer, containing dispersed hydrocortisone, i s surrounded by a hydrogel containing immobilized urease and release of hydrocortisone i s studied as a function of external urea concentration (2). Although this system has very l i t t l e therapeutic relevance, i t did establish the f e a s i b i l i t y of this concept and served as the basis for the development of a more useful system. Glucose-Glucose Oxidase Modulated System* C l e a r l y , a delivery system capable of modulating the release of i n s u l i n i n response to external glucose concentration i s of great interest i n managing diabetes* Two approaches are currently under study* One approach uses the glucose-glucose oxidase interaction to change the porosity of a r a t e - l i m i t i n g membrane (4); the other approach uses the desorption of a glycosilated i n s u l i n from Concanavalin A (J5). The approach described here uses the pH decrease caused by the gluconic acid generated i n the glucose-glucose oxidase reaction to change the erosion rate of a pH-sensitive polymer. Thus, for t h i s approach we need a polymer that not only s i g n i f i c a n t l y and reproducibly increases erosion rate with small decreases in external pH, but also undergoes surface erosion and i s capable of releasing i n s u l i n concomitantly with the erosion. One candidate polymer system i s the poly(ortho esters), which are pH-sensitive and undergo enhanced hydrolysis rates with decreasing external pH values (6). Furthermore, t h i s system has demonstrated surface erosion c h a r a c t e r i s t i c s and can be prepared i n both l i n e a r and crosslinked forms (7., 8). The crosslinked form i s p a r t i c u l a r l y interesting because sensitive macromolecules can be incorporated into the polymer under very mild conditions and without denaturation. These mild conditions are important because both i n s u l i n and glucose oxidase are proteins. Crosslinked poly(ortho esters) are prepared by a reaction sequence i n which an excess of the diketene acetal 3,9bis(ethylidene 2,4,8,10-tetraoxaspiro [5,5] undecane) i s reacted with a d i o l , and the ketene acetal terminated prepolymer i s then crosslinked with a t r i o l . Because the prepolymer i s a viscous l i q u i d at room temperature, the therapeutic agent and any excipients used are incorporated into the prepolymer by mixing at room temperature and then cured at temperatures that can be as low as 40°C. Figure 3 shows the results of an investigation of the pHs e n s i t i v i t y of a crosslinked poly(ortho ester) prepared from 3,9bis(ethylidene 2,4,8,10-tetraoxaspiro [5,5] undecane), triethylene g l y c o l , and 1,2,6-hexanetriol. In these studies, a marker molecule, p-nitroacetanilide was incorporated into the polymer, and the rate of release of p - n i t r o a c e t a n i l i d e was assumed to correspond to the rate of erosion of the polymer. This assumption i s not e n t i r e l y accurate because some d i f f u s i o n a l release occurs. However, the method i s a good measure for determining the changes i n erosion rates with changes i n external pH.


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pH SENSITIVE BIOERODIBLE

^I

POLYMER AND DRUG ^

HYDROGEL CONTAINING IMMOBILIZED ENZYME

(a) pH SENSITIVE BIOERODIBLE


POLYMER WITH DRUG AND ENZYME i

ENZYME - SUBSTRATE REACTION

(b) Figure 1. systems.

Schematic representation of i n s u l i n delivery

0

100 200 TIME - hours

300

Figure 2. Hydrocortisone release rate at 35°C from a n-hexyl half-ester of a copolymer of methyl v i n y l ether and maleic anhydride at pH 6.25 i n the absence and presence of external urea. • No external urea Φ10~ Μ external urea Ξ10" M external urea. (Reproduced with permission from Ref. 2. Copyright 1979, American Pharmaceutical Association.) 2


1

176



The data shown i n Figure 3 show a s i g n i f i c a n t increase i n erosion rate with decreasing external pH-values. However, neither the magnitude nor the response time for the change from pH 7·4 to 5, the region of i n t e r e s t , i s adequate for a self-regulated i n s u l i n delivery device, which should be able to respond to a decrease i n pH within about 15 minutes. In an attempt to increase the pH s e n s i t i v i t y of poly(ortho esters), we incorporated t e r t i a r y amine functions into the polymer structure. As shown i n Figures 4 through 6, the acid s e n s i t i v i t y was dramatically altered. Further i n t e r e s t i s that the useful pHrange and degree of acid s e n s i t i v i t y are a function of the percent of nitrogen i n the polymer. Thus, the polymer shown i n Figure 4 prepared from N-n-butyl diethanolamine and the non-nitrogen containing t r i o l , LG-650, has the lowest nitrogen content. It also has the lowest pH range and the lowest pH s e n s i t i v i t y . As the percent of nitrogen i n the polymer i s increased by using triethanolamine crosslinker and ultimately N-methyl triethanolamine, the useful pH-range of the polymer increases as does i t s pHs e n s i t i v i t y . In f a c t , the pH-sensitivity of the polymer shown i n Figure 6 i s extraordinary i n that a change as small as 0.05 pH unit produces a very s i g n i f i c a n t change i n erosion rate. The data shown i n Figures 4 through 6 indicate that aminecontaining poly(ortho esters) are suitable candidates for the development of a glucose-oxidase mediated i n s u l i n delivery system. However, to prevent a hypoglycemic condition, i n s u l i n delivery must rapidly decrease once the blood glucose l e v e l has decreased to the normal 100 mg $. To test the r e v e r s i b i l i t y of erosion rates with changing external pH values, we conducted an experiment i n which the device was repeatedly exposed to a pH 3 buffer and to a buffer t y p i c a l of the physiological pH of 7.4. Results of t h i s preliminary study shown i n Figure 7, indicate that the polymer responds reproducibly and repeatedly to these changes i n external pH. Because i n s u l i n i s a large molecule, the device configuration shown i n Figure 1(a) i s not suitable since i n s u l i n released from the polymer would probably be unable to traverse the glucose-oxidasecontaining hydrogel that surrounds the polymer. For this reason, the configuration shown i n Figure K b ) i s much preferred. In t h i s configuration, glucose w i l l i n t e r a c t with glucose oxidase dispersed i n the matrix, and the gluconic acid generated i n the surface layers w i l l accelerate surface erosion of the polymer i n proportion to i t s concentration. This expectation i s e n t i r e l y reasonable because previous work with a c i d i c excipients physically dispersed into the polymer has established a l i n e a r r e l a t i o n s h i p between concentration of the a c i d i c excipient and surface erosion of the poly(ortho ester) matrix ( 9 ) . However, because the device w i l l deliver not only i n s u l i n but also glucose oxidase, a n t i g e n i c i t y problems are a very r e a l concern. For t h i s reason, we w i l l use the procedure developed by Arbuchowski i n which poly(ethylene glycol) i s grafted onto the



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TIME - hours

Figure 3 · Effect of external pH on erosion rate of crosslinked poly(ortho ester) prepared from 3,9-bis(ethylidene 2,4,8,10-tetraoxaspiro [5,5] undecane), triethylene g l y c o l , and 1,2,6-hexanetriol. Disks 5.5 χ 0.75 mm; pnitroacetanilide loading 2 wt$.

80

TIME - hours

Figure 4. Effect of external pH on erosion rate of crosslinked poly(ortho ester) prepared from 3,9-bis(ethylidene 2,4,8,10-tetraoxaspiro [5,5] undecane), n-butyl diethanolamine, and LG-650. Disks 5.5 x 0.75 mm; pnitroacetanilide loading 2 wtt.


178



TIME - hours

Figure 5. Effect of external pH on erosion rate of crosslinked poly(ortho ester) prepared from 3,9-bis(ethylidene 2,4,8,10-tetraoxaspiro [5,5] undecane), n-butyl diethanolamine, and triethanolamine. Disks 5·5 x 0.75 mm; pn i t r o a c e t a n i l i d e loading 2 wtî.

TIME - hours

Figure 6. Effect of external pH on erosion rate of crosslinked poly(ortho ester) prepared from 3,9-bis(ethylidene 2,4,8,10-tetraoxaspiro [5,5] undecane), n-methyl triethanolamine, and triethanolamine. Disks 5.5 χ 0.75 mm; pn i t r o a c e t a n i l i d e loading 2 wtj.


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enzyme to prevent recognition of antigenic determinants on the enzyme (10). Triggered Drug Delivery Systems


Many applications are possible for an implantable delivery system that i s programmed to deliver a therapeutic agent at a preselected rate, but i s passive u n t i l a s p e c i f i c external molecule activates the device. Of p a r t i c u l a r i n t e r e s t i s the r e h a b i l i t a t i o n of individuals who have developed an opiate dependence 0 ) . In one application of such a device, the r e h a b i l i t a t e d individual would be implanted with a device containing the narcotic antagonist naltrexone, and the device would be designed so that i t can be triggered by morphine. As long as the i n d i v i d u a l r e f r a i n s from morphine intake, the device remains passive, and no naltrexone i s delivered. However, upon self-administration of morphine, the opiate appearing i n the tissues would activate the device and trigger release of naltrexone. Because naltrexone can displace morphine from i t s receptor s i t e s , the pleasurable morphine-induced high would be rapidly neutralized. The major advantage of this approach i s that drug i s released only when actually needed. Clearly, opiate addiction i s only one application and many other therapeutic uses can be envisioned. The device shown i n Figure 8 contains three separate components. One i s a pH-sensitive bioerodible polymer capable of releasing naltrexone by an erosion-controlled process. The polymer i s so designed that at the physiological pH of 7.4 i t erodes at the desired rate, but at a lower pH i t i s stable, undergoing no erosion. The second component i s an enzyme-degradable hydrogel that surrounds the bioerodible polymer with an environment having a pH low enough that no erosion takes place. The third component i s a reversibly inactivated enzyme that i n i t s active state i s capable of degrading the hydrogel, thus exposing the bioerodible polymer to the physiological pH. The f u n c t i o n a l i t y of the device i s based on the fact that morphine can d i f f u s e into the hydrogel and activate the enzyme. Thus, development of this device requires the development of three separate components, as discussed below. pH-Sensitive Polymer. The bioerodible polymer required i n t h i s application must meet two requirements: (1) i t must be stable at a pH lower than the physiological pH of 7.4 and erode at desired rates at that pH and (2) i t must be capable of releasing an incorporated therapeutic agent by an erosion-controlled process. A useful polymer system i s one already described i n the urea-urease modulated system (3.)· Because that polymer s o l u b i l i z e s by an i o n i z a t i o n of carboxylic acid groups, the s o l u b i l i z a t i o n i s highly pH dependent. Also, the pH at which s o l u b i l i z a t i o n takes place i s a function of the size of the a l k y l group i n the ester portion of the polymer. This l i n e a r dependence i s shown i n Figure 9. The polymer i s capable of releasing naltrexone; Figure 10 shows naltrexone release at pH 7.4. Because we wish to neutralize



100 80û LU

LU OC


ο < ζ α.

60-

I

10

TIME - hours

20

Figure 7. Effect of changing external from 3 to 7.4 pH on erosion rate of crosslinked poly(ortho ester) prepared from 3,9-bis(ethylidene 2,4,8,10-tetraoxaspiro [5,5] undecane), nbutyl diethanolamine, and LG-650. Disks 5.5 χ 0.75 mm; pnitroacetanilide loading 2 wtj.

PN sensitive hydrophobic bioerodible polymer and drug PH 7.4

enzyme-n>> ]

Figure 8. device.

In enzyme-degrodable ocldlc hydrogel

Schematic representation of triggered drug delivery


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2 -

ol

I

I

I

I

I

0

2

4

6

8

10

1

12

i—

14

NUMBER OF CARBONS IN ALKYL SUBSTITUENT

Figure 9. Relationship between pH of d i s s o l u t i o n and size of ester group i n half-esters of methyl v i n y l ether-maleic anhydride copolymers. (Reproduced with permission from Ref. 3. Copyright 1978, John Wiley & Sons.) 100 .TREX ONE RELE ASED-


10 I

s

80 6040200

H

0

10

20 TIME - minutes

30

40

Figure 10. Release of naltrexone from the n-hexyl half-ester of methyl v i n y l ether-maleic anhydride copolymer at pH 7·4 and 37°C. Disks 5.5 x 0.75 mm; naltrexone loading 10 w t i .


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morphine as soon as possible, the polymer was designed to completely degrade i n about 30 minutes. However, this time could be readily modified.


Enzyme-Degradable Hydrogel. Because lysozyme i s a well characterized enzyme, our f i r s t choice was a lysozyme-degradable hydrogel (11, 12). The natural substrate for lysozyme i s c h i t i n (13)» but because c h i t i n i s a r i g i d , hydrophobic material, i t i s c l e a r l y not suitable f o r t h i s work. The other natural substrates for lysozyme are certain b a c t e r i a l c e l l - w a l l peptidoglycans (13» 14), that are also unsuitable because they are crosslinked materials that cannot be fabricated into a coating suitable for our device. However, the h y d r o p h i l i c i t y of c h i t i n , an N-acetylglucosamine, can be greatly increased by deacetylation ( 15), and chitosan, the completely deacetylated c h i t i n i s actually water-soluble, but i t i s not degraded by lysozyme. P a r t i a l l y deacetylated c h i t i n , however, a highly hydrophilic material, i s a substrate for lysozyme (16) and can be r e a d i l y converted into a hydrogel by a crosslinking reaction between the free amino groups and glutaraldehyde. P a r t i a l l y deacetylated c h i t i n can be prepared by alkaline hydrolysis of c h i t i n , and as shown i n Figure 11, the degree of deacetylation can be r e a d i l y controlled by the length of the hydrolysis reaction. The rate of lysozyme-catalyzed degradation of the p a r t i a l l y deacetylated c h i t i n hydrogel i s indicated i n Figure 12, which measures the release of incorporated glucose oxidase as a function of time. Because the release was measured as a c t i v i t y of the enzyme, no absolute hydrogel degradation rates can be determined because only active enzyme was assayed i n these experiments. Nevertheless, the data show that rate of hydrogel degradation i s very slow. These findings are consistent with our previous work. The data shown i n Figure 12 indicate that the rate of glucose oxidase released from the hydrogel i s d i r e c t l y proportional to the concentration of the protein i n the hydrogel, suggesting that lysozyme catalyzed degradation of the hydrogel occurs predominantly by surface erosion process. Figure 13 shows the e f f e c t of lysozyme concentration on rate of degradation of the p a r t i a l l y deacetylated c h i t i n hydrogel, again measured as a c t i v i t y of the released glucose oxidase. As expected, the rate of degradation decreases as the concentration of the external enzyme increases. Note that no degradation takes place below a lysozyme concentration of 1 mg/mL. This finding indicates that complete i n h i b i t i o n of the enzyme may not be necessary, provided the t o t a l concentration of active enzyme remains below a c e r t a i n c r i t i c a l value. Reversibly Inactivated Enzyme. The reversible i n a c t i v a t i o n of an enzyme can be achieved by a procedure described by Schneider and coworkers (17)· In that procedure, a hapten i s chemically attached to the enzyme, close to i t s active s i t e , and the enzyme conjugate i s then complexed with an antibody to the hapten. Because the antibody




HELLER ET AL.

TIME OF ALKALINE HYDROLYSIS (hours) F i g u r e 11. R e l a t i o n s h i p o f c a r b o n - t o - n i t r o g e n c o n t e n t o f c h i t i n subjected t o a l k a l i n e h y d r o l y s i s for varying lengths o f time, ν , i d e a l i z e d c h i t i n (0Î d e a c e t y l a t i o n , N - a c e t y l - D g l u c o s a m i n e , M.W. = 203); Δ , i d e a l i z e d c h i t o s a n (100? d e a c e t y l a t i o n , g l u c o s a m i n e , M.W. = 161). (Reproduced w i t h p e r m i s s i o n from Ref. 12 C o p y r i g h t 1984, Academic P r e s s . )

0.8-1

0

1

1

2

3

4

5

6

TIME - hours F i g u r e 12. R e l e a s e o f g l u c o s e o x i d a s e from a g l u t a r a l d e h y d e crosslinked p a r t i a l l y deacetylated c h i t i n . E x t e r n a l lysozyme c o n c e n t r a t i o n 2 mg/mL. Weight o f g e l 18 mg. Amount o f g l u c o s e o x i d a s e i n g e l • 3 mg • 1.5 mg • 0.3 mg.



184


ENZYME CONCENTRATION - mg/ml

Figure 13. E f f e c t of external lysozyme concentration on the release of glucose oxidase from a glutaraldehyde-crosslinked, p a r t i a l l y deacetylated c h i t i n . Amount of glucose oxidase i n gel 3 mg/ 18 mg g e l .

Substrate Excluded Enzyme Inactive

Substrate Admitted Enzyme Active

Figure 14. P r i n c i p l e of homogeneous enzyme immunoassay (Reproduced with permission from Ref. 17. Copyright 1973, American Association f o r C l i n i c a l Chemistry, Inc. )


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i s a very large molecule, when complexed with the enzyme conjugate, i t s t e r i c a l l y i n h i b i t s access of the enzyme substrate to the enzyme active s i t e , thus rendering the enzyme i n a c t i v e . However, i n the presence of free hapten, the complex can disassociate, with consequent activation of the enzyme. This process i s shown schematically i n Figure 14.


The conjugation of lysozyme and carboxymethyl morphine i s shown schematically i n Figure 15. In this procedure, about four morphines are attached to the enzyme. Figure 16 shows the results of preliminary studies of the effect of antibody complexation with morphine-derivatized on the a c t i v i t y of lysozyme. Although i n these studies the exact concentration of the morphine antibody i n the morphine antiserum i s not known, the a c t i v i t y of lysozyme i s rapidly diminished with only a r e l a t i v e l y small decrease taking place upon addition of a large excess of antibody.

R-OÛOH • C l - C - O - i e o B u t y l II Ο

R

Ο •

\

c\ iio-lutyl-O-C

R-OOOH « O^-carboxymtthyl morphine

Figure 15. Lysozyme-morphine conjugation scheme


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CONTROLLED-RELEASE TECHNOLOGY 100 < ν £

80-

û


0 1 0

ι • • • • 1 5 10 15 ANTISERUM CONCENTRATION - mg/ml

Figure 16. Effect of adding morphine antibodies to an external lysozyme-morphine conjugate on the release of glucose oxidase from a glutaraldehyde-crosslinked, p a r t i a l l y deacetylated c h i t i n . Amount of glucose oxidase i n gel 1.5 mg/14.8 mg g e l . Acknowledgments The self-regulated i n s u l i n delivery work i s being funded by Grant GM 27164 and the triggered delivery system by Grant DA 03819·

Literature Cited 1. 2. 3.

Heller, J. Med. Device and Diagn. Ind. 1985, 7, 32-37. Heller, J . ; Trescony, P. V. J. Pharm. Sci. 1979, 68, 919-921. Heller, J.; Baker, R. W.; Gale, R. M.; Rodin, J. O. J. Appl. Polymer Sci. 1978, 22, 1991-2009. 4. Albin, G.; Horbett, J. T; Ratner, B. D. J. Controlled Release 1985, 2, 153-164. 5. Jeong, S. Y.; Kim, S. W.; Holmberg, D. L . ; McRea, J. C. J. Controlled Release 1985, 2, 143-152. 6. Heller, J.; Penhale, D.W.H.; Fritzinger, Β. K.; Ng, S. Y. In "Long-Acting Contraceptive Delivery Systems"; Zatuchni, G. I.; Goldsmith, A; Sheldon, J. D.; Sciarra, J., Eds.; Harper & Row: New York, 1984; pp. 113-128. 7. Heller, J.; Fritzinger, B. K.; Ng, S. Y.; Penhale, D.W.H. J. Controlled Release 1985, 1, 225-232. 8. Heller, J . ; Fritzinger, B. K.; Ng, S. Y.; Penhale, D.W.H. J. Controlled Release 1985, 1, 233-238. 9. Shih, C.; Higuchi, T.; Himmelstein, K. J. Biomaterials 1984, 5, 237-240. 10. Davis, F. F.; Abuchowski, Α.; van Es, T.; Palazuk, Ν. C.; Chen, R.; Sauoca, Κ.; Wieder, Κ. In "Enzyme Engineering"; Sauoca, K.; Wieder, Κ., Eds; Plenum Press, New York, 1978; Vol. 4, pp. 169-173. 11. Pangburn, S.H.; Trescony, P. V.; Heller, J. Biomaterials 1982, 3, 105-108. 12. Pangburn, S. H.; Trescony, P. V.; Heller, J. In "Chitin, Chitosan, and Related Enzymes;" Zikakis, J. P., Ed.; Academic Press, New York, 1984; pp. 3-19.


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13. Berger, L. R.; Weiser, R. S. Biochim. Biophys. Acta 1957, 26, 517-521. 14. Salton, M.R.J.; Ghuysen, J. M. Biochim. Biophys. Acta 1960, 45, 355-363. 15. Sannan, T.; Kurita, K.; Iwakura, Y. Macromol. Chem. 1976, 177, 3589-3600. 16. Amano, K.; Ito, E. Eur. J. Biochem. 1978, 85, 97-104. 17. Schneider, R. S.; Lindquist, P.; Wong, E. T.; Rubenstein, K. E.; Ullman. E. F. Clin. Chem. 1973, 19, 821-825. RECEIVED December 12, 1986


Use of Bioerodible Polymers in Self-Regulated Drug Delivery Systems

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