Ionian Polymer Jorwitnl l Volume 5 Number d (1996)

10261265796

Synthesis of Diclofenac Polymeric Prodrugs

and Their Hydrolysis Reactivity

Mohammad Hossein Nasir Tabrizi, Soodabeh Davaran and All Akbar EntezamiPolymer Laboratory, Faculty of Chemistry, University of Tabriz, Tabriz, LR.Iran

Received: 2B April 1996; accepted 16 October 1996

ABSTRACT

in order to minimize the sodium diclofenac side effects and to increase itstherapeutic efficiency, polymeric prodrugs of the diclofenac are prepared.

Diclofenac is attached to poychloromethylstyrene, polyvinyl chloroacetate

and polyethylene glycol through the hydrolyzable ester bonds, and assess-ed in vitro for usefulness as diclofenac prodrugs. Specially designed spacer

groups are introduced between the drug and polymer backbone.The polymers are characterized by FT1R, 1 H NMR and 11C NMR

spectroscopy . The hydrolysis of polymer-drug conjugates is carried out in

cellophane membrane dialysis bags containing aqueous buffer solutions(pH=B, 37 'C) . The hydrolysis solution was detected by UV spectrophoto-

metry at selected intervals. The results have shown that these polymers arepotentially useful as polymeric prodrugs . It is found that polyvinyl chioroacet-

ate is an appropriate carrier for release of the drug in human conditions.

Key Words : diclofenac , polymeric prodrugs, synthesis, controlled release, characterization

INTRODUCTION

Synthetic macromolecules with functional groupsof pharmacological potential have received consid-erable attention in the last 20 years [1, 2] . Themost interesting of all are investigations of pharma-cological active polymers which by themselves maybe active as drugs or alternatively used as carriersfor pharmaceutical agents widely used in medica-tion [3]. Such systems avoid the problems of low-molecular weight drugs which can produce general-ized toxic reactions. Our discussion in this paper islimited to the use of conjugates in which drugs, viaa degradable covalently bond attach to a non-

specific macromolecule carrier such as sodiumdiclofenac, (sodium [24(2, 6-dichlorophenyl)-amino]-phenyl]acetate), a potent non-steroidalanti-inflammatory drug (NSAID), which is thera-peutically used in inflammatory and painfuldiseases of rheumatic and nonrheumatic origin [4].

In order to counter the sodium diclofenacside effects many papers report the physicalcontrolled release of diclofenac [5, 6], but only afew reports have been published on the use of'theprodrugs. With this aim diclofenac was attached topoly(p-chloromethylstyrene) and polyvinyl chloro-acetate and polyoxyethylene by some esterificationmethods according to the Schemes (I—V) .

243

Synthesis of Didofenac Polymeric Prodrugs and Their Hydrolysis Reactivity

Dicln fenac(Drug-COOH)

(CH2 -CH)n - (CIH-CH)

MN -(CH2-CH),,- (CH2-CH),.- (CH2-CH) —µ

Drug-COOH(Et)3N

lCH 2qC-Drug

(1)

(PS.D)

0

Scheme I

CH2C1CH2Cl

,>r .,. -(CH 2 -CH)n-(CH2-I

CI H

bH

OCCH2C1

Drug-COOHI(Et)3Nor Drug-COONa/18-Crown-6 ether

(2)

M-- {CH2-CH),-(CH2 -CH)m-(CH2 -CH)1,-.-fOH

OCCH2CI qICH2qC-Drug11O

q 0

PVA .TD and PVACD

Scheme II

Drug-COO(CH2 CH2O)nH

Drug-COOH + HO(CH2CH2O),H C150 3 iior p-TSOH

n Drug-COO(CH2CH2O) aCH2CH20OGDrug

n -10000PEG .CD and PEG .PD

Scheme III

r-_ CH2CICOO(CH2CH2O)nH

CICH2 COOH + HO(CH2CH2q )AH C1SO3 Hor p-TSOH

n CH 2 CICOO(CH2CH2O)nCH 2CH20OCCH2C1

(3)Scheme IV

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;N.Ir Tabrai AN . et a1.

CII 2 CICOO(CH 2 CH 2O), H

+ Drug-COOH (Et) 3N }

CH 2 CI COO (CH2 CH 2O) aCH 2 CH2O O CCH2CI

n=10(x]0

r' Drug-COOCH2COO(CH 2 CH2 O)H

Drug-COOCH2 COO(CH2CH 2 O)„CH 2 CH 2OOCCI-I 2 OOC.-Drug

Scheme V

PEG .CC.D

The hydrolysis behaviour of polymer-drugadduct was studied in physiological conditions(aqueous phosphate buffer at 37 'C) . The pendantdiclofenac group was hydrolyzed under mild condi-tions and the quantity of released drug was detect-ed by spectrophotometric method.

The polymers were characterized by I, UR,'H NMR, 13C NMR and UV spectroscopies.

EXPERIMENTA L

Materials and InstrumentsSodium diclofenac was purchased from CHEMOS.A {Swiss), and all other chemicals were of reagentgrade . FTIR, UV and NMR spectra were recordedby Shimadzu 430 spectrophotometer, Shimadzu2100 spectrophotometer and JEOL 90 instruments,respectively.

SynthesesPoly (styrene-co-4-chlorornethylstyrene)The copolymer (structure 1) prepared according tothe method described by S .D Class and A.Eisenberg [7l.

Esterification of Po1y(styrene-co-4-chloromethyl-.sty-rene)A solution of 1 .8 g(6.17 mmol) diclofenac in 0.85mL(6 .17 mmol) triethylamine (Et) 3 N was pouredinto 35 roL of cthylacctate and 4 .0 g of poly-(styrcne-co-4-chloromethylstyrenc) and the mixturewas stirred under rellux temperature . After 30 hthe resin was filtered and washed thoroughly withethylacetate, ethanol, water and methanol and

dried under vacuum to yield 4 .5 g of product . Asample of 30 mg of the resulting esterified resinwas hydrolyzed by refluxing for 24 h in a mixture of5 mL of acetic acid and 5 mL of ON hydrochloricacid. FTIR (KBr) :1735 cm-'(s,C=O), 1100, 1275cm-'(s,C—O), 3300 cm-t(s,N—H).

Preparation of Poly(vinylalcohol)-co polyvinyl-chl-oroacetate)The title polymer (structure 2) was prepared byesterification of 10 g polyvinyl alcohol (M n =72000)with 35 .5 g (0.37 moi) chloroacetic acid in 50 mLdichloroethane in the presence of 10—12% p-tolu-enesulphonic acid at 75—80 ` C. The product wasprecipitated in cold ethanol, dissolved in solutionof Na2 COa in acetone and it was reprecipitatedfrom ethanol to give 6.9 g (70%) of product 181.

Esterification of Poly(vinylalcohol)-co-poly(vinyl-chloroacetate) with DiclofenacMethod A: A solution of 1 .8 g (6.17 mmol) ofdiclofenac in 0 .85 mL (6.17 mmol) tricthylamincwas poured into 35 mL of cthylacetate and it wasadded to 4 .0 g of poly(vinylalcohoI)-co-poly(vinyl-chloroacetate). The mixture was stirred underreflux temperature . After 30 h the resin wasfiltered and it was washed thoroughly with ethyl-acetate, ethanol, water and methanol and driedunder vacuum to yield 4.5 g of product.Method B: Poly(vinylalcohol)-co-poly(vinyi-chloro-acetate) (3 .0 g) was dissolved in DMF (160 mL) ina 500 mL round bottom flask. Sodium diclofenac,17 .8 g (56 mmol) and 18-crown-6 ether (1 .32 g)were added . The stoppered flasks were placed inan oil bath at 30±3 'C and stirred for 24 h . The

Iranian 1'utynu•r Journal ; Vulrane 5 Nrrn hrr 4 (1996)

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Syntlae..is of Diclofenan Polymrric l're.druOs and Ihcir Flydroly:is Reactn*ity

reaction mixtures were precipitated into methanol(3 L), washed with water (2 L) and precipitatedinto diethyl ether (3 L), and tinally they werefiltered and dried in vacuum at about 70 ' C. FTIR:(KBr) 1745 cm- (s,C=O), 1450, 1650 cm-'(s,aromatie C=C) . 1 H NMR(DMSO d6) : d 1 .1—! .2(chain CH2 , CH) 7—7.2(m, 7H, 2°ph) . ' 3 CNMR(DMSO d6) : 640—60(m, CHS, CH), 115—145(m, 2* ph), 170—172(C=O ester).

Esterification of Polyethylene Glycol with DiclofenacA solution of 1 .66 g (5 .6 mmol) of diclofenac, 20 gof polyethylene glycol (PEG), with molecular massof 10000, in 0.75 mL (11 .2 mmol) of chlorosulpho-nic acid (CISO1 H), or p-methylbenzenesulphonicacid (p-CH3C(,H5 SO3 H) (11 .2 mmol) in 25 mL ofchloroform was stirred under reflux temperature.After 2 h the solution was poured into a largeamount of diethyl ether and purified by repeatedreprecipitation . The resulting polymer was dissolv-ed in chloroform and reprecipitated by ether . Thepurification procedure was repeated twice . Thefinal product was dissolved in a mixed solventconsisting of ether-chloroform (50 :50 vlv) andprecipitated with ether to yield the pure esterifiedPEG (40%). FTIR (KBr) : 3500, 3200, 2900, 1735,1460, 1350, 900 cm- I .

Preparation of PEG with Chloromethylacetate End-

group

PEG with chloromethylacetate end-group (struc-ture 3) was prepared by esterification of PEG (10g) with chloroacetic acid 0.26 g (2 .S mmol) in 30 ` Cdichloroethane in the presence of 0 .3 mL (4x10 -3mmol) of chlorosulphonic acid under reflux temp-erature . After 5 h the solution was poured into alarge amount of diethyl ether . The purification ofresulting polymer was conducted according to themethod described above. FTIR (KBr) : 3500, 2900,1751, 1460, 1350, 900, 800 cm- f .

E.sierifccatinn of PEG with Chloroacetate End-groupA solution of 1 .66 g (5 .6 mmol) of diclofenac and20 g of PEG with chloroacetate end-group and0 .75 mL (11 .2 mmol) of chlorosulphonic acid was

poured in 25 mL of chloroform and the mixturewas stirred under reflux. After 2 h the solution waspoured into a large quantity of ether . Theprecipitated polymer was collected and purified bythe method described abos e . The yield was 30%.NMR(DMSO d6): 67—7.6(m, 25 ph), 3 .3—3 .6(m,CH 2 ).

Determination of the Extent of Ester Bonds Hydro-lysisThe polymers obtained were dried under vacuum,pulverized and sieved by a 200 mesh sieve . Thepowdered polymer (20 mg) was conducted into acellophane membrane hag containing 5 mL ofphosphate butler (pH=8) . The bag was transferredinto a solution of 25 mL of the same buffer. Theouter solution was stirred at 37 'C and a 3 mL ofsample was removed at selected intervals . Thequantity of released drug was determined by UWspectrophotometry at 275 nm and it was determin-ed from the calibration curve obtained previouslyunder the same conditions.

RESULTS AND DISCUSSION

Two different synthetic methods have beendeveloped in the preparation of polymers thatcontain pendant drug substituents . The drugmolecule is converted to a polymerizable derivativeand subsequently it is polymerized to produce apolymer-drug combination. In another method, thedrug is attached to a preformed polymer backbonevia degradable chemical bonds . In most cases thebest approach to obtain the desired system is tomodify the preformed polymers. Determining suit-able conditions for the reaction of the drug withthe polymer is an important process in which thesolvent selection is specially difficult, because manypolymers show only limited solubility in a fewnumber of solvents . Water soluble polymers usuallyhave a critical degree of substitution above whichthey become insoluble. The degree of substitutionin polymers was determined by elemental analysis.Such behaviour also limits the amount of agentsthat can be incorporated in this system. With

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Nasir Tabrizi M .H. et al.

respect to these problems, we have chosen special-ly designed synthetic routes for preparation ofpolymeric derivatives of diclofenac that have beenchemically bonded to preformed polymers.

Synthesis and polymerization of polymeric-able derivatives of diclofenac arc limited by somefactors such as unstability of the drug towardsapplied conditions for synthesis of the monomers,and also difficult polymerization process of adductmonomers associated with preparing systems frompolymerizable derivatives.

Synthesis of Polymeric ProdrugThe structure and synthetic routes for preparationof modified preformed polymers with diclofenacpendant groups are represented in Schemes I —V.

At the first approach, poly(chloromethylstyrenc-ro-styrenc) was reacted with the acidicform of drug in presence of triethylamine (SchemeI) . This reaction is a modified Merrifield reaction[9] . The polymer obtained is not soluble in waterand methanol . The degree of substitution is lowerthan 2%.

In order to increase the hydrophilic chara-cter of parent polymer, we attempt to preparepolyvinyl alcohol carriers. At first polyvinylchloroacetatc)-co-poly(vinylalcohol), Scheme II,was prepared according to the method described byclass [71.

Estcritication of the resulting polymer(Scheme III) with diclofenac was carried out bytwo methods . In the first method polymer wasreacted with acidic form of the drug in presence oftriethylamine . In the second method sodium diclo-fenac (salt form) was reacted with the polymer.This reaction was catalyzed by 1S-crown-6 ether.The yield of the product in the first method washigher than the second method, therefore, thewater solubility of resulting polymer that wasformed in the former method was less than thelatter. In another approach we have chosen thepreformed PEG as a parent polymer in order toincrease the degree of substitution and hydrophiliccharacter of polymer—drug conjugate.

In the first method the drug was directlyattached to the terminal hydroxyl groups of PEG .

PS.D

10

20

30

40

50

Time (h)

Figure 1 . Polystyrene–drug adduct hydrolysis.

The esterification of diclofenac with PEGwas explored using some of the con ventionalmethods for ester formation . Among thesemethods reactions were carried out in presence ofchlorosulphonic acid (PEG-CD) or p-toluenesul-phonic acid (PEG-PD) as catalysts . Chloromethyl-acetylation of PEG and esterification of resultingpolymer with diclofenac produced a polymer—drugconjugate containing a diestcr spacer group as inScheme V.

The esterification of drug with PEG wascatalyzed with p-toluenesulphunic acid orchlorosulphonic acid. The latter was a moreeffective method (Scheme IV) . In another methoda spacer group was introduced between the drugand hydroxyl group of the polymer (Scheme V)and the polymer was reacted with an acidic form ofthe drug in the presence of triethylamine.

0

2

4

6

0

10

12

Time (h)

Figure 2 . Poly(vinylalcohol)-co-poly(vinylchloroacetate)—drug hydrolysis in presence of crown ether.

4

3

2

1

0

10

a

6

4

2

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Synthesis of Diclofenac Polymeric Prodrug and Their Hydrolysis Reactivity

Figure 3 . Poly(vinylalcohol}-co-poly(vinylchloroacetate)–

drug hydrolysis in presence of triethylamine.

Hydrolysis of Polymeric Prodrug

The hydrolytic behaviour of polymeric prodrugswas studied in an aqueous phosphate buffer solu-tion at physiological condition (pH=$, 37 'C). Thehydrolysis was carried out in cellophane membranebags permeable to low molecular weight compoun-ds. The Figures 1-6 show the cumulative (weightgram ratio of drug/polymer) concentration ofreleased drug that was detected by UV spect-roscopy at 275 nm. All polymers released the druggradually under mild condition and in IH-I2 PD4 -Na2 HPD4 buffer (pH=8). Figure 1 shows thehydrolysis behaviour of polystyrene-drug adduct(PS .D) conjugates, and as it is shown in this figurethe degree of hydrolysis is low. This is because ofthe low hydrophilic character of polymer and lowdegree of substitution . Figures 2 and 3 show the

3.5

3.0

2.5

2.0

1 .5

1 .0

0.5

00

2

4

6

8

Time (h)

Figure 4. Polyethylene glycol–drug hydrolysis in presenceof p-toluenesulphonic acid .

Figure S . Polyethylene glycol–drug hydrolysis in presenceof chlorosulphonic acid.

release data for the poly(vinylalcohol)-co-(vinyl-chloroacetate) carriers, which were prepared bytwo methods, in presence of triethylamine andcrown ether (Scheme II) . The esterification pro-duct of poly(vinyl chloroacetate)-co-poly-(vinyl-alcohol) with sodium diclofenac had a low yieldwhen catalyzed with crown ether (Figure 2), butthe polymer obtained was hydrolyzed more rapidlycompared to the same polymer that was synthesiz-ed in presence of triethylamine (Figure 3) . In thiscase the precentage of substitution was higher.However, the resulting polymer was insoluble inwater and the hydrolysis rate was significantly lessthan that of low substituted polymer. The percent-age of substitution determines the degree of avail-able polymer sites that had diclofenac attached.Hence, an increase in degree of substitution results

4

6

8Time (h)

Figure 6 . Polyethylene glycol with chloroacetate end-group–drug hydrolysis.

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12

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in a decrease in the rate of hydrolysis . This isbecause of the increase in free hydroxyl groupsthat has led to high hydrophilicity of polymer asthe rate of hydrolysis is extremely sensitive to itsdegree of hydrophilicity. Two hydrolyzable esterbonds are present in polymer. Detection of hydro-lysis solution by UV spectroscopy was shown thatthe ester bond at the side chain of drug moiety canbe hydrolyzed during the reaction time (12 h) . Thedirect ester linkage between the main chain ofpolymer and spacer arm is less susceptible towardshydrolysis. This can be related to the sterichindrance of bulk polymer chains and decrease inbond mbility. Figures 4—6 show the release profilesof the drug from PEG derivatives as a function oftime. The direct attachment of drug to PEG wascarried out by two methods . At first the esterifi-cation reaction was catalyzed by p-toluenesulphon-ic acid (PEG.PD) and in the second method,chlorosulphonic acid was used as a catalyst(PEG.CD) . Hydrolysis profiles show that theloading of drug in PEG.PD is higher than PEG.CD(Figures 4 and 5) . In PEG.CCD, the drug was atta-ched to the polymer end-groups via spacer groups.In this case PEG was converted to chloro- acetatederivative and reacted with diclofenac . The hydrol-ysis of polymer and released diclofenac from thepolymer (PEG .CCD) proceeded rapidly comparedto (PEG.PD) and (PEG .CD) . The introducing ofspacer group increased the degree of hydrolysis.

CONCLUSION

In this article we have represented a chemicalmodel for prodrug formation of sensitive drugswith carboxylic acid functional groups . Hydrolysisstudies have been carried out in vitro . The resultscan determine the effect of chemical modificationon solubility and hydrolysis behaviour . Informationobtained gives insight into in vivo behaviour.However, the development of such systems into adrug product will require a truly multidisciplinaryapproach .

REFERENCES

1. Hastings O .w., Polymer, 26, 1331, 1985.

2. Akashi M . and Takemoto K ., Adv . in Polym . Sri., 97,

1990.

3. Vogel D ., Pure AppL Chem., 51, 2409, 1979.

4. Moser P., J. Med. Chem, 93, 2358. 1990.

5. Sagara K, Chem. Pharnt BulL, 40, 12, 3303, 1992.

6. SaIeh 1 ., Pharma. Sri., 2(3), 242, 1992.

7. S .D Class., Eisenberg A., J. Polym Ser., Polym Phys., Ed.,

2.4, 2757, 1986.

8. Cher. Abs., 71 :3(1947x.

9. Merrifield R .8 . f. Am. Chem. Soc., 85, 2149, 1963.

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