Controlled Release Polymeric Formulations

15 Effect of Cross-Linking Agents on the Release of Sodium Pentobarbital from Nylon Microcapsules

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 19, 2018 | https://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0033.ch015

M. D.

DEGENNARO

College of Pharmacy and Allied Health Professions, Northeast Louisiana University, Monroe, La. 71201 Β. B. THOMPSON School of Pharmacy, University of Georgia, Athens, Ga. 30602 L. A. LUZZI School of Pharmacy, West Virginia University, Morgantown, W.Va. 26506

The process of microencapsulation i s currently one of the most extensively investigated means for the storage, p r o tection, taste-masking, and prolongation of release for many pharmaceuticals. This i s attested to by the rapid accumulation of scientific articles and patents in the literature i n this and other countries. A n extensive listing of pharmaceuticals which have been encapsulated has been published by Bakan and Sloan (1). Methods available for encapsulation are numerous and an overview of many of them may be found i n a review of the sub ject by Ranney (2). Herbig (3) has enumerated some of the many materials which may be used as encapsulating m e m branes or as an adjunct to the encapsulating agent. Microencapsulation i s not a new process. In preparing "carbonless carbon paper," Green and Schleicher (4,5) made use of the first patented application of microencapsulation. In that process, oil soluble dyes were encapsulated i n a gelatinacacia membrane by the use of complex coacervation (6). L u z z i and Gerraughty investigated some of the variables involved i n the coacervation of oils and solids and i n addition Nixon et al. (10,11) have extensively studied the preparation and release of gelatin coacervate microcapsules. It i s evident from the literature that most of the pub lished material on microencapsulation pertains mainly to appli cations of the coacervate system and that very little has been reported, especially using nylon as the encapsulating material for pharmaceuticals. Chang et a l . (JL2) prepared s e m i permeable collodion and nylon microcapsules containing en zymes which can be used to treat enzyme deficiencies and also reported the use of semipermeable microcapsules i n an extra corporeal shunt system (13, 14). Kondo et a l . (15-18) p r e pared microcapsules v i a interfacial polymerization using a 195 Paul and Harris; Controlled Release Polymeric Formulations ACS Symposium Series; American Chemical Society: Washington, DC, 1976.


196

CONTROLLED RELEASE POLYMERIC

FORMULATIONS

variety of encapsulating membranes. The same authors also reported some of the characteristics of the formed m i c r o c a p sules. The only reference to the microencapsulation of a phar maceutical agent, using a nylon membrane as the encapsulating material, has apparently been made by L u z z i et a l . (19). Due to the potential which exists for the encapsulation of pharma ceuticals v i a interfacial polymerization, this work was under taken to determine what effect the incorporation of c r o s s linking agents into the membrane might have upon the release of sodium pentobarbital from nylon 6-10 microcapsules. Experimental Materials. The materials used i n this study were sodium pentobarbital, 1 carbon tetrachlorides (reagent grade), 1,6hexanediamine^ (Eastman grade), sebacyl chloride^ (Eastman grade), diethylenetriamine^ (95%), triethylenetetramine^ (technical grade), and Glycerin U . S. P . ^ A l l materials were used as supplied by the manufacturers without further purifica tion. Preparation of Drug-Containing Microcapsules. The method utilized for the preparation of the nylon microcapsules containing sodium pentobarbital i n this experimentation was a modification of the methods previously reported by Chang et a l . (12) and by L u z z i et a l . (19). The modified method consisted of adding 100 m l . of an aqueous solution containing the drug to be encapsulated, 1, 6-hexanediamine and glycerin to 500 m l . of carbon tetrachloride contained i n a 2000 m l . beaker. The m i x ture was then stirred at a speed setting of 60 using a counterrotating stirrer** for 15 seconds to f o r m a w a t e r - i n - o i l emul sion. Then, 500 m l . of the same organic solvent containing sebacyl chloride was added and the stirring was continued at A b b o t t Laboratories, N . Chicago, 111. ο J . T . Baker Chemical C o . , Phillipsburg, N . J . Eastman Kodak C o . , Rochester, N . Y . 4 A l d r i c h Chemical C o . , Milwaukee, W i s . 5 F i s h e r Scientific C o . , Fairlawn, N . J . Model L2994, Brookfield Engineering Laboratories, Stoughton, M a s s .

Paul and Harris; Controlled Release Polymeric Formulations ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

15.

DEGENNARO E T AL.

Cross-Linking

Agents

197


the s t i r r e r ' s maximum speed for a total of 10 minutes. The microcapsules thus formed were collected by filtration. While still damp, the filtered microcapsules were passed through a standard 12 mesh sieve. The microcapsules were then placed in an oven at 3 7 ° for 10-12 hours and were then placed in a vacuum desiccator over phosphorous pentoxide for an additional 10-12 hours. This procedure insured removal of residual solvent and moisture. Effect of C r o s s - L i n k i n g Agents. The effect of the c r o s s linking agents used in this study, diethylenetriamine and t r i ethylenetetramine, on the release of core material was determined by progressively incorporating them into the aqueous phase in exchange for the 1, 6-hexanediamine. This was done on both active site and molar bases. When the active sites were the c r i t e r i a , a given quantity of 1, 6-hexanediamine was removed and replaced with two-thirds the number of moles of diethylenetriamine and one-half the number of moles of t r i ethylenetetramine. A s s a y Procedure. Ultraviolet spectra for sample solutions of sodium pentobarbital in 0. IN ammonium hydroxide were recorded using a P e r k i n - E l m e r recording spectrophotometer with 0. IN ammonium hydroxide solution as a blank. The wavelength of maximum absorbance was found at 240 nm, and a l l measurements were made at this wavelength while employing appropriate blanks. Samples of empty nylon microcapsules treated in the same manner failed to exhibit absorbance at 240 n m . Absorbances of the solutions were obtained using a Beckman D U spectrophotometer. These data were then used to prepare a B e e r ' s law plot which was used for comparison to determine the amount of sodium pentobarbital released from the microcapsules. 8

A s s a y Procedure for Total Sodium Pentobarbital Content of Microcapsules. Triplicate samples of approximately 100 mg. of the microcapsules were accurately weighed and placed in a 150 m l . homogenizing flask containing 50 m l . of 0. IN a m monium hydroxide solution. The samples were then completely 7

g

Model 202,

The P e r k i n - E l m e r C o r p . , Norwalk, Conn,

Model D U - 2 , Beckman Instrument Inc., Fullerton, Calif.


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CONTROLLED RELEASE POLYMERIC FORMULATIONS

Table I.

C r o s s - L i n k i n g Replacements Carried Out On A n Active Site B a s i s *


Percent Diamine Replaced

Moles of Diamine Used

0

0. 032

Moles of Diethyl- 0 enetriamine Used

12

9

6

0.031

0.03

0.029

0.0012

0.0019

0.0025

Moles of T r i e t h y l - 0 enetetramine Used

0.0015

* Diethylenetriamine and triethylenetriamine replacements were not carried out simultaneously.

15

18

24

60

100

0.028

0.027

0.025

0.013

0.0032

0

0.0032

0.0038

0.0051

0.013

0.02

0.022

0.0039

0.0097

0.015

0.016

90


15.

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Agents

199


ruptured using a Virtis blender^ at its maximum speed. In each case, two samples were blended for 10 minutes and c o m plete rupture was assured by blending the third for 15 minutes with no observed increase in drug content. Complete collection from the flask assembly was insured by washing with 50 m l . of 0. IN ammonium hydroxide solution. Aliquots were taken and diluted to an appropriate volume for spectrophotometric assay. In-Vitro Dissolution Studies. Dissolution was followed by examining triplicate samples containing approximately 30 m g . of drug using the flask method as previously described (20). In each case, microcapsules were placed on the surface of 300 m l . of 0. IN hydrochloric acid in a 500 m l . round bottom flask the temperature of which had been brought to equilibrium at 37 + 0. 5 ° . The mixture was stirred at 50 r . p . m . and samples were withdrawn at appropriate time intervals using a pipet fitted with a cotton plug. A constant volume of dissolution medium was maintained by the addition of an equal volume of medium after each 2 m l . sample withdrawal. In each case, the cotton plug which had been used as a filter, was added to the d i s solution mixture. Appropriate dilutions were made, ultraviolet absorbances recorded, and comparisons to a Beer's law plot were made. Results and Discussion P r e l i m i n a r y investigations showed that stable m i c r o capsules could be prepared using an aqueous phase consisting of 10% sodium pentobarbital, 4. 5% 1, 6-hexanediamine, and 1% glycerin. A f t e r the initial emulsification period, 500 m l . of carbon tetrachloride containing 3. 38% sebacyl chloride was added to produce capsule formation. The effect of the c r o s s linking agents was determined by replacing preselected portions of 1, 6-hexanediamine with the two cross-linking agents on both an active site and molar basis. Table I shows the quantities of cross-linking agents, diethylenetriamine and t r i ethylenetetramine, on an active site basis used to replace the indicated percentages of 1, 6-hexanediamine. Table II shows the replacements made using diethylenetriamine on a molar basis. The amount of sebacyl chloride used was held constant Model 45, V i r t i s Research Equipment, Gardner, N . Y .



0. 031 0.0019

Moles of Diamine Used

Moles of Diethylenetriamine Used

6

0.0029

0.03

9

0.0038

0.029

12

0.0058

0.027

0.028 0.0048

18

15

C r o s s - L i n k i n g Replacements C a r r i e d Out On a Molar Basis

Percent Diamine Replaced

Table II.


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201

Agents


in a i l cases. Figure 1 is a plot of percent sodium pentobarbital r e leased at selected times for the replacement of 1, 6-hexanedia mine with the indicated quantities of diethylenetriamine on an active site basis. This figure shows that when diethylenetria mine was used to replace the diamine, the percent sodium pentobarbital released at a l l sampling intervals showed an irregularly increased release with increasing amounts of d i ethylenetriamine.

6

12

18

24

60

90

100

PERCENT DIAMINE REPLACED WITH DIETHYLENETRIAMINE ACTIVE SITE BASIS Figure 1. Percent sodium pentobarbital released at selected time intervals vs. di amine replacements with diethylenetriamine on an active site basis. Each value rep resents the average of at least three determinations on a minimum of two, and in most cases three or four, batches of microcapsules prepared at different times. Ο 5 ™in; · 20 min; Π 40 min; M 120 min.



202


FORMULATIONS

It may be seen from Figure 2 that there was little significant change in the amount of sodium pentobarbital r e leased when between 6 and 24% of the diamine were replaced. However, when 60-100% replacements were carried out, the amount of drug released became significantly greater. Figure 3 indicates that when triethylenetetramine was used to replace the diamine on an active site basis, the trend was also an increased release with increasing amounts of cross-linking agent. The one exception was at the 9% replace ment level. This level of replacement yielded only slightly slower release than did that which had no cross-linking agent. F r o m Figure 4 it may be seen that when diethylenetria100 -, 90 80 70 J

w CO

s I 8

60 J 50 40

w P-i

30 20 J 10 0

—ι—ι

5 10

1

r-

20

40

80

120

TIME CMINUTES) Figure 2. Extremes of the two release ranges observed when diamine was replaced with diethylenetriamine on an active site basis. Each value represents the average of at least three determinations on a minimum of two, and in most cases three or four, batches of microcapsules prepared at different times. — higher limits: φ 90% replaced, Ο 60% replaced; lower limits: • 6% replaced, • 9% replaced; - · - 0% cross-linking.



15.

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Cross-Linking

203

Agents

mine was used to replace 1, 6-hexanediamine on a molar basis, there was a general trend towards faster release rates as the amount of c r o s s - l i n k i n g agent was increased. Here again, as when replacements were carried out on an active site basis, the increase in release was not always directly related to the percentage of diamine replaced by diethylenetriamine. Since nylon 6-10 is a linear polymer, it was desired to add cross-linking agents to the aqueous phase in order to pro duce a non-linear polymer, and it was thought that the addition of cross-linking agents would have a profound effect upon both the porosity and strength of the nylon membrane. However, it appears that the addition of cross-linking agents is not a suit-

20 H 10 -Γ

5 10

20

40

80

120

TIME (MINUTES) Figure 3. Percent sodium pentobarbital released when diamine was replaced with triethylenetetramine on an active site basis. Each value represents the average of at least three determinations on at least two, and in some cases three or four, batches of microcapsules prepared at different times. • 9%; A 24%; Ο 60%; Φ 90%; Φ 100%; - · - 0% cross-linking.


204


FORMULATIONS


100 ^

5 10

20

40

80

120

TIME (MINUTES) Figure 4. Percent sodium pentobarbital released when diamine was replaced with diethylenetriamine on molar basis. Each value represents the average of at least three determinations on at least two, and in most cases three or four, batches of microcapsules prepared at different times. • 6%; • 9%; Ο 12%; Δ 15%; Φ 18%; 0% cross-linking.

able means for prolonging drug release from nylon 6-10 m i c r o capsules. This may be due to any one of a combination of factors. Kondo (18) revealed that microcapsules prepared using different monomer combinations were of different sizes. In the present report, the addition of diethylenetriamine and t r i ethylenetetramine to the aqueous phase could possibly have caused changes in the release rates by bringing about changes in porosity or in membrane thickness. Changes in both of these factors may have been due to the different polymerization characteristics of each combination of monomers.



15.

DEGENNARO E T AL.

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Agents

205

The addition of cross-linking agents to the aqueous phase also produces other changes which are not as obvious. Small additions of cross-linking agents to the aqueous phase produce changes in reactant concentrations as well as in the ratio of reactants to each other, and since the diamine concentration would be reduced, the cross-linking agents also serve as d i luting agents. It has been reported (2^) that while reactant concentrations determine membrane thickness, there exists an optimum proportion of diamine to acid halide for each pair of reactants in a given system (16, 22). Each addition of c r o s s linking agent produces new ratios of reactants for which optimum concentrations must be determined. Changes in the ratio of reactants may produce an excess or may alter the availability of the amines in such a way that products with rapid release rates are produced. The most plausible rationale which seems to explain the more rapid release rates observed in each case where diamine was replaced by diethylenetriamine or triethyltenetetramine i n volves the individual diffusion rate of the amines and the effects of the presence of diethylenetriamine and triethylenetetramine on the diffusion rate of the diamine. Although an orientation argument would be plausible, it would appear that on a functional group basis, the diamine would be more freely soluble in nonaqueous systems than either of the cross-linking agents investigated (23). It follows that diffusion into the nonaqueous phase for a mixture of these amines would allow a gradation of transfer, thus limiting the concentration of amine available for "nylon" formation at, or near, an aqueous/nonaqueous interface. This rationale is strengthened if there is a competition for available water molecules among the mixture of amines, the drug, and the glycerin, a l l of which are found in the aqueous phase. The result may be that a lower molecular weight nylon is initially formed by the early migration of the diamine. This is followed by the formation of higher molecular weight polymer brought about by the subsequent diffusion of the diethylenetriamine or triethylenetetramine. The higher molecular weight polymer may be less compact due to several factors including a possible predisposition to a combination of vertical and tangential polymer backbones. It is evident that the effects produced by the addition of cross-linking agents show definite trends towards increased rates of release. However, these trends are produced on an irregular basis, regardless of whether the cross-linking agent


206

CONTROLLED RELEASE POLYMERIC FORMULATIONS

is added on an active site or molar basis. It is apparent that the addition of cross-linking agents alters several factors, one or more of which might account for the changes in release rates observed. In order to determine which of these factors is p r i m a r i l y responsible for the observed results, additional studies must be carried out.


Conclusions The objective of this study was to determine the effect that the addition of cross-linking agents to the aqueous phase during microcapsule formation via interfacial polymerization might have upon the release of core material from the m i c r o capsules. In this study, two cross-linking agents, diethylenetriamine and triethylenetetramine, were investigated. Substitution of the cross-linking agents was carried out on an active site as well as molar basis. The conclusions resulting from this investigation are: 1. The progressive addition of diethylenetriamine or triethylenetetramine to the aqueous phase as c r o s s linking agents, on an active site basis, produced microcapsules having increased rates of release. The increase observed followed no discernible pattern, 2. The addition of diethylenetriamine to the aqueous phase, on a molar basis, also produced microcapsules with increased release rates as more diethylenetriamine was added. Again, the increase followed no discernible pattern. 3. Progressive addition of cross-linking agents into the aqueous phase probably produced encapsulating shells with increased porosity.


15.

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Agents

207

Literature Cited 1. 2.


3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

Bakan, J . A. and Sloan, F . D., Drug and Cosmetic Ind., (1972), 110, p. 34. Ranney, M . W., "Microencapsulation Technology," Noyer Development C o r p . , Park Ridge, 1969. Herbig, J . Α., "Encyclopedia of Chemical Technology," 2nd e d . , v o l . 13, p. 441, Interscience, New York, 1967. Green, Β . K . and Schleicher, L., U . S . Pat. 2,730,456 (1956). Green, Β . K . and Schleicher, L., U . S . Pat. 2,730,457 (1956). Bungenberg de Jong, H . G., "Colloid S c i e n c e , " E d . by H . R. Kruyt, V o l . II, E l s e v i e r , New Y o r k , 1949. L u z z i , L . A. and Gerraughty, R. J., J. P h a r m . Sci., (1964), 53, 429. I b i d . , (1967), 56, 634. I b i d . , (1967), 56, 1174. Khalil, S.A.H., Nixon, J . R. and C a r l e s s , J . Ε., J. P h a r m . P h a r m a c o l . , (1968), 20, 215. Nixon, J . R. and Walker, S. Ε., Ibid., (1971), 23, 1475. Chang, T.M.S., MacIntosh, F . C . and Mason, S. G., Can. J . Physiol. P h a r m a c o l . , (1966), 44, 115. Chang, T.M.S., T r a n s . A m e r . Soc. A r t i f . Int. O r g a n s . , (1966), 12, 13. Chang, T.M.S., Pont, Α., Johnson, L . J., and Malave, Ν . , I b i d . , (1968), 14, 163. Suzuki, S . , Kondo, T. and Mason, S. G., Chem. P h a r m . Bull., (1968), 16, 1629. Koishi, Μ . , Fukuhara, N . and Kondo, T., I b i d . , (1969), 17, 804. Shigeri, Y . and Kondo, T., I b i d . , (1969), 17, 1073. Shigeri, Υ., K o i s h i , Μ., Kondo, T., Shiba, M . and Tomioka, S . , Can. J. C h e m . , (1970), 48, 2047. L u z z i , L . Α., Zoglio, Μ . Α., and Maulding, Η. V., J. P h a r m . S c i . , (1970), 59, 338. Underwood, T . W . and Cadwallader, D . E., J. P h a r m . Sci., (1972), 61, 239. Chang, T.M.S., Science, (1964), 146, 524. Morgan, P . W . and Kwokek, S. L., J. P o l y m . Sci., (1959), 40, 299. Weast, R. C., Ed., "Handbook of Chemistry and Phys ics," 51st e d . , Chemical Rubber C o . , Cleveland, Ohio, 1970.


Controlled Release Polymeric Formulations

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