Application of pH- and Temperature-Sensitive Polymers for Controlled

Chapter 23 Application of pH- and Temperature-Sensitive Polymers for Controlled Drug Release Device 1

1

Soon HongYuk1,Jung Ki Seo , Jin Ho Lee , and Sun Hang Cho

2

1Department of Polymer Science and Engineering, Han Nam University, 133 Ojeong Dong, Daedeog Ku, Taejeon 300-791, Korea Advanced Materials Division, Korea Research Institute of Chemical Technology, P.O. Box 107, Yusung, Taejeon 305-600, Korea

Downloaded by UNIV OF ARIZONA on October 2, 2015 | http://pubs.acs.org Publication Date: May 15, 2000 | doi: 10.1021/bk-2000-0752.ch023

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A new pH/temperature-responsive polymer system with transitions resulting both from polymer-water and polymer-polymer interactions has been demonstrated using the copolymer composed of Ν,Ν'-dimethylaminoethyl methacrylate (DMAEMA) and ethyl acrylamide (EAAm) and the mixture of poly DMAEMA and poly EAAm. Based on the pH/temperature responsiveness of the copolymer and polymer mixture, glucose-controlled insulin delivery system and microsphere for temperature-sensitive solute release have been designed and characterized.

It has been recognized that the constant release is not the only way to maximum drug effect and minimum side effects and the assumption used for constant release rate sometimes fails its validity for physiological conditions. To overcome this difficulty, externally modulated or self-regulating drug delivery systems have been used as novel approaches to delivering drug as required. To achieve this drug delivery system, the phase transition polymers have been intensively exploited as a candidate materials. With the understanding of the phase transition in polymers, numerous polymer systems that show a phase transition in response to external stimuli such as temperature, pH (12), ionic strength (3), and electric potential (4-6) have been reported. For its remarkable properties, poly N-isopropylaerylamide has been studied widely for temperature-modulated device (7-14). Recently polymer systems that demonstrate the phase transition to more than one variable, in particular pH and temperature, have been studied intensively for realizing the more sophisticated device responding to external stimuli (15-16).

232

© 2000 American Chemical Society

In Controlled Drug Delivery; Park, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

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In this study, a new pH/temperature-responsive polymer system with transitions resulting both from polymer-water and polymer-polymer interactions has been demonstrated using the copolymer composed of Ν,Ν'-dimethylaminoethyl methacrylate (DMAEMA) and ethyl acrylamide (EAAm) and the mixture of poly DMAEMA and poly EAAm. Based on the pH/temperature responsiveness of copolymer and polymer mixture, glucose-controlled insulin delivery system and microsphere for temperature-sensitive solute release have been designed and characterized. Experimental Materials. DMAEMA monomer, ammonium persulfate (APS), and tetramethylethylene diamine (TEMED) were purchased from Aldrich. Bovine insulin, N,Nazobis(isobutyronitrile) (AIBN) and glucose oxidase (GOD) were purchased from Sigma Chemical Co. DMAEMA monomer was distilled before use. Other reagents were used as received. Synthesis. EAAm was synthesized in our laboratory as described previously (17). Poly(DMAEMA-co-EAAm) was prepared by free radical polymerization as follows: 7.8 g of distilled monomers (mixtures of DMAEMA and EAAm) and 0.02 g of AIBN as an initiator were dissolved in 100 mL of water/ethanol binary solvent (5/5 by volume). The ampoule containing the solution was sealed by conventional methods and immersed in a water bath held at 75 °C for 15 h. After polymerization, all polymers were dialyzed against distilled-deionized water at 4 °C and freeze-dried. PolyDMAEMA and polyEAAm were prepared using the preparation method of copolymer for the preparation of pH/temperature responsive polymer complex. To observe the effect of preparation method on polyDMAEMA, polyDMAEMA was prepared by free radical polymerization in water at room temperature using APS as initiator and TEMED as accelerator. Transmittance Measurements. The phase transition was traced by monitoring the transmittance of a 500 nm light beam on a Spectronic 20 spectrophotometer (Baush & Lomb). The concentration of the aqueous polymer solution was 5 wt%, and the temperature was raised from 15 to 70 °C in 2-deg increments every 10 min. To observe their pH/temperature dependence, the phase transitions of polymers in citric-phosphate buffer solution versus temperature at two pH values (4.0 and 7.4) were measured. Preparation ofInsulin-Loaded Matrix. Lyophilized copolymer was ground down to colloidal dimensions (< 1 μιη) using a laboratory planetary mill (Pulverisette, Fritsch GmbH). A 110 mg sample of copolymer powder, 20 mg of bovine insulin, and 20 mg of GOD were mixed, and the mixture was compressed into a disk-shaped matrix of 5-mm thickness and 15-mm diameter.


234

Measurement of Weight Loss of Insulin-Loaded Matrix in Response to Glucose. After immersion in phosphate buffer solution (PBS) for desired time at 37 °C, the insulin-loaded matrix was removed and dried in a vacuum oven at room temperature. The percent of weight of the matrix was determined as a function of time. Release Experiment for Insulin. Insulin release from the insulin-loaded matrix was measured in response to alternating changes of glucose concentration when 150 mg of an insulin-loaded matrix was introduced to 100 mL of phosphate buffer solution (PBS) at 37 °C. The amount of released insulin was measured by taking 1 mL of the release medium at a specific time and immersing the matrix in a fresh medium. Insulin was determined by reverse-phase HPLC, using a Resolvex C i (Fisher Scientific) and 0.01 Ν HCl/acetonitrile (80/20-50/50, v/v%) mobile phase over 30 min at a flow rate of 1 mL/min. The eiuate was monitored by optical absorption at 210 nm.


8

Preparation of Microsphere Using Temperature-Sensitive Polymers. Using 5 ml syringe (26 gauge needle), polymer solution mixture composed of polyDMAEMA, poly EAAm and model drug was transferred dropwise to the water as shown in Figure 1. The temperature of water should be higher than the LCST of polymer for the coagulation of polymer and hydrocortisone was used as a model drug. The microsphere was dried in vacuum oven at 40 °C and the diameter of microsphere was approximately 2 mm. The loading amount is approximately 20 wt %. Release Experiment for Hydrocortisone. The release of hydrocortisone from the microsphere was measured in response to pulsatile changes of temperature. 500 mg of microspheres were introduced into 200 ml of release medium (pH 7.4 PBS) at desired temperature. The amount of released hydrocortisone was measured by taking 1 mL of the release medium at a specific time, replacing the total release medium with fresh PBS to maintain sink conditions and assaying the drug concentration at 248 nm using a UV spectrophotometer (Shimadzu, Japan). Results and Discussions pH/Temperature-Induced Phase Transition. The temperature-induced behavior of poly(DMAEMA-co-EAAm) at two pHs (4.0 and 7.4) is observed as shown in Figure 2. At pH 7.4, the LCST of poly(DMAEMAco-EAAm) was shifted to a lower temperature with the increase of EAAm content. In general, the LCST should increase with increasing hydrophilicity of the polymer (16). However, a LCST shift to a lower temperature was observed with the incorporation of the hydrophilic EAAm. This is due to the formation of hydrogen bonds, which protect (N,N-dimethylamino)ethyl groups from exposure to water and result in a hydrophobic contribution to the LCST (18-19) At pH 4, no LCST was observed with polyDMAEMA and the LCST of all copolymers was increased compared to that at pH 7.4. At pH 4.0, (N,N-dimethylamino)ethyl groups of DMAEMA are fully ionized. An increasing electrostatic repulsion between charged sites on DMAEMA disrupts the hydrogen bonds between EAAm and DMAEMA. These interfere with the


235


PolyDMAEMA/ PolyEAAm/ Hydrocortisone Solution Mixture

Water

ο ο ο Ό Ο ο Ο Microsphere Figure 1. Preparation method of microsphere.



236 hydrophobic interactions between (N,N- dimethylamino)ethyl groups above the LCST and the hydrophobic contribution to the LCST due to hydrogen bonding. To prepare temperature-sensitive polymer complex, the mixtures of poly EAAm and polyDMAEMA in appropriate ratio with that of copolymer were prepared and their temperature-sensitive behaviors were observed. As presented previously, poly DMAEMA can be prepared in the water containing APS as an initiator and TEMED as an accelerator or water/ethanol solvent mixture containing AIBN as an initiator. Poly DMAEMA prepared in the water showed the LCST at 50 °C and that prepared in the water/ethanol mixture showed the LCST at 30 °C. The LCST of polyDMAEMA prepared in water was shifted to the lower temperature with the addition of polyEAAm, which was observed in the aqueous copolymer solution as shown Figure 2 (pH 7.4) and polymer aqueous solution became milky without aggregation above LCST. However, the polymer aggregate was formed with the addition of polyEAAm into the aqueous solution of polyDMAEMA prepared in the water/ethanol mixture above LCST. These indicate that there exists a difference in the stereochemical configuration of polyDMAEMA depending on the preparation method. To understand this behavior of polyDMAEMA, the tacticity of poly DMAEMA was investigated. Table 1 show the ct-CH portion of the C-NMR spectra of polyDMAEMA prepared at two different conditions. It shows that poly DMAEMA prepared in the water does not contain syndiotactic triads, however, the polymer prepared in water/ethanol mixture contains syndiotactic triads (13.6 ppm). According to the literature (20), the isotactic chain has no intramolecular interaction between side chains. On the other hand, every sequence in the syndiotactic chains brings two pendant groups into close proximity where they can form an intramolecular interaction which stabilizes the local chain conformation. This leads to the efficient formation of hydrogen bond between polyEAAm and polyDMAEMA prepared in water/ethanol mixture resulting in the formation of aggregation above LCST. The mechanism on this phenomenon is under investigation. 13

3

Glucose-Sensitive Insulin Release. Figure 3 shows the rationale of glucose-controlled insulin delivery system schematically. In the presence of glucose, gluconic acid generated by the glucose-GOD reaction protonates dimethylamino groups of poly(DMAEMA-co-EAAm), inducing the LCST shift to higher temperature from the surface of the insulin-loaded matrix. This leads to the disintegration of the matrix with polymer dissociation from the surface with the insulin release. Based on this concept, insulin-loaded matrix was designed and characterized. Poly(DMAEMA-co-EAAm) with 50 mol % of EAAm was selected as a model polymer for the preparation of an insulin-loaded matrix considering its pH/temperature responsiveness. Figure 4 shows the weight loss of the matrix in PBS at two different glucose concentrations. We found that 100 % of initial weight had been lost at 5 g/L of glucose concentration during 24 hours, whereas 10 % of initial weight had been lost at 0.5 g/L of glucose concentration. These results indicate that this polymer system responds to the change of glucose concentration in the presence of GOD.


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Table 1. Chemical shift for various (X-CH3 ôa-CH3 (ppm)

Area

Polymer


i Poly D M A E M A prepared in water/ethanol solvent mixture Poly D M A E M A prepared in water

h

s

18.36 16.49 13.63

18.32 16.45

-

i

h

s

3.28 5.49 2.95

3.34 5.42

-

i : isotactic triads, h: atactic triads, s: syndiotactic triads

Generation of Gluconic Acid L o c a l p H Decrease with L C S T Increase Disintegration of M a t r i x at L o c a l A r e a

Figure 3. Schematic representation of the glucose-controlled insulin release using poly(DMAEMA-co-EAAm).


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Figure 5 shows the alternating insulin release rate in response to an alternating exposure of the insulin-loaded matrix to high and low aqueous glucose solutions. Minimal release was observed at the lower concentration of glucose. The large deviation in the release rate was attributed to inhomogeneous mixing of the components in the insulin-loaded matrix causing its irregular dissolution. Temperature-Sensitive Release of Hydrocortisone from Microsphere. Firstly, poly(DMAEMA-co-EAAm) was used for the preparation of microsphere based on the preparation method presented previously. However, the formation of microsphere was not observed. The formation of micosphere was observed with the mixture of polyDMAEMA and polyEAAm. To understand this phenomenon more in detail, temperature-induced phase transition of polymer was observed in the various forms of polymer networks. As present previously, the LCST of linear (noncrosslinked) poly(DMAEMA-co-EAAm) was shifted to a lower temperature with the increase of EAAm content. This is due to the formation of hydrogen bonds, which protect (N,N-dimethylamino)ethyl groups from exposure to water and result in a hydrophobic contribution to the LCST. With the formation of crosslinked network, the polymer gel exhibited the temperatureinduced phase transition which was quite different from that of polymer aqueous solution. The transition temperature of poly(DMAEMA-co-EAAm) gel between the shrunken and swollen was shifted to the higher temperature with the increase of EAAm content, which was contrary to the LCST change of poly(DMAEMA-co-EAAm) aqueous solution as shown in Figure 6. This indicates that polymer-polymer interaction via hydrogen bond contributed hydrophobically to the LCST of poly(DMAEMA-coEAAm) aqueous solution but it contributed hydrophilically to the temperatureresponsive swelling behavior of poly(DMAEMA-co-EAAm) gel (21). With the formation of gel network, the degree of freedom of polymer chain is significantly decreased and this hinders the formation of the hydrogen bond between DMAEMA and EAAm. From this result, the temperature-induced phase transition of poly(DMAEMA-co-EAAm) is highly dependent on the change of hydrogen bond due to the structural difference. From this perspective, we can suggest that there exists the difference in the temperature-induced phase transition between poly(DMAEMA-coEAAm) and the mixture of polyDMAEMA and polyEAAm. Poly(DMAEMA-coEAAm) exhibit less drastic phase transition in response to temperature changes compared to the mixture of polyDMAEMA and polyEAAm. This arises from cooperative polymer-polymer interactions (zipper-like effect) in the mixture of polyDMAEMA and polyEAAm as shown schematically in Figure 7. Based on this characteristic of polymer complex, we can expect that the formation of microsphere using temperature-induced phase transition is observed with the polymer complex. When we applied a step function of temperature to the microsphere, hydrocortisone was released in a stepwise manner. Figure 8 exhibits high release rates at lower temperature due to the dissociation of temperature-sensitive intermolecular interaction in the polymer complex. This gave hydrocortisone release control in an onoff manner without considerable lag time and it was possible to switch hydrocortisone on-off with one degree temperature fluctuation.



239

Time (hours)

Figure 4, Disintegration of the polymer matrix in phosphate buffer solution at two glucose concentrations.

1200

Time (hours)

Figure 5. Insulin release from the insulin-loaded matrix in response to alternating change of glucose concentration.


240


n=3

0

20

40

60

80

100

TemperaturefC)

Figure 6. Equilibrium swelling change ofpoly(DMAEMA-co-EAAm) gel in respon ίο temperature change.

Poly(DMAEMA-co-EAAm)

Polymer complex

Figure 7. Schematic description of intra/intermolecular interaction in copolymer and polymer mixture.



241

50

Time (minutes)

35 °c

I

1

ι

1

Figure 8. Release of hydrocortisone from the micrpsphere in response to pulsatile temperature change.


242 Conclusions


pH/temperature-sensitive polymer systems with functions resulting from both polymer-water and polymer-polymer interactions was demonstrated and a transition mechanism was proposed. The formation of hydrogen bond in the polymer system played an important role in determining the pH/temperature-induced phase transition. Using these polymer systems, the delivery systems for glucose-controlled insulin release and temperature-controlled release of hydrocortisone were designed and characterized. These polymer systems have a variety of potential applications in the area of drug delivery. References 1. Siegel, R. Α.; Firestone, B. A. Macromolecules, 1988, 21, 3254. 2. Yuk, S. H.; Cho, S. H.; Lee, Η. B. J. Controlled Release, 1995, 37, 69. 3. Ricka, J.; Tanaka, T. Macromolecules, 1984, 17, 2916. 4. Tanaka, T.; Nishio, I., Sun, S. T.; Nishio, S. Science, 1982, 218, 467. 5. Kwon, I. C.; Bae, Y. H.; Kim, S. W. Nature, 1991, 354, 291. 6. Choi, O. S.; Yuk, S. H,; Jhon, M . S. J. Appl. Polym. Sci., 1994, 51, 375. 7. Heskins, M.; Guillet, J. E. J. Macromol. Sci.-Chem., 1986, A2(8), 1441. 8. Hoffman, A. S.; Afrassiabi, Α.; Dong, L. C. J. Controlled Release, 1986, 4, 213. 9. Bae, Y. H.; Okano, T.; Kim, S. W. Makromol. Chem., Rapid Commun., 1987, 8,481. 10. Bae, Y. H.; Okano, T.; Kim, S. W. Makromol. Chem., Rapid Commun., 1988,9, 185. 11. Stayton, P. S.; Shimoboji, T.; Long, C.; Chilkoti, Α.; Chen, G. H.; Harris, J. M.; Hoffman, A. S. Nature, 1995, 378, 472. 12. Ilman, F.; Tanaka, T.; Kokufuta, E. Nature, 1991, 349, 400. 13. Kim, S. W.; Bae, Y. H. Smart drug delivery system, in: Sam, A. P.; Fokkens, J. G. (Eds), innovation in drug delivery. Impact on Pharmacotherapy, The Anselmus Foundation, Houten, The Netherland, 1995,pp112. 14. Aoki, T.; Kawashima, M.; Katono, H.; Sanui, K.; Ogata, N.; Okano, T.; Sakurai, Y. Macromoecules, 1994, 27, 947. 15. Chen, G. H.; Hoffman, A. S. Nature, 1995, 373, 49. 16. Feil, H; Bae, Y. H.; Feijen, J.; Kim S. W. Macromolecules, 1993, 26, 1259. 17. McCormick, C. L.; Nonaka, T.; Johnson, C. B. Polymer 1988, 29, 371. 18. Cho, S. H.; Jhon, M . S.; Yuk, S. H.; Lee, H. B. J. Polym. Sci. B: Polym. Phys. 1997, 35, 595. 19. Yuk, S. H.; Cho, S. H.; Lee, S. H., Macromolecules, 1997, 30, 6856. 20. Russell, G. Α.; Hiltner, P. Α.; Gregonis, D. E.; deVisser, A. C.; Andrade, J. D., Polym. Sci.,: Polym. Phys Ed., 1980, 18, 1271. 21. Cho, S. H.; Jhon, M . S.; Yuk, S. H. Eur. Polym J. in press.


Application of pH- and Temperature-Sensitive Polymers for Controlled

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