Vitamin A Microencapsulation Within Poly(methyl methacrylate)-g-Polyethylenimine Microspheres: Localized Proton Buffering Effect on Vitamin A Stability Jong Suk Lee, Yoon Sung Nam, Byung-Young Kang, Sang-Hoon Han, Ih-Seop Chang Amore Pacific R&D Center, 314 –1, Bora-ri, Giheung-eup, Yongin-si, Gyounggi-do, 449 –729, South Korea Received 11 June 2003; accepted 5 September 2003 ABSTRACT: To stabilize vitamin A in a cosmetic/derma- tological formulation, we present here a new encapsulation method based on polymer microspheres having a localized “proton-buffering” capacity. Poly(methyl methacrylate)-g- polyethylenimine (PMMA-g-PEI) was prepared by direct condensation grafting of PEI onto poly(methyl methacry- late-co-methyl acrylic acid). The reaction was confirmed by FT-IR analysis showing the amide vibration at 1,550 cm -1 . Elemental analysis indicated that the weight content of the grafted PEI was 1.6% (w/w). Vitamin A was encapsulated into PMMA-g-PEI microspheres by using an oil-in-water (O/W) single emulsion method. The presence of PEI moiety dramatically improved the chemical stability of vitamin A in microspheres. Vitamin A encapsulated within PMMA-g-PEI microspheres maintained 91% of its initial activity after 30- day incubation at 40°C, while only maintaining 60% within plain PMMA microspheres. This study demonstrates that proton-buffering within hydrophobic polymer matrix is a useful strategy for stabilizing “acid-labile” active ingredi- ents. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 517–522, 2004 Key words: graft copolymers; microencapsulation; stabiliza- tion; functionalization of polymers; polyimines INTRODUCTION Vitamin A has been widely utilized as a popular active ingredient for antiaging dermatological treatments. It has been established that retinoids can act as impor- tant regulators in the proliferation and differentiation of cells. Topical application of retinoids is effective for stimulating collagen synthesis in the dermis and for treating skin diseases such as acne and psoriasis. 1–3 However, the use of vitamin A has been restricted because of high potency of skin irritation and intrinsic chemical instability. 4–6 Conventional cosmetic and pharmaceutical formu- lations usually have a large amount of water, which often mediates the degradation reaction of vitamin A. For that reason, recent research for stabilizing vitamin A has focused on isolating it from aqueous media by using various microencapsulation techniques, such as polymer microparticles, 7–10 liposomes, 6 solid-lipid nanoparticles (SLN), 11 etc. Although the chemical sta- bility can be partially improved by these physical encapsulation techniques, vitamin A still has an insta- bility problem in a water-based formulation for long- term storage. Therefore, a new technique is necessary to improve its chemical stability in commercial prod- ucts. The structural degradation of vitamin A can happen via various pathways in the presence of water, light, oxygen, high temperature, lipid peroxides, etc. Gener- ally, isomerization is induced by high temperature (all-trans form to 13-cis form) and ultraviolet light (all-trans form to 9-cis form), decomposition by oxygen and lipid peroxide, and dehydration by water. 5 In particular, dehydration is accelerated at low pH be- cause the terminal hydroxyl group of vitamin A is highly susceptible to proton-mediated hydrolysis. In our previous study, it was found that vitamin A mainly degrades to anhydro-vitamin A, an inactive form, via acid hydrolysis in a conventional emulsion cream. 12 Therefore, it is conceivable that vitamin A can be stabilized, if the accessability of proton molecules to vitamin A is minimized. In this contribution, we propose a new stabilization strategy for vitamin A, which is based on polymer microspheres having a locally alkaline microclimate. A similar concept was found in encapsulating thera- peutic proteins in biodegradable poly(d,l-lactic acid- co-glycolic acid) (PLGA) microspheres or implants, where acidification of the inner polymer environment is a critical issue for the protein instability prob- lem. 13–17 An acidic microclimate within PLGA matrix, which is suspected as one of main sources of protein inactivation, can be induced as polyester hydrolysis produces acidic degradation products. To overcome this problem, Schwendeman and colleagues coincor- porated an antacid, Mg(OH) 2 , into PLGA polymer, Correspondence to: Y. S. Nam (ysnam@amorepacific.com). Journal of Applied Polymer Science, Vol. 92, 517–522 (2004) © 2004 Wiley Periodicals, Inc.