pH-dependent sustained release characteristics of ...

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Journal of Biomaterials Science, Polymer Edition Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tbsp20

pH-dependent sustained release characteristics of disulfide polymers prepared by simple thermal polymerization a

Chul Ho Park & Jonghwi Lee

b

a

Jeju Global Research Center , Korea Institute of Energy Research , 200, Haemajihaean-ro, Gujwa-eup , Jeju Specific SelfGoverning Province , 695-971 , South Korea b

Department of Chemical Engineering and Materials Science , Chung-Ang University , 221, Heukseok-dong, Dongjak-gu, Seoul , 156-756 , South Korea Published online: 17 Jun 2013.

To cite this article: Journal of Biomaterials Science, Polymer Edition (2013): pH-dependent sustained release characteristics of disulfide polymers prepared by simple thermal polymerization, Journal of Biomaterials Science, Polymer Edition, DOI: 10.1080/09205063.2013.807458 To link to this article: http://dx.doi.org/10.1080/09205063.2013.807458

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Journal of Biomaterials Science, Polymer Edition, 2013 http://dx.doi.org/10.1080/09205063.2013.807458


pH-dependent sustained release characteristics of disulfide polymers prepared by simple thermal polymerization Chul Ho Parka and Jonghwi Leeb* a

Jeju Global Research Center, Korea Institute of Energy Research, 200, Haemajihaean-ro, Gujwa-eup, Jeju Specific Self-Governing Province 695-971, South Korea; bDepartment of Chemical Engineering and Materials Science Chung-Ang University, 221, Heukseok-dong, Dongjak-gu, Seoul 156-756, South Korea (Received 3 March 2013; accepted 14 May 2013) Biocompatible polymers have played an integral role in the advancement of drug delivery systems. The discovery of a novel polymer with innovative properties can provide great opportunities to enhance drug efficacy as well as reduce side effects. In this study, a novel disulfide polymer was synthesized and characterized. Its monomer is alpha-lipoic acid (ALA), which is synthesized in all cells in the human body. The disulfide polymer was obtained by the simple thermal polymerization of crystalline particles at a temperature higher than its melting point, followed by precipitation purification. It had rubbery and sticky characteristics. In vitro release tests demonstrated that the disulfide polymer had both pH-dependent degradation and related sustained release profiles, with a degraded form of ALA. Therefore, this novel class of responsive polymers that can be prepared by simple thermal polymerization has pronounced potential to contribute to future drug delivery systems. Keywords: lipoic acid; polymerization; disulfide polymer; sustainable release; pH-dependent degradation; hydrogel

Introduction The delivery of medicinal agents for disease healing purposes has been developed using various advanced materials and technologies.[1] Polymeric drug delivery systems offer some unambiguous advantages for localized and sustained drug delivery,[2] which is important to reduce various systemic side effects of toxic drugs. The chemical structures of the polymers as well as their physical formulations must be fine-tuned to achieve these desirable properties, meaning that the research of novel polymers with desired physicochemical properties such as biocompatibility, biodegradability, and controllablerelease is critical. Alpha-lipoic acid (ALA, 1,2-dithiolane-3-pentanoic acid) is a bio-synthesized material produced in most prokaryotic and eukaryotic cells. In human beings, it is present in all cells in the body as part of several multi-enzyme complexes for energy formation.[3] ALA is an essential element of a mitochondrial complex of proteins participating in glycine synthesis and degradation (the glycine cleavage system).[4] Recently, the pathway of its biosynthesis was unraveled in Escherichia coli.[5,6] In the last decade, *Corresponding author. Email: [email protected] Ó 2013 Taylor & Francis


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C.H. Park and J. Lee

many of the virtues of ALA have been studied, and there have been a substantial number of publications confirming the beneficial effects of ALA in the treatment of many diseases, including antioxidants, diabetes, atherosclerosis, degenerative processes in neurons, joint diseases, acquired immune deficiency syndrome, and obesity.[7,8] ALA contains a cyclic five-membered disulfide and a chiral center at the three position on the 1,2-ditholane. ALA is easy to reduce into dihydrolipoic acid (DHLA) by protonation due to its strong ring strain.[9] The generated DHLA induces a thiol– disulfide interchange reaction, so that the reduction of ALA reaches an equilibrium state. In addition, the high chemical potentials under certain conditions can produce disulfide polymers (where poly[3-(n-butane carboxylic acid) propyldisulfide] will be called a disulfide polymer in this paper).[10–16] Until recently, most researchers have thought that the formation of disulfide polymers in ALA formulations must be hindered. Often, simple tableting unit operation produced disulfide polymers, and they have been treated as undesirable byproducts to prevent polymerization, a composite tablet formula with various stabilizers has been considered in the pharmaceutical industry.[17] Chemical stabilizers might also be incorporated within the solution phases for injection.[18] To understand the chemical instability of pure ALA without any additives, our group has previously tried to study the chemical cleavage as well as polymerization under various experimental conditions.[16] However, this study demonstrates that the disulfide polymer (polymerized product) has unexpected and useful physical properties. It acts as a polymer matrix for a pH-dependent sustainable release at the first-order kinetic profile. The polymer also shows interesting pH-dependent degradation to return cyclic disulfide linkage of ALA monomer as a reversible inverse reaction. Experimental section Materials Yellowish ALA powder was purchased from Antibioticos S.P.A (Starada Rivoltana, Italy) and was used without any chemical purification. All organic solvents (tetrahydrofuran (THF), benzene, methanol, ethanol, acetone, cyclohexane, N,N-dimethylformaide, and dimethylsulfoxide) were purchased from Duksan Pure Chemical (Gyeonggi, South Korea). For the in vitro release test, KH2PO4, NaOH, and HCl were also purchased from Duksan Pure Chemical, and water was used after distillation. For the gel permeation chromatography (GPC) test, poly(methyl methacrylate) and THF (GPC grade) were purchased from Sigma–Aldrich (Missouri, USA). Polymerization The S–S bond in ALA may be homolytically cleaved by heat, especially at temperatures higher than its melting point of 63 °C. The crystalline ALA power was polymerized in a 20 mL vial in a 70 °C bath for 20 min or 1 h (immersing time, ±5 s) under nitrogen atmosphere. This condition was chosen based on our previous study.[16] The degree of polymerization was quantified by UV/vis spectroscopy (JASCO, V-500 series, Tokyo, Japan). This process is associated with the disappearance of UV absorption at 340 nm of a cyclic five-membered disulfide bond, indicating the destruction of the 1,2dithiolane ring.[16] Composites after polymerization, which contained crystalline particles of low molecular weight ALA in a disulfide polymer matrix, were dissolved in THF. A sticky, colorless, pure disulfide polymer was precipitated and aggregated within

Journal of Biomaterials Science, Polymer Edition

3


benzene. The obtained disulfide polymer was deposited to remove residual benzene/ THF in a vacuum oven at room temperature for 24 h. Characterization All samples were dissolved in various solvents to find the solubility parameter. The weight ratio of all of the samples to the solvent was 1:50. These solutions were stirred at room temperature for 48 h. The thermal phase changes of ALA as well as disulfide polymers/composites (10 mg) were measured by a differential scanning calorimeter (DSC, NF instrument, DSC2920, USA) at 5 °C/min heating/cooling rates from 50 to 80 °C. The holding time at each final temperature was 30 min. OriginLab 8.0 was used to calculate the enthalpy from the DSC data. The degree of crystallization was measured by X-ray scattering (Rigaku, D/MAX-2500/PC, Japan) and was calculated by OriginLab 8.0 software. The voltage and current were 40 kV and 100 mA, respectively. The range of detection was 5°–40°, and the scanning rate was 5°/min. The wavelength was 1.54 Å. A simulation program (Chem3D ultra 8.0, CambridgeSoft, USA) was used to calculate the atom-to-atom distance of the ALA molecules after running the molecular dynamics (MM2 to minimize energy). To check the molecular weight of a disulfide polymer, a GPC Chromatogram (eluent solvent: THF, column: PL gel Mixed-B, Agilent 1200 GPC/SEC series, USA) was used. The calibrant was poly(methyl methacrylate) (PMMA) in THF. Release test ALA (0.2 g) was polymerized at 70 °C for 20 and 60 min to produce ALA composites containing disulfide polymers of 20 and 60 wt.%, respectively. To set up equal conditions for release tests, all samples were formed into a pellet by a home-made compressor. Partially polymerized compounds (0.1 g) were pelletized at a pressure of 3 tons for 10 min, and the holder was put into liquid nitrogen for detachment. The diameter and the thickness of the pellets were 1 and 0.1 cm, respectively. For eliminating moisture, the pellets were deposited in a vacuum oven at room temperature for 24 h. The release tests were performed using the United States Pharmacopoeia paddle apparatus II method under a sink condition. The gastric fluid (no enzyme) was 0.1 N HCl, and the intestinal fluid consisted of KH2PO4 (8.805 g), NaOH (0.896 g), and distilled water (1 L). The volume of the dissolution medium was 500 mL at 37 ± 0.5 °C, and the paddle speed was 50 rpm. Specimens were withdrawn with a tolerance of ± 10 s. UV/vis spectroscopy quantified the amount of cumulative ALA released following the same method mentioned above. Results and discussion As shown in Figure 1, ALA, a widely used antioxidant and stabilizer for metal nanoparticles,[17–19] has a relatively high chemical potential and can be polymerized under certain conditions. To investigate the thermal instability and polymerization of ALA, DSC measurement was performed. Figure 2(a) and (b) shows the thermal properties of ALA during heating and cooling, respectively. The melting point of the ALA as received was 63 °C, which was measured during the first heating. After 30 min at 80 °C, liquid ALA was recrystallized during the first cooling (the exothermal transition).

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C.H. Park and J. Lee HO O

O

Polymerization


OH S

S

Disulfide polymer

Alpha-lipoic acid (ALA)

S

S

n

Disulfide polymer Poly[3-(n-butane carboxylic acid)propyldisulfide]

Thiol-disulfide Interchange reaction

Degradation (pH > 7)

O

OH SH

SH

Dihydrolipoic acid (DHLA)

Figure 1.

Schematic diagram of the chemical reactivity of ALA and the disulfide polymers.

During the second heating, one exothermic and one endothermic transition occur because the cooling rate might not be enough to rearrange the ALA molecules. The melting point of the recrystallized composite was 57 °C lower than the as-received ALA. The melting enthalpy also decreased with repeated heating (a 70% loss of crystallinity at the 2nd heating compared to the as-received ALA, followed by a 76% loss (3rd heating), 83% loss (4th heating), and 90% loss (5th heating)). This reduction in enthalpy resulted from decreasing crystal size as well as increasing disulfide polymer content. Although we did not study the crystallographic information of ALA in detail, the scattering vector of 7° of the diffraction peak corresponds to the longest endto-end distance (about 11 Å, see Figure 2(d)) calculated by the simulation program. The bilayer alignment of the ALA molecules may be attributed to a favorable interaction between the carboxylic acid groups, but this peak disappears after polymerization, as shown in Figure 2(c). Nevertheless, the three strongest diffraction peaks at around 22–26° still exist. Although the DSC data show a difference in the melting points before and after heating of the as-received ALA powder, the X-ray diffraction data indicated that the structural change in the short-range alignment is not remarkable. In addition, an amorphous polymer can be qualitatively elucidated by the existence of an amorphous halo using the X-ray diffraction pattern. The amorphous regions might result from the disulfide polymer or disordering of the ALA molecules. The calculated value of the amorphous region is around 58 ± 5% using a multi-peak fitting method from the OriginLab program. Also, the percentage of cyclic disulfide linkage (obtained by UV measurement) was about 40% (data not shown). Thus, both results show that the unreacted ALA is mostly crystalline and the disulfide polymer is amorphous.

Journal of Biomaterials Science, Polymer Edition st

nd

2 th

3

th

4 th

5 th

th

3

nd

-40

-20

0

20

4

40

(b) Heat flow (Endotherm)

Heat flow (Endotherm)

1

2

60

1 -40

-20

Temperature (oC)

0

st

20

40

60

Temperature (oC)

(c)

(d) Pure ALA

Intensity (a.u.)


(a)

5

H

~10.7Å 5

10

15

20

25

30

35

15

C O

40

S

10

O

20

25

S

30

Two theta

Figure 2. DSC data of the heating (a) and cooling (b) thermograms of ALA. Wide angle X-ray diffraction data of pure ALA (inset) and ALA/disulfide polymer composites after the first heating (c). 3D structure of ALA (d).

To check the physical properties of the pure disulfide polymer, the thermally reacted composites were dissolved in a good solvent, and the solution was then poured into a proper antisolvent. ALA has a wide solubility window, but thermally polymerized composites have a slightly narrower one. Through various tests, THF was selected as a relatively good organic solvent to dissolve ALA/disulfide polymer composites. The THF solution was dropped into various organic solvents, and benzene was selected to precipitate the disulfide polymers. Table 1 shows the solubility of ALA as well as the pure disulfide polymers obtained by an empirical precipitation method. From the empirical solubility experiments, the purified disulfide polymer was calculated to have a solubility parameter of around 20 MPa0.5. The polymer has a disulfide linkage in the main backbone and carboxylic acid on the side chains. The purified disulfide polymer had no crystal diffraction patterns on X-ray scattering, but only amorphous halo (data not shown). Also, the DSC data in Figure 3 show the existence of glass transition temperature. The data confirm that the disulfide polymer is a rubbery and amorphous polymer. In Figure 3, there are two glass transition temperatures (Tg) at around 10 and 43 °C. According to the Flory-Huggin theory, Tg is a function of molecular weight.[20] Our previous test found a bimodal molecular weight distribution (around 21,000 and 6000 g/ mol, measured by GPC),[16] which might correspond to the two Tg’s during the first heating.

6


Hilderbrand solubility parameterα

ALA

Disulfide polymer

ALA-40% composite

29.6 26.5 26.7 24.8 19.9 19.4 18.6 8.2

O O O O O O O X

X X O O X O X X

X X O O O O X X

Methanol Ethanol DMSO DMF Acetone THF Benzene Cyclohexane

Abbreviations: MeOH (methanol), EtOH (ethanol),qDMF (N,N-dimethylforamide), and DMSO (dimethyl sulfffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðd ¼ d2d þ d2p þ d2h, where, dd , dp , and dh are the energy from

oxide). a: Hilderbrand solubility parameter

dispersion forces, dipolar intermolecular force, and hydrogen bonds, respectivelyÞ. [26]

1.0

Heat flow (Endotherm)


Table 1. Solubilities of samples in various solvents. X is insoluble, and O is visually dissolved after 48 h.

o

2

nd

Tg : 10 C 1

0.5

st

Heating

0.0 -0.5 Cooling

-1.0 -20

0

20

40

60

Temperature (oC) Figure 3.

DSC thermograms of pure disulfide polymers.

Disulfide bonding exists extensively in proteins, hairs, and feathers in nature, as well as in cross-linkers in rubber industries. Many specialized chemical reactions have been studied for disulfides mainly associated with the cleavage of the S–S bond.[21] This bond is usually the weakest covalent bond in a molecule. The disulfide polymer might be thermally weak even though the linear disulfide linkage might be stronger than the cyclic chemical structure due to the lack of ring strain. This thermal degradation was proved by DSC. After the first heating, the sample was kept at 80 °C for 30 min. The higher Tg then disappeared during the second heating (Figure 3). Atsushi et al. characterized disulfide polymers using various analysis tools.[22] They demonstrated that the disulfide polymers could have two forms. Without thiol groups, disulfide polymers could have cyclic structures; but the polymers obtained in the presence of thiol compounds had linear structures. Our disulfide polymer might be in a cyclic conformation. Our previous GPC data exhibited that the molecular weight distribution of the disulfide polymer is bimodal.[16] Atsushi et al. showed a different unimodal distribution, possibly because they used different synthesis and precipitation

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conditions. Furthermore, ALA possesses two enantiomers (R-/S-lipoic acid) so that the disulfide polymers could have several different secondary chain structures. Thus, the differences in molecular weight distribution and chain configuration could result in the deviation of glass transition temperature (10 °C in [22]). Figure 1 shows that cleavage of the disulfide polymer could induce ALA recovery as a reversible inverse reaction. The pH environment influences the chemical change of the disulfide polymer. Carboxylic acid is widely used as a triggering moiety for pH-sensitive functions, and the logarithmic value of its acid dissociation constants might exist in weak acid regions. We prepared three pH-solutions (pH 4, 7, and 9). In the direct assessment of the degradation of the disulfide polymer under the different pH conditions, there was no difference in the degradation curves of the pH 4 and 7 solutions (Figure 4). However, when lumps of disulfide polymers were immersed in pH 9 solutions, rapid swelling occurred. The carboxylic acid of the side chains should be deprotonated. More interestingly, the size decreased with time, eventually disappearing. Following the reverse chemical reaction, the degraded samples returned to the cyclic disulfide linkage, which was confirmed by the existence of a unique UV absorption peak using a UV/vis spectrometer (data not shown).[16] This demonstrates that the disulfide polymer can be used as a pH-dependent degradable polymer. To further assess the pH-dependent characteristics of ALA release from disulfide polymers, the composites prepared by thermal polymerization were tested in simulated gastric/intestinal media. All samples were pelletized to eliminate the surface area effect on the release kinetics. In the gastric medium, all samples had unique first-order release kinetics, as described in Figure 5(a). As discussed before, protons in acid solutions can break down the five-membered disulfide linkage based on the thiol–disulfide interchange reaction. Thus, a part of the dissolved ALA might be in DHLA form in acidic conditions. The release rate of ALA decreased with an increase in the polymer concentration. A 20% increase in polymer content caused a threefold decrease in drug release. Furthermore, there was no remarkable ALA release in the 60% disulfide polymers. It might be good to keep in mind that, in Figure 5, oligomers could exist as intermediates between the original disulfide polymers and ALA.

100

ALA amount converted (%)



80 60 40

pH 4 pH 7 pH 9

20 0 0

5

10

Time (day) Figure 4.

pH-dependent degradation and recovery into ALA in various pH solutions.

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Cumulative ALA released (%)

ALA as received 20% disulfide polymer 60% disulfide polymer

80

60

40

20

0 0

5

10

15

20

Time (h)

(b) Cumulative ALA released (%)


(a) 100

100 80 60 40 20

ALA 20% disulfide polymer 60% disulfide polymer

0 0

5

10

15

20

Time (h) Figure 5.

Cumulative ALA released in simulated gastric (a) and intestinal fluids (b).

As mentioned before, disulfide polymers have pH-dependent swelling characteristics. Figure 5(b) shows that the release kinetic profiles in intestinal fluid without enzymes are much faster than in the gastric solutions. The pure ALA pellets perfectly dissolved without any chemical changes after 2 h. The release rates of ALA decreased with an increase in disulfide polymer content. Although there was a slight burst release during the initial 2 h, from then on they followed first-order release kinetics. The gastric release kinetics shows that disulfide polymers have enteric coating applicabilities to protect acid-sensitive substances/proteins and to deliver drugs only to intestinal targets. In addition, various targeting moieties can be covalently immobilized through conjugation with the functional groups of the carboxylic acid for colon targeting. With the development of intelligent drug delivery systems, multi-functional formulas, such as pH-dependent sustained release, are desired to control and maintain antioxidant efficacy for long time periods.[23] A few previous studies have suggested the possibility of using the polymerized form of ALA for enhanced functionalization of liposomes as a drug carrier.[24,25] This study shows that disulfide polymers could open



9

novel possibilities via simple thermal polymerization at relatively low processing temperatures. Oral solid dosage forms, currently the most important drug delivery form in the pharmaceutical market, could achieve the pH-dependent sustained release characteristics by the incorporation of ALA with other active ingredients and its subsequent thermal polymerization. Often, the thermal polymerization condition employed in here can be achieved in the tableting conditions of stable chemical drugs. With protein or peptide drugs, the disulfide polymers might probably be prepared prior to mixing, due to the thermal stability problems of drugs. Conclusions The applicability of disulfide polymers prepared from ALA via thermal polymerization to be used for sustainable release as well as pH-dependent degradation was studied. The polymerization was achieved by a simple thermal polymerization method at a temperature higher than the melting point of ALA crystals. The amorphous disulfide polymer was purified by a precipitation method. In pH 4 and 7 mediums, the disulfide polymer did not degrade, but did quite rapidly to a five-membered disulfide in pH 9 mediums. The release kinetics of ALA depends on the content of the disulfide polymer and the pH. Disulfide polymers could be potential candidates for pH-dependent sustainable release polymers as well as enteric coating materials. Acknowledgment This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean Government (MEST) (2012-014107), the R&D Program funded by the Ministry of Knowledge Economy (#10035574), Republic of Korea, and the Korea Healthcare Technology R&D Project, Ministry of Health & Welfare (Grant No: A103017).

References [1] Zhang Y, Chan HF, Leong KW. Advanced materials and processing for drug delivery: the past and the future. Adv. Drug Deliver Rev. 2013;65:104–120. [2] Allen TM, Cullis PR. Drug delivery systems: entering the mainstream. Science. 2004;303:1818–1822. [3] Fujiwara K, Okamura-Ikeda K, Motokawa Y. Assay for protein lipoylation reaction. Method Enzymol. 1995;251:340–347. [4] Hermann R, Niebch G, Borbe HO, Fieger-Büschges H, Ruus P, Nowak H, RiethmüllerWinzend H, Peukertd M, Blume H. Enantioselective pharmacokinetics and bioavailability of different racemic α-lipoic acid formulations in healthy volunteers. Eur. J. Pharm. Sci. 1996;4:167–174. [5] Cicchillo RM, Booker SJ. Mechanistic investigations of lipoic acid biosynthesis in Escherichia coli: both sulfur atoms in lipoic acid are contributed by the same lipoyl synthase polypeptide. J. Am. Chem. Soc. 2005;127:2860–2861. [6] Booker SJ. Unraveling the pathway of lipoic acid biosynthesis. Chem. Biol. 2004;11:10–12. [7] Kim M-S, Park J-Y, Namkoong C, Jang P-G, Ryu J-W, Song H-S, Yun JY, Namgoong IS, Ha J, Park IS, Lee IK, Viollet B, Youn JH, Lee HK, Lee KU. Anti-obesity effects of [alpha]-lipoic acid mediated by suppression of hypothalamic AMP-activated protein kinase. Nat. Med. 2004;10:727–733. [8] Bilska A, Włodek L. Lipoic acid – the drug of the future? Pharmacol. Rep. 2005;57:570–577. [9] Bachrach SM, Woody JT, Mulhearn DC. Effect of ring strain on the thiolatedisulfide exchange: a computational study. J. Org. Chem. 2002;67:8983–8990.


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[10] Burns JA, Whitesides GM. Predicting the stability of cyclic disulfides by molecular modeling: effective concentrations in thiol–disulfide interchange and the design of strongly reducing dithiols. J. Am. Chem. Soc. 1990;112:6296–6303. [11] Singh R, Whitesides GM. Degenerate intermolecular thiolate–disulfide interchange involving cyclic five-membered disulfides is faster by apprx.103 than that involving six- or sevenmembered disulfides. J. Am. Chem. Soc. 1990;112:6304–6309. [12] Houk J, Whitesides GM. Structure-reactivity relations for thiol–disulfide interchange. J. Am. Chem. Soc. 1987;109:6825–6836. [13] Thomas RC, Reed LJ. Disulfide polymers of DL-α-lipoic acid. J. Am. Chem. Soc. 1956;78:6148–6149. [14] Houk J, Whitesides GM. Characterization and stability of cyclic disulfides and cyclic dimeric bis(disulfides). Tetrahedron. 1989;45:91–102. [15] Nambu Y, Acar MH, Suzuki T, Endo T. Thermal and photoinitiated copolymerization of the cyclic disulfide lipoamide with styrene. Die Makromol. Chem. 1988;189:495–500. [16] Park CH, Kim AR, Yun HL, Lee J. Ring opening and polymerization of alpha-lipoic acid. Polymer. 2006;30:357–361. [17] Evans JL, Heymann CJ, Goldfine ID, Gavin LA. Pharmacokinetics, tolerability, and fructosamine-lowering effect of a novel, controlled-release formulation of alpha-lipoic acid. Endocrine Pract. Official J. Am. College Endocrinol. Am. Assoc. Clin. Endocrinol. 2002;8:29–35. [18] Huang EA, Gitelman SE. The effect of oral alpha-lipoic acid on oxidative stress in adolescents with type 1 diabetes mellitus. Pediatr. Diabetes. 2008;9:69–73. [19] Abad JM, Mertens SFL, Pita M, Fernández VM, Schiffrin DJ. Functionalization of thioctic acid-capped gold nanoparticles for specific immobilization of histidine-tagged proteins. J. Am. Chem. Soc. 2005;127:5689–5694. [20] Park CH, Kim JH, Ree M, Sohn B-H, Jung JC, Zin W-C. Thickness and composition dependence of the glass transition temperature in thin random copolymer films. Polymer. 2004;45:4507–4513. [21] Witt D. Recent developments in disulfide bond formation. Synthesis. 2008;2008:2491–2509. [22] Kisanuki A, Kimpara Y, Oikado Y, Kado N, Matsumoto M, Endo K. Ring-opening polymerization of lipoic acid and characterization of the polymer. J. Polym. Sci., Part A: Polym. Chem. 2010;48:5247–5253. [23] Bernkop-Schnürch A, Reich-Rohrwig E, Marschütz M, Schuhbauer H, Kratzel M. Development of a sustained release dosage form for α-lipoic acid. II. Evaluation in human volunteers. Drug Dev. Ind. Pharm. 2004;30:35–42. [24] Stefely J, Markowitz MA, Regen SL. Permeability characteristics of lipid bilayers from lipoic acid-derived phosphatidylcholines: comparison of monomeric, crosslinked and noncrosslinked polymerized membranes. J. Am. Chem. Soc. 1988;110:7463–7469. [25] Pax H, Blume A. Polymerizable phospholipids with lipoic acid as head group: synthesis and phase properties. Chem. Phys. Lipids. 1993;66:63–74. [26] Hansen CM. Hansen solubility parameters: a user’s handbook. Baco Raton (FL): CRC Press Taylor & Francis Group; 2007.