Synthetic Methods of Disulfide Bonds Applied in Drug Delivery Systems

Current Organic Chemistry

1477

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Current Organic Chemistry, 2016, 20, 1477-1489 ISSN: 1385-2728 eISSN: 1875-5348

Synthetic Methods of Disulfide Bonds Applied in Drug Delivery Systems Impact Factor: 2.157

a

a

a

a

a,b

Kaiming Wang , Na Liu , Pei Zhang , Yuanyuan Guo , Yongchun Zhang , Zhongxi Zhao Shanzhong Lic, Jianhua Caic and Jimin Caoc

a,c,*

a,*

, Yuxia Luan , BENTHAM SCIENCE

a

School of Pharmaceutical Sciences, Shandong University, 44 West Wenhua Road, Jinan, Shandong 250012, P.R. China; bSchool of Chemistry and Pharmaceutical Engineering, Qilu University of Technology, Daxue Road, Western University Science Park, Jinan, Shandong 250353, P.R. China; cJiangsu Shengshi Kangde Biotech Corporation, Lianyungang, Jiangsu 222006, China Abstract: Disulfide bond plays an important role in various fields, for example, biochemical processes, industrial and pharmaceutical chemistry, bioconjugates, peptidomimetics and selfassembled monolayers (SAMs) etc. In the past decades, drug delivery systems (DDS) have been widely investigated, and disulfide-based oxidation-reduction responsive DDS is the most promising one. Disulfide bonds have no physiological toxicity and remain stable in human body, and can be broken into a reduced form of glutathione (GSH) via the thiol-disulfide exchange reaction. Moreover, the GSH level in tumor tissues has been known to be at least 4-fold higher than that in normal tissues. Therefore, this specific redox potential in cells stimulates people to design oxidation-reduction responsive DDS for specifically releasing drugs in tumor cells and develop the new methods for the preparation of organic disulfanes. The present review has attempted to comprehensively summarize recent advances on the synthetic methods of disulfide bond in drug delivery systems.

Keywords: Disulfane synthesis, disulfide bonds, drug delivery systems, carrier, prodrug. 1. INTRODUCTION Disulfide bond is a covalent bond mainly formed from the oxidation of sulfhydryl (SH) group especially in cysteine residues in amino acid sequences [1]. It extensively exists in inorganic substances (e.g. mineral, sulfur, coal, oil, natural gas, and oil shale) and organisms (e.g. proteins and substances in the cytosol under oxidative stress) [2-5]. Disulfide bond plays an essential role in various fields, for example, biochemical processes, industrial and pharmaceutical chemistry [6, 7], bioconjugates, peptidomimetics, and selfassembled monolayers (SAMs) etc. [8-11], in particular, protein folding and the enhancement of protein stability, which ensure the biochemical function of some proteins [12]. In the oxidative condition, disulfide bonds can stabilize proteins by reducing the entropy of their denatured state [13], protect proteins from damage and increase the half-life of proteins [14]. The importance of disulfide bonds is known broadly, and their synthesis and applications have been widely studied and reported [15-19]. Disulfide bonds have no physiological toxicity and remain stable in human body. However, they will be broken by the reduced form of glutathione (GSH) via the thiol-disulfide exchange reaction. GSH is a cysteine-containing tripeptide produced in the cell cytoplasm. In biological systems, the concentrations of GSH in intracellular (1-10 mM) and extracellular (2-20 M) environment are significantly different [20-22]. Moreover, the GSH level in tumor tissues has been known to be at least 4-fold higher than that in normal tissues [23]. Disulfide bond remains stable in the blood circulation for long time, while it will be broken when entering into the tumor

*Address correspondence to these authors at the School of Pharmaceutical Sciences, Shandong University, 44 West Wenhua Road, Jinan 250012, China; Tel: +86-53188382187; Fax: +86-531-88382548; E-mails: [email protected], [email protected] 1875-5348/16 $58.00+.00

cells due to the high level of GSH. Therefore, this specific redox potential in cells stimulates people to design oxidation-reduction responsive drug delivery system for effective cancer therapy. The drug release rate was negligible under non-reducing conditions but increased with increasing GSH concentrations. In the past decades, drug delivery systems (DDS) have gained much attention in the field of pharmaceutical science. Controlled drug delivery system (CDDS) can achieve targeted delivery of the anticancer drug to lesion locations and release drugs in a controlled way, thus improve the treatment efficacy and reduce the side effects [24, 25] by conjugating stimuli-responsive materials. Various CDDSs [26-28] have been developed and they are responsive to the extracellular or intracellular stimuli [29], such as pH [30], redox gradients [31], the osmotic pressure [32], and the presence of enzymes [33]. Among them, disulfide-based oxidation-reduction responsive drug delivery system is the most promising one. Oxidation-reduction responsive drug delivery system includes the anticancer drug carriers [34] and the prodrugs. There are various carriers (Fig. 1) commonly used in drug delivery system, including organic polymers [35] (e.g. poly(ethylene glycol) (PEG)), inorganic nanoparticles [36-38] (e.g. mesoporous silica nanoparticle (MSN)), polysaccharides [39, 40] (e.g. chitosan, and dextran) and drugparticipating catanionic vesicles [41, 42]. Research on nanocarriers containing disulfide bonds is at the forefront of projects in nanomedicine. For example, a paclitaxel-conjugated polymeric micelle with poly(ethylene glycol) (PEG) and arginine-grafted bioreducible poly (disulfide amine) can stay in the blood long enough to increase the therapeutic efficacy [43]. Sun et al. [44] synthesized the micelles of poly(ethylene glycol)--poly(caprolactone) (PEG-SS-PCL) with PEG shells based on disulfanelinked. The biodegradable micelles formed from PEG-SS-PCL are

© 2016 Bentham Science Publishers

Current Organic Chemistry, 2016, Vol. 20, No. 14

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Organic polymers

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Inorganic nanoparticles O

H

O n

COOH

OH

drug-participating catanionic vesicles

Polysaccharides O

OH

n O

OH HO

OH HO

HO

PEG

HO

OH

HO

O

OH

HO O

N

dextran

O

O

N

NH3 O-

O n

OH

O O

CH2OH

OH mPEG

S

O

O

H

O

O

O H

OH

O H

H

O n

nanographene oxide

OH

H

NH2 chitosan

PLA MSN

OOC

OOC

O H

cytarabine-AOT

n

O

O

O n

OH

O O

mesoporous silica nanoparticle HO

PCL

OH

O HO

OH HN O

sodium alginate H2N

O

GO

NH2 NH

O

graphene oxide

HN N

H2N HO

OH OH

OS O

O O HO

amitriptyline-SDS

O F O2N NO2

polyethylenimine

disaccharide

Fig. (1). Various carriers used in drug delivery system.

sufficiently stable in water and have higher anticancer efficacy as compared to the reduction insensitive control. MSN has been employed in drug delivery systems as a stimuli-responsive carrier. MSNs have promising features in drug delivery because of their high surface area and large pore volume [45]. Dai et al. [46] reported a smart MSN based redox-responsive delivery system for delivering drugs to tumor cells. In this system, heparin as an endcapping agent was utilized to seal the mesopores of MSNs via disulfide bonds, which controls the drug releasing as intermediate linkers. To facilitate the uptake of doxorubicin (DOX) loaded MSNs, lactobionic acid molecules were conjugated to heparin endcapped MSNs. The study suggested a promising nanocarrier, which could deliver DOX to tumor tissue with high curative efficiency and minimal side effects in the cancer therapy. Moreover, Friedl et al. [47] designed a chitosan derivative (chitosan-TGA-MNA) that chitosan was conjugated to thioglycolic acid (TGA) and then conjugated to mercaptonicotinic acid (MNA) via disulfide bond, which is highly promising in the targeted delivery system, because chitosan and dextran are non-toxic, biocompatible materials. Besides in nanocarriers, disulfide bond has also been conjugated with the prodrug [48]. The method makes the prodrug stable during handling, storage, and in physiological conditions, as well as easily cleavable in the tumor intracellular compartment for the free parent drug releasing. A poly merizable disulfane paclitaxel (PTX) prodrug was synthesized, which consists of 2-hydroxyethyl methacrylate (HEMA) and PTX with active carboxyl groups by the consequential esterification reactions of 3,3’-dithiodipropionic (DTPA) [49]. The poly merizable prodrug was copolymerized with poly(ethylene glycol) methyl ether methacrylate (PEGMEA), which

can obtain a copolymer containing hydrophilic PEG side chains and PTX covalently linked onto the backbone by disulfide. Aissi et al. [50] described a prodrug (IUdR-SS-MTP) for melanoma-targeted therapy, which contains a Melanin-Targeting Probe (MTP) conjugated to 5-iodo-2’-deoxyuridine (IUdR) via a reduction-sensitive linker. Analytical results showed that IUdR can be released efficiently from prodrug bearing a self-immolative disulfane linker in the reductive state of disulfane. The disadvantages of CDDSs still draw much attention to researchers, such as leakage of drugs or low drug release ratio [51]. Disulfide bond is a potential way to solve the problems and there have been various ways to synthesize them. One way is using inherent oxidizing thiol groups to form disulfide bonds. If there is no thiol group in the molecule of interests, then it could be converted to other functional groups, such as amino groups, carbonyl groups, and carboxyl groups [29]. In addition, the inherent disulfane donor may give better than the conversion than other functional groups, as these linkers can provide higher yields because it usually requires fewer reaction steps. Figure 2 lists the common reagents with thiolating agents, disulphide cross-linkers and other functional groups such as aminating and carboxylating agents. This review article will focus on the synthetic methods of disulfide bond in drug delivery systems. 2. METHODS OF SYNTHESIS 2.1. Disulfane Synthesis from Thiols Thiols are used as the raw materials in most of the reactions of disulfanes preparation. Two reactions have been well studied. One

Synthetic Methods of Disulfide Bonds Applied


1479

Fig. (2). Thiolating agents, disulphide cross-linkers and other functional groups used in synthetic pathways and redox reactions.

is thioalkylation or thiolysis of S-containing compounds via nucleophilic displacement of a suitable leaving group by a thiol; another is the oxidative coupling of thiols in the presence of different oxidizing agents [8]. This section covers these methods of the preparation of disulfanes. 2.1.1. Oxidative Coupling of Thiols Using Air or DMSO Dimerization of highly active thiyl radicals (R–S), generated by abstraction of hydrogen from thiols, results in the formation of disulfide bonds from disulfanes [8]. Xiong et al. [52] reported a very mild procedure using air as the oxidant to conjugate methoxypoly(ethylene glycol)-thiol (mPEG-SH) to nanographene oxidethiol (NGO-SH) by a disulfane linkage. An ultrasonicator has also been employed for the preparation of disulfane from thiols through a "one-pot" process (Scheme 1). First, NaOH (1.2 g) and ClCH2COOH (1.0 g) were added to an NGO 1 aqueous suspension (10 mL, 2 mg/mL), followed by bath sonication to generate NGOCOOH 2. NGO-COOH (10 mg), mPEG-SH (50 mg) and NaSH (200 mg) in an aqueous solution were sonicated for 1 h and stirred for 10 h to produce NGO-SH 3. Then, another 50 mg of PEG-SH was added under vigorous stirring for an additional reaction to obtain the corresponding disulfane (NGO-SS-PEG) 4. Similarly, amphiphilic thiolated sodium alginate (TSA) was ultrasonicated for 3 min using an ultrasonic cell shredder to facilitate the oxidation reaction of thiol groups in deionized water, and the nanosphere sample was prepared (Scheme 2) [53].

A bioreducible polymer with disulfide bonds was reported as a tumor targeted nonviral gene carrier (Scheme 3) [54]. Ring-opening reaction of low molecular weight branched polyethylenimine (BPEI) 5 with propylene sulfide produced thiolated BPEI-SH 6 on reported method with some modifications [55]. Propylene sulfide, an effective thiolating agent, would not significantly change the acid-base property of the BPEI and can easily be purchased or synthesized by the existing procedure [56]. On one hand, BPEI-SHs were transferred to disulfane cross-linked polyethylenimines, BPEISS 7, by oxidation of the thiol groups with DMSO for 48 h at r.t. The key of this procedure is to keep the buffering property of PEI to a neutral pH in order to prevent the thiol groups from sulfide-forming polymerization because thiol groups and amino groups compete with each other in the ring-opening reaction of a propylene sulfide. In addition, the thiolation degree, which is the average number of thiol groups on a PEI molecule, can be readily adjusted by the propylene sulfide/PEI ratio. 2.1.2. Thioalkylation or Thiolysis of Thiols Using Dithiodipyridine and Related Derivatives A versatile synthetic route to functionalized unsymmetrical disulfanes from thiols (1 equiv.) with the aid of 2,2’-dithiodipyridine (2 equiv.) was reported. The intermediate 9 obtained from thiols 8 and 9 can undergo thiol–thiol coupling at room temperature in MeOH under nitrogen environment and further reacted with a dif-

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COOH COOH

O

OH

O

COOH

COOH

O Cl

OH

O NaOH

OH O

COOH 2

1

55oC

NaSH

S O

PEG S

S

S

O

PEG

S

O

O

O

S

SH

SH

PEG

SH

O

O

O

O

O

mPEG-SH

S

O

S

SH

O

PEG

O

O 4

3

Scheme 1. Synthesis of NGO-SS-PEG under sonication [52]. OOC

OOC

OOC

O A

NaIO4

O O

i) H2N

OOC O

O

SH

OOC

OOC

O O

O

O

O

O

O

ii) NaIO4 HO

OH

HO

OH

HO

OH

O

O

HO

OOC

OH

HO

OH

O

OH

O

HN

OOC O

B

self-assembly

O O

air oxidation S

O

S

cross-linking HO

SH

O O

OOC

TSA

OOC O

HO

O

HN

O

HN

OH

O

NH

SH TSA

O

O O

O COO

Scheme 2. (A) Synthetic scheme of TSA and (B) of disulfane cross-linked nanospheres [53].

COO



H2N

HS

NH

1481

HS

S NH2 NH

N

HN

MeOH, 60oC, 24h

N

N

HN N SH

5

6

HS

S NH

NH

S

HS

N

N N

N

HN

HN N

N S

SH

S

DMSO, RT, 48h

NH HS

H N

N

N N SH

HN

N

S

N

N

S

N H SH 7

Scheme 3. Synthesis of the disulfane-cross-linked polymer using propylene sulfide as an effective thiolating agent [54].

N

H

O n

S

S

N HS H

SH

O n

S

S

OH

N

O

O

H

MeOH, 25oC

n

S

S

THF, 25oC

8

OH O

9 95%

10 93%

Scheme 4. Synthesis of unsymmetrical disulfanes using dithiodipyridine [57]. NH2 MSN

S

S

N

HOOC

NH2

SH MSN

S

S

COOH

PBS, RT

Scheme 5. Oxidation of cysteine [58].

ferent thiol (2 equiv) in THF to form the unsymmetrical disulfane 10 in high yield, as outlined in Scheme 4 [57]. The key of this procedure is that the unsymmetrical disulfane 10 is obtained in appreciable yield via the formation of 9 without the symmetrical disulfane by-product detected. The procedure prac-

tically involves successive double SN2-type reactions, which gives the possibility to synthesize unsymmetrical disulfanes. The method is also extended to cysteine, which was converted to the corresponding disulfane (Mesoporous silica nanoparticle (MSN)-SS-cysteine) in phosphate saline buffer (PBS) solution (pH 7.4) at room temperature (Scheme 5) [58].

1482


O

O

HO

i) cysteamine, NaCNBH3 DMSO/H2O (v/v 3/1)

OH

n O HO

Wang et al.

OH HO

OH

HO

O

O

O

HO

ii) DTT, H2O

HO 12

S

N

O

O O

SH

N OH H

OH HO

HO

11

H

OH

n O

SH

O n

S

N

N

O

OH

n O OH HO

HO HO

pH 2.0, H2O, RT

HO

14

N OH H

S

S

13

DMF/LiCl, acetic acid, 40oC

O

O

OH

n O HO

HO

HO

O

S

N OH H

OH HO

O

S

H n

O 15

Scheme 6. Disulfane synthesis from Dex-SH derived from dextran and cysteamine [59]. HO

O

S

O

S

OH H

O

2n

O O

Sn(oct.)2, toluene,

100oC H

O

16

n

S

O

O

DTT, CH3ONa THF, 25oC

H

O n

O

SH

S

nO n

14

Scheme 7. Preparation of thiol from symmetrical disulfane by reduction with DTT [44].

Apart from thiol, aldehyde, alcohol and amine have always been transferred to thiol as the primary source for the preparation of disulfide. Sun et al. [59] demonstrated the synthesis of glycosyl disulfanes from dextran to disulfane-linked dextran--poly(-caprolactone) diblock copolymer (Dex-SS-PCL) (Scheme 6). The intermediate 13 was obtained from 2,2’-dithiodipyridine (2.0 equiv.) and Dex-SH 12 (1.0 equiv.). Under an argon atmosphere, Dex-SH 12 was synthesized by reduction with dithiothreitol (DTT, 18.0 equiv.) in H2O from Dex-SS-Dex (1.0 equiv.), derived from dextran 11 (1.0 equiv.) and cysteamine (1.0 equiv.) in DMSO/H2O (3/1 v/v) by NaCNBH3 (0.9 equiv.). Finally, the intermediate 13 (1.0 equiv.) reacted with PCL-SH 14 (1.1 equiv.) to produce the disulfane 15. The synthetic method for PCL-SH was reported by the same group in another similar disulfane synthesis [44], which was synthesized by ring-opening polymerization of -caprolactone 16, using bis(2hydroxyethyl)disulfane as an initiator and stannous octoate as a

catalyst under an argon atmosphere followed by reduction with DTT in high monomer conversion determined by 1H NMR (Scheme 7). Henne et al. [60] described another method of synthesis of the intermediate from camptothecin (CPT) (Scheme 8). Firstly, 2mercaptoethanol 18 was reacted with 2,2’-dithiodipyridine to generate 19, and then CPT 17 (1.0 equiv.) was treated with triphosgene (0.4 equiv.) in the presence of N, N-dimethyl-4-aminopyridine (DMAP) (6.0 equiv.) to form the chloroformate 20 of CPT. Finally, the addition of 19 to the chloroformate afforded the intermediate 21. Dithiodipyridine’s derivative, 2,2’-dithiodinicotinic acid (DTNA) 23, was achieved via oxidation of 2-mercaptonicotinic acid (MNA) 22 with hydrogen peroxide under neutral pH conditions as described previously by Iqbal et al. (Scheme 9) [61], and has also been employed to prepare the unsymmetrical disulfane 26 from the thiolated chitosan 25 (Scheme 10) [47]. The thiolated chi-



N

S

S

N

HO

HO

1483

S

S

SH

N

CH2Cl2, RT 18

19 O O

N

N

O

triphosgene, DMAP

N

O N

CH2Cl2, RT OH

17

O

O

O

O

20 Cl O

HO

S

S

N O

N

O N

CH2Cl2, RT

O O

N O

21

S S

Scheme 8. Synthesis of pyridyldithioethyl carbonate camptothecin [60]. COOH COOH

i) pH 7-8 N

N

S

S

N

ii) H2O2, pH 8-9

SH

HOOC

22

23

Scheme 9. Synthesis of DTNA [61]. COOH CH2OH

CH2OH O

O

H

O

HS

H

OH

N

NH2

HOOC

H

OH

N

CH2OH O

H O

O OH

H

H

NH

H

H

H H

S

O

H OH

S

EDAC, 0.1 M hydrochloric acid, RT n

H

H2O, 24h

NH

O S

SH HOOC 24

n

n

O

25

S N

26

Scheme 10. Synthesis of chitosan-TGA-MNA [47].

tosan was obtained from chitosan 0.1 M hydrochloric acid solution 24 and thioglycolic acid (TGA) activated by ethyl(dimethylaminopropyl)carbodiimide (EDAC) at r.t. under continuous stirring [62]. 2.2. Modification of Disulfanes Disulfanes have been synthesized by modifying some simple compounds with disulfide bonds such as cystamine dihydrochloride, dithiodipropionic anhydride and 2-hydroxyethyl disulfide. 2.2.1. Using Cystamine Dihydrochloride Cystamine dihydrochloride (Cys·2HCl) has been used as a disulfane donor in the past years [46, 63-66], which allows polymer

chains to be conjugated to another polymer chains or some drugs. The recent reaction [46] proceeds through amino-functionalization and followed carboxylation of mesoporous silica nanoparticles (MSNs) 27 and MSNs-COOH 28 (0.1 g) then reacts with 1 g of Cys·2HCl to produce MSNs-SS-NH2 29 (Scheme 11). The reaction is usually carried out at room temperature with reflux in 20 mL of EDC/NHS mixture solution (EDC: 0.015 M, NHS: 0.015 M in PBS buffer with pH = 5). Finally, MSNs-SS-heparin 30 was obtained by centrifugation from deacetylated heparin and 28 at room temperature for 24 h. Wu et al. [63] described a method of synthesis of unsymmetrical disulfanes from mPEG (Scheme 12). p-Nitrophenyl chlorofor-

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O O

Si

O

O

NH2

O

O

anhydrous toluene, 60oC

acetone, RT

27 S

H2N H

COOH

MSNs

HOOC

MSNs

28

NH2

S

Cl H

O

Cl H2N

S

S

H N

S

MSNs N H

EDC/NHS, RT

S

NH2

O 29 O

N-HP HP

S

EDC/NHS, RT

H N

MSNs

S

N H

S

S

HP

O 30 HOOC

HOOC

O

O N-deacetylated heparin (N-HP):

O

O O

NH2 OH

n

OSO3H

Scheme 11. Synthesis of disulfanes using cystamine dihydrochloride as a disulfane donor [46].

O N+

O

O n

Cl

OH

O

OO

O

n

O

O

O O

pyridine, CH2Cl2, RT

31

NO2

32 S

H2N H

NH2

S

Cl H

Cl

O n

H N

O

O

Et3N, DMSO, RT

S

S

NH2

O 33

Scheme 12. mPEG-SS-NH2 synthesis from mPEG through p-NPC [63]. O OH

O

n 34

Cl

O OH

KOH 40%, DMSO

O

O n

H2N

S

S

O

NH2 O

OH

O n

N H

S

S

NH2

DCC, NHS, CH2Cl2, RT 35

36

Scheme 13. mPEG-SS-NH2 synthesis from mPEG through chloroactic acid [64].

mate (p-NPC, 4 equiv.) with pyridine (5 equiv.) was used to activate mPEG 31 (1 equiv.) to generate PEG-NPC 32 under nitrogen in dichloromethane. Then, under a nitrogen atmosphere, 32 (1 equiv.) was treated with Cys·2HCl (7 equiv.) and Et3N (14 equiv.) to form mPEG-SS-NH2 33 at r.t for 24 h. Chloroactic acid was also employed in the synthesis of mPEGSS-NH2 from mPEG (Scheme 13) [64], which (5.3 equiv.) was added to a DMSO/H2O solution of mPEG 34 (1.0 equiv.) and KOH to produce the corresponding mPEG-COOH 35. After the reaction with Cys·2HCl and NaOH, cystamine was extracted by CH2Cl2 and

added dropwise to the CH2Cl2 solution of mPEG-COOH (1.0 equiv.), N, N-dicyclohexylcarbodiimide (DCC) (1.2 equiv.) and NHS (1.2 equiv.) to form mPEG-SS-NH2 36. 2.2.2. Using Dithiodipropionic Anhydride (DTDPA) It is important for this method that dithiodipropionic acid (DTDP) should firstly be converted to DTDPA in acetyl chloride through dehydration reaction (Scheme 14) [67, 68], which has high reactivity toward the reagents carrying either hydroxy or amino groups.



O

1485

O

O HO

S

Cl S

S

OH

O

S

O

O DTDP

DTDPA

Scheme 14. Preparation of DTDPA [67, 68]. O S

O

S O

O OH n

O

O

O

O n

O DMAP, Et3N, DMF, 35oC

S

S

OH

O

37

38 OH N

O

O

O

O

O O

O

S

n

S

N

O

O

O

DCC, DMF, RT 39 CH2OH

CH2OH O

H

O H

OH

H

SDS

NH

H

100-DD

O

H

O

O

H

OH

H

OH

H

H H

O

H

O OH

CH2OH

CH2OH O

H

NH2

H

DD

NH

O

H

100-DD

O

O

41 O

O O

O

n

O

O

CH2OH

CH2OH O

H OH

O

H

O H

H

R

Tris

OH

H

R

x

O

H

O H

H

H H

O

H

O OH

n

S S

O

O

O

O

S S

DD

S

40

O

DMSO, 40oC

NH3 O-

O

O

O

3 H

H

O OH

H

H

R'

H

H y

H

R'

x

R NHCOCH3 or NH3+ SO4- C12H25 R' NHCOCH3 or NH2 y

42

Scheme 15. Preparation of CS-SS-PCL [67, 69, 70].

Cao et al. [67] reported a formation of chitosan-SS-PCL from chitosan (CS) and PCL (Scheme 15). First, PCL-SS-COOH 38 was obtained from PCL 37 (1.0 equiv.) in the presence of DTDPA (1.5 equiv.) and DMAP (1.0 equiv.) in dried DMF. Secondly, PCL-SSCOOH 38 (1.0 equiv.) reacted with NHS (1.2 equiv.) and DCC (2.0 equiv.) in r.t for 24 h to yield PCL-SS-NHS 39, the active ester derivative of 38. Finally, after precipitated in 15% Tris-HCl solution (pH 9.0) to remove sodium dodecyl sulfate (SDS), CS-SS-PCL 42 was successfully synthesized from dodecyl sulfate-chitosan complexes (SCC) 41, which was prepared from chitosan (CS) 40 according to the previously procedure [69, 70], via the active ester.

Dithiodipropionic acid (DTDP) (4 equiv.) can be reacted with 2-hydroxyethyl methacrylate (1 equiv.) 43 in the presence of DMAP and N,N’-diisopropylcarbodiimide (DIC) in THF at r.t. to generate 44 (yield: 72%) purified by flash chromatography (Scheme 16) [49]. Then, the paclitaxel prodrug 45 (yield: 48%) was obtained from 44 in the presence of DMAP and DIC. Similarly, the reaction of amines with DTDPA can also yield DTDPA functionalized compound, as outlined in Scheme 17 [68]. N-(13-amino-4,7,10-trioxatridecanyl) biotinamide 46 (1.0 equiv.) was reacted with DTDPA (4.0 equiv.) in dry DMF to generate bi-


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Wang et al.

O HO

O

O HO

S

S

O

OH

O

HO

DMAP, DIC,THF, RT

S

S

O O

O

43

O

44 72%

O

O O O

S

S

O

O

O O

O

O

paclitaxel

OH DMAP, DIC,CH2Cl2, RT O

O

O

O

O

45 48%

O O

HO

Scheme 16. Syntheses of the paclitaxel prodrug [49]. O S H

H N

HN NH H

O H

O

S

S O

O

O

O

NH2

DMF, RT

O

46 O

S H N

HN NH H

O

O

O

O

H N

O

S

S

OH

O

47 F O

F F H

F

O

F

Et3N, DMF, RT

F

F

O

S H N

HN O

F

NH H

O

O

O O

H2N HO

O

OH OH

H N

S

S

F

O

O

48

F

F

F F

O O HO

O F

pyridine, DMF, RT O2N NO2

49 H

O

S H N

HN O

NH H

O

O

O

H N

O

S O

O S

N HHO

OH OH

O O HO

50

O F O2N NO2

Scheme 17. Syntheses of the cleavable biotinamide-SS-disaccharide [68].

otinamide-SS-COOH 47 purified by flash chromatography. Then, Et3N (1.5 equiv) and pentafluorophenyl trifluoroacetate (1.5 equiv) were added to a solution of biotinamide-SS-COOH 47 (1.0 equiv) in dry DMF under nitrogen atmosphere at r.t. and biotinamide-SSCOOC6F5 48 was obtained. Finally, 2,4-Dinitrophenyl 4’-amino2,4’-dideoxy-2-fluoro--xylobioside 49 (1.0 equiv) in dry DMF was added to biotinamide-SS-COOC6F5 48 (1.3 equiv) in pyridine with

stirring at r.t. for 4 h to produce biotinamide-SS-disaccharide 50 purified by flash chromatography. 2.2.3. Using 2-hydroxyethyl Disulfide The key of this procedure is to synthesize formyl chloride derivative 53 of 2-hydroxyethyl disulfide 51 (Scheme 18) [71]. Firstly, 2-hydroxyethyl disulfide was reacted with chloroacetyl


Current Organic Chemistry, 2016, Vol. 20, No. 14 O

O O

Cl HO

S

S

Cl

HO

OH

S

Et3N, CH2Cl2, 0oC to RT 51

S

Cl

Cl

Cl O

Cl

O

Cl

Cl

O

Hunig's base, CH2Cl2, 0oC

O

S

S

Cl

O

O

52

53

O

O O

O

1487

O

O

O

OH O

O

OH

O

3, DMAP

N H

O OH

OH

O

N H

O O

Hunig's base, CH2Cl2, 0oC to RT

O

O O

O

OH

O O

O

O O

O

O

S S

O Cl O

54

55 0oC

MeOH/NH3

O O

O O

O

OH O

O

O

O

O

O O

O

O

OH

O

O

DMAP, CH3CN, RT

O

S

HO

OH

O O

O O

O

S HO

O O

O

O

O

N H

O

O S

S

O

OH

O N H

O

O

56 57

O

Scheme 18. Synthesis of the free succinic acid-containing water-soluble paclitaxel prodrug [71]. O

O O

O

O

O

OH

O N H

O O

OH

O

OH

O O

OH

O N H

O

O

O O

O

OH

O O

O

O O S R

O

O

S R = OOC(CH3)NH2, NH2, OCH2COOH, etc.

Scheme 19. Synthesis of the other paclitaxel prodrugs [48].

chloride in presence of triethylamine to form the mono-chloroacetyl derivative 52, which was then converted to 53 via treatment with diphosgene in the presence of Hunig’s base. Secondly, in presence of Hunig’s base and DMAP, the addition of paclitaxel 54 to the mono-chloroacetyl derivative afforded the mono-adduct 55, and chloroacetyl group from 55 was removed by treating them with methanolic ammonia at 0°C to give the free hydroxyl group-

containing mono-adduct 56. Finally, the free succinic acidcontaining water-soluble paclitaxel prodrug 57 was prepared from the adduct 56 and succinic anhydride in presence of DMAP. Similarly, with the aid of 2-hydroxyethyl disulfide, paclitaxel can be converted to amino acid-containing paclitaxel prodrug, free terminal amino group-containing paclitaxel prodrug, glycolic acidcontaining paclitaxel prodrug, etc. (Scheme 19) [48].

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3. SUMMARY AND OUTLOOK From the above discussion, while the oxidation of thiols results in the formation of a disulfide bond in a symmetrical disulfane, the synthesis of unsymmetrical disulfane is indeed a subtle methodological challenge. There is an evidence that thiols, which are commercially available or easily synthesizable, remain the major choice as the starting materials and the thiol-disulfane exchange reaction remains the most popular method for the synthesis of disulfanes. The formation of the disulfide bond in disulfane generally involves thiolysis or occurs via radical formation and subsequent dimerization. Moreover, some disulfane donors bearing disulphide groups have also been commonly utilized, because of their high reactivity with amines and alcohols. Nevertheless, some of these conditions have certain disadvantages, especially for carriers and prodrugs with polymers. The main disadvantages are the long reaction time, the low yield, the harsh reaction condition, and the difficult work-up. It is therefore imperative that new efficient synthetic methods of organic disulfanes should be employed and more general, improved and eco-friendly methodologies should be developed to prepare both symmetrical and unsymmetrical disulfanes in drug delivery systems. We can predict that ceaseless experimentations are going on leading to extensive progresses towards future medical applications.

Wang et al. [12]

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CONFLICT OF INTEREST [24]

The authors confirm that this article content has no conflict of interest.

[25]

ACKNOWLEDGEMENTS

[26]

This work was supported by the funds from National “Major Science and Technology Project–Prevention and Treatment of AIDS, Viral Hepatitis, and Other Major Infectious Diseases” (Grant # 2013ZX10005004), Science & Technology Enterprise Technology Innovation Fund (Grant #BC2014172) of Jiangsu Province, Small & Medium Enterprise Technology Innovation Project (Grant #CK1333) of Lianyungang City. REFERENCES [1]

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Received: August 08, 2015

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Revised: October 22, 2015

Accepted: December 05, 2015