Direct synthesis of PLA-chlorphenesin prodrug and optimization

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Direct synthesis of PLA-chlorphenesin prodrug and optimization thereof with the aid of DOE Agnieszka Sebai, Pawe# Ru#kowski, Vanessa Bijak, Agnieszka Gadomska-Gajadhur, Aleksandra Kruk, and Ludwik Synoradzki Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.7b00266 • Publication Date (Web): 22 Nov 2017 Downloaded from http://pubs.acs.org on November 22, 2017

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Organic Process Research & Development

Direct synthesis of PLA-chlorphenesin prodrug and optimization thereof with the aid of DOE Agnieszka Sebai, Paweł Ruśkowski*, Vanessa Bijak†, Agnieszka Gadomska-Gajadhur, Aleksandra Kruk, and Ludwik Synoradzki Laboratory of Technological Processes, Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland

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TABLE OF CONTENTS GRAPHIC


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ABSTRACT

Direct, one-pot, solvent-free method for preparation of macromolecular prodrug composed of chlorphenesin (CF, 1) and polylactide (2) was developed. The procedure involves addition of chlorphenesin to L-lactide followed by ring-opening polymerization of the latter. The process was optimized by factorial design to maximize the conversion and to obtain sufficiently high average molecular mass of the prodrug. Proposed mathematical model, allows to control the length of the polymeric chain influencing duration of the therapeutic effect.

Keywords: chlorphenesin, DDS, polylactide, macromolecular prodrug, DOE. INTRODUCTION Chlorphenesin (3-(4-chlorophenoxy)-propane-1,2-diol, CF) (1, Figure 1) is an antifungal medication. It should not be mistaken for chlorphenesin carbamate (where the terminal –OH is replaced by –OC(O)NH2), which is a muscle relaxant drug and which is not the subject of this report.

Figure 1. Chemical structure of chlorphenesin (1)

CF or 1 has an antibacterial properties toward both Gram(+) and Gram(-) bacteria and is active towards yeasts like Candida albicans or Saccharomyces cerevisiae.1 The properties make it widely used as a preservative in cosmetic industry. The FDA reports that in 2011 CF was used in 1386 cosmetic formulations, mostly in leave-on dermal products.2. The World Health


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Organization classified 1 as an antifungal drug for topical use.3 In the European Union, the maximum concentration of chlorphenesin for cosmetics applications is established at 0,03%mass.4 1 can be delivered on skin or orally. When applied orally it is easily absorbed from digestive tract and metabolized in the liver with a half-life of 4 h. The main disadvantage of 1 is that it is completely excreted within 24 h and up to 85% of CF is eliminated from the organism in a form of glucuronides.5 In case of dermal application, its absorption half-life is 126 h and excretion half-life is 22 h.1 This results in an increase of effective dosage of the drug, causing therapy more expensive and less safe. At high concentration 1 may cause skin irritation in sensitive patients.6 Maintaining a therapeutic effect with a minimal risk of side effects can be achieved by applying 1 in the form of Drug Delivery System (DDS). DDS is a pharmaceutical form specially design to retain effective dose of API (Active Pharmaceutical Ingredient) over a long period of time thus extending duration of its therapeutic effect.7 Particular type of DDS is macromolecular prodrug in which API is chemically bonded to a polymeric chain. Macromolecular prodrugs are often biologically inactive or their biological activity is low. The therapeutic effect develops or intensifies after hydrolysis of a bond between the polymer matrix and API. To this day only prodrugs based upon of 1 and vinyl polymers are reported.8 Since lactic acid is a skin penetration promotor, prodrugs consisting of 1 and polylactide can be used in cosmetics to enhance their formulations. Polylactide (PLA) is an aliphatic biodegradable polyester made from a cyclic dimer of lactic acid, i.e. lactide and broadly used for medical applications. Moreover, PLA is obtained from natural resources. In the natural environment degradation of PLA is a two-step process that requires presence of water. It begins with hydrolytic degradation of polyester chains. Next, water molecules penetrate polymer coil, the structure of which was weakened in the first step, and


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cause further hydrolysis of ester bonds. As a result, PLA oligomers are produced, which are prone to be metabolised into carbon dioxide and water by microorganisms. In living organisms, degradation process is triggered by enzymes that transform the oligomers into lactic acid which then takes part in the citric acid metabolic cycle and ultimately is converted into carbon dioxide and water.9 Degradation in living organisms may be faster than in the environment due to diversified pH, increased temperature (37 °C) and abundance of enzymes. All products or byproducts of degradation are non-toxic and widespread in tissues. The most important goal in DDS development is achieving the desired release profile of API from the polymer matrix. It depends on a chemical structure and average molecular weight of the polymer matrix in a macromolecular prodrug. The length of the polymeric chain determines its degradation time. Another important factor that affects time of hydrolysis is crystallinity of the polymer. Polymers that contain more crystal domains degrade slower. In macromolecular prodrugs, average molecular weight should be sufficiently high to allow hydrolysis of API at a desired location. On the other hand, average molecular weight above 10 000 would typically result in significant slowdown of API release. As a result, concentration of the API in the organism would be too low and the prodrug would not be effective. Since the polymeric chain is only a support and plays role to the moment of hydrolysis of the API-polymer bond, its excessive lengthening can cause accumulation of the polymer (to the point of degradation). Therefore controlling the length of the polymer chain by controlling reaction parameters is extremely important and requires optimisation.10–13


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RESULT AND DISCUSSION Direct, one-pot condensation of chlorphenesin with L-lactide, combined with polymerization of the latter was carried out (Scheme 1). The reaction is practically solvent-free and only small volume of toluene, evaporated during the synthesis, is used to inject catalyst.

Cl

Cl

O

O

OH + n

O

O

4, ∆T O

O

O

O OH

HO

O

OH O n

1

2

3

O O Zn2+ 2

4

Scheme 1. Synthesis of chlorphenesin-PLA prodrug (3): condensation of chlorphenesin (1) with L-lactide (2) combined with Ring-Opening Polymerization catalyzed by zinc 2-ethylhexanoate (4).

The most common and the most effective catalyst for Ring-Opening Polymerisation (ROP) of lactide is tin(II) 2-ethylhexanoate.14-15 It is not recommended for medical and pharmaceutical applications since tin(II) accumulates in organism and can cause neurological problems.16 Thus, less active but biocompatible catalyst with less toxic metal17 – zinc(II) ethylhexanoate (4) – was used. Despite the fact that ethylhexanoic acid is reported teratogen18 it is not a concern in this case due to dermal application of a prodrug studied and low concentration of the catalyst. Preliminary research The preliminary studies of the prodrug preparation reaction were carried out (Table 1). When reaction of 1 with 2 was run at 160 °C for 6 hours, the polymer achieved average molecular weight above 3,000 g/mol and high lactide conversion (up to 96%) was observed. Keeping the


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reaction mixture for more than 5 h at high temperature caused browning of the polymer, which could negatively affect its technical specification. When the reaction was run at lower temperature (140 °C) for 5 h, both the lactide conversion (90%) and molecular weight of polymer (Mw < 3,000 g/mol) were lower. The obtained prodrugs were transparent in color. The hydroxyl group of chlorphenesin initiates ring opening of lactide more effectively in the presence of the catalyst. If the reaction was run without a catalyst, low molecular mass products were obtained (500–1400), regardless of temperature. Table 1. Preliminary trials of PLA-CF prodrug synthesis. No

Concentration of 1 (% mol)

Concentration of catalyst (% mol)

Temperature (°C)

Time (h)

Conversion (%)

Mw (g/mol)

I

5

0

100

4

0.4

505

II

5

0

120

4

2.5

509

III

0.6

0

140

4

11.9

1270

IV

0.6

0

150

4

87.6

1720

V

0.6

1

120

4

92.3

4060

VI

0.6

0.5

140

2

97.4

4920

VII

0.6

0.125

140

3.5

95.6

4950

VIII

0.6

0.05

150

6

96

3760

IX

3

0.05

150

6

88.8

4900

X

4

0.20

135

2

86

4750

XI

0.48

0.40

145

2

90

4950

XII

6

0.20

135

2

90.2

3270

The reaction progress was monitored in real-time using infrared spectroscopy (Figure 3). A significant decrease in intensity of the band that is characteristic for lactide (915 cm–1, Figure 2, b) indicates a considerable monomer conversion. The formation of polylactide is visible thanks to a new band (1120 cm–1, Figure 2, c) corresponding to vibrations of the ester C–O bond.


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Moreover, two bands characteristic for PLA at the wavenumber 860 cm–1 (Figure 2, a) and 1185 cm–1 (Figure 2, d) occurred. The IR spectra of the substrate and the product are shown at Figure 3.

d

c

b

a

Figure 2. Real-time IR spectra of the reaction (Z axis is time); characteristic bands are exposed: a – 860 cm–1 stretching vibration band of C–O in PLA, b – 915 cm–1 fingerprint band of lactide, c – 1120 cm–1 stretching band of C–O of ester in product, d – 1185 cm–1 fingerprint band of PLA.

Figure 3. The changes in IR spectrum during the reaction; blue line – spectrum of the substrate, grey line – spectrum of the product.


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Optimization The method for preparation of the polymeric prodrug PLA-chlorphenesin (optimization’s experiments procedure) is shown in a block diagram (Scheme 2). Polymerization of lactide with chlorphenesin underwent in inert gas atmosphere (argon), with absence of moisture, at 135– 145 °C for 2–3 h, counted from the time of catalyst injection (zinc 2-ethylhexanoate dissolved in toluene). After cooling down, the reaction mixture was dissolved in chloroform and precipitated in cold methanol (0–5 °C). The suspension was filtered off at room temperature under reduced pressure. The product was dried out at room temperature for ca. 24 h.


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chlorphenesin

L-lactide

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(Oct)2Zn in toluene

argon

Polymerisation T=135–145 oC, t=2–3 h argon chloroform

Dissolving T=25 oC

methanol

Precipitation T=0–5 oC, stirring

Filtration T=25 oC, vacuum methanol/chloroform filtrate

Drying T=25 oC vapours methanol/chloroform

PLA-chlorphenesin prodrug

Scheme 2. Block diagram of the prodrug 3 manufacturing at the laboratory scale.

Following process parameters were tested in the optimization protocol: the effect of temperature (x1), reaction time (x2), catalyst concentration (x3) and chlorphenesin concentration (x4) on conversion of lactide (y1) and weight-average molecular weight (Mw, y2). The optimization


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criterion was to obtain a polymer with the highest possible Mw but not higher than 10 000 g/mol. To achieve this goal, we decided to employ factorial design 24. The selected maximum and minimum levels for each factor are shown in Table 2. Table 2. Identification of input and output variables with codification of input.

Input

Output

Variable

Natural variable

x1

temperature

x2

Unit

Coded variables –1

0

+1

°C

135

140

145

time

h

2

2.5

3

x3

catalyst concentration

% mol

0.1

0.15

0.2

x4

chlorphenesin concentration

% mol

4

5

6

y1

lactide conversion

%

Max

y2

Mw

g/mol

Direct synthesis of PLA-chlorphenesin prodrug and optimization

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