in vitro characterization of polyorthoester microparticles containing bupivacaine

RESEARCH ARTICLE

In Vitro Characterization of Polyorthoester MicroparticlesContaining Bupivacaine

Jone-Shin Deng,* Luk Li, Youqin Tian, Eric Ginsburg, Matthew Widman,

and Alecia Myers

Advanced Drug Delivery, Hospital Products Division, Abbott Laboratories,

Abbott Park, Illinois, USA

ABSTRACT

Laboratory-scale spray-congealing equipment was utilized to fabricate injectable micro-

particles consisting of polyorthoester and bupivacaine. Operating conditions for the spray-

congealing process were optimized to produce microparticles with the desired shape and

particle size to yield acceptable syringeability and injectability. Characterizations were

performed to determine the chemico-physical properties of polyorthoester before and after

microparticle fabrication. Microparticles with different drug loadings and comparable particle

sizes were produced, and their in vitro drug-release profiles were determined. The in vitro

drug release of microparticles with a high drug loading was markedly faster than those with a

low drug loading. This is partially attributed to a more significant initial burst–drug release of

the microparticles with a high drug loading. The microparticles have demonstrated the

potential to be used for long-acting postsurgery pain management by local injection.

Key Words: Spray congealing; Injectable microparticles; Polyorthoester; Bupivacaine;

Particle size; Syringeability; Injectability; In vitro drug release; Initial burst–drug release;

Long acting.

INTRODUCTION

Microparticles consisting of a biodegradable poly-

mer and a local anesthetic agent have been extensively

studied for their application in long-acting post operative

pain management.[1 – 6] Microparticles with a desired

particle size can be suspended in an appropriate

liquid medium for injection. The sustained release of

the anesthetic agent at the administration site is mainly

achieved by the diffusion of the drug through the

polymeric matrix that undergoes in vivo erosion.

In this study, polyorthoester and bupivacaine free

base were used to fabricate sustained-release micro-

particles for injection. Polyorthoesters are bioerodable/

biocompatible polymers[7 – 14] that are specifically

designed to achieve sustained release of physically

31

DOI: 10.1081/PDT-120017521 1083-7450 (Print); 1097-9867 (Online)

Copyright q 2003 by Marcel Dekker, Inc. www.dekker.com

*Correspondence: Jone-Shin Deng, Advanced Drug Delivery, Hospital Products Division, Abbott Laboratories, Abbott Park,

IL 60064, USA; Fax: (847) 938-3645; E-mail: [email protected].

PHARMACEUTICAL DEVELOPMENT AND TECHNOLOGYVol. 8, No. 1, pp. 31–38, 2003

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016

©2003 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.

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incorporated drugs from products in the form of implants,

semi soilds, and microspheres.[15 – 24] Polyorthoester is an

amorphous polymer whose glass transition temperature

can be modified to yield optimal processability. Poly-

orthoesters undergo in vivo degradation through hydro-

lysis of the ester bonds. The composition of

polyorthoesters can be tailored to meet the desirable in

vivo biodegradability. Preliminary animal data have

shown that polyorthoesters are biocompatible and

produce nontoxic degradation products.[8,10] Their

applications[14] include peptide and protein delivery,

anti-cancer drug delivery, and periodontal and glaucoma

filtration surgery.

In this study, bupivacaine/polyorthoester micropar-

ticles were prepared by using a solvent-free fabrication

process: spray congealing. Their physical properties

were characterized and their in vitro drug-release profiles

were also determined.

MATERIALS

Polyorthoester, 1,2-propanediol/triethyleneglycol-

diglycolate (1,2-PD/TEG-diGL) in a 0.6/0.4 molar ratio

was supplied in a granular form by Advanced Polymer

Systems (APS), Redwood City, CA. Polyorthoester is

a random copolymer that was synthesized by reacting

3,9-di(ethylidene)-2,4,8,10-tetraoxaspiro [5,5]-undecane

(DETOSU)[7] with 1,2-propanediol (1,2-PD) and triethy-

lene glycol diglycolide (TEG-diGL). Figure 1 shows the

reaction scheme of the synthesis. Bupivacaine base

[C18H28N2O, Molecular weight ðMwÞ ¼ 288:4� is a

white crystalline powder and was purchased from

Organmol, Switzerland.

EQUIPMENT AND

PREPARATION PROCEDURES

Spray-congealing was performed using the spinning

disk technology developed by Particle and Coating

Technologies (PCT), Inc., St. Louis, MO. Bupivacaine

was first melted at a temperature above its melting point.

Polyorthoester granules were subsequently added into

the melted drug and were stirred until a homo-

geneous molten mixture was obtained. The polymer/drug

melt mixture was poured onto the spinning disk.

Figure 1. Reaction scheme of 1,2-PD/TEG-diGL synthesis.

Deng et al.32



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Microdroplets were first produced by the centrifugal

force generated by the spinning disk and microparticles

were subsequently formed when these droplets con-

gealed in the cooling chamber.

In this study, it was also found that the efficiency of

the spinning disk process was greatly reduced when the

viscosity of the melt mixture became relatively high. The

negative impact of high melt viscosity on this process can

be eliminated by installing a hot-air device atop the disk.

The hot-air device is used to maintain the temperature

around the disk above the congealing point of the material.

A schematic process diagram is shown in Fig. 2.

METHODS OF EVALUATION

Characterization of Starting Materials

Thermal Properties of Polyorthoester and

Bupivacaine Base

A differential scanning calorimeter (DSC), Seiko

Model 220C, was used to determine the glass transition

temperatures (Tg) of polyorthoester. Their hardness and

processability have been shown to correlate with their

Tg.[12] The melting point (Tm) and crystallization

behaviors of bupivacaine base were also evaluated using

DSC. The melting and crystallization characteristics of

bupivacaine base can be used to determine spray-

congealing conditions for the microparticles.

Characterization of Microparticles

Shape, Morphology, Size, and Size Distribution

of Microparticles

A Nikon Optiphot-2 microscope was used to examine

the shape of the microparticles. A Cambridge scanning

electron microscope (SEM) was used to examine the

surface morphology of the microparticles at a magnifi-

cation of 5000 £ . Size and size distribution of the

microparticles were determined by using an Aerosizer,

Model Mach2 manufactured by TSI, Inc. The particle size

measurement was carried out by determining the

aerodynamic diameter of the particles.

Copolymer Molecular Weight of Microparticles

The impact of the preparation process on the weight-

average Mw of polyorthoester was determined using gel

permeation chromatography (GPC). Three Waters Styr-

agel columns (HR-5E, HR-4E, and HR-3) were used in

series. A Shimadzu RID-6A RI was used as the detector.

The mobile phase was tetrahydrofuran (THF), the pump

Figure 2. Preparation of microparticles using the spinning disk technology.

In Vitro Characterization of Polyorthoester Microparticles 33



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rate was set at 1.0 mL/min, and the temperature of the

columns was maintained at 35 8C using a column heater.

Syringeability and Injectability of the Microparticles

Needles of 23-G and 25-G were used to evaluate the

syringeability/injectability of the microparticles. Micro-

particles (3% bupivacaine concentration) were added in

5 mL of an aqueous medium (0.75% CMC, 0.2% Tween

80 aqueous solution), gently shaken until uniformly

dispersed, and then 1 mL of the dispersion withdrawn

using a syringe attached to the testing needle. The

syringeability of the dispersion from the vial into the

syringe and its injectability through the needle into a vial

were evaluated.

In Vitro Dissolution Test

Microparticles were tested for their in vitro drug-

release profiles in a pH 7.4 phosphate buffer solution with

0.01% sodium lauryl sulfate used as the surfactant.

Sample bottles each containing about 12 mg of micro-

particles in 100 mL of solution were immersed in a

Precision shaker water bath at 37 8C and agitated by

reciprocating shaking at 100 rpm. At each time point, a

1-mL sample was drawn from the test bottle, which was

then replenished with the same volume of fresh dissolution

medium. The assay of bupivacaine in the dissolution

medium was performed using a high-performance liquid

chromatograph (HPLC) (Model HP 1100). A 5-mm C18

Alltima column (3.2 £ 150 mm) manufactured by Alltech

was used. The flow rate of the mobile phase was

1.5 mL/min, the ultraviolet (UV) detector wavelength was

263 nm, and the injection volume was 20mL. The drug

release was monitored for one week.

RESULTS AND DISCUSSION

Characterization of Starting Materials

Thermal Properties of Polyorthoester and

Bupivacaine Base

Glass transition temperature (Tg) of 1,2-PD/TEG-

diGL 60/40 was determined using DSC (Fig. 3).

This temperature was determined as the midpoint of the

thermodynamic transition curve. This transition

is attributed to the change in the heat capacity of

Figure 3. Glass transition temperature (Tg) of 1,2-PD/TEG-diGL (60/40).

Deng et al.34



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Figure 4. Melting temperature of bupivacaine base.

Figure 5. Crystallization temperatures of bupivacaine base.




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the material from a glassy state to a rubbery state during a

temperature scan. The Tg of polyorthoester was found to

be 37 8C, indicating moderate rigidity of the polymer at

room temperature.

The melting point and crystallization temperatures

of bupivacaine base were also determined by DSC using

a heat–cool cycle (Figs. 4 and 5, respectively). The

heating scan was conducted at a rate of 10 8C/min from

25 8C to 160 8C, and the melting point of bupivacaine

base was detected at 112.8 8C ðm:p: ¼ 107–1088C; ref.

The Merck Index, 11th ed., Merck & Co., Inc.) The

cooling scan was conducted at a rate of 10 8C/min from

160 8C to 270 8C, and two crystallization temperatures

(95.4 8C and 31.5 8C) of bupivacaine base were

determined. Since the Tg of polyorthoester and crystal-

lization temperatures of bupivacaine base are above

25 8C, it is feasible to conduct spray congealing of the

melted bupivacaine and polyorthoester mixture at

ambient temperatures.

Characterization of Microparticles

Size Distribution and Morphology of Microparticles

Polyorthoester (1,2-PD/TEG-diGL 60/40) micro-

particles with different drug loadings were prepared at a

disk speed of 10,000 rpm using temperatures ranging

from 185 8C to 200 8C (Table 1). The mean size of

microparticles slightly decreased with increasing drug

loading (Table 1). This could be attributed to a lower

melt viscosity of the material at a higher drug loading.

Microparticles were spherical in shape (Fig. 6) and

exhibited free-flowing properties. Microparticles with

different drug loadings (30%, 40%, and 75%) exhibited

significant differences in surface morphology when

examined by SEM at 5000 £ (Fig. 7). For samples

with a relatively low drug loading (e.g., 30% or 40%),

needle-like crystals of bupivacaine were seen

embedded in the polymeric domain (the matrix structure).

Table 1. Microparticles prepared using the spinning disk

technology.

Processing

temperature (8C)

Drug

loading (%)

Mean size

(size range) (mm)

200 for oil-bath

mixing and diska30 47 (22–81)

190 for oil-bath


190 for oil-bath


185 for oil-bath

mixing and diskb60 36 (17–62)

185 for oil-bath

mixing and diskb75 33 (14–61)

a With the hot-air process.b Without the hot-air process.

Figure 6. Representative microscopic image for

microparticles.

Figure 7. Surface morphology for samples with increasing drug loading.

Deng et al.36



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However, this two-phase matrix structure is not seen in

microparticles with a high drug loading (75%).

Copolymer Molecular Weight

The initial copolymer weight-average Mw of

polyorthoester was 50,070 daltons. A significant

reduction in copolymer molecular weight was observed

for the microparticles prepared by spray congealing

(Table 2). This may be attributed to heat degradation of

the polymer. However, the Mw values appeared to level

off around 14,000 daltons, regardless of processing

temperature and drug loading.

Syringeability/Injectability

All samples shown in Table 1 were tested, and results

indicated that microparticles with a mean size ranging

from 30 to 40mm exhibited acceptable syringeability

and injectability for needles of both 23-G and 25-G.

In Vitro Drug Release

A seven-day in vitro dissolution test was conducted

to determine the effect of drug loading on the in vitro

drug-release profiles of microparticles (samples shown in

Table 1 with mean size of 30 to 50mm) with 30% to 75%

bupivacaine (Fig. 8). Bupivacaine was completely

released in seven days for all the samples evaluated.

However, it is clear that an increase in drug loading

resulted in a faster drug-release rate. More than 70% drug

was released in one day for microparticles with a drug

loading of 50% or higher because of the marked initial

burst–drug release from these microparticles. Micro-

particles with a relatively lower bupivacaine loading

(30% or 40%) exhibited relatively slow drug-release

profiles with a moderate initial burst–drug release; less

than 50% of drug was released during the first 24 hours of

in vitro dissolution.

CONCLUSION

The properties of bupivacaine base and polyorthoe-

ster were evaluated in support of the design of a spray-

congealing process for fabrication of microparticles.

Bupivacaine and polyorthoester were found to be

miscible in their molten state and chemically compatible

in a wide range of processing temperatures. A spray-

congealing process utilizing the spinning disk technol-

ogy was found to be capable of producing microparticles

with desired sizes and shapes. The spherical micro-

particles yielded desired flow properties and a 3% drug

dispersion (30 to 40mm) exhibited acceptable syringea-

bility/injectability (23-G and 25-G needles). Although

the copolymer molecular weight dropped after spray

congealing, it was stabilized around 14,000. Micro-

particles consisting of copolymer with this Mw range

exhibited adequate physical rigidity and good powder

handling properties. Microparticles with a high drug

loading (75%) exhibited a significant in vitro burst–drug

release and a faster drug-release rate.

ACKNOWLEDGMENT

We gratefully acknowledge Dr. John Barr and

Advanced Polymer Systems for the polyorthoester

synthesis and supply.

Table 2. Copolymer molecular weights of microparticles.

Drug loading

(%)

Processing temperature

(8C)

Mw after

production

30 200 14,428

40 190 14,478

60 185 13,895

75 185 14,074

Figure 8. In vitro drug release for microparticles with

different bupivacaine loading.




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Deng et al.38



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in vitro characterization of polyorthoester microparticles containing bupivacaine

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