micellar formulations for drug delivery based on mixtures of hydrophobic and hydrophilic pluronic®...

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Journal of Controlled Release 94 (2004) 411–422

Micellar formulations for drug delivery based on mixtures of

hydrophobic and hydrophilic PluronicR block copolymers

Kyung T. Oh, Tatiana K. Bronich, Alexander V. Kabanov*

Department of Pharmaceutical Science, College of Pharmacy, 986025 University of Nebraska Medical Center, Omaha, NE 68198-6025, USA

Received 18 August 2003; accepted 25 October 2003

Abstract

Micelles formed by PluronicR block copolymers (PBC) have been studied in multiple applications as drug delivery systems.

Hydrophobic PBC form lamellar aggregates with a higher solubilization capacity than spherical micelles formed by hydrophilic

PBC. However, they also have a larger size and low stability. To overcome these limitations, binary mixtures from hydrophobic

PBC (L121, L101, L81, and L61) and hydrophilic PBC (F127, P105, F87, P85, and F68) were prepared. In most cases, PBC

mixtures were not stable, revealing formation of large aggregates and phase separation within 1–2 day(s). However, stable

aqueous dispersions of the particles were obtained upon (1) sonication of the PBC mixtures for 1 or 2 min or (2) heating at 70

jC for 30 min. Among all combinations, L121/F127 mixtures (1:1% weight ratio) formed stable dispersions with a small

particle size. The solubilizing capacity of this system was examined using a model water-insoluble dye, Sudan (III). Mixed

L121/F127 aggregates exhibited approximately 10-fold higher solubilization capacity compared to that of F127 micelles. In

conclusion, stable aqueous dispersions of nanoscale size were prepared from mixtures of hydrophobic and hydrophilic PBC by

using the external input of energy. The prepared mixed aggregates can efficiently incorporate hydrophobic compounds.

D 2003 Elsevier B.V. All rights reserved.

Keywords: Block copolymer; Drug delivery; PluronicR; Poloxamer; Solubilization

1. Introduction for various contrasting agents [4]. One of the exam-

Micelles formed from amphiphilic block copoly-

mers have recently attracted significant attention in

diverse fields of medicine and biology. In particular,

polymeric micelles have been developed in pharma-

ceutics as drug and gene delivery systems [1–3], as

well as in diagnostic imaging techniques as carriers

0168-3659/$ - see front matter D 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.jconrel.2003.10.018

* Corresponding author. Tel.: +1-402-559-9915; fax: +1-402-

559-9543.

E-mail addresses: [email protected] (T.K. Bronich),

[email protected] (A.V. Kabanov).

ples, PluronicR block copolymer (PBC), consists of

ethylene oxide (EO) and propylene oxide (PO) blocks

that are arranged in a basic EOx–POy–EOx structure

(often abbreviated as PEO–PPO–PEO). A prominent

feature of PBC, specifically related to drug delivery

applications, is the ability to self-assemble in aqueous

solutions into multimolecular aggregates having

spherical, rod-like or lamellar morphologies. The size

and morphology of the PBC aggregates strongly

depend on the block copolymer composition, specif-

ically, the lengths of EO and PO units as well as the

block copolymer concentration, and environmental

Table 1

Characteristics of PBC [13]

Copolymer MWa Average

number of

PO units

(NPO)b

Average

number of

EO units

(NEO)b

HLBc

L61 2000 31.03 4.55 3

F68 8400 28.97 152.73 29

L81 2750 42.67 6.25 2

P85 4600 39.66 52.27 16

F87 7700 39.83 122.50 24

L101 3800 58.97 8.64 1

P105 6500 56.03 73.86 15

L121 4400 68.28 10.00 1

P123 5750 69.40 39.20 8

F127 12,600 65.17 200.45 22

a The average molecular weights provided by the manufacturer.b The average numbers of PO and EO units were calculated

using the average molecular weights.c HLB values of the copolymers were determined by the

manufacturer.

K.T. Oh et al. / Journal of Controlled Release 94 (2004) 411–422412

parameters such as temperature and the quality of the

solvent [2,5,6].

The hydrophobic core of such aggregates serves as

a microenvironment for the incorporation of lipophil-

ic compounds, while the hydrophilic corona main-

tains the dispersion stability of the PBC aggregates.

Therefore, the noncovalent incorporation of drugs

into the hydrophobic PO core of the PBC micelles

results in increased solubility, increased metabolic

stability and increased circulation time [3,5,7,8].

Although not systematically studied for drug delivery

purposes, nonspherical polymeric micelles can pro-

vide potential benefits for formulation design. For

instance, the lamellar aggregates formed by hydro-

phobic PBC are likely to exhibit a higher solubiliza-

tion capacity than spherical micelles formed by

hydrophilic PBC [6]. The obvious drawbacks of such

systems are the formation of aggregates with a large

size, which falls outside of the apparent preferred size

range for drug delivery using nanoscale particles

(10–200 nm) and lack of stability in aqueous disper-

sion leading to phase separation. Lamellar aggregates

formed by PBC with long PO chains and short EO

chains usually have larger sizes (ca. 1000 nm).

Furthermore, even at low concentrations and at am-

bient temperatures these systems phase separate.

Recently, Schillen et al. reported vesicles from one

of the most hydrophobic PBC (L121) obtained by

extruding the dispersion of the block copolymer

through a 100 nm diameter membrane filter [9].

However, the PBC vesicles prepared in this way

had low stability and eventually reverted to the phase

separated state. One of the approaches to overcome

these limitations is to combine a lamella-forming

hydrophobic block copolymer with second hydrophil-

ic block copolymers that can stabilize the mixed

aggregates.

The steric stabilization of a lipid dispersion by EO

chains attached to the surface of lipid bilayer is very

well-known [10]. Incorporation of hydrophilic PBC

in lipid structures has also been shown to have a

steric stabilizing effect. Indeed, previous works

reported steric stabilization of lipid vesicles by

incorporation of hydrophilic PBC F127 into the lipid

membrane [11]. The incorporation of PBC with long

EO chains prevents the stacking of lamellae, main-

taining a dispersion of the lamellae and ultimately

resulting in the formation of stable structures [11].

A mixture of hydrophobic PBC L61 and hydro-

philic PBC F127 has already found practical applica-

tion in pharmaceutics and is currently being evaluated

in clinical trials as a doxorubicin delivery system

[12]. However, in this mixture hydrophobic L61

was a minor component, which was solubilized in

F127 micelles. In the current study, block copolymer

mixtures composed of a hydrophobic lamella-forming

PBC with very short EO chains and hydrophilic PBC

with long EO chains, where hydrophobic PBC is a

major or equal component of the mixture, are ex-

plored. These mixed PBC form small particles and

display elevated stability in dispersion compared to

the hydrophobic PBC alone. The factors governing

the formation and colloidal stability of such systems

and their capacity to solubilize a model hydrophobic

compound (Sudan III) are evaluated and discussed.

2. Materials and methods

2.1. Materials

PBC were commercially available from BASF

Corporation (Parsipanny, NJ) and were used without

additional purification. The molecular characteristics

of the block copolymers used in this study are

presented in Table 1. PBC are designated by their

K.T. Oh et al. / Journal of Controlled Release 94 (2004) 411–422 413

nomenclature indexes, e.g. ‘F127’ means ‘Pluronic

F127.’ Sudan III (Matheson Coleman & BeU) was

used as a hydrophobic model drug for solubility

studies. All other chemicals used were of analytical

grade.

2.2. Preparation of PBC mixtures

The experiment design used for preparation of

PBC mixtures is presented by the Scheme 1. Cold

stock solutions (4 jC) of PBC in distilled water or

phosphate buffer saline (PBS, pH 7.4) were mixed in

the proportions indicated, and then left in the refrig-

erator overnight. The solutions were then transferred

to room temperature for 12 h. Samples were divided

in three parts (5 ml each). One part was used as a no

energy input control. Another was sonicated for 1 or

2 min in polystyrene tubes (FALCONR, Becton

Dickinson, NJ) using a probe sonicator (Sonicator

XL, Misonix Farmingdale, NY) at 55 W. The third

part was heated in a water bath at 70 jC for 30 min

and then cooled at room temperature. The mixtures

obtained were characterized as described below.

2.3. Physicochemical characterization

2.3.1. Turbidity measurements

The turbidity experiments were carried out by

measuring the transmittance of the mixtures using a

Lambda 25 UV/Vis spectrophotometer (Perkin–Elmer

Scheme 1. Experimental design illustration how the P

instrument Co.) at k = 520 nm. The data are reported as

turbidity=(100� T)/100, where T is transmittance (%).

2.3.2. Size measurements

The effective hydrodynamic diameter (Deff) of the

particles was measured by photon correlation spec-

troscopy using a ‘‘Zeta-Plus’’ Zeta Potential Analyzer

(Brookhaven Instrument Co.) equipped with the Multi

Angle Sizing Option (BI-MAS). The sizing measure-

ments were performed in a thermostatic cell at a

scattering angle of 90j. Software provided by the

manufacturer was used to calculate Deff values. The

averaged Deff values were calculated from three meas-

urements performed on each sample (n = 3).

2.4. Solubilization of water-insoluble dye

Ten microliters of Sudan III stock solution (10 mg/

ml) in CHCl3 was added to empty vials and the

solvent was evaporated. Two milliliter aqueous sol-

utions of F127 and 1:1 wt. L121/F127 mixtures with

total PBC concentrations of 0.02, 0.1, 0.2, 0.3 and 0.4

wt.% were individually prepared, sonicated for 2 min,

added to the vials and then allowed to equilibrate in

the shaker (100 rpm) at room temperature for 2 days.

Absorption spectra of Sudan III in aqueous solutions

at 25 jC were recorded on a Lambda 25 UV/Vis

spectrophotometer. The data are reported as absor-

bance at 362 nm corresponding to the maximum in

the Sudan III spectra.

BC mixtures were prepared and characterized.


3. Results and discussions

3.1. Preparation of stable carrier from PBC mixture

Binary mixtures were prepared from hydrophobic

PBC (L121, L101, L81 and L61) and hydrophilic

PBC (F127, P105, F87, P85 and F68) in PBS (pH

7.4). The study of mixture stability was carried out

by measuring the size of the particles in the disper-

sions for several days. In most cases, the PBC

mixtures were not stable, revealing formation of large

aggregates (ca. 800–1000 nm) with a very wide

particle size distribution and high turbidity. Ulti-

mately, all samples phase separated within 1–2

day(s). However, stable aqueous dispersions of the

particles were obtained in selected cases when the

energy input (either sonication or heating) was ap-

plied during preparation (Table 2). For example,

when PBC with long PO, such as L121 or L101,

Table 2

The size and stability of the particles in PBC mixtures at various prepara

Components % wt. (A/B)a Size of aggregates upon prepara

Control Sonication

L121 0.1 1000 432

L121/F127 0.1/0.01 1193 322

0.1/0.1 1054 154

L121/P105 0.1/0.01 873 308

0.1/0.1 813 138

L121/P85 0.1/0.01 756 356

0.1/0.1 811 340

L101 0.1 701 337

L101/F127 0.1/0.01 586 321

0.1/0.1 578 210

L101/P105 0.1/0.01 645 279

0.1/0.1 762 126

L81 0.5 1151 1038

L81/F127 0.5/0.05 416 321

0.5/0.5 767 812

L81/F87 0.5/0.025 999 753

0.5/0.25 964 781

L61 1 1044 976

L61/F127 1.0/0.01 669 659

1.0/0.5 779 795

L61/F68 1.0/0.05 916 611

1.0/0.5 874 592

a Hydrophobic PBC % (wt.) (A)/Hydrophilic PBC % (wt.) (B) (A/B:b Size of the aggregates denote averaged effective hydrodynamic diam

(n= 3).c The first and second column indicates the effect of sonication and h

was used as a hydrophobic component, addition of

the hydrophilic PBC (P105 or F127) followed by

energy input resulted in a decrease of the turbidity

and formation of stable aqueous dispersions of mixed

aggregates with rather small particle sizes (ca. 150–

200 nm). The processing parameters (i.e. sonication,

temperature) did not significantly affect the polydis-

persity of the particles formed. The degree of poly-

dispersity of the mixed aggregates varied in the range

of 0.26–0.3. Consequent measurements of these sam-

ples showed that the size of the particles remained

practically unchanged for several days. Overall, the

dispersion stability of the binary PBC systems was

dependent on the structure of the PBC. In some cases,

no precipitation in the solutions was observed for

several days as indicated in Table 2. In other cases,

for example, L121 and P85, although the size of

particles significantly decreased after sonication, the

resulting dispersions were not stable and precipitated

tion conditions

tions (nm)b Effect of energy input

Heating Decrease

of sizecDecrease

of turbidity

Increase

of stability

1135 +� � �514 +� � �198 + + + + (7 days)

1018 +� � �157 + + + + (7 days)

676 +� � �244 + + � �979 +� � �584 +� � �213 + + + + (5 days)

440 +� � �171 + + + + (4 days)

801 �� 399 �� 723 �� 1548 �� 448 �� Precipitation �� Precipitation �� Precipitation �� Precipitation �� Precipitation �� e.g. L121/F127).

eter calculated from three measurements performed on each sample

eating, respectively.

Fig. 1. (a) Turbidity of solution and (b) effective diameter of the

particles formed in L121/F127 mixtures prepared (.) without anyalteration, (n) with sonication for 1 min, (E) with sonication for 2

min, and (��) upon elevation of temperature.Deff values are within F20 nm in the cases when sonication and heating are applied.


after 2–3 days. Mixtures prepared with L61 or L81

displayed low stability and high turbidity under all

conditions.

In addition, the sonication time is an important

factor affecting the size and stability of PBC mixtures.

In most cases, an increase of sonication time resulted in

a smaller size of the particles in the dispersions and an

increased stability. However, L121/F127 mixtures son-

icated for 3 min formed unstable dispersions, which

precipitated after 2 days (data not shown). Sonication

of L61 or L81 based mixtures for 2 min also resulted in

liquid-phase separation. Based on above results, mix-

tures of L121 and F127 that were characterized as

having both a small particle size and the highest degree

of stability were selected for further study.

3.2. Mixture of L121 and F127

L121 and F127 copolymers have average compo-

sitions of EO5PO68EO5 and EO100PO65EO100, respec-

tively. Both copolymers have a hydrophobic PPO

block of practically the same length, while the length

of the PEO blocks is drastically different. It is known

that hydrophobic L121 does not form micelles. Even

at low concentration and temperatures, unimers coex-

ist with larger unimer aggregates (an L1 phase) [9]. At

ambient temperature and low concentration (0.1

wt.%), L121 forms very turbid dispersions. The

aggregates detected in such dispersion were approxi-

mately 1 Am in diameter (Fig. 1a). In contrast, F127

forms only small spherical aggregates in a wide range

of concentrations. The mixtures of L121 and F127

were prepared using methods described above. The

concentration of hydrophobic L121 was kept constant

at 0.1 wt.%, while the concentration of hydrophilic

F127 was varied from 0.01 to 0.1 wt.%.

The L121/F127 mixtures prepared without energy

input (control group) were very turbid in the entire

range of compositions of the mixture. Conversely, in

the mixtures prepared with energy input (sonication or

temperature increase) the solution turbidity decreased

as the concentration of hydrophilic F127 increased

(Fig. 1a). The size of mixed PBC aggregates in the

control group remained large (ca. 1000 nm), or even

increased as the concentration of F127 was elevated.

However, sonication or temperature elevation of these

mixtures resulted in the formation of significantly

smaller aggregates (Fig. 1b). Cryo-TEM images of

L121/F127 (0.1%/0.1%) mixed aggregates prepared

upon sonication revealed that these aggregates repre-

sent spherical particles (data not shown). The sizes of

the particles calculated from the EM data were ap-

proximately 180 nm and are in a good agreement with

those determined by dynamic light scattering.

To characterize the stability of all the mixed

samples prepared by the previous methods, the size

measurements were repeated for several days for each

sample. All solutions were stored at room tempera-


ture. Once precipitation was detected visually, the

sample was considered phase-separated. The shaking

of those phase-separated PBC dispersions resulted in

resuspension of the particles. However, the mixtures

phase-separated again in few hours. Therefore, the

first appearance of flakes was considered as the onset

of precipitation. All mixtures prepared without energy

input phase separated within 1 day. However, an

increase in concentration of hydrophilic F127 and

input of energy (sonication or temperature increase)

resulted in formation of a stable dispersion (Fig. 2).

Subsequent measurements of these samples revealed

the particle size remained practically unchanged and

no precipitation was observed for at least a week. It is

important to note that these experiments were per-

formed repeatedly over a period of a year using stock

solutions prepared at various times. In all cases PBC

mixed aggregates prepared under similar conditions

(concentration of PBC, sonication time, annealing

temperature) had practically the same sizes. For ex-

ample, the variability in effective diameters of the

particles in 0.1% L121/0.1% F127 mixture was of the

order of 20 nm in the cases when sonication or heating

was applied.

Remarkably, the sonication of mixtures conducted

at low temperature (ice bath) did not result in either a

particle size decrease or stabilization of the disper-

sions. It is important to note that the results of the size

measurement appeared to be dependent on the time of

equilibration of PBC mixture at room temperature

Fig. 2. Stability of dispersion in the mixed L121/F127

before the input of energy. Specifically, incubation

for at least 12 h was necessary in order to obtain

reproducible data on the size of the particles formed.

This is consistent with the prior observation by

Schillen et al. [9], who noted that prolonged incuba-

tion of PBC prior to extrusion was essential for

formation of PBC vesicles.

3.3. The effect of temperature on mixed PBC

aggregates

Temperature dependent hydration of PPO and PEO

segments of the PBC is a key factor that determines the

unique self-assembly behavior of PBC and rich phase

diagrams of these copolymers in aqueous solutions.

Therefore, the effect of the temperature on formation of

the mixed PBC aggregates was also examined. Studies

using L121 alone without adding F127 demonstrated

that the size of the dispersion decreased as the temper-

ature increased (Fig. 3). This was also accompanied by

an increase in turbidity of the dispersion, so that at

approximately 35 jC the solution became milky. This

is consistent with the well-known low critical solution

temperature (LCST) behavior of L121.

Furthermore, the size of aggregates in L121/F127

dispersion prepared without sonication sharply de-

creased with a relatively small increase of tempera-

ture (from 23 to 27 jC) and then remained consistent

up to 70 jC (Fig. 3). The size of the particles in the

L121/F127 dispersion prepared using sonication was

solutions prepared using methods listed in Fig. 1.

Fig. 3. Temperature dependence of the size of the particles formed in 0.1 wt.% L121 prepared without sonication (diagonal bar) and 0.1 wt.%

L121/0.1 wt.% F127 mixtures: without sonication (filled bar) and with sonication at room temperature (opened bar) in PBS. Asterisk indicates

the measurements in the dispersions that were first heated to 70 jC (30 min) and then cooled down to 23 jC.

1 CMT values for different PBC referenced here were

determined using the pyrene solubilization technique [13,14] and

surface tension measurements [15]. The values obtained using these

techniques are in good agreement with each other as well as with

CMT values determined using differential scanning calorimetry

[16,17].


already small at 23 jC and it did not change as the

temperatures was elevated (Fig. 3, opened bars). It is

noteworthy that above 25 jC, the error in size

measurement for pure L121 increased significantly.

That was not the case for the L121/F127 (0.1%/

0.1%) mixture prepared with or without sonication

step, which remained transparent in the entire range

of temperature from 27 to 70 jC. Remarkably, in

both cases, the sizes of the L121/F127 aggregates at

elevated temperatures were practically the same, and

did not change when the dispersions were cooled to

ambient temperature. In contrast, the particle size in

L121 dispersion increased after cooling.

Overall, the energy input by a transient temperature

increase can result in formation of stable binary

dispersions of PBC. This procedure may be more

useful in pharmaceutical formulation processes com-

pared to sonication.

3.4. Possible explanation of the effect of energy input

To explain the unusual behavior observed in the

mixtures of hydrophobic and hydrophilic PBC, one

should take into account the differences in the critical

micelle temperature (CMT) of these polymers. Spe-

cifically, as schematically presented in Fig. 4, the

following is proposed:

(1) Hydrophobic PBC L121 at room temperature and

the concentrations used (0.1%) is above its CMT

[9]. As a result it forms large aggregates, which

were detected in the size measurement studies.

(2) Hydrophilic PBC F127 at room temperature and

the concentration used (0.1% and less) is below its

CMT [14]1. As a result it is not incorporated in the

L127 aggregates and does not stabilize their

dispersion.

(3) Once the temperature is increased, PBC form

mixed aggregates. The resulting mixed aggregates

have relatively small sizes due to steric stabiliza-

tion effect of long PEO chains of F127 blended

with L121. These aggregates may be micelles or

vesicles (closed or ruptured).

(4) According to the literature data, the CMT of F127

at 0.1% is approximately 31 jC [14], which is

generally consistent with the size decrease at

approximately 30–40 jC.(5) Some (slow) interaction between L121 and F127

is possible even at room temperature, which can

explain why incubation of the mixture at this

temperature before sonication affects the sizes of

the particles (as measured at high temperature).

Fig. 4. Schematic diagram of proposed mechanism for the formation of aggregates in the mixtures of hydrophilic and hydrophobic PBC. (a) At

room temperature hydrophobic PBC is above its CMT forming large lamellar aggregates. Hydrophilic PBC is below CMT, it does not

incorporate in the hydrophobic PBC aggregates. (b) Sonication or temperature increases results in dehydration of PO chains and incorporation of

hydrophilic PBC into mixed aggregates, which are sterically stabilized by EO chains. (c) Once the sonication is stopped or temperature

decreased the mixed aggregates become kinetically trapped and remain stable in dispersion for several days.


(6) After heating (or sonication) when the temperature

is decreased (energy input stopped), the small

mixed aggregates remain kinetically trapped. This

explains why the small particle size is observed at

room temperature for several days. The stabiliza-

tion effect of long PEO chains of hydrophilic F127

blended with L121 in mixed PBC aggregates

appears to be governed by kinetic factors. These

aggregates are thermodynamically unstable and

ultimately phase separate.

These considerations have predictive ability and

can, in particular, explain why mixtures containing

either hydrophobic or hydrophilic PBC with shorter

PPO (higher CMT) chains do not form small particles

under any condition. It is also predicted that hydro-

philic PBC with a lower CMT can form stable dis-

persions even at lower temperature. To evaluate this

prediction, mixtures of L121 with P123 (EO20PO69

EO20) were examined. As illustrated in Fig. 5, the

results show that below 0.05% P123, all mixtures

prepared without energy input produced aggregates

of large size, which phase separated after 2 days.

Conversely, above 0.05% P123, the dispersion was

stable without precipitation for several weeks and the

sizes of the aggregates were rather small. These results

are consistent with the proposed mechanism. Accord-

ing to Alexandridis et al. [14], the CMT of P123 at

0.05% is 22.5 jC. Indeed, at 25 jC, i.e. above the CMT

of 0.05% P123, the L121/P123 mixtures containing

Fig. 5. Size of particles in L121/P123 mixtures prepared by mixing of PBC without energy input at room temperature. All samples were

measured for several days. The shadow area corresponds to precipitation where no size measurement can be done.


0.05% (and more) P123 formed relatively small par-

ticles (250 nm). These mixtures remained stable with-

out precipitation for about 4 weeks (not shown). Inputs

of energy led to a further decrease of the particle size

below 200 nm.

Anomalous behavior associated with micellization

process, especially in the context of CMT, has been

observed in dilute solutions of various diblock and

triblock copolymers including PBC [18–21]. For

example, anomalous micellization of L64 in the

unimer –micelle transition region was reported

[19,20]. This effect was mainly attributed to compo-

sition heterogeneity of the PBC, particularly, the

presence of more hydrophobic diblock copolymers.

(The sizes of the aggregates detected were not affected

by batch-to-batch variation.)

However, it is unlikely that it could interfere with

the effects reported in this study. Indeed, the anom-

alous micellization is always observed in the inter-

mediate temperature regime between unimers and

micelles, i.e. before the ‘‘real’’ CMT. It has been

suggested that the large assembles formed in this

region represent the phase separated droplets of

minor insoluble components stabilized by the

adsorbed layer made up from the major component.

As soon as the ‘‘real’’ CMT of a major component is

reached, the insoluble components either solubilize

into the core of the micelles or form mixed micelles

with this major component (depending on their

molecular characteristics). In our study, hydrophobic

L121 at room temperature and the concentrations

used (0.1%) was already above its CMT. Therefore,

it is likely that hydrophobic minor contaminants such

as diblock copolymers are already entrapped in the

L121 aggregates and do not interfere with further

micellization. Furthermore, anomalous micellization

behavior has not been seen for hydrophilic PBC,

F127 and F88 used in these studies [14,20]. There-

fore, the effects of composition heterogeneity of

copolymers on the behavior of the PBC mixtures

under experimental conditions used (heating and

sonication) are unlikely.

3.5. Solubilization of a hydrophobic dye

The solubilization capacity of the mixed L121/

F127 aggregates was further evaluated using a model

water-insoluble dye, Sudan III that has a maximum


absorbance at 362 nm. The UV/Vis spectra of Sudan

III in water and PBC solutions are shown in Fig. 6 as

plot of UV absorbance (k = 362 nm), [A362 nm] versus

total PBC concentration. As is seen in Fig. 6, mixed

L121/F127 aggregates exhibit much greater solubili-

zation capacity compared to F127 dispersions. The

incorporation of insoluble dye into L121/F127 aggre-

gates did not result in a change of the particle size and

dispersions remained stable for at least a week (data

not shown). It appeared that the lamella-forming L121

in the mixture provides for approximately 10-fold

increase in the solubilization capacity compared to

the spherical micelle forming F127 alone. Specifical-

Fig. 6. Solubilization of Sudan III in F127 micelles (o) and 1:1 (% wt.) L12

spectra of Sudan III containing in water (1), in 0.4% solution of F127

solubilized Sudan III depending on total concentration of PBC in dispers

ly, the calculated weight-to-weight ratio of Sudan III

to copolymer in 0.1% L121/0.1% F127 mixed aggre-

gates is approximately 105 mg/g (10.5%). Therefore,

these mixtures of hydrophobic and hydrophilic PBC

can be used to improve the solubilization of poorly

soluble molecules.

4. Conclusions

In previous studies it was reported that aggregates

of hydrophobic lamella-forming PBC have higher

solubilization capacity than aggregates of hydrophilic

1/F127 mixtures (n) prepared with sonication for 2 min. (a) UV/Vis

(2), and in mixture of 0.2% L121/0.2% F127 (3). (b) Amount of

ion.


PBC that form spherical micelles. By properly

selecting the mixtures of a hydrophilic and hydro-

phobic PBC, it has been demonstrated that relatively

stable dispersions with relatively small particle sizes

can be produced. These dispersions have a higher

solubilization capacity with respect to a poorly

soluble organic compound compared to a dispersion

of hydrophilic PBC alone. A qualitative explanation

of the dispersion behavior of PBC mixtures has been

proposed, which facilitates selection of the mixture

components displaying high colloidal stability, small

particle size and good solubilization characteristics.

These findings may be useful for pharmaceutical

formulation development, particularly, for improve-

ment of solubilization capacity and stability of poly-

mer micelles. Furthermore, these mixtures can be

useful for preparation of stable pharmaceutical for-

mulation of hydrophobic PBC, which recently

attracted significant attention as functional excipients

in drug and gene delivery studies [5].

Acknowledgements

This work was in part supported by NSF DMR

award (0071682) to A.V. Kabanov. K.T. Oh has been

supported by a UNMC Fellowship. The authors

would like to thank R. Nessler (University of Iowa,

Central Microscopy Research Facility) for carrying

out cryo-TEM experiments and Dr. V. Alakhov

(Supratek Pharma Inc., Montreal, Canada) for valuable

discussions.

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micellar formulations for drug delivery based on mixtures of hydrophobic and hydrophilic pluronic®...

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