In vitro characterization studies of selfmicroemulsified bosentan systems

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In vitro characterization studies of self-
microemulsified bosentan systems
Harini Chowdary Vadlamudi, Prasanna Raju Yalavarthi, Basaveswara Rao M.
V., Arun Rasheed & Tejeswari N.
To cite this article: Harini Chowdary Vadlamudi, Prasanna Raju Yalavarthi, Basaveswara
Rao M. V., Arun Rasheed & Tejeswari N. (2017): In vitro characterization studies of self-
microemulsified bosentan systems, Drug Development and Industrial Pharmacy, DOI:
10.1080/03639045.2017.1287720
To link to this article: http://dx.doi.org/10.1080/03639045.2017.1287720
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2. RESEARCH ARTICLE
In vitro characterization studies of self-microemulsified bosentan systems
Harini Chowdary Vadlamudia,b
, Prasanna Raju Yalavarthic
, Basaveswara Rao M. V.d
, Arun Rasheede
and
Tejeswari N.c
a
Department of Pharmaceutics, Centre for Research Studies, Krishna University, Machilipatnam, India; b
Department of Pharmaceutics, PES
College of Pharmacy, Bangalore, India; c
Division of Pharmaceutics, Sri Padmavathi School of Pharmacy, Tirupati, India; d
Department of
Chemistry, Krishna University, Machilipatnam, India; e
Department of Chemistry, Al-Shifa College of Pharmacy, Poonthavanam, India
ABSTRACT
Context: Bosentan is a poorly soluble drug and pose challenges in designing of drug delivery systems.
Objective: The objective of this study is to enhance the solubility, dissolution and shelf-life of bosentan by
formulating it as S-SMEDDS capsules.
Materials and methods: Solubility of bosentan was tested in various liquid vehicles such as oils (rice bran
and sunflower), surfactants (span 20 and tween 80) and co-surfactants (PEG 400 and propylene glycol) and
microemulsions were developed. Bosentan was incorporated into appropriate microemulsion systems
which were previously identified from pseudo ternary phase diagrams. Bosentan-loaded SMEDDS were
evaluated for drug content, drug release, zeta potential, and droplet size. The selected liquid SMEDDS
were converted into solid SMEDDS by employing adsorption and melt granulation. Solid SMEDDS were
characterized for micromeritics and evaluated for drug content, drug release, and shelf-life.
Results: Isotropic systems R5, R13, S5, and S13 with submicron droplet size had exhibited 85.45, 94.12,
81.67, and 96.64% drug release, respectively. Solid SMEDDS of MR13 and AS13 formulations with rapid
reconstitution ability, exhibited 84.85 and 86.74% of on par drug release. The formulations were physico-
chemically intact for 1.02 and 1.56 years.
Discussion: Liquid SMEDDS composed with PEG400 had displayed optimal characters. Solid SMEDDS had
high-dissolution profiles than bosentan due to modification in the crystalline structure of drug upon
microemulsification.
Conclusion: Thus, solid SMEDDS addressed the solubility, dissolution, and stability issues of bosentan and
becomes an alternate for clinical convenience.
ARTICLE HISTORY
Received 29 September 2016
Revised 7 December 2016
Accepted 13 January 2017
KEYWORDS
Phase diagram;
microemulsion; zeta
potential; bioavailability;
endothelin
Introduction
Hypertension is the foremost threat and accounted for 9.4 million
deaths and 7% of disability adjusted life years (DALYs) in 2010
worldwide1
. Among the types of hypertension, pulmonary arterial
hypertension (PAH) is life-threatening with 20% mortality2
. Till
date, the enormity of research was not published on PAH.
Decreased release of vasodilators such as nitric oxide and prosta-
cyclin, and stimulation of vasoconstrictors such as thromboxane
and vascular endothelial growth factor (VEGF), led to the progres-
sive increase of pulmonary vascular constriction which subse-
quently lead to ventricular failure and premature death3
.
Conventional pharmacotherapy of PAH is aimed to increase the
release of prostacyclin and nitric oxide4
. Epoprostenol, a first
therapeutic moiety having such mechanism of action, was used
against PAH. Due to severe adverse-effects, the clinical usage of
epoprostenol was limited5
. Later on, treprostinil, iloprost, and bera-
prost found their effectiveness and also ineffectiveness in PAH
treatment6
. Clinical use of sildenafil and tadalafil was limited
owing to cost constraints7
. Inhaled NO can be an alternative treat-
ment but it is restricted to intensive care unit setting8
. Bosentan is
a potential inhibitor of the biosynthesis of protein matrix and sys-
temic sclerosis fibroblasts proliferation9,10
, hinders the vascular
smooth muscle cells proliferation11
. It was proven that bosentan
can prevent the deposition of collagen in pulmonary arteries12
,
and restrain the transcription factor nuclear factor-jB which arbi-
trates the tissue inflammation13
.
Bosentan, a BCS class-II moiety possesses moderate log P (3.8),
poor aqueous solubility (1 mg/100 ml), low bioavailability (50%),
and significant oral absorption in the presence of fat meal. Owing
such physico-chemical properties, bosentan becomes an interest
of research to develop effective oral dosage forms14
. Much
research was progressed in addressing solubility associated issues
of drug candidates in the contemporary periods. Among them,
approaches like complexation with cyclodextrins15
, microniza-
tion16
, solid dispersions17
, and nanonization18
have proved their
efficiencies appropriately19
.
In the past decennium, lipid-based drug delivery system
(LBDDS) approach gained its potential to enhance solubility
thereby dissolution efficacy because the energy input associated
with a solid–liquid phase transition can be avoided and thus sur-
mounting the slow dissolution progression after oral administra-
tion. Self-microemulsifying drug delivery systems (SMEDDS) belong
to class III b of LBDDS classification20
. SMEDDS are isotropic mix-
tures of oils (natural or synthetic), surfactants (solid or liquid), or
one or more hydrophilic solvents alternatively, and co-solvents/co-
surfactants which forms fine microemulsions (o/w type) upon gen-
tle agitation and dilution in aqueous GI fluid media21
. The mech-
anism by which enhanced absorption in GIT occurs, include
expediting the process of dissolution, decrease in the size of drug
CONTACT Harini Chowdary Vadlamudi vadlamudi.harini@gmail.com Centre for Research Studies, Krishna University, Machilipatnam 521001, India
Supplemental data for this article can be accessed here.
ß 2017 Informa UK Limited, trading as Taylor & Francis Group
DRUG DEVELOPMENT AND INDUSTRIAL PHARMACY, 2017
http://dx.doi.org/10.1080/03639045.2017.1287720

3. particles to molecular level, assisting solubilized phases formation,
formation of solid-state solution within the carrier, altering the
drug uptake, efflux and disposition by varying enterocyte-based
transport, and augmenting the drug transport via intestinal lymph-
atic system to the systemic circulation22–24
. These factors contrib-
uted in the selection of bosentan as model drug and this research
was focused on formulation and in vitro assessment of bosentan
solid SMEDDS.
Materials and methods
Materials
Bosentan was a gratis of M/s. Aurobindo Pharma Ltd., Hyderabad,
India. Double-refined olive, coconut, palm, ricebran, sun flower,
and gingelly oils were procured from the supermarket. Glycerin
and propylene glycol were purchased from Merck Specialties Ltd.,
Mumbai, India. Oleic acid, isopropyl myristate, PEG 400, PEG 600,
span 20, span 80 tween 20, and tween 80 were purchased from
SD Fine Chem Ltd., Mumbai, India. All other materials used in the
study were of pharmaceutical and analytical grade.
Methods
Solubility study
Solubility of bosentan was assessed in various liquids. Coconut oil,
gingelly oil, isopropyl myristate (IPM), oleic acid, olive oil, palm oil,
rice bran oil, sun flower oil, glycerin, PEG 400, PEG 600, propylene
glycol, span 20, span 80, tween 20, and tween 80 were selected as
liquid vehicles. Suitability of the liquid vehicles as solvent, surfac-
tant, and co-surfactants for the development of SMEDDS was
studied. An excess of bosentan was added to 5 ml of each liquid
vehicle in glass vials and vortexed for 30 s. Mixtures in glass vials
were equilibrated for 72 h at room temperature and centrifuged
for 15 min at 3000 rpm. Clear supernatant portion was collected
and filtered through 0.40 mm membrane. The filtrate was assayed
at 276 nm (Shimadzu UV 1700, Shimadzu Corporation, Tokyo,
Japan) upon suitable Beer’s dilutions.
Pseudo-ternary phase diagram
Suitable liquid vehicles from oils (sunflower oil and rice bran oil),
surfactants (span 20 and tween 80), and co-surfactants (propylene
glycol and PEG 400) selected to envisage the pseudo-ternary
phase diagrams. Varied ratios of volumes of oil to surfactant/cosur-
factant (S:CoS mix) were used in between 1:9 and 9:1. Distilled
water was added in increments to the systems of oil/S:CoS mix.
The mixtures were vortexed after each addition of water to obtain
isotropic systems and continued until the turbidity appears as end
point. The data of composition of transparent mixtures (microe-
mulsions) thus obtained were used to construct pseudo-ternary
phase diagrams.
Preparation of drug loaded liquid SMEDDS
Measured quantity of bosentan was added to the oil phase in a
graduated flask and stirred thermostatically. In another glass vial,
selected surfactant and co-surfactants were mixed at predeter-
mined ratios. The contents of graduated flask were transferred
carefully to the glass-vial containing S:CoS mix. The components
of glass-vial were agitated gently and equilibrated quickly by vor-
tex mixing at 37
C. The phase margin was drawn by noticing the
changes like conversion of sample from turbid to transparency or
from transparency to turbid. The composition of liquid SMEDDS is
given in Table 1.
Evaluation of liquid SMEDDS
Drug content
From each microemulsion formulation, 1 ml sample was collected
accurately, then diluted suitably with methanol and filtered
(0.45 mm). Concentration of bosentan present in the microemulsion
formulations was analyzed spectrophotometrically at 276 nm.
In vitro drug release
Liquid SMEDDS of bosentan (equivalent to 50 mg) were filled into
“size 0” hard gelatin capsules. In vitro drug release studies from
liquid SMEDDS were performed using 900 ml of 0.1N HCl as dissol-
ution medium at 37 ± 0.5
C in USP type I dissolution apparatus. At
predetermined time intervals, a 5 ml aliquot of dissolution medium
was withdrawn and assayed at 276 nm. The experiment was
repeated for six independent observations by maintaining the sink
condition.
Characterization of liquid SMEDDS
Fourier transform infra-red spectroscopy (FT-IR)
Chemical compatibility of bosentan with selected liquid
vehicles was analyzed by using attenuated total resonance
(ATR) FT-IR spectrophotometer (Agilent CARY 630 ATR-FTIR,
Table 1. Composition and drug content of liquid SMEDDS.
Formulations Smix, ratio (S: CoS)
% weight
of oil
% weight
of Smix
% drug
content
S1 Span 20:PG 1:1 17.77 78.8 50.06 ± 0.55
S2 1:2 16.04 83.96 58.30 ± 0.48
S3 2:1 13.88 86.12 70.71 ± 0.19
S4 2:3 18.18 81.82 63.26 ± 0.60
S5 Span 20: PEG 400 1:1 15.85 84.15 74.11 ± 0.48
S6 1:2 17.44 84.56 59.60 ± 0.46
S7 2:1 8.8 91.2 63.00 ± 0.60
S8 2:3 11.45 88.55 62.22 ± 0.55
S9 Tween 80: PG 1:1 16.47 83.53 75.42 ± 0.65
S10 1:2 14.28 85.72 70.13 ± 0.14
S11 2:1 11.23 88.77 67.71 ± 1.11
S12 2:3 9.89 90.11 65.09 ± 0.43
S13 Tween 80: PEG 400 1:1 18.36 81.64 78.24 ± 0.54
S14 1:2 12.22 87.78 70.19 ± 0.32
S15 2:1 13.33 86.67 70.98 ± 0.99
S16 2:3 16.27 83.73 55.94 ± 0.24
R1 Span 20:PG 1:1 12.9 87.1 53.92 ± 0.59
R2 1:2 8.88 91.12 76.60 ± 0.40
R3 2:1 12.35 87.65 68.49 ± 0.39
R4 2:3 14.6 85.4 55.94 ± 0.60
R5 Span 20: PEG 400 1:1 11.36 88.64 73.85 ± 0.48
R6 1:2 9.67 90.33 86.14 ± 0.96
R7 2:1 12.22 87.78 60.65 ± 1.60
R8 2:3 13.63 86.37 68.75 ± 0.53
R9 Tween 80: PG 1:1 10.34 89.66 56.86 ± 0.45
R10 1:2 13.04 86.96 50.84 ± 0.14
R11 2:1 15.73 84.27 54.77 ± 1.01
R12 2:3 7.86 92.14 76.60 ± 0.93
R13 Tween 80: PEG 400 1:1 8.6 91.4 76.47 ± 0.59
R14 1:2 10.84 89.16 64.18 ± 0.12
R15 2:1 10.86 89.14 50.45 ± 0.09
R16 2:3 11.22 88.78 58.56 ± 0.14
Values are expressed as mean ± SD n ¼ 6.
2 H. C. VADLAMUDI ET AL.

4. Agilent, Newark, DE). A sample of material was placed on the dia-
mond ATR crystal and analyzed by using Agilent resolutions pro
software (Agilent, Newark, DE). Each spectrum of sample was col-
lected from 32 single average scans at a resolution of 4 cmÀ1
in
the absorption region of 600–4000 cmÀ1
.
pH and viscosity
A measured (10 ml) sample of liquid SMEDDS was tested for its
hydronium ion concentration (pH) by membrane electrode (ELICO
LI 200, Hyderabad, India). The viscosity of SMEDDS was deter-
mined using Brookfield viscometer, Middleboro, MA (LDLV-E
Model) using spindle #63 at 10 rpm torque.
Zeta potential and droplet size analysis
Zeta potential and droplet size distributions were determined by
using Zeta sizer (HSA 3000, Horiba Scientific, Singapore,
Singapore). SMEDDS was diluted with excess amount of water in a
graduated vial, gently mixed by inverting the vial to form a fine
dispersion. The microdispersion was equilibrated for 30 min and
zeta profiles were estimated. Polydispersity index was also calcu-
lated using the equation, -DM¼ Mw/Mn, in which Mw is the weight-
average molar mass and Mn is the number-average molar mass.
Preparation of solid SMEDDS
The liquid SMEDDS displayed promised drug release was con-
verted into solid SMEDDS by employing adsorption and melt
granulation techniques. In adsorption technique, liquid SMEDDS
were allowed to adsorb onto an inert carrier aerosil 200.
Quantified aerosil 200 powder was admixed with liquid SMEDDS
until it becomes free flowing powder, sieved through #80 mesh
and vacuum desiccated. On the contrary, a measured quantity of
liquid SMEDDS was added drop-wise into the molten mass of PEG
4000. The resulting mixture was cooled and sifted through #22
mesh. Solid SMEDDS equivalent to 50 mg of bosentan was filled
into size “1” hard gelatin capsules. They were stored in vacuum
desiccator for further use25,26
. The compositions of S-SMEDDS are
presented in Table 2.
Evaluation of solid SMEDDS
Micromeritic analysis
Before encapsulation, a batch of S-SMEDDS was subjected to angle
of repose, bulk density, tapped density, and Hausner ratio
measurements.
Drug content
The solid SMEDDS filled-in capsule was dissolved in methanol
($100 ml) employing ultra-sonication (Thermolab, Waltham, MA).
The resulting solution was filtered through 0.45 mm nylon filter
and assayed.
In vitro drug release of solid SMEDDS
Rate and extent of bosentan release from solid SMEDDS were
studied by following the validated in vitro dissolution study proto-
cols using USP type I dissolution test apparatus. Solid SMEDDS
filled in capsule was inducted into the dissolution medium.
Aliquots of 5 ml dissolution medium were withdrawn at predeter-
mined time intervals, filtered, and amount of drug released was
estimated.
Scanning electron microscopy (SEM) of solid SMEDDS
The surface morphology of bosentan and solid SMEDDS (AR13
and MR13) were examined using a scanning electron microscope
(Thermoscientific SU 1510, Waltham, MA). The samples were fixed
to a brass specimen club using double-sided adhesive tape made
electrically conductive by coating with platinum using a
Thermoscientific Ion sputter (Thermoscientific SU 1510, Waltham,
MA) for 300 S at 15 Ma.
Stability studies
Bosentan S-SMEDDS filled in size capsules were placed in stability
chambers at 25
C/60% RH, 45
C/65% RH, and 60
C/75% RH for
90 d by following ICH zone-III guidelines for intermediate testing
of dosage forms. Samples in duplicate were withdrawn at 0, 1, 2,
and 3 months to evaluate their physical and chemical stabilities.
The physical stability was evaluated visually for physical changes
such as phase separation and drug precipitation upon dilution
with water. Chemical stability was expressed as the content of
bosentan present in the samples. Shelf-life of the formulations
were calculated by using the formulae t90 ¼0.1052/K, where K rep-
resents first-order rate constant.
Results
Solubility study
The solubility results in various vehicles are represented in
Figure 1. Based on the solubility data, rice bran oil (10.94 mg/ml),
and sunflower oil (9.52 mg/ml) as lipid vehicles; span 20
(12.71 mg/ml) and tween 80 (11.68 mg/ml) as surfactants; PEG 400
(12 mg/ml) and propylene glycol (7.59 mg/ml) as co-surfactants
were selected to develop the microemulsions.
Drug content
The drug content was in the range of 50–78% in rice bran oil and
sunflower oil formulations. The values are given in Table 1.
Table 2. The composition and drug content of S-SMEDDS.
Method Formulation
Liquid SMEDDS equivalent
to 50 mg of bosentan Aerosil 200 (mg) PEG 4000 (mg) Talc (mg) Percent of drug content
Adsorption AR5 S5 350 – 20 77.27 ± 0.68
AR13 S13 350 – 20 86.96 ± 0.87
AS5 R5 – 320 20 82.56 ± 0.78
AS13 R13 – 320 20 61.36 ± 0.46
Melt granulation MR5 S5 350 – 20 69.69 ± 0.75
MR13 S13 350 – 20 75.32 ± 0.87
MS5 R5 – 320 20 73.82 ± 0.98
MS13 R13 – 320 20 71.66 ± 0.76
Values are expressed as mean ± SD n ¼ 6.
DRUG DEVELOPMENT AND INDUSTRIAL PHARMACY 3

5. In vitro drug release
The in vitro drug release from the liquid dispersions is represented
in Figure 2. The results were evident that S5, S13, R5, and R13 for-
mulations had displayed higher drug release values of 83.67,
94.12, 85.45, and 96.64%. They were selected for further studies.
FT-IR
FT-IR spectra of bosentan SMEDDS showed peaks in the range of
3300–3500 cmÀ1
, 2900–2800 cmÀ1
, and 1260–1000 cmÀ1
indicating
secondary amine N–H bending, CC stretching, and C–O stretch-
ing, respectively, which is similar to the finger print region of
bosentan monohydrate. SMEDDS formulations showed a little vari-
ation in the wave numbers compared with bosentan but within
the same range. However, primary alcohol free O–H stretching at
3628 cm À1
and S–N asymmetric stretching at 1300 cmÀ1
present
in bosentan was absent in the SMEDDS formulation. Overall results
showed that there might be a negligible chemical interaction
between the drug and excipients.
pH and viscosity
The pH and viscosity values are given in Table 3. The pH of the
liquid SMEDD formulation was observed as 7 (neutral pH) and vis-
cosity in the range of 255–317 cps.
Zeta potential and droplet size analysis
The droplet size and PDI results represented in Table 3 indicate
that the droplet size was in the range of 6–489 nm and PDI of
Figure 1. Solubility of Bosentan in various vehicles.
Figure 2. % drug release from liquid SMEDDS formulations.
Table 3. Physicochemical properties of selected liquid SMEDDS.
Formulation pH
Viscosity
(mpa)
Droplet
size (nm)
Polydispersity
index
Zeta potential
(mV)
S5 7.1 255.5 431.0 0.604 À72.2
S13 7.3 316.2 356.4 0.830 À44.0
R5 7.0 297.4 6.2 0.729 À95.8
R13 7.1 275.5 488.7 0.481 À59.3
4 H. C. VADLAMUDI ET AL.

6. 0.4–0.8. The zeta potential values were in the range of À44 to
À95 mV as mentioned in Table 3.
Evaluation of solid SMEDDS
Micromeritic analysis
The micromeritics of S-SMEDDS are given in Table 4. Angle of
repose values of the powders was in the range of 22–29
that
indicated excellent flow property of S-SMEDDS. The bulk densities
of the formulations were in the range of 0.39–0.52 g/ml and
tapped density in between 0.44 and 0.6. Hausner’s ratio was found
to be around 1.1.
Drug content
The content of bosentan on an average in the liquid SMEDDS was
present in between 54 and 87% without much deviation. The data
given in Table 2 revealed the uniformity of drug content in the
formulations.
In vitro drug release of solid SMEDDS
The outcome of in vitro dissolution study of S-SMEDDS was evi-
denced that the AS13 formulation obtained from S13 of liquid
SMEDDS had the highest dissolution profile with 86.74% drug
release followed by MR13 with 84.85% at the end of 60 min as
demonstrated in Figure 3.
SEM of solid SMEDDS
SEM images of bosentan and promised solid SMEDDS namley
AR13 and MR13 of the highest drug release are shown in Figure 4.
Well-separated particles with definite shape appeared in SEM
images of S-SMEDDS in submicron range.
Stability studies
The stability testing of S-SMEDDS filled capsules was carried out in
triplicate. The capsules had shown some physical variations at the
end of 90 days storage with stress conditions. A little deformation
of capsules was observed at 60
C/75% RH condition. The capsules
kept at 25
C/60% RH and 45
C/65% RH conditions were found
intact. There was no significant change in the drug content of for-
mulations kept at 25
C/60% RH and 45
C/65% RH conditions. The
difference in the drug release was also not significant in the for-
mulations stored at all stress conditions. The mean shelf-lives (t90)
of AR13 and MR13 formulations were noticed as 1.56 and 1.02
years, respectively
Discussion
In the field of SMEDDS, many novel excipients have been
exploited for their specialties in recent times. Miscibility of drug in
the vehicle, solvent capacity of vehicle, self-dispersibility, capacity
to promote self-emulsification, regulatory clearances, and cost are
pivotal issues in exploring them. By following those lines, the solu-
bility of bosentan in selected oils, surfactants, and co-surfactants
was assessed to testify the miscibility and solvent capacity charac-
ters of liquid vehicles. Triglyceride vegetable oils were chosen as
oil phase owing to regulatory issues like safety as they are easily
digested and absorbed. Bosentan exhibited the highest solubility
in oils–rice bran oil (10.94 mg/ml) and sunflower oil (9.52 mg/ml).
These long-chain fatty acids stimulate the lipoprotein synthesis,
subsequently lymphatic absorption and thus increased bioavail-
ability of bosentan was expected. In other means, if selected oil
has good solubility of drug, in turn volume of oil required to dis-
solve the desired quantity of drug becomes less. Surfactants and
cosurfactants with HLB value 12 forms micelles even at low con-
centrations. Surfactants – span 20 (12.71 mg/ml), tween 80
(11.68 mg/ml); cosurfactants – PEG 400 (12 mg/ml) and propylene
glycol (7.59 mg/ml) were thus selected owing to their specialties.
Non-ionic surfactants were preferred as they are less affected by
changes in pH and ionic strength.
Concentrations of oil, ratio of surfactant, and cosurfactant play
key role in the formation of stable microemulsion.
Table 4. Micromeritic properties of S-SMEDDS.
Formulation
Angle of
repose (
)
Bulk density
(g/ml)
Tapped density
(g/ml)
Hausner’s
ratio
AS5 25.23 0.425 0.476 1.12
AS13 28.25 0.395 0.458 1.16
AR5 22.63 0.438 0.499 1.14
AR13 28.75 0.444 0.502 1.14
MS5 26.02 0.525 0.604 1.15
MS13 29.32 0.425 0.445 1.04
MR5 27.78 0.428 0.475 1.11
MR13 26.27 0.427 0.508 1.19
Figure 3. % drug release from S-SMEDDS formulations.
Figure 4. SEM images of bosentan and S-SMEDDS formulations.
DRUG DEVELOPMENT AND INDUSTRIAL PHARMACY 5

7. Thermodynamically stable systems were selected from microemul-
sion regions of respective pseudoternary phase diagrams (as pro-
vided in supplemental data) based on their phase behavior. In this
study, poor microemulsions were formed due to the high compos-
ition of oils. Hence, concentration of oil was kept as low as pos-
sible compared with Smix. Upon incorporation of bosentan,
microemulsion region was narrowed due to its expansion that
necessitated the use of a higher S/CoS ratio for stabilization. Effect
of surfactant:cosurfactant ratio (namely 1:1, 1:2, 2:1, and 2:3) was
clearly observed that the increased surfactant concentration lim-
ited microemulsion region. Equal quantities of surfactant and
cosurfactant resulted with the isotropic microsystems without any
irreversible chemical interactions as demonstrated in spectra.
Bosentan being a lipophilic moiety had moderate uniformity of
its content in the liquid formulations which was due to emulsified
bosentan entrapment in microdomains of SMEDDS. The postula-
tion may be supported by low SD values.
The low solubility of bosentan at pH range of 1–5 could pose a
question on obtaining the sink conditions. Liquid SMEDDS filled-in
capsules on introduction to dissolution media at sink condition,
capsules get disintegrated to leave their microemulsified contents
towards bulk. But some of the liquid SMEDDS were immiscible
with dissolution media. In such cases, Washburn’s equation is the
right choice to ascribe the drug-release patterns. However, appar-
ent ampiphilicity of bosentan SMEDDS due to surfactant and
cosurfactant mixture enabled the miscibility with GI fluids. Release
of bosentan into GI fluid was observed to follow diffusion and
convection principles.
Diffusion ability of bosentan microemulsions might be preju-
diced by the type of oil used. The surfactant’s hydrophobic group
and lipids of microemulsion may interact with the mucin of GI epi-
thelial layer in turn act as mucoadhesive nanoparticles. Alternate
possibility is PEG groups of surfactants surround the microemul-
sion surface thereby circumvent the interaction of microemulsions
with mucin and thus resulting in microemulsions that act as
mucus penetrating nanoparticles. The diffusion of drug from the
microemulsion droplets might also be affected by the droplet size.
This ambivalent status of microemulsions steered to the assump-
tion of two possible mechanisms. First case is that microemulsions
have no interactions with mucin and stay intact in the mucus
layer. If the microemulsion droplet size is small, they will diffuse
among the mucus meshes’ freely thus acting as mucus penetrat-
ing nanoparticles. In second case, microemulsions would interact
with mucins and behave as mucoadhesive nanoparticles. In both
the situations, bosentan loaded in microemulsions might be trans-
ported to the epithelium by the nanocarriers or diffuse through
the mucus layer in the form of free molecules27
.
Due to diffusion mechanism, the highest percent of drug was
released from formulations (S5, S13, R5, and R13) as PEG 400 cov-
ers the surface of microemulsion droplet and thus preventing its
interaction with mucin.
Influence of the droplet size on the magnitude of affinity
between droplets and GI mucosa has been formerly explored by
Gershanik et al.28
, who suggested that the best possible droplet
size must be in the range of 100–500 nm. Furthermore, it was
interpreted that microemulsions have a wide range of micro-drop-
let sizes because of the physical energy existing in the medium,
like the kinetic energy provided by the rotating paddle of the dis-
solution tester and in turn resulted with varied polydispersity indi-
ces. The polydispersibility index value above 0.5 and below 1
indicated that distribution of heterogeneous monodispersion in
SMEDDS due to slightly thick film of surfactant over the droplets.
Electrostatic forces of microemulsion droplets are critical for
assessing the stability of the SMEDDS formulation. An increase in
the electrostatic repulsive forces between microemulsion droplets
prevents the coalescence of droplets, and a decrease of electro-
static repulsive forces may cause phase separation. The globules
of bosentan SMEDDS displayed negative values of zeta potential
due to the presence negatively charged free fatty acid compo-
nents of both sunflower and rice bran oils. The formulations S5
exhibited high zeta potentials due to high affinity of span 20
towards bosentan’s surface with subsequent electric double-layer
establishment. There were several reports on the role of zeta
potential in the interactions of components of formulations with
the mucus of GIT. Since the emulsified droplets produced by
SMEDDS have high zeta potential, they are likely to facilitate intes-
tinal absorption of drug resulting in enhanced bioavailability29
.
As density is a key determinant in packing of powders, size “1”
capsules were selected for the encapsulation of solid SMEDDS
based on the values Hausner’s ratio and angle of repose.
Higher drug content values of S-SMEDDS indicated that less
amount of drug loss during solidification process due to inert and
hydrophilic characters of the carriers. In adsorption technique,
adsorbent aerosil 200 used was highly porous in nature and spe-
cific surface available is 200 m2
so that a large amount of liquid
gets adsorbed. Due to the capillary forces, the liquid is trapped
inside the pores and hence leaking was not observed and thus
promoted higher drug content. In melt granulation, bosentan was
admixed with the molten mass. By entrapping the liquid formula-
tion inside a pegylated solid matrix, the loss of drug was nullified
and the chemical stability of the formulation thus improved.
Outcome of dissolution study indicated that the S-SMEDDS
obtained from adsorption process had profound drug release
than that of S-SMEDDS of melt granulation. In adsorption tech-
nique, liquid SMEDDS adsorbed onto the surface of carrier. The
voids formed in remaining surface of carrier allowed the drug to
release through them. In melt granulation, hydrophilic base was
required to be melted first to release its drug from the solid
matrix. More than 75% of the drug was released within 60 min
from the selected liquid SMEDDS and but it was diminished after
the solidification. The silica (aerosil 200) is a potential proton
donor and acceptor due to silanol groups on its surface30
. It is
also postulated that the induction of low energy bonds like van
der Waal forces between adsorbent’s surface and hydrophobic
bosentan in SMEDDS. Furthermore, such forces are expected to
induce drug aggregation or coalescence, particularly in case of
SMEDDS containing hydrophobic moieties that are at equilibrium
solubility31
.
Lipid excipients, surfactants, and co-surfactants used in the for-
mulation are usually inert to impart the stability to the formula-
tions. In this study, there was a little deformation of capsules
which were stored at high relative humidity (75%) condition that
indicated the absorption of environmental moisture by gelatin, a
hydrophilic component of capsule shell. Thus, the drug content
and drug release of SMEDDS were affected.
Conclusions
The liquid SMEDDS of bosentan were developed by considering
solvent characteristics of oils, surfactants, and co-surfactants in
various proportions. The promised liquid SMEDDS were selected in
terms of drug content and its release. Bosentan liquid SMEDDS
were converted successfully into S-SMEDDS by adsorption and
melt granulation techniques to circumvent their stability limita-
tions. For consumers’ compliance, capsules of bosentan S-SMEDDS
were designed as unit dosage forms. The integrity of capsules of
AR13 and MR13 S-SMEDDS was maintained during their shelf-life
6 H. C. VADLAMUDI ET AL.

8. (t90) about 1.25 and 1.06 years and confirmed their extended sta-
bility. However, one cannot predict the drug release just from the
in vitro characteristics as in vivo drug-release invariably depends
on the effects of bile, GI motility, enzymatic action, etc. Hence, it
is required to extend the study on in vivo means to confirm the
clinical effectiveness of bosentan S-SMEDDS for therapeutic and
commercial applications.
Acknowledgements
The authors are thankful to M/s. Aurobindo Pharma Ltd.,
Hyderabad, for supplying of the sample of bosentan.
Disclosure statement
The authors report no declarations of interest.
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