Photochemistry of a novel antimuscarinic drug fesoterodine and identification of its photodegradation products by LC-ESI-MS studies

Article abstract of DOI:10.1016/j.jphotochem.2013.02.003

Fesoterodine (FESO) is a novel muscarinic receptor antagonist for the treatment of overactive bladder syndrome. The aim of this work was to study the photodegradation of FESO, to determine its kinetics and to identify the photodegradation products. The photochemistry of FESO was investigated in sample solutions exposed to a UV-A (320-400 nm) and UV-C irradiation (100-280 nm) at room temperature. The photodegradation process was monitored by means of liquid chromatography method equipped with monolithic column and photodiode array detector. This drug is more photolabile under UV-C light. In methanol-water solution (1:1, v/v), approximately 62.5% of FESO decomposes after 60 min of UV-C irradiation, whereas under 6 h of UV-A light only 4.9% decomposes. FESO was shown to be photolabile and its photodegradation reaction followed the zero-order kinetics with the rate constant k = 0.5503 min^-1, and t_1/2 and t_90% obtained were 46.92 min and 9.38 min, respectively. The main degradation products were isolated by semi-preparative liquid chromatography and identified by liquid chromatography coupled to an electrospray ionization mass spectrometry. The powdered and intact tablets were also exposed to UV-C irradiation. Based on the obtained results, a complete drug photodegradation pathway was proposed.

Full text of DOI:10.1016/j.jphotochem.2013.02.003

Journal of Photochemistry and Photobiology A: Chemistry 256 (2013) 16–22
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Photochemistry of a novel antimuscarinic drug fesoterodine and identification of
its photodegradation products by LC–ESI–MS studies
Maximiliano S. Sangoi∗, Vítor Todeschini, Gustavo K. Goelzer, Martin Steppe
Laboratory of Research in Pharmaceutical Quality Control, Faculty of Pharmacy, Federal University of Rio Grande do Sul, 90610-000 Porto Alegre, RS, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 August 2012
Received in revised form 19 January 2013
Accepted 9 February 2013
Available online 19 February 2013
Fesoterodine (FESO) is a novel muscarinic receptor antagonist for the treatment of overactive bladder
syndrome. The aim of this work was to study the photodegradation of FESO, to determine its kinetics
and to identify the photodegradation products. The photochemistry of FESO was investigated in sample
solutions exposed to a UV-A (320–400 nm) and UV-C irradiation (100–280 nm) at room temperature.
The photodegradation process was monitored by means of liquid chromatography method equipped
with monolithic column and photodiode array detector. This drug is more photolabile under UV-C light.
In methanol–water solution (1:1, v/v), approximately 62.5% of FESO decomposes after 60 min of UV-C
irradiation, whereas under 6 h of UV-A light only 4.9% decomposes. FESO was shown to be photolabile and
Keywords:
Fesoterodine
Photochemistry
Isolation and identification
Degradation products
LC–ESI–MS studies
its photodegradation reaction followed the zero-order kinetics with the rate constant k = 0.5503 min−1
,
and t1/2 and t90% obtained were 46.92 min and 9.38 min, respectively. The main degradation products were
isolated by semi-preparative liquid chromatography and identified by liquid chromatography coupled to
an electrospray ionization mass spectrometry. The powdered and intact tablets were also exposed to UV-
C irradiation. Based on the obtained results, a complete drug photodegradation pathway was proposed.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
drug formulations [7]. The second aspect of drug-light interactions
is that of the biological effects caused by the reaction of drugs, pho-
Fesoterodine (FESO), 2-(3-(diisopropylamino)-1-phenylpropyl)
-4-(hydroxyl-methyl)phenyl isobutyrate, is an oral, nonselec-
tive antimuscarinic that is rapidly and extensively converted
by ubiquitous nonspecific esterases to its active metabolite,
5-hydroxymethyltolterodine (5-HMT), indicated for overactive
bladder (OAB) treatment [1,2]. OAB is defined by the International
Continence Society as a syndrome associated with symptoms of
urgency with or without urgency urinary incontinence, which is
usually accompanied by frequency and nocturia [3].
The interest in the photostability studies has been increasing
steadily because is an integral part of the drug development process
and are widely recognized as one of the most important proce-
dures in registration of pharmaceutical products [4]. Knowledge of
the photochemical and photophysical properties of the compound
is necessary for appropriate handling, packaging and labeling the
drug substance and drug product [5]. Most pharmaceuticals contain
chromophores which absorb energy in the UV region through the
excitation of electrons within the molecular structure to a higher
energy state [6]. Radiation has two main effects on drugs. The first
is the influence of light on the stability of the drug substances and
toproducts or metabolites of drugs with light and biomolecules,
resulting in drug induced photosensitivity [8,9]. Nowadays, some
studies have been published about photochemical of the pharma-
ceuticals, identification of its degradation products and proposition
of its degradation pathways [10–12].
Preliminary stability investigations realized by our research
group revealed that FESO undergo degradation upon exposure to
light, and its photolability was established by forced degradation
testing. The stability-indicating LC method was developed and val-
idated for determination of FESO in commercial tablet dosage form
and forced degradation studies (hydrolysis, oxidation and photo-
lysis) have been reported; however, the photodegradation kinetics
of FESO in pharmaceutical formulations and the identification of
all photoproducts were not described [13]. Moreover, the drug was
also determined by validated liquid chromatography–tandem mass
spectrometry and capillary electrophoresis methods, but the degra-
dation products were not fully identified [14,15].
The identification of the chemical structures of the compounds
is not possible by LC alone. Therefore, the use of LC coupled to
electrospray ionization mass spectrometry (LC–ESI–MS) profiling
method which permits the rapid cataloging and identification of
potential degradants is an attractive alternative to rapidly char-
acterize degradation products and impurities [13,16]. Although
the traditional methods involving process scale-up, isolation, and
∗
Corresponding author. Tel.: +55 51 33085214.
E-mail address: maxsilvasangoi@yahoo.com.br (M.S. Sangoi).
1010-6030/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jphotochem.2013.02.003

M.S. Sangoi et al. / Journal of Photochemistry and Photobiology A: Chemistry 256 (2013) 16–22
17
purification of components such as degradants are expensive and
time-consuming, these methods, e.g., semi-preparative LC, are
essential for complete product isolation and identification.
The purpose of this study was to investigate the photochemical
transformation processes of FESO under UV-A and UV-C irradiation
in neutral conditions and to estimate the rate of the photodegra-
dation process by kinetic determinations. Besides, we also report
the structure identification of the main photoproducts by selective
LC–ESI–MS method, and the drug photodegradation pathway was
postulate.
2. Experimental
2.1. Chemicals
The fesoterodine fumarate reference substance was purchased
from Toronto Research Chemicals Inc. (Toronto, ON, Canada).
Toviaz® (Pfizer Inc., Zwickau, Germany) tablets for oral administra-
tion, containing 8 mg of fesoterodine fumarate per tablet (6.2 mg
fesoterodine base, excipients: glyceryl behenate, hypromellose,
indigo carmine aluminum lake, lactose monohydrate, soya lecithin,
microcrystalline cellulose, polyethylene glycol, polyvinyl alcohol,
talc, titanium dioxide, and xylitol), were obtained from commer-
cial sources. Phosphoric acid 85% and LC-grade acetonitrile and
methanol were obtained from Tedia Company Inc. (Fairfield, OH,
USA). Ammonium acetate was purchased from Spectrum Chem-
ical Corp. (Gardena, CA, USA). Ultrapure water (Milli Q Gradient
System, Millipore Corp., Bedford, MA, USA) was used for all the
analyses.
2.2. LC method
The LC apparatus was a Shimadzu LC system (Shimadzu
Corp., Kyoto, Japan) equipped with an SCL-10AVP system con-
troller, an LC-10ADVP binary pump, a SIL-10ADVP autosampler,
a CTO-10ACVP column oven, and an SPD-M10AVP photodiode
array (PDA) detector. The peak areas were integrated auto-
matically by computer using the Class VP software (v 6.12).
The experiments were performed on a reversed-phase Pheno-
menex Inc. (Torrance, CA, USA) Onyx C18 monolithic column
(100 mm × 4.6 mm i.d.). The Shimadzu LC system was operated
isocratically at controlled temperature (45 ◦C) using a mobile
phase of acetonitrile–methanol–0.03 mol L−1 ammonium acetate
(pH 3.8) (30:15:55, v/v/v), run at a flow rate of 2.4 mL min−1
with detection at 208 nm. The injection volume was 10 ␮L of a
solution containing 50 ␮g mL−1 for the reference substance and
sample solutions [13]. The quantitation was performed using the
absolute area of the peak. Standard curve was obtained by plot-
ting the peak area of FESO versus the theoretical concentration
over a range of 5–150 ␮g mL−1. The data were subjected to the
least squares regression analysis. Inspection of the plotted cal-
ibration curve described by equation: y = 11711.55x + 17019.76
and determination coefficient (r2 = 0.9995) confirmed that the
calibration curve was linear over the concentration range
[13].
Fig. 1. Plots of concentration (a; zero-order reaction), log of concentration (b;
first-order reaction) and reciprocal of concentration (c; second-order reaction) of
remaining fesoterodine versus time.
fine powder. Appropriated amounts were transferred into individ-
ual 50 mL volumetric flasks. After adding 30 mL of methanol, the
flasks were vortex mixed for 3 min. Then the samples were made
up to volume with methanol, transferred to appropriate tubes and
centrifuged at 3000 × g for 10 min. Aliquot of the clear supernatant
liquid at final concentration of 200 ␮g mL−1 of the active pharma-
ceutical ingredient was filtered through a 0.45 ␮m membrane filter
(Millipore Corp.).
2.3. Preparation of reference and sample solutions
2.4. Photodegradation studies
The stock solution of FESO was prepared by weighing accurately,
25 mg of fesoterodine fumarate (purity 98%) and diluted to volume
with methanol, obtaining a concentration of 1 mg mL−1 of fesotero-
dine base. The stock solution was stored at 2–8 ◦C, protected from
light and daily diluted to an appropriate concentration in mobile-
phase. To prepare the sample solutions, tablets containing 8 mg of
fesoterodine fumarate were accurately weighed and crushed to a
The effect of light was studied exposing the pharmaceutical
formulation solutions in 1 cm disposable cuvettes (Brand GmbH,
Germany). The light sources were an UV-A (center of emission
360 nm, bandwidth 40 nm) and an UV-C (emission spectra centered
at 254 nm) 30 W lamps (Philips, Amsterdam, Holland) fixed to the
chambers in a horizontal position, in order to compare the degra-
dation under the two different radiation sources. The chambers

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M.S. Sangoi et al. / Journal of Photochemistry and Photobiology A: Chemistry 256 (2013) 16–22
Table 1
Representation of kinetics models.
Reaction order
Kinetics parameters equations
Zero-order reaction
First-order reaction
Zero-order reaction
C = C0 − kt
t90% = 0.1C0/k
t90% = 0.106/k
t90% = 1/9kC0
t1/2 = C0/2k
t1/2 = ln 2/k
t1/2 = 1/kC0
ln C = ln C0 − kt
1/C = 1/C0 + kt
C, concentration; t, reaction time; C0, concentration of the reactants under consid-
eration at time zero; k, reaction rate constant; t1/2, half-life; t90%, time where 90% of
original concentration of the drug is left.
were internally coated with mirrors (16 cm × 16 cm × 100 cm) for
distributing the light uniformly. The distance between the lamps
and the samples were 10 cm. The temperature was controlled in
the chamber and kept always around 25 ◦C. A dark control was
carried out by exposing the FESO solutions in cuvettes, wrapped
in aluminum foil, for the same time period in both conditions.
2.5. Kinetics determination
The photodegradation kinetics of FESO in pharmaceutical for-
mulations was evaluated in methanol–water solution (1:1, v/v)
under the same conditions described earlier, but only the UV-C
lamp was used. The solutions of FESO in cuvettes were exposed
to the UV-C irradiation for time intervals of 0, 10, 20, 30, 40, 50,
and 60 min. Three samples were analyzed for each time interval.
After the required time, samples were diluted with mobile-phase
to 50 ␮g mL−1 for the LC determination. Placebo solution was pre-
pared at the same way to verify the influence of the excipients in the
degradation process. All solutions were injected in triplicate. Peak
purity tests were performed by the PDA detector. The concentration
of the remaining FESO determined at the different time intervals
were used in the plots. The plots were: values of concentration
versus time (zero-order reaction), log of concentration versus time
(first-order reaction), and reciprocal of concentration versus time
(second-order reaction). The determination coefficients (r2) were
obtained, and the best fit observed indicates the reaction order. The
kinetics parameters like apparent order degradation rate constant
(k), half-life (t1/2) and t90% (time where 90% of original concen-
tration of the drug is left) were obtained. The kinetics models are
represented in Table 1.
Fig. 3. Mass spectra obtained by direct injection into mass spectrometer of (a) fes-
oterodine (FESO) reference solution (2000 ng mL−1), (b) FESO after exposition to
UV-C light (t = 60 min), and (c) fraction of main photodegradation peak collected by
semi-preparative column. MS conditions are described in Section 2.
2.6. Isolation of main photodegradation peak by
semi-preparative LC
For the appropriate separation and isolation of main pho-
todegradation peak, different analytical conditions were employed.
Luna (Phenomenex Inc.) C18 (250 mm × 10.0 mm i.d.) semi-
preparative column packed with 5 ␮m particle size was employed
for better separation of the main degradants. The Shimadzu LC sys-
tem was operated isocratically at controlled temperature (40 ◦C)
using a mobile phase of acetonitrile–water (pH 3.0) (41:59, v/v),
run at a flow rate of 1.5 mL min−1 with detection at 208 nm. The
injection volume was 50 ␮L of a solution containing 3 mg mL−1
for the sample solution. After UV-C irradiation for 120 min, frac-
tions of photodegradation peak were collected at retention time
(tR) of 11.3 min in a 50 mL amber bottle. The isolated fractions hav-
ing purity greater than 99.0% when were analyzed by analytical LC,
using conditions mentioned in Section 2.2. The collected fraction
Fig. 2. Chromatogram (ꢀ = 208 nm) and absorption spectra of fesoterodine (FESO)
photodegraded solution after 60 min of UV-C irradiation obtained by monolithic
LC analysis. Experiments were performed using UV-C irradiation source and chro-
matographic system described in Section 2. MPP, main photodegradation peak; PP,
photodegradation product.

M.S. Sangoi et al. / Journal of Photochemistry and Photobiology A: Chemistry 256 (2013) 16–22
19
Fig. 4. LC–ESI–MS chromatograms of main photodegradation peak collected by semi-preparative column. MS was operated in SIR mode acquiring m/z values of 370.6, 311.5,
293.4, and 265.3. LC–ESI–MS conditions are described in Section 2. TIC, total ion count.
was concentrated on a rotaevaporator under vacuum. The residue
obtained was reconstituted with methanol for the analysis.
employed. The following protonated molecular ions were acquired
in SIR mode: m/z 412.7 for FESO; m/z 370.6, m/z 365.7, m/z 342.5,
m/z 311.5, m/z 293.4, m/z 284.5, and m/z 265.3 for photodegradation
products.
2.7. Identification of the main degradations products by
LC–ESI–MS
2.8. Tablets evaluation
The MS spectra were obtained by means of a triple quadrupole
mass spectrometer (Micromass UK Ltd., Manchester, UK), model
Quattro LC, equipped with an electrospray ionization (ESI) source
in positive mode using the Masslynx (v 3.5) software. The mass
spectrometer conditions were optimized with the direct injection
(syringe pump, Bioanalytical Systems Inc., West Lafayette, IN, USA)
of the FESO reference solution (2000 ng mL−1) into the system at
10 ␮L min−1. The best response was obtained with an ESI capil-
lary potential of 3.5 kV, cone voltage of 45 V, RF lens voltage of
0.4 V, source temperature of 120 ◦C, and ESI probe temperature of
500 ◦C, respectively. Degradation sample was injected directly into
the mass spectrometer and full scan spectra were acquired in the
mass-to-charge ratio (m/z) range between 100 and 600 atomic mass
unit (amu). When compared with the FESO mass spectrum, addi-
tional ions were observed changing the cone voltage to 80 V. The
selected ions were monitored in single-ion recording (SIR) mode
by the LC–ESI–MS system, carried out under the same analytical LC
conditions. An eluent split at approximately 1:10 post-column was
Toviaz® tablets were exposed to the UV-C lamp under two
forms: intact and powdered tablets. The presence of degradation
products was evaluated by LC (Section 2.2) after 1, 3, and 7 days of
exposition. After irradiation, the tablets were powdered, dissolved
in methanol and filtered using 0.45 ␮m nylon membrane before the
LC analysis. The final concentration was 50 ␮g mL−1
.
3. Results and discussion
The purpose of photostability testing is to provide evidence on
how the quality of a drug varies with the time under the influence of
the light. In this study, the photodecomposition of FESO was carried
out through the employment of a stress condition during different
periods of time. Many factors can affect the stability of a pharma-
ceutical product, such as the stability of the active ingredient, the
manufacturing process, the environmental conditions (heat, light,
and moisture during storage), as well as some chemical reactions

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M.S. Sangoi et al. / Journal of Photochemistry and Photobiology A: Chemistry 256 (2013) 16–22
Fig. 6. Time evolution of fesoterodine (FESO) concentration in methanol–water
solution (1:1, v/v). Experiments were performed using UV-C irradiation source and
chromatographic system described in Section 2. Inset: time evolution of the area
of main photodegradation peak (MPP) corresponding to photoproducts PP-4, PP-5,
PP-6 and PP-7.
phenomena are energy related, and consequently, they are be pre-
ceded by photons with specified wavelengths. On the other hand,
the rate of photodegradation reaction is dependent on light inten-
sity, so higher light intensity provides faster reaction rates. The
presence of chromophore groups in the chemical structure of FESO,
such as conjugated double bonds, and amine, suggests that FESO is
susceptible to UV-C radiation. The International Conference on Har-
monization (ICH) guideline requires that stress testing should be
carried out to elucidate the inherent stability characteristics of the
active substance in a pharmaceutical preparation. The light testing
should be an integral part of stress testing [21,22].
LC has become the most used method in drug analysis due to its
advantage in being a separating tool, and it is appropriate for stabil-
ity studies. The placebo chromatograms solutions did not show any
peak in the retention times of FESO and photodegradation products.
So, there is no influence of the excipients in the determination of
the photodegradation kinetics of this drug. The peak purity tool was
applied to verify FESO signal, showing that it was free from any co-
eluting compound, with values of a peak purity index higher than
0.9999, indicating the absence of interference in this condition.
Due to low decomposition of FESO (4.9%) under 6 h of UV-A
exposition at ambient temperature, all others experiments were
performed with UV-C light. The FESO photodegradation profile
(UV-C) was evaluated at different time intervals under the same
conditions described previously. Approximately 60% of drug degra-
dation was observed during 60 min of light exposure for tablets
solutions, and no presence of the decomposition products were
found in the control samples. No alteration of color solution was
observed. The temperature, which was controlled in the cham-
ber, was maintained below 27 ◦C. The photodegradation kinetics
was calculed for through the decrease in drug concentration by
time. The concentration, log, and reciprocal concentration values
of the remaining drug versus time are shown in Fig. 1. The FESO
concentration remaining was calculated at each time interval and
compared to freshly sample solution. According to the evaluation
of the determination coefficients obtained (Fig. 1), the FESO pho-
todegradation for pharmaceutical formulation in methanol–water
(1:1, v/v) solution show a zero-order kinetics under the experimen-
tal conditions applied.
Fig. 5. LC–ESI–MS chromatograms of fesoterodine (FESO) sample solution after
60 min of exposition to UV-C lamp. MS was operated in SIR mode acquiring m/z
values of 412.6, 370.6, 365.7, 342.5, 311.5, 293.4, 284.5, and 265.3. LC–ESI–MS con-
ditions are described in Section 2. TIC, total ion count.
like oxidation, reduction, hydrolysis, and racemization that might
occur [17,18]. Photochemical degradation is an important factor of
the pharmaceuticals stability. Since UV radiation has high energy,
it can be the cause of many degradation reactions [19,20]. Usually,
the photodegradation of a chemical compound is caused by oxi-
dation or by the breakdown of certain weak chemical bond. Both

M.S. Sangoi et al. / Journal of Photochemistry and Photobiology A: Chemistry 256 (2013) 16–22
21
Fig. 7. Proposed phototransformation pathway of fesoterodine (FESO) by LC–ESI–MS. PP, photodegradation product.
The results were used for quantitative assessment of the pho-
todegradation described by the rate constant of the zero-order
reaction (k). Their analysis revealed that under these experimental
conditions the rate constant value of the decomposition of FESO
was k = 0.5503 min−1, and the t1/2 and t90% were 46.92 min and
9.38 min, respectively.
Three minor photodegradation products detected were previ-
ously investigated by LC–ESI–MS and identified at 0.86 min (m/z
342.5) and at 0.66 min (m/z 284.5 and m/z 365.7) [13], namely in the
present study as PP-1, PP-2 and PP-3, respectively. Nevertheless,
the main degradation peak (tR = 3.30 min) was not identified due
to drug degradation to non-ionizable product in conditions evalu-
ated. The absorption spectra of FESO and PP-1 (5-HMT) showed a
little difference among them, especially at 235 and 280 nm, due to
hydrolyzation of the ester bond. However, a significative difference
was observed in the spectra of main photodegradation peak (MPP),
which presents a maximum at 222 nm and others bands centered
at 260 and 338 nm (Fig. 2).
In the present study, the ESI was optimized to found the ions
of main degradation peak previously unidentified. The FESO ref-
erence substance solution, FESO degraded solution at UV-C lamp
and isolated fraction were firstly analyzed by full-scan MS. The ESI
mass spectrum of degraded solution exhibited six main molecular
ion peaks (cone voltage of 80 V). The ion m/z 342.5 was detected
only with cone voltage adjusted to 45 V. Isolated fraction solution
analyzed by MS confirmed the presence of four new degradation
products revealed with m/z values of 265.3, 311.5, 370.6, and 293.4,
respectively namely as PP-4, PP-5, PP-6 and PP-7 (Fig. 3). Accord-
ing mean values of total ion count obtained in MS chromatograms,
PP-4, PP-5, PP-6 and PP-7 have 26.1%, 7.4%, 11.0% and 55.5% of MPP,
respectively.
An additional LC–ESI–MS study of isolated fraction was per-
formed by SIR mode, confirming the simultaneous elution of PP-4,
PP-5, PP-6, and PP-7 at 3.30 min. These compounds were co-
eluted due to specific interaction with octadecyl (C18) packing
materials of reversed-phase columns used in analytical and semi-
preparative analyses, Onyx and Luna (Phenomenex), respectively.
The Fig. 4 shows the mass chromatograms of respective molec-
ular ions. Furthermore, a FESO solution degraded after 60 min
of exposition to UV-C lamp was also analyzed by LC–ESI–MS
and the results proving the elution of all photodegradation
products identified in this work (Fig. 5), proving the high
selectivity of the MS in the analysis of a mixture of compo-
nents.
Concentration profile obtained by monolithic LC analysis of the
FESO solution showed that the drug concentration decreased with
the irradiation time and at least seven photoproducts were formed.
The minor photodegradation products (PP-1–PP-3) showed a low
increase and no proportional values of peak areas according
decrease of drug concentration. Moreover, a simultaneous incre-
ment in peak area of main degradation peak (PP-4–PP-7) was
observed (Fig. 6).
According obtained m/z values, the chemical structures of degra-
dation products were proposed. The compound PP-4 (m/z 265.3),
2-(3-(diisopropylamino)propyl)-4-(hydroxymethyl)phenol, was
obtained from the loss of phenyl group of 2-(3-(diisopropylamino)-
1-phenylpropyl)-4-(hydroxymethyl)phenol, named as PP-1 (m/z
342.5). The compound PP-5 (m/z 311.5), 2-(3-(diisopropylamino)-
1-phenylpropyl)phenol, was also obtained from PP-1 by
fragmentation of hydroxymethyl group. The more intense ion peaks
were PP-6 and PP-7. PP-6 (m/z 370.5), 4-(hydroxymethyl)-2-(3-
(isopropylamino)-1-phenylpropyl)phenyl isobutyrate, proceeds

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M.S. Sangoi et al. / Journal of Photochemistry and Photobiology A: Chemistry 256 (2013) 16–22
by loss of a single isopropylamine group. The PP-7 (m/z 293.4),
4-(hydroxymethyl)-2-(3-(isopropylamino)propyl)phenyl isobu-
tyrate, is a subproduct of PP-6 due to the loss of phenyl group. The
scheme of mass fragmentation, presented in Fig. 7, indicated the
full path of photofragmentation of FESO.
In general, the drug degradation process is often very com-
plicated and unpredictable, and to identify degradation products
directly from such complex mixtures is difficult. The conventional
process of isolation and spectral analysis, although very efficient,
is tedious and time-consuming [23]. Therefore, the emphasis today
is on techniques that allow characterization of very low quantities
of degradation products, against the traditional process. Thus, for
most compounds, molecular weight information is readily acquired
by LC-MS experiments, and structurally informative fragment ions
are generated by in-source collision-induced dissociation (CID)
processes or MSn experiments [24].
The analysis of the powdered tablets exposed to the UV-C lamp
demonstrated the photodegradation of FESO in the solid form
is slower than in solutions, but the products formed were the
same. It was possible to observe degradation products only at 7
days of exposition for powdered tablets; the LC analysis presented
FESO decomposition about 6% and the detection of major peak
(tR = 3.30 min; PP4–PP-7) and minor peaks of PP-1, PP-2 and PP-
3. On the other hand, the intact tablets did not show any significant
degradation after 7 days of irradiation. It is likely, that the excipients
protect the drug from the light effects. In the solid state, the pho-
tochemical process takes place on the formulation surface. In most
cases the interior of the preparation is unaffected independently of
the exposure time [7,17].
[3] B.T. Haylen, D. de Ridder, R.M. Freeman, S.E. Swift, B. Berghmans, J. Lee, A.
Monga, E. Petri, D.E. Rizk, P.K. Sand, G.N. Schaer, An International Urogyneco-
logical Association (IUGA)/International Continence Society (ICS) joint report
on the terminology for female pelvic floor dysfunction, Neurourology and Uro-
dynamics 29 (2010) 4–20.
[4] W. Aman, K. Thoma, The influence of formulation and manufacturing process
on the photostability of tablets, International Journal of Pharmaceutics 243
(2002) 33–41.
[5] S. Onoue, Y. Tsuda, Analytical studies on the prediction of photosen-
sitive/phototoxic potential of pharmaceutical substances, Pharmaceutical
Research 23 (2006) 156–164.
[6] M. Wrasse-Sangoi, L.T. Secretti, I.F. Diefenbach, C.M.B. Rolim, M.S. San-
goi, Development and validation of an UV spectrophotometric method
for the determination of aliskiren in tablets, Quimica Nova 33 (2010)
1330–1334.
[7] H.H. Tonnesen, Formulation and stability testing of photolabile drugs, Interna-
tional Journal of Pharmaceutics 225 (2001) 1–14.
[8] R.H. Clothier, Phototoxicity and acute toxicity studies conducted by the FRAME
alternatives laboratory: a brief review, Alternatives to Laboratory Animals 35
(2007) 515–519.
[9] J.R. Meunier, A. Sarasin, L. Marrot, J.R. Meunier, A. Sarasin, L. Marrot, Photogeno-
toxicity of mammalian cells: a review of the different assays for in vitro testing,
Photochemistry and Photobiology 75 (2002) 437–447.
[10] A. Maquille, J.-L.H. Jiwan, LC–MS characterization of metoclopramide pho-
tolysis products, Journal of Photochemistry and Photobiology A 205 (2009)
197–202.
[11] A. Maquille, S. Salembier, M.F. Hérent, J.-L.H. Jiwan, Photodegradation of flu-
pentixol in aqueous solution under irradiation at 254 nm: identification of the
photoproducts generated, Journal of Photochemistry and Photobiology A 214
(2010) 224–229.
[12] M.S. Sangoi, M. Wrasse-Sangoi, P.R. Oliveira, L.S. Bernardi, Determination of
lumiracoxib by a validated stability-indicating MEKC method and identification
of its degradation products by LC–ESI–MS studies, Journal of Separation Science
34 (2011) 1867–1874.
[13] M.S. Sangoi, V. Todeschini, M. Steppe, Fesoterodine stress degradation behavior
by liquid chromatography coupled to ultraviolet detection and electrospray
ionization mass spectrometry, Talanta 84 (2011) 1068–1079.
[14] M.S. Sangoi, M. Steppe, Determination of fesoterodine in pharmaceutical
formulations by using liquid chromatography–tandem mass spectrometry,
European Journal of Mass Spectrometry 16 (2010) 653–661.
[15] M.S. Sangoi, V. Todeschini, M. Steppe, Determination of fesoterodine in phar-
maceutical preparations by stability-indicating capillary zone electrophoresis
method, Journal of AOAC International, in press.
[16] K.J. Volk, S.E. Hill, E.H. Kerns, M.S. Lee, Profiling degradants of pacli-
taxel using liquid chromatography–mass spectrometry and liquid
chromatography–tandem mass spectrometry substructural techniques,
Journal of Chromatography B 696 (1997) 99–115.
[17] C.V. Garcia, N.S. Nudelman, M. Steppe, E.E.S. Schapoval, Structural elucidation
of rabeprazole sodium photodegradation products, Journal of Pharmaceutical
and Biomedical Analysis 46 (2008) 88–93.
[18] A.R. Breier, N.S. Nudelman, M. Steppe, E.E.S. Schapoval, Isolation and structure
elucidation of photodegradation products of fexofenadine, Journal of Pharma-
ceutical and Biomedical Analysis 46 (2008) 250–257.
[19] P. Grobelny, G. Viola, D. Vedaldi, F. Dall’Acqua, A. Gliszczynska-Swigło,
J. Mielcarek, Photostability of pitavastatin – a novel HMG-CoA reductase
inhibitor, Journal of Pharmaceutical and Biomedical Analysis 50 (2009)
597–601.
4. Conclusion
The photodegradation of FESO in aqueous methanol solution
followed a zero-order kinetic reaction and the kinetics parame-
ters of degradation rate constant, t1/2, and t90 can be predicted.
Seven photodegradation products were identified by LC–ESI–MS
technique and the complete photodegradation pathway of drug has
been proposed. FESO photolability showed by the present study
indicates that it is necessary to avoid exposure of the drug from
light effects, and special care must be taken during the preparation,
manufacture, and storage of this pharmaceutical drug.
Acknowledgments
[20] M.D. Malesuik, H.M.L. Gonc¸ alves, C.S. Paim, E.E.S. Schapoval, M. Steppe,
LC: analysis of photodegradation kinetics of nitazoxanide in pharma-
ceutical formulations, Journal of Chromatographic Science 47 (2009)
745–748.
[21] International Conference on Harmonization. ICH Q1A (R2): Stability Testing of
New Drug Substances and Products, Geneva, Switzerland, 2003.
[22] International Conference on Harmonization. ICH Q2 (R1): Validation of Analyt-
ical Procedures-Text and Methodology, Geneva, Switzerland, 2005.
[23] S. Mehta, R.P. Shah, S. Singh, Strategy for identification and characteriza-
tion of small quantities of drug degradation products using LC and LC–MS:
application to valsartan, a model drug, Drug Testing and Analysis 2 (2010)
82–90.
The authors wish to thank CAPES (Coordenac¸ ão de
Aperfeic¸ oamento de Pessoal de Nível Superior) and CNPq (Con-
selho Nacional de Desenvolvimento Científico e Tecnológico) for
the financial support and CBIM (Centro de Biodisponibilidade de
Medicamentos) for kindly provide the mass spectrometer.
References
[1] A.L. Smith, A.J. Wein, Recent advances in the development of antimuscarinic
agents for overactive bladder, Trends in Pharmacological Sciences 31 (2010)
470–475.
[2] J.-J. Wyndaele, Fesoterodine: individualised treatment of urgency urinary
incontinence across patient groups, European Urology Supplements 10 (2011)
14–22.
[24] Y. Wu, The use of liquid chromatography–mass spectrometry for the identifica-
tion of drug degradation products in pharmaceutical formulations, Biomedical
Chromatography 14 (2000) 384–396.