The Role of Excipients in the Stability of Triamcinolone Acetonide in Ointments
The Role of Excipients in the Stability of Triamcinolone Acetonide in Ointments
Anton J. P. van Heugten 1 2
Wouter S. de Vries 1
Marian M. A. Markesteijn 1
Roland J. Pieters 0
H. Vromans 1 2 3
0 Department of Chemical Biology and Drug Discovery, Utrecht Institute for Pharmaceutical Sciences, Utrecht University , P. O. Box 80082, 3508 TB, Utrecht , the Netherlands
1 Research and Development Department , Tiofarma B.V., Hermanus Boerhaavestraat 1, 3261 ME, Oud-Beijerland , the Netherlands
2 Department of Pharmaceutics, Utrecht Institute for Pharmaceutical Sciences, Utrecht University , 3584 CG, Utrecht , the Netherlands
3 Department of Clinical Pharmacy, Division of Laboratory Medicine & Pharmacy, University Medical Centre Utrecht , P/O Box 85500, 3508 GA, Utrecht , the Netherlands
Degradation of triamcinolone acetonide (TCA) in an ointment was investigated. TCA appeared to be concentrated in propylene glycol (PG) which in turn is dispersed in a lanolin-petrolatum mixture. Two predominant degradation products were identified: a 21-aldehyde and a 17-carboxylic acid. The 21-aldehyde is formed after TCA is oxidized by O2, a reaction that is catalyzed by trace metals. Logically, the content of trace metals has a profound effect on the degradation rate. It was shown that trace metals are extracted from lanolin and petrolatum by PG, increasing the concentration in PG. In accordance with these findings, TCA degrades faster in PG that is present in the ointment formulation than in regular PG. The 21-aldehyde was confirmed to be a primary degradation product, while the 17-carboxylic acid was identified as a secondary degradation product. Based on the mechanism of degradation, the ointment can be stabilized by the addition of sodium metabisulfite which was shown to reside also in the PG phase within the ointment.
Triamcinolone acetonide; Corticosteroid; Ointment; Trace metal; Degradation mechanism
Corticosteroids are widely used in a broad range of
products. A selection of these products is for dermal
application, such as ointments. In the Netherlands, TCA
ointment 0.1% FNA (Formulary of Dutch Pharmacists) was
available until it was withdrawn from the market after license
holders reported poor chemical stability (
). The ointment
consists of 0.1% TCA, 10% propylene glycol (PG), 10%
lanolin, and 79.9% petrolatum. Various similar products are
TCA and molecularly similar corticosteroids are prone to
oxidative degradation. This particular degradation
predominantly occurs at the 17-side chain of the corticosteroid
). Amongst other degradation products, two
are the most often mentioned for corticosteroids with the
same 17-side chain as TCA (Fig. 1); first, a 21-aldehyde
degradation product which formation is described in aqueous
and alcoholic solutions (Fig. 1, compound 2). This
21aldehyde is formed by a reaction between TCA and
molecular oxygen (O2) that is catalyzed by metal salts (4).
Second, for hydrocortisone and flurandrenolide the formation
of a 17-carboxylic acid in alkaline environment was
demonstrated by using O2 and OH− as reagents (
degradation products concern degradation of the A ring (
) or hydrolysis of the acetonide moiety (9). For a water-free
environment such as the TCA ointment, it was shown that the
21-aldehyde and 17-carboxylic acid are the main degradation
). The mechanism by which these are formed
remains unclear however.
The aim of this study was to investigate the degradation
mechanism of TCA in TCA ointment 0.1% FNA.
MATERIAL AND METHODS
Chemicals and Reagents
The following chemicals and reagents were used (all
materials complied with the quality requirements of the
European Pharmacopoeia (Ph. Eur. 2016)): TCA (Newchem,
Milan, Italy), PG (Brenntag, Dordrecht, the Netherlands),
lanolin (Stella Lanolines, Mouscron, Belgium), petrolatum
(Sonneborn, Amsterdam, the Netherlands), copper(II)
acetate (Cu(Ac)2) (Alfa Aesar, Haverhill, MA, USA), tert-butyl
peroxybenzoate (TBPB), sodium metabisulfite and
1,10phenanthroline (Sigma-Aldrich, St Louis, MO, USA), hexane
and acetonitrile (Avantor Performance Materials, Center
Valley, PA, USA), and phosphoric and acetic acid (Boom,
Meppel, the Netherlands). Distilled, deionized water was
prepared by an Elga Centra R 60/120 system (Woodridge, IL,
The Solubility of TCA in the Ointment Components and
Microscopical Structure of TCA Ointment
One hundred grams of PG was added to an Erlenmeyer
flask and heated to 60°C on a magnetic stirrer hot plate (IKA
C-MAG HS4, Staufen, Germany). TCA was added in 0.25 g
increments. One hundred grams of lanolin or petrolatum was
added to stainless steel mortars that were heated to 60°C on a
water bath (Ika Werke HB4 Basic, Strufen, Germany). TCA
was added in 0.010 g increments. End point (solubility) was
defined as the amount of TCA where solid dispersed particles
were still observed after intensive stirring for 10 min.
Additionally, TCA ointment sample was analyzed for
microscopic structure using a light microscope (Euromex
ME2880 microscope, Euromex, Arnhem, the Netherlands) with a
magnification of × 40.
The Metal Content in the Excipients of the Ointment
PG was mixed with lanolin and petrolatum in a ratio of
1:1:8 (i.e., the ratio of the ointment formulation). This
mixture was stored at 60°C for 1 month. At 60°C, the
ointment is phase separated in a PG and a
lanolinpetrolatum phase. The PG phase was separated from the
lanolin-petrolatum phase by using a syringe. 0.5 g of this PG
extract, normal PG, lanolin, and petrolatum were dispersed in
10 ml nitric acid and 2 ml of hydrogen peroxide in a Teflon
tube in a MDS 2000 lab microwave (CEM Corporation,
Matthews, NC, USA). After completing the microwave
program (15 min 60% power and 80 psi; 15 min 80% power
and 100 psi; and 30 min 90% power and 120 psi) the sample
tubes were cooled, transferred to 50-ml sample tubes and
diluted to 30 g with water. Samples were analyzed using an
inductively coupled plasma optical emission spectrometry
(ICP-OES) spectrometer Dual-View Prodigy 7 (Teledyne
Leemanlabs, Hudson, NH, USA) for iron, nickel, and copper
LC was conducted on a Shimadzu Prominence-i
LC2030C 3D liquid chromatograph with diode array detector
(Kyoto, Japan) and an Altima C18 column (250 × 4.6 mm,
with 5 μm particles) (Mandel Scientific Company, Ontario,
Canada). The flow rate was 1.5 mL/min, the injection volume
was 20 μL and UV detection was performed at 241 nm. The
mobile phase consisted of acetonitrile and water with the
addition of 20 mM phosphoric acid (acetic acid when
subsequently analyzed with MS). A gradient program was
used: 0% acetonitrile from start to 12 min, increased to 32%
at 12 min, maintained at 32% until 30 min, increased to 65%
at 40 min, decreased to 0% at 42 min, and maintained at 0%
until 47 min. Chromatograms were obtained and analyzed
with Shimadzu LabSolutions software version 5.5.7. This
method is similar to a recently published analytical method
for TCA ointment (
MS was conducted on an Agilent 1100 series ion-trap
system equipped with an electrospray ionization (ESI) source
and liquid chromatography sprayer (Agilent Technologies,
Waldbronn, Germany) and operated in positive mode.
Masses were scanned from m/z 50 to 600, nebulizer pressure
was 70 psi, gas flow was 12 L/min, gas temperature was 350°C
and capillary voltage was 1600 V. Data was analyzed with
Agilent LC/MSD Trap 4.1 version 5.0 (build 65) software.
RESULTS AND DISCUSSION
The Microscopical Structure of the TCA Ointment and the
Solubility of TCA in Ointment Excipients
Using a light microscope, the physical structure of the
ointment appeared to consist of two phases, a dispersed phase
in a more voluminous continuous phase. Since PG is more
polar than lanolin and petrolatum, it seems logical to state
that the dispersed phase consists of PG and the continuous
phase of lanolin and petrolatum. The latter two are mixable,
while these compounds hardly mix with PG. To study in what
phase TCA resides within the ointment, solubility tests in the
separate ingredients were conducted. The solubilities of TCA
in PG, lanolin, and petrolatum were 1.25, < 0.01, and < 0.01%,
respectively. From this, it can be concluded that TCA is
mainly present in the PG phase that is emulsified in a
lanolinpetrolatum mixture. Therefore, degradation experiments in
PG can be assumed to yield representative outcomes.
The Identity of the Major TCA Degradation Products in the
To elucidate the degradation mechanism of TCA,
degradation products in the ointment were determined.
Ointment was prepared and stored in closed glass containers
TCA content (± RSD)
at 60°C for 1 month. Samples were analyzed using LC-MS.
The two major degradation products were identified to be the
21-aldehyde and the 17-carboxylic acid (Fig. 1, compounds 2
and 3, respectively) based on the m/z ratios of 451.2 and
421.2, respectively. The 17-carboxylic acid showed a shift in
retention time in response to mobile phase pH confirming it
to be an acidic compound. To confirm that testing at 60°C
shows a realistic degradation pattern, an old ointment that
was stored for 5 years at room temperature was tested; the
same degradation products were formed.
The Degradation Mechanism of TCA
The two degradants, 21-aldehyde and 17-carboxylic acid,
are oxidative degradation products of TCA. They have been
described before as degradants of hydrocortisone and TCA in
aqueous and ethanolic solution in the presence of O2 and
metal salts (
). For a water-free environment, it is unclear
what reactants are present to oxidize TCA. In the ointment,
the excipient lanolin is known to contain peroxide impurities
(European Pharmacopoeia monograph 0134). Furthermore,
the presence of trace metals cannot be excluded since
excipients of natural origin are generally known to contain
low levels of these (
). Therefore, a stress study was
conducted to investigate whether TCA is oxidized by
peroxide residues and if its oxidation is catalyzed by trace
metals in the non-aqueous ointment. TBPB was used as a
model for organic peroxide residues (
). TCA was dissolved
in PG and exposed to varying combinations of Cu(Ac)2,
purging air, and TBPB (Table I). TCA content was
determined after storage at 60°C for 7 days.
The results in Table I clearly show that without the
presence of Cu(Ac)2, no clear degradation of TCA occurred.
This indicates that the presence of air or peroxides alone does
not initiate TCA degradation. However, when TCA was
exposed to Cu(Ac)2, significant decomposition occurred. This
is slightly increased by purging air in the presence of
Cu(AC)2. Previously, it has been shown that TCA
degradation is oxidative (
) and therefore, oxygen must be present
in PG to react with TCA. To study whether sufficient oxygen
is present in PG, it was purged for 1 hour with nitrogen in the
presence of Cu(Ac)2 in preliminary experiments (90.3% TCA
left for the purged and 85.9% for the untreated sample).
Clearly, less degradation occurred in the purged PG
compared to untreated PG. This clearly indicates that oxygen is
indeed necessary for TCA to decompose and that in
untreated PG sufficient oxygen is present to react with TCA.
Interestingly, the combination of Cu(Ac)2 and TBPB led
to less degradation than Cu(Ac)2 alone (96.1 versus 80.3%).
A similar amount of decomposition was expected since the
same amount of Cu(Ac)2 was present in the sample. It
therefore appears that the peroxide (TBPB) and Cu(Ac)2
interact. Such interaction can possibly be explained by the
Fenton-type reaction that occurs when trace metals (M) react
with peroxides (see Eqs. 1 and 2). Competition for Cu(Ac)2
between TBPB and TCA can explain the difference in
degradation between the samples with Cu(Ac)2 alone and
those with both Cu(Ac)2 and TBPB. Moreover, the hydroxyl
radicals (HO ) that form during Fenton-type reactions are
much more reactive than the natural oxidant (ROO ) (
Even in this environment, TCA degraded less than in the
presence of Cu(Ac)2 alone; affirming TCA oxidation is
mediated by trace metals and oxygen rather than peroxides.
As is shown above, the presence of air enforces TCA
degradation. Therefore, an in use stability study was
performed on a 0.5% TCA in PG:lanolin (1:1) mixture stored in
Fig. 3. Formation of the two major degradation products of triamcinolone acetonide
(TCA) in the ointment formulation. The predicted values are based on degradation
kinetics models presented in (
). For the experimental data (n = 3), a 95% CI was
calculated, error bars however were too small to be visible
closed glass containers and in glass containers that were
opened twice a week for 10 min. After 218 days at 60°C
96.8% (± 0.2) of TCA (± RSD) was left in the closed
containers while in the opened containers 73.1% (± 0.9) was
left (n = 3). Clearly, TCA decomposes significantly faster in
opened containers. When containers are opened, the sample
is exposed to fresh air, allowing the supply of oxygen. Since in
general, oxygen is more soluble in fatty environment
compared to polar environment (
), it seems likely that opening
containers twice a week is enough to replenish the oxygen
concentration in the sample. Therefore, more substrate
(oxygen) is available to react with TCA which can explain
the higher TCA degradation.
The Influence of Trace Metal Content on TCA Degradation
Metal salts greatly influence TCA degradation (
determine the relation between trace metal content and TCA
degradation, a 0.5% TCA solution in PG was exposed to
varying amounts of Cu(Ac)2 and equilibrated with air.
Cu(Ac)2 was used as a model for other metal salts which
are likely to react in a similar way (
). TCA content was
determined after 1 month. Figure 2 presents the results.
A clear concentration dependence of the degradation
constant can be observed. TCA degraded completely in the
10-ppm copper sample. In this sample, TCA is present in a
concentration which is 75 times higher than the copper molar
concentration. This shows that significantly more TCA
degraded than copper was present, indicating that copper
catalyzed the reaction. Copper, or other trace metals, as
oxidation catalysts is further supported by literature (
This indicates that TCA oxidation can be catalyzed and
hence, significantly increased even if only small traces of
metal impurities are present.
The Trace Metal Content in the Excipients of the Ointment
Limits for trace metals in topical products are set in the
European Pharmacopoeia (Ph. Eur. general text 5.20). Limits
for copper and iron are 250 and 1300 ppm, respectively.
Therefore, amounts < 250 ppm are allowed to be present in
lanolin, PG, and petrolatum. Commonly suspected trace
metals, copper, iron, and nickel (
) were determined
using ICP-OES in the individual excipients. Additionally,
lanolin and petrolatum were extracted with PG and analyzed
as well. Table II shows the results.
Trace metal content differed between excipients. It is
clear that lanolin and petrolatum contain more trace metals
than PG. Interestingly, the saturated PG extract shows a
higher level of trace metal content, showing that trace metals
transfer to PG when exposed to lanolin and petrolatum. This
extract was free from any undissolved lanolin and petrolatum
since at 60°C the PG phase is phase separated from the
lanolin-petrolatum mixture. Hence, it is to be expected that
the metal content in the PG phase of the ointment increases
when mixed with lanolin and petrolatum.
The Influence of Lanolin and Petrolatum Extracted with PG
As has been concluded above, TCA is predominantly
present in the PG phase of the ointment. To check the
influence of metals towards the TCA degradation, TCA was
dissolved in PG and in lanolin and petrolatum extracted with
PG. TCA content in this PG extract was determined after
16 days of storage at 60°C and compared to untreated PG.
The remaining relative content was 85.2 ± 0.5% and 97.2 ±
0.04% for the PG extract and in the untreated PG,
respectively. The faster degradation of TCA in the PG extract
confirmed that the additional trace metals originating from
the two excipients accelerate TCA degradation.
The Degradation Pathway
As described earlier, TCA degrades into the 21-aldehyde
and the 17-carboxylic acid. To study how this degradation
evolves, the ointment was stored at 60°C and samples were
analyzed at various time points for TCA, 21-aldehyde and
17carboxylic acid content. Figure 3 presents the results.
Figure 3 shows that the 21-aldehyde content increases
initially and subsequently levels and even slightly decreases
Fig. 4. Schematic overview of the TCA degradation route
after 3 days. The 17-carboxylic acid forms after a lag time of a
few hours and the content increases subsequently. This points
in the direction of a reaction from A (TCA) to B
(21aldehyde) to C (17-carboxylic acid) (Fig. 4). This
phenomenon has been described earlier by Waterman et al. (
degradation constants k1 and k2 can be calculated using Eqs.
3 and 4.
Bt ¼ A0 k2−k1
Ct ¼ A0 1 þ k1−k2
Using these equations, a degradation constant for the
formation of the 21-aldehyde (k1) was calculated to be
0.00576 ± 0.00048 day−1 and for the formation of the
17caboxylic acid (k2) 0.456 ± 0.144 day−1. The fit for the
equations was 0.946 further underlining the likelihood that
the 17-carboxylic acid is formed from the 21-aldehyde in the
Since k2 ≫ k1, more of the 17-carboxylic acid is present
than 21-aldehyde shortly after the start of TCA degradation.
Based on the previously shown influence of trace metals
and oxygen on the degradation of TCA, the following
degradation mechanism can be proposed (Fig. 5).
Figure 5 indicates that the suggested sequence of
degradation as shown in Fig. 4 is likely to occur according
to the mechanism shown. The transformation of TCA to a
21aldehyde is likely to involve Cu2+ to form both the
21aldehyde and Cu+, which subsequently forms Cu2+ using
). The degradation of a 21-aldehyde to a
17carboxylic acid has been reported for a corticosteroid with the
same 17-side chain as TCA, betamethasone, using LC-MS
isotope experiments using 18O2 as the oxidant (
). When the
two described mechanisms are linked, a water-free
transformation of TCA to a 21-aldehyde and a 17-carboxylic acid is
proposed. Potentially corticosteroids containing the same
C17moiety as TCA may degrade likewise. This is further
supported by literature on for example hydrocortisone
showing similar degradation products formed (
The Prevention of TCA Degradation
Since the degradation of TCA depends on O2 and trace
metal content, the addition of a sacrificing antioxidant and a
chelating agent could prevent degradation. To investigate the
influence of such compounds, 0.1% sodium metabisulfite (
Time in days
Standard ointment formulation
With sodium metabisulphite
With sodium metabisulphite and
and 0.1% 1,10-phenantroline as a chelating agent in organic
) were dissolved in PG in combination with
TCA and added to the lanolin-petrolatum mixture which was
then stored at 60°C for over 6 months in Erlenmeyer flasks.
An ointment without additional ingredients was stored as
well. Figure 6 presents the results.
Figure 6 clearly shows that sodium metabisulfite
stabilizes TCA in the ointment showing that it can work as a
sacrificing antioxidant in organic environment. The addition
of 1,10-phenantroline shows an additional stabilizing effect.
Logically, 1,10-phenantroline will sequester metals and
thereby decrease the available amount of catalytic metals to
react with TCA.
Oxidation of TCA takes place in the PG phase of the
ointment. This oxidation is catalyzed by trace metals which
are extracted from lanolin and petrolatum. An extensive
degradation mechanism is proposed based on these findings.
Sodium metabisulfite stabilized TCA in the ointment and
1,10-phenantroline shows an additional improvement when
combined with sodium metabisulfite.
Funding This work was supported by the R&D tax credit of the
Ministry of Economic Affairs, the Netherlands (grant number
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