Degradation of Paclitaxel and Related Compounds in
Aqueous Solutions III: Degradation Under Acidic pH
Conditions and Overall Kinetics
JIAHER TIAN1,2 VALENTINO J. STELLA1
1
Department of Pharmaceutical Chemistry, The University of Kansas, Lawrence, Kansas 66047
2
Forest Laboratories, Inc., 220 Sea Lane, Farmingdale, NewYork 11735
Received 11 July 2008; revised 22 July 2009; accepted 24 July 2009
Published online 9 September 2009 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/jps.21910
ABSTRACT: Paclitaxel and related taxanes are complex molecules with numerous
hydrolysable ester groups, possible epimerization at the 7-position, and possessing a
strained oxetane ring, a possible site for acid-catalyzed cleavage. Presented here is the
stability of paclitaxel, 10-deacetylbaccatin III, baccatin III, and N-benzoyl-3-phenyliso-
serine ethyl ester in aqueous solution over a pH range of 1 5 at various temperatures.
Analysis of various samples was by HPLC UV and LC MS. Baccatin III, 10-deacetyl-
baccatin III, and N-benzoyl-3-phenylisoserine ethyl ester were found to undergo acid
catalysis since pH-rate profiles all followed a first-order dependency in hydrogen ion
concentration. No evidence of any epimerization was noted at acidic pH values. Baccatin
III and 10-deacetylbaccatin III showed similar degradation rates with possible products
being possible dehydration around the 13-hydroxy group and cleavage of the oxetane
ring. Cleavage of the 10-acetyl group of baccatin III was a minor initial pathway.
N-Benzoyl-3-phenylisoserine ethyl ester degraded significantly slower than both 10-
deacetylbaccatin III and baccatin III. At pH 2, paclitaxel degraded at a rate between that
of N-benzoyl-3-phenylisoserine ethyl ester and 10-deacetylbaccatin III. The pH of
maximum stability for all compounds appeared to be around pH 4. ß 2009 Wiley-Liss,
Inc. and the American Pharmacists Association J Pharm Sci 99:1288 1298, 2010
Keywords: paclitaxel; 10-deacetylbaccatin III; baccatin III; oxetane ring cleavage;
hydrolysis; degradation; pH; stability; acid catalysis
INTRODUCTION routes of aqueous degradation of paclitaxel and
related taxanes under acidic pH conditions
Earlier articles1,2 in this series examined the have not been published. A number of articles
kinetics, pathways, and mechanisms of the did report the effect of acids and electrophiles on
epimerization and base-catalyzed degradation of the stability of paclitaxel and related materials
paclitaxel and related taxanes in aqueous solution in non-aqueous systems. These earlier studies
at near neutral and basic pH values. The purpose explored the biological activity of various pacli-
of the present study was to explore the degrada- taxel fragments formed under acidic reaction
tion of these compounds under acidic pH condi- conditions.3 5
tions. To our knowledge, the kinetics and major The structures of paclitaxel (1) and its related
compounds (2 7) used in this study are shown in
Figure 1. These were the same as those reported
Correspondence to: Valentino J. Stella (Telephone: 785-864-
in our earlier studies.1,2 By studying the kinetics
3755; Fax: 785-864-5736; E-mail: stella@ku.edu)
and pathways to degradation of fragments of
Journal of Pharmaceutical Sciences, Vol. 99, 1288 1298 (2010)
ß 2009 Wiley-Liss, Inc. and the American Pharmacists Association paclitaxel and taxotere, it was hoped that the
1288 JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 3, MARCH 2010
DEGRADATION OF PACLITAXEL AND RELATED COMPOUNDS 1289
could undergo ring-opening reaction under acidic
condition as was seen with some electrophilic
reagents.3 5
EXPERIMENTAL
Materials
All of the chemicals and solvents were as
described in detail earlier.1,2 The pH of the
solution was controlled throughout the reaction
by using dilute hydrochloric acid and appropriate
buffers. Reactions at pH 1, 2, and 3 were
performed in dilute solutions of hydrochloric acid.
Acetic acid/sodium acetate buffer was used for pH
4 and 5 and the buffer concentration was 1.0 mM.
No significant change of pH was observed
throughout the reaction. The ionic strength was
maintained at 0.15 with sodium chloride.
HPLC and Mass Spectrometry Assays
HPLC UV was employed to simultaneously
detect and quantify the presence of the starting
compound and its degradation products. The
stability indicating isocratic HPLC UV assay
and the HPLC system operating conditions
were described previously,1,2 as were mass
spectrometer conditions and are not repeated
here.
Kinetics
The kinetics of the degradation reaction was
investigated in aqueous solutions at pH 1 5. For
the kinetic experiments at 258C, 25 mL of the
appropriate buffer solutions were equilibrated in
a water bath at 25.0 0.18C, and the hydrolysis
study initiated by adding 0.8 mL stock solution
(125 mg/mL of the appropriate substrate in acet-
Figure 1. The structures of paclitaxel and related
compounds: paclitaxel (1) 7-epi-taxol (2), 10-deacetyl- onitrile) into the reaction buffer. This resulted in
taxol (3), 7-epi-10-deacetyltaxol (4), baccatin III (5), 10- an initial reaction concentration of 2.0 mg/mL
deacetylbaccatin III (6), and N-benzoyl-3-phenyliso-
(6.4 10 6 M for N-benzoyl-3-phenylisoserine
serine ethyl ester (7).
ethyl ester, 3.7 10 6 M for 10-deacetylbaccatin
III, 3.4 10 6 M for baccatin III, and 2.3 10 6 M
for paclitaxel). At various time intervals, aliquots
degradation complexity seen with larger mole- (0.8 mL) of the reaction solutions were withdrawn,
cules like paclitaxel would be better understood. and assayed immediately by HPLC.
Paclitaxel has four hydrolysable ester bonds, For the stability studies at elevated tempera-
which might be expected to undergo hydrolysis tures at 43.5, 50.0, and 70.08C respectively,
promoted by acid. In addition to possible ester vials containing reaction solutions were placed
hydrolysis, the oxetane ring (D-ring) of paclitaxel in thermostatically controlled ovens. Solutions
DOI 10.1002/jps JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 3, MARCH 2010
1290 TIAN AND STELLA
were prepared at 258C and the reported pH values
were those determined at 258C. No corrections for
temperature effects on pH were applied. The
reaction solutions were maintained at the desired
temperature throughout the kinetic study. The
vials were removed from the ovens at appropriate
time intervals and quickly cooled in ice water to
quench the reaction, and followed by immediate
HPLC analysis.
The individual rate constant values were
obtained from the best multi-variance regression
of experimental data to various equations using
the SigmaPlot program (v 7.101, SPSS, Inc.,
Chicago, IL). Numbers are reported standard
deviation ( SD) where statistics could be deter-
mined.
RESULTS AND DISCUSSION Figure 2. Semi-log plots for the degradation of 10-
deacetylbaccatin III in aqueous solution at pH 1.09
(*), pH 1.99 (!), and pH 3.05 (&), T ź 258C showing
Paclitaxel (1) is a complex molecule capable of
the loss of 10-deacetylbaccatin III follows pseudo-first-
undergoing both epimerization and hydrolysis of
order kinetics.
various ester bonds under neutral to basic pH
conditions in aqueous buffers. Tian and Stella1,2
were better able to understand the degradation of
Although ester hydrolysis might be expected to
paclitaxel and taxotere under these conditions by
be the major route of degradation, HPLC UV
also studying the hydrolysis of 10-deacetylpacli-
analysis showed the appearance of six quantifi-
taxel (3), baccatin III (5), 10-deacetylbaccatin III
able peaks with time. The peak area for loss of
(6), and N-benzoyl-3-phenylisoserine ethyl ester
10-deacetylbaccatin III and formation of the six
(7) and some of their related epimers (2 and 4, see
degradation peaks is shown in Figure 3. For
Fig. 1).
example, at pH 2 and 708C, two initial products
were observed, one eluting before the parent peak
(P2) and one eluting later than the parent peak
(P1). A sample HPLC UV MS chromatogram is
Degradation of 10-Deacetylbaccatin III (6)
shown in Figure 4. These products, P1 and P2,
The degradation of the 10-deacetylbaccatin III (6), were formed by parallel competing reactions from
the taxane fragment of taxotere and 10-deacetyl- 10-deacetylbaccatin III as shown by the open
paclitaxel (3), was monitored in aqueous solutions square and circle symbols. These products were
of pH 1.09, 1.99, 3.05, 4.53, and 5.18 at 258C and followed by other products with time (open and
higher temperatures. No epimerization was noted filled upside down triangles and the filled circle
in any studies under acidic pH conditions. This symbols) that are clearly secondary or tertiary
observation held for all the substrates studied (the upside down triangle) products as indicated
under these conditions. In the pH range1 3, the by the delay in their appearance, that is, they
overall loss of the compound demonstrated good were formed from one or both of the initial
linearity on the semi-logarithmic plots, as shown products. An effort was only made to identify P1
in Figure 2. For those reactions where only limited and P2 from HPLC UV MS data. Unfortunately,
degradation occurred, first-order kinetics was because of the low concentrations used and the
assumed based on the observation over many small quantity of 10-deacetylbaccatin III available
half-lives for the more completely degraded to work with, it was not possible to isolate enough
samples. The reactions at pH 4 5 were too slow of these degradation products to run NMR studies
to follow at 258C so no reliable results were to confirm the proposed structures.
achieved over the time period of study at this The late eluting peak (P1, open square symbol
temperature and are, therefore, not reported here. in Fig. 3) was not the epimer when compared to a
JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 3, MARCH 2010 DOI 10.1002/jps
DEGRADATION OF PACLITAXEL AND RELATED COMPOUNDS 1291
been made up of multiple products. The contents
of this peak were not characterized.
Based on the information gathered, the pro-
posed initial degradation pathways of 10-deace-
tylbaccatin III (6) in the acidic pH range, 1 3, is
shown in Scheme 1. The primary reactions were
probable dehydration of the C13 OH (P1) and
hydrolytic opening of the oxetane D-ring (P2).
Once the D-ring is opened, two additional OH
groups are formed. Due to the close proximity, it is
likely the nearby C4 acetyl group is transferred to
either of the two OH positions. Consistent with
this speculation was the observation that two of
the additional, but secondary products, showed
the same mass number but slightly different
HPLC retention times as P2. Overall, a simplified
treatment of the kinetic data can be made for the
degradation of 10-deacetylbaccatin III (6) in
Figure 3. Plot of the peak area versus time course for
low pH can be defined Scheme 1, where Eq. 1
various peaks seen in the chromatogram for the degra-
can define the overall loss of 10-deacetylbaccatin
dation of 10-deacetylbaccatin III (&) and the appear-
III (6)
ance of its initial degradation products, P1 (&) and P2
d½DŠ=dt ź k1½DŠ þk2½DŠ (1)
(*) in aqueous solution at pH 2, T ź 708C. The HPLC
UV chromatogram shows more decomposition products
where the proposed dehydration and D-ring
with time. That is, two primary products (P1, &, mass
opening are parallel reactions, with constants k1
number ź 527 and P2, *, mass number ź 563, see
and k2, respectively.
Scheme 1 for proposed structures) were followed by
further degradation to secondary products indicated
½DŠ ź½DŠo exp½ ðk1 þ k2ÞtŠ ź½DŠo exp kobst (2)
by the symbols *, !, and 5.
The overall rate constant kobs is the sum of k1 and
k2, and can be obtained by slope of linear fitting of
standard (earlier shown to also elute later than the semi-log plot for the loss of starting material
the parent peak) of 10-deacetylbaccatin III (6) but against time. These initial reaction products (P1
showed a loss of mass number 18 (consistent with and P2) degrade further to subsequent products,
the loss of water) supporting the conclusion that with apparent constants k3 and k4, respectively.
ester hydrolysis (loss of acetate or benzoate) As such, all k values could be estimated from the
was not responsible for this product. The longer peak area versus time profiles of products P1 and
retention time indicated this intermediate having P2 (see Fig. 5) if the assumption were made that
reduced polarity, thus, it was considered a the response factors for P1 and P2 were identical
possible dehydrated product. to that of 10-deacetylbaccatin III. If one makes the
By contrast, the mass number of the other assumption that the response factor for P1 and P2
initial product (P2, open circle symbol in Fig. 3), are similar, it appears from peak area versus time
which eluted faster than 10-deacetylbaccatin III, plot seen in Figure 5 that the dehydration and
gave a mass number for the addition of 18 mass hydrolytic opening of D-ring have comparable
units, again not consistent with ester hydrolysis. rates at pH 2.
This structure was considered to be the product Theoretically, the oxetane ring structure of
from the oxetane ring opening, consistent with paclitaxel should be much less reactive than an
findings by others under acidic reaction condi- epoxide, its three-membered analogue. However,
tions.3 5 With longer reaction time, both initial this ring-opening step to form a D-secotaxol
products degraded to further products. Among derivatives appears to occur quite readily. The
the later products analyzed, no ester hydrolytic estimated acid-catalyzed rate constant is 10
products including the presence of benzoic acid times faster than the estimated value for the
were observed. From Figure 3, there did appear to opening of a similar epoxide structure based on
be one major final product but this was an early previously reported data.6 The ring-opening
eluting peak near the solvent front and may have reaction to form D-secotaxols results in a tricyclic
DOI 10.1002/jps JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 3, MARCH 2010
Figure 4. HPLC chromatogram and MS data for the degradation of 10-deacetyl
baccatin III (at 9.1 min) and formation of the primary products P1 and P2 in aqueous
solution at pH 2, T ź 708C, t ź 11 h. Two primary products (early and late eluting, 5.7 (P2)
and 16.2 (P1) min, respectively) followed by further degradation to secondary products
(the peak at 6.4 min was only seen after the appearance of P1 and P2).
Scheme 1. Proposed degradation pathway for 10-deacetylbaccatin III (6) at
low pH values in which the primary reaction pathways resulting in P1 and P2, are
dehydration of C13 OH and hydrolytic opening of oxetane D-ring, respectively. Once the
D-ring is opened, transesterification is proposed with the acetyl group transferred to either of
the two newly formed OH positions followed by other complex reactions.
JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 3, MARCH 2010 DOI 10.1002/jps
DEGRADATION OF PACLITAXEL AND RELATED COMPOUNDS 1293
Moreover, the pH dependency for 10-deacetyl-
baccatin III (6) and baccatin III (5) degradation,
Figure 6, shows that both compounds undergo
apparent acid catalysis since the slopes of the pH-
rate profiles are close to minus unity. The
degradation rates for the two compounds are
almost identical, indicating the hydrolysis of the
additional acetyl ester group on 10-position seen
with baccatin III is minimal compared to the two
other reactions seen with 10-deacetylbaccatin III
although a small peak for 10-deacetylbaccatin III
was observed. This indicates that ester hydrolysis
does not contribute significantly to the overall
initial degradation in the acidic pH range. That is,
if hydrolysis of the 10-acetate group was faster
than the dehydration of the C13 OH group or the
Figure 5. Experimental data and fitted lines describ-
D-ring opening seen with 10-deacetylbaccatin III,
ing the appearance of the two initial products P1 (*)
baccatin III should exhibit faster degradation
and P2 (*) for the degradation of 10-deacetylbaccatin
than 10-deacetylbaccatin III at the same acidic pH
III at pH 2, T ź 708C.
value.
From these results, the concentration-time
ring system which is considerably more flexible profiles at acidic pH values are thought to be a
than the rigid, inverted cup-shaped, tetracyclic consequence of reactions pathways illustrated in
1
ring system. It was reported that the H-NMR Scheme 2 where three reactions occur in parallel:
signals of the protons of the A ring underwent a dehydration at the C13 OH, opening of oxetane
noticeable shift on opening of the D-ring, indicat- ring (D-ring), and minor hydrolysis of the 10-acetyl
ing this increased structural flexibility and group. With longer reaction time, these initial
conformational change.5 products P5, P6, and P7 (6), degrade further to
The pseudo-first-order degradation rate con- secondary products (profiles not characterized).
stants for 10-deacetylbaccatin III under acidic pH
increased rapidly with increasing temperature.
An Eyring plot (not shown) of 10-deacetylbaccatin
III degradation at pH 1.96 was performed. The
6
enthalpy of degradation, DHź, was determined to
be 28 2 kcal mol 1 from the slope of the straight
6
line, and the entropy of activation, DSź, was found
to be 4.2 0.6 e.u.
Degradation of Baccatin III (5) and Comparison to
10-Deacetylbaccatin III (6)
The time courses of degradation of baccatin III (5)
were measured in aqueous solutions of pH 1.12
and 1.98 at 258C. Linear semi-logarithmic plots
indicated that the overall degradation followed
pseudo-first-order kinetics, while again no epi-
merization was observed. Upon analysis of the
final products, hydrolysis of the ester bonds was
Figure 6. Partial pH-rate profiles for the degradation
considered insignificant under these acidic pH
of 10-deacetylbaccatin III (*) and baccatin III (*) in
conditions compared to other reactions as seen
the acidic pH range 1 3, T ź 258C. The solid line for
with 10-deacetylbaccatin III with the products
10-deacetylbaccatin III is the best fit to Eq. 4 while
showing similar relative retention times to those
the line for baccatin III is the straight line joining the
seen with 10-deacetylbaccatin III. two points.
DOI 10.1002/jps JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 3, MARCH 2010
1294 TIAN AND STELLA
Scheme 2. Proposed degradation profile for baccatin III in the acidic pH range. The
degradation of baccatin III at low pH undergoes three pathways in parallel: apparent
dehydration, D-ring opening, and hydrolysis of acetyl groups followed by a series of
secondary reactions. Formation of 10-deacetylbaccatin III constituted a minor initial
pathway as little 10-deacetylbaccatin III was detected in the HPLC assay.
The overall loss of the starting compound follows of the linear semi-logarithmic plot of degradation
pseudo-first-order kinetics of the starting material.
In the acidic pH range, 1 3, acid-catalyzed
½DŠ ź½DŠo exp½ ðk5 þ k6 þ k7ÞtŠ degradation is dominant compared to water
catalysis, and the total shape of log kobs
ź½DŠo exp½ kobsÞtŠ (3)
versus pH profiles can be expressed by the
following equation:
where the overall rate constant kobs, the sum of k5,
k6, and k7, can be readily obtained from the slope
kobsźkHaH (4)
JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 3, MARCH 2010 DOI 10.1002/jps
DEGRADATION OF PACLITAXEL AND RELATED COMPOUNDS 1295
where kH is an acid catalysis second-order rate
constant and aH is the activity of hydrogen ion
that was approximated to concentration. In
Figure 6, the solid line for 10-deacetylbaccatin
III represent the theoretical line calculated from a
linear fit to Eq. 4, while the data points are the
experimental results. The slope of this line is close
to unity. For baccatin III, the solid line is just the
straight line joining the two experimental values
with again the slope being very close to the
expected unity value. The apparent second-order
rate constants for the acid-catalyzed degradation,
kH, are 7.7 10 2 and 6.8 10 2 M 1 h 1 at 258C,
for 10-deacetylbaccatin III (6) and baccatin III (5),
respectively. No statistical analysis was per-
formed due to the limited quantity of data.
Obviously, the apparent dehydration reaction
seen with baccatin III and 10-deacetylbaccatin III
cannot be a primary pathway for paclitaxel and
Figure 7. Partial pH-rate profile for the degradation
taxotere degradation under acidic pH conditions
of N-benzoyl-3-phenylisoserine ethyl ester at acidic pH
without first hydrolysis of the side chain ester values at 708C. The filled symbol (*) represents the
experimental data determined at 708C, while the open
group. Once the side chain is cleaved, however,
symbol (*) represents the experimental point deter-
the apparent dehydration reaction is possible and
mined at 508C.
such a reaction pathway would be seen in
secondary degradation products. This points out
one of the occasional dangers in using model was 0.958 0.138 M 1 h 1. If one were to use the
compounds such as baccatin III and 10-deacetyl- 70 and 508C data points generated at pH 0.96 a
baccatin III in studying and predicting the value 1.7 10 2 M 1 h 1 for kH at 258C can be
degradation rate of more complex molecules such estimated. This value is lower compared to a value
as paclitaxel and taxotere, that is, a pathway seen of 6.8 10 2 M 1 h 1 for baccatin III degradation
in the model compounds that are not possible in under similar acidic pH conditions.
the more complex molecule.
Degradation of Paclitaxel Under
Degradation of N-Benzoyl-3-Phenylisoserine Acidic pH Conditions
Ethyl Ester
Because of its low solubility (<1 mg/mL) in water
N-benzoyl-3-phenylisoserine ethyl ester (7) is at 258C, it is difficult to follow the stability of
selected as a mimic of the side chain of paclitaxel. paclitaxel at this temperature. When the time
The loss of the compound was followed kinetically course of paclitaxel at pH 2.03 was followed at 50
at pH 0.96, 1.94, and 2.68 at 708C, as well as pH and 708C, the increased solubility at these
0.96 at 508C. The semi-logarithmic plots of the elevated temperatures was sufficient for quanti-
concentration of the starting compound versus tative analysis. Semi-logarithmic plots of the total
time were linear and indicated that the overall loss of the starting compound versus time were
degradation followed pseudo-first-order kinetics. linear and indicated that the overall degradation
Figure 7 is the pH-rate profile for the degrada- followed pseudo-first-order kinetics, unlike those
tion generated by plotting the rate constants seen under basic pH conditions where the
determined at 708C and the data point generated reversible epimerization reaction leads to complex
at 508C. In the pH range, 1 3, acid-catalyzed kinetics.1,2 The experimental data yielded
degradation seems the dominant pathway at 708C rate constant values of 3.62 0.19 10 3 and
(and is expected to be similar at 258C) and is 2.63 0.13 10 2 h 1 at 50 and 708C, respec-
adequately defined by Eq.4. The second-order rate tively, at pH 2.03. Figure 8 illustrates the
constant for the acid-catalyzed degradation, kH, at comparison of the degradation of 10-deacetylbac-
708C, determined from the experimental data, catin III, N-benzoyl-3-phenylisoserine ethyl ester,
DOI 10.1002/jps JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 3, MARCH 2010
1296 TIAN AND STELLA
oxetane ring cleavage is probably the predomi-
nant reaction under this condition.
The acid-catalyzed degradation of 10-deacetyl-
baccatin III and baccatin III were discussed
earlier. Both compounds can undergo apparent
dehydration and oxetane ring opening, and
the overall reaction is fast. By contrast, the
N-benzoyl-3-phenylisoserine ethyl ester goes
through a slower acid-catalyzed ester hydrolysis.
Paclitaxel shows a reaction rate in between,
indicating multiple pathways that are faster than
side chain ester hydrolysis alone under these
conditions. These results suggest paclitaxel fol-
lows the reaction pathways described in Scheme 3.
Degradation of paclitaxel under acidic pH under-
goes hydrolysis of the side chain and D-ring
opening simultaneously. Once the side chain is
cleaved, the baccatin III being produced degrades
through dehydration and hydrolytic D-ring open-
Figure 8. Semi-log plots showing the degradation of
ing and a small contribution from 10-deacetyla-
N-benzoyl-3-phenylisoserine ethyl ester (!), paclitaxel
tion.
(&), and 10-deacetylbaccatin III (*) in pH 2 aqueous
Dehydration of C13 OH and the hydrolysis
solution maintained at 708C
opening of oxetane ring (D-ring) are the primary
steps for the degradation of 10-deacetylbaccatin
and paclitaxel at pH 2.03 and 708C. If one were to III and baccatin III at pH values <3, while no
correct for the fact that paclitaxel cannot undergo epimerization is observed under these conditions.
the dehydration reaction seen with 10-deacetyl- These two reactions are acid-catalyzed, with
baccatin and baccatin III, one would conclude that comparable rate constants at the same pH value.
Scheme 3. Proposed degradation scheme for paclitaxel in the acidic pH range.
Paclitaxel undergoes hydrolysis of the side chain and D-ring opening simultaneously.
Once the side chain is cleaved, the baccatin III being produced degrades further through
apparent dehydration and hydrolytic D-ring opening as indicated in Scheme 2.
JOURNAL OF PHARMACEUTICAL SCIENCES, VOL. 99, NO. 3, MARCH 2010 DOI 10.1002/jps
DEGRADATION OF PACLITAXEL AND RELATED COMPOUNDS 1297
Acid-catalyzed hydrolyses of ester bonds on C2, basic pH range was those determined experimen-
C4, and C10 positions are relatively slow under tally from earlier work.1,2 A very similar plot
acidic pH conditions and make little to no major is seen in the neutral to basic pH range for 10-
contribution to the overall degradation kinetics. deacetylbaccatin III (not shown) and from more
Paclitaxel undergoes some hydrolysis of its side limited data for paclitaxel.1,2 The plots show a
chain and oxetane ring opening simultaneously, V-shape profile, with base-catalyzed reversible
without epimerization. After the side chain is epimerization to C7 OH and hydrolytic degrada-
cleaved, the initial product baccatin III degrades tion of ester groups at high pH. Compared to basic
further through the pathways described above. condition, the compounds are much more stable
under acidic pH conditions, in which the primary
degradation appears to be through dehydration at
the C13 OH and hydrolytic opening of the
Overall pH Dependency of the Degradation of
oxetane ring. The maximal stability is obtained
Paclitaxel-Rated Compounds
near pH 4 5.
Based on the similarity of the chemical struc-
The epimerization of paclitaxel and several
ture, the measured kinetic data of smaller partial
related taxanes have been examined in aqueous
structures such as 10-deacetylbaccatin III and
solution, and the rate constants of the intercon-
baccatin III allows one to make reasonable and
version of the R- and S-epimers determined in
reliable assumptions about the chemical stability
near neutral and basic pH range.1,2
of the more complex molecules. Paclitaxel, tax-
Figure 9 shows the overall pH-rate profile for
otere, and their other derivatives likely have
the epimerization and degradation of baccatin III
similar V-shape profiles. The rates for epimeriza-
from pH 1 12 at 258C. The data for the neutral to
tion of paclitaxel and taxotere are comparable to
baccatin III and 10-deacetylbaccatin III, respec-
tively, while paclitaxel showed faster hydrolysis
under basic pH due to the more labile side-chain
ester bond. Lacking the dehydration reaction seen
with 10-deacetylbaccatin III, paclitaxel showed
better chemical stability at pH values <3.
ACKNOWLEDGMENTS
The authors greatly appreciate the contribution
and support by Dr. Richard Schowen. The authors
also gratefully acknowledge the support of
Dr. Gunda Georg and Tapestry Pharmaceuticals,
Inc. as suppliers of paclitaxel and other related
compounds.
Figure 9. pH-rate profile for baccatin III degradation
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