Degradation of Paclitaxel and Related Compounds
`in Aqueous Solutions II Nonepimerization Degradation
`Under Neutral
`to Basic pH Conditions
`
`JIAHER TIAN12 VALENTINO J STELLA1
`
`1Department of Pharmaceutical Chemistry The University of Kansas Lawrence Kansas 66047
`2Wyeth Research 401 N Middletown Road Pearl River New York 10965
`
`Received 11 July 2007 revised 20 August 2007 accepted 29 August 2007
`
`Published online in Wiley InterScience wwwintersciencewileycom DOI 101002 ljps21214
`
`ABSTRACT
`have complex structures that
`Paclitaxel and other taxanes
`include the
`presence of numerous hydrolytically sensitive ester groups The present study attempts
`to make sense of the kinetics of the base catalyzed hydrolysis of various ester groups found
`in paclitaxel by also studying the hydrolysis of 7epitaxol 10deacetyltaxol 7epi10
`III and Nbenzoy13phenylisoserine ethyl
`deacetyltaxol baccatin III 10deacetylbaccatin
`ester Kinetics were studied as function of pH buffer concentration
`and temperature and
`analyzed using a stability indicating HPLC assay and LCMS to identify degradation
`products The kinetics were complicated by the epimerization reaction occurring at the
`7position but isolation of the hydrolytic components of the kinetics was possible All ester
`hydrolysis reactions observed above pH 67 were as expected base catalyzed After
`the C7 paclitaxel hydrolysis occurs mainly due to cleavage of the
`epimerization at
`side chain with further hydrolysis of the ester bonds at C10 C2 and C4 with the C10
`acetate hydrolysis being the relatively next most facile By studying the hydrolysis
`and
`7epi10deacetyltaxol
`baccatin
`of 10deacetyltaxol
`III
`10deacetylbaccatin
`III
`Nbenzoy13phenylisoserine
`ethyl ester good insight
`into the hydrolysis of the larger
`more complex taxanes was possible © 2007 Wiley Liss Inc and the American Pharmacists
`Association J Pharm Sci 9731003108
`2008
`Keywords
`taxol 7epitaxol baccatin
`paclitaxel
`degradation pH stability
`
`epimerization hydrolysis
`
`III
`
`INTRODUCTION
`
`This study was undertaken
`the
`to investigate
`degradation kinetics of paclitaxel and related com
`pounds in aqueous solutions in the neutral
`to basic
`pH range where both epimerization at
`the C7 site
`and hydrolysis were observed An earlier article
`described the role of pH on the C7 epimerization
`and identified the likely mechanism for this base
`
`to Valentino J Stella Telephone 785864
`Correspondence
`3755 Fax 7858645736 Email stellakuedu
`Journal of Pharmaceutical Sciences Vol 97 31003108 2008
`0 2007 Wiley Liss Inc and the American Pharmacists Association
`
`catalyzed reaction An additional goal was to pro
`vide insight into the stability of other taxanes and
`paclitaxel analogues in aqueous solution
`The
`and therapeutic
`chemistry
`benefits
`of
`paclitaxel and related taxanes has been studied
`extensively since the 1980s27 Limited quanti
`tative information is available
`on the chemical
`in aqueous solution because
`stability of paclitaxel
`of its poor aqueous solubility its complex structure
`and its nonadherence to first order kinetics under
`to basic pH conditions However
`some
`neutral
`useful studies have been published81° Tian and
`Stella quantified the C7 epimerization of pacli
`taxel and several
`related taxanes
`in aqueous
`
`3100
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`JOURNAL OF PHARMACEUTICAL SCIENCES VOL 97 NO 8 AUG
`
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`PACLITAXEL AND RELATED COMPOUNDS IN AQUEOUS SOLUTIONS
`
`3101
`
`solution and the rate constants for the interconver
`sion of the 7R and 7Sepimers The focus of the
`present study was to investigate the kinetics of the
`further degradation of these epimers under neutral
`and basic pH conditions and to study the influence
`of the ring system and substituents
`and its related
`The structures of paclitaxel
`compounds
`used
`in this study are illustrat
`ed in Figure 1 Baccatin
`and 10deacetyl
`III
`baccatin III represent the diterpene ring structure
`and Nbenzoy1
`of paclitaxel and 10deacetyltaxol
`3phenylisoserine ethyl ester was selected as a
`mimic of the side chain of paclitaxel
`
`EXPERIMENTAL
`
`Materials
`
`H3C
`
`OH
`
`1 R1 = CH3C0 R2 = OH
`2 R1 = CH3C0 R2 = H
`3 R1 = H
`R2 = OH
`4 R1 = H
`R2 = H
`
`R3 = H
`R3 = OH
`R3 = H
`R3 = OH
`
`Rig
`
`0
`
`R2
`
`CH
`
`All of the chemicals solvents and buffer solutions
`to those des
`used in this study were identical
`in an earlier study
`cribed in greater detail
`
`HC
`
`HO
`
`pH
`
`The pH of the solutions was controlled through
`the reaction by using appropriate buffer
`out
`solutions and dilute sodium hydroxide solutions
`The preparation of buffer solutions was described
`previously The kinetic measurements at pH 11
`and 12 were performed in dilute sodium hydroxide
`solutions of appropriate concentration Buffer
`concentration when not varied was 10 mM
`for the measurement at pH 12 No signi
`except
`ficant change of pH was observed throughout
`the
`reaction
`
`5 R1=CH3CO R2 = OH
`6 R1 = H
`R2 = OH
`
`R3 = H
`R3 = H
`
`Analytical HPLC Assays and Mass Spectrometry
`
`OH
`
`7 F
`
`igure 1 The structures of paclitaxel and related
`paclitaxel 1 7epitaxol 2 10deacetyl
`compounds
`taxol 3 7epi10deacetyltaxol 4 baccatin III 5 10
`III 6 and Nbenzoy13phenylisoser
`Me ethyl ester 7
`
`deacetylbaccatin
`
`18 kV For HPLCUVMS mode the flow rate was
`0833 mLmin versus 0167 mLmin for UV and MS
`respectively The molecules undergo electron spray
`ionization in the positive ion mode
`
`their epimers and other
`The starting compounds
`products were simultaneously mea
`degradation
`sured using an isocratic HPLCUV assay The
`stability indicating assay and the HPLC system
`employed in this study were described earlier as
`was the identification and profiling of epimeriza
`tion and degradation products using a Waters
`Alliance 2690 HPLC system connected to a Micro
`mass Quattro Micro Tandem Quadruple mass
`spectrometer The system was operated at an
`electrospray source block temperature of 120°C a
`desolvation temperature of 50°C a cone voltage of
`
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`JOURNAL OF PHARMACEUTICAL SCIENCES VOL 97 NO 8 AUGUST 2008
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`3102
`
`TIAN AND STELLA
`
`Kinetic Procedure
`
`The reaction kinetics for further degradation of
`both R and Sepimers was investigated in aqueous
`solutions at pH 612 For the kinetic experiments
`at 25°C 242 mL appropriate buffer solutions were
`equilibrated in a water bath at 250 ± 01°C and
`the epimerization was initiated by adding 08 mL
`stock solution 125 RgmL in acetonitrile
`into
`the reaction buffer This resulted in an initial
`of 20 agmL At various
`reaction concentration
`time intervals aliquots 08 mL of the reaction
`solutions were withdrawn and assayed by HPLC
`The reactions at basic pH were quenched by adding
`dilute hydrochloric acid solution to adjust the pH
`close to 5 before HPLC analysis
`For the stability studies at elevated temperatures
`37 50 and 70°C respectively vials containing
`sample solutions were placed in thermostatically
`The
`ovens
`solutions were
`controlled
`reaction
`maintained at the desired temperature throughout
`the stability study Portions were removed from
`
`070304005
`I nn
`
`628
`
`the reaction solution at appropriate intervals The
`samples were quickly cooled in ice water
`to quench
`the reaction followed by immediate HPLC analysis
`The time interval between sampling and HPLC
`injection was less than three minutes so that the
`experimental error was minimized
`
`RESULTS AND DISCUSSION
`
`III
`
`Total Degradation of 10Deacetylbaccatin
`Figure 2 shows HPLCUVMS chromatograms
`the degradation of 10deacetyl baccatin
`of
`III
`The starting compound the Sepimer and the
`and degradation products were
`epimerization
`from one another After
`adequately
`separated
`the C7 hydroxyl
`the initial
`epimerization of
`both 7S and 7R epimers further degraded into
`more fragmented products For example some
`benzoic acid was confirmed by its strong UV
`
`1
`
`Channel
`An1
`130e5
`
`Scan ES+
`TIC
`312e8
`
`aCc0
`
`5
`Sa
`
`c 0
`
`5
`
`1C89 03
`
`03C
`
`V30
`cr
`900 L
`
`a c
`
`T a
`
`847
`
`156
`
`302
`434407
`
`070604005
`
`IU
`
`357
`
`495
`
`654 729
`
`10111247128816201591 A
`10114W916410441 045r
`
`17 23
`
`1972
`
`6442239lwmv22792440150
`
`2498
`2689
`2875
`3063 3214
`34A7
`4044t4050440451101445
`
`1250
`
`1500
`
`1750
`
`2000
`
`225o
`
`2500
`
`2750
`
`3000
`
`3250
`
`Time
`3500
`
`Figure 2 HPLC chromatogram and MS data for
`the hydrolytic degradation of
`10deacetyl baccatin III and its epimer in aqueous solution at pH 108 50°C reaction
`time =3 h After the initial epimerization of the C7 hydroxyl both 10deacetylbaccatin
`III and its epimer degrade to further products such as benzoic acid
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`PACLITAXEL AND RELATED COMPOUNDS IN AQUEOUS SOLUTIONS
`
`3103
`
`at various pH values 612 The experimental
`results were plotted in Figures 3 and 4 The
`pseudo first order
`follows
`overall degradation
`pH and temperature
`kinetics at any constant
`The pseudo first order
`k3 are
`rate constants
`linear regression using Sig
`obtained
`from best
`maPlot v 7101
`
`Total Degradation of Baccatin III
`
`Under basic pH condition baccatin
`III quickly
`forms its 7Repimer followed by further hydro
`lysis of various ester bonds leading to multiple
`products such as 10deacetylbaccatin III 7epi
`III and other minor hydro
`10deacetylbaccatin
`lytic products The degradation time course was
`described earlier The ester groups at C2 C4
`and C10 positions all undergo hydrolysis simul
`and
`the value
`the
`taneously
`therefore
`of
`k3 is the sum
`overall degradation rate constant
`of all hydrolytic rate constants
`contributing to
`hydrolysis Among
`the hydrolytic
`products
`III and 7epi10deacetylbac
`10deacetylbaccatin
`catin III were observed as the primary products
`indicating that
`the 10 acetyl ester hydrolyzed
`faster than the other two ester groups Therefore
`degradation pathways of
`the proposed
`initial
`
`100
`
`10
`
`ResidualPercent
`
`2000
`
`4000
`
`6000
`
`8000
`
`10000
`
`Reaction Time minute
`
`Figure 3 Semilog plot of the total loss of 10deacetyl
`
`baccatin III sum of 7R and 7Sepimers at pH 904 0
`1078 y 1182 a and 25°C The total
`
`including both epimers follows
`10deacetylbaccatin
`III
`pseudo first order kinetics in the high pH range
`
`loss of
`
`absorbance and by added standards while it was
`hardly detected by MS due to its low molecular
`indicated some hydrolytic
`weight This result
`the benzoate ester bond at C13
`cleavage of
`this was a relatively
`position occurred however
`minor overall pathway
`Tian and Stella discussed the C7 epimerization
`III proceeds accord
`reaction of 10deacetylbaccatin
`ing to Scheme 1 kl
`is the epimerization rate
`from the 7Sepimer
`to the 7Repimer
`constant
`and k2 is the reverse The observed base catalyz
`to higher pH
`ed epimerization in near neutral
`range suggests a possible rapid deprotonation
`protonation of the C7 OH followed by a structural
`through a retroaldoValdol mecha
`rearrangement
`nism to form the 7epimer The ratelimiting
`step of structure rearrangement most
`likely
`occurs with the formation of an enolate inter
`is the sum of all
`mediate k3
`the degradation
`rate constants for primary hydrolytic deacylation
`both 7R and 7S
`of 10deacetylbaccatin
`III
`epimers Due to the similarity of
`the chemical
`structures of the R and Sepimers it
`is assumed
`that they have identical rate constants for hydro
`lytic degradation Despite neither epimer following
`simple first order kinetics due to the relatively fast
`preequilibrium the overall degradation
`loss of
`the sum of the two epimers did follow pseudo
`first order kinetics Thus
`R + 5 = R0 + No exPk3t
`
`1
`
`where 1310 and So are the initial concentration
`of the 7R and 7Sepimers and k3 is the overall
`apparent degradation rate constant Therefore
`the values of k3 were readily calculated
`from
`the slope of the linear semilogarithmic plot of the
`residual
`10deacetylbaccatin
`percent
`total
`III
`sum of both epimers versus time Similarly the
`total loss of 10deacetylbaccatin III was measured
`
`Sepimer
`
`Repimer
`
`k2
`
`k31
`
`Products
`
`ik3
`
`Products
`
`Scheme 1 Proposed reaction scheme for the C7 epi
`merization and subsequent hydrolysis of paclitaxel and
`related analogues
`
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`

`baccatin III
`Figure 5
`
`in the basic pH range is shown in
`
`pH Rate Profiles for Total Degradation of
`Ill and Baccatin Ill
`10Deaceylbaccatin
`
`The first order rate constants for total degradation
`and baccatin
`k3 for 10deaceylbaccatin
`III
`III
`were determined in aqueous solution at various
`The
`pH conditions
`pH dependencies
`the
`of
`pseudo first order degradation rate constants of
`III and baccatin III at 25°C
`10deacetylbaccatin
`are shown in Figure 6 The observed rates of the
`degradation increased rapidly and uniformly with
`increasing pH Since the slopes of these straight
`line portions of log k versus pH profiles are close to
`is likely that
`the overall degradation
`unity it
`is
`base catalyzed with little evidence of a water term
`in this pH range The solid lines in Figure 6
`were drawn from fits with the slope fixed at unity
`to show the relative stability of the derivatives
`In addition no significant buffer catalysis was
`
`Rproducr
`Rproduct
`
`3104
`
`TIAN AND STELLA
`
`100
`
`10
`
`ResidualPercent
`
`100
`
`200
`
`300
`
`400
`
`Reaction Time hi
`Figure 4 Semilog plot of the total loss of 10deacetyl
`baccatin III sum of 7R and 7Sepimers at pH 758 at
`
`25°C 0 50°C y and 70°C a
`
`I
`
`5 product
`Sproduct
`
`major
`
`Ho
`
`OH
`
`4
`
`k1
`
`k2
`
`k2
`
`more product
`
`more product
`
`Figure 5 The proposed degradation pathways of baccatin III
`in the basic pH range
`A fast epimerization forms its 7Repimer followed by further hydrolysis of ester bonds
`The value of the overall degradation rate constant k3 is the sum of all hydrolytic rate
`constants contributing to hydrolysis Thus k3 = k3 +14+
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`PACLITAXEL AND RELATED COMPOUNDS IN AQUEOUS SOLUTIONS
`
`3105
`
`k3B values for 10deacetylbaccatin
`calculated
`and baccatin III at 25°C are 372± 113 x 102 and
`537 ± 106 x 102 M1111 respectively The higher
`is consistent with the more
`constant
`for baccatin III
`rapid hydrolysis of the 10 acetyl group compared to
`the other ester groups
`
`III
`
`Effect of Temperature on Degradation
`1 0 Deacetylbaccatin
`of
`
`III
`
`pH
`
`The total degradation of 10deacetylbaccatin III was
`kinetically followed in aqueous solution of pH 761 at
`25 50 and 70°C and the first order rate constants
`the linear semi
`were obtained from the slopes of
`logarithmic plots According to the transition state
`theory the reaction rate constants are related with
`the activation enthalpy
`and activation entropy
`through the Eyring equation In Figure 7 the
`Eyring plots effect of temperature on the degrada
`III were generated by
`tion of 10deacetylbaccatin
`plotting lnkT versus 1T and the solid straight
`linear fit The values of
`the best
`lines represent
`ALP and 6S are 266± 27 kcal mo11 and 13±
`02 eu respectively
`
`for 10deacetylbaccatin
`
`III
`
`hydrolysis
`
`Figure 6 pH rate profiles for the total degradation
`sum of 7S and 7Repimers of 10deacetylbaccatin
`
`III
`
`0 baccatin III y and Nbenzoy13phenylisoserine
`ethyl ester M at various pH values 25°C The symbols
`
`fit
`
`total
`
`pseudo first order
`are experimentally obtained
`degradation rate constant k3 while the solid line repre
`to Eq 2 where values of k3B are
`sents the best
`372 + 113 x 102 537 ± 106 x 102 and 761 ± 069 x
`102 A41 h1 respectively The filled symbols represent
`data actually measured at 25°C while the open symbols
`to 25°C from experimental data
`are results extrapolated
`measured at 70°C and AW
`
`observed although minimal testing was done The
`total shape of log k3 versus pH profiles of degrada
`to basic pH range can
`tion under neutral
`be
`simplified and expressed as the following
`
`C
`
`k3 = k3B
`
`01H
`
`2
`
`where
`k3 is the first order
`rate constant
`for
`the total hydrolysis k3B is the second order rate
`constants for the base catalyzed hydrolysis Kw is
`the autoprotolysis constant of water and 044 is the
`activity of hydrogen ion measured by the glass
`electrode
`In Figure 6 the solid lines represent the theo
`retical curves slopes fixed at unity calculated by
`Eq 2 while the data points are the experi
`mentally obtained first order rate constants The
`second order
`rate constants
`of base catalyzed
`hydrolysis k3B are obtained with the best
`fits of
`
`the observed rate pH profiles using Eq 2 The
`
`0 0028
`
`00029
`
`00031
`
`00032
`
`00033
`
`00034
`
`1T K
`III 0 at pH 761 and 25
`ethyl ester V studied
`
`Figure 7 The Eyring plots for the hydrolytic degra
`dation of 10deacetylbaccatin
`50 and 70°C compared with the hydrolytic degradation
`of Nbenzoy13phenylisoserine
`at pH 991 and 25 435 and 50°C The symbols are
`experimental data while the solid straight
`line repre
`to the Eyring equation
`sents the best
`linear fit
`
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`

`3106
`
`TIAN AND STELLA
`
`Degradation of NBenzoy13Phenylisoserine Ethyl
`Ester and pH Rate Profile
`
`ester was
`NBenzoy13phenylisoserine
`ethyl
`studied as a mimic of
`the side chain of pacli
`taxel structure Figure 8 shows the degradation
`of Nbenzoy13phenylisoserine
`in
`ethyl ester
`aqueous solution at pH values 991 1104 and
`1205 at 250°C
`The first order rate constants of degradation for
`ethyl ester under
`Nbenzoy13phenylisoserine
`various pH conditions are determined in aqueous
`solution The loss of starting compound at pH 991
`1104 and 1205 were determined at 25°C while
`the reaction at lower pH values 781 and 616 were
`slower and therefore were determined at elevated
`temperatures and extrapolated to 25°C Figure 6
`includes the pH rate profile for the hydrolysis of
`ethyl ester at 25°C
`Nbenzoy13phenylisoserine
`from pH 612 The observed rates of the degrada
`tion increased rapidly and uniformly with increas
`ing pH Since the slope of
`the straightline
`portions of log k versus pH profiles is close to
`unity the hydrolysis is clearly base catalyzed
`For the hydrolysis of a simple ester group base
`catalyzed reaction is dominant compared to water
`and basic
`promoted catalysis in near neutral
`pH range Eq 2 can express the pH rate profile
`The solid line in Figure 6 represent the theoretical
`
`curves calculated by Eq 2 while the data points
`
`are experimentally determined The second order
`rate constants of base catalyzed degradation kB is
`761 ± 069 x 102 M1111 at 25°C obtained from
`the best fits of the observed rate pH profiles using
`Eq 2
`
`Effect of Temperature on Degradation
`of NBenzoy13Phenylisoserine Ethyl Ester
`
`To study the temperature effect
`the degradation
`ethyl ester was
`of Nbenzoy13phenylisoserine
`followed in aqueous solutions of pH 991 at 250
`435 and 50°C Based on the Eyring plots in
`Figure 7 the values of Al and
`184± 06
`kcal mot and 18 ± 1 eu are obtained from the
`slope of the best linear fit and from they intercept
`respectively
`
`Degradation of Paclitaxel and Other Taxanes
`
`The total degradation of paclitaxel 7epitaxol
`10deacetyltaxol 7epi10deacetyltaxol were mea
`sured at pH 772 and 70°C At
`this elevated
`temperature the solubility of
`these compounds
`was sufficient for quantitative analysis The time
`course of paclitaxel degradation is shown
`in
`Figure 9 Following the initial
`epimerization at
`
`40000
`
`V
`V
`
`30000
`
`I
`
`20000
`
`coU
`
`VT
`
`10000
`
`I
`
`v
`
`vv 7
`77
`
`600 v
`
`0
`
`6 g ° v
`
`V v V 7 7 7
`
`0
`
`7
`
`0
`
`0
`
`20
`
`40
`
`60
`
`80
`
`100
`
`120
`
`140
`
`160
`
`Time min
`
`Figure 9 Time courses for the loss of paclitaxel y
`formation and loss of 7epitaxol 7 and formation of
`baccatin III 0 and 7epibaccatin III 0 in aqueous
`
`solution at pH 772 T
`
`70°C
`
`200
`
`400
`
`600
`
`800
`
`1000
`
`1200
`
`Reaction Time minute
`
`100
`
`ResidualPercent
`
`Figure 8 Semi log plot showing the pseudo first order
`degradation ofNbenzoy13phenylisoserine ethyl ester in
`
`aqueous solution at pH 991 0 1104y 1205 a and
`
`250°C
`
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`

`PACLITAXEL AND RELATED COMPOUNDS IN AQUEOUS SOLUTIONS
`
`3107
`
`Table 1 Pseudo First Order Rate Constant k3 and
`Second Order Base Catalyzed Degradation Constant
`k3B for the Loss of Taxanes at pH 772 and 70°C
`k3 hr1
`k3B M1 h1
`117±005x 105
`100 ± 004
`0966 ± 0075
`113 ± 009 x 105
`0762 ± 0022
`895 ± 026 x 104
`916±031x 104
`0780 ± 0027
`
`Compound
`
`Paclitaxel
`
`7epitaxol
`
`10Deacetyltaxol
`7epi10deacetyltaxol
`
`and 70°C Thus the second order rate constants
`k3B are
`the based catalyzed
`degradation
`for
`calculated from k3 in Table 1
`
`Effect of Structure on Degradation
`Table 2 lists the second order rate constants km
`the basecatalyzed
`hydrolysis of paclitaxel
`of
`and its analogues at 25 and 70°C The values
`are either experimentally determined or derived
`from their temperature dependence These com
`pounds except
`for Nbenzoy13phenylisoserine
`ethyl ester have the same fundamental diterpene
`the
`ring skeleton but different substituents at
`C10 and C13 position By comparing the value of
`k3B the substituent
`effects on the hydrolysis
`can be determined For example
`rate constants
`baccatin III exhibits faster basecatalyzed hydro
`III at 25°C This
`lysis than 10deacetylbaccatin
`is due to the hydrolysis of the C10
`difference
`acetate ester bond of baccatin III The hydrolytic
`cleavage of the C10 acetate ester bond is faster
`so than the other two
`but not overwhelmingly
`ester groups on C2 and C4 positions The C10
`acetate is one of the most easily accessed position
`with less steric hindrance for the nucleophilic
`attack of the ester bond Moreover
`the electron
`from the nearby C9 carbonyl
`withdrawing effect
`and C11C12 double bond may also accelerate the
`reaction Compared to 10deacetylbaccatin III and
`III Nbenzoy13phenylisoserine
`baccatin
`ethyl
`ester has a better leaving group thus exhibits the
`fastest hydrolysis rate
`In addition 10deacetyltaxol and paclitaxel show
`little difference in their base catalyzed rate con
`stants at 70°C This supports the observation that
`step other than the C7 epime
`the major initial
`rization of paclitaxel hydrolysis is side chain
`cleavage not hydrolysis of the C10 acetate If these
`rate constants
`the
`are cumulative
`
`hydrolytic
`
`deacetyltaxol
`
`the C7 baccatin III and 7epibaccatin III are found
`to be the primary hydrolytic products indicating a
`relatively rapid cleavage of the side chain under
`this condition
`In Figure 10 the total degradation
`of pacli
`and 7epi10
`taxel 7epitaxol 10deacetyltaxol
`The
`one graph
`are plotted on
`the total
`the
`semilogarithmic plots of
`loss of
`starting compound including both epimers versus
`time are reasonably linear and indicated that the
`overall degradation followed
`pseudo first order
`kinetics when
`epimerizations were
`corrected
`for The experimental data yielded good fits to
`
`the theoretical solid lines calculated from Eq 1
`The determined first order hydrolysis rate con
`stants k3 are listed in Table 1 Figure 10 also
`shows that S and Repimers degrade at virtually
`identical rates at a fixed pH and temperature
`The pH rate profiles for
`the hydrolysis of
`III and baccatin III at basic
`10deacetylbaccatin
`pH values
`indicate that base catalysis is the
`primary degradation pathway above pH 7 Due
`taxane ring structure similarity and
`to the
`comparable degradation mechanism compounds
`like paclitaxel 10deacetyltaxol and their epimers
`likely have the same predominant term at pH 772
`
`100
`
`150
`
`Time minute
`
`PercentResidual
`
`Figure 10
`
`Semi log plot of the loss sum of 7S and 7R
`
`epimers of paclitaxel 0 7epitaxol 0 10deacetyl
`taxolV 7epi10deacetyltaxol 7 at pH 772 70°C The
`
`loss of each taxane sum of 7S and 7Repimers
`total
`shows good linearity on this semi log plot and follows
`pseudo first order kinetics
`
`DOI 101002jps
`
`JOURNAL OF PHARMACEUTICAL SCIENCES VOL 97 NO 8 AUGUST 2008
`
`

`

`3108
`
`TIAN AND STELLA
`
`Compound
`
`10Deacetylbaccatin
`
`III
`
`Baccatin III
`
`k3B M1 h1 at 70°C
`248 ± 005 x 104
`ND
`509 ± 046>< 104
`895 ± 026 x 104
`117 ± 005 x 105
`
`Table 2 Comparison of Base Catalyzed Degradation Constant k3B for the
`Degradation of Taxanes at 25 and 70°C
`k3B M1 h1 at 25°C
`372 ± 113 x 102
`537 ± 106x 102
`761 ± 069 x 102
`ND
`ND
`
`Phenylisoserine ethyl ester
`
`10Deacetyltaxol
`
`Paclitaxel
`
`ND not determined
`
`III and Nbenzoy13
`data for 10deacetylbaccatin
`phenylisoserine ethyl ester hydrolysis may effec
`tively predict the hydrolysis of 10deacetyltaxol
`
`CONCLUSIONS
`
`the C7
`Following their initial epimerization of
`hydroxyl paclitaxel and some related taxanes
`including 7epitaxol 10deacetyltaxol 7epi10
`deacetyltaxol baccatin III and 10deacetylbacca
`tin III degrade mainly by hydrolysis under basic
`and neutral conditions in aqueous solution while
`no other reaction such as hydrolytic opening of the
`oxetane ring was observed The hydrolysis for each
`compound above pH 7 is base catalyzed All ester
`to hydrolysis simulta
`bonds
`are susceptible
`neously The primary step of paclitaxel hydrolysis
`is side chain cleavage yielding baccatin III and
`baccatin V 7epibaccatin III with parallel and
`sequential but slower hydrolysis of the other ester
`bonds That is the initial products undergo further
`hydrolysis at ester bonds on C10 C2 and C4
`positions while hydrolysis of C10 acetate is the
`most significant The kinetic data of smaller partial
`III and
`such as 10deacetylbaccatin
`structures
`Nbenzoy13phenylisoserine
`ethyl ester
`the hydrolysis of
`good estimation of
`the larger
`more complex molecule like 10deaceyltaxol
`
`led to
`
`ACKNOWLEDGMENTS
`
`The authors greatly appreciate
`the contribu
`tion and support by Dr Richard Schowen The
`authors also gratefully acknowledge the support
`of Dr Gunda George and Tapestry Pharma
`ceuticals Inc as suppliers of paclitaxel and other
`related compounds
`
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`JOURNAL OF PHARMACEUTICAL SCIENCES VOL 97 NO 8 AUGUST 2008
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`DOI 101002jps
`
`