A novel approach of water-soluble paclitaxel prodrug with no auxiliary and no byproduct: Design and synthesis of isotaxel

Article abstract of DOI:10.1021/jm034112n

A novel water-soluble paclitaxel prodrug, isotaxel 2, that realizes a higher water-solubility and the formation of paclitaxel through a simple pH-dependent chemical mechanism via the O-N acyl migration was synthesized and showed promising results in water

Full text of DOI:10.1021/jm034112n

3782
J . Med. Chem. 2003, 46, 3782-3784
A Novel Ap p r oa ch of Wa ter -Solu ble
P a clita xel P r od r u g w ith No Au xilia r y
a n d No Byp r od u ct: Design a n d Syn th esis
of Isota xel
Yoshio Hayashi, Mariusz Skwarczynski,
Yoshio Hamada, Youhei Sohma, Tooru Kimura, and
Yoshiaki Kiso*
Department of Medicinal Chemistry, Center for Frontier
Research in Medicinal Science, Kyoto Pharmaceutical
University, Yamashina-Ku, Kyoto 607-8412 J apan
Received May 25, 2003
Abstr a ct: A novel water-soluble paclitaxel prodrug, isotaxel
2, that realizes a higher water-solubility and the formation of
paclitaxel through a simple pH-dependent chemical mecha-
nism via the O-N acyl migration was synthesized and showed
promising results in water-solubility and kinetics. This pro-
drug, a 2′-O-benzoyl isoform of paclitaxel, has no additional
functional auxiliaries released during conversion to paclitaxel,
which would be a great advantage in toxicology and medical
economics.
F igu r e 1. The O-N acyl migration of isotaxel 2 to paclitaxel.
Paclitaxel (Taxol, 1) is one of the most important
chemotherapeutic agents with promising antitumor
activity, especially against ovarian, breast, and lung
cancers.1 However, its sparing water-solubility (0.00025
mg mL-1)2 requires coinjection of a detergent, Cremo-
phor EL, which was suggested to cause hypersensitivity
reactions, and patients receiving this drug require
premedication.3 To resolve these problems, many at-
tempts such as water-soluble prodrugs,2,4 nonprodrug
water-soluble analogues,5 or orally active analogues6
were reported. In water-soluble prodrugs, the hydroxyl
groups at the C-2′ and/or C-7 were extensively modified
with hydrophilic and/or charged solubilizing moieties.
The targeting of paclitaxel using hydrophilic enzymati-
cally cleavable groups4e and tumor targeting moieties4f-i
has also been reported. None of these applications are
presently in clinical use, although some of them are
currently undergoing clinical evaluation.4j-m In addition,
the released auxiliary moieties may have some unfavor-
able effects in vivo. These factors suggest that novel
approaches for water-soluble prodrugs are needed.
We report herein a new prodrug isotaxel 2 (Figure 1)
with improved water-solubility. This prodrug, having
no additional water-solubilizing auxiliaries and forming
no byproduct during conversion to the parent drug, is a
2′-O-benzoyl isoform of 1, was designed to increase
water-solubility with an ionized 3′-amino group and
allows for conversion to 1 via O-N acyl migration of
the benzoyl group under physiological conditions.7
N-O intramolecular acyl migration is known as a side
reaction of Ser- or Thr-containing peptides.8 The â-
hydroxyl groups are acylated by the N-O shift under
acidic conditions and the resulting O-acyl products can
be readily converted to N-acyl compounds under neutral
or slightly basic conditions in aqueous buffer. The
liberated ammonium ion enhances the water-solubility
of O-acyl products. Considering these features, we
designed a novel class of “O-N intramolecular acyl
migration”-type water-soluble prodrugs of HIV-1 pro-
tease (PR) inhibitors having allophenylnorstatine (Apns,
(2S,3S)-3-amino-2-hydroxy-4-phenylbutanoic acid).9 Hur-
ley et al. also reported a study on the O-N acyl
migration of renin inhibitors.10 The O-acyl prodrug of
HIV-1 PR inhibitors at the 2-hydroxyl group of the Apns
moiety completely released their parent drug in a few
minutes under physiological conditions.
First, we examined the effect of the benzoyl group and
stereochemistry of R-hydroxy-â-amino acids on the
kinetics of the O-N acyl migration using a series of
model compounds (see Supporting Information). It was
revealed that the migration of O-benzoylphenylisoserine
derivative corresponding to the amino acid part of
paclitaxel showed relatively slower migration with a t1/2
value of 12 min under physiological conditions. This
value appeared to be suitable for systemic distribution
after injection and not long enough for metabolism and
elimination. Moreover, under the acidic aqueous condi-
tions, these compounds were maintained stably without
any migration. These promising results prompted us to
apply this strategy to paclitaxel. Hence, we next inves-
tigated the synthesis of the O-acyl form of paclitaxel,
namely isotaxel 2.
As depicted in Scheme 1, Nâ-Troc-phenylisoserine
methyl ester 4,11 was prepared from commercially
available (2R,3S)-phenylisoserine‚HCl 3. 1,3-Oxazoli-
dine derivative 5 was obtained from ester 4 in the
reaction with 4-methoxybenzaldehyde dimethyl acetal
in the presence of a catalytic amount of PPTS. Hydroly-
sis of 5 gave carboxylic acid 6 which was used in the
next step without further purification. The coupling of
6 with 7-Troc-baccatin III12 in the presence of DCC
afforded the corresponding ester 7 in a nearly quantita-
tive yield without any detectable epimerization at the
* To whom correspondence should be addressed. Tel: +81-75-595-
4635. Fax: +81-75-591-9900. E-mail: kiso@mb.kyoto-phu.ac.jp.
10.1021/jm034112n CCC: $25.00 © 2003 American Chemical Society
Published on Web 07/26/2003

Letters
J ournal of Medicinal Chemistry, 2003, Vol. 46, No. 18 3783
Sch em e 1a
a
Reagents and conditions: (a) succinimidyl-2,2,2-trichloroethyl carbonate, NaOH, NaHCO3, dioxane, rt, 1 h; (b) SOCl2, MeOH, 0 °C to
room temperature, 14 h, 97% over two steps; (c) 4-methoxybenzaldehyde dimethyl acetal, PPTS, toluene, distillation 30 min, 92%; (d)
KOH, MeOH, rt, 30 min. 99%; (e) 7-Troc-baccatin III, DCC, DMAP, toluene:CH2Cl2 2:1, rt, 3 h, 98%; (f) PTS, MeOH, rt, 24 h, 94%; (g)
benzoic acid, EDC‚HCl, DMAP, CH2Cl2, rt, 2 h, 92%; (h) Zn (dust), MeOH:AcOH 1:1, rt, 4 h, then HCl, 77%.
decomposition. Moreover, incubation in 0.035% citric
acid saline (pH 4.0) at room temperature showed very
slow migration of 2‚HCl (<3% of paclitaxel was released
after incubation for 3 h), suggesting a possible condition
of the injectable solution for practical clinical use.4c,14
In addition, the 2′-O-benzoyl ester of 2 was biologically
stable since this bond did not cleave at all in the
experiment using porcine liver esterase (see Supporting
Information).
In conclusion, we synthesized a water-soluble pacli-
taxel prodrug, isotaxel 2, that realized a higher water-
solubility and the formation of paclitaxel through a
simple pH-dependent chemical mechanism via the O-N
acyl migration. This prodrug, a 2′-O-benzoyl isoform of
paclitaxel, has no additional functional auxiliaries
released during conversion to paclitaxel. This would be
a great advantage in toxicology and medical economics,
since the potential side effects caused by reported
auxiliaries and the use of detergent for solubilization
can be omitted.
F igu r e 2. Migration of prodrug 2 in different pH conditions
at 37 °C; c ) concentration, t ) incubation time.
C-2′ position.13 The oxazolidine ring in 7 was cleaved
with PTS. Finally, after benzoylation of the 2′-hydroxyl
group with benzoic acid by the EDC-DMAP method,
deprotection of both Troc groups using Zn-AcOH, and
following purification and ion-exchange by HPLC eluted
with 12 mM aq HCl gave isotaxel 2 as a HCl salt with
a good total yield (58%).
The water-solubility of 2‚HCl was determined as a
value of 0.45 ( 0.04 mg mL-1, which was 1800-fold
higher than that of paclitaxel (0.00025 ( 0.00004 mg
mL-1). To study the kinetics of O-N benzoyl migration,
2‚HCl was dissolved in PBS at different pH and
incubated at 37 °C. The migration was monitored by
HPLC (see Supporting Information). A complete migra-
tion was observed at pH 7.4 with a t1/2 value of 15.1 (
1.3 min (Figure 2), and this value is suggested to be
appropriate for the systemic distribution. On the other
hand, a slower migration was observed at pH 4.9 with
a t1/2 value of 252.2 ( 37.7 min and no migration at pH
2.0 after 6 h of incubation.
Ack n ow led gm en t. This research was supported in
part by the Frontier Research Program of the Ministry
of Education, Science and Culture of J apan, and grants
from the Ministry of Education, Science and Culture of
J apan. We thank Dr. S. Ogawa and Ms. K. Oda for the
measurement of NMR and mass spectra.
Su p p or tin g In for m a tion Ava ila ble: The synthetic and
spectroscopic data of 2, 4-9 and O-benzoyl-R-hydroxy-â-amino
acid derivatives, O-N benzoyl migration study of O-benzoyl-
R-hydroxy-â-amino acid derivatives, the modeling of migration
intermediates of model compounds, and water-solubility and
stability studies of isotaxel. This material is available free of
charge via the Internet at http://pubs.acs.org.
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