May 2005
Chem. Pharm. Bull. 53(5) 537—540 (2005)
537
Isomerization Kinetics of Panipenem in Aqueous Solution
,a
a
a
b
Nobuya ITO,* Masahiko SUZUKI, Akira KUSAI, and Kozo TAKAYAMA
a
Pharmaceutical Development Laboratories, SANKYO CO., LTD.; 1–12–1 Shinomiya, Hiratsuka-shi, Kanagawa
54–0014, Japan: and Department of Pharmaceutics, Hoshi University; 2–4–41 Ebara, Shinagawa-ku, Tokyo 142–8501,
b
2
Japan. Received January 13, 2005; accepted February 28, 2005; published online March 3, 2005
The isomerization kinetics of panipenem (INN: (ꢀ)-(5R,6S)-3-[(S)-1-(acetimidoylpyrrolidin-3-yl)thio]-6-
[(R)-hydroxyethyl]-7-oxo-1-azabicyclo[3.2.0]hept-2-ene-2-carboxylic acid, CAS No. 87726-17-8) in aqueous solu-
tion were investigated. An equilibrium between the Z-form and E-form was observed, and it was found that the
isomerization rates were affected by the pH of the solution. Under acidic conditions, the isomerization rates were
small. However, the isomerization rates were increased with the pH value. This phenomenon resulted from the
extent of proton dissociation from the acetimidoyl group.
Key words panipenem; Z-isomer; E-isomer; equilibrium; kinetics; mechanism
Panipenem (PAPM, Fig. 1) is a synthesized carbapenem in the reaction solutions was 1 mg/ml in every experiment (1 mg/mlꢀ3 mM).
Samples were taken out from the test tube at specified times, and were di-
luted with ice-cooled buffer (pH 5.0). The concentration of each form of pa-
nipenem (Z-form or E-form) was determined by high-performance liquid
chromatography (HPLC).
High-Performance Liquid Chromatography Analysis A m-Bondas-
phere C18 (3.9 mm I.D.ꢂ150 mm, Nihon Waters K.K.) column was ice-
cooled and kept at 0 °C during analysis. The mobile phase was 0.05 M ac-
antibiotic discovered and developed at Sankyo Co., Ltd. This
compound has a broad antibacterial spectrum and potent bac-
tericidal activity against both Gram-positive and Gram-nega-
tive bacteria, and this compound is not readily hydrolyzed by
1—4)
microbial b-lactamase.
The pyrrolidinylthio group of the C2 side chain of the car-
bapenem nuclear structure, enhances the activity against
Gram-negative bacteria, while the acetimidoyl group, at the 1
position of the pyrrolidinylthio group, enhances the activity
against Pseudomonas aeruginosa.
etate buffer (pH 5.0) : CH CNꢀ94 : 6 (v/v). The flow rate of the mobile
3
phase was 1.0 ml/min. The elution profiles were monitored and recorded by
a UV detector at 280 nm.
Theoretical Analysis
k
f
→
In the previous report, the stability of panipenem in aque-
ous solution was investigated in terms of pH, ion strength,
temperature and buffer catalysis, and compared with other b-
Z -isomer ← E -isomer
(1)
k
r
Scheme 1 represents the reversible reaction between the Z-form isomer
13)
5—11)
and E-form isomer. The constant values, k and k , are the rate constants of
f r
lactam antibiotics.
the forward reaction and reverse reaction, respectively. According to Scheme
As for its stereochemistry, panipenem exists as either the
1, the reaction can be represented by Eq. 2.
Z-form or E-form depending on the position of the acetimi-
12)
d
doyl group. A certain isomer type can be obtained depend-
ing on the solvent used in the recrystallization process; the
E/Z mixture, E-form, and Z-form can be obtained, by using
[
Z]ꢀꢄk [Z]ꢃk [E]
(2)
f
r
dt
where [Z] and [E] are the concentration of the Z-form and E-form, respec-
acetone, methanol, and ethanol as the recrystallization sol- tively. The degradation of panipenem was negligible in all the experiments
vent, respectively. In this report, the isomerization kinetics of in this report. Thus, the total concentration was maintained during the ki-
netic runs.
panipenem in aqueous solution was investigated. The data in
this report were obtained as the basic information for the for-
mulation development.
[Z]ꢃ[E]ꢀconst.ꢀ[Z]
0
ꢃ[E]
0
(3)
where [Z] and [E] are the initial concentrations of the Z-form and E-form,
0
0
respectively. From Eqs. 2 and 3, Eq. 4 can be obtained.
Experimental
Materials Panipenem (E-form and Z-form) was obtained from Sankyo
Co., Ltd. and was used without further purification. 2-(N-morpholino)
d
[Z]ꢀꢄ(k
ꢃk
)⋅[Z]ꢃk
⋅([Z]
ꢃ[E]
)
(4)
f
r
r
0
0
dt
ethanesulfonic acid (MES, pK ꢀ6.15) and 3-(N-morpholino) propanesul-
a
fonic acid (MOPS, pK ꢀ7.20) were purchased from Dojin Chemicals Co.,
Ltd. All other chemicals were of reagent grade.
Kinetic Runs The buffer used as the kinetic medium was 0.05 M MES
a
Equation 5 can be obtained by integrating Eq. 4, with “[Z]ꢀ[Z] at tꢀ0”
as the initial condition.
0
(
pH 5—6) or 0.05 M MOPS (pH 7—9), which were adjusted to the desired
k [Z] ꢄk [E]
0
f
0
r
(
k ꢃk )⋅t ꢀln
(5)
pH values with concentrated aqueous sodium hydroxide. The buffered solu-
tions in the test tube were maintained at the reaction temperature (ꢁ0.1 °C)
with a thermostatic water bath. The pH value of the buffered solutions was
measured before and after the kinetic runs. The concentration of panipenem
f
r
(
k ꢃk )⋅[Z]ꢄk ⋅([Z] ꢃ[E] )
f r r 0 0
Equation 5 can be transformed to Eq. 6, and [Z] can be expressed as a
function of time.
Fig. 1. Chemical Structure of Panipenem (Left, Z-Form; Right, E-Form)
∗
© 2005 Pharmaceutical Society of Japan