Photo-degradation products of pramipexole

Article abstract of DOI:10.1016/j.bmcl.2012.02.044

Two pyrrolidine compounds (1 and 2) were isolated from photo-degradation of Bi-Sifrol tablets. Compound 1 was esterified to p-bromophenacyl ester as single-crystal, and then the structure was elucidated by single-crystal X-ray study. Compound 2 was determined by 2D NMR and mass spectra. Otherwise, we established that the photo-degradation of pramipexole was smoothly carried out in the methanol solution, and elucidate the degradation mechanism.

Full text of DOI:10.1016/j.bmcl.2012.02.044

Bioorganic & Medicinal Chemistry Letters 22 (2012) 2951–2953
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Bioorganic & Medicinal Chemistry Letters
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Photo-degradation products of pramipexole
⇑
Eiji Nishimura , Aya Kugimiya, Hideo Naoki, Nobuyuki Hamanaka
Sanyo Chemical Laboratory Co. Ltd, Tajii, Mihara-ku, Sakai, Osaka 587-0012, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Two pyrrolidine compounds (1 and 2) were isolated from photo-degradation of Bi-Sifrol tablets. Com-
pound 1 was esterified to p-bromophenacyl ester as single-crystal, and then the structure was elucidated
by single-crystal X-ray study. Compound 2 was determined by 2D NMR and mass spectra. Otherwise, we
established that the photo-degradation of pramipexole was smoothly carried out in the methanol solu-
tion, and elucidate the degradation mechanism.
Received 26 December 2011
Revised 2 February 2012
Accepted 15 February 2012
Available online 1 March 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Pramipexole
Photo-degradation
Pyrrolidine
NMR
LC/MS
Bi-Sifrol tablet is a dopamine receptor agonist which consists of
pramipexole hydrochloride hydrate, and is licensed by Boehringer
Ingelheim (Fig. 1). This drug is used as an anti-Parkinson’s disease
agent, stimulating D2 receptor at postsynaptic membrane of stria-
tum. Pramipexole free base and its hydrochloride salt are known to
be stable against a beam of light, while Bi-sifrol tablets are rela-
tively unstable, therefore blister pack of aluminum is used for
shading in Bi-sifrol tablets.
We predicted that the additive in Bi-sifrol tablets assists to decom-
pose the active pharmaceutical ingredient (API). Therefore, we
mixed pramipexole and the additive used in Bi-sifrol tablets (povi-
done K25, magnesium stearate, mannitol, and silica gel), and then
the mixture was irradiated under a fluorescent lamp.
Althought there are
a few reports about pramipexole
impurities,1 isolation and characterization of those impurities are
not mentioned. Thus, it is important to propose a degradation
mechanism for pramipexole. In this Letter, the characterization of
the photo-degradation products of Bi-sifrol tablets is provided.
Some Bi-sifrol tablets were ground with a mill, and then irradi-
ated under a fluorescent lamp for 2 days. The time-dependent
change of the degradation profiles was examined by HPLC/ESI-
MS. The LC/MS chromatogram displayed two remarkable (M+H)+
ion peaks at m/z 228.13 and, 210.12, respectively. On the other
hand, pramipexole hydrochloride salt and free base powders were
also irradiated in the same way, but they did not decompose at all.2
H
N
S
N
NH2
2HCl H2O
Figure 1. Structure of pramipexole hydrochloride hydrate.
⇑
Corresponding author.
E-mail address: nishimura@sanyokagaku.co.jp (E. Nishimura).
Figure 2. ORTEP view of 1a.
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2012.02.044

2952
E. Nishimura et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2951–2953
CN
irradiated in the same way. This mixture produced the same deg-
O
N
radation products as the Bi-sifrol tablets, and the rate of degrada-
tion was faster than the hydrochloride salt mixture. It has been
reported that when the aspirin crystal was mixed with a silica
gel, the stability of the aspirin decreased.3 Accordingly, we ex-
pected that the same phenomenon may be observed in Bi-sifrol
tablets. Based on these results, we carried out photo-irradiation
to the mixture of pramipexole free base and silica gel, and two deg-
radation compounds were isolated by silica gel chromatography
(CHCl3–TEA–MeOH).4
The molecular formula of 1 was determined as C10H15N3O2 based
on molecular related ion (M+H)+ m/z 210.1218 (calculated
210.1243) by high-resolution positive ion ESI-MS. The 1H NMR
spectrum of 1 showed three pairs of non-equivalent methylene
groups and one methine signal derived from –CH2–CH2–CH–CH2–.
The signals of n-propyl group were observed. The 13C NMR spec-
trum of 1 exhibited seven signals at d 11.5, 21.0, 26.1, 31.7, 38.0,
45.4, and 60.7, respectively, in the aliphatic region, one nitril carbon
at d 120.1, and also two sp2 carbons at d 173.8 and 178.1. Further-
more, the IR spectrum revealed strong absorption for a nitrile group
at 2183 cmÀ1. To confirm the structure, 1 was esterified to p-Brom-
ophenacyl ester 1a, and recrystallized from n-hexane–AcOEt to ob-
tain as single-crystal.5 The structure of 1a was determined by a
single-crystal X-ray study as in Figure 26.
N
HO
Figure 3. Structure of 1.
O
NH2
O
N
N
HO
Figure 4. Structure of 2.
O
N
CN
N
NH2
O
O
N
N
MeO
MeO
3
4
Consequently, 1 was elucidated to be (S,E)-2-(5-(cyanoimino)-
1-propylpyrrolidin-2yl) acetic acid (Fig. 3).
Figure 5. Structure of 3 and 4.
The molecular formula of 2 was determined as C10H17N3O3
based on molecular related ion (M+H)+ m/z 228.1330 (calculated
228.1348) by high-resolution positive ion ESI-MS. The 1H NMR
spectral data was similar to that of 1. The 13C NMR spectrum of 2
exhibited a new amide signal at d 168.3 instead of a nitrile carbon
at d 120.1. From these results, we expected that the structure of 2
was as 1 with the nitrile group of 1 replaced with an amide group.
Therefore, we hydrolysed 1 with HCl7, and the NMR spectral data
of the product was perfectly consistent with that of 2. Compound
2 was determined as (S,E)-2-(5-(carbamoylimino)-1-propylpyrroli-
din-2yl) acid (Fig. 4). In conclusion, the structures of two unknown
compounds were determined in photo-degradation of the bi-sifrol
tablet.
S
NH2
O
N
N
MeO
Figure 6. Structure of 5.
In the mixture of the pramipexole hydrochloride and silica gel,
we only detected the same two degradation products that were
detected in Bi-sifrol tablets. Furthermore, pramipexole free base
instead of hydrochloride salt was mixed with silica gel and
Photo-degradation of pramipexole caused vanish of 4,5,6,7-tet-
rahydrobenzothiazole skelton to give the pyrrolidine compounds.
liquid state
H
N
H
N
OH
OH
O2
S
N
S
N
O
O
NH2
NH2
NH2
6
O
N
S
N
O
N
CN
N
NH2
solid state
NH2
MeOH
O
O
S
N
O
N
N
N
MeO
MeO
MeO
OH
3
5
4
O
N
S
N
CN
N
NH2
NH2
O
O
O
N
N
N
HO
HO
HO
1
2
Scheme 1. Photo-degradation mechanism of pramipexole.

E. Nishimura et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2951–2953
2953
study, BI-II 546CL and 2-aminobenzothiazole were detected from Bi-sifrol
tablets, but others were not detected.
Compound 1 and 2 increased the oxidation stage compared with
pramipexole, we expected that the singlet oxygen involve the
photo-degradation. Therefore the methanolic solution of pramip-
exole and catalytic rosebengal were subjected to photo-irradiation
under an atmosphere of oxygen. Using 2.0 g of pramipexole, the
reaction was completely finished in 6 hours, and we isolated com-
pound 3 (455 mg, 21.5%) and 4 (129 mg, 5.6%)8 (Fig. 5). Hydrolysis
of 3 was performed by treatment with LiOH to give 1 in high
yields.9
O
H
OH
N
H
N
S
N
S
N
S
N
NH2
NH2
NH2
HCl
BI-II 546CL
2-aminobenzothiazole
BI-II 820BS
The two spots (compounds 5 and 6) on TLC examination at an
early stage of photo-irradiation were purified with silica gel chro-
matography.10 The HR-ESI MS of 5 indicated the C11H20N3O2S
based on molecular related ion (M+H)+ m/z 258.1259 (calculated
258.1276), which supported that the compound includes sulfur
atom. Compound 5 showed similar spectrum in 1H NMR to that
of 2, however, the amide signal of 5 in 13C NMR spectrum observed
down field shift to d 196.1 ppm. Therefore, we determined as (S,E)-
methyl 2-(5-(carbamolimino)-1-propylpyrrolidin-2yl) acetate
(Fig. 6). Compound 5 was photoirradiated to give 3.11
HN
S
O
S
O
N
HN
H
N
N
N
H
S
H2N
NH2
N
BI-II 751xx
BI-II 786BS
The 1H NMR spectrum of compound 6 showed two A2B2-type
signals at d 6.64 and 6.73 only in the aromatic region. The GC-MS
analysis of 6 was detected (M)+ at m/z 110, and the fragment ion
of 6 was consistent with authentic pyrocatechol spectrum in EI-
MS. From these photo-degradation products of pramipexole, we
consider the reaction mechanism to be as illustrated in Scheme 1.
The reaction starts from addition of the singlet oxygen to thia-
zole ring as [2+2]cycloaddition. The C–C bond cleavage of thiazole,
followed by secondary amine attack to carbonyl group affords the
pyrrolidine structure.12 Then it was solvolyzed to give carboxylic
acid and methyl ester.
In summary, we carried out photo-irradiation experiments of
the Bi-sifrol tablets and succeeded in the characterization of four
pyrrolidine compounds as photo-degradation products. Further-
more, we observed that the photo-degradation of pramipexole
was smoothly carried out in the methanolic solution, and eluci-
dated the possible degradation mechanism.
2. Panditrao, V. M.; Sarkate, A. P.; Sangshetti, J. N.; Wakte, P. S.; Shinde, D. B. J.
Braz. Chem. Soc. 2011, 22, 1253.
3. Nakai, Y.; Yamamoto, K.; Terada, K.; Oguchi, T.; Yamamoto, M. Yakugaku Zasshi
1987, 107, 294.
4. Isolation of compound 1 and 2: pramipexole free base (1.0 g) was mixed with
Silica gel (5.0 g) and irradiated under
a fluorescent lamp in ambient
atmosphere for 1 week. Then the irradiated mixture was extracted with
MeOH (100 mL). After concentration, the MeOH extract was chromatographed
over silica gel, using CHCl3–Triethylamine–MeOH (9:1:0.5) as eluent to give 1
(147 mg) and 2 (30 mg).
5. Preparation of p-bromophenacylester 1a as single crystal: To a solution of 1
(7 mg) in CH3CN (2 mL) was added 4-bromophenacyl bromide (22 mg) and
triethylamine (0.2 mL), and the mixture was allowed to react overnight at room
temperature. The reaction mixture was evaporated and purified with
preparative TLC (AcOEt as eluent) to give 1a (8 mg). 1a was recrystallized
with n-hexane-AcOEt to afford single crystal of 1a.
6. Crystallographic data for the structures reported in this paper have been
deposited with the Cambridge Crystallographic Data Centre following deposit
number CCDC-865055 for compound 1a. Copies of these data can be obtained,
free of charge on application to the Director. CCDC 12 Union Road, Cambridge
CB2 1EZ, UK (fax: (+44) 1223 336033: of e-mail: deposit@ccdc.cam.uk).
7. Hydrolysis of 1: 1 (70 mg) was dissolved in 35% HCl (2 mL) and then stirred at
room temperature for 2 h. The reaction mixture was concentrated, and purified
with preparative TLC (CHCl3–MeOH (1:1) as eluent, Rf = 0.35) to give 2 (27 mg).
8. Photo-degradation in liquid state: Pramipexole free base (2.0 g) and rosebengal
(0.1 g) were dissolved in MeOH (500 mL). The mixture was irradiated with
bubbling oxygen under fluorescent lamp for 6 h. Then the reaction mixture was
concentrated, and chromatographed over silica gel, using AcOEt–MeOH (9:1 to
8:2) as eluent to give 3 (455 mg) and 4 (129 mg).
Acknowledgments
We thank the late Professor M. Nishizawa and Professor H.
Imagawa (Tokushima Bunri University) for X-ray analysis, Profes-
sor T. Honda (Hoshi Pharmaceutical University) and Professor K.
Mizuno, and Professor H. Ikeda (Osaka Prefecture University) for
the valuable discussion of photo-degradation.
9. Hydrolysis of 3:
3 (4.2 g) was reacted with LiOH (1.3 g) in MeOH–H2O
(25 mL:20 mL) for 2 h in an ice bath. The reaction mixture was added to H2O
(100 mL), and then pH was adjusted to 1.0 by adding HCl. The mixture was
shaken with CHCl3 (three times of 100 mL), and concentrated to give 1 (3.6 g).
10. Isolation of 5 and 6: pramipexole free base (1.0 g) and rosebengal (0.05 g) were
dissolved in MeOH (500 mL). The mixture was irradiated with bubbling oxygen
under fluorescent lamp for 1 h. Then the reaction mixture was concentrated,
and chromatographed over silica gel, using n-Hexane–EtOAc (2:1?1:2) as
eluent to give 5 (26 mg) and 6 (1 mg).
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2012.02.044.
References and notes
11. Photo-irradiation of 5: 5 (1 mg) and catalytic rosebengal were dissolved in
MeOH (0.7 mL), irradiated under fluorescent lamp for 1 h to afford 3 (1 mg).
12. (a) Wasserman, H. H.; Druckrey, E. J. Am. Chem. Soc. 1968, 90, 2440; (b)
Matsuura, T.; Saito, I. Bull. Chem. Soc. Jpn. 1969, 42, 2973; (c) Wasserman, H. H.;
Vinick, F. J.; Chang, Y. C. J. Am. Chem. Soc. 1972, 94, 7180.
1. (a) Jancic, B.; Medenica, M.; Ivanovic, D.; Malenovic, A. Acta Chim. Slov. 2007, 54,
49; (b) Jancic, B.; Medenica, M.; Ivanovic, D.; Malenovic, A. Chromatographia
2007, 65, 633; (c) Malenovic, A.; Jancic, B.; Vemic, A.; Ivanovic, D. J. AOAC Int.
2010, 93, 1102. Five impurities were described in that article1c as below. In our