A novel drug interaction between the quinolone antibiotic ciprofloxacin and a chiral metabolite of pentoxifylline

Article abstract of DOI:10.1016/j.bcp.2007.05.013

Pentoxifylline (PTX), a methylxanthine derivative, is metabolized to seven compounds in vivo, with metabolites 1 and 5 possessing biologic activity. Metabolite-1 is a chiral molecule and its S-enantiomer is selectively formed during PTX metabolism in vivo. We have developed a reproducible method of synthesizing a racemic mixture of the chiral metabolite-1 (M-1) of PTX. In this study, we examined the kinetics of racemic M-1 in mice compared to PTX. An interaction between PTX and the quinolone antibiotic ciprofloxacin has been demonstrated. A goal of this study was to determine if a similar interaction occurs between ciprofloxacin and M-1 in vivo. M-1 and PTX had similar absorption and elimination rates. M-1 was rapidly converted to PTX, while very little PTX was converted to M-1 in vivo. The peak concentration of biologically active drug (PTX + M-1) was 36% higher when M-1 was administered compared to PTX. Combination of ciprofloxacin and PTX significantly increased serum concentrations of both PTX and M-1 (2-fold) compared to controls. The combination of M-1 and ciprofloxacin significantly increased serum concentration of M-1 (3-fold) and PTX (2-fold). The ciprofloxacin/M-1 combination produced a significantly higher sera concentration of bioactive drug compared to all other groups suggesting that this combination may enhance the anti-fibrogenic effect.

Full text of DOI:10.1016/j.bcp.2007.05.013

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A novel drug interaction between the quinolone antibiotic
§
ciprofloxacin and a chiral metabolite of pentoxifylline
Jennifer M. Raoul , Marc R. Peterson , Theresa C. Peterson a,b,*
b
a
a
Department of Medicine, Faculty of Medicine, Dalhousie University, Halifax, Nova Scotia, Canada
b
Department of Pharmacology, Faculty of Medicine, Dalhousie University, Halifax, Nova Scotia, Canada
a r t i c l e i n f o
a b s t r a c t
Article history:
Pentoxifylline (PTX), a methylxanthine derivative, is metabolized to seven compounds in
vivo, with metabolites 1 and 5 possessing biologic activity. Metabolite-1 is a chiral molecule
and its S-enantiomer is selectively formed during PTX metabolism in vivo. We have
developed a reproducible method of synthesizing a racemic mixture of the chiral metabo-
lite-1 (M-1) of PTX. In this study, we examined the kinetics of racemic M-1 in mice compared
to PTX. An interaction between PTX and the quinolone antibiotic ciprofloxacin has been
demonstrated. A goal of this study was to determine if a similar interaction occurs between
ciprofloxacin and M-1 in vivo. M-1 and PTX had similar absorption and elimination rates. M-
Received 19 April 2007
Accepted 16 May 2007
Keywords:
Pentoxifylline
Ciprofloxacin
Metabolite-1
Fibrosis
1
was rapidly converted to PTX, while very little PTX was converted to M-1 in vivo. The peak
concentration of biologically active drug (PTX + M-1) was 36% higher when M-1 was admi-
nistered compared to PTX. Combination of ciprofloxacin and PTX significantly increased
serum concentrations of both PTX and M-1 (2-fold) compared to controls. The combination
of M-1 and ciprofloxacin significantly increased serum concentration of M-1 (3-fold) and PTX
Metabolism
Drug interaction
(
2-fold). The ciprofloxacin/M-1 combination produced a significantly higher sera concen-
tration of bioactive drug compared to all other groups suggesting that this combination may
enhance the anti-fibrogenic effect.
#
2007 Elsevier Inc. All rights reserved.
1
.
Introduction
system, results from our lab. and others have shown that PTX
is a potent anti-fibrogenic compound. PTX inhibits prolifera-
tion and collagen synthesis by cultured fibroblasts, myofibro-
blasts, hepatic stellate cells, renal myofibroblasts, intestinal
smooth muscle cells and vascular smooth muscle cells in the
presence of various fibrogenic stimuli [12–18]. PTX also
attenuated fibrosis in animal models of glomerulonephritis
[19], Peyronie’s disease [20] and hepatic fibrosis [13,21–23]. The
combined anti-inflammatory and anti-fibrotic effects of this
drug, along with its safety and affordability, make PTX an
attractive candidate for treatment of chronic inflammatory
and fibrotic diseases.
Pentoxifylline (PTX; 1-(5-oxyohexyl)-3,7-dimethylxanthine) is
a methylxanthine derivative and non-specific phosphodies-
terase (PDE) inhibitor [1,2]. It is widely prescribed for the
treatment of certain vascular disorders such as intermittent
claudication due to its ability to improve blood flow through
narrow vessels [2,3]. In addition to its effects on the
microcirculation, PTX is known to have diverse anti-inflam-
H
matory actions, including in vitro and in vivo inhibition of T 1
cytokine synthesis (e.g. TNF-a, IL-12, IL-2 and IFN-g) by various
immune cells [4–11]. In addition to its effects on the immune
§
Grant support: Canadian Institutes of Health Research; Nova Scotia Health Research Foundation.
*
Corresponding author at: Department of Medicine, Tupper Medical Building, 11-C1, Dalhousie University, Halifax, Nova Scotia B3H4H7,
Canada. Tel.: +1 902 494 2571; fax: +1 902 494 1388.
E-mail address: tcpmed@dal.ca (T.C. Peterson).
0006-2952/$ – see front matter # 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.bcp.2007.05.013

640
b i o c h e m i c a l p h a r m a c o l o g y 7 4 ( 2 0 0 7 ) 6 3 9 – 6 4 6
PTX is metabolized to at least seven molecules in mammals
that elevated serum drug levels are not due to down-regulation
of the CYP1A2 gene, but rather to inhibition of the catalytic
activity of CYP1A2 and an induction of CYP2E1, liver enzymes
involved in PTX metabolism [31]. Since both PTX and M-1 are
biologically active compounds, elevated levels of these drugs in
mice in vivo would likely improve the therapeutic effects of PTX
administration. A direct interaction between M-1 and cipro-
floxacin has not been examined. The goals of the present study
were to compare the kinetics of racemic M-1 and PTX in mice,
since racemic M-1 had not previously been administered in
vivo, and to determine if a pharmacologic interaction between
ciprofloxacin and the novel M-1 occurs in mice.
with metabolites 1 and 5 being the major products formed [24].
Metabolite-1 (M-1; 1-(5-hydroxyhexyl)-3,7-dimethylxanthine)
is formed from PTX in vivo by the reduction of a single carboxyl
group to a corresponding hydroxyl group, forming a chiral
centerinthe molecule(Fig. 1). The S-enantiomerofM-1 (M-1S)is
selectively formed during PTX metabolism in vivo and is
thought to have biological properties similar to PTX although
few studies have examined this [1,25,26]. We have been
investigating the in vitro effects of racemic M-1 (R/S) and have
shown that it is as effective as PTX in inhibition of PDGF-
stimulated human skin fibroblast proliferation [27] and is more
potent than PTX in inhibiting collagen synthesis by PDGF-
stimulatedporcine hepaticmyofibroblasts[13,28]. These results
indicate that M-1 has potential as an anti-fibrogenic agent.
A pharmacologic interaction between PTX and the antibiotic
ciprofloxacin in humans was reported to produce elevated
serum levels of both PTX and M-1 in vivo with increased
formation of the transient M-1R enantiomer [29]. We have
shown that this interaction can be reproduced in mice [30], and
2.
Methods
2.1.
Animals
Male CD1 mice weighing approximately 25 g were obtained
from Charles River Laboratories and maintained in the Carlton
Fig. 1 – Structures of pentoxifylline and its major metabolite (M-1). M-1 is formed from the reduction of a carboxyl group
O) on the PTX molecule (arrow) to a corresponding hydroxyl group (–OH), forming a chiral center in the metabolite. The
(
C
different spatial orientations of the hydroxyl group around the chiral carbon atom of M-1 give rise to the two enantiomers
of the molecule (M-1S and M-1R). The S-enantiomer is the major metabolite formed in vivo during PTX metabolism. Also
shown are the metabolic pathways of pentoxifylline that we propose are affected by ciprofloxacin. The effect is shown as (+)
or (S) above the CYP enzyme.

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641
Animal Care Facility at Dalhousie University. Animals were
Table 1 – Treatment groups in ciprofloxacin experiment
housed six per cage with a 12 h light:12 h dark cycle and ad
libitum access to water and standard chow. Animals were
given 1 week to acclimatize to the facility before experimenta-
tion began. The experimental protocol was approved by the
University Committee on the use of Laboratory Animals at
Dalhousie University.
DAY 1–9
DAY 9
N (mice)
Saline (i.p.)
Saline (i.p.)
Saline (i.p.)
Saline (i.p.)
5
5
5
5
5
5
PTX (100 mg/kg i.p.)
M-1 (100 mg/kg i.p.)
Saline (i.p.)
1
Cipro (25 mg/kg i.p.)
1
Cipro (25 mg/kg i.p.)
1
PTX (100 mg/kg i.p.)
M-1 (100 mg/kg i.p.)
Cipro (25 mg/kg i.p.)
2
.2.
Chemical synthesis of metabolite-1 from pentoxifylline
Metabolite-1 is a chiral molecule derived from PTX by the
reduction of a single ketone functional group to a hydroxyl
animals from each group received an injection of PTX (100 mg/
kg i.p.), five animals received an injection of M-1 (100 mg/kg
i.p.), and the remaining animals received an equivalent
volume of saline (ꢀ200 ml). Table 1 gives a summary of the
treatment groups. Animals were sacrificed at 30 min post-
injection and blood was collected by cardiac puncture for
serum analysis by HPLC. This timepoint was chosen based on
the kinetics data (Fig. 2) since it is a point beyond the peak
absorption of both PTX and M-1 by the i.p. route.
group (Fig. 1).
A racemic mixture of metabolite-1 was
synthesized from PTX (Sigma; Oakville, ON, Canada) in a
chemical reduction reaction carried out in the Department of
Chemistry at Dalhousie University with the assistance of Dr.
Jim Pincock. PTX (10 g) was dissolved in HPLC-grade methanol
4
(500 ml) and sodium borohydride (NaBH ; 1.8 g) was slowly
added as the reducing agent. The reaction proceeded for 1 h at
room temperature with constant stirring and the methanol
was evaporated using a rotary evaporator. The crude product
2.5.
High performance liquid chromatography (HPLC)
was dissolved in 150 ml of distilled water (dH
extracted four times using dichloromethane (4 ml  100 ml
CH Cl ). A powdered drying agent (Na SO ) was added to the
organic phase to absorb water and the solution was gravity
filtered. The remaining organic solvent (CH Cl ) was evapo-
2
O) and was
Serum drug levels were measured using reverse-phase HPLC
[33,34] by comparing samples of unknown drug concentration
2
2
2
4
to standard curves prepared using
a range of known
2
2
concentrations of PTX and M-1 (final concentration 1–15 mg/
ml) referenced to phenacetin, the internal standard. Phena-
cetin was used because it has a retention time similar to PTX
and can be visualized on the same HPLC chromatogram.
Samples and standards were prepared for HPLC analysis by
extraction through 1 ml C-18 Solid Phase Extraction cartridges
(JT Baker) into glass test tubes and were dried by nitrogen
evaporation. Dried samples could be stored for extended
rated leaving a powdered compound (M-1) behind. This
product was recrystallized from 2-propanol overnight at
4
8C, dried by suction filtration the following day, and was
allowed to air dry for several days before use. A yield greater
than 80% was consistently achieved with this method. The
product was identified as M-1 by proton nuclear magnetic
1
resonance spectroscopy ( H NMR). The synthesis method
employed was a non-selective chemical reduction reaction
producing a racemic mixture (1:1) of M-1 enantiomers [32].
2
.3.
Kinetics of M-1 and PTX in vivo
Mice were injected with heparin (1000 units/kg i.p.; Leo
Pharma Inc., Thornhill, ON, Canada) to prevent blood from
clotting during the procedure. After 40 min, animals received
an injection of either PTX or M-1 (100 mg/kg i.p.). Blood
samples (ꢀ50 ml) were collected at 5, 10, 15, 25, 40, 60 and
1
20 min post-injection from a small puncture at the tip of the
tail. Blood was centrifuged and serum was collected for HPLC
analysis.
2
.4.
Interaction between M-1 and ciprofloxacin in vivo
Combination of ciprofloxacin and PTX in vivo is reported to
elevate serum levels of PTX in humans with increased
formation of the rare R-enantiomer of M-1, which is not
generally produced during PTX metabolism [29]. A direct
interaction between ciprofloxacin and racemic M-1 has never
been investigated. In order to assess the interaction of
ciprofloxacin with PTX and the novel M-1 in mice, animals
were first divided into two groups of 15. One group received
daily injections of the antibiotic ciprofloxacin (25 mg/kg i.p.;
Bayer) for 9 days while the control group received daily
injections of sterile 0.9% saline solution. On the ninth day, five
Fig. 2 – Kinetics of M-1 and PTX in vivo. Animals were
injected with M-1 (dashed lines) or PTX (solid lines)
(100 mg/kg i.p.) and serum samples were analyzed by
HPLC at 5, 10, 15, 25, 40, 60 and 120 min. Interconversion
of PTX and M-1 occurs in vivo thus both drugs are detected
by HPLC analysis following injection of either drug alone.
Rate of elimination between the 20 and 40 min timepoints
was approximately 0.75 mg/ml/min for both PTX and M-1.
Each point is mean drug concentration (mg/ml) W S.E.M. for
four separate animals.

642
b i o c h e m i c a l p h a r m a c o l o g y 7 4 ( 2 0 0 7 ) 6 3 9 – 6 4 6
periods at 4 8C until HPLC analysis. Dried samples were
dissolved in HPLC mobile phase (100 ml; 75% mH O: 25%
2
acetonitrile) and 10 ml was injected into the HPLC injection
port using a glass syringe. The HPLC instrument used was
Beckman System Gold consisting of an Analogue Interface
Module 406 coupled to a UV detector Module 166 (set at
2
73 nm) and Programmable Solvent Module 126. The sta-
tionary phase used was a C-18 column (250 mm length, i.d.
.6 mm; Econosource) and the flow rate was 1 ml/min with a
4
run-time of 15 min. HPLC printouts were obtained consisting
of a chromatograph and relevant data for each sample,
including retention times, peak height and peak area. The
ratio of the area under the drug peak to the area under the
phenacetin peak (peak areadrug/peak areaphen) was used to
construct a standard curve, with drug concentration (mg/ml)
on the x-axis and peak area ratio (peak areadrug/peak areaphen
)
on the y-axis. Serum drug levels from treated animals were
determined by calculating peak areadrug/peak areaphen for
each sample and comparing to the standard curves (x = y À b/
m).
Fig. 3 – Kinetics of M-1 and PTX in vivo graphed as total
drug (PTX + M-1) in serum. Combined levels of PTX and M-
1
give an estimate of relative bioactivity in vivo. Each point
is mean drug concentration (mg/ml) W S.E.M. for four
separate animals.
3.
Results
3
.1.
Kinetics of M-1 and PTX
Kinetics of M-1 and PTX in vivo are shown in Fig. 2. HPLC
analysis of serum drug levels following M-1 (dashed lines) and
PTX (solid lines) injection indicated that both drugs were
detectable in serum following injection of either drug alone.
This was expected since the conversion between PTX and M-1
is a reversible reaction that takes place in the blood. M-1 had
reached a peak by 10 min post-injection while PTX peaked at
therapeutic effects of PTX or M-1 depend on the total amount
of both active compounds in the body, results were graphed as
total drug (PTX + M-1) in the serum at 30 min post-injection
(Fig. 6). Total drug levels following M-1 injection alone were
significantly higher than following PTX injection alone
(23.6 Æ 1.7 mg/ml versus 15 Æ 1.0 mg/ml; p < 0.05). Ciprofloxa-
cin pre-treatment significantly elevated total drug levels
following PTX or M-1 injection ( p < 0.05). The combination
of ciprofloxacin and M-1 produced a significantly higher total
drug level (57 Æ 3.3 mg/ml) compared to PTX with ciprofloxacin
pre-treatment (28 Æ 4.6; p < 0.05).
1
5 min. The rate of elimination for both M-1 (following M-1
injection) and PTX (following PTX injection) was 0.75 mg/ml/
min between the 20 and 40 min timepoints. Serum half-lives
of PTX and M-1 were 15 and 20 min, respectively. Levels of PTX
and M-1 in serum following M-1 injection reached near
equilibrium by 25 min, whereas only a small amount of PTX
was converted to M-1 following PTX injection (Fig. 2). This
resulted in 36% higher peak combined drug levels (M-1 + PTX)
following M-1 injection compared to PTX (Fig. 3). Since both
drugs are active in vivo, the combined levels of PTX and the M-
4.
Discussion
We have previously demonstrated that racemic M-1 inhibits in
vitro proliferation and collagen synthesis in fibroblasts
stimulated with PDGF, implicating M-1 as a potential anti-
fibrogenic compound [13,27]; however, its effects in vivo were
previously unexplored. Injection of M-1 (32 and 100 mg/kg i.p.)
into 25 g mice revealed no signs of toxicity. Signs of toxicity
would include dizziness (rotating head movements), lethargy
and hypersalivation (excessive licking movements). No signs
of toxicity were observed in CD1 mice with these doses of drug.
Examination of M-1 kinetics compared to PTX revealed that
both drugs reached peak serum levels 10–15 min after i.p.
injection with similar a-elimination rates of 0.75 mg/ml/min.
The serum half-lives of M-1 and PTX were 15 and 20 min,
respectively. Differences in the metabolic profiles of the two
compounds were observed by analyzing the conversion of M-1
to PTX in vivo and vice versa. Following injection of PTX, very
little M-1 was detected in the serum. Following M-1 injection,
the levels of M-1 and PTX in serum reached equilibrium by
40 min, indicating that PTX is a major metabolite formed from
1
will determine the overall biological effects of drug
administration. While no drugs were detected in serum 120-
minutes after PTX injection, detectable amounts of both M-1
and PTX remained following injection of the metabolite.
3
.2.
Interaction between M-1 and ciprofloxacin in vivo
To investigate the possibility of an interaction between
ciprofloxacin and M-1 in vivo, mice were pre-treated with
ciprofloxacin or saline for 9 days before receiving an injection
of M-1 or PTX (100 mg/kg i.p.). Ciprofloxacin pre-treatment
increased serum drug levels following injection of either PTX
or M-1 (Fig. 4). Serum M-1 levels were increased 3-fold
(
p < 0.05; Fig. 5) compared to serum M-1 levels in mice not
pre-treated with ciprofloxacin. A statistically significant 2-fold
increase in serum drug levels was achieved in all other groups
due to ciprofloxacin pre-treatment ( p < 0.05). Since the

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643
Fig. 4 – Representative HPLC chromatographs demonstrating the effect of ciprofloxacin pre-treatment on serum levels of PTX
and M-1. This figure shows the visual difference in HPLC peak area for each drug between treatment groups. Animals were
treated with ciprofloxacin (25 mg/kg) or saline for 9 days before receiving an injection of PTX or M-1 (100 mg/kg i.p.). Serum
samples were collected 30 min after PTX or M-1 injection. (A) PTX injection (saline pre-treatment). (B) M-1 injection (saline
pre-treatment). (C) PTX injection (ciprofloxacin pre-treatment). (D) M-1 injection (ciprofloxacin pre-treatment). Phenacetin
(
30 mg/ml) was used as the internal standard for all samples so that serum drug levels could be calculated from a standard
curve.
racemic M-1 in vivo. A recent study demonstrated that
reversible interconversion of PTX and M-1 takes place rapidly
in human erythrocytes (CYP450-independent mechanism)
with the conversion of M-1 to PTX being favored 4-fold over
the conversion of PTX to M-1 [35], consistent with our findings
in mice. The rate of conversion of M-1S to PTX was 4-fold
higher than conversion of M-1R in this study, and the authors
indicate that liver enzymes likely participate in conversion of
M-1R to PTX in vivo. In support of this, conversion of M-1R back
to PTX has been reported in human liver microsomes and is
thought to occur partially via CYP1A2 and also via an
unidentified CYP450-independent mechanism, which could
include cytosolic oxidases [36,37].
an anti-fibrogenic drug (IC50 value 35 mM) compared to PTX
(IC50 value 161 mM) for the inhibition of collagen synthesis in
hepatic stellate cells. The higher levels of M-1 achieved in this
study by injecting the metabolite directly therefore could
result in enhanced anti-fibrogenic effects in vivo.
An in vivo drug interaction between the quinolone
antibiotic ciprofloxacin and PTX has been demonstrated in
humans that resulted in elevated plasma levels of PTX and M-1
[29]. Our group has reproduced this interaction in mice and
demonstrated that pre-treatment of CD1 mice with cipro-
floxacin (25 mg/kg) resulted in a 2-fold increase in serum
concentrations of PTX and M-1 compared to controls [12,30].
Increased serum drug levels were due to inhibition of CYP1A2
catalytic activity and induction of CYP2E1 [12], consistent with
ciprofloxacin as a competitive inhibitor of the CYP1A2 enzyme
[38,39].
Since both PTX and racemic M-1 are active in vivo, the
higher total drug levels (M-1 + PTX) achieved in serum
following M-1 injection would likely result in enhanced
therapeutic effectiveness of this compound. The total drug
levels achieved in mouse serum following M-1 and PTX
injection were 171 and 125 mM, respectively, concentrations at
which significant anti-fibrogenic effects have been reported in
vitro [13,27]. In a previous study [13], M-1 was more potent as
A goal of the present study was to determine if a similar
interaction exists between ciprofloxacin and M-1. Consistent
with our previous published results, a 2-fold increase in serum
drug levels was measured 30 min after PTX injection (100 mg/
kg i.p.) in the ciprofloxacin pre-treated group [12]. In the

644
b i o c h e m i c a l p h a r m a c o l o g y 7 4 ( 2 0 0 7 ) 6 3 9 – 6 4 6
Fig. 5 – Effect of ciprofloxacin pre-treatment on serum levels of PTX and M-1 30 min after injection of either drug. Animals
were treated with ciprofloxacin (25 mg/kg) or saline for 9 days before receiving an injection of M-1 or PTX (100 mg/kg i.p.).
Interconversion of PTX and M-1 takes place in vivo thus both compounds are detected in serum after injection of either
drug alone. Each bar represents mean drug concentration (mg/ml) W S.E.M. for five animals. Figure is representative of two
separate experiments yielding similar results. (*) Significant difference from same group without ciprofloxacin pre-
treatment; p < 0.05.
ciprofloxacin/M-1 group, there was
a significant 2-fold
increase in PTX serum levels and a 3-fold increase in serum
M-1 levels seen at 30 min post-injection. The total drug levels
(PTX + M-1) achieved in serum following M-1 injection were
higher than the total drug levels achieved following PTX
injection. The interaction between ciprofloxacin and M-1
significantly elevated total serum drug levels 2-fold greater in
the ciprofloxacin/M-1 group (57 Æ 3.3 mg/ml which is equiva-
lent to 240 mM) compared to all other groups (Fig. 6). The drug
concentration achieved (240 mM) is a concentration that has
previously been shown to significantly inhibit proliferation
[40] and collagen synthesis [13] in hepatic stellate cells in vitro,
key parameters in fibrosis. This suggests that the drug
combination of M-1 and ciprofloxacin has tremendous
potential as an anti-fibrotic therapy.
The 3-fold increase in M-1 levels in the ciprofloxacin/M-1
group suggests that an additional pathway of M-1 metabolism
was blocked since a 2-fold increase was seen in all other
groups. CYP1A2 is important for the N-demethylation of PTX
and M-1 to M-6 and M-7, respectively [12]. Also, CYP1A2 plays a
partial role in the conversion of M-1R back to PTX [37]. CYP3A4
mediates the conversion of M-1R to M-3b (trans-4,5-diol of M-
1), a metabolite of M-1R formed in vivo [41]. Ciprofloxacin is a
Fig. 6 – Effect of ciprofloxacin pre-treatment on total drug
PTX + M-1) levels in serum 30 min after M-1 or PTX
competitive inhibitor of CYP3A4 [42] and likely contributes to
the dramatic elevation in M-1 levels seen with the ciproflox-
acin/M-1 combination since pathways to three of its metabo-
lites (M-6, M-3b and PTX) are likely inhibited by ciprofloxacin.
Inhibition of CYP1A2 and CYP3A4 by ciprofloxacin is compe-
titive in nature and therefore pre-treatment with ciprofloxacin
is likely not necessary to achieve increased serum levels of
PTX and M-1 solely by its action on CYP1A2 and CYP3A4 but
recent evidence suggests that pre-treatment with ciproflox-
acin induced CYP2E1 [12], which would result in greater
(
injection. Mice were treated with ciprofloxacin (25 mg/kg)
of saline for 9 days before receiving an injection of PTX or
M-1 (100 mg/kg i.p.). Results are expressed as mean drug
concentration (mg/ml) W S.E.M. for five animals. Figure is
representative of two separate experiments yielding
similar results. (*) Significant difference from same group
without ciprofloxacin pre-treatment; p < 0.05. (**)
Significant difference between groups indicated; p < 0.05.

b i o c h e m i c a l p h a r m a c o l o g y 7 4 ( 2 0 0 7 ) 6 3 9 – 6 4 6
645
conversion of pentoxifylline to metabolite-1. These combined
effects would explain the 3-fold elevation in blood levels of
M-1 when it is administered following ciprofloxacin.
The combination of M-1 and ciprofloxacin should be
examined in animal models of disease, specifically diseases
involving hyperproliferation, inflammation and fibrosis. Since
ciprofloxacin is commonly prescribed to patients with Crohn’s
disease to prevent or treat bacterial overgrowth [43,44], the
combination of ciprofloxacin with M-1 in these patients
warrants further investigation as an aid to preventing stricture
formation.
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[18] Hewitson TD, Martic M, Kelynack KJ, Pedagogos E, Becker
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19] Lin SL, Chen YM, Chien CT, Chien WC, Tsai CC, Tsai TJ.
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