Type 4 phosphodiesterase (PDE4) inhibitors reportedly induce apoptosis in chronic lymphocytic leukemia (CLL) cells. Following clinical improvement of one previously untreated CLL patient with sildenafil therapy, we evaluated the in vitro induction of apoptosis in CLL cells by 4 PDE5/6 inhibitors, including sildenafil, vardenafil, zaprinast, and methoxyquinazoline (MQZ). After 24 hours of culture, the various PDE inhibitors differed in their ability to induce apoptosis, with zaprinast displaying no killing effect. Normal B cells isolated from control donors were totally resistant to PDE-induced apoptosis. Vardenafil was 3 and 30 times more potent an inducer of apoptosis than sildenafil and MQZ, respectively. Both vardenafil and sildenafil failed to elevate adenosine 3′5′ cyclic monophosphate (cAMP) levels, largely excluding an inhibitory effect on cAMP-PDE3, -PDE4, and -PDE7. Vardenafil- or sildenafil-treated B-CLL cells displayed up to 30% intracellular active caspase 3. Drug-induced apoptosis was inhibited by the caspase inhibitor z-VAD.fmk, prevented by interleukin-4 (IL-4), and significantly reduced by stromal-derived factor1-α (SDF-1α). We conclude that vardenafil and sildenafil induce caspase-dependent apoptosis of B-CLL cells in vitro and thus might be considered in the treatment of CLL patients. However, further in vivo investigations should be warranted.

It has been extensively reported that defective programmed cell death in B-cell chronic lymphocytic leukemia (B-CLL) is responsible for the relentless accumulation of malignant B cells in blood, bone marrow, and lymphoid organs and that it plays a key role in the pathogenesis of the disease.1 However, the discrepancy between the in vivo resistance of leukemic cells to apoptosis and their high sensitivity to in vitro spontaneous or induced apoptosis remains unclear. Corticosteroids,2 alkylating agents,3 purine analogs,4irradiation,5 methylxanthine derivatives,6interleukin-5 (IL-5)7 and IL-10,8salicylates,9 mitoxantrone,10 ubiquitin proteasome inhibitors,11 arsenic trioxyde,12colchicine,13 hydroxychloroquine,14flavopiridol,15 monoclonal antibodies such as CD20,16 CD47,17 and CD52,16immunoglobulin M (IgM),16 and mitochondrial benzodiazepine receptor antagonist PK1119518 were all successively demonstrated to elicit in vitro apoptosis in B-CLL cells. Clinical efficacy has been shown for most of them.

Cyclic adenosine monophosphate (cAMP) is catabolized within cells to 5′ AMP by 3′5′ cAMP phosphodiesterases (PDEs). The PDE family includes 10 classes of enzymes that are differentially expressed in various cell types. Normal lymphocytes express at least cAMP-PDE3 and -PDE4.19,20 Increases in cAMP levels induced growth arrest or cell death of malignant lymphoid cells. We and others21,22 reported that PDE4 inhibitors induced the apoptosis of B-CLL cells. Sildenafil (Viagra; Pfizer, Paris, France) and vardenafil (a generous gift from Bayer, Puteaux, France) are known to be potent and specific inhibitors of PDE5A, expressed mainly in human vascular smooth muscle cells and platelets.23 However, they also inhibit PDE6 in retina.24 We here present evidence, following one clinical observation, that sildenafil and vardenafil are new inducers of apoptosis in B-CLL cells in vitro. This may be relevant for the development of therapeutic strategies to treat CLL.

Patient samples

Nineteen peripheral blood samples from 17 patients with CLL were studied after informed consent was obtained. According to the Binet classification, 15 patients had stage A disease, one (patient 14) had stage B disease, and one (patient 11) had stage C disease. Patients received no treatment for at least 6 months before the study.

Cell culture conditions and reagents

Peripheral blood mononuclear cells (PBMCs) were isolated from patients with CLL by density-gradient centrifugation of heparinized blood using Lymphoprep (Nycomed, Olso, Norway). B cells were prepared from CLL PBMCs or tonsillar lymphocytes by one cycle of rosetting with S-(2 aminoethyl) isothiouronium bromide (Aldrich, Milwaukee, MN)–treated sheep red blood cells to deplete T cells. B-cell purity was shown to be greater than 98% by flow cytometry (FACScan; Becton Dickinson, Le-Pont-de-Claix, France). Cells were cultured in RPMI 1640 10% fetal calf serum (FCS) at 2 × 106/mL with or without PDE inhibitors at the indicated concentrations: sildenafil; vardenafil, zaprinast, and 4 (3′4′) methylendioxybenzyl (amino) 6 methoxyquinazoline (MQZ) (Calbiochem; Merck Eurolab, Fontenay-sous-Bois, France); and theophylline (Sanofi; Département Hôpital, Gentilly, France). Recombinant IL-4 was a generous gift of Immunex (Seattle, WA); SDF-1α was purchased from Peprotech (Rocky Hill, NJ).

Total cellular cAMP measurement

Cells were incubated in various conditions (sildenafil, vardenafil, and theophylline) at 106 cells/mL for 0.5, 1, and 24 hours as described in “Cell culture conditions and reagents”. Briefly, 0,1 mL was lysed by dodecyl trimethylammonium bromide, and cAMP was measured by enzyme-linked immunosorbent assay (ELISA) nonacetylation protocol, using the Biotrak cAMP EIA System (Amersham, Orsay, France).

Assays for apoptosis

Detection of phosphatidylserine (PS) exposure and decrease in ΔΨm were performed by flow cytometry using a XL analyzer (Beckman-Coulter, Villepinte, France). For PS exposure, cells were double stained with fluorescein isothiocyanate (FITC)–labeled annexin-V (R&D Systems, Minneapolis, MN) and propidium iodide (PI) at 2 μg/mL (Sigma). Percentages of annexin-V–positive cells were calculated by combining annexin V+/PI(early apoptotic) and annexin V+/PI+ (late apoptotic) cells. Decreased levels of ΔΨm were assessed using 3,3′-dihexyloxacarbocyanine iodide (DiOC6)3 (Molecular Probes, Eugene, OR). Caspase 3 activity was measured by flow cytometry using cell-permeable fluorogenic caspase substrate (Phiphilux kit; Medical and Biological Laboratory, Watertown, MA).25 

Case report

A 47-year-old man had a history of chronic bronchitis associated with a peripheral blood lymphocytosis level of 30 × 109/L in October 1993. No other clinical or biologic abnormalities were found. Diagnosis of B-CLL was documented according to National Cancer Institute criteria, stage A in the Binet classification. The patient was given no therapy. In March 1998, after 5-year follow-up, his hematologic status was unchanged, and his lymphocyte count was 20 × 109/L. One year later, in May 1999, a routinely performed laboratory test showed a clear-cut decrease of lymphocyte count at 4 × 109/L. Questioning revealed that the only treatment received by the patient was sildenafil, 50 mg once a week during the last 3 months. Immunophenotyping showed the persistence of the leukemic clone (CD19+/CD5+/CD23+/CD79b, low-intensity monotypic lambda lymphocytes). The patient remained on 50 mg sildenafil once a week for 3.5 years, and his lymphocyte count remained between 3 × 109/L and 4 × 109/L, without any other therapy. To demonstrate the possible role of the drug in the decrease of lymphocytosis, it was suggested that the patient discontinue sildenafil treatment for a couple of months, but he categorically refused. A blood test in April 2002 revealed 3.8 × 109/L lymphocytes with a persistence of 39% of B-CLL cells with the leukemic phenotype.

Types 5 and 6 PDE inhibitors induce in vitro apoptosis in B-CLL without elevation in cAMP level

This clinical observation prompted us to investigate the in vitro effect of 4 PDE types 5 and 6 inhibitors, including sildenafil, vardenafil, zaprinast, and the chemical compound MQZ, on the induction of apoptosis in CLL cells. As depicted in Figure1A, sildenafil (50 μg/mL) induced apoptosis after 24 hours of culture in 14 of 14 CLL PBMC cultures. The mean percentage of sildenafil (50 μg/mL)–induced annexin V+ cells was 33.4% ± 3.5% (n = 18). This level was similar to that observed with vardenafil (10 μg/mL) (42.3% ± 10.2) (n = 4) or the PDE4 inhibitor theophylline (60 μg/mL) (38.1% ± 3.3%) (n = 18) (Figure 1B). Interestingly, 2 other PDE5 inhibitors resulted in differential induction of apoptosis, with MQZ (10 μg/mL), significantly inducing cell death (16.6% ± 2.2%) (n = 24) and zaprinast (10 μg/mL) displaying no effect (9.1% ± 1.7%) (n = 7). Of note, combinations of optimal concentrations of sildenafil and theophylline did not further increase the level of apoptosis (data not shown). As depicted in Figure2A, the induction of apoptosis increased up to 5 days of culture. The EC50 (effective concentration of drug that inhibited viability of treated B-CLL cells to 50% of untreated cells) for vardenafil was 3 and 30 times lower than for sildenafil and MQZ, respectively (Figure 2B). PDE inhibitor–induced apoptosis appeared to be selective for the leukemic B cells because increasing concentrations of either sildenafil or vardenafil augmented the percentage of annexin V+ cells in B-CLL but not in normal B lymphocytes isolated from control donors (Figure3 and data not shown). Because B-CLL expressed cAMP-PDE3 and -PDE4,19 20 we next evaluated whether the induction of apoptosis by sildenafil or vardenafil was caused by an inhibition of PDE other than PDE5. Hence, we measured alterations in cAMP levels following incubation with PDE5 inhibitors. Sildenafil and vardenafil (75 μM) failed to augment cAMP above background in B-CLL cells, whereas an approximately 65-fold increase was observed with theophylline in the same cells (Table1), largely excluding an inhibition of PDE3 and PDE4. We conclude that sildenafil and vardenafil induce B-CLL apoptosis in vitro and do not increase intracellular cAMP levels.

Fig. 1.

Type 5/6 PDE inhibitors induce apoptosis of CLL PBMCs in vitro.

(A) PBMCs isolated from 14 patients with CLL were cultured for 24 hours in the presence (░) or absence (□) of sildenafil (50 μg/mL). (B) Differential induction of apoptosis in CLL PBMCs by various phosphodiesterase inhibitors (mean percentages ± SEMs). Apoptosis was determined by double staining with FITC-labeled annexin V and PI analysis by flow cytometry. Percentage annexin V+ cells are calculated as described in “Patients, materials, and methods.” Statistical analysis was performed using the paired Student t test (*P < .05; **P < .01; ***P < .001).

Fig. 1.

Type 5/6 PDE inhibitors induce apoptosis of CLL PBMCs in vitro.

(A) PBMCs isolated from 14 patients with CLL were cultured for 24 hours in the presence (░) or absence (□) of sildenafil (50 μg/mL). (B) Differential induction of apoptosis in CLL PBMCs by various phosphodiesterase inhibitors (mean percentages ± SEMs). Apoptosis was determined by double staining with FITC-labeled annexin V and PI analysis by flow cytometry. Percentage annexin V+ cells are calculated as described in “Patients, materials, and methods.” Statistical analysis was performed using the paired Student t test (*P < .05; **P < .01; ***P < .001).

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Fig. 2.

Kinetics and dose-response curves of apoptosis induced by PDE inhibitors.

(A) Time-course experiment. (B) Dose-reponse curves (EC50) for sildenafil, vardenafil, and MQZ. For both panels, apoptosis was determined as described in the legend to Figure 1, and results from 1 experiment representative of 2 are shown.

Fig. 2.

Kinetics and dose-response curves of apoptosis induced by PDE inhibitors.

(A) Time-course experiment. (B) Dose-reponse curves (EC50) for sildenafil, vardenafil, and MQZ. For both panels, apoptosis was determined as described in the legend to Figure 1, and results from 1 experiment representative of 2 are shown.

Close modal
Fig. 3.

No killing effect of sildenafil on normal B cells.

B lymphocytes isolated from one patient with CLL or from one control donor were cultured for 24 hours with increasing concentrations of sildenafil (ranging from 0.5 to 100 μg/mL). Results from 1 experiment representative of 2 are shown. Similar results were obtained with vardenafil.

Fig. 3.

No killing effect of sildenafil on normal B cells.

B lymphocytes isolated from one patient with CLL or from one control donor were cultured for 24 hours with increasing concentrations of sildenafil (ranging from 0.5 to 100 μg/mL). Results from 1 experiment representative of 2 are shown. Similar results were obtained with vardenafil.

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Table 1.

Variations of cAMP levels by PDE inhibitors

TimeMediumSildenafil
(50 μg/mL)
Vardenafil
(20 μg/mL)
Vardenafil
(50 μg/mL)
Theophylline
(60 μg/mL)
30 min 24.4 24.4 28.7 28.1 1423  
60 min 20.0 24.6 14.9 18.6 1340  
24 h 23.5 20.5 24.8 21.4 1340 
TimeMediumSildenafil
(50 μg/mL)
Vardenafil
(20 μg/mL)
Vardenafil
(50 μg/mL)
Theophylline
(60 μg/mL)
30 min 24.4 24.4 28.7 28.1 1423  
60 min 20.0 24.6 14.9 18.6 1340  
24 h 23.5 20.5 24.8 21.4 1340 

CLL PBMCs (1 × 106/mL) were incubated with the various PDE inhibitors for the indicated times. Total cellular cAMP (fmol/well) was measured, as described in “Patients, materials, and methods.”

Sildenafil- and vardenafil-induced apoptosis is dependent on caspase activation

We next determined the apoptotic pathway elicited by sildenafil and vardenafil. We found that both drugs induced the cytoplasmic (cell shrinkage and decrease in ΔΨm, as measured by DiOC6 staining) and the nuclear events of apoptosis (appearance of hypodiploid DNA, as detected by PI staining of fixed cells) (Figure4A and data not shown). PDE inhibitor–induced apoptosis was caspase dependent (Figure 4B-C). Apoptosis was abrogated when cultures were grown in the presence of the broad caspase inhibitor z-VAD.fmk (Figure 4B), and sildenafil and vardenafil significantly increased the level of active caspase 3 as measured by flow cytometry (Figure 4C).

Fig. 4.

Sildenafil- and vardenafil-induced apoptosis in CLL B cells is caspase dependent.

(A) B-CLL cells were cultured for 24 hours with or without sildenafil (50 μg/mL). Cell viability was determined by FSC/SSC (i,iv); double staining with FITC-labeled annexin V/PI (ii,v) or by DiOC6 staining (iii,vi). Percentage of dead cells (annexin+ cells) and percentage viable cells (DiOC6high) are also shown. (B) B-CLL cells (n = 2) were cultured as in panel A in the absence or presence of the caspase inhibitor z-VAD.fmk (50 μM). (C) B-CLL lymphocytes were cultured for 24 hours with or without sildenafil (50 μg/mL) or vardenafil (10 μg/mL), and caspase 3 activity was detected by flow cytometry as described in “Patients, materials, and methods.” Results shown are from 1 experiment representative of 3 performed on samples from separate donors.

Fig. 4.

Sildenafil- and vardenafil-induced apoptosis in CLL B cells is caspase dependent.

(A) B-CLL cells were cultured for 24 hours with or without sildenafil (50 μg/mL). Cell viability was determined by FSC/SSC (i,iv); double staining with FITC-labeled annexin V/PI (ii,v) or by DiOC6 staining (iii,vi). Percentage of dead cells (annexin+ cells) and percentage viable cells (DiOC6high) are also shown. (B) B-CLL cells (n = 2) were cultured as in panel A in the absence or presence of the caspase inhibitor z-VAD.fmk (50 μM). (C) B-CLL lymphocytes were cultured for 24 hours with or without sildenafil (50 μg/mL) or vardenafil (10 μg/mL), and caspase 3 activity was detected by flow cytometry as described in “Patients, materials, and methods.” Results shown are from 1 experiment representative of 3 performed on samples from separate donors.

Close modal

B-CLL cells undergo rapid and spontaneous apoptosis in vitro, contrasting with their prolonged lifespan in vivo. It was suggested that in vitro apoptosis could be the result of the absence of several humoral and cellular factors present in vivo.26,27 The protective role of IL-4 in spontaneous or drug-induced apoptosis has been extensively demonstrated.27 In agreement with these reports, we here show that IL-4 completely prevented sildenafil-induced apoptosis (Figure 5A). Contact with stromal cells and, more recently, the presence of SDF-1α protected the leukemic cells from undergoing spontaneous apoptosis.26 28 Similarly, the addition of SDF-1α significantly reduced sildenafil-induced apoptosis. Finally, culture in 10% autologous plasma failed to rescue leukemic B cells from PDE inhibitor–induced apoptotic cell death (Figure 5B).

Fig. 5.

Sildenafil-induced apoptosis in B-CLL cells is prevented by IL-4 and SDF-1α but not by autologous plasma.

(A) B-CLL lymphocytes (n = 3) were cultured for 24 hours with or without sildenafil (50 μg/mL) in the absence or presence of either IL-4 (20 ng/mL) or SDF-1α (1 μg/mL). (B) Cultures were performed in RPMI supplemented with 10% FCS or 1% autologous plasma. Percentages of dead cells (annexin+ cells) were determined as described in the legend to Figure 1.

Fig. 5.

Sildenafil-induced apoptosis in B-CLL cells is prevented by IL-4 and SDF-1α but not by autologous plasma.

(A) B-CLL lymphocytes (n = 3) were cultured for 24 hours with or without sildenafil (50 μg/mL) in the absence or presence of either IL-4 (20 ng/mL) or SDF-1α (1 μg/mL). (B) Cultures were performed in RPMI supplemented with 10% FCS or 1% autologous plasma. Percentages of dead cells (annexin+ cells) were determined as described in the legend to Figure 1.

Close modal

In previous reports, 2 PDE4 inhibitors—theophylline and rolipram—were shown to induce in vitro apoptosis of B-CLL cells.21,22 However, the successful therapeutic use of theophylline in those patients might be hampered by side effects resulting from its adenosine receptor antagonistic activity. The present case report suggested that sildenafil, known as a selective PDE5 inhibitor, leads to clinical improvement in the absence of any other treatment, and the in vitro findings support this hypothesis. We found that sildenafil and vardenafil induced apoptosis in B-CLL cells. Normal B cells were resistant to killing by both PDE inhibitors. PDE5 inhibitor–induced apoptosis was caspase 3-dependent, as reported for the PDE4 inhibitor rolipram.29 

Consistent with its biologic activity—more potent than sildenafil on human corpus cavernosum smooth mucle30,31—vardenafil was approximately 3 times more potent than sildenafil in inducing B-CLL cell apoptosis. However, optimal in vitro induction of apoptosis required micromolar-range concentrations of sildenafil or vardenafil compared with lower doses used in clinical trials and other cell-based assays, not excluding the possibility that other PDE5 subtypes might also have been inhibited in our in vitro assay. Freshly isolated B-CLL cells preferentially expressed PDE1B, PDE3B, PDE4A, and PDE4B subtypes, all described as cAMP-specific phosphodiesterases.19,22Moreover, PDE3B inhibition augmented PDE4 inhibitor–induced apoptosis in B-CLL.20 Leukemic B cells expressed PDE7A, which is further up-regulated by cAMP.32 We therefore attempted to determine whether sildenafil and vardenafil targeted cAMP-PDE by measuring alterations in intracellular cAMP levels in treated B-CLL cells. Up to 75 μM of either drug failed to produce cAMP levels above background in B-CLL cells, which is consistent with the drug's selectivity for cGMP-PDE5/6.23 In that regard, exisulind, a type 2/5 PDE inhibitor, induced apoptosis in tumor cells without increasing cAMP levels.33 Our data did not rule out an effect on PDE1 because low doses of sildenafil reportedly affected PDE1,23 though one would also have anticipated an elevation in cAMP level. Furthermore, despite the presence of the PDE1 transcript in B-CLL, there was no evidence of PDE1 activity in the same samples.22 Unexpectedly and most interestingly, ongoing microarray analysis revealed no expression of PDE5 in B-CLL but did reveal the presence of PDE6, an isoenzyme selectively expressed in the photoreceptors of the retina (J.D. and F.D., personal observations). Abnormal vision and retinal side effects have been reported in patients receiving the maximum recommended dose of sildenafil.34-36This effect was attributed to a weak inhibitory activity of sildenafil on PDE6.24 Indeed, sildenafil and vardenafil (values in brackets for vardenafil) inhibited hydrolysis of cGMP with an IC50 of 6.6 nm (0.7 nm) for PDE5 and 48 nm (11 nm) for PDE6.31 Because antibodies to PDE5/6 were not commercially available, we were unable to evaluate the expression of these subtypes at the protein level in B-CLL cells.

It is reasonable to postulate that B-CLL cells express PDE6 isoenzymes, which might be the target of sildenafil and vardenafil activity in our in vitro assay. The clinical efficacy of sildenafil and vardenafil, which showed tolerable side effects in vivo,37 should be further investigated in patients with early-stage CLL or perhaps in combination with chemotherapeutic agents in patients with advanced-stage disease.

We thank Dr A. Dalloul for his helpful suggestions.

Prepublished online as Blood First Edition Paper, June 28, 2002; DOI 10.1182/blood-2002-01-0075.

The publication costs of this article were defrayed in part by page charge payment. Therefore, and solely to indicate this fact, this article is hereby marked “advertisement” in accordance with 18 U.S.C. section 1734.

1
Andreeff
 
M
Darzynkiewicz
 
Z
Sharpless
 
TK
Clarkson
 
BD
Melamed
 
MR
Discrimination of human leukemia subtypes by flow cytometric analysis of cellular DNA and RNA.
Blood.
55
1980
282
293
2
McConkey
 
DJ
Aguilar-Santelises
 
M
Hartzell
 
P
et al
Induction of DNA fragmentation in chronic B-lymphocytic leukemia cells.
J Immunol.
146
1991
1072
1076
3
Begleiter
 
A
Lee
 
K
Israels
 
LG
Mowat
 
MR
Johnston
 
JB
Chlorambucil induced apoptosis in chronic lymphocytic leukemia (CLL) and its relationship to clinical efficacy.
Leukemia.
8(suppl 1)
1994
S103
S106
4
Robertson
 
LE
Chubb
 
S
Meyn
 
RE
et al
Induction of apoptotic cell death in chronic lymphocytic leukemia by 2-chloro-2′-deoxyadenosine and 9-beta-D-arabinosyl-2-fluoroadenine.
Blood.
81
1993
143
150
5
Delic
 
J
Magdelenat
 
H
Barbaroux
 
C
et al
In vivo induction of apoptosis in human lymphocytes by therapeutic fractionated total body irradiation.
Br J Radiol.
68
1995
997
1003
6
Mentz
 
F
Merle-Beral
 
H
Ouaaz
 
F
Binet
 
JL
Theophylline, a new inducer of apoptosis in B-CLL: role of cyclic nucleotides.
Br J Haematol.
90
1995
957
959
7
Mainou-Fowler
 
T
Craig
 
VA
Copplestone
 
JA
Hamon
 
MD
Prentice
 
AG
Interleukin-5 (IL-5) increases spontaneous apoptosis of B-cell chronic lymphocytic leukemia cells in vitro independently of bcl-2 expression and is inhibited by IL-4.
Blood.
84
1994
2297
2304
8
Fluckiger
 
AC
Durand
 
I
Banchereau
 
J
Interleukin 10 induces apoptotic cell death of B-chronic lymphocytic leukemia cells.
J Exp Med.
179
1994
91
99
9
Bellosillo
 
B
Pique
 
M
Barragan
 
M
et al
Aspirin and salicylate induce apoptosis and activation of caspases in B-cell chronic lymphocytic leukemia cells.
Blood.
92
1998
1406
1414
10
Bellosillo
 
B
Colomer
 
D
Pons
 
G
Gil
 
J
Mitoxantrone, a topoisomerase II inhibitor, induces apoptosis of B-chronic lymphocytic leukaemia cells.
Br J Haematol.
100
1998
142
146
11
Masdehors
 
P
Omura
 
S
Merle-Beral
 
H
et al
Increased sensitivity of CLL-derived lymphocytes to apoptotic death activation by the proteasome-specific inhibitor lactacystin.
Br J Haematol.
105
1999
752
757
12
Zhang
 
W
Ohnishi
 
K
Shigeno
 
K
et al
The induction of apoptosis and cell cycle arrest by arsenic trioxide in lymphoid neoplasms.
Leukemia.
12
1998
1383
1391
13
Forbes
 
IJ
Zalewski
 
PD
Giannakis
 
C
Cowled
 
PA
Induction of apoptosis in chronic lymphocytic leukemia cells and its prevention by phorbol ester.
Exp Cell Res.
198
1992
367
372
14
Lagneaux
 
L
Delforge
 
A
Carlier
 
S
Massy
 
M
Bernier
 
M
Bron
 
D
Early induction of apoptosis in B-chronic lymphocytic leukaemia cells by hydroxychloroquine: activation of caspase-3 and no protection by survival factors.
Br J Haematol.
112
2001
344
352
15
Pepper
 
C
Thomas
 
A
Hoy
 
T
Fegan
 
C
Bentley
 
P
Flavopiridol circumvents Bcl-2 family mediated inhibition of apoptosis and drug resistance in B-cell chronic lymphocytic leukaemia.
Br J Haematol.
114
2001
70
77
16
Keating
 
MJ
Chronic lymphocytic leukemia [review].
Semin Oncol.
14(suppl)
1999
107
114
17
Mateo
 
V
Lagneaux
 
L
Bron
 
D
et al
CD47 ligation induces caspase-independent cell death in chronic lymphocytic leukemia.
Nat Med.
5
1999
1277
1284
18
Auer RL, Fennel DA, Corbo M, Fegan CD, Lister A, Cotter FE. PK11195, a mitochondrial benzodiazepine receptor antagonist, as a new therapy for CLL: B-cell lymphoproliferative disorders II. Presented at: Congress Amsterdam; June 5-7, 2001.
19
Jiang
 
X
Li
 
J
Paskind
 
M
Epstein
 
PM
Expression and regulation of mRNA for distinct isoforms of cAMP-specific PDE-4 in mitogen-stimulated and leukemic human lymphocytes.
Cell Biochem Biophys.
28
1998
135
160
20
Moon
 
E
Lee
 
R
Near
 
R
Weintraub
 
L
Wolda
 
S
Lerner
 
A
Inhibition of PDE3B augments PDE4 inhibitor-induced apoptosis in a subset of patients with chronic lymphocytic leukemia.
Clin Cancer Res.
8
2002
589
595
21
Mentz
 
F
Mossalayi
 
MD
Ouaaz
 
F
et al
Theophylline synergizes with chlorambucil in inducing apoptosis of B-chronic lymphocytic leukemia cells.
Blood.
88
1996
2172
2182
22
Kim
 
DH
Lerner
 
A
Type 4 cyclic adenosine monophosphate phosphodiesterase as a therapeutic target in chronic lymphocytic leukemia.
Blood.
92
1998
2484
2494
23
Wallis
 
RM
Corbin
 
JD
Francis
 
SH
Ellis
 
P
Tissue distribution of phosphodiesterase families and the effects of sildenafil on tissue cyclic nucleotides, platelet function, and the contractile response of trabeculae carneae and aortic rings in vitro.
Am J Cardiol.
83
1999
3C
12C
24
Ballard
 
SA
Gingell
 
CJ
Tang
 
K
Turner
 
LA
Price
 
ME
Naylor
 
AM
Effects of sildenafil on the relaxation of human corpus cavernosum tissue in vitro and on the activities of cyclic nucleotide phosphodiesterase isozymes.
J Urol.
159
1998
2164
2171
25
Liu
 
L
Chahroudi
 
A
Silvestri
 
G
et al
Visualization and quantification of T cell-mediated cytotoxicity using cell-permeable fluorogenic caspase substrates.
Nat Med.
8
2002
185
189
26
Panayiotidis
 
P
Jones
 
D
Ganesnhaguru
 
K
Foroni
 
L
Hofbrand
 
AV
Human bone marrow stromal cells prevent apoptosis and support the survival of chronic lymphocytic leukaemia cells in vitro.
Br J Haematol.
92
1996
97
103
27
Dancescu
 
M
Rubio-Tujillo
 
M
Biron
 
G
Biron
 
D
Delespesse
 
G
Sarfati
 
M
Interleukin 4 protects chronic lymphocytic leukemic B cells from death by apoptosis and upregulates Bcl-2 expression.
J Exp Med.
176
1992
1319
1326
28
Burger
 
JA
Tsukada
 
N
Burger
 
M
Zvaifler
 
NJ
Dell'Aquila
 
M
Kipps
 
TJ
Blood derived nurse-like cells protect chronic lymphocytic leukemia B cells from spontaneous apoptosis through stromal cell-derived factor-1.
Blood.
96
2000
2555
2563
29
Siegmund
 
B
Welsch
 
J
Loher
 
F
et al
Phosphodiesterase type 4 inhibitor suppresses expression of anti-apoptotic members of the Bcl-2 family in B-CLL cells and induces caspase-dependent apoptosis.
Leukemia.
15
2001
1564
1571
30
Kim
 
NN
Huang
 
YH
Goldstein
 
I
Bischoff
 
E
Trais
 
AM
Inhibition of cyclic hydrolysis in human corpus cavernosum smooth muscle cells by vardenafil, a novel, selective phosphodiesterase type 5 inhibitor.
Life Sci.
69
2001
2249
2256
31
Saenz de Tejada
 
I
Angulo
 
J
Cuevas
 
P
et al
The phosphodiesterase inhibitory selectivity and the in vitro and in vivo potency of the new PDE5 inhibitor vardenafil.
Int J Impot Res.
13
2001
282
290
32
Lee
 
R
Wolda
 
S
Moon
 
E
Esselstyn
 
J
Hertel
 
C
Lerner
 
A
PDE7A is expressed in human Blymphocytes and is up-regulated by elevation of intracellular cAMP.
Cell Signal.
14
2002
277
284
33
Thompson
 
WJ
Piazza
 
GA
Li
 
H
et al
Exisulind induction of apoptosis involves guanosine 3′, 5′-cyclic monophosphate phosphodiesterase inhibition, protein kinase G activation, and attenuated β-catenin.
Cancer Res.
60
2000
3338
3342
34
Vobig
 
MA
Klotz
 
T
Staak
 
M
Bartz-Schmidt
 
KU
Engelmann
 
U
Walter
 
P
Retinal side effects of sildenafil [letter].
Lancet.
353
1999
375
35
Marmor
 
M
Kessler
 
R
Sildenafil (Viagra) and ophthalmology.
Surv Ophthalmol.
44
1999
153
162
36
Zrenner
 
E
No cause for alarm over retinal side effects of sildenafil [comment].
Lancet.
353
1999
340
341
37
Herrmann
 
HC
Chang
 
G
Klugherz
 
B
Mahoney
 
PD
Hemodynamic effects of sildenafil in men with severe coronary artery disease.
N Engl J Med.
342
2000
1622
1626

Author notes

Hélène Merle-Béral, Department of Hematology, Pavillon Laveran, Hôpital Pitié-Salpêtrière, 47, Blvd de l'Hôpital, 75651 Paris Cedex 13, France; e-mail:helene.merle-beral@psl.ap-hop-paris.fr.

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