Enhanced efficacy of gemcitabine in combination with anti-CD20 monoclonal antibody against CD20+ non-Hodgkin's lymphoma cell lines in vitro and in scidmice
© Smith et al; licensee BioMed Central Ltd. 2005
Received: 10 March 2005
Accepted: 18 August 2005
Published: 18 August 2005
Despite exciting new targeted therapeutics against non-Hodgkin's lymphoma (NHL), chemotherapy remains a cornerstone of therapy. While purine nucleoside analogs have significant activity in low grade NHL, the pyrimidine nucleoside analog gemcitabine has been less extensively studied, but has important activity. Use of the anti-CD20 monoclonal antibody rituximab in combination with chemotherapy for B-NHL is becoming prevalent in clinical practice, but has not been extensively studied in pre-clinical models.
We have tested the activity of gemcitabine ± rituximab in vitro and in scid/human NHL xenograft models. We used two t(14;18)+, CD20+ follicular B cell NHL cell lines, DoHH2 a transformed NHL line and WSU-FSCCL isolated from pleural fluid of a patient with indolent NHL.
Gemcitabine is cytotoxic to DoHH2 and WSU-FSCCL cells in vitro, and the IC50 is 2–3 fold lower in the presence of rituximab. Apoptosis is also enhanced in the presence of rituximab. Clearance of NHL cells from ascites in scid mice is prolonged by the combination, as compared with either agent alone. Most importantly, survival of scid mice bearing human NHL cells is significantly prolonged by the combination of gemcitabine + rituximab.
Based on our pre-clinical data showing prolonged survival of mice bearing human lymphoma cell line xenografts after treatment with gemcitabine + anti-CD20 antibody, this combination, expected to have non-overlapping toxicity profiles, should be explored in clinical trials.
Non-Hodgkin's lymphoma (NHL) is increasing in incidence and is now the fifth most common malignancy in the U.S. Despite novel targeted biologic treatment options, chemotherapy remains an important component of therapy. Furthermore, most patients with indolent lymphoma and at least half of all patients with aggressive NHL are not cured . Improved therapeutic approaches are needed.
Gemcitabine is a pyrimidine nucleoside analog with clinical anti-cancer activity. Purine nucleoside analogs such as fludarabine, cladribine and pentostatin have been extensively studied and have significant activity against certain non-Hodgkin's lymphoma subtypes, particularly indolent forms. Though less well studied, an increasing body of data indicates activity of gemcitabine against lymphoma, both Hodgkin's and NHL [2–7]. The precise place of gemcitabine in the therapeutic armamentarium for NHL remains to be elucidated.
The chimeric anti-CD20 monoclonal antibody rituximab is active as a single agent in B cell NHL . In addition, it may sensitize cells to the action of chemotherapeutic and other biologic agents pre-clinically [9–12], as well as in patients [13, 14]. While mechanisms of rituximab action include direct apoptotic induction, complement activation and antibody dependent cytotoxicity, which of these is important may depend on the experimental conditions, and the relative importance in patients remains to be determined (reviewed in ). The same mechanisms, as well as intracellular signaling , may account for chemosensitization, but again the exact means by which this occurs in patients remains to be fully elucidated. One previous report has demonstrated in vitro sensitization of aggressive B cell NHL cell lines to gemcitabine by rituximab . Here we extend these in vitro results to additional human CD20+ lymphoma cell lines that carry the t(14:18) translocation, perhaps more analogous to the clinical use of rituximab. More importantly, we demonstrate that gemcitabine + rituximab enhances survival in vivo in a human B-NHL cell line/scid mouse xenograft model.
Cell culture and growth assay
Cells are incubated under standard conditions and cell numbers are determined by methyl thiazol tetrazoliumbromide (MTT) assay as before . Briefly, DoHH2 , a t(14;18)+ transformed lymphoma cell line, was obtained from Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH (DMSZ, German Collection of Microorganisms and Cell Cultures, Braunschweig, Germany). WSU-FSCCL, as previously reported , was isolated from pleural fluid of a patient with follicular grade I lymphoma, contains t(14;18), is EBV negative and behaves in scid mice as a more indolent disease , although it does contain a c-myc translocation.
Anti-CD20 monoclonal antibody
Chimeric anti-CD20 (clone C2B8, rituximab) obtained from IDEC (San Diego, CA), and gemcitabine (generously supplied by Lilly, Indianapolis, IN) were injected intraperitoneally (ip).
Apoptosis and cell cycle
Cell cycle was analyzed by DNA content per cell, by propidium iodide (PI) staining of nuclei from hypotonically lysed cells . Apoptosis was determined by dual staining of 1 × 105 intact cells in 100 μl calcium binding buffer containing 5 μl of fluoroscein isothiocyanate (FITC)-labeled annexin V (Pharmingen) and 5 μg/ml PI for 15 minutes in the dark, followed by analysis by flow cytometry (FACScan).
Poly(ADP-ribose) polymerase (PARP) cleavage assay
DoHH2 and WSU-FSCCL cells were lysed in cold radioimmunoprecipitation assay (RIPA) buffer  containing 100 μg phenylmethanesulfonyl fluoride (PMSF)/ml and 1 μg aprotinin/ml for 30 min on ice, pelleted and the supernatant separated by 4–20% SDS-PAGE. Transfer was to Immobilon-P, blocked with 1% casein-0.04% Tween-20 and probed with anti-PARP C2–10 antibody (Trevigen, Gaithersburg, MD). Secondary antibody was horseradish peroxidase labeled anti-mouse IgG (1:3000) for 30 min, detected by chemiluminescence and Hyperfilm ECL (Amersham, Piscataway, NJ). Densitometry utilized NIH Image 1.61 software.
Female CB17 scid mice were bred, housed and treated in the Fox Chase Cancer Center Laboratory Animal Facility under an approved protocol. Mice 4–8 weeks old were injected ip with either 1 × 107 WSU-FSCCL cells or 5 × 106 DoHH2 cells. Mice were observed daily and euthanized when they appeared ill. Lymphoma involving diffuse adenopathy, splenomegaly, infiltration of liver and bone marrow, along with ascites developed in untreated mice with each model, at 8–11 weeks with WSU-FSCCL and 4–6 weeks with DoHH2. Cells were collected sequentially from ascites by peritoneal washings and analyzed, or mice were followed for survival. For ascites clearance, mice bearing DoHH2 received 20 μg rituximab intraperitoneally (ip) on day 3 after lymphoma cell injection and/or gemcitabine 120 μg/gm ip on day 4, while mice bearing WSU-FSCCL received 100 μg rituximab intraperitoneally (ip) on day 7 after lymphoma cell injection, and/or gemcitabine 120 μg/gm ip on day 8. For survival, mice bearing DoHH2 received 5 μg rituximab on days 2, 9 and 16 and/or gemcitabine 120 μg/gm ip on days 3, 10 and 17.
Cell growth inhibition by gemcitabine and rituximab
Cell cycle alteration by gemcitabine and rituximab
Apoptosis induced by gemcitabine and rituximab
In vivoefficacy of gemcitabine ± rituximab
This report demonstrates the anti-lymphoma activity of the pyrimidine analog gemcitabine in vitro and in a scid mouse human lymphoma xenograft model. Rituximab is an active therapy for CD20+ NHL as a single agent and in combination with some biologic and chemotherapeutic agents [9–12]. The generalizability and mechanism of chemosensitization by rituximab has not been fully explored. Evidence for several mechanisms of rituximab mediated lymphoma cell death has been presented including direct induction of apoptosis, complement mediated killing and antibody-dependent cell mediated cytotoxicity (ADCC). Much of the data addressing these processes comes from in vitro systems, and the relevance to activity in patients remains uncertain. Our in vitro data in this report rely only on direct apoptosis, as we do not add human serum as a complement source nor effector cells. Direct apoptosis can be affected by degree of crosslinking of the antibody, which in turn can be controlled by humoral or cell surface molecules. Complement activity may be altered by inhibitory factors and may differ between human serum in vitro and mouse complement in murine models. We have found that crosslinking does enhance the degree of apoptosis after rituximab treatment in our two cell lines, while addition of complement has little effect (data not shown). ADCC depends on effector cells, and even the precise effector cells remain uncertain. Scid mice have residual granulocytes and NK cells, and depletion of these cells can abolish rituximab efficacy . To understand the mechanisms of resistance to rituximab will require more complete knowledge of which of these mechanisms of action is or are most important in patients . Recent data suggests that rituximab can alter intracellular signaling, even without inducing apoptosis, in ways that can sensitize cells to chemotherapy effects .
Our results demonstrate that the combination of gemcitabine and rituximab inhibits NHL cell growth, induces apoptosis in these cells, and, most importantly, is effective in prolonging survival of mice bearing human t(14;18)+ lymphoma cells. Prior reports have shown additive effects of gemcitabine and rituximab in aggressive NHL cell lines in vitro . In that report, aggressive NHL cell lines that were relatively resistant to both gemcitabine and rituximab were pre-treated with rituximab for 24 hours and then treated with gemcitabine for 18 hours and found to have modest increases in hypodiploid cells and PI positive cells after PI staining.
The cell cycle changes seen after gemcitabine treatment alone, and in combination with rituximab, are cell line dependent. WSU-FSCCL cells, in contrast to most cells, are not blocked in S phase by gemcitabine alone, but are after rituximab is added. This may reflect altered intracellular signaling that could restore a putative S phase DNA damage sensor, such as the ATM pathway . Similarly, induction of early versus late apoptosis is also cell line dependent. Further exploration of the biochemical basis of these changes is warranted to better understand which of the many potential targets of a pyrimidine nucleoside analog are the important determinants of gemcitabine activity against NHL cells, and how these targets are affected by rituximab.
Concurrent treatment of B cell lymphoproliferative disorders (NHL and CLL) with rituximab and chemotherapy is becoming more common, and data suggests benefit in time to disease progression in indolent disease  and also in overall survival in aggressive disease . Questions remain regarding the optimal way to combine rituximab and chemotherapy and whether therapeutic efficacy of specific chemotherapeutic agents is enhanced by rituximab. Pre-clinical studies of the interaction of chemotherapy agents and rituximab may provide insight to guide the development of appropriate clinical trials.
This work was supported by NIH RO1CA71552, Fox Chase Cancer Center Core Grant NIH CA06927 and grants from the Mary L. Smith Charitable Lead Trust and the Martha Rogers Charitable Trust. Additional support came from the Janice Charach Epstein Research Fund and the Lester I. Smith Research Fund. Kathy Zenszer provided excellent secretarial assistance.
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