Potential Therapeutic Applications of Oblimersen in CLL
Potential Therapeutic Applications of Oblimersen in CLL
B-cell chronic lymphocytic leukemia
(B-CLL) is a disorder
characterized by the unremitting
accumulation of small, slowly
(weak)/CD23+/sIg+ (weak) monoclonal
CLL is very rare in people younger
than age 40. However, its incidence increases
sharply after the fourth decade
of life, to the point of being the predominant
leukemic type in the elderly in the
Western world, across race and sex differences
(Figure 1). It is estimated that
7,300 new cases were diagnosed in the
United States in 2003. Statistics from
the nine population-based cancer registries
of the National Cancer Institute's
Surveillance, Epidemiology and End
Results (SEER) Program for 2000
showed significantly better survival differences
for patients younger than age
65 (average presentation age: 64 years)
than for those 65 years of age and older,
although CLL shortens life expectancy
considerably, even in younger patients
Neither the Rai nor the Binet
staging systems, which discriminate
CLL by the sites of disease and/or the
degree of cytopenias induced by leukemic
marrow replacement, enable
physicians to identify patients in the
good-prognosis group who will eventually
progress. However, over time,
these systems have proved to be extremely
useful for categorizing patients,
particularly those participating
in clinical trials.
The original Rai system consisted
of five stages (0 to IV) but was
modified to a three-stage scheme in
1987, when analysis of survival
data in a large number of patients established
three distinct risk categories
(low, intermediate, and high), which
differ significantly in terms of median
survival. Patients in the low-risk group
(stage 0) have lymphocytosis with
no other abnormality; patients in the
intermediate-risk group have enlarged
lymph nodes (stage I) and/or spleen
(stage II) in addition to lymphocytosis;
and patients in the high-risk
group have anemia (hemoglobin value
< 11.0 g/dL, stage III) and/or thrombocytopenia
(platelet count of < 100 *
109/L, stage IV) (Table 2).
Serum levels of beta2-microglobulin,[
7] lactate dehydrogenase,
soluble CD23, and the cell-surface
expression of CD38 can help predict
disease activity. However, the presence
of certain cytogenetic abnormalities in
the leukemic B cells [10,11] and/or somatic
mutations in the immunoglobulin
heavy-chain genes seem to predict
rapid disease progression and survival
CLL B cells with mutated immunoglobulin
heavy-chain genes are associated
with the more favorable
genetic defect deletion 13q14. Conversely,
heavy-chain genes are associated with
trisomy 12 and the high-risk 17p and
11q genomic aberrations. CLL B
cells with unmutated immunoglobulin
heavy-chain genes generally show
the distinctive expression of the zetaassociated
protein of 70 kD (ZAP-70),
an intracellular tyrosine kinase, which
has been found to be associated with
enhanced immunoglobulin receptor
signaling in CLL B cells. Mounting
data indicate that expression of ZAP-
70 is a more reliable marker of the risk
of early disease progression than the
mutational status of the expressed immunoglobulin
heavy-chain genes in
Based on these recent findings, it
has been proposed that there are two
types of CLL (Table 3). The first type
arises from relatively less differentiated
or "immunologically naive"
pregerminal B cells with unmutated
immunoglobulin heavy-chain genes,
displays atypical CLL morphology,
and has a poor prognosis. The other
type evolves from more differentiated
postgerminal "memory" B cells with
somatically mutated immunoglobulin
heavy-chain genes and has a good
Treatment With Chemotherapy
Approximately one-third of patients
with CLL never require treatment. In
another third, an initial indolent phase
is followed by disease progression. The
remaining third show aggressive disease
at the outset and need immediate treatment.
Ultimately, 50% of patients with
CLL will require treatment. Deferring
therapy until progression of CLL does
not affect survival. Neither single-agent
chlorambucil (Leukeran) nor a variety
of combination chemotherapy regimens
(some including anthracyclines) have
been shown to increase response rates
or improve survival in clinical studies.[
19-21] A meta-analysis of 2,048
patients with early-stage disease included
in seven randomized trials comparing
immediate or deferred treatment
with chlorambucil demonstrated no
benefit in either arm. A summary
of clinical experience with chemotherapy
and chemoimmunotherapy in
CLL is provided in Table 4.[23-30]
Fludarabine (Fludara), a purine
analog, yields better response rates
than chlorambucil but causes more
myelosuppression and greater reduction
in CD4+ lymphocytes. The
results of three phase III randomized
trials in previously untreated patients
with CLL demonstrated the superiority
of fludarabine over other alkylating
agent-based therapy in terms of
response, duration of remission, and
disease progression-free survival (but
not overall survival).[24,25,27]
A phase II trial conducted by Flinn
and colleagues showed a complete
response rate of 47% in 17 untreated
patients with CLL who received
fludarabine and cyclophosphamide
plus filgrastim (Neupogen) support.
In a subsequent randomized
phase II trial that was conducted for
the Cancer and Leukemia Group B
(CALGB 9712), Byrd et al demonstrated
that the administration of
fludarabine concurrently with
rituximab (Rituxan) for 6 monthly
courses followed by rituximab
2 months later for 4 weekly courses
also produced a complete response
rate of 47% in 51 untreated patients
with B-CLL. These results compared
favorably with results that
were observed in 53 patients who
received fludarabine alone for
6 months followed sequentially
2 months later by rituximab consolidation.[
Of note, two studies[32,33] testing
higher doses of single-agent rituximab
in patients with pretreated and untreated
B-CLL demonstrated a doseresponse
relationship for this agent. In
addition, investigators at the University
of Texas M. D. Anderson Cancer
Center recently reported a complete
response rate of 67% in 135 patients
with CLL receiving fludarabine (25
mg/m2/d) for 3 days, cyclophosphamide
(250 mg/m2/d) for 3 days, and
rituximab (375 to 500 mg/m2) on day
1 of each treatment cycle. These
responses included a high proportion
of patients with molecular remissions,
indicating the possibility to consolidate
the response with autologous
hematopoietic cell transplantation.
Another treatment option available
for patients with chronic lymphocytic
leukemia is conventional allogeneic
bone marrow transplantation, which
may be curative in some cases.
However, only 10% of patients are eligible
for this treatment, which is associated
with significant morbidity
and mortality. Allogeneic bone
marrow transplantation in which the
patient's marrow is not completely ablated
by chemotherapy with or without
low-dose radiotherapy (often
termed a minitransplant) is another
option now being evaluated for patients
Treatment With Oblimersen
A variety of investigational agents that include an assortment of chemical compounds, monoclonal antibodies, and biologic approaches have been evaluated for the treatment of CLL and await confirmation of their clinical usefulness in comparative studies (Table 5). Notable among them is oblimersen sodium injection (Genasense, formerly known as G3139), an innovative treatment modality in the form of an antisense oligonucleotide targeted to the Bcl-2 molecule. Bcl-2 protein is upregulated in a wide variety of lymphoid malignancies, including CLL. The protein is thought to be responsible for maintaining the viability of malignant lymphoid cells and may contribute to chemotherapy and radiotherapy resistance.[36,37] Higher levels of Bcl-2 protein expression have been inversely correlated with survival in previously untreated patients with CLL. In B-CLL cells, expression levels of Bcl-2 protein were significantly elevated over those of normal B cells and correlated inversely with those of the proapoptotic protein BAX. In addition, quantitative expression analysis by reverse transcription polymerase chain reaction showed the relative ratio of Bcl-2 protein to BAX to be markedly increased in CLL and also in mantle cell lymphoma. Previous studies have shown that reduction of Bcl-2 protein expression by antisense therapy sensitizes cells to chemotherapy- induced apoptosis.[41,42] The activity of oblimersen in CLL has been evaluated in vitro and in vivo. Auer et al showed that in cells obtained by CD19 selection of peripheral blood samples from patients with CLL, Bcl-2 protein expression was significantly downregulated and markers of apoptosis upregulated by oblimersen in a sequence-specific manner, compared with sense and nonsense oligonucleotide controls. In this system, oblimersen (2 μM) was more active than either fludarabine (50 μM) or dexamethasone (1 μM) as an inducer of apoptosis, and potentiation of this activity was noted when oblimersen was combined with either fludarabine or dexamethasone. Furthermore, pretreatment with oblimersen sensitized CLL cells to the apoptotic effect of rituximab in a doseresponse relationship. With similar culture systems, synergism to induce apoptosis has been shown in vitro for oblimersen combined with the novel proteasome inhibitor bortezomib (PS-341, Velcade)  and with the humanized anti- CD52 monoclonal antibody alemtuzumab (Campath 1H). In these studies, CD19 antibody-selected CLL cells from 12 patients were put in short-term culture with or without oblimersen (at a concentration of 5 μM) to reduce Bcl-2 protein levels. Control sense and nonsense oligonucleotides were also used. Alemtuzumab was added at concentrations ranging from 1 μg/mL to 10 μg/mL. All CLL samples showed some apoptosis with alemtuzumab alone; however, those cells pretreated with oblimersen showed an enhanced level of apoptosis. The addition of oblimersen greatly enhanced the effectiveness of alemtuzumab with similar cell kill at 20% of the dose compared with alemtuzumab alone. Clinical Experience
Oblimersen was evaluated in a nonrandomized phase I/II trial as monotherapy for heavily pretreated patients with relapsed or refractory CLL to determine the maximum tolerated dose and evaluate single-agent activity. Fourteen patients received oblimersen as a continuous intravenous (IV) infusion at a daily dose ranging from 3 to 7 mg/kg for 5 to 7 days every 3 weeks. Several patients exhibited antitumor effects, including tumor lysis, reduction in circulating CLL cells, and decreases in lymphadenopathy and splenomegaly. One patient with Richter's syndrome developed a stable partial response that was reported to last several years. In cycle 1, patients received one of four different oblimersen doses: 3, 4, 5, or 7 mg/kg/d. Six of the patients receiving higher dosages (3 receiving 7 mg/kg/d, 1 receiving 5 mg/kg/d, and 2 receiving 4 mg/kg/d) showed doselimiting toxicity requiring treatment discontinuation (high fever, severe hypotension, hypoglycemia, and back pain requiring opiate analgesics) despite reduction of leukocytosis. In cycle 2, two patients who were escalated from a lower dose to 7 mg/kg/d also showed dose-limiting toxicity. These findings clearly demonstrated that patients with CLL are more sensitive to the side effects of oblimersen than patients with solid tumors, in whom these doses are generally well tolerated. Either tumor lysis or direct oligonucleotide immunostimulation of the malignant B cells is thought to be responsible for this distinct toxicity pattern.[47,48] In a subsequent phase I/II trial in 26 patients with relapsed or refractory CLL who had previously received fludarabine, oblimersen was administered as a continuous IV infusion at a dosage of 3 mg/kg/d for 5 to 7 days every 3 to 4 weeks. Six patients in phase I received the phase II dose of oblimersen (3-4 mg/kg/d). Patients had received a median of 3 (range: 1- 13) prior chemotherapy regimens for CLL. Thirteen, 7, and 4 patients had Rai stages II, III, and IV, respectively, and 2 patients had Richter's transformation. The median age was 61 years (range, 44-70). The evaluable population included 23 patients who received the phase II dose for ≥ 2 cycles (6 enrolled in phase I, and 17 enrolled in phase II). Two patients (9%) achieved partial response, 11 (48%) showed stable disease, and the remaining 10 patients (43%) had progressive disease. Reductions in circulating CLL cells of ≥ 50% from baseline were seen in 9 of 23 patients (39%). In addition ≥ 50% decrease in lymphadenopathy was seen in 8 of 19 patients (42%), and a ≥ 50% reduction of hepatomegaly or splenomegaly was seen in 8 of 16 patients (50%). Oblimersen (3 to 4 mg/kg/d) was well tolerated, with only rare grade 3 or higher toxicities reported. The most common symptoms were fatigue, night sweats, increased dyspnea, and pneumonia. Results confirmed that oblimersen has antitumor activity in pa- tients with CLL, even in the absence of other cytotoxic drugs, and that a daily dose of 3 mg/kg was well tolerated. Subsequently, a phase III trial evaluating fludarabine (25 mg/m2) and cyclophosphamide (250 mg/m2; days 1-3) with or without oblimersen (3 mg/kg/d continuous IV infusion; days 1-7) was initiated in patients with relapsed or refractory CLL. Stratification at recruitment grouped patients according to three criteria: 1) "responsiveness" or "refractoriness" to fludarabine; 2) number of previous chemotherapy regimens (1-2 vs ≥ 3); and 3) duration of response to prior therapy (≥ 6 vs < 6 months). The primary end point was response rate (complete response plus nodular partial response), and secondary end points were overall response (complete response plus nodular partial response plus partial response), overall survival, and time to disease progression. This trial recently completed accrual of 241 patients. Conclusion The rationale for evaluating oblimersen in CLL is that Bcl-2 protein is highly expressed and in all likelihood is a key survival factor in this disease. Oblimersen has shown limited single-agent activity in patients with previously treated CLL. In addition, patients with CLL appear to have considerably more sensitivity to the development of significant hypotension and fever than patients with solid tumors who receive oblimersen at similar dosages, who exhibit better tolerance. Oblimersen 3 mg/kg/d was well tolerated in phase II trials, and this dosage was used in the phase III trial. A trial of oblimersen combined with fludarabine and rituximab in patients with previously untreated or relapsed CLL has recently been initiated.
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