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Confirmation of the Activity of the Interleukin-2 Fusion Toxin Denileukin Diftitox against Chemorefractory Chronic Lymphocytic Leukemia, including Cases with Chromosome 17p Deletions and without Detectable CD25 Expression

The study by Frankel et al. (1) is the first published report of a novel treatment for fludarabine-refractory CD25-positive chronic lymphocytic leukemia (CLL), using the immunotoxin denileukin diftitox (Ontak). This fusion protein is proposed to kill cells expressing the interleukin 2 (IL-2) receptor via internalization of diphtheria toxin bound to IL-2 (2) . The IL-2 receptor is a trimeric complex composed of CD25 ( subunit), CD122 (ß subunit), and CD132 ( subunit). Although denileukin diftitox is registered in the United States only for the treatment of cutaneous T-cell lymphoma, its evaluation in CLL is logical, because approximately one-half of CLL patients have tumor cells that express CD25 (3) . In addition, previous trials of the parent compound, DAB₄₈₆IL-2, demonstrated an objective response in 2 of 13 patients with CLL (4, 5, 6, 7) .

We wish to report an additional seven CLL patients treated with denileukin diftitox and to highlight its efficacy in instances of CD25-negative and highly chemorefractory disease with a chromosome 17p abnormality. Two patients were treated as part of a clinical trial using denileukin diftitox in B-cell lymphoproliferative disorders; the remaining five were not eligible for the trial and were treated on a compassionate use basis after obtaining informed consent with national regulatory and institutional ethics approval. Patient characteristics are presented in Table 1 . Six patients were initially given a standard regimen of daily 18 µg/kg i.v. doses for 5 days, repeated at three-weekly intervals. Because of advanced age and poor performance status, patient 5 initially received 9 µg/kg/day for two courses, with a subsequent increase to 13.5 µg/kg/day for the remainder of the treatment.

All of the patients suffered some adverse effects, of various severity (Table 2) . Patient 1 required treatment cessation after developing severe vascular leak syndrome, manifested as symptomatic ascites, bilateral pleural effusions, and a hemorrhagic pericardial effusion with tamponade requiring pericardiocentesis. Pretreatment hypoalbuminemia in this patient is likely to have predisposed her to this complication. Patient 4 tolerated only one cycle of therapy, subsequently requiring prolonged hospital admission for grade III fatigue; grade IV anorexia requiring enteral feeding; clinical hepatitis with abdominal pain, nausea, and diarrhea, followed by disseminated Herpes simplex infection. The patient died of pneumonia 3 months after this course of denileukin diftitox.

Of the seven patients treated, two had an objective partial response, and there were two minor responses (Table 2) . Patient 1 (with the poor prognosis 17p deletion) had a clinically significant hematological response, becoming platelet independent after previously requiring weekly platelet transfusions. Patient 2 has had an excellent response overall, with normalization of peripheral lymphocyte count, >90% reduction in nodal size, and conversion from a diffuse bone marrow infiltrate to scattered lymphoid nodules with intervening normal hemopoiesis (Figs. 1 and 2 ). Six of the seven patients had a significant and rapid reduction in peripheral blood lymphocytosis (Fig. 3) , indicating drug activity even in two patients who otherwise had no significant response. The observed responses have been relatively durable. One partial response lasted 16 months, and the other is ongoing at 15+ months. One minor response patient died of infectious complications in ongoing response at 3 months, and the other developed progressive disease at 10 months.

View larger version (87K): [in this window] [in a new window]   Fig. 1. A, patient 2, bone marrow trephine before therapy showing diffuse infiltrate of small lymphocytes; B, patient 2, bone marrow trephine after denileukin diftitox showing nodular lymphocytic infiltrate and partial restoration of normal hemopoiesis.

View larger version (65K): [in this window] [in a new window]   Fig. 2. A, patient 2 (computed tomography abdomen), splenomegaly and multiple lymph node masses before therapy; B, patient 2 (computed tomography abdomen), partial resolution of abdominal lymphadenopathy and reduction in splenic size after six cycles denileukin diftitox.

View larger version (20K): [in this window] [in a new window]   Fig. 3. Response of lymphocyte count to administration of Ontak (denileukin diftitox). All patients except patients 4 and 6 had a significant and rapid reduction in lymphocyte count.

In contrast to the series by Frankel et al. (1) , we have demonstrated that denileukin diftitox has activity in cases without detectable CD25 expression (one partial response, one minor response, two no response). There are a number of potential reasons for this. Re et al. (8) have demonstrated that the ß subunit of the IL-2 receptor (and not CD25 positivity) is critical for denileukin diftitox-mediated cytotoxicity in vitro. The ß- and -chains dimerize to form intermediate-affinity receptors that are capable of signal transduction independent of -chain expression. Perhaps flow cytometric CD122 detection may be more helpful in determining which CLL patients are likely to respond, but this was unfortunately not performed on the current cases. It is possible that CD25 expression is below levels of detection given that cytokine receptors are typically expressed at low levels in unstimulated cells (10–1000 molecules/cell), whereas the limit of sensitivity for flow cytometry is typically around 200–500 molecules/cell (9) . Another possibility is that denileukin diftitox may act indirectly via inactivation of "bystander" nontumor cells. Increased numbers of cytotoxic "antitumor" suppressor cells targeting the CLL clone could theoretically result from immunotoxin-mediated destruction of their CD25+/CD4+ endogenous inhibitors.

Regardless of the true mechanism of action, denileukin diftitox provides another alternative for treatment of patients with chemotherapy-refractory CLL, including those in whom cytopenias preclude any other form of therapy. Also encouraging is the activity of this agent in a patient with the 17p deletion, historically resistant to all forms of therapy except the anti-CD52 antibody alemtuzumab (Campath) and allografting (10) .

FOOTNOTES

Note: Affiliations of authors: Department of Haematology, Peter Mac Callum Cancer Institute, East Melbourne, Victoria 8006, Australia.

Requests for reprints: John F. Seymour, Division of Haematology/Medical Oncology, Peter MacCallum Cancer Institute, St. Andrew’s Place, East Melbourne, Victoria, 3002 Australia. Phone: 613-9656-1700; Fax: 613-9656-1408; E-mail: John.Seymour{at}petermac.org

Received 11/20/03; revised 2/ 3/04; accepted 2/18/04.

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