IL-2 Appears to Increase Rituximab Antitumor Effects

MILAN, Italy—Giving interleukin-2 (IL-2, aldesleukin, Proleukin) in combination with the anti-CD20 monoclonal antibody rituximab (Rituxan) may increase the antibody’s efficacy in lymphoma patients, apparently because it increases the number of natural killer (NK) cells. Researchers at a Clinical Development Conference sponsored by Chiron Corporation suggested that IL-2 be studied as a regular addition to rituximab therapy and also as an addition to rituxi-mab/chemotherapy regimens.

Joseph Rosenblatt, MD, Pierluigi Porcu, MD, and Maurice Wolin, MD, discussed the rationale for cytokine/rituximab combinations and early clinical trial results at the meeting.

Dr. Rosenblatt, chief of the Division of Hematology/Oncology, University of Miami School of Medicine, said that the strategy of adding IL-2 emerged as a means of further heightening the antibody-dependent cell-mediated cytotoxicity (ADCC) of rituximab.

Rituximab is an anti-CD20 antibody and thus very efficient at removing B cells, but also directly initiates apoptosis by triggering intracellular signaling pathways or by perturbing calcium channels. The antibody also fixes complement. Investigators have been aware for some time that rituximab is associated with a "delayed response" that evolves or increases over time in some patients, and this is assumed to be immune mediated.

Rituximab in Bulky Disease

The need to improve rituximab efficacy is particularly apparent in bulky disease, low-grade non-Hodgkin’s lymphoma (NHL). "Rituximab is a useful drug, but as tumors exceed 5 cm in diameter, the overall response rates decrease, there are partial rather than complete re-sponses, and times to progression are less than satisfactory," Dr. Rosenblatt said. "When rituximab is given alone, a relatively small number of patients have molecular complete remissions, even in the setting of low tumor burden."

Interestingly, about 40% of lymphoma patients who relapse after rituximab respond if treated again, and Dr. Rosenblatt said that the duration of these second responses often exceeds that of the first response, contrary to the result typically seen with chemotherapy.

"Presumably most of the rituximab effect is mediated through ADCC and most is mediated by NK cells through engagement of FcgRIII immunoglobulin receptors," Dr. Rosenblatt said (see Figure 1). This receptor activates various intracellular functions. However, the FcgRIIB receptor inhibits many of those functions.

Dr. Wolin, vice president for oncology research and development, Chiron Corporation, said that adding IL-2 to rituximab originally evolved out of a simple hope that increasing the number of NK cells would shift the balance toward the activating FcgRIII receptor and thus increase ADCC.

"Administration of IL-2 results in expansion of NK cells with FcgRIII receptors capable of participating in ADCC," Dr. Rosenblatt said. "Combining IL-2 and rituximab should be clinically feasible, the NK cell population should increase with this regimen, and responses appear to correlate with the degree of NK activation and expansion."

Genetic polymorphisms for the FcgRIII receptor exist such that patients can be classified as possessing either "high-affinity" or "low-affinity" receptors. "Clinical responses to rituximab appear to break down according to the prevalence of the high-affinity version of the FcgRIII receptor," Dr. Rosenblatt said. "There is also a suggestion that molecular complete response is much more likely if the patient is homozygous for the high-affinity FcgRIII receptor."

Combinations of rituximab with chemotherapy are already widely used, and Dr. Rosenblatt said that one of the issues facing clinical trial designers is whether adding chemotherapy will diminish some of the IL-2 efficacy.

Pulsed IL-2

Dr. Porcu’s group is testing rituximab/IL-2 regimens in NHL, and is also attempting to prevent acute infusion reactions—thought to be mediated by tumor necrosis factor-a (TNF-a)—that are sometimes seen with monoclonal antibodies by combining the anti-TNF antibody etanercept (Enbrel) with rituximab. Dr. Porcu is associate professor of hematology/oncology, Ohio State University Comprehensive Cancer Center.

Early data led to design of a clinical trial in which patients with CD20-positive B-cell malignancies were given rituximab daily for 4 weeks, followed by a 2-week break, then initiation of daily low-dose IL-2 (1 × 106 IU/m²/d) with periodic intermediate-dose pulses (3 to 15 × 106 IU/m²/d) thought to be required for NK cell activation. Of 12 patients enrolled to date, 10 had prior rituximab. Of nine evaluable patients, one achieved a complete response (CR), and two had a partial response (PR). One patient each with a CR and PR had previous disease recurrence or progression despite treatment with rituximab alone.

The dose-limiting toxicity in Dr. Porcu’s study was acute renal failure in a patient with low-grade lymphoma who developed profound hypercalcemia and renal insufficiency, but whose renal insufficiency eventually resolved.

"Side effects with IL-2 include fatigue, flu-like symptoms, and local cutaneous reaction at the injection site. These are completely reversible but can be quite bothersome to the patient and require a lot of local care," Dr. Porcu said.

Daily or Thrice-Weekly Dosing

Dr. Wolin reported data from two phase I trials assessing the combination of IL-2 and rituximab—NHL 01 using daily IL-2 dosing and NHL 02 with thrice- weekly dosing (Figure 2) in patients with relapsed stage III/IV NHL. The data showed a strikingly clear association between clinical response and NK cell counts (Figure 3).

"Every patient who had above 200 NK cells/µL responded," he said. The early studies also showed that patients tolerate a thrice-weekly IL-2 dosing schedule much better than having to inject themselves every day, he said.

Responses may also be dose dependent. "At the lowest dose of IL-2, we saw a couple of patients with stable disease. As we increased the dose, we saw CRs. Significantly, we had no patients who developed progressive disease at dose levels above 10 million IU/m²/d," Dr. Wolin said.

In NHL 01, 5 patients receiving IL-2 at 6 MIU daily achieved a PR, including 3 with intermediate-grade NHL. In NHL 02, 7 of 10 patients receiving IL-2 at a dosage of 10 MIU thrice weekly or higher achieved an objective tumor response (including 4 CRs), with 5 re-sponses in patients with intermediate-grade NHL (see Table).

"Another surprise," Dr. Wolin noted, "was that at the higher doses, there was a sustained high level of NK cells, despite the fact that patients had been off IL-2 for a month." Only responding patients exhibited sustained ADCC activity beyond the treatment period. These data demonstrated that expansion of NK cell populations increased following IL-2 therapy, and that clinical response appeared to correlate with the degree of NK cell expansion.

The dose-limiting toxicities were grade 3-4 asthenia and pyrexia (n = 1) with daily IL-2 dosing, and grade 3-4 increases in alanine aminotransferase (n = 1) and hypersensitivity reactions (n = 1) with thrice weekly dosing. The mean tolerated dose of IL-2 in combination with rituximab (375 mg/m² IV) was 6 MIU given subcutaneously (SC) daily or 14 MIU SC three times a week.

Chiron’s Proleukin IL-2, which was originally developed as an intravenous formulation, is being used for subcutaneous dosing in some clinical trials.  Dr. Wolin noted that Chiron Corporation is also developing an IL-2 formulation specifically for SC use.

Future trials with IL-2 plus rituximab combination therapy will include patients who were refractory to rituximab mono-therapy. "The hypothesis is that this might be due to a lack of immune robustness that IL-2 could restore," Dr. Wolin said. Other studies will be conducted in NHL patients who failed previous conventional chemotherapy.

In summary, the data presented at the meeting showed that the combination of IL-2 and rituximab is clinically feasible. Patients tolerated IL-2 thrice-weekly dosing better than daily dosing and achieved more tumor responses with thrice-weekly dosing. Ongoing phase II studies will further define the safety and efficacy of combinations of IL-2 and rituximab.