Despite a compelling logic, tumor immunotherapy has generally failed. Work by us and others suggests that this failure owes in part to tumor-mediated immune dysfunction. CD4+CD25+ regulatory T cells (Tregs) contribute to this dysfunction. In support, we showed that in human ovarian cancer, Tregs block tumor-specific immunity, promoted tumor growth and predicted poor survival. We hypothesize that depleting Tregs will improve immunity and improve therapeutic outcomes in cancer patients. Ontak is a recombinant protein fusing interleukin-2 to the active domain of diphtheria toxin. It was designed to kill CD4+CD25+ leukemia cells. We tested the hypothesis that Ontak kills CD4+CD25+ Tregs and thus improves immunity in cancer patients. In an institutional review board (IRB)-approved study, patients 1-3 (advanced-stage cancer of the ovary, breast or lung) received a single intravenous dose of Ontak 9 μg/kg. Patient 4 had relapsed, metastatic ovarian cancer and received one dose of 12 μg/kg Ontak. Blood immune cells and cytokines were studied before and after treatment by flow cytometry. Functional T cell suppression was tested in vitro. Prior to Ontak, mean blood CD4+CD25+ cell concentration was 126/mm3 and prevalence was 26.8 % of CD4+ T cells, which decreased to 78 cells/mm3 and 19.0 % of CD4+ T cells 3-5 days after Ontak. Interferon-γ+ T cells increased approximately 50% and FOXP3 message by RT-PCR (a functional Treg marker) was significantly reduced in CD4+CD25+ cells after Ontak. CD4+CD25+ T cells were 4.2-fold less potent in inhibiting T cell proliferation in vitro after treatment. In patient 4, a single Ontak dose reduced blood CA-125 (an ovarian cancer marker) from 94 to 38 Units/ml 4 weeks later (normal <35), suggesting clinical efficacy. Patient 4 then received 6 additional doses of Ontak 12 μg/kg in an approved study amendment. Blood CA-125 dropped to 16 Units/ml 4 weeks after the final dose. Bony and visceral metastases were reduced or eliminated on PET/CT scan, except an increased inguinal mass that was necrotic tumor on biopsy. CA-125 is 17 Units/ml 6 months after treatment. Our data demonstrate for the first time that Ontak depletes functional Tregs in human cancer, associated with improved T cell immunity and clinical efficacy. We propose that Treg depletion improves immunity and mediates clinical efficacy, but mechanistic specifics remain to be determined. Our mouse model suggests that the Ontak IL-2 moiety does not mediate observed effects. Treg depletion alone, or in combination with other modalities may be a useful strategy to treat cancer.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]