CTLA-4 blockade following relapse of malignancy after allogeneic stem cell transplantation is associated with T cell activation but not with increased levels of T regulatory cells

Ipilimumab: a novel immunostimulatory monoclonal antibody for the treatment of cancer

Ipilimumab (Yervoy, developed by Medarex and Bristol-Myers Squibb) is a fully human monoclonal IgG1κ antibody against the cytotoxic T-lymphocyte antigen-4 (CTLA-4), an immune-inhibitory molecule expressed in activated T cells and in suppressor T regulatory cells. Interaction of the monoclonal antibody with CTLA-4 blocks inhibitory signals generated through this receptor and enhances T cell activation, leading to increased antitumor responses. Ipilimumab has been approved by FDA in March 2011 as monotherapy (3mg/kg every 3 weeks for 4 doses) for the treatment of advanced (unresectable or metastatic) melanoma both in pre-treated or chemotherapy naïve patients. Four months later, ipilimumab has received a rapid approval by the European Commission, after a positive opinion from the Committee for Medicinal Products for Human Use. However, the indication in the EU is limited to previously-treated patients with advanced melanoma. Ipilimumab is the first agent that has demonstrated to improve overall survival in patients with metastatic melanoma, which has a very poor prognosis, in randomized phase III clinical trials. The patterns of tumour response to ipilimumab differ from those observed with cytotoxic chemotherapeutic agents, since patients may have a delayed yet durable response and obtain long-term survival benefit despite an initial tumour growth. The major draw-back of ipilimumab is the induction of immune-related adverse effects; the latter can be life-threatening, unless promptly managed with immunosuppressive agents (most frequently corticosteroids) according to specific guidelines. Further development of ipilimumab includes its use in the neoadjuvant or adjuvant high-risk melanoma setting and for the treatment of other refractory and advanced solid tumours, either as single agent or in combination with additional immunostimulating agents or molecularly targeted therapies.

Ipilimumab (Yervoy) and the TGN1412 catastrophe

The development of the anti-CTLA-4 antibody (ipilimumab; marketed as Yervoy) immune regulatory therapy was based on the premise that "Abrogation of the function of CTLA-4 would permit CD28 to function unopposed and might swing the balance in favor of immune stimulation, tolerance breakdown and tumor eradication…" (Weber, 2009). By now, the vast majority of data collected from more than 4000 patients proves that this prediction was entirely correct. Paradoxically, the successful blockade of immune checkpoints raises the question whether an anti-CTLA-4 antibody could ever become an important therapy against cancer. T cells lost their ability to discriminate between self and non-self. Thus, tolerance to self tissues was broken in ∼70% of the patients. In the recent industry-sponsored phase III clinical trial of ipilimumab, 147 (38.7%) of the patients experienced severe adverse events and 6.8% suffered dose-limiting events (8.4%, in the ipilimumab-alone group). There were 14 deaths related to the study drugs and 7 of these were associated with immune-related adverse events. In contrast, the complete response rate was only 0.2%, in one patient out of 403 who received ipilimumab plus a peptide vaccine. Promoters of ipilimumab appear to be unmindful of the clinical trial catastrophe in London. Then, a humanized "superagonist" anti-CD28 monoclonal antibody, TGN1412, which "preferentially" activated regulatory T cells, at a higher dose, also activated all CD28 positive T cells. This precipitated a "cytokine storm" leading to life-threatening multiple organ failure in the six healthy human volunteers. Neither anti-CD28 nor anti-CTLA-4 therapies rely on antigen-specificity. Both release free antibody into the body against common molecular targets that are expressed on the targeted as well as on the non-targeted T cells. At lower antibody doses specific T cells are preferentially activated. With increasing antibody dose, however, the kinetics of the interaction is pushed in favor of widespread non-specific T cell expansion. Using the law of mass action we calculated that the vast majority of the CTLA-4 receptors on all activated T cells (including melanoma specific T cells) in the phase III clinical trial of ipilimumab will have been saturated. This would explain the runaway immune response observed. The conclusions drawn by the authors of the ipilimumab trial paper could bear an independent inspection and reassessment concerning the validation of the blockade of immune checkpoints as an important therapeutic strategy against cancer.