Abstract 68: Methotrexate prevents primary immune responses against recombinant immunotoxin in murine models

Abstract

Abstract Recombinant immunotoxins (rITs) are composed of a tumor antigen-targeting antibody fragment fused to a portion of Pseudomonas exotoxin A. rITs have been effective in clinical trials for patients with hematologic malignancies. The CD22 targeting rIT Moxetumomab Pasudotox has achieved overall response rates of 86% and complete remission rates of 46% in patients with relapsed-refractory Hairy Cell Leukemia. However, the therapeutic efficacy of rITs against solid tumors is limited by their immunogenicity in immune-competent patients. In clinical trials to treat mesothelioma patients with SS1P, a rIT targeting mesothelin, 90% of patients developed neutralizing antibodies against SS1P after one cycle of treatment. When immunosuppressive chemotherapy and SS1P were combined, more cycles of rIT could be given and several patients with advanced chemo-refractory mesothelioma had striking tumor regressions. This implicates high therapeutic potential for rITs against solid tumors once immunogenicity is surmounted. Methotrexate (MTX) is a folate antagonist which interferes with purine biosynthesis, and is used to treat osteosarcomas and other cancers. MTX also interferes with T cell responses and is used to treat autoimmune diseases. Based on its immunosuppressive properties, Joly et al. demonstrated that low-dose MTX prevented the formation of ADAs against recombinant human alglucosidase alfa in mice in an antigen-specific manner. We hypothesized that MTX would similarly prevent the formation of ADAs against rITs in an antigen-specific manner. To test our hypotheses, mice were treated with the mesothelin-targeting rIT RG7787 with or without MTX given 0, 24, and 48 hours after RG7787 treatment. Serum was collected and anti-RG7787 ADAs were measured by direct ELISA. We found that six doses of RG7787 combined with low dose MTX (1 mg/kg) inhibited the formation of ADAs against RG7787. This inhibition was sustained through six challenges with RG7787 without additional MTX. Further, we found that immunization with RG7787 plus MTX induced RG7787-specific tolerance, and had no effect on the ADA response against a second protein, ovalbumin. We conclude that combination of MTX and RG7787 is effective at preventing primary immune responses in a durable, antigen-specific manner. We propose to combine this agent in immune-competent cancer patients receiving rIT therapy to prevent rIT immunogenicity. Citation Format: Emily M. King, Ronit Mazor, Ira Pastan. Methotrexate prevents primary immune responses against recombinant immunotoxin in murine models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 68. doi:10.1158/1538-7445.AM2017-68