Abstract SY03-04: Novel imaging approaches for immunotherapy and targeted therapy of melanoma in animal models and humans

Molecular imaging may allow gaining a better understanding of the pharmacodynamic (PD) effects of immunotherapy strategies and novel targeted therapies for cancer. Two general approaches for imaging can be used, the administration of radiolabeled small molecule tracers that specifically accumulate in cells modulated by the therapy or the genetic labeling of immune cells followed by the administration of a detection probe. The use of targeted agents that block the MAPK pathway is providing unprecedented response rates in melanoma with the BRAF V600 mutation. The positron emitting tomography (PET) probe [ 18 F]fluorodeoxy-glucose ([ 18 F]FDG) can noninvasively image an effect of the therapy in inhibiting glucose metabolism, and effects on cell replication can be studied with the PET probe 3′-deoxy-3′-[18F]fluorothymidine ([ 18 F]FLT). In vitro studies in a panel of BRAF V600E mutant melanoma cell lines showed differential effects of the BRAF inhibitor PLX4032 with most cell lines being highly sensitive and a minority being resistant. Oncogenic analysis did not discern between these cell lines. Despite evidence of blocking the MAPK pathway by detecting cell cycle arrest both by flow cytometry or the incorporation of FLT in nearly all cell lines, some cell lines failed to lose viability when exposed to the BRAF inhibitor. The best way to discern between poorly responsive and highly responsive cells was the differential uptake of 2DG. In the clinic, PLX4032 has resulted in response rates up to 80% in patients with BRAF V600+ metastatic melanoma, which are preceded by dramatic decrease in [ 18 F]FDG uptake in PET scans. Areas of progression due to acquired resistance to PLX4032 are visualized by reactivation of [ 18 F]FDG uptake in PET scans. Therapy with anti-CTLA4 antibodies results in durable objective responses in approximately 10% of patients with metastatic melanoma, and the anti-CTLA4 antibody ipilimumab has demonstrated improvement in overall survival in a randomized clinical trial. A key issue is the location where lymphocyte proliferation occurs after the release of the CTLA4 cell cycle inhibitory checkpoint in lymphocytes. Immune monitoring assays had failed to detect a large lymphocytic expansion in blood, while tumors of patients responding to this therapy had markedly increased lymphocytic intratumoral infiltrates. We analyzed changes in glucose metabolism using the PET probe [ 18 F]FDG, and cell replication with the PET probe [ 18 F]FLT, after the administration of the CTLA4-blocking antibody tremelimuamb. In these studies, molecular imaging with [ 18 F]FLT allowed mapping and noninvasive imaging of cell proliferation in the spleen, the largest secondary lymphoid organ, after CTLA4 blockade in patients with metastatic melanoma. A better understanding of the distribution and tumor targeting of lymphocytes may allow advancing adoptive cell transfer (ACT) immunotherapy for cancer. We have used T cell receptor (TCR) engineering together with molecular imaging to study the antitumor activity in an animal model of TCR-engineered ACT therapy. Splenocytes were activated ex vivo and genetically modified with a retroviral vector expressing a TCR for tyrosinase and either luciferase for bioluminescence imaging (BLI) or sr39tk for positron emitting tomography (PET) imaging. Genetic labeling with the BLI and PET reporter genes allowed visualization of the distribution and antigen-specific tumor homing of TCR transgenic T cells, with trafficking correlated with antitumor efficacy. After an initial brief stage of systemic distribution, TCR redirected and genetically labeled T cells demonstrated an early pattern of specific distribution to antigen-matched tumors and locoregional lymph nodes, followed by a more promiscuous distribution one week later with additional accumulation in antigen-mismatched tumors. Therefore, the molecular imaging reporter gene labeling approach allowed the non-invasive imaging of the in vivo dynamics of the TCR transgenic cells. The approach of TCR engineering can result in responses in patients with advanced cancer. In an ongoing clinical trial we have treated patients with metastatic melanoma with the ACT of lymphocytes genetically modified to express a high affinity TCR for the melanosomal antigen MART-1. This approach has led to high initial antitumor responses followed by tumor progression once the T cells lost their immunological potency in vivo. PET imaging with [ 18 F]FDG demonstrated profound effects on tumor lesions beyond the changes in size criteria provided by CT scans, allowing the study of T cell responses to cancer. In conclusion, the use of molecular imaging in preclinical models and in the clinic allows longitudinal analysis of biological processes without altering their course, thereby providing important information to advance immunotherapy and targeted therapy strategies for cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr SY03-04. doi:10.1158/1538-7445.AM2011-SY03-04