Imaging of CTLA4 Blockade–Induced Cell Replication with ¹⁸F-FLT PET in Patients with Advanced Melanoma Treated with Tremelimumab

Abstract

Preclinical models predict that blockade of the coinhibitory molecule cytotoxic T lymphocyte-associated antigen 4 (CTLA4) on lymphocytes results in the release of a cell cycle inhibitory checkpoint, allowing lymphocyte proliferation, tumor targeting, and regression. However, there is a paucity of data demonstrating that lymphocyte proliferation does occur in humans treated with CTLA4-blocking antibodies. Methods: We tested the role of whole-body molecular imaging in patients with advanced melanoma receiving the CTLA4-blocking antibody tremelimumab, allowing the analysis of changes in glucose metabolism using the PET probe 18 F-FDG and cell replication with the PET probe 39-deoxy-39-18 F-fluorothymidine ( 18 F-FLT). Results: PET/CT scans obtained at a median of 2 mo after initial dosing did not demonstrate significant changes in lesion size or 18 F-FDG or 18 F-FLT uptake when focusing on metastatic lesions. Similarly, there was no difference in 18 F-FDG uptake in the non-melanoma-involved spleen. However, there were significant increases in standardized uptake values for 18 F-FLT in the spleen using post-and pretremelimumab treatment scans. Conclusion: Molecular imaging with the PET probe 18 F-FLT allows mapping and noninvasive imaging of cell proliferation in secondary lymphoid organs after CTLA4 blockade in patients with metastatic melanoma. The cytotoxic T lymphocyte-associated antigen 4 (CTLA4) is a coinhibitory activation-induced surface receptor on T cells that functions as a major negative regulator of anti-self-immune responses. It provides a dominant negative signaling to T cells on binding to the costimulatory molecules CD80 (B7.1) and CD86 (B7.2) expressed on the surface of antigen-presenting cells (1). Most naïve T cells do not express surface CTLA4 because of its binding to AP50, a subunit of the clathrin adaptor AP-2 protein (2). Once a T cell is activated through its T-cell receptor, downstream T-cell receptor signaling through Src kinases results in tyrosine phosphorylation of CTLA4 and the uncoupling of CTLA4 from AP50, allowing its surface expression, which peaks at 48 h after activation. Cell surface CTLA4 has 100-1,000 times higher affinity for the costimulatory molecules expressed by antigen-presenting cells, thereby efficiently competing with the positive costimulatory receptor CD28 (1). The engagement of CTLA4 by costimulatory molecules results in decreased T-cell receptor signaling, interleukin 2 transcription (3), and cell cycle arrest at the G1 stage, with the final result of inducing T-cell anergy (4,5). A clear example of the critical role of CTLA4 on tolerance is the striking phenotype of CTLA4 knock-out mice, which develop rapid T-cell proliferation and autoimmune infiltration of multiple organs shortly after birth (6,7).Monoclonal antibodies blocking CTLA4 induce regression of immunogenic tumors in mice (8) and are being pursued as a treatment approach for patients with cancer. Two fully human antibodies with CTLA4-blocking activity-ipilimumab and tremelimumab-are in clinical develo...

PET Imaging of Proliferation with Pyrimidines

“…The authors concluded that 18 F-FLT PET has been able to detect immune system activation after melanoma vaccination. The use of 18 F-FLT PET to detect activation was also demonstrated in a study of CTLA4 inhibition (81). Significant increases in the uptake of 18 F-FLT was seen in tumor-free spleen after treatment with tremelimumab in 12 patients with advanced melanoma, suggesting activation of immune response in the healthy tissue.…”

Engineered antibody fragments for immuno-PET imaging of endogenous CD8 + T cells in vivo

“…[ 18 F] FLT-PET suffers from high uptake in proliferating bone marrow, therefore limiting detection of lesions in bone. However, [ 18 F] FLT-PET was used clinically to detect cytotoxic T-lymphocyte antigen-4 blockade-induced cell replication in the spleens of patients with melanoma (13) and, more recently, to detect antigen-specific immune responses in patients with melanoma who have lymph node metastases using dendritic cell therapy (14). [ 18 F]FAC-PET can distinguish between innate and adaptive immune cells due to the up-regulation of deoxycytidine kinase in proliferating T cells, but the uptake in a Moloney murine sarcoma virus/murine leukemia virus complex-induced sarcoma model was limited to proliferating T cells in the draining lymph nodes and spleen (15).…”

The role of F-18-fluorothymidine PET in oncology

“…Response assessment based on 18 F-FLT PET was not found to be advantageous over 18 F-FDG. Cobben et al [94] and Ribas et al [100] showed promising results in patients with stage III or advanced melanoma; Cobben et al studied 10 patients and reported that all locoregional lymph-node metastases were correctly visualized by 18 F-FLT PET with a sensitivity of 88 %; Ribas et al showed that 18 F-FLT allowed mapping and non-invasive imaging of cell proliferation in secondary lymphoid organs in 12 patients. In the study by Pfannenberg et al [99], 11 patients with metastatic germ cell tumors were examined with 18 F-FDG PET/CT and 18 F-FLT PET/CT before chemotherapy, after the first cycle, and 3 weeks after completion.…”