Macrophage Response to Locally-delivered Topotecan in Glioblastoma

Introduction: Glioblastoma (GBM) is the most common primary central nervous system tumor, with a median survival of approximately 15 months following diagnosis. We recently completed a phase 1b trial with convection-enhanced delivery (CED) of topotecan for recurrent GBM. Comparison of pre- vs post-treatment biopsies identified a clinically favorable significant decrease in tumor proliferative signatures following treatment but also a significant increase of inflammatory cells within the treatment volume. To identify therapeutic targets, the aim of this study is to analyze the topotecan-induced inflammatory myeloid cell response, identified by Iba1 and MSR1 proteins, with a focus on immunosuppressive macrophages identified by the MARCO protein.

Methods: Tumor biopsies were collected at surgical implant and explant of the CED catheter therefore providing tissue for analysis from before and after CED of topotecan. Samples were processed into 5µm paraffin sections and stained with multiplex fluorescence immunohistochemistry for MARCO, Iba1, and MSR1. Z-stack images were obtained using a confocal microscope. Tissue quantification and cell detection was performed via QuPath, and statistical significance was assessed using Welch’s t-tests.

Results: A significant increase of myeloid cell response of Iba1+ cells was identified after treatment (P = 0.001). These myeloid cells, labeled by the activated microglia/macrophage marker MSR1, rose significantly following treatment (P = 0.025). A distinct subpopulation of MARCO+ immunosuppressive macrophages was characterized. The percentage of MARCO+ cells, as percent total cells, significantly increased from pre- to post-treatment (P < 0.001). MARCO+ cells colocalized with MSR1 and/or Iba1, and MARCO+ cells negative for MSR1/Iba1, also significantly increased (P = 0.033, P = 0.004).

Conclusion : A robust expansion of tumor-associated myeloid cells significantly enriched for a subpopulation of immunosuppressive MARCO+ cells occurs following response to locally delivered topotecan therapy. Ongoing studies are targeting this inflammatory response to improve the treatment of CED of chemotherapy.