Tumor Suppression by Anti-Fibroblast Activation Protein Near-Infrared Photoimmunotherapy Targeting Cancer-Associated Fibroblasts

Cancer-associated fibroblasts (CAFs) constitute a prominent cellular component of the tumor stroma, with various pro-tumorigenic roles. Numerous attempts to target fibroblast activation protein (FAP), a highly expressed marker in immunosuppressive CAFs, have failed to demonstrate anti-tumor efficacy in human clinical trials. Near-infrared photoimmunotherapy (NIR-PIT) is a highly selective tumor therapy that utilizes an antibody-photo-absorbing conjugate activated by near-infrared light. In this study, we examined the therapeutic efficacy of CAF depletion by NIR-PIT in two mouse tumor models. Using CAF-rich syngeneic lung and spontaneous mammary tumors, NIR-PIT against FAP or podoplanin was performed. Anti-FAP NIR-PIT effectively depleted FAP+ CAFs, as well as FAP+ myeloid cells, and suppressed tumor growth, whereas anti-podoplanin NIR-PIT was ineffective. Interferon-gamma production by CD8 T and natural killer cells was induced within hours after anti-FAP NIR-PIT. Additionally, lung metastases were reduced in the treated spontaneous mammary cancer model. Depletion of FAP+ stromal as well as FAP+ myeloid cells effectively suppressed tumor growth in bone marrow chimeras, suggesting that the depletion of both cell types in one treatment is an effective therapeutic approach. These findings highlight a promising therapy for selectively eliminating immunosuppressive FAP+ cells within the tumor microenvironment.

Nanobody-Based ImmunoPET for Hepatocellular Carcinoma

PURPOSE/OBJECTIVE(S)

HCC accounts for 75-90% of all primary liver cancers, the majority of which are treated with liver-directed therapy. Treatment response and recurrence are difficult to discern using conventional imaging with MR/CT. Tumor-selective PET imaging could help with clinical management in this setting. Here, we engineer HN3, a single-domain antibody (nanobody) specific to GPC3, a histopathologically-defining HCC marker, as an immunoPET agent. We compared both conventional and sortase-based site-specific modification methods for synthesizing HN3 immunoPET tracers.

MATERIALS/METHODS

Stochastic lysine conjugation with deferoxamine (DFO-NCS) was done to synthesize nHN3-DFO. ssHN3-DFO was engineered utilizing sortase-mediated conjugation of HN3 containing an LPETG C-terminal tag and a triglycine-DFO chelator. Biolayer interferometry (BLI) and radioligand saturation assays were done to determine binding affinity pre- and post-Zirconium-89 labeling. Following, PET/CT with a terminal 3-hour biodistribution was done in mice inoculated with isogenic A431 and A431-GPC3+ xenografts to determine conjugate specificity for GPC3. Finally, conjugates were evaluated in a HepG2 liver cancer model via ex vivo biodistribution studies and a comparative PET/CT study in mice bearing HepG2 tumors that were imaged with both [18F]FDG and 89Zr-ssHN3.

RESULTS

Both conjugates exhibited nanomolar binding affinity for GPC3 in vitro (11-30 nM for nHN3 and 10-15 nM for ssHN3). A431 and A431-GPC3+ PET/CT and biodistribution studies showed specificity to GPC3 by both probes, with more favorable tumor uptake by 89Zr-ssHN3 at 3 hours post-injection (14% IA/g vs. 7% IA/g for nHN3). Both tracers also displayed uptake in HepG2 (GPC3+) liver tumors, again with the site specifically conjugated probe having higher tumor accumulation and lower liver signal than the conventionally modified HN3 (7% IA/g vs. 5 % IA/g for tumor and 2% IA/g vs. 4% IA/g for liver at 1-hour post-injection). PET/CT studies in mice imaged with [18F]FDG and 89Zr-ssHN3 demonstrated more consistent tumor accumulation for the nanobody conjugate (4/4 mice had uptake by the tumor vs. 1/4 for FDG).

CONCLUSION

We successfully designed, synthesized, and characterized novel GPC3-selective nanobody PET probes that can image liver tumors in vivo. The site-specifically conjugated tracer showed more favorable biodistribution and pharmacokinetic properties, resulting in a much higher tumor: liver signal compared to 89Zr-nHN3. We also show the superiority of the 89Zr-ssHN3 imaging over conventional [18F]FDG, highlighting a clear advantage in using targeted tumor imaging for this cancer type. Successful translation of the site-specifically conjugated nanobody may ultimately aid in characterizing lesions following liver-directed therapy and allow for more comprehensive screening, early diagnosis, and post-treatment surveillance of HCC.

Site-Specifically Conjugated Single-Domain Antibody Successfully Identifies Glypican-3-Expressing Liver Cancer by Immuno-PET

Primary liver cancer is the third leading cause of cancer-related deaths, and its incidence and mortality are increasing worldwide. Hepatocellular carcinoma (HCC) accounts for 80% of primary liver cancer cases. Glypican-3 (GPC3) is a heparan sulfate proteoglycan that histopathologically defines HCC and represents an attractive tumor-selective marker for radiopharmaceutical imaging and therapy for this disease. Single-domain antibodies are a promising scaffold for imaging because of their favorable pharmacokinetic properties, good tumor penetration, and renal clearance. Although conventional lysine-directed bioconjugation can be used to yield conjugates for radiolabeling full-length antibodies, this stochastic approach risks negatively affecting target binding of the smaller single-domain antibodies. To address this challenge, site-specific approaches have been explored. Here, we used conventional and sortase-based site-specific conjugation methods to engineer GPC3-specific human single-domain antibody (HN3) PET probes. Methods: Bifunctional deferoxamine (DFO) isothiocyanate was used to synthesize native HN3 (nHN3)-DFO. Site-specifically modified HN3 (ssHN3)-DFO was engineered using sortase-mediated conjugation of triglycine-DFO chelator and HN3 containing an LPETG C-terminal tag. Both conjugates were radiolabeled with 89Zr, and their binding affinity in vitro and target engagement of GPC3-positive (GPC3+) tumors in vivo were determined. Results: Both 89Zr-ssHN3 and 89Zr-nHN3 displayed nanomolar affinity for GPC3 in vitro. Biodistribution and PET/CT image analysis in mice bearing isogenic A431 and A431-GPC3+ xenografts, as well as in HepG2 liver cancer xenografts, showed that both conjugates specifically identify GPC3+ tumors. 89Zr-ssHN3 exhibited more favorable biodistribution and pharmacokinetic properties, including higher tumor uptake and lower liver accumulation. Comparative PET/CT studies on mice imaged with both 18F-FDG and 89Zr-ssHN3 showed more consistent tumor accumulation for the single-domain antibody conjugate, further establishing its potential for PET imaging. Conclusion: 89Zr-ssHN3 showed clear advantages in tumor uptake and tumor-to-liver signal ratio over the conventionally modified 89Zr-nHN3 in xenograft models. Our results establish the potential of HN3-based single-domain antibody probes for GPC3-directed PET imaging of liver cancers.

Rapid Depletion of Intratumoral Regulatory T Cells Induces Synchronized CD8 T- and NK-cell Activation and IFNγ-Dependent Tumor Vessel Regression

Regulatory T cells (Tregs) are known to inhibit antitumor immunity, yet the specific mechanism by which intratumoral Tregs promote tumor growth remains unclear. To better understand the roles of intratumoral Tregs, we selectively depleted tumor-infiltrating Tregs using anti-CD25-F(ab')2 near-infrared photoimmunotherapy. Depletion of tumor-infiltrating Tregs induced transient but synchronized IFNγ expression in CD8 T and natural killer (NK) cells. Despite the small fraction of CD8 T and NK cells contained within examined tumors, IFNγ produced by these CD8 T and NK cells led to efficient and rapid tumor vessel regression, intratumoral ischemia, and tumor necrosis/apoptosis and growth suppression. IFNγ receptor expression on vascular endothelial cells was required for these effects. Similar findings were observed in the early phase of systemic Treg depletion in tumor-bearing Foxp3DTR mice; combination with IL15 therapy further inhibited tumor growth and achieved increased complete regression. These results indicate the pivotal roles of intratumoral Tregs in maintaining tumor vessels and tumor growth by suppressing CD8 T and NK cells from producing IFNγ, providing insight into the mechanism of Treg-targeting therapies. SIGNIFICANCE: Intratumoral Treg depletion induces synchronized intratumoral CD8 T- and NK-cell activation, IFNγ-dependent tumor vessel regression, and ischemic tumor necrosis/apoptosis, indicating the roles of intratumoral Tregs to support the tumor vasculature. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/81/11/3092/F1.large.jpg.