Granzyme B PET Imaging in Response to In Situ Vaccine Therapy Combined with αPD1 in a Murine Colon Cancer Model

Abstract

Immune checkpoint inhibitors (ICIs) block checkpoint receptors that tumours use for immune evasion, allowing immune cells to target and destroy cancer cells. Despite rapid advancements in immunotherapy, durable response rates to ICIs remains low. To address this, combination clinical trials are underway assessing whether adjuvants can enhance responsiveness by increasing tumour immunogenicity. CpG-oligodeoxynucleotides (CpG-ODN) are synthetic DNA fragments containing an unmethylated cysteine-guanosine motif that stimulate the innate and adaptive immune systems by engaging Toll-like receptor 9 (TLR9) present on the plasmacytoid dendritic cells (pDCs) and B cells. Here, we have assessed the ability of AlF-mNOTA-GZP, a peptide tracer targeting granzyme B, to serve as a PET imaging biomarker in response to CpG-ODN 1585 in situ vaccine therapy delivered intratumourally (IT) or intraperitoneally (IP) either as monotherapy or in combination with αPD1. [18F]AlF-mNOTA-GZP was able to differentiate treatment responders from non-responders based on tumour uptake. Furthermore, [18F]AlF-mNOTA-GZP showed positive associations with changes in tumour-associated lymphocytes expressing GZB, namely GZB+ CD8+ T cells, and decreases in suppressive F4/80+ cells. [18F]AlF-mNOTA-GZP tumour uptake was mediated by GZB expressing CD8+ cells and successfully stratifies therapy responders from non-responders, potentially acting as a non-invasive biomarker for ICIs and combination therapy evaluation in a clinical setting.

Molecular imaging to support cancer immunotherapy

“…Several studies evaluating granzyme B PET for ICI therapy in different mouse models accurately predicted response to ICI therapy. [76][77][78][79][80][81][82] Granzyme B levels were higher in human melanoma tumors responding to ICI therapy. 78 There is one ongoing clinical trial evaluating granzyme PET with [ 68 Ga]Ga-NOTA-hGZP in patients with melanoma and NSCLC treated with pembrolizumab (NCT04169321), two other trials are listed (NCT04721756, NCT05000372), evaluating [ 18 F]-LY3546117 and [ 68 Ga]Ga-grazytracer respectively for granzyme PET imaging, but not yet recruiting.…”

Radionuclide Imaging of Cytotoxic Immune Cell Responses to Anti-Cancer Immunotherapy

“…A distinct advantage of using 18 F-tracers, instead of 68 Ga-based tracers, is the improved imaging quality associated with 18 F, and, in addition, the radiation burden is lower despite having a longer half-life, which is also preferable [74,99]. However, 68 Ga is eluted from a generator, 68 Ga yielding [ 68 Ga]Ga-NOTA-GZP [24,96]. The in vitro results indicated that the tracer has high specificity for granzyme B over other granzyme family members, yet selectivity over closely related proteases was not demonstrated [24,97].…”

“…Created with BioRender.com.Larimer et al developed a granzyme B-targeted PET activity-based probe (GZP), bearing the granzyme B recognition sequence, Ile-Glu-Phe-Asp, attached to an aldehyde warhead. This ABP contained the radiometal chelator NOTA for radio-labelling with68 Ga yielding [68 Ga]Ga-NOTA-GZP[24,96]. The in vitro results indicated that the tracer has high specificity for granzyme B over other granzyme family members, yet selectivity over closely related proteases was not demonstrated[24,97].…”

Functional imaging of immune cell subpopulations in the tumor microenvironment: clinical implications

“…Additional PET approaches for tracing activated T cells and quantifying the number of T cells have used [ 18 F]F-AraG ( 3 ) and 89 Zr-Df-IAB22M2C ( 4 ), respectively. Attempts to develop PET imaging of granzyme B have been ongoing ( 5 , 6 ), but the study by Zhou et al is the first to demonstrate its clinical utility ( 1 ).…”