Sensitive in vivo imaging technologies applicable to the clinical setting are still lacking for adoptive T-cell-based immunotherapies, an important gap to fill if mechanisms of tumor rejection or escape are to be understood. Here, we propose a highly sensitive imaging technology to track human TCR-transgenic T cells in vivo by directly targeting the murinized constant TCR beta domain (TCRmu) with a zirconium-89 ( 89 Zr)-labeled anti-TCRmu-F(ab') 2 fragment. Binding of the labeled or unlabeled F(ab') 2 fragment did not impair functionality of transgenic T cells in vitro and in vivo. Using a murine xenograft model of human myeloid sarcoma, we monitored by Immuno-PET imaging human central memory T cells (T CM ), which were transgenic for a myeloid peroxidase (MPO)-specific TCR. Diverse T-cell distribution patterns were detected by PET/CT imaging, depending on the tumor size and rejection phase. Results were confirmed by IHC and semiquantitative evaluation of T-cell infiltration within the tumor corresponding to the PET/CT images. Overall, these findings offer a preclinical proof of concept for an imaging approach that is readily tractable for clinical translation.
A number of different technologies have been developed to monitor in vivo the distribution of gene-modified T cells used in immunotherapy. Nevertheless, in-depth characterization of novel approaches with respect to sensitivity and clinical applicability are so far missing. We have previously described a novel method to track engineered human T cells in tumors using 89Zr-Df-aTCRmu-F(ab')2 targeting the murinized part of the TCR beta domain (TCRmu) of a transgenic TCR. Here, we performed an in-depth in vitro characterization of the tracer in terms of antigen affinity, immunoreactivity, influence on T-cell functionality and stability in vitro and in vivo. Of particular interest, we have developed diverse experimental settings to quantify TCR-transgenic T cells in vivo. Local application of 89Zr-Df-aTCRmu-F(ab')2-labeled T cells in a spot-assay revealed signal detection down to approximately 1.8x104 cells. In a more clinically relevant model, NSG mice were intravenously injected with different numbers of transgenic T cells, followed by injection of the 89Zr-Df-aTCRmu-F(ab')2 tracer, PET/CT imaging and subsequent ex vivo T-cell quantification in the tumor. Using this setting, we defined a comparable detection limit of 1.0x104 T cells. PET signals correlated well to total numbers of transgenic T cells detected ex vivo independently of the engraftment rates observed in different individual experiments. Thus, these findings confirm the high sensitivity of our novel PET/CT T-cell tracking method and provide critical information about the quantity of transgenic T cells in the tumor environment suggesting our technology being highly suitable for further clinical translation.
Cancer immunotherapy has proven high efficacy in treating diverse cancer entities by immune checkpoint modulation and adoptive T-cell transfer. However, patterns of treatment response differ substantially from conventional therapies, and reliable surrogate markers are missing for early detection of responders versus non-responders. Current imaging techniques using 18F-fluorodeoxyglucose-positron-emmission-tomograpy (18F-FDG-PET) cannot discriminate, at early treatment times, between tumor progression and inflammation. Therefore, direct imaging of T cells at the tumor site represents a highly attractive tool to evaluate effective tumor rejection or evasion. Moreover, such markers may be suitable for theranostic imaging.Methods: We mainly investigated the potential of two novel pan T-cell markers, CD2 and CD7, for T-cell tracking by immuno-PET imaging. Respective antibody- and F(ab´)2 fragment-based tracers were produced and characterized, focusing on functional in vitro and in vivo T-cell analyses to exclude any impact of T-cell targeting on cell survival and antitumor efficacy.Results: T cells incubated with anti-CD2 and anti-CD7 F(ab´)2 showed no major modulation of functionality in vitro, and PET imaging provided a distinct and strong signal at the tumor site using the respective zirconium-89-labeled radiotracers. However, while T-cell tracking by anti-CD7 F(ab´)2 had no long-term impact on T-cell functionality in vivo, anti-CD2 F(ab´)2 caused severe T-cell depletion and failure of tumor rejection.Conclusion: This study stresses the importance of extended functional T-cell assays for T-cell tracer development in cancer immunotherapy imaging and proposes CD7 as a highly suitable target for T-cell immuno-PET imaging.
Objectives: Radiohybrid prostate-specific membrane antigen (rhPSMA)-ligands are applicable as radiochemical twins for both diagnostic PET imaging and endoradiotherapy.Based on preliminary data as diagnostic ligand, the isomer rhPSMA-7.3 is a promising candidate for potential endoradiotherapy. The aim of this preclinical evaluation was to assess biodistribution, dosimetry and therapeutic efficacy of 19 F/ 177 Lu-rhPSMA-7.3 in comparison to the established therapeutic agent 177 Lu-PSMA I&T (imaging & therapy). Methods: Biodistribution of 19 F/ 177 Lu-rhPSMA-7.3 and 177 Lu-PSMA I&T was performed in LNCaP tumor bearing SCID-mice after sacrifice at defined time points up to 7 days (n=5). Organs and tumors were dissected, injected dose per gram (%ID/g) was determined and dosimetry calculations were performed using OLINDA/EXM1.0. The therapeutic efficacy of a single dose of 30 MBq 19 F/ 177 Lu-rhPSMA-7.3 (n=7) was compared with 177 Lu-PSMA I&T (n=7) and control groups (n=6-7) using C4-2 tumor bearing SCID-mice by evaluating tumor growth and survival over 6 weeks post treatment.
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