Early Detection in a Mouse Model of Pancreatic Cancer by Imaging DNA Damage Response Signaling

How much do we know about the metastatic process?

Cancer cells can leave their primary sites and travel through the circulation to distant sites, where they lodge as disseminated cancer cells (DCCs), even during the early and asymptomatic stages of tumor progression. In experimental models and clinical samples, DCCs can be detected in a non-proliferative state, defined as cellular dormancy. This state can persist for extended periods until DCCs reawaken, usually in response to niche-derived reactivation signals. Therefore, their clinical detection in sites like lymph nodes and bone marrow is linked to poor survival. Current cancer therapy designs are based on the biology of the primary tumor and do not target the biology of the dormant DCC population and thus fail to eradicate the initial or subsequent waves of metastasis. In this brief review, we discuss the current methods for detecting DCCs and highlight new strategies that aim to target DCCs that constitute minimal residual disease to reduce or prevent metastasis formation. Furthermore, we present current evidence on the relevance of DCCs derived from early stages of tumor progression in metastatic disease and describe the animal models available for their study. We also discuss our current understanding of the dissemination mechanisms utilized by genetically less- and more-advanced cancer cells, which include the functional analysis of intermediate or hybrid states of epithelial-mesenchymal transition (EMT). Finally, we raise some intriguing questions regarding the clinical impact of studying the crosstalk between evolutionary waves of DCCs and the initiation of metastatic disease.

Discovery, Synthesis, and Evaluation of Novel Dual Inhibitors of a Vascular Endothelial Growth Factor Receptor and Poly(ADP-Ribose) Polymerase for BRCA Wild-Type Breast Cancer Therapy

Poly(ADP-ribose) polymerase (PARP) inhibitors have been approved for the treatment of breast cancer (BC) with breast cancer susceptibility (BRCA) gene mutation. Leveraging new synthetic lethal interactions may be an effective way to broaden the indication of PARP inhibitors for BC patients with wild-type BRCA. Vascular endothelial growth factor receptor (VEGFR)-mediated suppression of angiogenesis has been reported to improve the sensitivity of wild-type BRCA cells to PARP inhibitors through synthetic lethality. Herein, we reported the conjugation of a PARP inhibitor with a VEGFR inhibitor pharmacophore to construct dual VEGFR and PARP inhibitors. The most potent compound 14b is identified to exert promising activities against VEGFR and PARP in the nanomolar range and possesses significant in vitro and in vivo antitumor and antimetastasis features. It also presented a favorable pharmacokinetic characteristics in rats with an oral bioavailability of 60.1%. Collectively, 14b may be a promising therapeutic agent of BRCA wild-type BC.

Imaging DNA damage response by γH2AX in vivo predicts treatment response to Lutetium-177 radioligand therapy and suggests senescence as a therapeutically desirable outcome

Rationale: An effective absorbed dose response relationship is yet to be established for Lutetium-177 based radionuclide therapies such as 177Lu-DOTATATE and 177Lu-PSMA. The inherent biological heterogeneity of neuroendocrine and prostate cancers may make the prospect of establishing cohort-based dose-response relationships unobtainable. Instead, an individual-based approach, monitoring the dose-response within each tumor could provide the necessary metric to monitor treatment efficacy. Methods: We developed a dual isotope SPECT imaging strategy to monitor the change over time in the relationship between 177Lu-DOTATATE and 111In-anti-γH2AX-TAT, a modified radiolabelled antibody that allows imaging of DNA double strand breaks, in mice bearing rat pancreatic cancer xenografts. The dynamics of γH2AX foci, apoptosis and senescence following exposure to 177Lu-DOTATATE was further investigated in vitro and in ex vivo tumor sections. Results: The change in slope of the 111In-anti-γH2AX-TAT to 177Lu signal between days 5 and 7 was found to be highly predictive of survival (r = 0.955, P < 0.0001). This pivotal timeframe was investigated further in vitro: clonogenic survival correlated with the number of γH2AX foci at day 6 (r = -0.995, P < 0.0005). While there was evidence of continuously low levels of apoptosis, delayed induction of senescence in vitro appeared to better account for the γH2AX response to 177Lu. The induction of senescence was further investigated by ex vivo analysis and corresponded with sustained retention of 177Lu within tumor regions. Conclusions: Dual isotope SPECT imaging can provide individualized tumor dose-responses that can be used to predict lutetium-177 treatment efficacy. This bio-dosimeter metric appears to be dependent upon the extent of senescence induction and suggests an integral role that senescence plays in lutetium-177 treatment efficacy.

A Model System to Explore the Detection Limits of Antibody-Based Immuno-SPECT Imaging of Exclusively Intranuclear Epitopes

Imaging of intranuclear epitopes using antibodies tagged to cell-penetrating peptides has great potential given its versatility, specificity, and sensitivity. However, this process is technically challenging because of the location of the target. Previous research has demonstrated a variety of intranuclear epitopes that can be targeted with antibody-based radioimmunoconjugates. Here, we developed a controlled-expression model of nucleus-localized green fluorescent protein (GFP) to interrogate the technical limitations of intranuclear SPECT using radioimmunoconjugates, notably the lower target-abundance detection threshold. Methods: We stably transfected the lung adenocarcinoma cell line H1299 with an enhanced GFP (EGFP)-tagged histone 2B (H2B) and generated 4 cell lines expressing increasing levels of GFP. EGFP levels were quantified using Western blot, flow cytometry, and enzyme-linked immunosorbent assay. An anti-GFP antibody (GFP-G1) was modified using dibenzocyclooctyne-N3-based strain-promoted azide-alkyne cycloaddition with the cell-penetrating peptide TAT (GRKKRRQRRRPPQGYG), which also includes a nuclear localization sequence, and the metal ion chelator N3-Bn-diethylenetriamine pentaacetate (DTPA) to allow radiolabeling with 111In. Cell uptake of 111In-GFP-G1-TAT was evaluated across 5 cell clones expressing different levels of H2B-EGFP in vitro. Tumor uptake in xenograft-bearing mice was quantified to determine the smallest amount of target epitope that could be detected using 111In-GFP-G1-TAT. Results: We generated 4 H1299 cell clones expressing different levels of H2B-EGFP (0-1 million copies per cell, including wild-type H1299 cells). GFP-G1 monoclonal antibody was produced and purified in house, and selective binding to H2B-EGFP was confirmed. The affinity (dissociation constant) of GFP-G1 was determined as 9.1 ± 3.0 nM. GFP-G1 was conjugated to TAT and DTPA. 111In-GFP-G1-TAT uptake in H2B-EGFP-expressing cell clones correlated linearly with H2B-EGFP expression (P < 0.001). In vivo xenograft studies demonstrated that 111In-GFP-G1-TAT uptake in tumor tissue correlated linearly with expression of H2B-EGFP (P = 0.004) and suggested a lower target-abundance detection threshold of approximately 240,000 copies per cell. Conclusion: Here, we present a proof-of-concept demonstration that antibody-based imaging of intranuclear targets is capable both of detecting the presence of an epitope of interest with a copy number above 240,000 copies per cell and of determining differences in expression level above this threshold.

Molecular Imaging of Apoptosis: The Case of Caspase-3 Radiotracers

The molecular imaging of apoptosis remains an important method for the diagnosis and monitoring of the progression of certain diseases and the evaluation of the efficacy of anticancer apoptosis-inducing therapies. Among the multiple biomarkers involved in apoptosis, activated caspase-3 is an attractive target, as it is the most abundant of the executioner caspases. Nuclear imaging is a good candidate, as it combines a high depth of tissue penetration and high sensitivity, features necessary to detect small changes in levels of apoptosis. However, designing a caspase-3 radiotracer comes with challenges, such as selectivity, cell permeability and transient caspase-3 activation. In this review, we discuss the different caspase-3 radiotracers for the imaging of apoptosis together with the challenges of the translation of various apoptosis-imaging strategies in clinical trials.

Positron range-free and multi-isotope tomography of positron emitters

Despite improvements in small animal PET instruments, many tracers cannot be imaged at sufficiently high resolutions due to positron range, while multi-tracer PET is hampered by the fact that all annihilation photons have equal energies. Here we realize multi-isotope and sub-mm resolution PET of isotopes with several mm positron range by utilizing prompt gamma photons that are commonly neglected. A PET-SPECT-CT scanner (VECTor/CT, MILabs, The Netherlands) equipped with a high-energy cluster-pinhole collimator was used to image ¹²⁴I and a mix of ¹²⁴I and ¹⁸F in phantoms and mice. In addition to positrons (mean range 3.4 mm) ¹²⁴I emits large amounts of 603 keV prompt gammas that-aided by excellent energy discrimination of NaI-were selected to reconstruct ¹²⁴I images that are unaffected by positron range. Photons detected in the 511 keV window were used to reconstruct ¹⁸F images. Images were reconstructed iteratively using an energy dependent matrix for each isotope. Correction of ¹⁸F images for contamination with ¹²⁴I annihilation photons was performed by Monte Carlo based range modelling and scaling of the ¹²⁴I prompt gamma image before subtracting it from the ¹⁸F image. Additionally, prompt gamma imaging was tested for ⁸⁹Zr that emits very high-energy prompts (909 keV). In Derenzo resolution phantoms 0.75 mm rods were clearly discernable for ¹²⁴I, ⁸⁹Zr and for simultaneously acquired ¹²⁴I and ¹⁸F imaging. Image quantification in phantoms with reservoirs filled with both ¹²⁴I and ¹⁸F showed excellent separation of isotopes and high quantitative accuracy. Mouse imaging showed uptake of ¹²⁴I in tiny thyroid parts and simultaneously injected ¹⁸F-NaF in bone structures. The ability to obtain PET images at sub-mm resolution both for isotopes with several mm positron range and for multi-isotope PET adds to many other unique capabilities of VECTor's clustered pinhole imaging, including simultaneous sub-mm PET-SPECT and theranostic high energy SPECT.