Investigation of Radiotracer Metabolic Stability In Vitro with CYP-Overexpressing Hepatoma Cell Lines

Preclinical evaluation of the potential PARP-imaging probe [carbonyl-11C]DPQ

BACKGROUND

Poly (ADP-ribose) polymerase (PARP) enzymes are crucial for the repair of DNA single-strand breaks and have become key therapeutic targets in homologous recombination-deficient cancers, including prostate cancer. To enable non-invasive monitoring of PARP-1 expression, several PARP-1-targeting positron emission tomography (PET) tracers have been developed. Here, we aimed to preclinically investigate [carbonyl-11C]DPQ as an alternative PARP-1 PET tracer as it features a strongly distinct chemotype compared to the frontrunners [18F]FluorThanatrace and [18F]PARPi.

RESULTS

[carbonyl-11C]DPQ was synthesised in a GE TracerLab FXC2 module, yielding sufficient activity (940 ± 410 MBq), molar activity (53 ± 16 GBq/µmol) and radiochemical purity (> 97%) for subsequent preclinical evaluation. [carbonyl-11C]DPQ showed high stability in formulation, in human plasma, and when incubated with human liver microsomes. In vitro, similar specific uptake was observed in both PC3 prostate cancer cells and CHO-K1 Chinese hamster ovary cells. However, in vivo studies using fertilised chicken eggs (in ovo model) revealed poor and non-displaceable tumour accumulation in PC3-derived xenografts, despite confirmed vascularisation and PARP-1 expression. Rapid uptake was observed in the liver (10 min), with less than 30% of the intact compound remaining in the liver 70 min post-injection.

CONCLUSIONS

Although [carbonyl-11C]DPQ demonstrated metabolic stability and specific binding in vitro, suboptimal tumour-targeting properties and pronounced liver metabolism were observed in ovo. Therefore, further animal experiments with mammalian models were not indicated.

Impact of Viscosity on Human Hepatoma Spheroids in Soft Core-Shell Microcapsules

The extracellular environment regulates the structures and functions of cells, from the molecular to the tissue level. However, the underlying mechanisms influencing the organization and adaptation of cancer in three-dimensional (3D) environments are not yet fully understood. In this study, the influence of the viscosity of the environment is investigated on the mechanical adaptability of human hepatoma cell (HepG2) spheroids in vitro, using 3D microcapsule reactors formed with droplet-based microfluidics. To mimic the environment with different mechanical properties, HepG2 cells are encapsulated in alginate core-shell reservoirs (i.e., microcapsules) with different core viscosities tuned by incorporating carboxymethylcellulose. The significant changes in cell and spheroid distribution, proliferation, and cytoskeleton are observed and quantified. Importantly, changes in the expression and distribution of F-actin and keratin 8 indicate the relation between spheroid stiffness and viscosity of the surrounding medium. The increase of F-actin levels in the viscous medium can indicate an enhanced ability of tumor cells to traverse dense tissue. These results demonstrate the ability of cancer cells to dynamically adapt to the changes in extracellular viscosity, which is an important physical cue regulating tumor development, and thus of relevance in cancer biology.

Preclinical evaluation of an 18F-labeled Nε-acryloyllysine piperazide for covalent targeting of transglutaminase 2

BACKGROUND

Transglutaminase 2 (TGase 2) is a multifunctional protein and has a prominent role in various (patho)physiological processes. In particular, its transamidase activity, which is rather latent under physiological conditions, gains importance in malignant cells. Thus, there is a great need of theranostic probes for targeting tumor-associated TGase 2, and targeted covalent inhibitors appear to be particularly attractive as vector molecules. Such an inhibitor, equipped with a radionuclide suitable for noninvasive imaging, would be supportive for answering the general question on the possibility for functional characterization of tumor-associated TGase 2. For this purpose, the recently developed 18F-labeled Nε-acryloyllysine piperazide [18F]7b, which is a potent and selective irreversible inhibitor of TGase 2, was subject to a detailed radiopharmacological characterization herein.

RESULTS

An alternative radiosynthesis of [18F]7b is presented, which demands less than 300 µg of the respective trimethylammonio precursor per synthesis and provides [18F]7b in good radiochemical yields (17 ± 7%) and high (radio)chemical purities (≥ 99%). Ex vivo biodistribution studies in healthy mice at 5 and 60 min p.i. revealed no permanent enrichment of 18F-activity in tissues with the exception of the bone tissue. In vivo pretreatment with ketoconazole and in vitro murine liver microsome studies complemented by mass spectrometric analysis demonstrated that bone uptake originates from metabolically released [18F]fluoride. Further metabolic transformations of [18F]7b include mono-hydroxylation and glucuronidation. Based on blood sampling data and liver microsome experiments, pharmacokinetic parameters such as plasma and intrinsic clearance were derived, which substantiated the apparently rapid distribution of [18F]7b in and elimination from the organisms. A TGase 2-mediated uptake of [18F]7b in different tumor cell lines could not be proven. Moreover, evaluation of [18F]7b in melanoma tumor xenograft models based on A375-hS100A4 (TGase 2 +) and MeWo (TGase 2 -) cells by ex vivo biodistribution and PET imaging studies were not indicative for a specific targeting.

CONCLUSION

[18F]7b is a valuable radiometric tool to study TGase 2 in vitro under various conditions. However, its suitability for targeting tumor-associated TGase 2 is strongly limited due its unfavorable pharmacokinetic properties as demonstrated in rodents. Consequently, from a radiochemical perspective [18F]7b requires appropriate structural modifications to overcome these limitations.