Carbon-11 labeled cathepsin K inhibitors: syntheses and preliminary in vivo evaluation

Development and Application of Reversible and Irreversible Covalent Probes for Human and Mouse Cathepsin-K Activity Detection, Revealing Nuclear Activity

Cathepsin-K (CTSK) is an osteoclast-secreted cysteine protease that efficiently cleaves extracellular matrices and promotes bone homeostasis and remodeling, making it an excellent therapeutic target. Detection of CTSK activity in complex biological samples using tailored tools such as activity-based probes (ABPs) will aid tremendously in drug development. Here, potent and selective CTSK probes are designed and created, comparing irreversible and reversible covalent ABPs with improved recognition components and electrophiles. The newly developed CTSK ABPs precisely detect active CTSK in mouse and human cells and tissues, from diseased and healthy states such as inflamed tooth implants, osteoclasts, and lung samples, indicating changes in CTSK's activity in the pathological samples. These probes are used to study how acidic pH stimulates mature CTSK activation, specifically, its transition from pro-form to mature form. Furthermore, this study reveals for the first time, why intact cells and cell lysate exhibit diverse CTSK activity while having equal levels of mature CTSK enzyme. Interestingly, these tools enabled the discovery of active CTSK in human osteoclast nuclei and in the nucleoli. Altogether, these novel probes are excellent research tools and can be applied in vivo to examine CTSK activity and inhibition in diverse diseases without immunogenicity hazards.

The Tumor Proteolytic Landscape: A Challenging Frontier in Cancer Diagnosis and Therapy

In recent decades, dysregulation of proteases and atypical proteolysis have become increasingly recognized as important hallmarks of cancer, driving community-wide efforts to explore the proteolytic landscape of oncologic disease. With more than 100 proteases currently associated with different aspects of cancer development and progression, there is a clear impetus to harness their potential in the context of oncology. Advances in the protease field have yielded technologies enabling sensitive protease detection in various settings, paving the way towards diagnostic profiling of disease-related protease activity patterns. Methods including activity-based probes and substrates, antibodies, and various nanosystems that generate reporter signals, i.e., for PET or MRI, after interaction with the target protease have shown potential for clinical translation. Nevertheless, these technologies are costly, not easily multiplexed, and require advanced imaging technologies. While the current clinical applications of protease-responsive technologies in oncologic settings are still limited, emerging technologies and protease sensors are poised to enable comprehensive exploration of the tumor proteolytic landscape as a diagnostic and therapeutic frontier. This review aims to give an overview of the most relevant classes of proteases as indicators for tumor diagnosis, current approaches to detect and monitor their activity in vivo, and associated therapeutic applications.

Preclinical evaluation of [11 C]L-235 as a radioligand for Positron Emission Tomography cathepsin K imaging in bone

The cathepsin K (CatK) enzyme is abundantly expressed in osteoclasts, and CatK inhibitors have been developed for the treatment of osteoporosis. In our effort to support discovery and clinical evaluations of a CatK inhibitor, we sought to discover a radioligand to determine target engagement of the enzyme by therapeutic candidates using positron emission tomography (PET). L-235, a potent and selective CatK inhibitor, was labeled with carbon-11. PET imaging studies recording baseline distribution of [¹¹ C]L-235, and chase and blocking studies using the selective CatK inhibitor MK-0674 were performed in juvenile and adult nonhuman primates (NHP) and ovariectomized rabbits. Retention of the PET tracer in regions expected to be osteoclast-rich compared with osteoclast-poor regions was examined. Increased retention of the radioligand was observed in osteoclast-rich regions of juvenile rabbits and NHP but not in the adult monkey or adult ovariectomized rabbit. Target engagement of CatK was observed in blocking studies with MK-0674, and the radioligand retention was shown to be sensitive to the level of MK-0674 exposure. [¹¹ C]L-235 can assess target engagement of CatK in bone only in juvenile animals. [¹¹ C]L-235 may be a useful tool for guiding the discovery of CatK inhibitors.

Synthesis and preliminary radiopharmacological characterisation of an 11 C-labelled azadipeptide nitrile as potential PET tracer for imaging of cysteine cathepsins

An O-methyltyrosine-containing azadipeptide nitrile was synthesised and investigated for its inhibitory activity towards cathepsins L, S, K, and B. Labelling with carbon-11 was accomplished by reaction of the corresponding phenolic precursor with [¹¹ C]methyl iodide starting from cyclotron-produced [¹¹ C]methane. Radiopharmacological evaluation of the resulting radiotracer in a mouse xenograft model derived from a mammary tumour cell line by small animal PET imaging indicates tumour targeting with complex pharmacokinetics. Radiotracer uptake in the tumour region was considerably lower under treatment with the nonradioactive reference compound and the epoxide-based irreversible cysteine cathepsin inhibitor E64. The in vivo behaviour observed for this radiotracer largely confirms that of the corresponding ¹⁸ F-fluoroethylated analogue and suggests the limited suitability of azadipeptide nitriles for the imaging of tumour-associated cysteine cathepsins despite target-mediated uptake is evidenced.

Cysteine cathepsins: their role in tumor progression and recent trends in the development of imaging probes

Papain-like cysteine proteases bear an enormous potential as drug discovery targets for both infectious and systemic human diseases. The considerable progress in this field over the last two decades has also raised interest in the visualization of these enzymes in their native context, especially with regard to tumor imaging. After a short introduction to structure and general functions of human cysteine cathepsins, we highlight their importance for drug discovery and development and provide a critical update on the current state of knowledge toward their involvement in tumor progression, with a special emphasis on their role in therapy response. In accordance with a radiopharmaceutical point of view, the main focus of this review article will be the discussion of recently developed fluorescence and radiotracer-based imaging agents together with related molecular probes.

Well-designed bone-seeking radiolabeled compounds for diagnosis and therapy of bone metastases

Bone-seeking radiopharmaceuticals are frequently used as diagnostic agents in nuclear medicine, because they can detect bone disorders before anatomical changes occur. Furthermore, their effectiveness in the palliation of metastatic bone cancer pain has been demonstrated in the clinical setting. With the aim of developing superior bone-seeking radiopharmaceuticals, many compounds have been designed, prepared, and evaluated. Here, several well-designed bone-seeking compounds used for diagnostic and therapeutic use, having the concept of radiometal complexes conjugated to carrier molecules to bone, are reviewed.