Cathepsin K (CatK) is a part of the family of cysteine proteases involved in many important processes, including the degradation activity of collagen 1 and elastin in bone resorption. Changes in levels of CatK are associated with various pathological conditions, primarily related to bone and cartilage degradation, such as pycnodysostosis (associated with CatK deficiency), osteoporosis, and osteoarthritis (associated with CatK overexpression). Recently, the increased secretion of CatK is being highly correlated to vascular inflammation, hypersensitivity pneumonitis, Wegener granulomatosis, berylliosis, tuberculosis, as well as with tumor progression. Due to the wide spectrum of diseases in which CatK is involved, the design and validation of active site-specific inhibitors has been a subject of keen interest in pharmaceutical companies in recent decades. In this review, we summarized the molecular background of CatK and its involvement in various diseases, as well as its clinical significance for diagnosis and therapy.

Cysteine cathepsins F and W are members of the papain-like cysteine protease family, which have distinct structural features and functional roles in various physiological and pathological processes. This review provides a comprehensive overview of the current understanding of the structure, biological functions, and pathological implications of cathepsins F and W. Beginning with an introduction to these proteases, we delve into their structural characteristics and elucidate their unique features that dictate their enzymatic activities and substrate specificity. We also explore the intricate involvement of cathepsins F and W in malignancies, highlighting their role as potential biomarkers and therapeutic targets in cancer progression. Furthermore, we discuss the emerging roles of these enzymes in immune response modulation and neurological disorders, shedding light on their implications in autoimmune and neurodegenerative diseases. Finally, we review the landscape of inhibitors targeting these proteases, highlighting their therapeutic potential and challenges in clinical translation. This review brings together the diverse facets of cysteine cathepsins F and W, providing insights into their roles in health and disease and guiding future investigations for therapeutic advances.

Cathepsin Z/X (Cat Z/X) is a distinct member of the cysteine cathepsin family, known for its distinctive structural and functional properties, such as strict carboxypeptidase activity and integrin-binding motifs. These features set Cat Z/X apart from other cysteine proteases and underlie its involvement in diverse physiological and pathological processes, including immune regulation, cell signaling, and extracellular matrix remodeling. Dysregulated Cat Z/X expression and activity have been associated with various diseases, including cancer, neurodegenerative disorders, and metabolic conditions. This review summarizes current knowledge on Cat Z/X, focusing on its structural characteristics, biological functions, and roles in disease pathogenesis, particularly in malignancies, neuroinflammation, and metabolic disorders. It also explores natural and synthetic Cat Z/X inhibitors and their potential for therapeutic development. Despite growing research interest, the precise molecular mechanisms and context-specific functions of Cat Z/X are not yet fully understood. Further research is required to elucidate its regulatory networks, refine detection methods, and develop selective modulators targeting its proteolytic and non-proteolytic activities. A deeper understanding of Cat Z/X biology could pave the way for its application as a diagnostic biomarker and therapeutic target in numerous diseases.