Models of myeloma bone disease: In vivo and in vitro approaches.

This review focuses on experimental models developed to study myeloma bone disease (MBD), a major cause of morbidity in multiple myeloma (MM). Under physiological conditions, bone remodeling is regulated by osteoclasts (OCs) and osteoblasts (OBs); in MM, this balance is disrupted, resulting in enhanced bone resorption and suppressed bone formation. Myeloma cells alter the bone marrow (BM) microenvironment by increasing the RANKL/OPG ratio and secreting Wnt pathway inhibitors such as DKK-1 and sclerostin, thereby promoting osteoclastogenesis and inhibiting osteoblast differentiation. To dissect these mechanisms and evaluate therapeutic strategies, diverse preclinical systems have been developed. Syngeneic murine models, notably the 5T series, remain the most established for reproducing both osteolysis and impaired bone formation, though interspecies differences limit translational relevance. Humanized mouse systems and three-dimensional (3D) in vitro models increasingly address these constraints by incorporating human stromal and hematopoietic elements. Emerging induced pluripotent stem cell-derived bone marrow organoids (iBMOs) offer a fully human platform capable of modeling both osteoclast and osteoblast dynamics. While current iBMOs lack mineralized bone and mature vascular or immune components, advances in differentiation control and matrix engineering are expected to bridge these gaps, providing physiologically relevant and ethically sustainable models for studying MBD and testing therapeutic interventions.