DNA Damage Response Pathways and Functional Adaptations Associated with Melphalan Resistance in Multiple Myeloma Cell Line Models

Background/Objectives: Multiple myeloma (MM) is a genetically heterogeneous plasma cell malignancy characterized by progressive genomic instability and therapeutic resistance. Melphalan remains a cornerstone of MM treatment, particularly as high-dose conditioning before autologous stem cell transplantation; however, acquired resistance significantly limits its efficacy. This study aimed to identify molecular determinants of melphalan responsiveness by evaluating redox status and DNA damage response (DDR)-associated parameters in human MM cell line models. Methods: Melphalan-sensitive (RPMI-8226, U266) and melphalan-resistant (LR5, LR6) human MM cell lines were comparatively analyzed for baseline oxidative stress, accumulation of abasic sites, DNA damage, chromatin organization, and apoptotic sensitivity following melphalan exposure. UVC-induced DNA damage resolution and the kinetics of melphalan-induced γH2AX foci removal were also assessed. Results: Melphalan-sensitive cell lines exhibited significantly higher baseline DNA damage, γH2AX foci, abasic sites, and oxidative stress compared with resistant counterparts. In contrast, melphalan-resistant cells demonstrated more efficient resolution of UVC-induced DNA damage, faster removal of melphalan-induced γH2AX foci, more accessible chromatin, and markedly reduced apoptotic sensitivity following melphalan treatment. Multivariate statistical analyses identified distinct DDR-related molecular signatures associated with melphalan-sensitive and melphalan-resistant phenotypes. Conclusions: Melphalan resistance in MM is associated with coordinated alterations in redox status, DNA damage dynamics, chromatin organization, and apoptotic sensitivity. These findings suggest that DDR-associated parameters may contribute to the molecular characterization of melphalan responsiveness and provide a framework for understanding resistance-associated phenotypes. Further functional studies are required to establish whether these alterations directly mediate melphalan resistance and represent actionable therapeutic targets.