Exploiting Apoptotic Vulnerabilities in AL Amyloidosis

Abstract

AL Amyloidosis (AL) is a rare disorder characterized by the excessive production and secretion of misfolding light chains by a clonal population of abnormal plasma cells. This results in the formation of amyloid fibrils in extracellular spaces within healthy tissues, which compromises organ function and is eventually fatal. Despite its low incidence, AL significantly impairs patients' quality of life and carries a poor prognosis; for individuals diagnosed at stage IV, the median survival is approximately five months. Currently, no standard-of-care therapies have been developed specifically for AL. Instead, treatments are adapted from those for multiple myeloma (MM), a more prevalent plasma cell malignancy that shares many similarities in pathophysiology with AL. Standard AL treatments typically include chemotherapeutic agents that modulate the activity of BCL-2 family proteins to induce apoptosis in the abnormal clonal plasma cells, followed by autologous stem cell transplantation. However, after successful initial responses, many patients experience relapses, and late-stage patients with multi-systemic organ failure exhibit poor tolerance to these regimens and are unable to receive transplantation. This highlights the critical need for more effective and tolerable therapeutic strategies for AL, especially those that can potentially leverage the unique biology and vulnerabilities of AL clonal plasma cells. This dissertation investigates novel vulnerabilities in AL as potential apoptosis-promoting therapeutic targets. Initially, we explored the use of BH3 mimetics—synthetic small molecules designed to mimic the pro-apoptotic activity of BH3-only proteins in the BCL2 family—in combination with selective standard-of-care agents. Our findings demonstrated that the BCL-XL inhibitor A1331852 and the BCL-XL degrader DT2216 sensitize the AL cell line to dexamethasone-induced apoptosis, highlighting BCL-XL as a viable therapeutic target in AL. Subsequently, we examined the potential of targeting light chain secretion as a novel therapeutic approach. We observed that brefeldin A, a commercially available general secretion inhibitor, along with GB-001 and GB-002, two novel Sec61 translocon inhibitors, effectively induce apoptosis in AL cells while sparing healthy peripheral blood mononuclear cells (PBMCs). Mechanistic studies revealed that brefeldin A induces apoptosis through light chain-mediated endoplasmic reticulum (ER) stress and MCL-1 depletion, whereas GB-001 and GB-002 trigger apoptosis by inducing proteasomal stress and upregulation of Noxa. These agents produce apoptotic signaling and increase apoptotic priming, sensitizing AL cells to other standard-of-care therapies. Finally, we evaluated the in vivo efficacy of brefeldin A in AL xenograft mouse models. Low-dose brefeldin A treatment significantly inhibited xenograft growth, while a higher dose treatment resulted in xenograft remission comparable to the standard therapy dexamethasone and was well tolerated. Together, our results uncover previously unexplored apoptotic vulnerabilities, including BCL-XL dependency and light chain secretion, in AL and provide a framework for developing targeted therapies with the potential for clinical application.