The BH3 Mimetic, Small Molecule, ABT-737, Was Effective As Lympholytic Therapy in the MRL/lpr^−/− Mouse Model of Autoimmune Lymphoproliferative Syndrome (ALPS)

Abstract 695

Autoimmune Lymphoproliferative Syndrome (ALPS) is an inherited disease associated with defects of lymphocyte apoptosis leading to lymphoproliferation and autoimmunity. The majority of ALPS patients have mutations in the FAS gene. MRL/lpr^−/− (lpr) mice are used as a model for ALPS and are homozygous for FAS mutations, presenting with massive lymphadenopathy and splenomegaly associated with hypergammaglobulinemia, glomerulonephritis, and expansion of a rare population of TCR ab⁺CD4⁻CD8⁻ (Double-negative, DN) T cells that are characteristic of ALPS. Currently, there are no proven therapies for the lymphoproliferation underlying ALPS besides immunosuppressive agents like high dose steroids and rapamycin. ABT-737 is a BH3 mimetic agent that binds and inactivates anti-apoptotic proteins of the BCL-2 family such as BCL-X_L and BCL-2. An oral equivalent to this drug, Navitoclax, is currently being tested in Phase 1 studies for the treatment of diverse tumors. ABT-737 is potentially of interest for the treatment of ALPS, given that it directly activates the intrinsic pathway of apoptosis, independent of the FAS pathway. We tested the BH3 mimetic ABT-737, with and without dexamethasone, in the lpr mouse model as a potential therapeutic agent for the lymphoproliferative disease. Lpr mice (N=30) were treated daily via IP injections with dexamethasone (4mg/kg) plus ABT-737 (50mg/kg) (Dex+ABT-737), dexamethasone alone (Dex), ABT-737 alone (ABT-737) or vehicle only (VEH) for 8 weeks. Vehicle treated mice developed very severe lymphoproliferation, with mean lymph node (LN) and spleen (SPL) weights of 750 mg and 465 mg, respectively. Cellularity reached LN= 85×10⁶ cells; SPL=290×10⁶ cells. Dex+ABT-737 treated mice had the greatest reduction in lymph node and spleen weights (final weight of 101 mg, p<0.0001; and 213 mg, p<0.005, respectively) and cellularity (LN=4×10⁶, SPL=63×10⁶, p<0.001), followed by ABT-737 alone (LN 453 mg, p=0.02; SPL 118 mg, p=ns; LN=20×10⁶, p=0.001; SPL= 118×10⁶, p=0.01), then Dex (LN 542 mg, p=ns; SPL 450mg, p=ns; LN 45×10⁶, p=ns; SPL 241×10⁶, p=ns). Although lymphoid cellularity was significantly lower with treatment, there was no bone marrow toxicity based on cell numbers. Additionally, there were significant reductions in DNT cells in the spleen, lymph nodes and blood with Dex+ABT-737 (p<0.005) and ABT-737 only (p<0.01) compared to vehicle. The DNT cells also appeared to be more sensitive to treatment as they were reduced to a greater extent than CD4, CD8 or B cells. ABT-737 was also able to kill patient derived EBV-transformed B cell lines in vitro, through induction of uncoupling of BAX and BAK from BCL-X_L and consequent activation of the intrinsic pathway of apoptosis with release of cytochrome c and AIF. These studies provide the first evidence for the utility of this class of drugs in the treatment of a non-malignant lymphoproliferative disorder.