Valproic Acid (VPA), a Histone Deacetylase (HDAC) Inhibitor, Diminishes Lymphoproliferation in the Fas Deficient MRL/lpr−/− Murine Model of Autoimmune Lymphoproliferative Syndrome (ALPS).

ALPS is an inherited disorder of apoptosis leading to lymphoproliferation and autoimmunity. ALPS Type Ia, Ib and II are associated with germline mutations in Fas, FasL and Casapase 8 or 10, respectively; patients in whom no mutations have been identified are classified as Type III. The vast majority of patients are ALPS Type Ia (greater than 70%). They often present with childhood onset autoimmune cytopenias associated with lymphadenopathy, splenomegaly, increased double negative T cells (DNT; TCRα/β+CD3+CD4−CD8−), defective apoptosis by in vitro assay, and have an increased risk of lymphoma. Similarly, MRL/lpr−/− mice homozygous for Fas mutations develop an ALPS-like disease with massive lymphadenopathy, splenomegaly, hypergammaglobulinemia, autoimmune glomerulonephritis, and expansion of DNT cells secondary to defective lymphocyte apoptosis leading to lymphomagenesis. Currently, there are no proven therapies for the lymphoproliferation underlying ALPS itself, although complications like autoimmune cytopenias and post-splenectomy sepsis are manageable. Hence, studies were conducted to determine the efficacy of valproic acid (VPA) to control the lymphoproliferation associated with ALPS. VPA is a histone deacetylase (HDAC) inhibitor in clinical use for the last four decades as an anticonvulsant in children and adults, and recently being explored as an anti-neoplastic agent.

PBMCs from normal controls and ALPS Type Ia patients were cultured in vitro with 0–4 mM VPA in the presence or absence of 50 uM of the pan-caspase inhibitor Z-VAD-FMK. A dose response was observed with a high degree of cell death noted at 4 mM after 48 hours, with an LD50 of 2 mM. VPA appeared to induce cell death by both caspase-dependent and -independent mechanisms based on partial inhibition of VPA-induced cell death by Z-VAD-FMK. Further preclinical studies were conducted in the MRL/lpr−/− murine model of ALPS. Twenty, 8-week old female MRL/lpr−/− mice were treated intraperitoneally (i.p.) with 500 mg/kg of VPA in sterile PBS or PBS alone five days per week for 8 weeks. Significant reduction of the spleen weight (p=0.034) and cellularity (p=0.0001) was observed in VPA treated (n=10) mice compared to controls (n=10). Reductions in size and cellularity were also observed in the lymph nodes (p=0.09 and 0.0002, respectively). A concomitant decrease (p<0.05) in DNT cells was observed in the spleen (11.2±0.6 vs 8.1±0.4) and blood (9.3±0.96 vs 5.5±0.6). Serum drug levels peaked (462±10 ug/mL) by 2 hours post-i.p. injection of VPA, where-as a 2.5 fold increase in histone acetylation was observed in the spleen at 4 hours, following a single injection. Analysis of the effects of VPA on autoimmune renal disease in these animals is underway. Based on our in vitro and in vivo data, VPA is effective at reducing lymphoproliferative activity in Fas deficient MRL/lpr−/− mice. It is being further explored in early phase clinical trials as a lympholytic agent to shrink lymph nodes and abrogate hypersplenism in ALPS patients.