Atacicept (TACI-Ig) Inhibits Growth of TACIhigh Primary Myeloma Cells in SCID-Hu Mice.
Authors: Yaccoby Shmuel , Dillon Stacey R. , Ling Wen , Perkins Paul , Tisdale John R. , Pennisi Angela , Saha Rinku , Li Xin , Barlogie Bart , Tricot Guido , Zhan Fenghuang , Shaughnessy John 
Published: April 2020

Abstract
Two TNF family members known to play key roles in normal B cell biology, BLyS/BAFF (B-lymphocyte stimulator/B cell activating factor) and APRIL (A PRoliferation-Inducing Ligand), also promote the survival of various malignant B cell types, including multiple myeloma (MM). BLyS binds to 3 TNF-R-related receptors, BCMA (B-cell maturation antigen, TACI (transmembrane activator and CAML interactor), and BAFF-R (BAFF-receptor), whereas APRIL binds to TACI and BCMA and to heparan sulfate proteoglycans (HSPG) such as CD138. All MM cells express HSPG and one or more of these 3 receptors, and incubation of MM cells with BLyS and/or APRIL leads to enhanced survival of these malignant cells in vitro. Stromal cells and osteoclasts residing in the bone marrow (BM) produce BLyS and APRIL and thus provide a paracrine source of these survival factors MM cells. Inhibition of BLyS and APRIL in vitro using a soluble receptor, TACI-Ig, causes cultured MM cells to die rapidly. It has been shown (Moreaux et al, Blood 106:1021) that differences in TACI gene expression can distinguish tumors with a BM microenvironment dependence signature (TACIhigh) from those with a plasmablastic signature (TACIlow), suggesting that TACIhigh MM cells may be more sensitive to growth factor withdrawal. We tested the ability of atacicept (TACI-Ig) to inhibit MM growth in the SCID-hu model of MM (Yaccoby et al, Blood 92:2908). Myelomatous SCID-hu mice were constructed by implanting a human fetal bone into which primary MM cells were directly injected. Changes in levels of tumor burden were monitored by weekly measurements of serum human monotypic immunoglobulins (hIg), and confirmed by histology. We compared the ability of atacicept and BAFFR-Ig (which binds BLyS, but not APRIL) to inhibit MM cell growth in this model. Upon establishment of myeloma growth, mice were injected intraperitoneally with atacicept (5 or 10 mg/kg, 3 times a wk; n=7), BAFFR-Ig (10 mg/kg, 3 times a wk; n=5) or with vehicle (PBS; n=7) for 6 wks. Compared to controls, in which tumor burden (hIg) increased by 1007±260% (avg±SEM) from pre-treatment levels, atacicept treatment markedly reduced tumor burden in 5 experiments in which TACIhigh MM cells were used, and delayed growth or had no effect in 2 experiments using TACIlow MM cells (avg increase in tumor burden of 302±184% for all 7 experiments; p&lt;0.01 vs. control). Inhibition of tumor infiltration was confirmed by histology. Preliminary data also indicate that atacicept can inhibit growth of bortezomib-resistant MM cells in vivo. The level of TACI gene expression by MM cells generally correlated with response to atacicept. In vitro MM cell/osteoclast co-culture experiments confirmed these results, as atacicept inhibited growth of TACIhigh MM cells, but had a minimal effect on growth of TACIlow MM cells. In contrast, BAFFR-Ig treatment in the SCID-hu model resulted in tumor reduction in 1 experiment, delayed growth in 2 experiments and no effect in 2 experiments (avg increase in hIg of 762±298 and 208±46% in control and BAFFR-Ig groups, respectively; p&lt;0.078), suggesting that APRIL plays a non-redundant role in survival of tumor cells from certain MM patients. Our results suggest that atacicept might represent an important new treatment for MM, and that TACI gene expression may prove to be a useful predictive marker for this therapeutic intervention.