FTY720 Increases CD74 Expression and Sensitizes Mantle Cell Lymphoma Cells to Milatuzumab-Mediated Cell Death

Abstract 600

Mantle cell lymphoma (MCL) is an aggressive B-cell malignancy with a short median survival despite multimodal therapy. FTY720, an immunosuppressive drug approved for the treatment of multiple sclerosis, promotes MCL cell death via down-modulation of phospho-Akt and Cyclin D1, and subsequent cell cycle arrest (1). However, the mechanism of FTY720-mediated MCL cell death remains to be fully clarified. Here we show features of autophagy blockage by FTY720 treatment, including accumulation of autolysosomes, increased LC3-II and p62 levels. FTY720 is phosphorylated in vivo by sphingosine kinase 2 and converted to p-FTY720, which binds to sphingosine-1-phosphate (S1P) receptors. A non-phosphorylatable FTY720 derivative (OSU-2S) was recently developed at the Ohio State University (2): OSU-2S treatment induces MCL cell death and shows features of autophagy blockage that led us to conclude that FTY720 phosphorylation and its interaction with SP1 receptors are not required for FTY720-mediated cell death and blockage of autophagy in MCL cells. We also demonstrate that FTY720-induced cell death is mediated by lysosomal membrane permeabilization with subsequent translocation of lysosomal hydrolases to the cytosol. FTY720-mediated disruption of the autophagic-lysosomal pathway led to increased levels of CD74, a potential therapeutic target in MCL that is degraded in the lysosomal compartment. We have recently reported CD74 to be expressed on MCL cells and that milatuzumab (Immunomedics, Morris Plains, NJ), a humanized anti-CD74 monoclonal antibody, has significant anti-MCL activity in vitro and in vivo (3). This finding provided the rationale for examining combination therapy with FTY720 and milatuzumab.

The in vitro survival of 4 MCL cell lines treated with FTY720, immobilized milatuzumab, and the combination was determined at 24 hours by Annexin-V/PI staining and flow cytometry. Incubation of 4 MCL cell lines with FTY720 and milatuzumab (1 μg/ml) resulted in a statistically significant decrease in cell viability compared to either agent alone for each of the four cell lines (P< 0.01), despite using FTY720 at concentrations lower than the LC₅₀ previously published [Jeko-1 FTY720: 10 μM (LC₅₀: 12.5 μM), Z-138 and UPN-1: 6 μM (LC₅₀: 7.5 μM); Mino 3.75 μM (LC₅₀: 7.5μM)] (1). Notably, combination treatment resulted in synergistic killing in cell lines derived from patients with blastoid variant MCL (Jeko-1, Z-138, UPN-1), despite the fact that both FTY720 and milatuzumab as single agents showed only modest activity. Incubation of primary tumor cells from 6 MCL patients (3 blastoid variant and 3 classic MCL) with FTY720 (2.5 μM, LC₅₀: 5 μM) and miltauzumab induced an average 78.5% cell death compared to 47% of FTY720 treated cells and 50% the milatuzumab-treated cells (P=0.0005 and P=0.0014, respectively).

To examine the in vivo activity of FTY720 and milatuzumab, a preclinical model of human MCL using the SCID (CB17 scid/scid) mouse depleted of NK cells was used. In this model, i.v. injection of 40×10⁶ JeKo cells results in disseminated MCL 3 weeks after engraftment. The primary end-point was survival, defined as the time to develop cachexia/wasting syndrome or hind limb paralysis. Mice (n=10/group) were treated starting at day 15 post engraftment. Twenty control mice received either placebo (saline) or trastuzumab (15 mg/kg) treatment. The third group was treated with FTY720 (5 mg/kg) every day for 2 weeks via i.p injection. The fourth group received milatuzumab (15 mg/kg) every three days, via i.p. injection. The fifth group received the combination of FTY720 and milatuzumab. The median survival for the combination-treated group was 36 days (95% CI:31,36), compared to 28 days for the saline-treated mice (95% CI:24,31), 27 days for the trastuzumab-treated mice (95% CI:23,29), 31 days for the FTY720-treated mice (95% CI:28,32), and 33.5 days for the milatuzumab-treated mice (95% CI:23,34). The combination treatment significantly prolonged survival of this group compared to control groups (P<0.0001), FTY720 (P=0.0001) and milatuzumab (P=0.0048).

The most clinically relevant aspect of these findings is that we demonstrate that a potent anti-MCL agent (FTY720) has also the ability to modulate a druggable target (CD74) by preventing its degradation in the autophagic-lysosomal pathway. We believe these findings support clinical evaluation of this combination in patients with MCL.