YM155 Induces Apoptosis through Proteasome-Dependent Degradation of MCL-1 in Primary Effusion Lymphoma

Primary effusion lymphoma (PEL) is a subtype of non-Hodgkin lymphoma caused by human herpes virus 8 (HHV-8), which mainly occurs in patients with acquired immunodeficiency. It is highly refractory to conventional chemotherapies, and has a very poor prognosis. We recently developed patient-derived xenograft (PDX) screening, a novel high-throughput drug screening system using PDX cells that were established by transplantations of primary tumor cells into immunodeficient mice and maintained primary cell phenotype. PDX screening is expected to discover anti-tumor drugs that have been overlooked by conventional screenings using cell lines. Here, we performed a PDX screening to develop a new therapeutic agent for PEL.

We previously established a PDX and a cell line designated as GTO from the same primary cells of PEL. We performed screenings of a library containing 3518 known pharmacologically active substance and off-patent drugs using the PDX cells (PDX screening) and GTO (Cell-line screening). We compared the results of both screenings and found that PDX cells and cell lines had quite different drug sensitivity profiles. The correlation coefficient between them was 0.67. Twenty-six drugs (0.7%) were at least 2 times more effective for PDX cells than for GTO and designated as PDX-preferred drugs (Figure A). The opposites were named as cell line-preferred drugs and existed 80 (2.2%). We found that PDX-preferred drugs significantly higher activity to induce reactive oxygen species (ROS) production (P<0.001), indicating the sensitivity of PDX cells to oxidative stress.

We examined the reproducibility of anti-tumor effect of top 10 compounds of PDX screening in different system including in vivo mouse model and finally selected YM155, a possible survivin inhibitor, as the best candidate for an anti-tumor drug for PEL. It showed strong and dose-dependent anti-tumor effect on both PDX cells and cell lines of PEL. Its GI₅₀ was 7.8 nM in the PDX cells, and 1.2 - 7.9 nM in three kinds of PEL cell lines.

YM155 treatment increased the cleavage of caspase-3, caspase-7, and PARP and caused apoptosis of GTO, which was inhibited by a caspase inhibitor, Z-VAD-FMK. Although YM155 was discovered as a survivin inhibitor, we observed that YM155 reduced myeloid cell leukemia-1 (MCL-1) protein prior to survivin reduction by time course experiments. Observed MCL-1 reduction by YM155 was attenuated by a proteasome inhibitor, MG132, suggesting that MCL-1 reduction was due to proteasome-dependent degradation. Furthermore, we confirmed the importance of MCL-1 for survival by its knockdown by siRNA in PEL cell line.

Finally, we assessed the in vivo effect of YM155. NOD/SCID/IL-2Rgnull mice were injected intraperitoneally with PEL-PDX cells and were treated with vehicle or YM155 (5mg/kg) from day 1 to 21. YM155 was administered by continuous subcutaneous injection using osmotic pumps. Treatment with YM155 significantly inhibited progression of ascites compared with control mice (Figure B). These results suggested that YM155 was a promising anti-cancer agent for PEL.

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