Inhibition of ATR Overcomes Chemotherapy Resistance in p53 Deficient Myeloma Cells

In MM, as well as in most hematological malignancies, deficiency in p53 pathway (mainly because of TP53 deletion and/or mutation) is associated with resistance to treatments (Tessoulin Blood Reviews 2017; 31:251). Recent clinical studies have shown that deletion or mutation of TP53 are the most adverse prognostic values for patients (Thakurta Blood 2019;133:1217). Although these patients are easily identified, there is no dedicated therapies for them. p53 pathway is central for homeostasis and cell adaptation/response to many stresses, including DNA repair orchestration and survival regulation. In p53 deficient cells, DNA damaging drugs don't induce massive apoptosis and cells escape to death. In normal cells, DNA damaging drugs induce cell cycle arrest and DNA repair, mainly orchestrated by p53 target genes. Cell cycle arrest in S phase, which is critical for allowing homologous DNA repair, is activated by cell cycle check-point inhibitor such as Chk1, an ATR target. In p53 deficient cells, inhibiting check point inhibitor using ATR inhibitor should allow DNA damaged cells to progress into cell cycle despite the lack of repair and in fine induce replicative/mitotic catastrophe. The aim of this study was to assess whether inhibiting ATR in p53 deficient myeloma cells could overcome chemotherapy resistance.

ATR inhibitor, VE-821, was assessed in 13 human myeloma cell lines (HMCLs) alone and in combination with DNA damaging agents, CX5461, a G quadruplex inhibitor, or melphalan, the « myeloma » alkylating drug. The HMCL cohort included 8 HMCLs, 5 TP53^Abn and 5 TP53^wt. Cell viability was assessed using Cell Titer Glo assay or using flow cytometry (loss of AnnexinV or CD138 staining in HMCLs or primary myeloma cells, respectively).

In our cohort of 13 HMCLs and by contrast to previous results, CX5461 was more efficient in TP53^wt than in TP53^abn HMCLs (mean of death at 0.5mM was 43% versus 24%, p=0.04). Melphalan was also more potent in TP53^wt than in TP53^abn HMCLs (LD₅₀ values were 26 mM versus 10 mM, p=0.008). By contrast, ATR inhibitor VE-821 (2.5mM) was efficient in both types of HMCLs (mean of death in TP53^wt was 45% and 28% in TP53^abn HMCLs, p=0.6). Combination of CX5641 (0.5mM) with VE-821 (2.5mM) was more efficient than each drug alone and efficacy was not dependent on TP53 status (mean of death in TP53^wt was 69% versus 56% in TP53^abn HMCLs, p=0.6): interestingly, combo was efficient in all TP53^abn HMCLs, being either additive (n=5) or even synergistic (n=3). By contrast, combo was not efficient in all TP53^wt HMCLs (either additive or antagonist). Combination of melphalan (10 mM) with VE-821 (2.5mM) was also synergistic in TP53^abn HMCLs (mean of cell death was 9% with melphalan and 73% for combo, p<0.05). Preliminary results of combos in 6 consecutive primary samples with MM or plasma cell leukemia (3 TP53^wt and 3 TP53^abn) demonstrated efficacy. Indeed, in the 3 TP53^abn samples, both CX5641/VE-821 and melphalan/VE-821 combos displayed synergism or additivity: median of expected values versus observed values was 61% versus 74% for CX5641/VE-821, and 98% versus 89% for melphalan/VE-821, respectively. In the 3 TP53^wt samples, combos displayed additivity or antagonism: median of expected versus observed values was 15% versus 15% for CX5641/VE-821, and 100% versus 62% for melphalan/VE-821, respectively. In normal peripheral blood cells (n=2), both combos were not cytotoxic (mean values of cell death were 0% with CX5641/VE-821 and 3% with melphalan/VE-821).

To decipher the molecular pathway involved in cell response, we monitored cell cycle using BrdU/IP assay, replicative stress response using Chk1 phosphorylation and DNA double strand breaks using Comets assays in 3 TP53^abn HMCLs. At 24h, CX5641 induced an increase of cells in S (mean of increase 12%) and G2M phases (11%), while VE-821 didn't modify cell cycle. Combination of CX5641 with VE-821 induced a dramatic increase of cells in G2M (20%) (and in subG2 phase), and a decrease of cells in S phase (10%), indicating that cells blocked in S phase by CX5641 were released by VE-821.CX5641 induced Chk1 phosphorylation, which was inhibited by addition of VE-821, confirming the CX5641/ATR/Chk1 signaling. Finally, CX5641 and VE-821 induced comets, confirming irreversible DNA double strand breaks.

All these results show that inhibition of ATR after inducing DNA damage in TP53^abn myeloma cells efficiently induces cell death, while preserving normal cells.