PROJECT SUMMARY

Myelodysplastic syndrome (MDS) is a lethal disease that is difficult to treat. The most established risk factors for MDS are older age and prior exposure to cancer therapy given for a previous solid tumor. When MDS occurs after exposure to chemotherapy it is referred to as therapy-related MN (tMN). This accounts for 15-20% of all MDS cases. In recent years we have learned that the origins of MDS stem from clonal hematopoiesis (CH). Our blood cells are made by hematopoietic stem cells (HSCs) in the bone marrow. CH is characterized by the over- representation of blood cells derived from a single HSC (clone). CH is common with normal human aging. However, in a small subset of individuals, CH will lead to MDS. The environmental factors that lead to CH progression to MDS are incompletely characterized. However, one clear stressor is exposure to cancer therapy. We have previously shown that CH mutations in the DNA damage response (DDR) pathway including TP53 , PPM1D and CHEK2 prevent cells that are damaged from chemotherapy from dying. Thus, in the presence of chemotherapy, DDR CH clones have a better survival compared to normal HSCs. This leads to CH mutations in the DDR pathway is particularly strong in the setting of environmental stressors such as cytotoxic therapy.

While we know the types of CH that put individuals at high risk of developing MDS, there are no established therapeutic strategies to target CH clones to prevent transformation. Given how difficult it is to treat MDS, ultimately we believe that the best therapeutic strategy might be to MDS through targeting CH. To this end, we have recently identified that inhibition of CDK4/6 shows promise in preventing CH expansion. While many pathways drive the competitive advantage of specific CH mutations, a common pathway is through disrupting normal regulation of cell-cycle division in HSCs. This is known to be tightly regulated by the proteins CDK4 and CDK6. CDK4/6 inhibitors are drugs designed to interrupt the growth of cancer cells. We therefore hypothesized we might be able to use drugs that inhibit CDK4/6 to prevent the chemotherapy-induced expansion of CH mutations. We were able to test this in human blood samples taken from clinical trials of the CDK4/6 inhibitor trilaciclib.

Trilaciclib is a CDK4/6 inhibitor that halts HSCs in the G1 phase of the cell cycle thus preventing cells from dividing. When given in combination with chemotherapy, trilaciclib appears to protect HSCs from chemotherapy- induced damage. Trilaciclib is currently approved by the Federal Drug Administration as a therapy that can be given in combination with chemotherapy in small cell lung cancer to prevent the development of chemotherapy- induced low blood cell counts (myelosuppression). In preliminary data, we sequenced pre- and post-treatment blood samples obtained from a randomized, placebo-controlled trial of trilaciclib in patients with lung cancer. Trilaciclib showed a protective effect against therapy-related expansion of DDR clones compared to placebo. Interestingly, we also observed inhibition of the growth of CH clones bearing epigenetic mutations (DNMT3A, TET2, and ASXL1).

These preliminary data suggest that CDK4/6 inhibitors may mitigate CH expansion. Here we seek to: 1) further characterize the potential of CDK4/6 inhibitors to prevent CH expansion in additional clinical settings and to establish whether the impact of CDK4/6 inhibition extends beyond completion of therapy; and 2) evaluate the potential of CDK4/6 inhibitors to inhibit CH independent of chemotherapy. Because trilaciclib is currently used clinically only in conjunction with chemotherapy, we will model its use independent of chemotherapy in mouse models of CH. Furthermore, we will investigate whether other commonly used CDK4/6 inhibitors including Palbociclib and Ribociclib also show an inhibitory effect on CH. We hypothesize that CDK4/6 inhibition extends beyond the protection of HSPCs during chemotherapy to actually prevent CH expansion and transformation to MDS. If successful, this innovative work will establish trilaciclib as a novel therapy to prevent the progression of CH to MDS.