Background Acute myeloid leukemia (AML) is an aggressive cancer of the blood, where malignant myeloid blasts accumulate in the bone marrow. One of the challenges of effective AML treatment is resistance to cytarabine (or ara-C), a standard AML chemotherapeutic drug used in front-line treatment today. In 2017, Schneider et al . reported the dNTPase sterile alpha motif and HD-domain-containing protein 1 (SAMHD1) to be a targetable biomarker for ara-C treatment response.¹ The intracellular triphosphorylated active form of ara-C, ara-CTP, was recognized as a substrate by SAMHD1 and is hydrolyzed back to ara-C. This led to a decrease in the amount of ara-CTP within the cells and consequently reduced cytotoxicity.¹ SAMHD1 can be targeted by the lentiviral accessory protein Vpx for proteasomal degradation by interacting with the proteasomal degradation complex and SAMHD1. This study aims to use Vpx to target SAMHD1 in AML cells to improve ara-C sensitivity.

Materials and Methods In order to manipulate SAMDH1 levels using Vpx, different Vpx delivery systems were developed. These are virus-like particles (VLPs) packaged with different homologs of Vpx from Simian Immunodeficiency Viruses (SIV) and HIV-2, and cell-penetrating peptides (CPPs) bound to either a 67 amino acid truncated SIVmac Vpx (67aaVpx) or to the WT full-length form. Two different CPPs were used in the synthesis: TAT and CPP44. The latter was chosen, as significantly better uptake of the CPP was observed in AML cell lines and primary blasts compared to healthy PBMCs.².

Results Upon treating AML cell lines with the VLPs, we observed different SAMHD1-degradation capacities of the different Vpx homologs. SIVmac239 Vpx and HIV-2 7312a Vpx were most efficiently loaded into the VLPs, showed the highest SAMHD1-degradation and improved ara-C sensitivity up to 80-fold. In contrast, HIV-2 Rod9 Vpx did not show any SAMHD1 degradation or improvement in ara-C sensitivity despite its high packaging efficiency in the VLPs. As for the CPPs, CPP44 bound to 67aaVpx showed better uptake and SAMHD1 degradation compared to the TAT bound 67aaVpx in THP-1 cells, which is an AML cell line with high SAMHD1 expression levels. Upon co-treatment with ara-C, up to a 5-fold reduction in IC50 was observed when treated with CPP44-bound 67aaVpx. In order to increase the efficiency further, full-length Vpx-bound CPPs will be prepared, and trials using these CPPs are currently underway. Conclusions

We demonstrate that inducing SAMHD1 degradation by Vpx delivered via VLPs or CPPs efficiently improved ara-C sensitivity in AML cell lines. Combining a Vpx delivery system with treatments containing ara-C might improve treatment outcomes in SAMHD1-high patients who are otherwise non-responsive.

References

1. Schneider C, Oellerich T, Baldauf HM, Schwarz SM, Thomas D, Flick R, et al. SAMHD1 is a biomarker for cytarabine response and a therapeutic target in acute myeloid leukemia. Nat Med 2017 Feb 1;23(2):250–5.

2. Kondo E, Saito K, Tashiro Y, Kamide K, Uno S, Furuya T, et al. Tumour lineage-homing cell-penetrating peptides as anticancer molecular delivery systems. Nat Commun 2012 Jan 17;3(1):951.

Disclosure Information R. Nair: None. H. Baldauf: None.