Targeting the PNP - SAMHD1 synthetic-lethal combination in cancer

Abstract The enzymatic transition state structures of mammalian purine nucleoside phosphorylases (PNPs) were solved experimentally in this laboratory by a combination of intrinsic kinetic isotope effects (KIEs) and computational quantum chemistry. The resulting bond geometry and electrostatic potential maps enabled the design and synthesis of transition state analogs. Immucillin-H (aka BCX1777, Forodesine, Mundesine) inhibits human PNP with picomolar affinity. The PNP blockade causes dGTP accumulation and apoptotic cell death specifically in activated and malignant T cells. Forodesine entered clinical trials and was approved as an oral therapy for refractory or relapsed peripheral T-cell lymphoma in Japan in 2017. Approximately 10% of otherwise refractory patients treated with Forodesine therapy undergo complete remission. Others show stable disease or no response. Genomic analysis of Forodesine responder vs non- responder T-cells from cancer patients revealed drug efficacy inversely proportional to SAMHD1 expression (sterile alpha motif and HD-domain-containing protein 1). SAMHD1 is a dNTP triphosphohydrolase (dNTP + H2O  dNuc + PPPi), preventing dGTP accululation from inducing apoptosis in targeted cancer T cells. Therefore, the combination of SAMHD1 inhibitors with Forodesine is expected to extend the therapeutic profile of human PNP inhibitors, a synthetic lethal interaction. Drug design by transition state analysis will be extended to SAMHD1, a rare triphosphohydrolase activity. SAMHD1 inhibition has potential to enhance T-cell anticancer outcomes. The privilidged enzyme-reactant geometry at the transtion state will be enlisted as one inhibitor design element. Drug candidates designed to stablize the SAMHD1 protein geometry at the transition state will be powerful inhibitors. Chemical screening with fragment libraries, some with covalent potential, coupled by click chemistries, will be used in combinatorial inhibitor design as an alternative approach. SAMHD1 inhibitors, used in combination with Forodesine, are expected to provide improved therapeutic approaches to the spectrum of T-cell malignancies. SAMHD1 docked to its transition state to define the enzymatic cavity at the transition state will be used to grow complimentary, protein-stabilizing structures from library fragments to mimic the transition state configuration. SAMHD1 conformational traps of transition state geometry will be tight-binders. A Bronx-based novel patient-derived cancer T-cell library will be tested in culture and in mouse xenografts with SAMHD1 inhibitors to explore the scope of synthetic lethal interactions across genetically defined human T-cell cancers. The program will be achieved with specific aims to: 1) solve the transition state structure of SAMHD1 from reactant and catalytic site perspectives, 2) discover SAMHD1 inhibitors by conformational transition state stabilization, fragment screening and growth into the catalytic site cavity, structural biology approaches and 3) use cell culture and mouse xenografts to characterize the biological effects of this synthetic lethal interaction.