In humans, platelet count within the normal range is required for physiological hemostasis, but, adversely, platelets also support pathological thrombosis. Moreover, by releasing growth factors, they may enhance neoplastic proliferation. We hypothesize that platelet count correlates with platelet-dependent pathologies, even within the range of hemostatic competence. Because platelet production is promoted by thrombopoietin signaling through the myeloproliferative leukemia virus oncogene (cMPL), a receptor expressed on megakaryocytes, we evaluated the feasibility of selective targeting of hepatic thrombopoietin production to test this hypothesis. We synthesized murine- and primate-specific antisense oligonucleotides (THPO-ASO) that silence hepatic thrombopoietin gene (THPO) expression without blocking extrahepatic THPO. Repeated doses of THPO-ASO were administered to mice and a baboon, causing a sustained 50% decline in plasma thrombopoietin levels and platelet count within 4 weeks in both species. To investigate whether reducing platelet count within the translationally relevant hemostatic range could alter a neoplastic process, we administered THPO-ASO to 6-week-old transgenic MMTV-PyMT mice that develop early ductal atypia that progresses into cMPL-negative fatal metastatic breast cancer within 2 to 3 months. THPO-ASO treatment increased the average time to euthanasia (primary humane endpoint) at 2 cm3 combined palpable tumor volume. Our results show that THPO-ASO reduced blood platelet count, plasma platelet factor 4, vascular endothelial growth factor, thrombopoietin levels, bone marrow megakaryocyte density, tumor growth rate, proliferation index, vascularization, platelet and macrophage content, and pulmonary metastases vs untreated controls. These findings confirm that sustained and moderate pharmacological platelet count reduction is feasible with THPO-ASO administration and can delay progression of certain platelet-dependent pathological processes within a safe hemostatic platelet count range.
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Acknowledgments This project has been sponsored by Aronora, Inc and Ionis Pharmaceuticals. The investigators were also supported, in part, by National Institutes of Health, National Heart, Lung, and Blood Institute grants HL117589 and HL095315 (A.G. and E.I.T.), and HL101972 (O.J.T.M.), and National Cancer Institute grants
Conflict-of-interest disclosure: A.G. and E.I.T. are employees of Oregon Health & Science University, which may have financial interest in the results of this study. A.G., E.I.T. (Aronora, Inc), A.S.R., and B.P.M. (Ionis Pharmaceuticals) are inventors of a pending patent on the chemistry and composition of hepatocyte-specific THPO-ASO. L.M.C. is a paid consultant for Cell Signaling Technologies; receives reagent and/or research support from Plexxikon, Inc, Pharmacyclics, Inc, Decipereha Pharmaceuticals, LLC, Gen-entech, Inc, Roche Glycart AG, and NanoString Technologies; and is a member of the Scientific Advisory Boards of Syndax Pharmaceuticals, Inc, Carisma Therapeutics, Zymeworks, Inc, and Verseau Therapeutics, Inc. The remaining authors declare no competing financial interests.