The Ribosomal Biogenesis Inhibitor CX-5461 Is an Anti-Cancer Therapeutic That Increases Platelet Count in Mice and in Humans
Authors: Kaur Amandeep , Hannan Katherine M , Nazir Samina , Maclachlan Kylee H , Poortinga Gretchen , Man Si Ming , Harrison Simon , Hannan Ross D , Gardiner Elizabeth E. 
Published: November 2019

Background: The ribosomal biogenesis inhibitor CX-5461 is a first-in-class selective inhibitor of ribosomal RNA gene transcription with single-agent antitumor activity against advanced hematologic cancers and synergistic activity when combined with front-line and emerging agents (Devlin et al. 2015, Sornkom et al. 2018, Maclachlan et al. 2018). It has predictable pharmacokinetics and a safety profile allowing prolonged dosing (Khot et al, Cancer Discov 2019). Unexpectedly, CX-5461 treatment increases platelet count by ~50% in tumour diseased mice; an effect that was maintained for up to 6 months of treatment.
Aims: We investigated mechanisms that may contribute to increased platelet counts following CX-5461 treatment in a liver carcinoma mouse model or disease-free mice and evaluated blood test data from patients with haematological malignancies treated with CX-5461.
Methods: We evaluated blood cell numbers in blood from vehicle- or CX-5461 treated mice with liver carcinoma, and blood cell numbers, bone marrow megakaryocytes, platelet receptors, immature platelets, platelet function, thrombopoietin (TPO) and inflammatory cytokine levels in disease-free C57BL/6 KaLwRij mice treated with 35 mg/kg CX-5461 or vehicle (n=15). Human platelet receptors and function were assessed in CX-5461-treated platelet-rich plasma by flow cytometry, platelet spreading assays and light transmission aggregometry. We evaluated temporal platelet count, and the platelet activation marker soluble glycoprotein (GP) VI in patient plasma samples who had received a single dose of 25-250 mg/m2 CX-5461 (n=16) with 1-way ANOVA.
Results: In a model of liver carcinoma, mice with disease displayed significantly elevated (2.3-fold) platelet counts after 3 months treatment with biweekly injections of 35 mg/kg CX-5461 compared with vehicle-treated diseased mice (p = 0.0013). Disease-free mice treated for 7 or 14 days (3 or 6 x 35 mg/kg doses) with CX-5461 showed ~60% increase in platelet count; in comparison white and red cells were mildly suppressed. The CX-5461-mediated platelet increase at d7 (p<0.0005) was reversible within 1 week. At d14 (p<0.01, 1.7-fold increase in platelet count) a significant increase in megakaryocytes (p<0.05) and immature platelets (p<0.01) was observed. CX-5461 treatment had no effect on plasma TPO, platelet lifespan or platelet GPVI or GPIbα levels. Integrin αIIb was significantly elevated. Inflammatory cytokines interleukin (IL)-6 (p<0.05) and TNFα (p<0.01) increased at d7 in plasma. After pre-treatment for 30 min with CX-5461 ex vivo, human platelet function (aggregation and platelet spreading on collagen and fibrinogen) and receptor levels were normal. In 8/16 patients receiving CX-5461, increases of up to 34% in platelet counts were measured at day 15 of CX-5461 treatment, with no change in sGPVI.
Conclusions: CX-5461 treatment of mice for two weeks increased platelet counts, megakaryocyte number and immature platelet fraction by an unknown mechanism however IL-6 has been reported by others to enhance megakaryopoeisis. Brief CX-5461 treatment in a group of patients with advanced haematological malignancy resulted in small increases in platelet count with retained platelet activation and treatment ex-vivo demonstrated platelet function to be unaffected. Future work will explore the link between CX-5461 treatment and megakaryopoiesis and thrombopoiesis, including the potential for CX-5461 to ameliorate drug-induced thrombocytopenia.

Disclosures
Harrison: Takeda: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen Cilag: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; GSK: Consultancy, Research Funding; Novartis: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; AbbVie: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: investigator on studies, Research Funding; Celgene: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Amgen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding. Hannan:Pimera: Membership on an entity's Board of Directors or advisory committees, Research Funding.