Oral Presentation 50 Years Shine-Dalgarno Symposium 2023

Inhibition of RNA polymerase I stimulates platelet production and may address chemotherapy-related thrombocytopenia (#13)

Vijay Bhoopalan 1 , Amandeep Kaur 1 , James Hearn 1 , Rita Ferreira 1 , Kylee Maclachlan 2 3 , Si Ming Man 1 , Nadine Hein 1 , Kate Hannan 1 , Ross Hannan 1 , Elizabeth Gardiner 1
  1. Australian National University, Canberra, ACT, Australia
  2. Cancer, Memorial Sloan-Kettering, New York, NY, USA
  3. Cancer Haematology, Peter MacCallum Cancer Centre, Melbourne, Vic, Australia

Background: Chemotherapy-induced thrombocytopenia remains a major dose-limiting factor in cancer therapy. CX-5461, an RNA Polymerase I inhibitor, is a strong candidate anti-cancer therapeutic that has completed phase I/II clinical trials. Pre-clinical studies identified that CX-5461 treatment increased circulating platelet counts in both mice and humans, potentially counteracting thrombocytopenia side effects.

Aim: To explore mechanisms underpinning CX-5461-mediated thrombopoiesis.

Methods: C57BL/6 wild-type or TPO-receptor-deficient mice received three doses/week of 35 mg/kg CX-5461 or vehicle. Blood cells were enumerated and platelet lifespan, receptor levels, and function were assessed. Bone marrow (BM) was isolated, and MK number/ploidy and haematopoietic cell subpopulations were quantified. Plasma, serum cytokines, and TPO levels were assessed by ELISA, and liver TPO mRNA by RT-qPCR. Whole BM or FACS-sorted mouse Lineage-Sca1+c-Kit+ (LSK) cells were cultured to quantify MK colony forming units (CFU).

Results: A single-dose of CX-5461 increased platelet counts 1-34% in 9/16 haematological malignancy patients. CX-5461-treated mice showed rapid, reversible 1.7-fold increases in platelets (d7); lymphocytes and neutrophils remained unchanged, erythrocytes decreased. Key platelet receptors showed normal levels and function. New platelets were increased 2.3-fold, but platelet lifespan was unchanged. Inflammatory cytokines (IL-6, TNFα, IL-1β) in plasma and in BM remained within normal ranges. A >2-fold increase in MK numbers and hyperploidy (>8N) was detected together with increased Sca1+MK (p<0.01), platelet/MK-biased multipotent progenitor-2 (LSK CD135-CD48+CD150+) (p> 0.0005). CX-5461 treatment led to a >3-fold increase in MK-CFU. CX-5461-induced platelet production was independent of TPO, as plasma TPO and liver TPO mRNA levels were unaffected. CX-5461 treatment of TPO receptor-/- mice resulted in 5-fold (p< 0.0005) increased circulating platelets.

Conclusions: CX-5461 treatment rapidly increases circulating platelet levels. CX-5461 promotes a platelet/MK-biased haematopoietic pathway via a pathway unrelated to TPO signalling or inflammation. RNA-Polymerase I inhibitors offer a novel target for treating thrombocytopenia and a tool to further our understanding of thrombopoiesis.