Pidnarulex HCl is the salt form of CX-5461, a first-in-class non-genotoxic small molecule targeted inhibitor of RNA polymerase I (Pol I) that activates the p53 pathway without causing DNA damage. CX-5461 selectively inhibits rRNA synthesis by Pol I in the nucleolus, but does not inhibit mRNA synthesis by RNA Polymerase II (PolII) and does not inhibit DNA replication or protein synthesis. Inhibition of Pol I results in nucleolar stress and release of ribosomal proteins (RP) from the nucleolus. The RP bind to Mdm2 and liberate p53 to orchestrate apoptosis in cancer cells. CX-5461 demonstrates a favorable preclinical profile, potently and selectively kills cancer cells, demonstrates robust in vivo efficacy in multiple models, and has demonstrated oral bioavailability in multiple species.
MFH290 is a novel cysteine (Cys)-directed covalent inhibitor of CDK12 13. MFH290 forms a covalent bond with Cys-1039 of CDK12, exhibits excellent kinome selectivity, inhibits the phosphorylation of serine-2 in the C-terminal domain (CTD) of RNA-polymerase II (PolII), and reduces the expression of key DNA damage repair genes. Importantly, these effects were demonstrated to be CDK12-dependent as mutation of Cys-1039 rendered the kinase refractory to MFH290 and restored PolII CTD phosphorylation and DNA damage repair gene expression. Consistent with its effect on DNA damage repair gene expression, MFH290 augments the antiproliferative effect of the PARP inhibitor olaparib.