The Replication protein A (RPA) complex consists of the three subunits Rpa 14, Rpa 32 and Rpa70 (Rpa 3, 2 and 1 in mice). The Rpa complex represents the major single-stranded DNA binding protein in eukaryotic cells. It plays important roles in several pathways of DNA metabolism such as DNA replication, DNA repair and DNA recombination and interacts with a high number of proteins involved in genome and telomere maintenance (Zou et al., 2006). During replication the RPA complex controls the global response of new origin firing via the ATR-CHK1 pathway. Toldeo et al. further underlined a role of RPA in genomic stability by showing that a progressive RPA depletion after ATR inhibition leads to massive DNA breakage due to exposure of unprotected ssDNA. These observations suggest a therapeutic window for RPA inhibition in cancer therapy as many cancers harbor high levels of replicative stress.
Specifically, we hypothesized that the Rpa complex should be an attractive therapeutic target, as in cells with high levels of replicative stress small molecule inhibition of the Rpa complex should result in the exposure of unprotected ssDNA with subsequent accumulation of double strand DNA breaks and cell death. In cooperation with A. Poso (TüCAD2, University of Tuebingen), we performed in silico screening approaches and after the identification and validation of first lead compounds iterative fast learning cylces and chemical optimization was applied.
Strikingly, our most advanced compounds show EC50 concentrations in cellular assays in aggressive pancreatic cancer cell lines of 100nM. EC50 values of 100nM can be regarded as excellent for compounds disrupting protein-protein interactions (PPI). For comparison, most advanced PPI compounds like Navitoclax, a bcl-2 and bcl-xL inhibitor, show EC50 values in cellular systems ranging between 110nM and 22μM.
The current focus of the project lies on preclinical drug development (improving ADME-properties, pharmacokinetic studies, toxicity testing etc.) and we are aiming to bring Rpa3 inhibiting compounds into clinical application for the treatment of aggressive and fast proliferating cancers.
