The vast majority of cancer patients receive DNA-damaging drugs or ionizing radiation (IR) during their course of treatment, yet the efficacy of these therapies is tempered by DNA repair and DNA damage response (DDR) pathways. Aberrations in DNA repair and the DDR are observed in many cancer subtypes and can promote de novo carcinogenesis, genomic instability, and ensuing resistance to current cancer therapy. Additionally, stalled or collapsed DNA replication forks present a unique challenge to the double-strand DNA break (DSB) repair system. Of the various inducible DNA lesions, DSBs are the most lethal and thus desirable in the setting of cancer treatment. In mammalian cells, DSBs are typically repaired by the error prone non-homologous end joining pathway (NHEJ) or the high-fidelity homology directed repair (HDR) pathway. Targeting DSB repair pathways using small molecular inhibitors offers a promising mechanism to synergize DNA-damaging drugs and IR while selective inhibition of the NHEJ pathway can induce synthetic lethality in HDR-deficient cancer subtypes. Selective inhibitors of the NHEJ pathway and alternative DSB-repair pathways may also see future use in precision genome editing to direct repair of resulting DSBs created by the HDR pathway. In this review, we highlight the recent advances in the development of inhibitors of the non-phosphatidylinositol 3-kinase-related kinases (non-PIKKs) members of the NHEJ, HDR and minor backup SSA and alt-NHEJ DSB-repair pathways. The inhibitors described within this review target the non-PIKKs mediators of DSB repair including Ku70/80, Artemis, DNA Ligase IV, XRCC4, MRN complex, RPA, RAD51, RAD52, ERCC1-XPF, helicases, and DNA polymerase θ. While the DDR PIKKs remain intensely pursued as therapeutic targets, small molecule inhibition of non-PIKKs represents an emerging opportunity in drug discovery that offers considerable potential to impact cancer treatment.
Molecularly targeted cancer therapies substantially improve patient outcomes, although the durability of their effectiveness can be limited. Resistance to these therapies is often related to adaptive changes in the target oncoprotein which reduce binding affinity. The arsenal of targeted cancer therapies, moreover, lacks coverage of several notorious oncoproteins with challenging features for inhibitor development. Degraders are a relatively new therapeutic modality which deplete the target protein by hijacking the cellular protein destruction machinery. Degraders offer several advantages for cancer therapy including resiliency to acquired mutations in the target protein, enhanced selectivity, lower dosing requirements, and the potential to abrogate oncogenic transcription factors and scaffolding proteins. Herein, we review the development of proteolysis targeting chimeras (PROTACs) for selected cancer therapy targets and their reported biological activities. The medicinal chemistry of PROTAC design has been a challenging area of active research, but the recent advances in the field will usher in an era of rational degrader design.
Despite remarkable advances within the field of molecular cancer therapeutics over the preceding decades, DNA-damaging agents retain a central role in the pharmacotherapy of neoplastic diseases. The clinical response to DNA-damaging therapies is varied within and across cancer subtypes where a robust DNA damage response (DDR) predicts recalcitrance. Replication protein A (RPA) is the predominant human single-stranded DNA (ssDNA)-binding protein, playing essential roles in DNA replication, repair, recombination, and the DNA-damage response (DDR). Inhibition of the RPA−DNA interaction is an approach to cancer drug discovery that holds potential to provide single-agent activity and/or synergy with existing therapeutics. Proteolysis targeting chimera (PROTAC) based drug design presages a paradigm shift within drug discovery as PROTACs begin to carve out a space in the clinical landscape. The PROTAC platform is particularly suited for drugging overexpressed proteins such as RPA because iterative target degradation improves drug:target stoichiometry and precludes RPA’s role in protein scaffolding. Herein, we describe the synthesis and biological evaluation of an RPA targeted PROTAC library conceived by conjugation of the diaryl pyrazole RPA inhibitor TDRL-551 and an E3-ligand via a linker of variable length and composition. Residing within the beyond rule of five (bRo5) chemical space, careful consideration was given to minimize the liability to cell permeability imposed by the high molecular weight of PROTACs. Excitingly, cellular uptake across the compound library was broadly improved relative to the unconjugated warhead TDRL-551. Similarly, RPA engagement by the PROTAC series was widely unhindered and, in many cases, improved relative to the warhead TDRL-551. A subset of the compound library unified by flexible linkers and alkyne attachment of the E3-ligand (typified by GL-3311) display low single-digit micromolar IC50 values in viability assays across H460, A549, and A2780 cell lines. Citation Format: Jeremy M. Kelm, Jitender Dev Gaddameedi, Pamela S. VanderVere-Carozza, Amirreza Samarbakhsh, Nivisa Vakeesan, Hussein W. Kansou, Sara Serafimovski, Katherine S. Pawelczak, John J. Turchi, Navnath S. Gavande. Development of replication protein A (RPA) targeted PROTACs for the treatment of lung and ovarian cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 802.
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