Itarnafloxin

CX-3543 (Quarfloxin) and CX-5461 (Pidnarulex) were originally derived from a group of fluoroquinolones that were shown to have dual topoisomerase II (Top2) and G-quadruplex (G4) interactions, and QQ58 was the starting structure for their design. Quarfloxin was initially shown to inhibit c-MYC mRNA expression. Studies at Cylene Pharmaceuticals showed that the primary mechanism of action of Quarfloxin is due to displacement of nucleolin from quadruplexes on the non-template strand of rDNA, causing rapid redistribution of nucleolin from nucleoli, inhibition of rRNA synthesis, and apoptotic death in cancer cells. At Cylene a follow-up compound to Quarfloxin, named Pidnarulex (CX-5461), was optimized for targeting RNA Pol 1. Significantly, in more recent work published in Proc Natl Acad Sci USA and Cell in 2020 and in eLIFE and Nat Comm in 2021, it has been shown that the real molecular target for Pidnarulex is Top2 at transcribed regions containing G4s, rather than RNA Pol 1. These results support the original design strategy published in Mol Cancer Ther in 2001, which was to rationally design a G4-targeting drug (QQ58) starting from a fluoroquinolone duplex-targeting Top2 poison (A-62176) that had good drug-like properties. A very important breakthrough was realized when homologous recombination (HR) was found to be important in the repair of DNA damage caused by G4-interactive compounds, suggesting that a synthetic lethal approach might be useful in identifying cancer patients sensitive to these agents. Through use of an unbiased screen, this mechanistic insight was shown to directly apply to Cylene compounds, which were found to induce DNA damage and to be dependent on BRCA1/2-mediated HR and the DNA-PK-mediated nonhomologous end-joining (NHEJ) pathway for damage repair. To evaluate how this mechanistic insight involving a synthetic lethal approach might be applied clinically, a recent Canadian Phase I clinical trial with Pidnarulex in breast and ovarian cancer patients with known BRCA1/2 germline mutations was carried out. Because of the G4 stabilizer function of Pidnarulex, patient populations that responded well to this compound were identified: they are cancer patients with BRCA1/2 deficiency or deficiency in other DNA damage response pathways. Clinically observed resistance to Pidnarulex resulted from reversion to WT BRCA2 and PALB2 ("partner and localizer of BRCA2," because it partners with another gene, called BRCA2), thus providing strong evidence for the underlying synthetic lethal hypothesis proposed for G4-targeting compounds that cause DNA damage.

The clinical applicability of G-quadruplexes (G4s) as anticancer drugs is currently being evaluated. Several G4 ligands and aptamers are undergoing clinical trials following the notable examples of quarfloxin and AS1411, respectively. In this review, we summarize the latest achievements and breakthroughs in the use of G4 nucleic acids as both therapeutic tools ('friends', as healing anticancer drugs) and targets ('foes', within the harmful cancer cell), particularly using aptamers and quadruplex-targeted ligands, respectively. We explore the recent research on synthetic G4 ligands toward the discovery of anticancer therapeutics and their mechanism of action. Additionally, we highlight recent advances in chemical and structural biology that enable the design of specific G4 aptamers to be used as novel anticancer agents.