Quarfloxin (also known as CX-3543) is a fluoroquinolone derivative patented by Cyclene Pharmaceuticals, Inc. as an antitumor agent. Quarfloxin selectively disrupts nucleolin/rDNA G-quadruplex complexes in the nucleolus, thereby inhibiting DNA polymerase I transcription and inducing apoptosis in cancer cells. CX-3543 was evaluated in phase II clinical studies for the treatment of low or intermediate grade neuroendocrine carcinoma, including carcinoid and islet cell cancer. In 2008, a trial for the treatment of chronic lymphocytic leukemia (CLL) was withdrawn prior to patient enrollment. In 2010, phase I clinical studies for the treatment of solid tumors and for the treatment of lymphoma were terminated upon the observation that the modified dose schedule presented no advantage over previously studies schedule solid tumors/ Cylene discontinued development of quarfloxin in 2010.