Enrichment and Quantitative Determination of 5-(Hydroxymethyl)-2′-deoxycytidine, 5-(Formyl)-2′-deoxycytidine, and 5-(Carboxyl)-2′-deoxycytidine in Human Urine of Breast Cancer Patients by Magnetic Hyper-Cross-Linked Microporous Polymers Based on Polyionic Liquid

5-(Hydroxymethyl)-2′-deoxycytidine (5-hmdC), 5-(formyl)-2′-deoxycytidine (5-fodC), and 5-(carboxyl)-2′-deoxycytidine (5-cadC) are crucial intermediate products of the DNA demethylation pathway, which can also act as potential biomarkers reflecting the diagnosis and prognosis in multiple tumors. Detecting 5-hmdC, 5-fodC, and 5-cadC in human urine has various advantages including readily available samples and being noninvasive to patients. However, few works have reported the detection of 5-fodC and 5-cadC due to their trace amounts. Here we developed a novel magnetic hyper-cross-linked microporous polymer (HMP) material based on polyionic liquid (PIL) for the enrichment of 5-hmdC, 5-fodC, and 5-cadC. These magnetic PIL-HMP materials provided specific enrichment superiority for three modified cytidines. After enrichment, the signal intensity of 5-hmdC, 5-fodC, and 5-cadC increased 10–75-fold with lower limits of quantitation (LLOQ) of 0.049, 0.781, and 0.781 ng/mL, respectively. The recoveries were approximately 86.5–95.2% for 5-hmdC, 95.2–107.8% for 5-fodC, and 99.4–102.4% for 5-cadC under the relative standard deviation (RSD) of 0.2–10.3%. Finally, we successfully applied magnetic PIL-HMP materials coupled with high-performance liquid chromatography–electrospray ionization tandem mass spectrometry (HPLC-ESI-MS/MS) in enrichment and quantitative determination of 5-hmdC, 5-fodC, and 5-cadC in human urine of 10 breast cancer patients and 10 healthy people. We found that the level of 5-hmdC decreased in breast cancer patients (p < 0.05), while the levels of 5-fodC and 5-cadC increased (p < 0.05, p < 0.01). Our results demonstrated that the levels of metabolic 5-hmdC, 5-fodC, and 5-cadC in human urine are closely related to breast cancer, which could contribute to the clinical diagnosis and investigation of breast cancer and its occurrence and development mechanisms.