DDEL-04. Time-of-day effects on blood-brain barrier/blood-tumor barrier permeability to chemotherapeutics in breast cancer brain metastases

Abstract Breast cancer is a leading cause of cancer diagnoses in women. Although the five-year survival rate is >85% for regional breast cancer, this drops to <10% in the presence of brain metastases. Breast cancer brain metastases (BCBM) are difficult to treat due to the restrictive nature of the blood-brain barrier (BBB) and blood-tumor barrier (BTB) to therapeutics. Previous studies demonstrate circadian regulation of BBB/BTB permeability, specifically, activity/expression of ATP-binding cassette (ABC) transporters at the BBB. Murine studies demonstrate time-of-day (TOD) differences in the ABC transporter Abcb1 (P-glycoprotein) expression at the BBB in response to cortical neuronal activity. In mice bearing BCBM, intratumoral 14C-paclitaxel concentrations fluctuate throughout the day, peaking mid-active phase. Paclitaxel is a substrate for p-glycoprotein (Pgp), one of the most prominently studied ABC transporters. However, it remains unknown if there are TOD differences in BBB/BTB permeability to chemotherapeutics that are substrates for multiple ABC transporters. We hypothesized that intratumoral concentrations of 14C-Etoposide (effluxed by Pgp, BCRP, and MRP2) in BCBM varies based on the TOD of injection. Female NU/NU mice received an intracardiac injection of 1.75x105 JIMT-1BR3 cells (human HER2+ BC cell line with enhanced brain tropism). Twenty days later, mice were injected intravenously with 7.5 mg/kg of 14C-etoposide at zeitgeber time (ZT, a 24-hour circadian notation; ZT0 = lights onset) 0, 6, 12, or 18. After two hours, brains were collected and sectioned. Mice receiving chemotherapy during the active phase exhibited higher intratumoral 14C-Etoposide concentrations than those receiving injections during the inactive phase. Similar to previous studies, concentrations peaked within BCBM at ZT18; trough concentrations were observed at ZT6. Immunostaining for Pgp, BCRP, and MRP2 will determine if there are TOD differences in expression of ABC transporters at the BBB/BTB. These data provide evidence supporting consideration of TOD in BCBM treatment.