Borneol and Tetrandrine Modulate the Blood–Brain Barrier and Blood–Tumor Barrier to Improve the Therapeutic Efficacy of 5-Fluorouracil in Brain Metastasis

The efficacy of chemotherapeutic drugs for the treatment of brain metastasis may be compromised by the blood–brain barrier (BBB) and blood–tumor barrier (BTB). P-glycoprotein (P-gp) is a multidrug resistance protein that potentially limits the penetration of chemotherapeutics through the BBB and BTB. 5-Fluorouracil (5-FU) is widely used to treat cancer. Bioactive constituents of medicinal herbs, such as borneol and tetrandrine, potentially improve drug penetration through the BBB and BTB. We hypothesized that borneol and tetrandrine might modulate the BBB and BTB to enhance 5-FU penetration into the brain. To investigate this, in vitro and in vivo models were developed to explore the modulatory effects of borneol and tetrandrine on 5-FU penetration through the BBB and BTB. In the in vitro models, barrier integrity, cell viability, barrier penetration, P-gp activity, and NF-κB expression were assessed. In the in vivo brain metastasis models, cancer cells were injected into the internal carotid artery to evaluate tumor growth. The experimental results demonstrated that borneol and borneol + tetrandrine reduced BBB integrity. The efflux pump function of P-gp was partially inhibited by tetrandrine and borneol + tetrandrine. In the in vivo experiment, borneol + tetrandrine effectively prolonged survival without compromising body weight. In conclusion, BBB and BTB integrity was modulated by borneol and borneol + tetrandrine. The combination of borneol and tetrandrine could be used to improve the chemotherapeutic control of brain metastasis.