Trastuzumab has shown positive results in many patients with metastatic HER2-positive breast cancer, but it is less effective for controlling metastases in the CNS, which remains a site of relapse. The poor prognosis for patients with brain metastases is thought to be largely due to the presence of the blood-brain barrier (BBB) that prevents delivery of most drugs to the CNS and to the heterogeneous and limited permeability of the blood-tumor barrier (BTB). Focused ultrasound (FUS) bursts combined with circulating microbubbles can temporarily permeabilize both the BBB and the BTB. This technique has been investigated as a potential noninvasive method for targeted drug delivery in the brain. Here, we investigated whether BBB/BTB permeabilization in the tumor and surrounding brain tissue induced by FUS and microbubbles can slow tumor growth and improve survival in a breast cancer brain metastases model. HER2/neu-positive human breast cancer cells (BT474) were inoculated in the brains of 41 nude (nu/nu) rats. Animals in the treatment group received six weekly treatments of BTB/BBB permeabilization under MRI guidance combined with IV administration of trastuzumab (2 mg/kg). Tumor growth and survival rates were monitored via MRI for seven weeks after sonication. Starting at week seven and continuing through the end of the study, the mean tumor volume of the FUS+trastuzumab group was significantly (P<0.05) less than those of the three control groups (no treatment, FUS alone, trastuzumab alone). Furthermore, in four out of 10 rats treated with FUS+trastuzumab, the tumor appeared to be completely resolved in MRI, an outcome which was not observed in any of the 31 rats in three control groups. Trastuzumab improved median survival by 13% compared to the no treatment group, a difference which was significant (P=0.044). Treatment with FUS+trastuzumab produced the most significant benefit compared to the no-treatment controls (P=0.0084). More than half (6/10) animals survived at the study endpoint, leading to a median survival time greater than 83 days (at least 32% longer than the untreated control group). Overall, this work suggests that BBB/BTB permeabilization induced by FUS and microbubbles can improve outcomes in breast cancer brain metastases.
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