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The blood-brain barrier is the selectively permeable vasculature of the brain vital for maintaining homeostasis and neurological function. Low permeability is beneficial in the presence of toxins and pathogens in the blood. However, in the presence of metastatic brain tumors, it is a challenge for drug delivery. Although the blood-tumor barrier is slightly leaky, it still is not permissive enough to allow the accumulation of therapeutic drug concentrations in brain metastases. Herein, we discuss the differences between primary brain tumors and metastatic brain tumors vasculature, effects of therapeutics on the blood-tumor barrier, and characteristics to be manipulated for more effective drug delivery.

The blood-brain barrier (BBB) is a major anatomical and physiological barrier limiting the passage of drugs into brain. Central nervous system tumors can impair the BBB by changing the tumor microenvironment leading to the formation of a leaky barrier, known as the blood-tumor barrier (BTB). Despite the change in integrity, the BTB remains effective in preventing delivery of chemotherapy into brain tumors. Focused ultrasound is a unique noninvasive technique that can transiently disrupt the BBB and increase accumulation of drugs within targeted areas of the brain. Herein, we summarize the current understanding of different types of targeted ultrasound mediated BBB/BTB disruption techniques. We also discuss influence of the tumor microenvironment on BBB opening, as well as the role of immunological response following disruption. Lastly, we highlight the gaps between evaluation of the parameters governing opening of the BBB/BTB. A deeper understanding of physical opening of the BBB/BTB and the biological effects following disruption can potentially enhance treatment strategies for patients with brain tumors.

Background Brain metastasis is a devastating stage of cancer progression, occurring in ~30% of metastatic breast cancer patients. Two-year survival rates for these patients is low, and most typically survive less than one year. Treatments for these women are limited by the blood-brain barrier, but include cytotoxic chemotherapy, surgical resection, and radiation therapy (whole-brain radiotherapy or stereotactic radiosurgery). Radiotherapy is considered to be capable of inducing disruption of the blood-brain barrier and eliciting an abscopal response to extracranial tumors. Methods A combination of ionization chamber and Gafchromic film dosimetry was used to commission and determine dose outputs for our experimental design. Dose deposition in-vivo was verified by immunohistochemistry. To evaluate the effects of ionizing radiation at the normal blood-brain barrier and the blood-tumor barrier, athymic nude and FVB mice were used. Athymic nude mice were injected with MDA-MB-231Br cells. Lesions were allowed to develop for ~28 days. Mice were then irradiated at the prescribed dose. Prior to tissue collection, mice were injected with Texas red, followed by a vascular washout with physiological buffer. Fluorescence in normal and diseased brain was quantified by fluorescent microscopy. Results Using a 10mmx10mm collimator, determined to have adequate field homogeneity as determined by Gafchromic film analysis, we were able to successfully treat a single hemisphere in mice. The blood-brain-barrier remained undisrupted in athymic nude mice at doses up to 12Gy compared to untreated brain and radiation naïve controls. Immune competent FVB mice treated with radiation showed significant blood-brain barrier disruption at a dose of 12Gy only. The blood-tumor barrier showed significant disruption at 24hrs following radiation treatment (6 or 12Gy). Conclusion Our study demonstrated that radiation therapy disrupts the blood-tumor barrier, but fails to disrupt the normal blood-brain barrier in athymic nude mice. However, in FVB immune competent mice, the blood-brain barrier was disrupted at a dose of 12Gy, suggesting an abscopal-like response impacts extent of barrier leakage.

Lung cancer is the most prevalent malignancy to affect both men and women. Around 80% of all lung cancers are classified as non-small cell lung cancer (NSCLC). This subtype of lung cancer is also the most likely to metastasize to the brain. Clinically, the common treatment for NSCLC is epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs), due to the high occurrence of EGFR mutations. However, the cancer cells quickly develop resistance to the EGFR TKIs. This resistance and the added difficulty of delivering drugs across the blood-tumor barrier in efficacious concentrations to treat brain lesions are important to consider when developing treatment strategies for lung cancer brain metastases. Our study utilizes a NSCLC cell line, PC-9-Br6, which was developed in our laboratory to preferentially metastasize to the brain. This cell line was demonstrated by our collaborator to express higher levels of Bcl-2 in comparison to the parental PC-9-P cell line. We hypothesized combining novel Bcl-2 inhibitors (ABT-199/ABT-263) with an EGFR inhibitor (gefitinib) would increase survival and decrease tumor burden in our clinically relevant mouse model of lung cancer brain metastases.

Brain metastases encompass nearly 80% of all intracranial tumors. A late stage diagnosis confers a poor prognosis, with patients typically surviving less than two years. Poor survival can be equated to limited effective treatment modalities. One reason for the failure rates is the presence of the blood-brain (BBB) and blood-tumor barriers (BTB) that limit the access of potentially effective chemotherapeutics to metastatic lesions. Strategies to overcome these barriers include new small molecule entities capable of crossing into the brain parenchyma, novel formulations of existing chemotherapies, and disruptive techniques. Herein, we review BBB physiology and BTB pathophysiology. Additionally, we review the limitations of routinely practiced therapies and three current methods being explored for blood-brain/blood-tumor barrier disruption for improved delivery of chemotherapy to brain tumors.