No abstract

Radiation therapy is a mainstay in the treatment of brain metastasis, yet some tumors are resistant, and elsewhere brain recurrence outside the radiation field is common. Phototherapy using UV light-activated compounds can both kill cancer cells directly and trigger an immune response to extend tumor control. Poor penetration depth of ultraviolet light, however, has limited this treatment to superficial tumors. High-energy photon beams create high energy electrons within the patient which in turn produce Cherenkov radiation in the UV spectrum while traveling through tissue. Given that this Cherenkov radiation is generated deep within the patient and has the ability to activate photosensitive compounds, we therefore developed a platform to test this phenomenon to enhance radiation therapy for brain metastasis. We first tested UV-activated psoralen derivatives in combination with UV light in vitro for activity against murine 4T1 breast cancer cells, and then irradiated an ex vivo organotypic brain slice platform using a high energy linear accelerator to generate Cherenkov radiation. We tested the survival of 4T1 cells expressing fluorescent and bioluminescent reports in the presence and absence of these psoralen compounds in this ex vivo brain metastasis model. 8-methoxypsoralen (8-MOP) and 4’-Aminomethyltrioxsalen hydrochloride (AMT) both showed 365nm UVA light-specific cell killing in vitro. We optimized AMT cell loading (1 hour) and concentrations [1μM] AMT to maximize cytotoxicity. Testing of AMT using the organotypic brain slice platform and high-energy irradiation to generate Cherenkov-UV light demonstrated similar enhanced cell death of 4T1 cells despite high baseline levels of radiation-induced tumor cell kill. Cherenkov radiation-induced photo-activation of AMT improved cell killing in an ex vivo model of breast cancer brain metastasis. This application holds promise for the re-treatment of refractory tumors with high-energy, low dose radiation, and enhanced elsewhere brain metastasis control through activation of the immune system.