The intraoperative clear delineation between brain tumor and normal tissue in real time is required to ensure near-complete resection without damaging the nearby eloquent brain. Tumor Paint BLZ-100, a tumor ligand chlorotoxin (CTX) conjugated to indocyanine green (ICG), has shown potential to be a targeted contrast agent. There are many infrared imaging systems in use, but they are not optimized to the low concentration and amount of ICG. The authors present a novel proof-of-concept near-infrared (NIR) imaging system using a standard charge-coupled device (CCD) camera for visualizing low levels of ICG attached to the tumors. This system is small, inexpensive, and sensitive. The imaging system uses a narrow-band laser at 785 nm and a notch filter in front of the sensor at the band. The camera is a 2-CCD camera, which uses identical CCDs for both visible and NIR light.
The NIR system is tested with serial dilution of BLZ-100 from 1 μM to 50 pM in 5% Intralipid solution while the excitation energy is varied from 5 to 40 mW/cm2. The analog gain of the CCD was changed from 0, 6, and 12 dB to determine the signal-to-noise ratio. In addition to the Intralipid solution, BLZ-100 was injected 48 hours before euthanizing the mice that were implanted with the human glioma cell line. The brain was removed and imaged using the NIR imaging system.
The authors' results show that the NIR imaging system using a standard CCD is able to visualize the ICG down to 50 nM of concentration with a high signal-to-noise ratio. The preliminary experiment on human glioma implanted in mouse brains demonstrated that BLZ-100 has a high affinity for glioma compared with normal brain tissue. Additionally, the results show that NIR excitation is able to penetrate deeply and has a potential to visualize metastatic lesions that are separate from the main tumor.
The authors have seen that BLZ-100 has a very high affinity toward human gliomas. They also describe a small, cost-effective, and sensitive NIR system for visualizing brain tumors tagged using BLZ-100. The authors hope that the use of BLZ-100 along with NIR imaging will be useful to delineate the brain tumors in real time and assist surgeons in near-complete tumor removal to increase survival and reduce neurological deficits.