Glioblastoma Multiforme (GBM) is the most aggressive brain cancer with an abysmal prognosis.¹ It is believed that the poor prognosis is due to a population of stem-like cells called brain tumor initiating cells (BTIC). These cells are, in part, responsible for chemo-radiation resistance and tumor recurrence.¹ It has been shown that monocytes can be activated to suppress BTICs using Amphotericin B (Amp B).¹ The traditional method of measuring tumor volume using MRI requires long treatment periods to detect Amp B’s therapeutic response. We aimed to develop an MRI method that can rapidly detect a drug response associated with stimulation of the immune system to assist with studies of drugs less toxic than Amp B. Monocyte trafficking to brain is part of the Amp B mediated anti-tumor response.¹ We propose using intravenous injection of ultra-small iron oxide nanoparticles (USPIO) can label monocytes.² We show that the efficacy of Amp B can be visualized by tracking monocyte trafficking into brain using USPIO-MRI.10,000 human derived BTICs (line BT048) were implanted into the right striatum of immune compromised mice, as described previously.³ Treatment with Amp B (0.2mg/kg, n = 4) or vehicle (sodium deoxycholate, n = 5) was initiated 35 days after tumor implantation and continued until sacrifice. MRI was performed 42-45 days post implantation. MRI was done with a Bruker 9.4T MRI using a T1w RARE sequence (TE = 7ms, TR = 500 ms, RARE factor = 4, voxel size = 0.15 mm x 0.15 mm x 0.75 mm) and a multiecho gradient echo (MEGE) sequence (TR = 1500ms, TE = 3.1, 7.1, 11.1, 15.1, 19.1 ms, voxel size =0.15 mm x 0.15 mm x 0.75 mm, Flip Angle = 30o). A cannula was placed into the tail vein before the start of the scan and gadolinium (0.1mmol/kg) was injected as a 100 uL bolus. The T1w RARE sequence was repeated, and Ferumoxytol (30mg/kg) was then injected as a 100 uL bolus. The multiecho gradient echo was repeated 24 hours post Ferumoxytol injection. T2* maps (before and after Ferumoxytol) were calculated, and changes in T2* calculated as post contrast – pre contrast. Amp B animals had a significant decline in changes in tumor T2* compared to vehicles (p<0.05, independent t-test) (Figure 1). We assessed the integrity of the BBB using the T1w RARE post-Gd sequence and found similar degrees of gadolinium enhancement, which is seen exclusively in the tumor (Figure 2).These results support our hypothesis that monocytes are engulfing USPIO particles and then migrating to the site of the tumor. Since both the vehicle and Amp B animals had a similar degrees of gadolinium we argue that the changes are not caused by increased damage to the BBB.The decreased T2* in the tumor after USPIO supports the hypothesis that Amp B increases lymphocyte trafficking into brain tumors and that this method can detect the treatment response. Using USPIO-MRI, we were able to observe the pharmacological activities of Amp B after only 7 days of treatment. This is promising method of monitoring immune-activating anti-cancer treatments that can be easily translated into clinical use.