Abstract LB-161: Monitoring early therapeutic response by measuring extracellular pH in a tumor model with acidoCEST MRI
Authors: Akhenblit Paul J. , Howison Christine M. , Malm Scott W. , Chen Liu Qi , Baker Amanda F. , Pagel Mark D. 
Published: May 2015

Abstract
Introduction: The purpose of the present study is to examine whether tumor extracellular pH (pHe) is a sensitive and specific biomarker for measuring immediate, short term drug response to an mTOR inhibiting agent (RAD001, everolimus) by a novel, non-invasive MRI-based pH imaging method (acidoCEST).
Methods: Two groups of SCID mice with Granta 519 human mantle cell lymphoma flank xenograft models were used in the study. First, to investigate if RAD001 has anti-tumor activity in the Granta 519 model, mice were injected IP with 5 mg/kg per day with RAD001 or vehicle (DMSO) (N=12 per group) every day until tumors reached 2000 mm3. Survival analysis was conducted using GraphPad software. In a separate correlative imaging study mice with tumors averaging 400 mm3 (N=8) were scanned with acidoCEST MRI pretreatment, one day post initial treatment, and one week after continuous treatment. To prepare for each acidoCEST MRI scan session, a mouse was anesthetized with 1.5% isoflurane, respiration rate and body temperature were monitored, and body temperature was maintained at 37.0᠑C with warm air. A bolus of 200 μL of 370 mgI/mL iopamidol was injected IV prior to scanning, followed by continuous infusion of iopamidol at 150 uL/hour during scanning. A CEST-FISP MRI protocol was performed with a Bruker Biospec 7T MRI scanner with a 72 mm volume coil using previously published procedures. The results were analyzed using Matlab, following previously published procedures. The average pHe was determined for each tumor. Pixels which registered a pHe of 6.00 to 6.99 were grouped together as the “acidic fraction,” and those pixels with pH ≥7 were collectively grouped together as the “neutral fraction.” The percent uptake of the contrast agent in the tumor area was measured to assess vascular perfusion in the tumor microenvironment.
Results: RAD001 treatment results in a significant survival advantage in Granta 519 flank xenograft tumors. AcidoCEST imaging of tumors showed pretreatment an average pHe value of 6.82, demonstrating that the tumor model was moderately acidic. The standard deviation of pixelwise pHe values was 0.24, indicating moderate heterogeneity of pHe in the measured tumor area. Post initial treatment, the pHe had a statistically significant increase to 6.92 with a standard deviation of 0.14, indicating a decrease in average acidosis and a decrease in heterogeneity of acidosis. After continuous treatment, the pHe dropped to 6.74 with a standard deviation of 0.26, suggesting a return to a more normal metabolic rate. In keeping with the trend, the acidic fraction of the tumor decreased from 33.1% to 19.6%, before increasing to 49% during this study. The tumor growth study showed a temporary growth delay before returning to a more normal growth rate, which paralleled the acidoCEST MRI results.
Conclusion: Tumor pHe measured with acidoCEST MRI can detect early response to RAD001. Additional pre-clinical studies are warranted to determine if this biomarker is a robust measure of early therapeutic response. In addition, translation of acidoCEST MRI to the clinic should be pursued to provide this biomarker for clinical studies.
Citation Format: Paul J. Akhenblit, Christine M. Howison, Scott W. Malm, Liu Qi Chen, Amanda F. Baker, Mark D. Pagel. Monitoring early therapeutic response by measuring extracellular pH in a tumor model with acidoCEST MRI. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr LB-161. doi:10.1158/1538-7445.AM2014-LB-161