¹³C-MR Spectroscopic Imaging with Hyperpolarized [1-¹³C]pyruvate Detects Early Response to Radiotherapy in SCC Tumors and HT-29 Tumors

Purpose: X-ray irradiation of tumors causes diverse effects on the tumor microenvironment, including metabolism. Recent developments of hyperpolarized ¹³C-MRI enabled detecting metabolic changes in tumors using a tracer [1-¹³C]pyruvate, which participates in important bioenergetic processes that are altered in cancers. Here, we investigated the effects of X-ray irradiation on pyruvate metabolism in squamous cell carcinoma (SCCVII) and colon cancer (HT-29) using hyperpolarized ¹³C-MRI. Experimental Design: SCCVII and HT-29 tumors were grown by injecting tumor cells into the hind legs of mice. [1-¹³C]pyruvate was hyperpolarized and injected intravenously into tumor-bearing mice, and ¹³C-MR signals were acquired using a 4.7 T scanner. Results: [1-¹³C]pyruvate and [1-¹³C]lactate were detected in the tumor-bearing legs immediately after hyperpolarized [1-¹³C]pyruvate administration. The [1-¹³C]lactate to [1-¹³C]pyruvate ratio (Lac/Pyr) increased as the tumors grew in nonirradiated SCCVII tumors. The increase in Lac/Pyr was suppressed modestly with a single 10 Gy of irradiation, but it significantly decreased by further irradiation (10 Gy × 3). Similar results were obtained in HT-29; Lac/Pyr significantly dropped with fractionated 30 Gy irradiation. Independent ex vivo measurements revealed that the lactate dehydrogenase (LDH)activity and protein level were significantly smaller in the irradiated SCCVII tumors compared with the nonirradiated tumors, indicating that a decrease in LDH activity was one of the main factors responsible for the decrease of Lac/Pyr observed on ¹³C-MRI. Conclusions: Robust changes of Lac/Pyr observed in the HT-29 after the radiation suggested that lactate conversion from pyruvate monitored with hyperpolarized ¹³C-MRI could be useful for the evaluation of early response to radiotherapy.