UCL Discovery

Abstract

The effective magnetic resonance imaging (MRI) transverse relaxation rate R-2* was investigated as an early acute marker of the response of rat GH3 prolactinomas to the vascular-targeting agent, ZD6126. Multigradient echo (MGRE) MRI was used to quantify R-2*, which is sensitive to tissue deoxyhemoglobin levels. Tumor R-2* was measured prior to, and either immediately for up to 35 minutes, or 24 hours following administration of 50 mg/kg ZD6126. Following MRI, tumor perfusion was assessed by Hoechst 33342 uptake. Tumor R-2* significantly increased to 116 +/- 4% of baseline 35 minutes after challenge, consistent with an ischemic insult induced by vascular collapse. A strong positive correlation between baseline R-2* and the subsequent increase in R-2* measured 35 minutes after treatment was obtained, suggesting that the baseline R-2* is prognostic for the subsequent tumor response to ZD6126. In contrast, a significant decrease in tumor R-2* was found 24 hours after administration of ZD61126. Both the 35-minute and 24-hour R-2* responses to ZD6126 were associated with a decrease in Hoechst 33342 uptake. Interpretation of the R-2* response is complex, yet changes in tumor R-2* may provide a convenient and early MRI biomarker for detecting the antitumor activity of vascular-targeting agents.