Abstract 2068: RRx-001 oxidation of redox sensitive protein thiols in tumors measured by Gd-LC7-SH enhanced MRI In preclinical tumor models
Authors: Raghunand Natarajan , Scicinski Jan , Oronsky Bryan , Guntle Gerald , Bruckheimer Elizabeth , Korn Ron 
Published: May 2015

RRx-001 is a member of the novel dinitroazetidine class of anticancer agents that profoundly perturbs the thiol redox potential of cancer cells while inducing damaging ROS/RNS. A Phase 1 study investigating the safety and tolerability of RRx-001 has been completed and Phase 2 studies are ongoing. Pre-clinical studies indicate that RRx-001 selectively alkylates glutathione and a specific thiol on hemoglobin, resulting in a pro-oxidant effect in tumors. We have previously reported the development of thiol-containing DOTA-based chelates of gadolinium as redox-sensitive MRI contrast agents. In the present preclinical MRI study we investigated the pharmacodynamics and mechanism of action of RRx-001 using the novel thiol-bearing contrast agent Gd-LC7-SH.

SCID mice were inoculated in the flank with either CHP-100, HT-29, or PANC-1 cells. Mice were imaged on a 7 Tesla Bruker Biospec® small animal MRI scanner when tumors had grown to 250-400 mm3 in size. Mice were anesthetized using isoflurane and cannulated at the tail vein for injections using a zero dead volume i.v. line for administration of Gd-LC7-SH or RRx-001. Longitudinal relaxation time (T1) maps of the tumor were acquired pre-contrast and at various times post-contrast to 60 min post-injection of 0.05 mmol/kg Gd-LC7-SH. Mice were imaged before treatment and at 1h, 24 h and 72 h post-treatment with 10 mg/kg RRx-001.

Gd-LC7-SH spontaneously binds to thiol targets following i.v. administration. The fraction of gadolinium that is bound to macromolecular targets such as plasma albumin and exofacial protein thiols (EPTs) is protected from renal clearance, and produces a prolonged decrease in tumor T1 on MRI. We have quantified this decrease in tumor T1 by the parameter -T1, calculated as pre-contrast tumor T1 minus tumor T1 at 40-60 min post Gd-LC7-SH. The larger the magnitude of -T1, the greater is the retention of Gd-LC7-SH in the tumor. In all 3 tumor types, the tumor -T1 at 1 h post-drug was significantly smaller than pre-drug tumor -T1 (p&lt;0.02). In the HT-29 and PANC-1 tumors the -T1 at 72 h post-drug remained smaller than baseline -T1 (p&lt;0.05). These observations indicate decreased tumor retention of Gd-LC7-SH following treatment with RRx-001, which is consistent with a decrease in availability of reduced albumin and EPTs in the tumor.

The previously reported redox activity of RRx-001 together with its very short half-life in vivo suggests an indirect effect on albumin and exofacial thiols that manifests as smaller -T1 values on Gd-LC7-SH MRI imaging. The anti-proliferative activity of RRx-001 may not only be due to glutathione depletion and NO release under hypoxia, but also to an increase in intratumoral ROS burden leading to a direct redox modulation of exofacial thiols integral to tumor protein function. Additional studies are planned to confirm this postulate.

Citation Format: Natarajan Raghunand, Jan Scicinski, Bryan Oronsky, Gerald Guntle, Elizabeth Bruckheimer, Ron Korn. RRx-001 oxidation of redox sensitive protein thiols in tumors measured by Gd-LC7-SH enhanced MRI In preclinical tumor models. [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 2068. doi:10.1158/1538-7445.AM2014-2068