Nitrite may serve as a combination partner and a biomarker for the anti-cancer activity of RRx-001.

Abstract

RRx-001 is an anti-cancer immunotherapeutic that increases the sensitivity of drug resistant tumors via multiple mechanisms which involve binding to hemoglobin and enhancing nitrite reductase activity of deoxyhemoglobin. In the present study, the effect of clinically used doses of RRx-001 on erythrocyte deformability was examined. A dose dependent effect of RRx-001 (1-1000 micro molar) on erythrocyte deformability was measured by ektacytometer under hypoxia (n =8). Low dose RRx-001 (20 micro molar) in the presence of ODQ (1H-[1,2,4]Oxadiazolo[4,3-a]quinoxalin-1-one), L-NAME (L-NG-Nitroarginine methyl ester) or nitrite were examined both in normoxia and hypoxia. Intracellular nitric oxide (NO) levels were measured fluorometrically with DAF-FM-DA. Higher doses of RRx-001 (100, 1000 micro molar) significantly decreased erythrocyte deformability under hypoxia (p <0.01; p <0.05, respectively). RRx-001 (20 micro molar), alone or in combination with ODQ or L-NAME, did not change deformability. However, RRx-001 and nitrite caused an increase in deformability (p <0.01) under hypoxia. RRx-001 induced NO production was more pronounced in the presence of nitrite (p <0.05). Co-administration of RRx-001 and nitrite under hypoxic conditions results in a significant increase in erythrocyte deformability that is related to increased NO production. We suggest that measurement of serum nitrite level in RRx-001 treated cancer patients should be routinely undertaken and supplemented if levels are low for maximal activity.