Sickle Erythrocytes Target Cytotoxics to Hypoxic Tumor Microvessels and Potentiate a Tumoricidal Response

Abstract

Resistance of hypoxic solid tumor niches to chemotherapy and radiotherapy remains a major scientific challenge that calls for conceptually new approaches. Here we exploit a hitherto unrecognized ability of sickled erythrocytes (SSRBCs) but not normal RBCs (NLRBCs) to selectively target hypoxic tumor vascular microenviroment and induce diffuse vaso-occlusion. Within minutes after injection SSRBCs, but not NLRBCs, home and adhere to hypoxic 4T1 tumor vasculature with hemoglobin saturation levels at or below 10% that are distributed over 70% of the tumor space. The bound SSRBCs thereupon form microaggregates that obstruct/occlude up to 88% of tumor microvessels. Importantly, SSRBCs, but not normal RBCs, combined with exogenous prooxidant zinc protoporphyrin (ZnPP) induce a potent tumoricidal response via a mutual potentiating mechanism. In a clonogenic tumor cell survival assay, SSRBC surrogate hemin, along with H2O2 and ZnPP demonstrate a similar mutual potentiation and tumoricidal effect. In contrast to existing treatments directed only to the hypoxic tumor cell, the present approach targets the hypoxic tumor vascular environment and induces injury to both tumor microvessels and tumor cells using intrinsic SSRBC-derived oxidants and locally generated ROS. Thus, the SSRBC appears to be a potent new tool for treatment of hypoxic solid tumors, which are notable for their resistance to existing cancer treatments.

Drug-loaded sickle cells programmed ex vivo for delayed hemolysis target hypoxic tumor microvessels and augment tumor drug delivery

“…These findings indicate an affinity of allogeneic SSRBCs for tumor microvasculature and is consistent with earlier findings using human SSRBCs in mice [2]. The enhanced ability of SSRBCs to sequester in tumor microvasculature may be attributed in part to SSRBCs’ expression of multiple adhesion receptors which bind cognate ligands in hypoxic tumor microvessels [2, 4]. Ongoing investigation is addressing this point together with a biodistribution and toxicity follow-on study.…”

“…2 shows that the drug-loaded SSRBCs localized in tumors to a significantly greater degree (≥4.75x) than liver and spleen, sites of known SSRBC accumulation in human sickle cell anemia patients [31-33]. These findings indicate an affinity of allogeneic SSRBCs for tumor microvasculature and is consistent with earlier findings using human SSRBCs in mice [2]. The enhanced ability of SSRBCs to sequester in tumor microvasculature may be attributed in part to SSRBCs’ expression of multiple adhesion receptors which bind cognate ligands in hypoxic tumor microvessels [2, 4].…”

“…In the current studies we used a small volume of photo-oxidized SSRBCs (200μL) in mice which when translated to humans amounts to less than 1 unit of packed red blood cells. In our previous studies, a therapeutic effect was induced in mice with a single SSRBC infusion in the identical volume with no trace of hemolysis [2]. Thus, alloimmunization and transfusion reactions with our system could be minimized by using a single small volume infusion of SSRBCs coupled with careful screening of the SS cells for ABO, Rh C, D, E, and Kell antigens.…”

“…However, the risk of SSRBC treatment in these settings may be no greater than those of many widely used cancer cytotoxics (e.g., doxorubicin), antiangiogenic agents (e.g., Avastin), and biologics (e.g., IL-2) which are known to exacerbate these very same vascular conditions. We previously reported that SSRBC infusions in tumor bearing mice showed a tumoricidal response with no significant constitutional toxicity or histologic evidence of tissue injury [2]. This evaluation also included histology of brain microvasculature which was devoid of inflammation, thrombosis, or necrosis.…”

“…Treatment failure of such hypoxic tumors is largely attributable to the nature of the tumor microenvironment whose neovasculature is poorly perfused leading to impaired drug transport and low tumor cell drug concentrations [1]. Previous studies have shown that sickle red blood cells (SSRBCs) but not normal RBCs (nRBCs) possess the unique ability to selectively target such hypoxic solid tumors wherein they form microaggregates, occlude tumor microvessels, and release powerful intrinsic pro-oxidants that contribute to the tumoricidal response[2-3]. The unique ability of activated SSRBCs, but not normal RBCs (nRBCs), to target and occlude tumor microvasculature is based in part on their expression of multiple adhesion receptors which bind cognate ligands expressed in tumor microvessels in response to hypoxic conditions [2, 4].…”