Preclinical Evaluation of Safety and Biodistribution of Red Cell Microparticles (RMP): A Novel Hemostatic Agent

BACKGROUND: Uncontrollable bleeding is a major cause of mortality and morbidity worldwide. Effective hemostatic agents are urgently needed. Red cell microparticles (RMP) have been introduced as a promising new hemostatic agent [Thromb Haemost 2013, 110:751-60]. It acts to promote both primary (platelet function) and secondary (coagulation, thrombin generation) hemostasis, thus meeting the criteria for a universal hemostatic agent. However, although no overt adverse effects were observed in limited animal studies, more rigorous evaluation of safety as well as biodistribution of RMP were addressed in this report.

METHODS: RMP were prepared from Type O+ human RBC by high-pressure extrusion. Male rats were treated with either a 1× dose (=6x10¹⁰ RMP/kg BW), or 4×, or 20× dose, then monitored for at least 60 min for physiological responses of blood pressure, body and head temperature, hematocrit, and blood gases. A group of RMP-treated rats (n =6) were perfused with saline followed by formaldehyde - acetic acid - methanol mixture (1:1:8) at 4 hr post-RMP. Heart, brain and lung were harvested, sectioned and evaluated in blinded fashion by independent animal pathologist for signs of thrombosis or other pathology. For biodistribution study, RMP were labeled with Alexa-488 and unbound dye was removed by dialysis. At 15 min and 24 hours after infusion of labeled RMP, animals were perfused with saline for 2 min to remove labeled RMP in blood vessels, then organs were harvested, homogenized and sonicated. Fluorescence of the homogenates was measured by a fluorescence plate reader.

RESULTS: (i) Toxicity. No adverse effects of RMP were detected by monitoring the above physiological parameters at any of the 3 dose regimens. In addition, histological examination of brain, heart and lung sections by an animal pathologist revealed no microthrombi or other abnormality. (ii) Biodistribution. After injection of fluorescence-labeled RMP, levels of RMP in blood rose rapidly (1 min), then declined to baseline at 15 min, demonstrating rapid clearance. Fluorescence of tissue homogenates at 15 min revealed the highest fluorescence (FAU/g tissue) in liver, followed by spleen, lymph nodes, and lung. At 24 hr, fluorescence of the above-mentioned organs had decreased by 20 - 60%. Surprisingly, the organ of greatest fluorescent intensity at 24 hr had shifted to the kidneys, followed by liver, spleen, and lymph nodes.

DISCUSSION. Results further support safety of RMP as hemostatic agent. This is unexpected in view of reports finding proinflammatory properties of RMP obtained from stored blood or by calcium ionophore. This apparent discrepancy may be explained by the high-pressure extrusion method of production, since preliminary experiments show absence of lipid rafts on these RMP compared to RMP prepared by other ways. Lipid rafts are implicated as inflammatory mediators. Our biodistribution data demonstrated that RMP were sequestered mainly by liver, spleen, lymph nodes, and lung at 15 min post-RMP infusion. Our results also suggest that at 24 hr post-RMP infusion, many sequestered RMP have been degraded, leading to redistribution of fluorescent label to the kidney.