Pete Lollar, MD
Dr. Lollar indicated no relevant conflicts of interest.
Korin N, Kanapathipillai M, Matthews BD , et al. Shear-activated nanotherapeutics for drug targeting to obstructed blood vessels. Science. 2012;337:738-742.
If you hold a wet ball of sand and rub your hands together, the applied shear stress disperses the sand into its individual grains. Korin et al. in the laboratory of Donald Ingber used this analogy to describe novel particles they call shearactivated nanotherapeutics (SA-NTs). Shear stresses can increase more than 100-fold to greater than 1,000 dynes/cm2 at sites of vascular constriction due to stenosis or thrombosis. Platelets are activated by high shear stress in these regions, adhere to the surface of narrowed vessels, and contribute to the pathologic process.
The authors used the idea of this phenomenon to target SA-NTs to constricted blood vessels. To do this, they fabricated SA-NTs as platelet-sized, ~4 μm aggregates composed of ~0.2 μm biodegradable poly(lactic-co-glycolic acid) “nanoparticles” (NPs). Flow experiments revealed that SA-NTs remained intact at physiologic levels of shear stress but broke up into their constituent NPs at greater than 100 dynes/cm2. Using a microfluidic device containing a constricted lumen lined with cultured endothelial cells, the authors demonstrated that released NPs accumulated on cells downstream but not upstream of the constriction.
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The authors constructed SA-NTs containing fluorescent NPs coated with the fibrinolytic agent, recombinant tissue plasminogen activator (tPA). The properties of the tPA SANTs were studied in a ferric chloride-induced model of mesenteric artery thrombosis in mice. Vessel narrowing in this model produced shear stresses of 450 dynes/cm2 as measured by optical Doppler velocitometry. When injected intravenously after vascular injury, tPA SA-NTs accumulated at sites of thrombosis and produced thrombolysis. When injected intravenously before vessel injury, tPA SA-NTs produced a significant increase in vessel occlusion time compared with an equal dose of soluble tPA. The authors also injected preformed fibrin clots into their constricted lumen microfluidic device and found that tPA SA-NTs produced clot lysis significantly faster than an equivalent dose of soluble tPA. Fibrin clots also were injected into mouse lungs in an ex vivo ventilation-perfusion system. tPA NPs localized selectively at regions of vascular occlusion. Lysis of the fibrin emboli by the tPA-NPs resulted in normalization of pulmonary artery pressure levels at a dose in which an equivalent amount of soluble tPA had no effect. The dose of soluble tPA required to normalize pulmonary artery pressures (5 μg/ml) was 100-fold greater than the effective dose of tPA NPs. Finally, the authors produced an in vivo pulmonary embolism model by infusing fluorescent fibrin clots intravenously into mice. When infused intravenously after embolization, tPA SA-NTs resulted in a reduction of both total clot area and clot number. Additionally, a fatal pulmonary embolism model was developed by injecting larger fibrin clots that lodged in the main pulmonary arteries. Mice then were infused with tPA SA-NTs or a control vehicle. All control animals died within one hour of embolization (n = 7), whereas six out of seven tPA SA-NT mice survived.
The study by Korin et al. provides proof-of-principle that a physical phenomenon – shear stress-dependent accumulation of biologically active agents – can be exploited to target drugs to thrombotic sites. Local delivery of drug-laden SA-NTs may lower required doses and decrease adverse events such as bleeding. This approach is potentially applicable to the clinical use of tPA and other antithrombotic agents.
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