Exosomes are small (50-100 nm in diameter) vesicles secreted from various mammalian cells. Exosomes have been correlated with tumor antigens and anti-tumor immune responses and may represent cancer biomarkers. Herein, we report on the development of an aptamer-based electrochemical biosensor for quantitative detection of exosomes. Aptamers specific to exosome transmembrane protein CD63 were immobilized onto gold electrode surfaces and incorporated into a microfluidic system. Probing strands pre-labeled with redox moieties were hybridized onto aptamer molecules anchored on the electrode surface. In the presence of exosomes these beacons released probing strands with redox reporters causing electrochemical signal to decrease. These biosensors could be used to detect as few as 1×10(6) particles/mL of exosomes, which represents 100-fold decrease in the limit of detection compared to commercial immunoassays relying on anti-CD63 antibodies. Given the importance of exosome-mediated signal transmission among cells, our study may represent an important step towards development of a simple biosensor that detects exosomes without washing or labeling steps in complex media.

Targeted cancer therapies are designed to deactivate signaling pathways used by cancer cells for survival. However, cancer cells are often able to adapt by activating alternative survival pathways, thereby acquiring drug resistance. An emerging theory is that autocrine or paracrine growth factor signaling in the cancer microenvironment represent an important mechanism of drug resistance. In the present study we wanted to examine whether paracrine interactions between groups of melanoma cells result in resistance to vemurafenib - an FDA approved drug targeting the BRAF mutation in metastatic melanoma. We used a vemurafenib-resistant melanoma model which secretes fibroblast growth factor (FGF)-2 to test our hypothesis that this is a key paracrine mediator of resistance to vemurafenib. Sensitive cells treated with media conditioned by resistant cells did not protect from the effects of vemurafenib. To query paracrine interactions further we fabricated a microfluidic co-culture device with two parallel compartments, separated by a 100 μm wide hydrogel barrier. The gel barrier prevented resorting/contact of cells while permitting paracrine cross-talk. In this microfluidic system, sensitive cells did become refractive to the effects of vemurafenib when cultured adjacent to resistant cells. Importantly, incorporation of FGF-2 capture probes into the gel barrier separating the two cell types prevented onset of resistance to vemurafenib. Microfluidic tools described here allow for more sensitive analysis of paracrine signals, may help better understand signaling in the cancer microenvironment and may enable development of more effective cancer therapies.