ABSTRACT Nearly 1 million percutaneous coronary interventions (PCIs) are performed each year in the US. Approximately 70% of these cases are considered complex and require the treatment of calcified lesions, tortuous vessels, and multi-vessel disease, often with total or sub-totally occluded lesions. Many operators avoid these more complex cases, which are often not suitable for radial access approaches due to the limitations of smaller size guide catheters and lack of support required to effectively deliver coronary stents and other interventional devices. To overcome this limitation, guide extension catheters (GEC) have been developed to help deliver stents in these challenging anatomies. GECs are currently utilized in approximately 18% of all coronary interventions. However, GECs have several key shortcomings that have prevented their usage other than as a bailout option. Specifically, current GECs are unable to access more distal tortuous lesions in 28% of cases, resulting in the inability to deliver a stent in many cases. Blunt ended tubular GECs also have serious safety issues with coronary artery dissection and/or plaque or thrombus embolization. Given that GECs are used in ~18% of all PCI cases, these data represent a significant volume of patients that are not treated effectively. Hence, CrossLiner Inc. has developed a novel multi-functional GEC that combines the requirement for deeper catheter intubation with a microcatheter leading tip, and pre-dilatation balloon on the leading element, to allow safe crossing of severe lesions in tortuous coronary vessels, with balloon pre-dilatation, followed by crossing the lesion with the guide extension and then seamless stent delivery. The combination of a GEC with a seamless transition from the outer to the inner microcatheter-balloon provides the capability for easier stent delivery, which can be unsheathed at the lesion as opposed to being delivered from a poorly supported proximal location. This approach enabled by the CrossLiner GEC will substantially improve PCI in patients with complex lesions while reducing risks, procedural duration, radiation/contrast exposure, stent dislodgement, coronary dissection and perforation that exist with current GECs. Data from bench and in vivo Phase I studies support the utility of CrossLiner?s multi-functional GEC by demonstrating a significant reduction in the force required to pass a highly angulated segment while increasing the intubation depth compared to first-generation devices. Results in vivo showed an increase in intubation depth from 5.6 cm to 12.9 cm and a 41% reduction in the time to deliver a stent. Importantly, the CrossLiner GEC was able to successfully cross stented segments in all vessels in a swine model of complex lesions, while first-generation devices were stuck proximal to the stent or at the stent edge in ~75% of the coronary vessels tested. The CrossLiner system is now ready for final process refinement and design control development (Aim 1) followed by usability evaluation, and regulatory clearance (Aim 2). Successful execution of these Phase II studies will position the CrossLiner GEC system for full commercialization of this highly innovative approach to improve the treatment for complex PCI patients.