**1. Introduction**

Drug-coated balloons (DCBs) represent a new therapeutic approach to treat peripheral arterial disease (PAD) [1–5]. In the United States, PAD a ffects more than eight million people, with an annual cost of roughly \$21 billion [6]. Traditionally, endovascular treatment of PAD has been performed by balloon angioplasty or the placement of a permanent metallic stent [7,8]. However, results are poor, with 50–85% of patients developing hemodynamically significant restenosis (re-occlusion), and 16–65% developing occlusions within 2 years post-treatment [9,10]. The use of anti-proliferative drugs in combination with bare metal stents, i.e., drug-eluting stents (DES), was a major breakthrough and highly successful in treating coronary artery disease [11,12]. However, stents have shown very poor clinical outcomes in treating PAD, as they are subjected to biomechanical stress and severe artery deformation (twisting, bending, and shortening), leading to high fracture rates (up to 68%) and restenosis [13].

DCBs, which were FDA-approved for the treatment of PAD in late 2014, provide a new therapeutic approach for interventionalists to practice a 'leave nothing behind' procedure, preserving future treatment options DCBs are angioplasty balloons directly coated with an anti-proliferative therapeutic drug and an excipient (drug carrier) [1,14–18]. The excipient enhances the adhesion of the drug to the balloon surface, increases the stability of the drug coating during handling and delivery, and maximizes drug retention to the targeted arterial segment. [18–24] Current DCBs excipients include polysorbate and sorbitol, urea, polyethylene glycol (PEG) and butyryl-tri-hexyl citrate (BTHC). The rationale for the selection of these various excipients varies. For example, excipients such as polysorbate and PEG are known cosolvents of paclitaxel [25,26], which can alter the vessel interaction of the drug with the DCB device. Conversely, urea acts to increase paclitaxel release at the lesion [18] and PEG has been shown to bind to hydroxylapatite, a primary component of calcified atherosclerotic lesions [17,19,23,24], thereby improving local pharmacodynamics.

However, more recent pre-clinical studies have demonstrated the potential of DCB excipients to embolize and travel downstream to distal tissue post-treatment [27,28]. As peripheral arteries undergo severe mechanical deformation, excipients should aid in maintaining drug residency on the luminal surface, in particular at the early time phase, prior to the buildup of tissue, following delivery onto the luminal surface of the artery. Therefore, novel excipients that are capable of maintaining drug residency while minimizing downstream or o ff-target e ffects are needed. Keratins are a class of proteins that can be derived from numerous sources, including from human hair. Keratins have been shown to achieve the sustained release of small-molecule drugs and growth factors [29,30]. Further, keratin films have been reported for use in vascular grafts to reduce thrombosis, suggesting their utility in cardiovascular applications [31]. The goal of this study was thus to examine the ability to use an oxidized form of keratin (known as keratose (KOS)) as a new drug carrier excipient to aid in the delivery and retention of the anti-proliferative drug, paclitaxel. Specifically, the mobility, retention and biological impact of a KOS–paclitaxel-coated DCB was determined using *ex vivo* and *in vivo* models.
