J. Khoury, S. Kirkpatrick, D. Shashkov
Keywords: ANAB, polymer-free, drug elution
Summary:Much discussion has focused on inflammatory and thrombogenic influences of polymer coatings on drug-eluting stents and their potential for late stent thrombosis. Similarly, a means to deliver proteins such as BMP or antibiotics on orthopedic or dental implants without a binding polymer is highly desirable. ANAB, a process which employs accelerated neutral atoms under vacuum, has been developed for polymer-free drug delivery devices. To study the ANAB-controlled elution of drugs, CoCr stents were surface coated with rapamycin (RAP); one group received an ANAB processing while another group did not. Stents were placed in human plasma to simulate in vivo drug elution; drug weights were recorded daily and compared to historical data of the commercially available Cypher® stent. XPS analysis of ANAB-treated RAP surface reveals increased carbon concentration, indicating formation of carbon-rich scaffolds on the surface. Stents were placed in plates and 10,000 endothelial cells were seeded, allowed to attach for 24 hours, and counts were measured by MTS assay. RAP on control stents eluted off within 24 hours and increased cell attachment from 576±403 cells (pre-eluted) to 8876±1170 (24h post-eluted). ANAB-modified stents resulted in a slower elution of RAP and did not allow additional cell attachment on post-eluted stents over the 7-day study. Importantly, traditional polymer coatings maintain drug availability for an extended time, while ANAB-treatment results in a progressive and complete elution, enabling re-endothelialization. To study the effects of ANAB processing on potential protein degradation, 1µg BMP-2 was applied on the surface of titanium. BMP from untreated or ANAB-treated titanium was extracted from the surface, and concentrations of BMP-2 were evaluated by ELISA or subjected to SDS-PAGE to visualize degraded protein products. ELISA results revealed 0.8μg±0.03μg remaining after ANAB-treatment, and the remaining protein displayed no degradation as assayed by silver-stain. To study the effects of ANAB on temperature sensitive therapeutics such as antibiotics, kanamycin was applied to titanium disks. Untreated and ANAB-treated disks were then placed on Nutrient agar dishes containing S.aureus and incubated for 4 days. Measurements of the zone of inhibition (ZI) was recorded daily and compared to kanamycin filter disks and bare titanium. Studies revealed an 8mm ZI by kanamycin on filter paper at 48 hours which decreased to 6mm at 72h and 0mm at 96h post placement on S.aureus. Kanamycin placed on titanium disks revealed a 6mm ZI at 48h, 4mm at 72h, and 2mm at 96h. ANAB-treated disks revealed a 6mm ZI at 48h, 5mm at 72h, and 4mm at 96h. Bare Ti disks had no ZI; ANAB-treated disks showed slower degradation of the antibiotic as compared to untreated or filter disks by means of ZI. Therefore, ANAB processing results in a controlled release of drugs and therapeutics without the use of a binding polymer. ANAB offers the potential to create implants containing predetermined doses of drugs or therapeutics. Controlling the release rate of the therapeutic may be achieved by multi-layering drug and ANAB treatments. Such controlled drug release without the use of binding polymers is crucial in development of next generation implants.