Poster O13 - The influence of hyperlipidemia on magnesium biocorrosion in vitro and in vivo

Introduction: Cardiovascular disease is the leading cause of death globally, affecting an estimated 127 million people in the United States alone[1]. In the case of atherosclerosis, the standard of care includes deployment of metallic stents into stenotic lesions, although it is widely accepted that the permanent nature of metal stents is not required. Recent advancements have led to the development of temporary bioabsorbable metal stents. Studies have demonstrated the safety of absorbable metals such as magnesium (Mg), which has ultimately led to the European market approval of a Mg based coronary stent. Although the clinical application of absorbable metal stents has been generally successful, acute Mg stent failure via rapid degradation has been seen in certain clinical cases. We hypothesize that the heterogenous degradation behavior of Mg stents in the clinic is driven by the perturbed atherosclerotic microenvironment, consisting largely of changes in the blood lipid profile and increases of lipid laden macrophage-based inflammation in atherosclerotic plaques. Unfortunately, current in vitro and in vivo testing methods do not fully recapitulate the complex atherosclerotic microenvironment experienced by Mg materials in patients. Therefore, the goal of this study is to clarify the contribution of the atherosclerotic microenvironment on Mg material performance using disease specific in vitro and in vivo testing methodologies. We utilize a well characterized hypercholesterolemic transgenic mouse model (APOE-/-), wherein a Mg wire is surgically implanted within the abdominal aorta, and a lipid mimicking in vitro corrosion assay to elucidate changes in Mg degradation from clinically relevant microenvironments.

Materials and

Methods: In vitro- Mg-Y-Nd alloys—WE43 (4% Y, 3% Nd) and WE22 (2% Y, 2% Nd)—100 um diameter wires were supplied from Fort Wayne Metals. Wires were cleaned with a chromic acid (200g/L) and silver nitrate (2 g/L) solution for 10 minutes, followed by DI water for 1 minute and dried in 200 proof ethanol. Wires were suspended from microcentrifuge tube caps and 1mL of DMEM + 10%FBS + 1% p/s or Lipid Medium (DMEM + 10%FBS + 1% p/s supplemented with 100 mg/dL LDL, 25 mg/dL HDL, 200 mg/dL triglycerides) in a CO2 incubator. Media was replaced every 24 hours; wire was removed after 1, 3, and 7 days, then dried and mounted in epoxy for cross sectional analysis. The mounted wires were polished, then imaged using an optical microscope capable of z axis stepping. Images were analyzed in ImageJ. Scanning electron microscopy (SEM) images and energy dispersive x-ray spectroscopy (EDX) maps were taken with a JEOL JSM-6510LV with an accelerating voltage of 20keV.
In vivo- WE22 and WE43 wires were implanted into the abdominal aorta of APOE-/- and C57BL/6J mice for 1-30 days (n=3 per time point). Implants were excised and examined with histology and metallographic analysis.

Results, Conclusions, and Discussions: Mg based stents implanted into coronary vessels with atherosclerotic lesions will experience a highly heterogeneous corrosion environment. Additionally, the blood profile of patients with atherosclerosis routinely displays elevated levels of cholesterol, triglycerides, and low-density lipoprotein. This is further confounded by the fact that the blood chemistry presentation between males and females with atherosclerosis is markedly different. In this study, the biocorrosion performance of two clinically relevant Mg alloys in media recapitulating elevated lipid levels was evaluated. The materials are microstructurally distinct yet come from the same alloy “WE” family. We use planimetry, SEM-EDX imaging, and histology to evaluate corrosion and inflammation respectively. Interestingly, lipid supplementation had antithetical effects on WE43 vs. WE22 at acute timepoints, with WE22 in lipid medium experiencing a reduction in corrosion parameters when compared to DMEM alone (7.18±6.20% vs. 25.06±4.40% cross sectional area reduction, Fig. 1) at 1 day, and WE43 incubated in lipid medium displaying increased corrosion activity at 1 day when compared to DMEM (26.27±11.06% vs. 10.03±1.95% cross sectional area reduction, Fig. 1). Surprisingly for WE22, this early term delay in corrosion preludes a significant increase in activity at 3 days for the lipid supplemented group. We evaluated the corrosion progression of WE22 in the abdominal aorta of a normocholesterolemia and hypercholesterolemia mouse model (APOE-/-), and show that indeed WE22 corrosion is accelerated in mice with atherosclerotic lesions (Fig. 2). Histological analysis demonstrated an increased macrophage presence in the neointima of APOE-/- animals, and markedly increased lipid laden plaques. We speculate that the early term formation of the oxide film on WE22 wires in lipid medium is structurally and chemically distinct, resulting in reduced passivation of the alloy. While the corrosion progression of WE43 in lipid media does not appear modified at 3 days, ongoing experiments out to longer timepoints will determine whether a change in corrosion kinetics manifests at later timepoints in vitro and in vivo. These studies demonstrate the significant impact of atherosclerotic microenvironments on the biocorrosion progression of Mg alloys, and future work will decouple the contribution of lipid laden macrophages towards Mg corrosion progression.

Acknowledgements (Optional): 1. Martin, S. S., et al. (2024). "2024 Heart Disease and Stroke Statistics: A Report of US and Global Data From the American Heart Association." Circulation 149(8): e347-e913.