The Human Acellular Vessel for Vascular Reconstruction or Bypass: A Novel Biologic Conduit for Vascular Bypass and Repair

Manufacturing and validation of small-diameter vascular grafts: A mini review

The field of small-diameter vascular grafts remains a challenge for biomaterials scientists. While decades of research have brought us much closer to developing biomimetic materials for regenerating tissues and organs, the physiological challenges involved in manufacturing small conduits that can transport blood while not inducing an immune response or promoting blood clots continue to limit progress in this area. In this short review, we present some of the most recent methods and advancements made by researchers working in the field of small-diameter vascular grafts. We also discuss some of the most critical aspects biomaterials scientists should consider when developing lab-made small-diameter vascular grafts.

Comparative outcomes of arterial bypass using the human acellular vessel and great saphenous vein in patients with chronic limb ischemia

OBJECTIVE

The Human Acellular Vessel (HAV) is a novel, off-the-shelf biologic conduit being evaluated for arterial reconstructions. Regulatory studies in peripheral arterial disease (PAD) to date have consisted of single-arm cohorts with no comparator groups to contrast performance against established standards. This study aimed to compare outcomes of the HAV with autologous great saphenous vein (GSV) in patients with advanced PAD undergoing infrageniculate bypass.

METHODS

Patients with advanced PAD and no autologous conduit who underwent bypass with the 6-mm diameter HAV (Group 1; n = 34) (March 2021-February 2024) were compared with a multicenter historical cohort who had bypass with single-segment GSV (group 2; n = 88) (January 2017-December 2022). The HAV was used under an Investigational New Drug protocol issued by the Food and Drug Administration (FDA) under the agency's Expanded Access Program.

RESULTS

Demographics were comparable between groups (mean age 69 ± 10 years; 71% male). Group 1 had higher rates of tobacco use (37 pack-years vs 28 pack-years; P = .059), coronary artery disease (71% vs 43%; P = .007), and prior coronary artery bypass grafting (38% vs 14%; P = .003). Group 1 had more patients classified as wound, ischemia, and foot infection clinical stage 4 (56% vs 33%; P = .018) and with previous index leg revascularizations (97% vs 53%; P < .001). Both groups had a similar number of patients with chronic limb-threatening ischemia (Rutherford class 4-6) (88% vs 86%; P = .693) and Global Anatomic Staging System stage III (91% vs 96%; P = .346). Group 1 required a composite conduit (two HAV sewn together) in 85% of bypasses. The tibial vessels were the target in 79% of group 1 and 100% of group 2 (P < .001). Group 1 had a lower mean operative time (364 minutes vs 464 minutes; P < .001). At a median of 12 months, major amputation-free survival (73% vs 81%; P = .55) and overall survival (84% vs 88%; P = .20) were comparable. Group 1 had lower rates of primary patency (36% vs 50%; P = .044), primary-assisted patency (45% vs 72%; P = .002), and secondary patency (64% vs 72%; P = .003) compared with group 2.

CONCLUSIONS

Implanted under Food and Drug Administration Expanded Access provisions, the HAV was more likely to be used in redo operations and cases with more advanced limb ischemia than GSV. Despite modest primary patency, the HAV demonstrated resilience in a complex cohort with no autologous conduit options, achieving good secondary patency and providing major amputation-free survival comparable with GSV at 12 months.

Outcomes of lower extremity arterial bypass using the Human Acellular Vessel in patients with chronic limb-threatening ischemia

OBJECTIVE

Patients with chronic limb-threatening ischemia (CLTI) and no great saphenous vein to use as a conduit for arterial bypass have a high risk for amputation despite advances in medical and endovascular therapies. This report presents findings from a U.S. Food and Drug Administration (FDA) supported study of the Human Acellular Vessel (HAV) (Humacyte Inc.) used as a conduit for arterial bypass in patients with CLTI and inadequate or absent autologous conduit.

METHODS

The HAV is a 6-mm, 40-cm vessel created from human vascular smooth muscle cells seeded onto a polyglycolic acid scaffold pulsed in a bioreactor for 8 weeks as cells proliferate and the scaffold dissolves. The resultant vessel is decellularized, creating a nonimmunogenic conduit composed of collagen, elastin, and extracellular matrix. The FDA issued an Investigational New Drug for an intermediate-sized, single-center study of the HAV under the agency's Expanded Access Program in patients with advanced CLTI and inadequate or absent autologous conduit. Technical results and clinical outcomes were analyzed and reported.

RESULTS

Between March 2021 and July 2023, 29 patients (20 males; mean age, 71 ± 11 years) underwent limb salvage operation using the HAV as a bypass conduit. Most patients had advanced CLTI (Rutherford class 5/6 in 72%; wound, ischemia, and foot infection stage 3/4 in 83%), and 97% had previously failed revascularization(s) of the extremity. Two HAVs were sewn together to attain the needed bypass length in 24 patients (83%). Bypasses were to tibial arteries in 23 patients (79%) and to the popliteal artery in 6 (21%). Technical success was 100%, and the 30-day mortality rate was 7% (2 patients). With 100% follow-up (median, 9.3 months), the limb salvage rate was 86% (25/29 patients). There were 16 reinterventions to restore secondary patency, of which 15 (94%) were successful. Primary and secondary patency of the HAV at 9 months were 59% and 71%, respectively.

CONCLUSIONS

The HAV has demonstrated short- to intermediate-term safety and efficacy as an arterial bypass conduit in a complex cohort of patients with limb-threatening ischemia and no autologous options. This experience using the FDA's Expanded Access Program provides real-world data to inform regulatory deliberations and future trials of the HAV, including the study of the vessel as a first-line bypass conduit in less severe cases of chronic limb ischemia.

Outcomes of Arterial Bypass With the Human Acellular Vessel for Chronic Limb-Threatening Ischemia Performed Under the FDA Expanded Access Program

OBJECTIVE

To report outcomes of the human acellular vessel (HAV) implanted for limb salvage through the Food and Drug Administration (FDA) Expanded Access Program for patients with chronic limb-threatening ischemia with no autologous conduit.

METHODS

The HAV is a bioengineered vascular conduit designed with human vascular smooth muscle cells. The product is under regulatory study. From April 2019 to November 2021, the HAV was implanted in 14 patients (12 men; mean age, 62±14 years) at 3 US centers. Each case was performed with a single-use investigational new drug Expanded Access Program issued by the FDA. Institutional review board approval was obtained; technical and clinical outcomes were analyzed.

RESULTS

A single 6-mm-diameter (40-cm-long) HAV was implanted in 9 patients; 5 patients required 2 HAVs sewn together as a composite. Technical success was 100%. Median follow-up was 12 (range, 1 to 41) months. Primary and secondary patency rates were 72% and 81% at 12 months; assisted primary patency was attained in 4 patients. Amputation-free survival was 93% at 6 months and 77% at 12 months. All patients with a patent HAV experienced clinical improvement with no HAV-related infections or adverse events. There were 4 deaths in the cohort, late mortality unrelated to the HAV.

CONCLUSION

The HAV is a safe and effective "off-the-shelf" biologic conduit. This experience from the FDA Expanded Access Program in this population with few alternative limb salvage options will help guide regulatory deliberations for patients with lower extremity ischemia and no autologous bypass conduit options.

Photochemical Modification of the Extracellular Matrix to Alter the Vascular Remodeling Process

Therapeutic interventions for vascular diseases aim at achieving long-term patency by controlling vascular remodeling. The extracellular matrix (ECM) of the vessel wall plays a crucial role in regulating this process. This study introduces a novel photochemical treatment known as Natural Vascular Scaffolding, utilizing a 4-amino substituted 1,8-naphthimide (10-8-10 Dimer) and 450 nm light. This treatment induces structural changes in the ECM by forming covalent bonds between amino acids in ECM fibers without harming vascular cell survival, as evidenced by our results. To further investigate the mechanism of this treatment, porcine carotid artery segments were exposed to 10-8-10 Dimer and light activation. Subsequent experiments subjected these segments to enzymatic degradation through elastase or collagenase treatment and were analyzed using digital image analysis software (MIPAR) after histological processing. The results demonstrated significant preservation of collagen and elastin structures in the photochemically treated vascular wall, compared to controls. This suggests that photochemical treatment can effectively modulate vascular remodeling by enhancing the resistance of the ECM scaffold to degradation. This approach shows promise in scenarios where vascular segments experience significant hemodynamic fluctuations as it reinforces vascular wall integrity and preserves lumen patency. This can be valuable in treating veins prior to fistula creation and grafting or managing arterial aneurysm expansion.

Expanded Utility of Human Acellular Vessel in Hemodialysis Access Surgery and Arterial Aneurysm Repair

Vascular access is essential for hemodialysis (HD) in patients with end-stage renal disease (ESRD). When the standard of care arteriovenous fistula (AVF) is limited, secondary to aneurysmal degeneration, trauma, and thrombus, interposition grafting is a reasonable reconstruction approach. As these grafts and comorbidities place ESRD patients at sustained risk of complications, reconstructions with regenerative medicine biologic conduits hold promise in improving safety and efficacy. Here, a biocompatible human acellular vessel (HAV) is our conduit of interest. With United States Food and Drug Administration use authorization under the Expanded Access Program, we report three cases of complex vascular access surgery with four aneurysm repairs using HAV. Patient selection focused on meeting unmet needs for those without adequate care alternatives, including active access and endoprosthetic stent graft infections, right heart failure due to high-output AVF, and arterial and access outflow aneurysms. In this high-risk expanded access population, operative technical success and interval success for patients given their inherent comorbidities, offer potential expanded utility of HAV in HD access surgery and arterial aneurysm repair.

The Human Acellular Vessel (HAV) as a vascular conduit for infrainguinal arterial bypass

Autologous vein is the optimal conduit for peripheral arterial bypass surgery, a standard recently highlighted by findings from the BEST-CLI trial. The Human Acellular Vessel is a novel biologic conduit produced using regenerative medicine technologies with structural and mechanical properties like a human blood vessel. Not yet approved by the United States Food and Drug Administration, the Human Acellular Vessel is being studied as an alternative bypass conduit in patients with peripheral arterial disease, vascular injury, and those in need of arteriovenous access for hemodialysis. This report describes and illustrates the technical aspects of intraoperative handling specific to the use of this new and innovative technology.

Application of decellularized vascular matrix in small-diameter vascular grafts

Coronary artery bypass grafting (CABG) remains the most common procedure used in cardiovascular surgery for the treatment of severe coronary atherosclerotic heart disease. In coronary artery bypass grafting, small-diameter vascular grafts can potentially replace the vessels of the patient. The complete retention of the extracellular matrix, superior biocompatibility, and non-immunogenicity of the decellularized vascular matrix are unique advantages of small-diameter tissue-engineered vascular grafts. However, after vascular implantation, the decellularized vascular matrix is also subject to thrombosis and neoplastic endothelial hyperplasia, the two major problems that hinder its clinical application. The keys to improving the long-term patency of the decellularized matrix as vascular grafts include facilitating early endothelialization and avoiding intravascular thrombosis. This review article sequentially introduces six aspects of the decellularized vascular matrix as follows: design criteria of vascular grafts, components of the decellularized vascular matrix, the changing sources of the decellularized vascular matrix, the advantages and shortcomings of decellularization technologies, modification methods and the commercialization progress as well as the application prospects in small-diameter vascular grafts.