Extending the availability of living allogeneic heart valves for transplantation

Abstract

There remains an urgent clinical need to develop durable heart valve replacements which can act as life-long substitutes for the native heart valve, especially for pediatric patients wherein the current standard of care is multiple reoperations to repair or replace failed valve prostheses throughout a child’s lifetime. These reinterventions are often required within just months of the valve replacement’s original implantation. This is because there are currently no valve prostheses with the capacity for growth or self-repair. Both bioprosthetic valves (including cryopreserved human allografts, which represent the historic gold-standard for pediatric patients) and mechanical valves suffer from limited durability and are unable to support the somatic growth of a child. Living heart valve transplantation is a recently introduced surgical technique wherein valvular allografts are procured from a heart deemed unsuitable for orthotopic transplantation or from a living valve donor who is receiving a heart transplant for a non-valvular indication. In heart valve transplantation, these living valves are semi-emergently transplanted in the recipient, with the core advantage of functioning as a viable, growth-capable valve. The in vivo adaptive growth and durability of living heart valve transplants is currently being studied in single and multi-center clinical studies, with promising early outcomes. However, there remain key translational challenges to this technique’s implementation – namely, lack of donor availability as well as the time, resource, and logistic constraints involved in urgent transplantation. These challenges are exacerbated by the difficulties of donor-recipient size matching in pediatric patients. In this dissertation, we therefore work towards establishing a strategy for the storage and preservation of living allogeneic valves, with the hypothesis that integrating physiologic biochemical and mechanical cues will enable the preservation of valvular tissue physiology ex vivo. In Aim 1, we develop a custom preservation solution aimed at preserving valvular tissue viability. We thoroughly evaluate the preservation solution’s capacity to maintain valvular leaflet and pulmonary artery viability, metabolism, microarchitecture, and cell physiology for up to 7 weeks ex vivo. Our experimental model employed freshly sourced porcine valves that had been subject to the upper limit of static cold storage currently used in clinical practice. We found that our preservation solution could preserve valvular viability and metabolism for at least 7 weeks, with maintenance of key phenotypic markers of valvular interstitial cells. However, the leaflet microarchitecture began to degrade by 3 weeks ex vivo, with increasing collagen content. Building on these results, we sought to integrate physiologic mechanical cues to more effectively preserve valvular microarchitecture. In Aim 2, we designed, built, and tested a custom bioreactor for the storage of valvular allografts. With clinical translation as a primary goal, a key design specification of our bioreactor was to avoid the use of a pump. We created a low-cost, user-friendly, compact system which was composed of a closed loop, wherein the valve is maintained in a valvular housing chamber and fluid flow through the valve is dependent on axial rotation of the bioreactor. We validated that rotation could introduce native-like open/close cycles in porcine valves and obtained fine-tuned control over the hydrodynamic environment by manipulating the bioreactor’s rotation protocol. Finally, we demonstrated the capacity to maintain valvular viability in the bioreactor for up to 4 weeks. We found that valves exposed to mechanical stimulation demonstrated maladaptive remodeling, characterized by leaflet retraction and fibroblast proliferation. Therefore, we then performed a parameter-controlled study which identified a unique biochemical treatment which could prevent valvular fibrosis in response to mechanical stimulation ex vivo. In Aim 3, we evaluated the in vivo response to living valvular allografts in the preclinical and clinical settings. We established a porcine model for heart valve transplantation and then applied this model towards evaluating the impact of static cold storage on valvular allograft function. We found that fresh valve transplants demonstrated preserved function for 2 months of follow-up, versus a valve that had been subject to 4 weeks of static cold storage which showed severe insufficiency and diffuse calcification throughout the pulmonary artery on explant. We next investigated clinical outcomes following heart valve transplantation at Columbia, following two patients who received “domino” heart valve transplant at our center. Our outcomes demonstrated maintained valve function with adaptive valvular growth in both patients. We also report the first-ever evidence of immunosensitization following heart valve transplant, with both patients developing donor specific antibodies within 100 days post-transplant. Collectively, this dissertation functions as a foundation for normothermic storage and preservation of living allogeneic valves through integration of key homeostatic signals in the ex vivo environment. Moving forward, the strategy developed in Aims 1 and 2 will be evaluated in vivo, as per the preclinical model establish in Aim 3. In tandem, we are continuing to evaluate the in vivo performance of heart valve transplantation via multicentric studies including centers from across the United States, as well as through the development of a dedicated registry aimed at the long-term follow up of heart valve transplant recipients