A groundbreaking phase 1 trial at Stanford Medicine has demonstrated promising results for a non-myeloablative hematopoietic stem cell transplantation (HSCT) protocol that could revolutionize kidney transplant outcomes. The innovative approach, combining total lymphatic irradiation (TLI), total body irradiation (TBI), and antithymocyte globulin (ATG) conditioning, shows potential for achieving immune tolerance in patients receiving haploidentical living donor kidney transplants.
Clinical Protocol and Study Design
The trial investigated three distinct treatment arms, building upon previously published protocols. The base conditioning regimen included 10 doses of 120 centigrays TLI and 5 doses of 1.5 mg/kg ATG following kidney transplant, with HSCT performed on day 11. Living donors underwent hematopoietic stem cell mobilization and leukapheresis 6 weeks before kidney donation.
The study enrolled 37 patients across three arms: 27 patients in arm 1 (TLI/ATG), 6 patients in arm 2 (TLI/TBI/ATG), and 4 patients in arm 3 (TLI/TBI/ATG + regulatory T cells). The median age of participants ranged from 34.7 to 41.3 years across the cohorts.
Safety and Efficacy Outcomes
The protocol demonstrated an exceptional safety profile, with 100% one-year overall survival rates in both the second and third cohorts. Only one patient in the second cohort experienced grade 2 skin graft-vs-host disease (GVHD), which resolved with treatment. No GVHD cases were reported in the third cohort.
Dr. Everett Meyer, associate professor at Stanford Medicine and senior study author, emphasized the mechanism of action: "The mixed chimerism involves the donor and recipient immune systems interacting together for the reinduction of peripheral regulatory pathways, notably with T regulatory cells, inducing a lot of this function."
Mixed Chimerism and Immunosuppression Withdrawal
The addition of low-dose TBI significantly enhanced the levels and stability of mixed chimerism during the first year post-treatment. Notably, two patients achieved complete immunosuppressive drug withdrawal during the second year after treatment. The TLI/TBI/ATG conditioning group demonstrated particular success, achieving optimal outcomes in approximately one-third of patients.
Future Directions
The research team is now exploring several optimization strategies, including:
- Re-timing T regulatory cell infusion to day 6 post-bone marrow transplant
- Incorporating third-generation donor graft products
- Testing new immunosuppressive agents
- Investigating delayed transplant cohorts
These findings build upon recent success in a phase 3 trial where kidney transplant recipients receiving MDR-101 achieved donor mixed chimerism and functional immune tolerance for over two years, with improved quality of life compared to standard treatment.
Dr. Meyer noted, "Over the past 20 years, we've had success being able to induce stable and transient mixed tolerance with long-term immunosuppression withdrawal, first on a phase 1 basis, and then now very recently, reported as a positive phase 3 trial outcome."
