Safety Profile Of The First Pediatric Cardiomyopathy Gene Therapy Trial: RP-A501 (AAV9:LAMP2B) For Danon Disease

Background Danon disease (DD) is a rare, X-linked monogenic cardiomyopathy caused by mutations in the LAMP2 gene which is essential for autophagy. In male patients, DD is characterized by a severe, progressive hypertrophic cardiomyopathy and arrhythmias resulting in median mortality under 20 years (y). This open-label Phase 1 trial in DD males evaluates systemic in vivo gene therapy (RP-A501) with adeno-associated virus 9 (AAV9) and a normal copy of the human LAMP2B gene. Pediatric evaluation was preceded by treatment of two adult DD cohorts with evidence of manageable safety, LAMP2B cardiac gene and protein expression and preliminary clinical efficacy. In this initial pediatric cohort, there was specific emphasis on minimization of immune responses, particularly complement activation and thrombotic microangiopathy (TMA) that have been associated with systemic AAV therapy in pediatric and adult evaluations in monogenic neuromuscular, metabolic and coagulation disorders. Methods An enhanced immunosuppressive regimen incorporating rituximab, sirolimus and limited corticosteroids was implemented to minimize the risk of complement-mediated events and steroid exposure after a single IV infusion of RP-A501 (AAV9.LAMP2B) at 6.7 × 1013 GC/kg with weight-based viral titer caps. Results Two pediatric patients (ages 11 and 12y) have received IV RP-A501 (6.7 × 1013 GC/kg). Platelet counts remained within normal range with modest decreases during days 6-14 post-infusion. Complement soluble membrane attack complex (sC5b-9) increased during days 5-14 post-infusion; no increases >200ng/mL over any 24 hour period were observed; peak values remained <450ng/mL (ULN 250 ng/mL) and were markedly lower than those observed in the adult cohort. Hemoglobin and creatinine levels remained stable during the initial weeks post-infusion. Transaminases and liver-focused parameters (GGT and bilirubin) were also largely stable. Limited and transient troponin increases were noted, consistent with known AAV9 cardiotropism. Increases in neutralizing and anti-capsid antibodies were observed and were more modest than those in the adult cohort. Managed with a steroid-sparing regimen, steroid induced exacerbations of DD-related skeletal myopathy in the pediatric patients were limited relative to those reported in the adult cohort. As previously reported, all adult patients with observed immunosuppressive regimen compliance (N=4) had evidence of cardiac LAMP2B expression within 6 months. Evaluation of gene expression in the pediatric patients is currently underway. Conclusions Robust immunomodulation during the initial days post-infusion enabled administration of this first-in-pediatric RP-A501 gene therapy for DD cardiomyopathy without evidence of immune-mediated clinical sequelae. Transient inhibition of humoral and cell-mediated immune response is intended to enable optimal RP-A501 cardiomyocyte delivery and transduction in order to arrest and potentially reverse the rapidly progressive cardiomyopathy associated with DD. Danon disease (DD) is a rare, X-linked monogenic cardiomyopathy caused by mutations in the LAMP2 gene which is essential for autophagy. In male patients, DD is characterized by a severe, progressive hypertrophic cardiomyopathy and arrhythmias resulting in median mortality under 20 years (y). This open-label Phase 1 trial in DD males evaluates systemic in vivo gene therapy (RP-A501) with adeno-associated virus 9 (AAV9) and a normal copy of the human LAMP2B gene. Pediatric evaluation was preceded by treatment of two adult DD cohorts with evidence of manageable safety, LAMP2B cardiac gene and protein expression and preliminary clinical efficacy. In this initial pediatric cohort, there was specific emphasis on minimization of immune responses, particularly complement activation and thrombotic microangiopathy (TMA) that have been associated with systemic AAV therapy in pediatric and adult evaluations in monogenic neuromuscular, metabolic and coagulation disorders. An enhanced immunosuppressive regimen incorporating rituximab, sirolimus and limited corticosteroids was implemented to minimize the risk of complement-mediated events and steroid exposure after a single IV infusion of RP-A501 (AAV9.LAMP2B) at 6.7 × 1013 GC/kg with weight-based viral titer caps. Two pediatric patients (ages 11 and 12y) have received IV RP-A501 (6.7 × 1013 GC/kg). Platelet counts remained within normal range with modest decreases during days 6-14 post-infusion. Complement soluble membrane attack complex (sC5b-9) increased during days 5-14 post-infusion; no increases >200ng/mL over any 24 hour period were observed; peak values remained <450ng/mL (ULN 250 ng/mL) and were markedly lower than those observed in the adult cohort. Hemoglobin and creatinine levels remained stable during the initial weeks post-infusion. Transaminases and liver-focused parameters (GGT and bilirubin) were also largely stable. Limited and transient troponin increases were noted, consistent with known AAV9 cardiotropism. Increases in neutralizing and anti-capsid antibodies were observed and were more modest than those in the adult cohort. Managed with a steroid-sparing regimen, steroid induced exacerbations of DD-related skeletal myopathy in the pediatric patients were limited relative to those reported in the adult cohort. As previously reported, all adult patients with observed immunosuppressive regimen compliance (N=4) had evidence of cardiac LAMP2B expression within 6 months. Evaluation of gene expression in the pediatric patients is currently underway. Robust immunomodulation during the initial days post-infusion enabled administration of this first-in-pediatric RP-A501 gene therapy for DD cardiomyopathy without evidence of immune-mediated clinical sequelae. Transient inhibition of humoral and cell-mediated immune response is intended to enable optimal RP-A501 cardiomyocyte delivery and transduction in order to arrest and potentially reverse the rapidly progressive cardiomyopathy associated with DD.