S158 Abstracts / Neuromuscular Disorders 29 (2019) S37–S208 P.311 BIO101 demonstrates combined beneficial effects on skeletal muscle and respiratory functions in a mouse model of Duchenne muscular dystrophy P. Dilda 1 , M. Latil 1 , B. Didry-Barca 1 , S. On 1 , M. Serova 1 , K. Mamchaoui 2 , S. Veillet 1 , R. Lafont 3 1 Biophytis, Paris, France; 2 Center for Research in Myology, Paris, France; 3 Sorbonne University, Paris, France DMD is a X-linked inherited muscular disease characterized by progressive muscle weakness, cardiomyopathy and respiratory failure leading to premature death. Muscles undergo repeated cycles of necrosis/regeneration and are replaced by connective tissues. Glucocorticoids and supportive therapies are the current standard of care leaving many patients, primarily those suffering from respiratory function defect, with an unmet medical need. Twelve-week-old C57BL10-mdx mice were orally treated with vehicle or BIO101 at 50mg/kg * day for 8 weeks. BIO101 treatment improved mdx running distance by 2.4-fold when compared to vehicle-treated mdx mice (+136%) and significantly increased in situ Tibialis anterior absolute maximal force by 15.3% supported by a reduction in muscle lesion profile with fewer necrotic or fibrotic area associated with less inflammatory cells infiltration. DMD-patient-derived myoblasts (KM571DMD10FL) treated with BIO101 showed an improvement of differentiation according to increased myotubes diameter, number of nuclei per myotube and fusion index (+21%, p<0.001, +34%, p<0.01 and +7%, p<0.01, respectively). In addition, whole body plethysmography experiments demonstrated that BIO101 improved respiratory functions and airway responsiveness of mdx mice in a time- dependent manner. Interestingly, this effect on respiratory function is not only associated with breathing parameters (inspiratory and expiratory time, relaxation time) as demonstrated by enhanced pause (Penh) measurements but also with a modification of deep airway structure as shown by improvement of lungs mechanical properties (resistance, elastance and compliance) in mdx mice shown by FlexiVent experiments. These results demonstrate the efficacy of BIO101 in the improvement of mdx muscle and respiratory functions. BIO101, which received Orphan Drug Designation in the US and the Europe Union, could offer a new option, alone or in combination with gene therapies, for the treatment of DMD. http://dx.doi.org/10.1016/j.nmd.2019.06.425 P.312 Functional and histological improvements comparing 4 micro-dystrophin constructs in the mdx mouse model of DMD R. Potter 1 , D. Griffin 1 , K. Heller 2 , J. Mendell 2 , L. Rodino-Klapac 1 1 Sarepta Therapeutics, Inc., Cambridge, USA; 2 Nationwide Children’s Hospital, Columbus, USA Duchenne muscular dystrophy (DMD) is the most common severe childhood form of muscular dystrophy. More than 2000 mutations of the DMD gene are responsible for progressive loss of muscle strength, and ultimately respiratory and cardiac failure. Through head-to-head comparison, functional and histological benefit across different micro- dystrophin constructs was evaluated. We designed 4 unique constructs of rAAVrh74 vector, including use of the MHCK7 promoter in comparison to a less active MCK promoter with the same micro-dystrophin transgene that contains the N-terminus and spectrin repeats R1, R2, and R3, respectively (rAAVrh74.MHCK7.micro-dystrophin; rAAVrh74.MCK.micro- dystrophin), a mini-dystrophin construct that contains the nNOS binding site (rAAVrh74.DV.minidystrophin), and a micro-dystrophin containing the C-terminus (rAAVrh74.MHCK7.microdys.Cterm). To test the efficacy of the 4 constructs of rAAVrh74.micro-dystrophin, we evaluated both functional and histological benefit 4 weeks post intramuscular vector delivery in the mdx mouse model. Delivery of the rAAVrh74.MHCK7.micro-dystrophin construct is the most advantageous in normalizing histologic and functional outcome measures among these constructs. Specific force output significantly increased in the tibialis anterior muscle compared with the other 3 constructs and there was no difference from wild-type levels. Muscle environment was normalized, as demonstrated by reductions in centralized nucleation and normalized myofiber diameters. Transgene expression through immunofluorescent staining and western blot was significantly increased compared with the other constructs, indicating functional and histological advantages of the rAAVrh74.MHCK7.micro-dystrophin construct. Findings from this preclinical study provided proof-of-principle for safety and efficacy of systemic delivery of rAAVrh74.MHCK7.micro-dystrophin in a dose- escalation study in the mdx mouse model for DMD. http://dx.doi.org/10.1016/j.nmd.2019.06.426 P.313 Improvement of human induced pluripotent stem cells (hiPSCs) - based therapy of Duchenne muscular dystrophy by using mesenchymal stem cells (MSCs) A. Elhussieny , K. Nogami, F. Takemura, Y. Maruyama, Y. Miyagoe-Suzuki, S. Takeda NCNP, Tokyo, Japan Despite exhaustive clinical effort, there is currently no effective treatment for Duchenne muscular dystrophy (DMD). Different strategies are needed for treating DMD because while the deficiency of dystrophin protein is the primary cause of damage to muscles, other secondary mechanisms are also involved Thus, the multiple functions of stem cells make them a candidate for treating the different pathological mechanisms underlying DMD. Human induced pluripotent stem cells (hiPSCs) can be induced to differentiate into various cell types, including skeletal muscle, even after extensive expansion in vitro. Therefore, hiPSCs are expected to be an inexhaustible source of myogenic stem cells for cell therapy. MSCs may play a role in improving the survival, migration, and engraftment of transplanted myogenic cells because they have anti-inflammatory, immunomodulatory properties, promote angiogenesis, provide protection from apoptosis, and secrete paracrine factors necessary for skeletal muscle growth. In this study, we induced MSCs (iMSC) from hiPSCs lines. iMSC were able to undergo trilineage differentiation into osteogenic, chondrogenic, and adipogenic cells. Flow cytometric analysis revealed that iMSC highly express MSC markers, CD73, CD90 and CD105 but not CD34 and CD45. interestingly iMSCs showed higher proliferative potential compared with human bone marrow derived MSCs (BM-MSCs). We next examined the differentiation of human myoblasts co-cultured with human iMSCs, or BM-MSCs, using the transwell method. The results showed that both iMSCs and BM-MSCs promote myoblast differentiation. At the WMS meeting, we will present the engraftment efficacy of co transplantation of hiPSC-derived muscle progenitors with iMSCs or BM-MSCs into skeletal muscle of immunodeficient mdx mice. http://dx.doi.org/10.1016/j.nmd.2019.06.427 P.314 Pharmacological activation of SERCA2a SUMOylation improves cardiac function and pathology in models of Duchenne muscular dystrophy J. Oh, P. Lee, S. Watanabe, R. DeVita, R. Hajjar, C. Kho, A. Lee Icahn School of Medicine at Mount Sinai, New York, USA Abnormal calcium overload is a critical pathogenic abnormality of Duchenne muscular dystrophy (DMD). Previously, we demonstrated that the SUMOylation plays an essential role in regulating sarcoplasmic reticulum calcium-ATPase (SERCA2a) pump, a prime target for modulation of cardiac contractility. Furthermore, our group identified a first-in-class SERCA2a SUMOylation enhancing compound, N106. This agent improves contractile dysfunction in a mouse model of heart failure. In this study, we investigated whether N106 treatment can improve DMD-associated cardiac dysfunction and muscle damages. Studies were applied to both dystrophin deficient mdx and utrophin heterozygous mdx (mdx/utrn+/-) mice as murine models for DMD. 16-month-old DMD mice were treated with either N106 (10