The rapid pace of precision medicine development is illustrated by the progress of new therapies for the incurable Duchenne muscular dystrophy (DMD), which has wide-ranging implications for genetic neuromuscular diseases. The current state-of-the-art is an antisense oligonucleotide-based gene therapy designed to skip specific exons to restore the expression of shorter but functional dystrophin. We previously completed an investigator-initiated phase I study with systemic administration of the morpholino antisense NS-065/NCNP-01 (viltolarsen) for exon-53 skipping in patients with DMD, which was recently conditionally approved under accelerated approval mechanisms in Japan and the US. However, a weakness of the current oligonucleotide-based exon-skipping approach is that it is mutation-specific. NS-089/NCNP-02 was developed as the world’s first exon 44-skipping drug to expand the number of patients who can benefit from this approach. This skip is expected to benefit 6·2% of all patients with DMD. Here we report the findings of a first-in-human phase I/II study that evaluated the safety, efficacy, and pharmacokinetics of NS-089/NCNP-02 in patients with DMD amenable to exon 44 skipping. Additionally, this was the first study to use mesenchymal urine-derived cells from study patients to evaluate the efficacy of a drug in a DMD clinical trial. Findings from these assays suggest that exon 44 skipping and recovery of dystrophin protein levels can be achieved when an adequate concentration of NS-089/NCNP-02 reaches the skeletal muscle, consistent with the in vivo efficacy. This is of clinical significance, as future clinical trials can utilize this methodology to evaluate the pharmacological efficacy and sequence optimization of morpholino antisense drugs and determine patients’ eligibility for specific morpholino antisense treatments. We also discuss the potential of urine-derived cell-based approaches to model human neuromuscular and neurodegenerative diseases.