Ubenimex is being developed for the treatment of pulmonary arterial hypertension (PAH) (World Health Organization [WHO] Group 1) to improve exercise capacity and delay clinical worsening. This study is a Phase 2, open-label, multicenter, extension study to evaluate long-term safety and efficacy of ubenimex in patients with PAH (WHO Group 1) who complete Study EIG-UBX-001 (Study EIG-UBX-001 is a Phase 2, randomized, double-blind, placebo-controlled clinical trial) and meet the eligibility criteria for Study EIG-UBX-002. The primary objective for this study is: