This is an interventional study to assess the safety, PK, and efficacy of HST5040 in 12 subjects - 6 with Methylmalonic Acidemia (MMA) and 6 with Propionic Acidemia (PA). The study consists of 3 parts: - Part A: Open-label, within-subject, dose escalation study in PA and MMA subjects ≥ 2 years old to identify a safe and pharmacologically active (optimal) dose of HST5040 for use in Part B. Subjects will continue in a Part A open-label extension until all subjects complete Part A and the optimal dose of HST5040 is identified for use in Part B. - Part B: 6-month, randomized, double-blind, placebo-controlled, 2-period crossover in the same subjects from Part A to evaluate safety and efficacy of the optimal dose of HST5040 in addition to standard of care (SoC). - Part C: open-label long-term extension study in PA and MMA subjects ≥ 2 years old (N = approximately 12, 6 each) to evaluate the long-term safety and efficacy of the optimal dose of HST5040. This study will determine whether HST5040 can improve levels of disease-associated toxins that accumulate in patients with PA and MMA.