A Study in Adult Volunteers to Understand the Safety and Effects of a New Drug, PTC612 - Part 1 (Single Ascending Dose) and Part 3 (Multiple Ascending Dose)

Part 1 Single Ascending Dose (SAD): Part 1 of the study will evaluate single ascending oral doses of PTC612 in healthy participants. PTC612 will be administered as a powder formulation for oral administration (powder is mixed with a liquid vehicle prior to administration). This part is designed to assess the safety, tolerability, and pharmacokinetics of PTC612 following single-dose exposure across increasing dose levels. The following dose levels may be explored and administered orally (note these are estimated doses and subject to change): • Group 1: 3 milligram doses of PTC612 or matching placebo • Group 2: 10 milligram doses of PTC612 or matching placebo • Group 3: 30 milligram doses of PTC612 or matching placebo • Group 4: 90 milligram doses of PTC612 or matching placebo In Part 1, the safety and pharmacokinetics (PK) of single doses of PTC612 will be evaluated in a randomized, double-blind, placebo-controlled design across approximately four dose groups in eligible participants. In each Single Ascending Dose (SAD) cohort, a sentinel dosing approach will be implemented. For the initial two participants, randomization will assign one participant to receive PTC612 and the other to receive placebo. For the remaining six participants within the same cohort, randomization will allocate five participants to receive PTC612 and one participant to receive placebo. Each Single Ascending Dose (SAD) group will be dosed according to a sentinel dosing design to ensure optimal safety. Unique Subject will be enrolled in each cohort After each group completes dosing and PK blood sample collection, the Safety Review Committee (SRC) will review the available safety data, tolerability findings, and pharmacokinetic results through at least Day 3 (and up to Day 8). Based on this review, the Safety Review Committee (SRC) will determine the appropriate dose escalation for the subsequent group, including any required dose adjustments. Each subsequent group, with unique subjects, will commence dosing approximately two weeks following final dose administered in the previous group. Participants will receive study drug directly from the Investigator (or designee), under medical supervision. Part 3 Multiple Ascending Dose (MAD): Unique participants will be enrolled in each part and each cohort. In Part 3, the safety and PK of multiple PTC612 doses will be assessed in a randomized, double-blind, placebo-controlled design with 3 dose groups in eligible participants. In each group, 8 participants will be randomized to treatments in a ratio of 6:2 (active: placebo). PTC612 is planned to be dosed as oral tablets in Part 3, although an oral powder may be used for dosing if needed (this will be determined based on the doses selected and agreed upon by the Safety Review Committee (SRC). The first dose group in Part 3 may be initiated upon completion of Cohort 2 in Part 1 (i.e., 10 milligrams in Single Ascending Dose), if supported by the Safety Review Committee (SRC). The second dose group in Part 3 may be initiated upon completion of Cohort 3 in Part 1. In Part 3, PTC612 will be administered in the fasted condition. Oral doses of PTC612 or matching placebo will be administered once daily in the morning from Day 1 through Day 14. In each MAD cohort, participants will be randomized in a 3:1 ratio, with six participants receiving PTC612 and two receiving placebo. Sentinel dosing will not be implemented in Part 3 (Multiple Ascending Dose) of the study. The starting dose for this part of the study will be chosen based on safety and drug-level results from earlier participants. Any increase in dose will only happen after careful review of safety data by the Study Review Committee to ensure the dose remains safe for participants. Maximum dose to be tested will be no more than 90mg. Oral Powder will be dissolved in a liquid Participants will receive study drug directly from the Investigator (or designee), under medical supervision.