A Study to Investigate PORT-77 Oral Dosing Compared With Placebo in Participants With Erythropoietic Protoporphyria (EPP) or X-linked Protoporphyria (XLP)

Inclusion Criteria: * Aged 12 years or older. * Clinical history of EPP, XLP, or EPP2 supported by genetic confirmation or historical laboratory test results * History of consistent, non-painful prodrome within approximately 45 minutes of sunlight exposure and prior to phototoxic attacks * Demonstrates ≥85% compliance with daily symptom diary during run-in period. * Body weight or BMI at Screening as follows: 1. For participants aged 12 to \<18 years: body weight ≥32 kg 2. For participants aged ≥18 years: BMI ≥18.5 kg/m2 * AST and ALT \<3 × ULN and total bilirubin \<2 × ULN (unless documented Gilbert syndrome) at Screening * Willing and able to provide informed consent and/or assent for the study. * Willing and able to comply with study visits, study procedures, and contraception guidance * Intends to remain in the same approximate geographic latitude for the duration of the placebo-controlled period, with no more than 14 days spent outside this region. Exclusion Criteria: * Diagnosis of another porphyria or another photodermatosis that may confound the evaluation of PORT-77 * Any evidence of clinically significant organ dysfunction or any clinically significant deviation from normal in the clinical or laboratory assessments * Major surgery within 8 weeks before Screening, incomplete recovery from any previous surgery, or major surgery planned to occur during the study * History of, or anticipated need for, liver transplantation or history of bone marrow transplantation * Active infection with hepatitis B or C * Unable to swallow tablets or has a disease that significantly affects gastrointestinal function * Any other disease, condition, or circumstance that, at the discretion of the Investigator or Sponsor, would interfere with the evaluation of PORT-77 or study participation, or would make study participation not in the best interest of the participant * History of drug or alcohol abuse within the last 12 months, or current or planned use of prohibited or illegal substances * Has taken any medication, vitamin, or supplement that alters sensitivity to light exposure (eg, afamelanotide, melanotan, beta carotene, dersimelagon) within 90 days of Day 1; or has taken bitopertin within 120 days of Day 1; or has taken iron within 30 days of Day 1 * Drugs or supplements that may impact or be impacted by PORT-77 * Concurrent or anticipated participation in an interventional clinical trial during the study period. * Received another investigational therapy within 5 half-lives, if the half-life is known, or within 30 days, if the half-life is unknown, prior to Day 1. * Known hypersensitivity to PORT-77 or excipients * Female who has a positive pregnancy test at Screening or Day 1 or is breastfeeding