Key Inclusion Criteria for Parts A and B * Documentation of PH1 as determined by genetic analysis confirming pathogenic mutations in the alanine-glyoxylate aminotransferase (AGXT) gene (valid historical laboratory data will be reviewed and approved by the Sponsor) * Age at time of signing the informed consent/assent form: * Part A: ≥18 years to ≤64 years * Part B: ≥6 years to \<18 years * 24-hour UOx ≥0.7 mmol/24 hours/1.73 m² * eGFR ≥30 mL/min/1.73m² * Weight ≤90 kg Key Exclusion Criteria for Parts A and B * Confirmed diagnosis of primary hyperoxaluria type 2 or type 3 * History of a liver, kidney or combined liver/kidney transplant * Currently on dialysis * Participant has previously used (within past 24 months) or is currently receiving an approved or investigational urinary oxalate lowering RNA interference (RNAi) or siRNA therapy * Female participants who are pregnant or breastfeeding (or are planning either during the first 12 months)