Key Inclusion Criteria:

Part A:

• Women of non-childbearing potential (surgically sterilized or post menopausal) or men, aged ≥18 to ≤ 55 years
• BMI of ≥ 18 to ≤ 32 kg/m²
• When engaging in sex with a woman of child bearing potential, two forms of birth control are required

Part B:

• Women of non-childbearing potential (surgically sterilized or post menopausal) or men, aged ≥18 to ≤ 80 years. Women who are of childbearing potential but on highly effective, low user dependent contraceptive methods will be allowed.
• BMI of ≥ 18 to ≤ 32 kg/m²
• Have a Clinical Dementia Rating® plus National Alzheimer's Coordinating Center frontotemporal lobar degeneration global score ≥ 0.5
• Have confirmed granulin (GRN) mutation via genetic testing or historical records available for review by investigator
• When engaging in sex with a woman of child bearing potential, both the male participant and his female partner must use highly effective contraception

Part C:

• All participants who completed Part B of this trial are eligible for an 18-month OLE if the participant has no unresolved clinically significant TEAEs, where continued dosing may represent a risk to participant safety.

Key Exclusion Criteria:

• Have any history of clinically significant neurologic, psychiatric, endocrine, pulmonary, cardiovascular, gastrointestinal, hepatic, pancreatic, renal, metabolic, hematologic, immunologic, or allergic disease, or other major disorders
• Have a history of malignancy, except fully resected basal cell carcinoma or other malignancies at low risk of recurrence
• Have a clinically significant history of stroke, cognitive impairment due to causes other than FTD, seizure within 5 years of screening, or head trauma with loss of consciousness within 2 years of screening
• Have a positive serum pregnancy test or are currently lactating or breastfeeding