This is a Phase 2a study to assess the the safety and tolerability of TPN-101 in patients with Amyotrophic Lateral Sclerosis (ALS) and/or Frontotemporal Dementia (FTD) Associated with Hexanucleotide Repeat Expansion in the C9orf72 gene (C9ORF72 ALS/FTD).

This is a Phase 2a multi-center, randomized, double-blind, placebo-controlled parallel-group, 2-arm study with a long-term, open-label treatment phase in patients with C9ORF72 ALS and/or FTD. This study includes a 6-week Screening Period, a 24-week Double-blind Treatment Period, a 24-week Open-label Treatment Period, and a Follow-up Visit 4 weeks post-treatment.

Inclusion Criteria:

• Documentation of a clinical genetic test demonstrating a hexanucleotide repeat expansion (HRE) in the C9orf72 gene
• Has a reliable caregiver/informant to accompany the patient to all study visits

For patients with ALS (with or without FTD):

• Diagnosis of ALS (probable, possible, laboratory-supported probable or definite) according to the World Federation of Neurology revised E1 Escorial criteria
• Onset of weakness within 3 years prior to Screening
• Slow vital capacity (SVC) ≥ 60% of predicted normal adjusted for sex, age, and height (from the sitting position)
• Able to perform reproducible pulmonary function tests.
• ALS Functional Rating Scale-Revised (ALSFRS-R) ≥ 30 and score of 3 or 4 on Item #3 (swallowing) at Screening

For patients with FTD:

• A gradual, progressive decline in behavior, language, or motor function consistent with mild cognitive impairment, mild behavioral impairment, mild cognitive/behavioral impairment, behavioral variant FTD, primary progressive aphasia, or amnestic syndrome
• CDR Dementia Staging Instrument plus National Alzheimer's Coordinating Center Behavior and Language Domains (CDR plus NACC FTLD) global score of 0.5-2.0 at Screening

Exclusion Criteria:

• Presence of other significant neurological or psychiatric disorders
• History of clinically significant brain abnormality
• Clinically significant medical illness
• Tracheostomy or diaphragmatic pacing
• Autoimmune disease requiring treatment or management (quiescent rheumatoid arthritis, psoriasis, or controlled Type 1 diabetes are acceptable)
• History of human immunodeficiency virus (HIV) or hepatitis B infection, or any active infection during Screening, unless the patient will have been symptom-free for at least 30 days prior to randomization