An antisense oligonucleotide (ASO) targeting the interferon regulatory factor 4 (IRF4) mRNA, with potential immunomodulating and antineoplastic activities. Upon intravenous administration, frenlosirsen hybridizes with IRF4 mRNA, which blocks translation of the IRF4 protein. Reduction of IRF4 levels prevents the expression of IRF4-controlled tumor promoter genes, and may enhance tumor cell apoptosis and prevent T-cell exhaustion. IRF4, a transcription factor expressed in lymphocytes and essential for plasma cell differentiation, is involved in immune cell development and plays a key role in T-cell functions. It is overexpressed in certain tumor cell types and is a key regulator of multiple genes controlling tumor cell survival.