PDE5A and cancer: Previous randomized studies of breast and prostate cancer patients have not shown any significant overlapping toxicities of PDE5 inhibitors with concurrent administration of RT or cytotoxic chemotherapy.21,22 Tadalafil has also been safely combined with vaccine therapy and anti-PD1 antibody for head and neck cancer patients.11,23 Given its safety profile, inexpensive cost, and long half-life, tadalafil may be a promising drug to be repurposed as an immune-modulating agent to treat various cancers.