MPO and chronic rhinosinusitis with nasal polyps: A double-blinded prospective study by Van Zele et al. investigated the effect of doxycycline on 47 participants with CRSwNP and showed that doxycycline significantly lowered nasal secretion of MMP-9, ECP, and myeloperoxidase, as well as decreased polyp size with moderate effect for up to 12 weeks [107].