MMP2 and cancer: In 2017, Ji’s group reported a pH-responsive AuNP for targeted delivery of ALA to kill A549 cancer cells (Wu et al., 2017b), and they later reported a pH and matrix metalloproteinase-2 (MMP-2) dual-responsive ALA prodrug nanoparticle for photodynamic killing of SCC-7 cancer cells (Wu et al., 2017a).