MMP9 and neoplasm: SPECT/CT images showed that the nanoparticles progressively accumulated in 4T1Luc tumours with low expression of MMP9, reaching 48 h upon intravenous injection a SUV value of 2.8 ± 0.11 (10.2 ± 0.33% ID/g), while a lower SUV of 1.75 ± 0.2 (6.23 ± 0.72% ID/g) was observed in the same period in the A431 tumours with high expression of MMP9.