In vivo biodistribution of radiolabeled MMP‐2/9 activatable cell‐penetrating peptide probes in tumor‐bearing mice

Matrix metalloproteinases (MMPs) play a pivotal role in cancer progression and present therefore an interesting biomarker for early diagnosis, staging and therapy evaluation. Consequently, MMP‐specific molecular imaging probes have been proposed for noninvasive visualization and quantification of MMP activity. An interesting approach is MMP‐2/9 activatable cell‐penetrating peptides (ACPP) that accumulate in the tumor tissue after activation. However, a recent study revealed that probe activation occurred already in the vasculature followed by nonspecific tumor targeting. In the latter study, biodistribution was determined 6 and 24 h post‐ACPP injection. An alternative explanation could still be that the kinetics of tumor‐specific activation is faster than that of blood activation plus subsequent nonspecific uptake in tumor. The aim of this study was to assess if tumor‐specific ACPP activation occurs in mice with MMP‐2/9 positive subcutaneous HT‐1080 tumors at 3 h post‐injection. As control, we studied the MMP‐2/9 sensitive ACPP in mice bearing subcutaneous BT‐20 tumors with low MMP‐2/9 expression to test if probe cleavage correlates with tumoral MMP expression. Ex vivo biodistribution showed no improved tumoral ACPP activation in HT‐1080 tumor‐bearing mice at 3 h post‐injection compared with previous reported data collected at 24 h post‐injection. Furthermore, tumoral uptake and relative tumoral activation for ACPP were similar in both BT‐20 and HT‐1080 tumor‐bearing mice. In conclusion, this study suggests that tumoral ACPP uptake in these tumor models originates from probe activation in the vasculature instead of tumor‐specific MMP activation. Novel ACPPs that target tissue‐specific proteases without nonspecific activation may unleash the full potential of the elegant ACPP concept. Copyright © 2014 John Wiley & Sons, Ltd.