PT  - JOURNAL ARTICLE
AU  - Sander M.J. van Duijnhoven
AU  - Marc S. Robillard
AU  - Klaas Nicolay
AU  - Holger Grüll
TI  - Tumor Targeting of MMP-2/9 Activatable Cell-Penetrating Imaging Probes Is Caused by Tumor-Independent Activation
AID  - 10.2967/jnumed.110.082503
DP  - 2011 Feb 01
TA  - Journal of Nuclear Medicine
PG  - 279--286
VI  - 52
IP  - 2
4099  - http://jnm.snmjournals.org/content/52/2/279.short
4100  - http://jnm.snmjournals.org/content/52/2/279.full
SO  - J Nucl Med2011 Feb 01; 52
AB  - Activatable cell-penetrating peptides (ACPPs) are a new class of promising molecular imaging probes for the visualization of enzymes in vivo. The cell-penetrating function of a polycationic peptide is efficiently blocked by intramolecular electrostatic interactions with a polyanionic peptide. Proteolysis of a cleavable linker present between the polycationic cell-penetrating peptide and polyanionic peptide affords dissociation of both domains and enables the activated cell-penetrating peptide to enter cells. Here, we aimed to develop an ACPP sensitive to matrix metalloproteinase-2 and -9 (MMP-2/9) for nuclear imaging purposes. Methods: MMP-2/9 ACPPs and nonactivatable cell-penetrating peptides (non-ACPP) were prepared by 9-fluorenylmethyloxycarbonyl solid-phase peptide synthesis and labeled with 177Lu or 177Lu/125I for dual-isotope studies. The in vivo biodistribution of these probes was assessed in MMP-2/9–positive tumor-bearing mice (n = 6) and healthy mice (n = 4) using γ-counting. Furthermore, a radiolabeled cell-penetrating peptide serving as a positive control was evaluated in tumor-bearing mice (n = 6). Results: Biodistribution studies showed a 5-fold-higher retention of ACPP in tumor than in muscle (P < 0.01) and a 6-fold-higher tumor retention relative to non-ACPP (P < 0.01), supporting earlier studies on fluorescently labeled ACPPs proposing activation by tumor-associated MMP-2/9. Surprisingly, however, the uptake of ACPP was significantly higher than that of non-ACPP in almost all tissues (P < 0.01). To unravel the activation process of ACPP in vivo, we developed dual-isotope ACPP analogs (dACPPs) that allowed us to discriminate between uncleaved dACPP and activated dACPP. In vivo biodistribution of dACPP indicated that the tissue-associated counts originated from activated dACPP. Interestingly, dACPP administration to healthy mice, compared with MMP-2/9–positive tumor-bearing mice, resulted in a similar dACPP biodistribution. Furthermore, a radiolabeled cell-penetrating peptide showed tumor-to-tissue ratios equal to those found for ACPP (P > 0.05). Conclusion: This study demonstrates that the tumor targeting of radiolabeled MMP-2/9 ACPPs is most likely caused by the activation in the vascular compartment rather than tumor-specific activation, as suggested earlier. The results in the present paper indicate that different and more tissue-specific enzyme–ACPP combinations are needed to unleash the full potential of the elegant ACPP concept in living animals.