Synthesis and evaluation of novel Tc-99m labeled probestin conjugates for imaging APN/CD13 expression in vivo

The enzyme aminopeptidase N (APN, also known as CD13) is known to play an important role in tumor proliferation, attachment, angiogenesis, and tumor invasion. In this study, we hypothesized that a radiolabeled high affinity APN inhibitor could be potentially useful for imaging APN expression in vivo. Here, we report synthesis, radiolabeling, and biological evaluation of new probestin conjugates containing a tripeptide, N,N-dimethylglycyl-l-lysinyl-l-cysteinylamide (N(3)S), chelator. New probestin conjugates were synthesized by solid-phase peptide synthesis method, purified by reversed-phase HPLC, and characterized by electrospray mass spectrometry. The conjugates were complexed with Re(V) and (99m)Tc(V) by transmetalation using corresponding Re(V) or (99m)Tc(V) gluconate synthon. The mass spectral analyses of ReO-N(3)S-Probestin conjugates were consistent with the formation of neutral Re(V)O-N(3)S complexes. Initial biological activity of ReO-N(3)S-Probestin conjugates determined by performing an in vitro APN enzyme assay using intact HT-1080 cells demonstrated higher inhibition of APN enzyme activity than bestatin. In vivo biodistribution and whole body planar imaging studies of (99m)TcO-N(3)S-PEG(2)-Probestin performed in nude mice xenografted with human fibrosarcoma tumors derived from HT-1080 cells demonstrated a tumor uptake value of 2.88 ± 0.64%ID/g with tumor-to-blood and tumor-to-muscle ratios of 4.8 and 5.3, respectively, at 1 h postinjection (p.i.). Tumors were clearly visible in whole body planar image obtained at 1 h p.i., but not when the APN was competitively blocked with a coinjection of excess nonradioactive ReO-N(3)S-PEG(2)-Probestin conjugate. These results demonstrate the feasibility of using high affinity APN inhibitor conjugates as targeting vectors for in vivo targeting of APN.