CTSB and neoplasm: Cat K 680 FAST and VM249 are self-quenched imaging agents with an amino acid recognition sequence that is flanked on both sides by Vivotag S 680; however, the substrate of VM249 is susceptible to cleavage by cathepsins B, K, L, and S. These imaging agents have potential advantages over Prosense 680, including a smaller molecular weight, the ability to image tumors several hours after administration, and, for VM249, a higher tumor-to-muscle signal ratio than Prosense 680 (22.7 ± 2.3) (Fig. 2a).