Figure 4.2.

Hypothetical scheme for molecular-genetic imaging utilizing bioluminescence resonance energy transfer (BRET): a cancer-specific or cancer-selective promoter (CSP) drives luciferase gene expression (luc) with the C-terminal end containing a cDNA sequence corresponding to amino acids for the substrate of a matrix metalloproteinase (sMMP) or collagen (i.e., CSP-luc-sMMP) forming Luc-sMMP. The construct generates bioluminescent signal by adding substrates for luciferin (D-luciferin or coelenterazine). The addition of Q-dots with overlapping absorption spectra with respect to the emission spectra of the version of luc chosen will undergo BRET enabling fluorescence detected in the NIR region to visualize the presence of tumor (upper sequence). As the tumor progresses, the tumor microenvironment changes, and the involvement of extracellular matrix proteins or MMPs lead to subsequent cleavage of sMMPs from Luc-sMMP, releasing the Q-dot. The Q-dot will no longer be activated by bioluminescence because the molecular distance (<1000 nm) between it and the bioluminescent construct will no longer enable BRET. BRET signal turns off during tumor invasion and metastasis, where MMPs abound. This is an example of an imaging reporter that can measure tumor growth while maintaining sensitivity to its microenvironment, via loss of BRET.