Bio-activated reporters to visualize, in real time, specific gene therapy products

A method of monitoring gene therapy in a mammalian subject, comprising:

• a) providing a composition comprising a complex for use in magnetic resonance imaging (MRI) to the subject, wherein the subject is deficient in a target gene and has received or is receiving gene therapy to restore activity of the target gene, wherein during said gene therapy the target gene is co-expressed with a reporter gene that produces a reporter gene product;
• and
• b) performing magnetic resonance imaging on the subject,
• wherein the complex comprises:
• (i) an MRI contrast agent;
• (ii) a blocking substrate cleavable by the reporter gene product;
• and
• (iii) a self-immolative linker attaching the blocking substrate to the MRI contrast agent,
• wherein the self-immolative linker provides a functional group that coordinatively saturates the MRI contrast agent, thereby preventing a water access to the MRI contrast agent resulting in no detectable signal in vivo,
• wherein the functional group is attached to the self-immolative linker by a spacer comprising at least 4 carbons, and
• wherein cleavage of the blocking substrate by the reporter gene product, if present in the subject, initiates a spontaneous electron cascade of the self-immolative linker that results in removal of the functional group from the MRI contrast agent, thus activating the MRI contrast agent to generate a detectable signal in vivo by magnetic resonance imaging, wherein the detectable signal is produced within 1 hour following cleavage of the blocking substrate,
• wherein the detectable signal indicates that the reporter gene product was expressed and the activity of the target gene was restored by the gene therapy, or wherein no detectable signal indicates that the reporter gene product was not expressed and the activity of the target gene was not restored by the gene therapy.