A novel technique to monitor carboxypeptidase G2 expression in suicide gene therapy using 19F magnetic resonance spectroscopy

Progress and problems with the use of suicide genes for targeted cancer therapy

Among various gene therapy methods for cancer, suicide gene therapy attracts a special attention because it allows selective conversion of non-toxic compounds into cytotoxic drugs inside cancer cells. As a result, therapeutic index can be increased significantly by introducing high concentrations of cytotoxic molecules to the tumor environment while minimizing impact on normal tissues. Despite significant success at the preclinical level, no cancer suicide gene therapy protocol has delivered the desirable clinical significance yet. This review gives a critical look at the six main enzyme/prodrug systems that are used in suicide gene therapy of cancer and familiarizes readers with the state-of-the-art research and practices in this field. For each enzyme/prodrug system, the mechanisms of action, protein engineering strategies to enhance enzyme stability/affinity and chemical modification techniques to increase prodrug kinetics and potency are discussed. In each category, major clinical trials that have been performed in the past decade with each enzyme/prodrug system are discussed to highlight the progress to date. Finally, shortcomings are underlined and areas that need improvement in order to produce clinical significance are delineated.

Sensitive luciferin derived probes for selective carboxypeptidase activity

Highly selective luminescent probes, QLUC-TYR and LUC-GLU, for detection of carboxypeptidase activity were synthesized. Caged substrates were first cleaved by corresponding carboxypeptidases, and then they were activated by luciferase to emit light. Enzymatic activities of biologically important carboxypeptidases can be determined using this technology.

Noninvasive detection of carboxypeptidase G2 activity in vivo

The pseudomonad protein, carboxypeptidase G2 (CPG2), is a prodrug-activating enzyme utilized in the targeted chemotherapy strategies of antibody- and gene-directed enzyme prodrug therapy (ADEPT and GDEPT). We have developed a noninvasive imaging approach to monitor CPG2 activity in vivo that will facilitate the preclinical and clinical development of CPG2-based ADEPT and GDEPT strategies. Cleavage of the novel reporter probe, 3,5-difluorobenzoyl-L-glutamic acid (3,5-DFBGlu), by CPG2, in human colon adenocarcinoma WiDr xenografts engineered to stably express CPG2, was monitored using (19)F MRSI. The high signal-to-noise ratio afforded by the two MR-equivalent (19)F nuclei of 3,5-DFBGlu, and the 1.4 ppm (19)F chemical shift difference on CPG2-mediated cleavage, enabled the dynamics and quantification of the apparent pharmacokinetics of 3,5-DFBGlu and its CPG2-mediated cleavage in the tumor to be evaluated. In addition, the apparent rate of increase of 3,5-difluorobenzoic acid concentration could also provide a biomarker of CPG2 activity levels in tumors of patients undergoing CPG2-based therapies, as well as a biomarker of treatment response. The addition of in vivo reporter probes, such as 3,5-DFBGlu, to the armamentarium of prodrugs cleaved by CPG2 affords new applications for CPG2 as a gene reporter of transgene expression.

(19)F MRI for quantitative in vivo cell tracking

Cellular therapy, including stem cell transplants and dendritic cell vaccines, is typically monitored for dosage optimization, accurate delivery, and localization using noninvasive imaging, of which magnetic resonance imaging (MRI) is a key modality. (19)F MRI retains the advantages of MRI as an imaging modality, and also allows direct detection of labeled cells for unambiguous identification and quantification, unlike typical metal-based contrast agents. Recent developments in (19)F MRI-based in vivo cell quantification, the existing clinical use of (19)F compounds and current explosive interest in cellular therapeutics have brought (19)F imaging technology closer to clinical application. We review the application of (19)F MRI to cell tracking, discussing intracellular (19)F labels, cell labeling and in vivo quantification, as well as the potential clinical uses of (19)F MRI.