Ld. Stegman et al., Noninvasive quantitation of cytosine deaminase transgene expression in human tumor xenografts with in vivo magnetic resonance spectroscopy, P NAS US, 96(17), 1999, pp. 9821-9826
Citations number
49
Categorie Soggetti
Multidisciplinary
Journal title
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Analysis of transgene expression in vivo currently requires destructive and
invasive molecular assays of tissue specimens. Noninvasive methodology for
assessing the location, magnitude, and duration of transgene expression in
vivo will facilitate subject-by-subject correlation of therapeutic outcome
s with transgene expression and will be useful in vector development. Cytos
ine deaminase (CD) is a microbial gene undergoing clinical trials in gene-d
irected enzyme prodrug gene therapy. We hypothesized that in vivo magnetic
resonance spectroscopy could be used to measure CD transgene expression in
genetically modified tumors by directly observing the CD-catalyzed conversi
on of the 5-fluorocytosine (5-FC) prodrug to the chemotherapeutic agent 5-f
luorouracil (5-FU). The feasibility of this approach is demonstrated in sub
cutaneous human colorectal carcinoma xenografts in nude mice by using yeast
CD (yCD). A three-compartment model was used to analyze the metabolic flux
es of 5-FC and its metabolites. The rate constants for yCD-catalyzed prodru
g conversion (k(1)(app)), 5-FU efflux from the observable tumor volume (k(2
)(app)), and formation of cytotoxic fluorinated nucleotides from 5-FU (k(3)
(app)) were 0.49 +/- 0.27 min(-1), 0.766 +/- 0.006 min(-1), and 0.0023 +/-
0.0007 min(-1), respectively. The best fits of the 5-FU concentration data
assumed first-order kinetics, suggesting that yCD was not saturated in vivo
in the presence of measured intratumoral 5-FC concentrations well above th
e in vitro K-m, These results demonstrate the feasibility of using magnetic
resonance spectroscopy to noninvasively monitor therapeutic transgene expr
ession in tumors. This capability provides an approach for measuring gene e
xpression that will be useful in clinical gene therapy trials.