Pharmacokinetics and the bystander effect in CD::UPRT/5-FC bi-gene therapy of glioma.
BACKGROUND
Cytosine deaminase (CD) converts 5-fluorocytosine (5-FC) to 5-fluorouracil (5-FU) in CD/5-FC gene therapy, 5-FU will be mostly converted into nontoxic beta-alanine without uracil phosphoribosyltransferase (UPRT). UPRT catalyzes the conversion of 5-FU to 5-fluorouridine monophosphate, which directly kills CD::UPRT-expressing cells and surrounding cells via the bystander effect. But the pharmacokinetics and the bystander effect of CD::UPRT/5-FC has not been verified in vivo and in vitro. Before the CD::UPRT/5-FC bi-gene therapy system is used in clinical trial, it is essential to monitor the transgene expression and function in vivo. Thus, we developed a preclinical tumor model to investigate the feasibility of using (19)F-magnetic resonance spectroscopy ((19)F-MRS) and optical imaging to measure non-invasive CD and UPRT expression and its bystander effect.


METHODS
C6 and C6-CD::UPRT cells were cultured with 5-FC. The medium, cells and their mixture were analyzed by (19)F-MRS. Rats with intracranial xenografted encephalic C6-CD::UPRT glioma were injected intraperitoneally with 5-FC and their (19)F-MRS spectra recorded. Then the pharmacokinetics of 5-FC was proved. Mixtures of C6 and C6-CD::UPRT cells at different ratios were cultured with 5-FC and the cytotoxic efficacy and survival rate of cells recorded. To determine the mechanism of the bystander effect, the culture media from cell comprising 25% and 75% C6-CD::UPRT cells were examined by (19)F-MRS. A comparative study of mean was performed using analysis of variance (ANOVA).


RESULTS
(19)F-MRS on samples from C6-CD::UPRT cells cultured with 5-FC showed three broad resonance signals corresponding to 5-FC, 5-FU and fluorinated nucleotides (F-Nuctd). For the C6 mixture, only the 5-FC peak was detected. In vivo serial (19)F-MRS spectra showed a strong 5-FC peak and a weak 5-FU peak at 20 minutes after 5-FC injection. The 5-FU concentration reached a maximum at about 50 minutes. The F-Nuctd signal appeared after about 1 hour, reached a maximum at around 160 minutes, and was detectable for several hours. At a 10% ratio of C6-CD::UPRT cells, the survival rate was (79.55 +/- 0.88)% (P < 0.01). As the C6-CD::UPRT ratio increased, the survival rate of the cells decreased. (19)F-MRS showed that the signals for 5-FU and F-Nuctd in the culture medium increased as the ratio of C6-CD::UPRT in the mixture increased.


CONCLUSIONS
(19)F-MRS studies indicated that C6-CD::UPRT cells could effectively express CD and UPRT enzymes. The CD::UPRT/5-FC system showed an obvious bystander effect. This study demonstrated that CD::UPRT/5-FC gene therapy is suitable for 5-FC to F-Nuctd metabolism; and (19)F-MRS can monitor transferred CD::UPRT gene expression and catalysis of substrates noninvasively, dynamically and quantitatively.
Shi
De-zhi
DZ
Department of Neurosurgery, First Affiliated Hospital of Nanjing Medical University, Nanjing, Jiangsu 210029, China.
Hu
Wei-xing
WX
Li
Li-xin
LX
Chen
Gong
G
Wei
Dong
D
Gu
Pei-yuan
PY
D000818:Animals; D000963:Antimetabolites / Q000493:pharmacokinetics / Q000627:therapeutic use; D002460:Cell Line; D043525:Cytosine Deaminase / Q000235:genetics / Q000502:physiology*; D005437:Flucytosine / Q000493:pharmacokinetics* / Q000627:therapeutic use*; D015316:Genetic Therapy / Q000379:methods*; D005910:Glioma / Q000188:drug therapy / Q000628:therapy*; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D010430:Pentosyltransferases / Q000235:genetics / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley
D000963:Antimetabolites; D005437:Flucytosine; D010430:Pentosyltransferases; C023269:uracil phosphoribosyltransferase; D043525:Cytosine Deaminase