In Vitro and in Vivo Evaluation
of an Innocuous Drug
Cocktail To Improve the Quality of Folic Acid Targeted Nuclear Imaging
in Preclinical Research
- Publication date
- Publisher
Abstract
Folate receptor (FR) targeting is an attractive strategy
for nuclear
imaging of cancer and activated macrophages through application of
folic acid radioconjugates. However, significant renal accumulation
of folate radioconjugates and hence exceedingly high emission of radiation
from the kidneys may mask uptake of radioactivity at sites of interest
such as small metastases in the abdominal region of animal models
of ovarian cancer. Recently it was observed that the antifolate pemetrexed
(PMX) reduces undesired renal uptake of radiofolates. A disadvantage
of this strategy is the fact that pemetrexed is a chemotherapeutic
agent which may have toxic side effects. The aims of this study were
therefore to investigate whether the desired effect of PMX to reduce
renal accumulation of folate radioconjugates would be maintained if
it was applied as a cocktail together with its antidote, thymidine,
and to explore whether thymidine was an effective rescue agent against
PMX’s related toxicity in vitro and in vivo. In vitro internalization
of <sup>67</sup>Ga-EC0800 was investigated using FR-positive KB tumor
cells and embryonic monkey MA104 kidney cells in the absence and presence
of PMX alone and in combination with thymidine. Uptake of <sup>67</sup>Ga-EC0800 into KB cells was increased by coincubation of the cells
with PMX. In contrast uptake of <sup>67</sup>Ga-EC0800 into MA104
cells was reduced under the same conditions. In both cell lines coincubation
of thymidine did not change the results obtained with PMX. Biodistribution
and SPECT/CT imaging studies of <sup>67</sup>Ga-EC0800 were performed
with KB tumor bearing mice injected with PMX alone or with a cocktail
of PMX and thymidine. The radiofolate’s kidney uptake reducing
effect of PMX in mice was maintained also if PMX was employed together
with its antidote thymidine. The tumor uptake of <sup>67</sup>Ga-EC0800
remained unchanged independent of the administration of PMX or a combination
of PMX and thymidine. The effect of thymidine to abrogate PMX-induced
cytotoxicity was demonstrated in vitro with an MTT assay using KB
and MA104 cells. Cell viability was reduced to 50% (KB cells) and
70% (MA104 cells) of untreated controls if PMX (5 μM and 15 μM, respectively)
was coincubated. Addition of thymidine
(10 μM or 100 μM) compensated PMX’s toxic effects
in a dose-dependent manner. The effect of thymidine was also investigated
in non-tumor bearing mice treated with high-dosed PMX. Rescue of mice
with side effects after the third and fourth cycles of PMX application
(1 mg/mouse) was achieved by application of thymidine (20 mg/mouse)
at five consecutive days starting the day of PMX injection. Application
of PMX together with thymidine as a cocktail is effective to improve
the tissue distribution of radiofolates while preventing pharmacological
and potentially toxic side effects of the chemotherapeutic agent PMX.
These findings open new perspectives for folate-based nuclear imaging
in preclinical research potentially allowing longitudinal investigations
and monitoring therapies in animal models of cancer and inflammatory
diseases
