The aim of this study was to identify targets for rational
chemotherapy of
glioblastoma. In order to elucidate differences in the biochemistry of
tumor and normal human brain, in vivo pool sizes of
purine nucleotides,
nucleosides, and nucleobases and of
purine metabolizing
enzymes in biopsy material from 14
grade IV astrocytomas and 4 normal temporal lobe samples were analyzed. Specimens were collected during surgery using the freeze-clamp sampling technique and analyzed by high pressure liquid chromatography. Total
purine nucleotides, adenylates, and guanylates in the
tumors were 2186, 1865, and 310 nmol/g (wet weight), respectively, which corresponds to 61, 60, and 71% of normal brain tissue concentrations. Relative to normal brain the
tumors had significantly lower
ATP and
GTP levels, essentially normal pool sizes of
purine nucleosides and bases, unchanged activities of the salvage
enzymes hypoxanthine-guanine phosphoribosyltransferase,
adenine phosphoribosyltransferase, and
adenosine kinase (659, 456, and 98 nmol/h/mg
protein, respectively) and 4-fold higher activities of
IMP dehydrogenase (11.6 nmol/h/mg
protein); the latter is the rate limiting
enzyme for guanylate de novo synthesis.
IMP pools in the
tumors were 64% of values in normal brain. Modulation of the guanylate pathway in
glioblastoma by inhibition of
IMP dehydrogenase with
tumor specific agents such as
tiazofurin (2-beta-D-ribofuranosylthiazole-4-carboxamide) appears to be a rational therapeutic approach. Preliminary in vitro experiments with normal and malignant tissue specimens from 2 additional patients revealed that significant amounts of the active metabolite
thiazole-4-carboxamide adenine dinucleotide are formed from
tiazofurin. At a concentration of 200 microM this
drug was able to deplete guanylate pools in the
tumors to a median of 54% of
phosphate buffered saline treated controls. Flux studies with [14C]
formate showed that
tiazofurin strongly inhibited de novo synthesis of guanylates in
glioblastoma to an average of 10% of controls. This effect was more pronounced in the
tumors as compared to normal brain. No inhibition of salvage of [14C]
guanine by
tiazofurin could be observed in normal and malignant tissues. Supportive measures have to be considered to inhibit the highly active salvage
enzyme hypoxanthine-guanine phosphoribosyltransferase that can partly antagonize a
tiazofurin induced decrease in
guanine nucleotides.