The hypothesis was tested that the increased
IMP dehydrogenase activity in human myelocytic leukemic cells, and along with it guanylate biosynthesis, might be a sensitive target to
chemotherapy by
tiazofurin. 1.
IMP dehydrogenase activity in normal leukocytes was 3.1 +/- 0.5 (means +/- S.E.) nmol/hr/mg
protein and in leukemic cells it was elevated 15- to 41-fold. The activity of
guanine phosphoribosyltransferase in normal leukocytes was 389 +/- 27 nmol/hr/mg
protein and in the leukemic cells it increased 2.8- to 6.8-fold. 2.
IMP dehydrogenase was purified 4,900-fold to homogeneity from rat
hepatoma 3924A with a yield of 30%. The kinetic properties of the
hepatoma enzyme were similar to those of the
enzyme in human myelocytic leukemic blast cells because of the similarity of the Km's for
IMP (23 microM),
NAD (44 and 65 microM); the Ki for TAD was 0.1 microM in both
enzymes. 3. There was a selectivity of the in vitro response to
tiazofurin in human normal and leukemic leukocytes. When labeled
tiazofurin was incubated with leukocytes from normal, healthy volunteers and from leukemic patients, the leukemic leukocytes made 20- to 30-fold more TAD and the
GTP content decreased as compared to normal leukocytes. This procedure proved to be a suitable predictive test in a clinical setting because patients with positive tests responded to
tiazofurin whereas those with negative ones did not. 4. The National Cancer Institute approved a chemotherapeutic phase I/II trial which concentrates on treatment of refractory
acute myelocytic leukemia.
Tiazofurin is infused in a 60-minute period with a pump to insure uniform delivery. A novel aspect of the trial was that it was directed primarily by the biochemical impact of
tiazofurin on
IMP dehydrogenase activity and
GTP concentration and the
tiazofurin doses were to be adjusted accordingly. Patients received
allopurinol as a routine precaution against possible accumulation of
uric acid in the kidney. 5. In the first eight patients, there was one complete remission, two entered the chronic phase, two entered into partial remission, one did not respond, and two were not evaluable. In the five patients who responded, there was a rapid, profound decrease in
IMP dehydrogenase activity of the blast cells and a gradual decline in
GTP concentrations. The blast cell count followed the decrease in the
GTP concentration. The white blood cell count was largely preserved. 6. Bone marrow aspirates and peripheral blood samples showed that with
tiazofurin treatment there was an induced differentiation of the myelocytes.(ABSTRACT TRUNCATED AT 400 WORDS)