Is there a role for thymidine in cancer chemotherapy?

Resistance to differentiation affects ribo- and deoxyribonucleotide pools and sensitivity to pyrimidine metabolism antagonists in HL60 cells

HL60 myeloid leukemia cells are extensively used as a differentiation model. We investigated a variant of HL60 which is resistant to differentiation induction (HL60-R) by standard differentiation inducers such as retinoic acid and dimethylsulfoxide (DMSO). To find an explanation for this resistance, we examined nucleotide (NTP) and deoxynucleotide (dNTP) pools in HL60-R and its parent cell line, sensitive to differentiation, HL60-S. We also explored whether these differences led to a difference in sensitivity to various antimetabolites. Drug sensitivity was measured with the tetrazolium (MTT) assay, while nucleotides were measured with anion-exchange HPLC. HL60-R cells were between 2- and 5-fold resistant to the antimetabolites 5-fluorouracil, Brequinar, hydroxyurea and N-(phosphonacetyl)-L-aspartate (PALA), but more sensitive to aza-2'-deoxycytidine (DAC), cytarabine and thymidine (5- to 10-fold). The NTP pools in both HL60 variants showed a normal pattern with ATP being the highest (2530-2876 pmol/10⁶ cells) and CTP being lowest. However, UTP pools were 2-fold higher in the HL60-S cells (p < .01), while CTP and GTP pools were 30% higher (p < .01) compared to HL60-R cells. For the dNTP pools, larger differences were observed, with dATP (50 pmol/10⁶ cells) being highest in HL60-R cells, but dATP was 4-fold lower in HL60-S cells. In HL-60-R, the triple combination retinoic acid, DMSO and DAC increased all NTPs almost 2-fold in contrast to HL60-S. Uridine increased UTP (1.4-fold), CTP (2-fold) and dCTP (1.4.-fold) pools in both cell lines, but thymidine increased only dTTP pools (4- to 7-fold), with a depletion of dCTP. PALA decreased UTP and CTP in both cell lines, but increased ATP (only in HL60-R). Hydroxyurea decreased dNTP especially in HL60-S cells. In conclusion, the pronounced differences in NTP and dNTP pools between HL60-S and HL60-R possibly play a role in the induction of differentiation and drug sensitivity.

Folinic acid modulation of fluorouracil: tissue kinetics of bolus administration

Thymidylate synthase (TS) is the enzyme target of 5-fluorouracil (FUra) that recent laboratory and clinical studies with folinic acid (calcium leucovorin) suggest may mediate important antitumor cytotoxicity. Measurement in carcinoma tissue of parameters related to TS inhibition by 5-fluorodeoxyuridylate (FdUMP), by analogy to hormone receptor analysis, should be useful to determine which patients should receive fluoropyrimidine drug therapy and to evaluate folinic acid requirements. Folinic acid is metabolized to 5,10-methylenetetrahydropteroylglutamine (CH2FH4), which must be present in large excess to effect desired levels of maximal inhibition of TS, by promoting formation and stabilization of TS-FdUMP-CH2FH4 ternary complexes. In patients with metastatic disease, serial biopsies of tumor and normal tissues for studies of pharmacodynamic responses to test-dose FUra or folinic acid are shown to be easily added to routine intraoperative management. A suitable methodologic approach is described and examples given of assays of free TS, FdUMP, dUMP, and CH2FH4 levels after FUra or folinic acid, that may be useful in future studies aimed at improving the cost-effectiveness of FUra-folinic acid combinations.

A sensitive non-radioactive assay for pyrimidine nucleoside phosphorylase using reversed-phase high performance liquid chromatography

A new sensitive assay for pyrimidine nucleoside phosphorylase using non-radioactive substrates is described. With the natural substrate uridine (UR) and the analog, 5'-deoxy-5-fluorouridine (5'dFUR) conditions have been optimized to measure the product formation with reversed-phase high-performance liquid chromatography. Using automated injection large series of samples may be analyzed. The assay for UR phosphorylase appeared to be comparable to existing methods with radiolabeled UR as substrate regarding sensitivity and linearity. The assay has been used to measure kinetic parameters for 5'dFUR and UR in two cell lines from intestinal origin.

Low concentrations of thymidine modulate allogeneic responses

The effect of the addition of low concentrations of thymidine on the development of primary allogeneic immune responses in culture was studied. It was found that, dependent upon day of addition to culture, allogeneic cytotoxic T-lymphocyte generation was augmented by thymidine in the concentration range of 10(-9) to 10(-8) M while specific T-suppressor cell development was inhibited by 10(-7) M thymidine. Based on the concentration of thymidine in serum, its addition in the concentration range of 10(-9) to 10(-7) M results in 0.1 to 10 fold concentration change in cultures supplemented with 10% serum. While the mechanism(s) by which such minor changes in thymidine concentrations induced the modulations seen have not been elucidated, it is clear that such changes are easily attainable. In fact, such variations in the concentration of thymidine could contribute to the variable results often observed with different lots of fetal calf serum and workers investigating immunomodulations in culture systems should be aware of this possibility.

Production of the effector molecule thymidine by human lung alveolar macrophages

Cultured human alveolar macrophages (HAM phi) were found to produce soluble factors which inhibit tritiated thymidine [( 3H]-TdR) incorporation into tumour cells in vitro. We present evidence that thymidine (TdR) can be detected in HAM phi culture supernatants by thin-layer chromatography. Moreover, TdR secretion by HAM phi is an active process. Using [6-14C]-orotic acid as an early precursor to TdR and [3H]-TMP as the phosphorylation product, we were able to show that cultured HAM phi transformed them into labelled TdR. The very efficient thin-layer chromatography of the labelled metabolites was backed up by high-pressure liquid chromatography of nucleotides. HAM phi produce TdR by de novo synthesis and dephosphorylation. This phenomenon is due to the lack of thymidine kinase in normal mature macrophages. Since TdR, in high concentrations, can inhibit DNA synthesis through the 'TdR blockade' phenomenon, it is suggested that TdR secretion by HAM phi is a mechanism of non-specific modulation of tumour cell growth but highly restricted to the immediate macrophage microenvironment in vivo. The effectiveness of the thymidine gradient may thus be quite narrow, but is worthy of interest.

Clinical and biochemical studies of high-dose thymidine treatment in patients with solid tumors

In a clinical study of high-dose thymidine (TdR) treatment, toxic effects, TdR metabolism, and the influence of TdR on pyrimidine and purine metabolism were examined. Ten patients with solid tumors were treated with continuous infusion of TdR at 34-75 g/m2/day for 3 to 5 days. Hematologic toxicity occurred with 5-day TdR infusion at 75 g/m2/day but not when plasma TdR concentration failed to reach millimolar levels. In three patients who received similar TdR doses, plasma TdR levels were related to elimination rates of TdR and its metabolites from plasma. In one patient in whom urinary excretion was studied, 100% of the TdR dose given was recovered in the form of TdR, thymine (Thy), beta-aminoisobutyrate, and 5-hydroxymethyluracil (5-HMUra). The latter metabolite, which had not been previously described in high-dose TdR treatment, was also found in plasma at levels from 5% to 10% of those of TdR. No effects of high-dose TdR infusion on purine levels in plasma were observed, while a substantial increase in uracil levels was noted both in plasma and urine. These data provide further information on high-dose TdR treatment with regard to clinical, pharmacokinetic, and biochemical effects.