PubMed Health⌕ Search

Biomedical subjects

A H Calvert

Publications and source records attributed to A H Calvert.

At least 127 records · Page 7Linked to original sources
PubMed1987

Recent preclinical and clinical studies with the thymidylate synthase inhibitor N10-propargyl-5,8-dideazafolic acid (CB 3717).

CB 3717, N10-propargyl-5,8-dideazafolic acid, is a tight-binding inhibitor of thymidylate synthase (TS) whose cytotoxicity is mediated solely through the inhibition of this enzyme. Recent preclinical studies have focused on the intracellular formation of CB 3717 polyglutamates. Following a 12-hour exposure of L1210 cells to 50 microM [3H]CB 3717, 30% of the extractable radioactivity could be accounted for as CB 3717 tetra- and pentaglutamate, as determined by high-pressure liquid chromatography (HPLC) analyses. As inhibitors of isolated L1210 TS, CB 3717 di-, tri-, tetra- and pentaglutamate are 26-, 87-, 119- and 114-fold more potent than CB 3717, respectively, and their formation may, therefore, be an important determinant of CB 3717 cytotoxicity. In early clinical studies with CB 3717, activity has been seen in breast cancer, ovarian cancer, hepatoma, and mesothelioma. Toxicities included hepatotoxicity, malaise, and dose-limiting nephrotoxicity. This latter effect is thought to be due to drug precipitation within the renal tubule as a result of the poor solubility of CB 3717 under acidic conditions. In an attempt to overcome this problem, a clinical trial of CB 3717 administered with alkaline diuresis is under way. Preliminary results at 400 and 500 mg/m2 suggest that a reduction in nephrotoxicity may have been achieved with only 1 instance of renal toxicity in 10 patients. Hepatotoxicity and malaise are again the most frequent side effects. Evidence of antitumor activity has been seen in 3 patients. Pharmacokinetic investigations have shown that alkaline diuresis does not alter CB 3717 plasma levels or urinary excretion and that satisfactory urinary alkalinization can be readily achieved.

Adult↗
PubMed1986

The effect of the nucleoside transport inhibitor dipyridamole on the incorporation of [3H]thymidine in the rat.

Dipyridamole is a non-specific inhibitor of nucleoside transport into mammalian cells. It is currently undergoing clinical evaluation in combination with various antimetabolites in an attempt to enhance the activity of these anticancer drugs by blocking the salvage of extracellular nucleosides, an important determinant of their cytotoxicity. In the present study, the effect of i.v. infusions of dipyridamole on [3H]thymidine incorporation into DNA has been examined in the anaesthetized rat. The tissues studied were bone marrow, gastrointestinal tract epithelium and the ascitic form of the Walker carcinosarcoma. Dipyridamole at 10 mg/kg, given over 3 hr, led to plasma levels of less than 5 microM and did not reduce [3H]thymidine incorporation into any of the tissues studied. At 40 mg/kg dipyridamole (plasma levels 10-15 microM) [3H]thymidine incorporation into the DNA of bone marrow and gastrointestinal tract epithelium was reduced to 20-30% of control values. Increasing the dose to 100 mg/kg did not lead to a further suppression of incorporation. Measurement of [3H]thymidine plasma pharmacokinetics and the intracellular distribution of tritium suggested that the inhibition of [3H]thymidine incorporation was due to reduced cellular uptake. In contrast to the effects on normal tissues, even at a lethal dose (200 mg/kg) dipyridamole did not significantly inhibit [3H]thymidine incorporation into Walker tumour cells. The levels of dipyridamole found in the ascitic fluid, at 100 mg/kg approximately half those in plasma, argue against a pharmacokinetic basis for this difference. Dipyridamole was found to bind extensively (97%) to rat plasma proteins, which may explain the discrepancy between the concentrations of dipyridamole required to inhibit nucleoside incorporation in vitro, in serum-free media, and those needed in vivo. From a comparison of the plasma levels of dipyridamole which cause an inhibition of [3H]thymidine incorporation in the rat with those which can be achieved safely in patients, it is concluded that dipyridamole is unlikely to markedly reduce nucleoside salvage in man.

Animals↗
PubMed1986

Phase II trial of carboplatin (JM8) in treatment of patients with malignant mesothelioma.

Seventeen patients with malignant mesothelioma were treated in a phase II study with carboplatin, a cisplatin analogue without significant nephrotoxicity or neurotoxicity. The drug was given in a dose of 300-400 mg/m2 by i.v. infusion, repeating at 28-day intervals. One patient achieved a complete clinical and radiological remission of 15 months' duration, and a second patient achieved a partial response of 11 months' duration (overall response rate 12%; overall response rate in previously untreated patients 20%). Four other previously untreated patients achieved symptomatic relief. Treatment was well tolerated without severe side-effects. Carboplatin, like most other cytotoxic drugs, is active only in a small minority of patients with mesothelioma, but its ability to achieve occasional good responses and frequent symptomatic relief, combined with low toxicity, may justify a short therapeutic trial in patients whose tumour is symptomatic.

Adult↗
PubMed1986

Quinazoline antifolates inhibiting thymidylate synthase: benzoyl ring modifications.

Four new analogues of the antifolate N10-propargyl-5,8-dideazafolic acid were prepared that were substituted in the benzoyl ring. The 2'-chloro and 2'-methyl analogues were prepared from the appropriately substituted p-nitrobenzoic acids. The route to the 3'-chloro and 3',5'-dichloro analogues was by chlorination of diethyl N10-propargyl-5,8-dideazafolate and diethyl N-[4-(prop-2-ynylamino)benzoyl]-L-glutamate, respectively, using sulfuryl chloride. The compounds were tested for their inhibition of purified L1210 thymidylate synthase (TS), for their inhibition of purified L1210 dihydrofolate reductase (DHFR), and for their inhibition of the growth of L1210 cells in culture. The 2'-chloro substituent reduced the TS inhibition by twofold and the 2'-methyl substituent reduced it by 20-fold; the 3'-chloro and 3',5'-dichloro derivatives were very poor inhibitors. The substituents only slightly affected the DHFR inhibition. None of the compounds improved upon N10-propargyl-5,8-dideazafolic acid in inhibiting the growth of L1210 cells in culture.

Folic Acid↗
PubMed1986

Quinazoline antifolates inhibiting thymidylate synthase: variation of the amino acid.

Five new analogues (1c-g) of the antifolate N10-propargyl-5,8-dideazafolic acid (1a) are described in which the benzoyl-L-glutamate moiety was replaced by benzoic acid (desglutamyl-N10-propargyl-5,8-dideazafolic acid), benzoyl-L-aspartate, 4-phenylbutyrate, benzoylglycine, and benzoyl-L-alanine. The esters of the appropriate 4-aminophenyl (benzoyl) starting materials were sequentially alkylated upon nitrogen, first with a propargyl halide and then with 2-amino-6-(bromomethyl)-4-hydroxyquinazoline hydrobromide. Saponification of the antifolate esters so produced gave the desired analogues. The new derivatives (1c-g) and also the known diethyl ester of 1a (1b) were tested for their inhibition of purified L1210 thymidylate synthase (TS) and for their inhibition of the growth of L1210 cells in culture. The TS inhibition of the analogues 1b-g was estimated by calculating the inverse relative potency, defined as the ratio IC50(compound)/IC50(1a). The results obtained were as follows: greater than 62, 84, 9, 333, 21, and 5, respectively. All were thus less inhibitory than 1a. None of the compounds improved upon 1a in inhibiting the growth of L1210 cells in culture.

Animals↗
PubMed1986

A phase I evaluation of the quinazoline antifolate thymidylate synthase inhibitor, N10-propargyl-5,8-dideazafolic acid, CB3717.

CB3717 is a quinazoline antifolate whose cytotoxic activity is mediated by inhibition of thymidylate synthase (TS). A phase I clinical trial commenced in September 1981 and 99 patients have received 296 treatments. Doses were dissolved in 0.15 mol/L NaHCO3 (pH 9.0) at a concentration of 4 mg/mL infused over one hour or in a total volume of 1 L infused over 12 hours. Doses were repeated every 3 weeks. The starting dose of 140 mg/m2 was escalated to 600 mg/m2. Renal toxicity, detected by a decrease in the 51Cr EDTA clearance, was dose-related and occurred in seven of ten patients receiving greater than 450 mg/m2. Reversible hepatic toxicity often associated with malaise occurred in 223 of 288 assessable courses (77%). Fifty-nine courses (20%) were associated with increases in alanine transaminase (ALT) levels to greater than 2.5 times the upper limit of the normal laboratory range. Increases in alkaline phosphatase levels also occurred, but were less marked. The severity and prevalence of these elevations were unaffected by the duration of the infusion. A self-limiting rash appeared in 12 patients and a radiation recall reaction was seen in two. Leukopenia developed in 17 patients (WBC less than 3 X 10(9)/L), and thrombocytopenia occurred in six patients (platelets less than 100 X 10(9)/L). The mean leucocyte nadir occurred on day 10 and was followed by recovery at 11 to 19 days. Neither the incidence nor the severity of any of these latter toxicities was dose related. The maximum tolerated dose was in the region of 600 mg/m2 with renal toxicity being dose limiting, although the inter-patient variation did not allow a precise definition. Seventy-six patients were evaluable for response. Responses occurred at doses greater than or equal to 200 mg/m2 and were ovary, one complete response (CR), one partial response (PR), seven minor responses (MR) in 30 cases; breast, two PRs and one MR in eight cases; adenocarcinoma of the lung, one MR in 5 cases; mesothelioma, one PR in five cases; and colon, two MRs in four cases. CB3717 has activity in heavily pretreated patients. The recommended phase II dose for good-risk patients is 400 mg/m2 using the one-hour infusion schedule of administration.

Acetylglucosaminidase↗
PubMed1986

Increased thymidylate synthase in L1210 cells possessing acquired resistance to N10-propargyl-5,8-dideazafolic acid (CB3717): development, characterization, and cross-resistance studies.

The properties are described of a mutant L1210 cell line (L1210:C15) with acquired resistance (greater than 200-fold) to the thymidylate synthase (TS) inhibitor N10-propargyl-5,8-dideazafolic acid. TS was overproduced 45-fold and was accompanied by a small increase in the activity of dihydrofolate reductase (2.6-fold). Both the level of resistance and enzyme activities were maintained in drug-free medium (greater than 300 generations). Failure of N10-propargyl-5,8-dideazafolic acid to suppress the [3H]-2'-deoxyuridine incorporation into the acid-precipitable material of the resistant line supported the evidence that TS overproduction was the mechanism of resistance; consequently the L1210:C15 cells were largely cross-resistant to another (but weaker) TS inhibitor, 5,8-dideazafolic acid. Minimal cross-resistance was observed to the dihydrofolate reductase inhibitors methotrexate and 5-methyl-5,8-dideazaaminopterin (5- and 2-fold, respectively). L1210 and L1210:C15 cells were, however, equally sensitive to 5-fluorodeoxyuridine (FdUrd), an unexpected finding since a metabolite, 5-fluorodeoxyuridine monophosphate, is a potent TS inhibitor; however, this cytotoxicity against the L1210:C15 cells was antagonized by coincubation with 5 microM folinic acid although folinic acid potentiated the cytotoxicity of FdUrd to the N10-propargyl-5,8-dideazafolic acid-sensitive L1210 line. Thymidine was much less effective as a FdUrd protecting agent in the L1210:C15 when compared with the L1210 cells; however, a combination of thymidine plus hypoxanthine was without any additional effect (compared with thymidine alone) against the sensitive line but effectively protected L1210:C15 cells such that the concentration of FdUrd necessary to reduce the cell count to 50% of control at 48 h was increased greater than 11,000-fold. We propose that the elevated TS levels result in sequestration of the reduced-folate pool (as N5,10-methylene tetrahydrofolic acid) into the TS ternary complex with 5-fluoro-2'-deoxyuridine 5'-monophosphate. Despite "free" TS, the de novo synthesis of thymidylate and purines is inhibited by substrate depletion. The fact that folinic acid is able to reverse the inhibition of [3H]-2'-deoxyuridine incorporation by FdUrd into the resistant cells supports this hypothesis.

Animals↗
PubMed1986

Pharmacokinetics of the thymidylate synthase inhibitor N10-propargyl-5,8-dideazafolic acid (CB3717) in the mouse.

The tissue distribution, excretion, and metabolism of the thymidylate synthase inhibitor N10-propargyl-5,8-dideazafolic acid (CB3717) have been investigated in the mouse. Following 100 mg/kg of 2-14C-CB3717 ip, levels of radioactivity in the brain, testes, muscle, heart, and lung equilibrated slowly with those in the plasma and were no longer significantly lower 5 hours (lung) and 12 hours (brain, testes, muscle, and heart) after administration. In contrast, concentrations of 14C in the liver and kidney were markedly higher than those in the plasma at all time points studied (1.3 hours-23 days). High-performance liquid chromatographic (HPLC) analysis of livers removed 5 hours after drug administration and kidneys excised 24 hours after treatment indicated that, at these time points, greater than 50% of the radioactivity was in the form of unchanged CB3717. Furthermore, HPLC analysis of plasma removed over the period 0.5-6 hours demonstrated that all of the 14C could be accounted for as CB3717. Although the accumulation and retention of radioactivity in the liver and kidney were also apparent following 20 and 200 mg/kg of 14C-CB3717, the effect was less marked at the lower dose, thereby suggesting dose-dependent pharmacokinetics. In excretion studies (0-48 hours), the major route of elimination was found to be via the feces, with 46% of the 14C recovered; 26% of the dose was recovered as unchanged CB3717. Radioactivity excreted in the urine accounted for 20% of the administered 14C, while CB3717 eliminated via this route represented 15% of the dose. In addition to CB3717, a metabolite was detected in the feces which comprised 8% of the dose administered. The metabolite was shown to be 4-(N-((2-amino-4-hydroxy-6-quinazolinyl)methyl)prop-2-ynylamino) benzoic acid (CB3751) by HPLC and mass spectrometry. The formation of CB3751 could be catalyzed in vitro by the contents of the cecum and prevented in vivo by antibiotic pretreatment and is therefore considered to be the result of bacterial metabolism. CB3717 binds extensively to plasma proteins (92%; concentration range, 25-250 microM). These studies have shown that CB3717 does not apparently undergo extensive host metabolism in vivo, and therefore the biological properties of this novel antimetabolite are probably a function of the parent compound. In addition, the accumulation of CB3717 in the liver and kidney may be related to the hepatotoxic and nephrotoxic effects of this drug.

Animals↗
PubMed1985

The clinical pharmacokinetics of the novel antifolate N10-propargyl-5,8-dideazafolic acid (CB 3717).

The pharmacokinetics of the new antifolate CB 3717 were studied in 20 patients during its phase-I clinical evaluation. The drug was administered at doses of 100-550 mg/m2 in 1-h and 12-h infusions, resulting in peak plasma concentrations of CB 3717 of 40-200 microM. There was a linear relationship between the dose and both CB 3717 AUC and peak plasma levels. Following a 1-h infusion, drug levels in the plasma decayed biphasically (t1/2 alpha = 49 +/- 9 min, t1/2 beta = 739 +/- 209 min). 27% +/- 2% of the dose was excreted in urine in the 24-h period after treatment, suggesting that the major route of elimination was via the bile. Furthermore, the parent compound CB 3717 and its desglutamyl metabolite, CB 3751, were found in a faecal collection although the metabolite was not detected in plasma or urine samples. Plasma protein binding of CB 3717 was extensive (97.6% +/- 0.1%). Significant quantities of CB 3717 penetrated into ascitic fluid but not into cerebrospinal fluid. Residual drug was detected in postmortem kidney tissue from a patient who died of progressive disease 8 days after treatment with 330 mg/m2 CB 3717. Thus, dose-limiting renal toxicity (maximum tolerated dose 600 mg/m2) may be due to drug precipitation in the renal tubules. Elevation of liver enzymes, in particular transaminases, occurred frequently as a toxic manifestation of CB 3717 therapy. In 11 patients studied after their first treatment there was a positive correlation between the rise in serum alanine transaminase and peak drug levels (r = 0.69, P = 0.02). These pharmacokinetic studies have shown that, by analogy with experimental systems, cytotoxic plasma levels of CB 3717 are archieved in man. In addition, they have been valuable in interpreting toxicities observed during phase-I clinical studies.

Antineoplastic Agents↗
PubMed1985

Phase I studies with carboplatin at the Royal Marsden Hospital.

Carboplatin was evaluated in a Phase I and pharmacokinetic study at the Royal Marsden Hospital between April 1981 and November 1981. Sixty patients were entered of whom 16 had impaired renal function. No evidence of ototoxicity or nephrotoxicity was found. Nausea and vomiting were much reduced compared to cisplatin. The dose limiting toxicity was delayed myelosuppression with thrombocytopenia being more severe than leucopenia. A number of clinical responses were seen, particularly in patients with ovarian carcinoma. Pharmacokinetic studies suggested that the dose of JM8 should be adjusted according to the glomerular filtration rate.

Antineoplastic Agents↗
PubMed1985

Quinazoline antifolates inhibiting thymidylate synthase: variation of the N10 substituent.

The synthesis of 12 new 5,8-dideazafolates with isopropyl, cyclopropylmethyl, 2-fluoroethyl, carbamoylmethyl, phenacyl, 3-fluorobenzyl, 5-uracilylmethyl, carboxymethyl, 2-carboxyethyl, 3-cyanopropyl, 3-hydroxypropyl, and cyanomethyl substituents at N10 is described. In general, the synthetic route involved monoalkylation of diethyl N-(4-amino-benzoyl)-L-glutamate, coupling of the resulting secondary amine with 2-amino-6-(bromomethyl)-4-hydroxyquinazoline hydrobromide in N,N-dimethylacetamide with calcium carbonate as the base, and deprotection using mild alkali. The cyanomethyl derivatives was found to be unexpectedly base labile and was therefore prepared by mild acid deprotection of a di-tert-butyl ester. The compounds were tested as inhibitors of purified L1210 thymidylate synthase (TS). Four members of the series were more potent that the N10-hydrogen compound, but none was superior to the previously described N10-propargyl-5,8-dideazafolic acid. Selected compounds were examined as inhibitors of purified L1210 dihydrofolate reductase (DHFR). As desired, N10 substitution in general reduced DHFR inhibitory activity; these results are discussed. As a measure of cytotoxicity, the compounds were examined for their inhibition of the growth of L1210 cells in culture. None of the new substituents conferred enhanced potency relative to N10-propargyl-5,8-dideazafolic acid (ID50 = 5 microM), which, as the best TS inhibitor and a relatively poor DHFR inhibitor, continues to lead this series.

Alkylation↗
PubMed1985

Carboplatin: a very active new cisplatin analog in the treatment of small cell lung cancer.

Carboplatin, a cisplatin analog without significant clinical nephrotoxicity, has been evaluated in the treatment of 56 patients with small cell lung carcinoma at a dose of 300-400 mg/m2 iv monthly in a phase II study. Twenty-three patients (41%) achieved a response, including five (9%) complete remissions. Of 30 previously untreated patients, 18 (60%) achieved a response, including three (10%) complete remissions. Median response duration was 4.5 months (range, 2-9). No nephrotoxicity was seen and hydration was not required. Nausea or vomiting occurred in only 24 patients (43%) and was rarely severe. Myelosuppression was dose-limiting: 20 patients (36%) developed leukopenia and eight (14%) developed thrombocytopenia, but leukopenic infections occurred in only three patients. Carboplatin is a very active new agent in the treatment of small cell lung cancer, with less toxicity and better tolerance than cisplatin. It merits further investigation in combination chemotherapy and against non-small cell lung cancer.

Adult↗
PubMed1984

Modulation of anti-metabolite effects. Effects of thymidine on the efficacy of the quinazoline-based thymidylate synthetase inhibitor, CB3717.

CB3717 (N-(4-(N-((2-amino-4- hydroxy-6-quinazolinyl)methyl)prop-2-ynylamino ) benzoyl)-L-glutamic acid) is an antitumour agent that inhibits thymidylate synthetase (TS). A dose-dependent fall in plasma thymidine (dThd) (1.43 microM to 0.47 microM) occurred in non-tumour-bearing mice following the administration of CB3717. Further, in mice carrying the L1210/CBRI tumour, the drug's antitumour properties were ablated by co-administration of dThd, an effect consistent with TS being the cytotoxic locus. In vitro studies of protection by dThd against CB3717 cytotoxicity were carried out in an attempt to quantify this reversal. The metabolism of [14C]-dThd was measured in cultures of L1210 cells (10(4)/ml) exposed to a completely cytotoxic dose of CB3717 (50 microM). The cytotoxicity of the drug was only expressed when the dThd concentration (0.5-2 microM) had fallen to less than 0.1 microM in the media. This reduction was due to: (1) dThd incorporation into DNA, (2) catabolism of dThd to thymine. By reducing the initial cell concentration to 10(3)/ml the depletion of dThd was substantially reduced and consequently cells continued to grow for a longer period. The critical concentration of dThd, below which growth in the presence of CB3717 could not be supported was estimated to be between 0.026 and 0.1 microM. Thus even the minimum level of dThd achieved in vivo was still in excess of that required for protection from CB3717 toxicity in vitro. There was a small accumulation of deoxyuridine (dUrd) (approximately 2-fold) in mouse plasma 24 hr after completion of a 5-day course of CB3717 (200 mg/kg) but in vitro studies demonstrated that this was unlikely to modulate CB3717 toxicity in the presence of dThd. We caution against the use of rodent tumour models (or human tumour xenografts) for antitumour or toxicity testing of compounds designed to inhibit the de novo synthesis of thymidylate; they may be misleading because the high dThd levels found in these animals compared with man may mask the cytotoxic effects of these drugs.

Animals↗
PubMed1984

Pharmacokinetics of cis-diammine-1,1-cyclobutane dicarboxylate platinum(II) in patients with normal and impaired renal function.

cis-Diammine-1,1-cyclobutane dicarboxylate platinum(II) (CBDCA, JM8) is a nonnephrotoxic analogue of cisplatin currently undergoing clinical evaluation. Pharmacokinetic studies have been performed in patients receiving CBDCA (20 to 520 mg/sq m) as a 1-hr infusion without hydration or diuresis. Following the end of the infusion, plasma levels of total platinum and ultrafilterable (Mr less than 50,000) platinum (free platinum) decayed biphasically with first-order kinetics (total platinum t alpha 1/2 = 98 min; t beta 1/2 range, 399 to greater than 1440 min; free platinum t alpha 1/2 = 87 min; t beta 1/2 = 354 min). During the first four hr, binding of platinum to plasma protein was limited (24%), with most of the free platinum in the form of unchanged CBDCA (94%). However, by 24 hr, the majority of platinum was protein bound (87%). The major route of elimination was renal, 65% of the platinum administered being excreted in the urine within 24 hr, with 32% of the dose excreted as unchanged CBDCA. No evidence was found from studies on the renal clearance of free platinum to indicate renal tubular secretion (mean free platinum renal clearance, 69 ml/min). However, the plasma clearance of free platinum did correlate positively with glomerular filtration rates (p = 0.005). None of the pharmacokinetic parameters determined were dose dependent. In vitro studies with plasma and urine demonstrated that, in contrast to cisplatin, CBDCA is a stable complex [t 1/2 - 37 degrees; plasma, 30 hr, and urine (range), 20 to 460 hr]. The differences in the pharmacokinetics of cisplatin and CBDCA may explain why the latter complex is not nephrotoxic.

Antineoplastic Agents↗
PubMed1983

Biochemical effects of a quinazoline inhibitor of thymidylate synthetase, N-(4-(N-(( 2-amino-4-hydroxy-6-quinazolinyl)methyl)prop-2-ynylamino) benzoyl)-L-glutamic acid (CB3717), on human lymphoblastoid cells.

The biochemical effects of the antitumor agent N-(4-(N-(( 2-amino-4-hydroxy-6-quinazolinyl)methyl)prop-2-ynylamino) benzoyl)-L -glutamic acid (CB3717) were studied in WI-L2 cultured human lymphoblastoid cells. CB3717 was a potent inhibitor of human thymidylate synthetase; the inhibition was competitive with 5,10-methylenetetrahydrofolate (Ki = 4.9 X 10(-9) M). CB3717 also inhibited human dihydrofolate reductase, competitively with dihydrofolate (Ki = 2.3 X 10(-8) M). The growth-inhibitory effect of CB3717 could be prevented completely by 10 microM thymidine. Administration of thymidine could be delayed for up to 8 hr after CB3717 treatment without cytotoxicity but, if thymidine was delayed for 24 hr, severe toxicity resulted. Incubation for 16 hr in the presence of a growth-inhibitory concentration of CB3717 did not result in the appearance of dihydrofolate in WI-L2 cells. These results indicate that, in the presence of CB3717, thymidylate synthetase, rather than dihydrofolate reductase, became rate-limiting for the cycle of dihydrofolate oxidation and reduction. Treatment of cells for 16 hr at an IC50 concentration of CB3717 caused a decrease of 88% in cellular dTTP and a 2,300% increase in dUMP. The level of dUDP also increased, and traces of dUTP appeared in treated cells. No large changes were seen in ribonucleotide pools. A kinetic analysis was made, by computer simulation, of predicted consequences of metabolic effects of compounds that inhibit both dihydrofolate reductase and thymidylate synthetase. It was concluded that, even if the Ki of the inhibitor for thymidylate synthetase were 3 orders of magnitude higher (weaker) than the Ki for dihydrofolate reductase, thymidylate synthetase could still become rate-limiting.

Animals↗