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Resistance to antifolates in Bacillus subtilis strains results from the presence of an antifolate resistance mutation (afo). Strains which are thyA(+)afo are unconditionally resistant to antifolates. The conditional resistance of thyA afo strains is hypothesized to be due to the thyB(+) gene product (thymidylate synthetase B) having a high K(m) for the folate substrate, thus leading to thymineless death in the presence of antifolates. An alternative model for conditional antifolate resistance was shown to be incorrect by analysis of folate metabolism in methotrexate-treated cells. Genetic analysis and studies of the response of afo(+) cells to methotrexate suggested that most, if not all, B. subtilis thymine-requiring mutants are afo. Analysis of dihydrofolate reductase from afo cells did not reveal an obvious mechanism for antifolate resistance in those cells.
One hundred thirty-eight adults with advanced cancers were treated with Baker's Antifol. The complete response + partial response rate was only 10%. Best responses were obtained in 31 patients with lung adenocarcinoma (complete response + partial response, 13%), in 25 patients with colorectal carcinoma (partial response, 16%), and in 6 patients with renal cell carcinoma (partial response, 50%). Two partial responses occurred in 15 patients with squamous cancer. No significant responses were seen in 27 patients with other adenocarcinomas, 13 with sarcomas, 14 with melanomas, and 8 with miscellaneous tumors. The most frequent toxicities were dermatitis, stomatitis, gastrointestinal symptoms, and mild myelosuppression. The incidence of dermatitis was significantly decreased by shortening the schedule of Baker's Antifol administration from 5 to 3 days. Baker's Antifol has some degree of antitumor activity, and studies of combination of this agent with other effective chemotherapeutic agents are indicated.
Fifty-two untreated patients with colorectal cancer were randomized to receive 5-fluorouracil (5-FU) alternating either with methotrexate (MTX) or Baker's Antifol (BAF) with or without the immunostimulant, levamisole (Program I). Fifty-five patients who had received prior treatment were randomized to receive methyl-CCNU (Me) with MTX or BAF (Program II). Fifteen of these patients had failed to respond to initial therapy with 5-FU plus MTX or BAF and subsequently received Me plus the alternate antifol. Overall response rate for each of programs I and II was 10%. The responses were 1/11 with 5-FU-MTX plus levamisole, 2/12 with 5-FU-MTX, 1/8 with 5-FU-BAF plus levamisole, 0/8 with 5-FU-BAF, 2/20 with Me-MTX and 2/21 with Me-BAF. The median survival times (MST) for patients receiving Programs I and II were 10 and 5 months, respectively. The MST for all patients receiving MTX was significantly longer than that of patients receiving BAF Survival was not influenced by levamisole administration. Both chemotherapy programs were well tolerated. The sequential administration of 4 active agents failed to improve the results of treatment of colorectal cancer.
As part of the search for new antimalarial drugs, a screening program was developed using sensitive and chlorguanide triazine (CGT, cycloguanil) resistant strains of the folate-requiring bacteria, Streptococcus faecium durans, Lactobacillus casei, and Pediococcus cerevisiae. The activities of 40 compounds have been studied against these strains and Escherichia coli. Observations have been made on the points of 50% growth inhibition, the fold increase of resistance shown to each compound by the resistant strains as compared with the parent sensitive strains, and the reversal of growth inhibition by folic acid with S. faecium and L. casei by folinic acid with P. cerevisiae and by p-aminobenzoic acid with E. coli. Comparisons have been made of the activities of the test compounds with those of the standard antimalarial antifoltes, CGT and pyrimethamine (PM), and the antibacterial results have been compared with the activities of the compounds against Plasmodium berghei infections in the mouse and against human malaria infections where data are available. Of the 17 compounds reversed by folates, five had patterns of activity similar to CGT and PM in that they were most active against S. faecium and nine compounds exhibited a different pattern, being highly active against all four test bacteria. This suggests that these latter compounds either have different pharmacokinetic properties or have additional modes of action. The three CGT-resistant organisms responded to antifolates in different ways. S. faecium (R) and P. cerevisiae (R) strains were cross resistant to 4,6-diaminotriazines, 2,4-diaminopyrimidines, 2,4-diaminoquinazolines, and active 2,4-diaminopteridines. L. casei (R) was cross resistant to the triazines but was collaterally sensitive to all the other antifolates. Most of the compounds not reversed by folates were much less inhibitory for the test organisms; they were most active against L. casei. In general, their growth inhibitory concentrations varied less for the four test organisms and the responses of the sensitive and CGTR strains were similar. However, there was some cross resistance to five compounds and some collateral sensitivity to five others. Comparison of the bacteriological data with the activities of the compounds against Plasmodium berghei in the mouse showed little correlation between the two test systems; each appears to provide independent and useful information.
Mutations in the Plasmodium falciparum genes, pfdhfr and pfdhps, drive antifolate resistance and threaten malaria control in regions where sulfadoxine-pyrimethamine (SP) is the primary chemoprevention strategy. The spatial patterns and evolutionary dynamics of these mutations in high-transmission settings remain incompletely understood. Here we genotyped 11 resistance-associated mutations in pfdhfr and pfdhps in 4,725 P. falciparum isolates collected from 16 Ugandan health facilities as part of annual surveillance between 2016 and 2022. Notably, we show that the frequency of PfDHFR I164L, which confers higher pyrimethamine resistance, increased over time from 19.4% to 32.4%. Using identity-by-descent, haplotype structure, and extended haplotype homozygosity analyses, we show that PfDHFR I164L is present on multiple haplotype backgrounds and undergoes localised expansions, without detectable signatures of recent positive selection at all but one site. Our results suggest that the evolution of antifolate resistance, driven by PfDHFR I164L, is spatially heterogeneous and complex in regions that primarily use SP chemoprevention programmes.
Carboxypeptidase G1 (CPDG1), an enzyme that degrades folates but not the nonclassical folate antagonists triazinate (TZT, Baker's antifol) and 2,4-diamino-5-(3',4'-dichlorophenyl)-6-methylpyrimidine (DDMP), enhanced the antineoplastic activity of these antifolates. In 6-day cell culture experiments with Walker 256 carcinosarcoma, CPDG1 at levels up to 0.54 unit/ml showed very little inhibitory effect on growth. In the presence of 10(-7) M DDMP, the grown of Walker 256 cells was similar to that of controls, but in combination with CPDG1 (0.1 unit/ml) 80% growth inhibition was observed. TZT at a concentration of 10(-8) M did not affect the growth of Walker 256 cells. The combination of 10(-8) M TZT with CPDG1 (0.1 unit/ml), however, strongly inhibited cell growth. In experiments with rats bearing Walker 256 carcinosarcoma, the administration of CPDG1 (800 units/kg/day) from Day 1 to Day 6 resulted in no significant increase in life span. Administration of TZT in doses up to 0.05 mg/kg on Day 1 or that of DDMP up to 15 mg/kg on Days 1, 3, and 5 had no antitumor effects as measured by survival of the animals. However, CPDG1 (800 units/kg/day) from Day 1 to Day 6 in combination with TZT (0.05 mg/kg on Day 1) or DDMP (15 mg/kg on Days 1, 3, and 5) resulted in increases of 50 and 30%, respectively, in the survival of the tumor-bearing animals. These results demonstrate that CPDG1 enhances the antitumor effects of TZT or DDMP.
Ten patients with disseminated adenocarcinoma were treated with combination chemotherapy employing Adriamycin and Baker's Antifolate (BAF). There were seven patients with lung adenocarcinoma, two of whom achieved partial remission while the remaining five had their disease stabilized. Drug toxicity to the bone marrow, gastrointestinal mucosa, and skin was dose-limiting and was greater than the known toxicities of the individual drugs. Pharmacological studies of both drugs were performed on five patients to determine whether abnormal pharmacokinetics could explain this collateral toxicity. Adriamycin plasma concentrations and disappearance seemed to be unaffected by BAF. However, BAF levels were prolonged, apparently due to an Adriamycin effect on the plasma elimination of BAF, resulting in a prolonged exposure of sensitive tissues and organs to BAF. Consequently, when BAF and Adriamycin are used in combination, appropriate dose and schedule changes must be made to avoid any potentially serious side effects.
Four cell lines, SK-N-SH, SK-N-MC, SK-N-BE(2), and IMR-32, established in vitro from tumor tissue of patients with neuroblastoma were analyzed by trypsin-Giemsa banding methods. In two of the lines a large, abnormally staining chromosome region was observed. This "homogeneously staining region" (HSR) was considerably longer than any of the bands present in normal human cells and, as revealed by both G- and Q-banding, stained with an intermediate intensity. It was located on chromosomes No 6, 10, 17, or 19 of the SK-N-BE(2) cell line and on chromosome No 1 of the IMR-32 line. In concurrent studies, long HSR's were also observed in Chinese hamster sublines that had been exposed to and had developed high levels of resistance to methotrexate or methasquin and high levels of activity of target enzyme dihydrofolate reductase. For several sublines with the highest levels of enzyme activity, approximately 2% of the total cell protein was dihydrofolate reductase. Of 13 independently derived sublines with acquired resistance to antifolate, only those 7 with greater than 100-fold increases in enzyme activity consistently exhibited HSR's. These regions comprised 2-5% of the total length of the chromosome complement and were specifically localized, as demonstrated by G-banding. Analysis of chromosome replication patterns of the HSR in human neuroblastoma and in drug-resistant Chinese hamster cells by tritiated thymidine radioautography indicated that the long, abnormally staining region replicated relatively rapidly and synchronously and terminated replication before the midpoint of the S phase. The HSR thus appeared to represent a novel chromosome abnormality that may be present in cells with specialized functions. Drug-resistant Chinese hamster cells were characterized by overproduction of target enzyme, whereas human neuroblastoma cells had phenotypes of normal neuronal cells. Whether the HSR is transcriptionally active was not elucidated.
The antifolate activity and the transport characteristics of isoaminopterin (IA) in HeLa cells were studied and compared with those of methotrexate (MTX). Both IA and MTX inhibited the incorporation of [2-(14)C]deoxyuridine into HeLa cell DNA by 50% at a concentration of 0.09 muM by 8 h postaddition of the compounds. Unlike MTX, the inhibition of DNA synthesis by IA was time dependent and reached a maximum at 24 h. IA-induced inhibition was due to interference with folate metabolism, since it could be completely reversed with N(5)-formyl-tetrahydrofolate. Competitive transport experiments between IA and either radiolabeled MTX or radiolabeled folate showed that IA preferentially uses the reduced folate/MTX transport system. IA inhibited MTX uptake by 50% at a concentration of 6.8 muM but had a negligible effect on folate transport.
The action of antimetabolites in vivo involves the consideration of several interrelated factors. If the mechanism of biochemical action is understood, the next items of priority are whether the active form of the drug gets to the site of action, its concentration at this site, the time course of removal of active drug from this site, and how these kinetics relate to the biochemical inhibition. Studies of antifolates represent some of the approaches used to further understand antimetabolite action in vivo. The interrelationships of blood flow, mixing volumes of tissue compartments, membrane resistance, binding of drug, binding site formation, transport competition, and biochemical kinetics of rate-limiting steps, substrate competition, and salvage pathways are considered from an in vivo point of view.
The general principles of ligand-binding radioassays are reviewed and a specific noncompetitive system is described for the measurement of methotrexate (MTX) using dihydrofolate reductase as the binding determinant. This type of radioassay can also be exploited to measure any antifolate compound which binds to this enzyme even if [3H]MTX is used as the tracer. The radioassay has now been used to measure MTX in all types of body fluids, tissue extracts, and rbc lysates from patients receiving this drug.
Iv Baker's antifol (BAF) (250 mg/m2/day X 3 consecutive days) was administered to 34 patients with metastatic sarcoma. All patients had received extensive prior therapy including prior chemotherapy and had progressive disease at the start of the study. Liver and renal functions were normal in all patients. Of 29 patients evaluable for response, 25 demonstrated progressive disease and four had stable disease for periods of from 1 to 6 months. No objective responses were observed. The other five patients died from 3 to 12 days after initiation of therapy. Toxicity included myelosuppression of significant degree in nine patients, gastrointestinal effects of nausea and vomiting in seven, stomatitis in three, and dermatitis in four. Most toxicity was mild to moderate, although one drug-related death due to marked myelosuppression was seen. In conclusion, BAF is considered to be insignificantly active in the secondary treatment of metastatic sarcomas at the dose and schedule studied.
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Blood levels of three aryldihydro-s-triazines in rats were followed: 4,6-diamino-1,2-dihydro-2,2-dimethyl-1-phenyl-s-triazine (I), the prototype of the series; 4,6-diamino-1-(3,4-dichlorophenyl)-1,2-dihydro-2,2-dimethyl-s-triazine (II); and N-(m-tolyl)-p-(4,6-diamino-1,2-dihydro-2,2-dimethyl-s-triazin-1-yl)hydrocinnamide (III). The blood profiles obtained provide substantial evidence that III, but not II, was precipitated in the peritoneal cavity where it was injected. Precipitation after intraperitoneal injection may explain why III and similar triazines with long nonpolar chains have been reported to be more active against intraperitoneal Walker 256 tumor than is II, even though the latter compound is a far more potent inhibitor of Walker 256 dihydrofolate reductase and of tumor cell cultures in vitro. Precipitation in the peritoneal cavity also may be involved in the difficulty of obtaining the toxicity-free antineoplastic activity expected from certain aryldihydrotriazines selectively inhibiting neoplastic dihydrofolate reductase.
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