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Biomedical subjects

B A Chabner

Publications and source records attributed to B A Chabner.

At least 109 records · Page 6Linked to original sources
PubMed1987

Developmental therapeutics and the acquired immunodeficiency syndrome.

Patients with the acquired immunodeficiency syndrome (AIDS) die of overwhelming infections as a consequence of the destruction of the T4 subset of lymphocytes. Approaches to the treatment of AIDS have involved attempts to reestablish immune competence as well as treat opportunistic infections. The discovery of the human T-lymphotropic virus type III, which causes AIDS, has provided a specific target for screening antiviral drugs. There are many potential screening targets, from surface-binding proteins to viral integration and assembly, but most of the recent efforts have been aimed at developing drugs to inhibit the unique viral DNA polymerase (reverse transcriptase). The early studies with 3'-azido-3'-deoxythymidine (AZT) have provided encouraging results.

Acquired Immunodeficiency Syndrome↗
PubMed1987

Antimetabolites.

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Animals↗
PubMed1987

Multidrug resistance.

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ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1987

Antifolate metabolism and sites of action: implications for the design of new antifolates.

Recently, folate-dependent enzymes in the de novo thymidine and purine biosynthetic pathways have come under scrutiny as potential sites for chemotherapeutic exploitation by antifolates. In this manuscript we report on the progress that has been made in designing inhibitors of these pathways. In addition, a molecular model is proposed for the design of new antifolates directed against thymidylate synthase (TS).

Fluorodeoxyuridylate↗
PubMed1987

Preliminary results of a phase II trial for the treatment of metastatic breast cancer with 5-fluorouracil and leucovorin.

The active metabolite of FUra, 5-fluorodeoxyuridine monophosphate (5-FdUMP), requires the presence of reduced folates to form a covalent ternary complex with the target enzyme thymidylate synthase (TS). In vitro and in vivo studies have demonstrated a potentiation of the cytotoxic effects of FUra when combined with the reduced folate, leucovorin. We have applied this concept to the treatment of metastatic breast cancer in a phase II trial, as recent clinical studies on patients with colorectal carcinoma have suggested an enhanced efficacy for the combination of FUra plus leucovorin. Patients entered on the present study are undergoing treatment with a 5-day daily regimen of leucovorin (500 mg/m2, iv) followed by FUra (375 mg/m2, iv). Toxicity and response data are currently being collected on patients who have failed "standard" combination regimens that included FUra. In patients with accessible tumor, serial biopsies are being obtained during treatment with the combination of FUra and leucovorin and during therapy with FUra alone to assess the degree of 5-FdUMP binding to the target enzyme, TS, in the presence and absence of exogenously administered leucovorin. Preliminary results from the biochemical studies suggest an enhanced saturation of TS by the fluorinated pyrimidine when administered with leucovorin.

Adult↗
PubMed1986

The effect of methotrexate on intracellular folate pools in human MCF-7 breast cancer cells. Evidence for direct inhibition of purine synthesis.

This report details the effects of methotrexate on the intracellular folate pools of the MCF-7 human breast cancer cell line. To achieve this goal, we designed a high-pressure liquid chromatography system capable of separating the physiologic folates. The folate pools were quantitated following growth and equilibration in 2.25 microM radiolabeled folic acid. Each of the intracellular folates was identified by coelution with standard folates and by chemical/biochemical tests unique to each of the various folates. The 10-formyl-H4PteGlu (where H4PteGlu represents dl-tetrahydrofolic acid) pool accounted for 20.5% of the total intracellular folate pool in untreated cells, whereas 5-formyl-H4PteGlu and H4PteGlu accounted for 6.5 and 10.6%, respectively. The levels of these three folates remained stable throughout cell growth. The 5-methyl-H4PteGlu pool accounted for less than 10% in early growth phase cells but assumed greater than 60% of the total pool by the mid- and late-log phases of cell growth. When the MCF-7 cells were exposed to 1 microM methotrexate, de novo purine synthesis and de novo thymidylate synthesis were rapidly inhibited to less than 20% of control within 3 h. During this time period, rapid alterations in the folate pools also occurred such that dihydrofolic acid levels rose from less than 1% in untreated cells to greater than 30% of the total pool. This rise was accompanied by a parallel fall in 5-methyl-H4PteGlu. H4PteGlu and 5-formyl-H4PteGlu were undetectable following 2 h of methotrexate exposure, but 10-formyl-H4PteGlu, the required cosubstrate for de novo purine synthesis, was preserved at greater than 80% of pretreatment values following a 1 microM methotrexate exposure of up to 21 h. The rapid inhibition of de novo purine synthesis in these cells following methotrexate exposure coupled with a relatively preserved 10-formyl-H4PteGlu pool suggests direct inhibition of this synthetic pathway by the temporally coincident accumulation of dihydrofolic acid and/or methotrexate polyglutamates. This inhibition cannot be ascribed to depletion of the folate cofactor 10-formyl-H4PteGlu.

Breast Neoplasms↗
PubMed1986

Progress and perspectives of systemic anticancer treatment.

Cancer chemotherapy embodies an expanding breadth of molecular and biological disciplines and is evolving from approaches based solely on DNA synthesis to include approaches based on a vast number of cellular factors that may differ between malignant and normal tissues. These factors include response to growth factors, antigenic determinants on cell membranes, the ability to respond to signals of differentiation and/or proliferation, and the ability to stimulate cytotoxic recognition responses. Concurrently, the use of agents designed to interfere with DNA synthesis has led to a better understanding of the molecular events that cause drug resistance. The use of these drugs demonstrated that mechanisms operating in antimetabolite resistance may be quite different from those operating in resistance to alkylating agents. Further, multiple changes may frequently coexist in the same cells. Conceptual advances in the area of drug resistance continue to support the long-held concept that the probability of cure is greatest when the tumor burden is small. National Cancer Institute efforts in drug discovery are now designed to explore these recent developments using screening systems that employ human tumors that are usually resistant to therapy. This approach is expected to contribute substantially to the ability to cure malignant disease.

Antineoplastic Agents↗
PubMed1986

Potential roles for preclinical pharmacology in phase I clinical trials.

Concepts elucidated from preclinical pharmacology studies have made a substantial impact on the clinical use of anticancer drugs. However, the majority of animal pharmacology results have not been available until after drugs have entered clinical trials. Since clinical pharmacokinetic measurements are already part of many phase I trials, human data could be directly compared with mouse data if mouse pharmacology studies were completed before clinical trials were initiated. Once the starting dose in a phase I clinical trial has been evaluated, subsequent doses are escalated until the maximum tolerated dose is reached. The rate of escalation is empirically defined by a modified Fibonacci series. This universal escalation scheme is applied to all drugs, with no modifications based upon pharmacology or other factors. If the starting dose is far removed from the maximum tolerated dose, a large number of dose escalations are required. Consequently, most patients receive subtherapeutic doses, and the amount of resources allocated to each drug increases. We are exploring potential strategies for controlling the rate of dose escalation based upon pharmacokinetic determinations in mouse and man. Retrospective analyses indicate that 20%-50% savings in the total number of dose escalations are possible.

Aminoacridines↗
PubMed1985

Enhanced inhibition of thymidylate synthase by methotrexate polyglutamates.

We have studied the effects of methotrexate (MTX-Glu1) and the polyglutamate derivatives of methotrexate (MTXPGs) with 2, 3, 4, and 5 glutamyl residues on the catalytic activity of thymidylate synthase purified from MCF-7 human breast cancer cells and on the kinetics of the ternary complex formation by 5-fluoro-2'-deoxyuridine 5'-monophosphate, folate cofactor, and thymidylate synthase. MTX-Glu1 exhibited uncompetitive inhibition of thymidylate synthase when reaction kinetics were analyzed by either double reciprocal plots or a computerized mathematical model based on nonlinear least-squares curve fitting. The Ki for MTX-Glu1 inhibition was 13 microM and the I50 was 22 microM, irrespective of the degree of polyglutamation of the folate. In contrast, the polyglutamated derivatives of MTX all acted as noncompetitive inhibitors. The MTXPGs had 75-300-fold greater potency than MTX-Glu1 as inhibitors of thymidylate synthase catalytic activity, with Ki values from 0.17 to 0.047 microM for MTX-Glu2 to MTX-Glu5, respectively. Neither MTX-Glu1 nor MTXPGs promoted the formation of a charcoal-stable ternary complex with thymidylate synthase and 5-fluoro-2'-deoxyuridine 5'-monophosphate. CH2-H4PteGlu5 (where PteGlu represents pteroylglutamic acid) was found to be 40-fold more potent than CH2-H4PteGlu1 in participating in the formation of a ternary complex, and 10 microM MTX-Glu5 significantly inhibited the formation of a ternary complex containing this folate as cofactor. The inhibition was determined to be due to a reduction in the kon. The potency of this inhibition was markedly greater in the presence of CH2-H4PteGlu1 as compared to CH2-H4PteGlu5. This finding suggests that the degree of interference with complex formation in intact cells would depend on the state of polyglutamation of available folate cofactor. Ternary complex formation with H2PteGlu5 as the folate cofactor was also investigated, and a 50% reduction in complex formation was found in the presence of a 2 microM concentration of MTX-Glu5. These findings have significant implications regarding the mechanism of action of MTX-Glu1 and contribute to an understanding of the complex interactions of MTX-Glu1 and 5-fluorouracil.

Breast Neoplasms↗
PubMed1985

Inhibition of phosphoribosylaminoimidazolecarboxamide transformylase by methotrexate and dihydrofolic acid polyglutamates.

We report the enhanced inhibitory potency of methotrexate (MTX) polyglutamates and dihydrofolate pentaglutamate on the catalytic activity of phosphoribosylaminoimidazolecarboxamide (AICAR) transformylase purified from MCF-7 human breast cancer cells. In the present work, MTX (4-amino-10-methylpteroylglutamic acid) and dihydrofolate, both monoglutamates, were found to be weak competitive inhibitors of AICAR transformylase with Kis of 143 and 63 microM, respectively, and their inhibitory capacity was largely unaffected by the glutamated state of the folate cosubstrate. In contrast, MTX polyglutamates were found to be potent competitive inhibitors, with an approximately 10-fold increase in inhibitory potency with the addition of each glutamate group up to four (i.e., the pentaglutamate derivative). MTX tetra-and pentaglutamates were the most potent, with equivalent Kis of 5.6 X 10(-8) M or 2500-fold more potent than MTX. Dihydrofolate pentaglutamate was as potent an inhibitor as MTX pentaglutamate, with a Ki of 4.3 X 10(-8) M. The potent inhibitory effects demonstrated by the polyglutamate compounds when tested against the folate monoglutamate substrate were sharply curtailed when folate pentaglutamate was used as the substrate. MTX and dihydrofolate pentaglutamates were only 7- and 25-fold more potent than their monoglutamate counterparts under these conditions. A model depicting these complex interactions is postulated. These findings have significant implications regarding the mechanism of action of MTX.

Acyltransferases↗
PubMed1985

Formation of methotrexate polyglutamates in purified myeloid precursor cells from normal human bone marrow.

Immature myeloid precursor cells were preferentially selected from normal human bone marrow by using immune rosette techniques that employed monoclonal antibodies against mature granulocytes, monocytes, T and B lymphocytes, and erythroid precursors (Mo5, M3, OKT3, B1, and EP1, respectively). We examined the formation, retention, and cytotoxic effects of methotrexate (MTX) polyglutamates (MTX-PGs) in these purified myeloid precursor cells. After 1- and 24-h exposures to MTX, with thymidine and deoxyinosine as rescue, the intracellular MTX-PG profile was examined by high-pressure liquid chromatography. Efflux patterns of MTX-PGs were also studied after additional 1- and 24-h incubations in drug-free media. Cytotoxic effects of retained MTX-PGs on bone marrow myeloid precursors were examined by colony formation in drug-free semisolid agar. Normal myeloid precursor cells converted MTX to MTX-PGs in a concentration- and time-dependent manner, preferentially retaining MTX-PGs with three to five glutamyl moieties. At low concentrations of MTX (1 microM), MTX-PG formation was insufficient to maintain saturation of the target enzyme dihydrofolate reductase after removal of drug from the incubation medium, and there was no decrease in myeloid colony formation. At higher concentrations of MTX (10 microM), formation of higher molecular weight polyglutamates was sufficient to allow for 24-h saturation of intracellular binding capacity after removal of extracellular drug and resulted in a 35% reduction in the formation of colony-forming units in culture. Comparison of MTX metabolism in normal bone marrow cells and the MTX-sensitive HL-60 human leukemia cell line showed twofold greater PG formation by these tumor cells after 24-h exposure to 1 or 10 microM MTX, and a marked (greater than 30-fold) increase in cytotoxicity for the HL-60 cells as compared with normal myeloid precursors, suggesting that the MTX polyglutamation may be important to its selective antitumor action.

Bone Marrow↗