Cellular pharmacology of MTA: a correlation of MTA-induced cellular toxicity and in vitro enzyme inhibition with its effect on intracellular folate and nucleoside triphosphate pools in CCRF-CEM cells.

The mechanism of action of an antifolate may be investigated using a variety of experimental methods. These include experiments in a cell culture setting to observe possible protection against drug effects afforded by the end products of metabolic pathways, assessing the activity of purified target enzymes in the presence of the antifolate, and, finally, the measurement of drug effects on intracellular folate and nucleoside triphosphate pools. The current discussion is focused on studies using CCRF-CEM leukemia cells that were designed to compare and contrast mechanisms of action of the antifolates methotrexate, which is primarily a dihydrofolate reductase inhibitor, raltitrexed, a thymidylate synthase inhibitor, LY309887, a glycinamide ribonucleotide formyltransferase inhibitor, and MTA (multitargeted antifolate), which is a novel antifolate antimetabolite. The results of these studies support the hypothesis that MTA affects multiple enzymatic targets and has a distinct mechanism of action from methotrexate, raltitrexed, and LY309887.

Preclinical cellular pharmacology of LY231514 (MTA): a comparison with methotrexate, LY309887 and raltitrexed for their effects on intracellular folate and nucleoside triphosphate pools in CCRF-CEM cells.

LY231514 (N-[4-[2-(2-amino-3,4-dihydro-4-oxo-7H-pyrrolo[2,3-d]pyrimidin-5-yl)ethy l]-benzoyl]-L-glutamic acid) is a new folate-based antimetabolite currently in broad phase II clinical evaluation. Previous in vitro studies (C. Shih et al, CancerRes 57: 1116-1123, 1997) have suggested that LY231514 could be a multitargeted antifolate (MTA) capable of inhibiting thymidylate synthase (TS), dihydrofolate reductase (DHFR) and glycinamide ribonucleotide formyltransferase (GARFT). The present study compared LY231514 with methotrexate, raltitrexed and a glycinamide ribonucleotide formyltransferase inhibitor, LY309887, at 300, 100, 30 and 100 nM, respectively, for their effects on intracellular folate and at 100, 66, 20 and 30 nM respectively, for their effects on nucleoside triphosphate pools in CCRF-CEM cells. Methotrexate induced an accumulation of dihydrofolate species, together with a rapid depletion of ATP, GTP and all of the deoxynucleoside triphosphates. LY309887 caused an accumulation of 10-formyltetrahydrofolate, a rapid loss of ATP, GTP and dATP, but a slower loss in dCTP, dTTP and dGTP. Both LY231514 and raltitrexed had minimal effects on folate pools. In contrast, they caused rapid depletion of dTTP, dCTP and dGTP, but induced an accumulation of dATP at different rates, with raltitrexed doing so about 2.5 times faster. Most of the observed metabolic changes could be understood on the basis of current knowledge of folate and nucleotide metabolism. We concluded that LY231514 was distinct from methotrexate, LY309887 and raltitrexed based on their metabolic effects in CCRF-CEM cells, and that in this cell line the inhibitory effects of LY231514 were exerted primarily against the thymidylate cycle and secondarily against de novo purine biosynthesis.

Phase I study of gemcitabine using a once every 2 weeks schedule.

Gemcitabine (2',2'-difluorodeoxycytidine) is a novel nucleoside analogue. As part of a series of studies to determine the maximum tolerated dose (MTD) of gemcitabine and the most appropriate schedule, a two-centre phase I study of gemcitabine was undertaken in patients with advanced refractory solid tumours using a once every 2 weeks schedule. Fifty-two patients were entered into the study at 14 different dose levels (40-5700 mg m-2). Weekly evaluations for toxicity were performed and the MTD for this once every 2 weeks schedule was 5700 mg m-2. The dose-limiting toxicity was myelosuppression, with neutropenia being most significant. Other toxicities were nausea, vomiting, fever and asthenia. One minor response was seen in a heavily pretreated breast cancer patient treated at 1200 mg m-2. Preclinical studies suggest that the efficacy of gemcitabine is more schedule than dose related, and it is concluded that this is not the most appropriate dosing schedule for gemcitabine. However, this study demonstrates the safety profile of gemcitabine, as doses over fourfold greater than that recommended for the weekly schedule of 1000 mg m-2 could be tolerated.

Phase II study of sulofenur (LY 186641). A novel antineoplastic agent in advanced non-small cell lung cancer.

Sulofenur is a member of a new class of antineoplastic agents with a novel chemical structure and unique pharmacological and biological properties. Preclinical studies have demonstrated a wide spectrum of anti-tumor activity against murine solid tumors and human tumor xenografts. In phase I trials, only mild toxicities were observed. Twenty-six patients (pts), two of whom were inevaluable, with advanced non small cell lung cancer without prior chemotherapy were entered on this phase II trial. Pts received 800 mg/m2 sulofenur po Monday-Friday x 21 days, q 28 days. Seventeen male and 9 female pts with median performance status 1 received a median of 2 courses. Twenty pts had stage IV disease and 19 pts had adenocarcinoma, 6 squamous cell and 1 undifferentiated carcinoma. The main toxicity was grade 1 to 3 anemia in 16 (62%) pts, with hemolysis noted in 9 pts. Although methemoglobinemia was observed in 19 pts, it was severe in only 3 pts. Transient elevation of alkaline phosphatase was seen in 11 pts and one pt had a minor abnormality in glucose metabolism. Other common chemotherapy related side effects such as granulocytopenia or alopecia were not encountered with this agent. Of 24 evaluable pts, two pts had stable disease or minor response and 22 pts had progressive disease. In conclusion although sulofenur had only minor side effects, in the dosage and schedule used, it did not produce any significant response in advanced non-small cell lung cancer.

Biological characterization of a new radioactive labeling reagent for bacterial penicillin-binding proteins.

Radiolabeled penicillin G is widely used as the imaging agent in penicillin-binding protein (PBP) assays. The disadvantages of most forms of labeled penicillin G are instability on storage and the long exposure times usually required for autoradiography or fluorography of electrophoretic gels. We investigated the utility of radioiodinated penicillin V as an alternative reagent. Radioiodination of p-(trimethylstannyl)penicillin V with [125I]Na, using a modification of the chloramine-T method, is simple, high yielding, and site specific. We demonstrated the general equivalence of commercially obtained [3H]penicillin G and locally synthesized [125I]penicillin V (IPV) in their recognition of bacterial PBPs. Profiles of PBPs in membranes from Bacteroides fragilis, Escherichia coli, Providencia rettgeri, Staphylococcus aureus, Streptococcus pyogenes, Enterococcus faecalis, and Enterococcus faecium labeled with IPV or [3H]penicillin G were virtually identical. Use of IPV as the imaging agent in competition experiments for determination of the affinities of various beta-lactam antibiotics for the PBPs of E. coli yielded results similar to those obtained in experiments with [3H]penicillin G. Dried electrophoretic gels from typical PBP experiments, using IPV at 37.3 Ci/mmol and 30 micrograms/ml, exposed X-ray film in 8 to 24 h. The stability of IPV on storage at 4 degrees C was inversely proportional to specific activity. At 37.3 Ci/mmol and 60 micrograms/ml, IPV retained useful activity for at least 60 days at 4 degrees C. IPV represents a practical and stable reagent for rapid PBP assays.

Clinical oncology and chemical thermodynamics.

The development of oncolytic agents for cancer chemotherapy is often based on chance discovery and intensive structure modification. A mechanistic understanding of the essential biochemistry of many anticancer drugs remains elusive because of the biological complexity of drug-drug and drug-target interactions. The potential of computational science to analyze and quantify these interactions may provide a rational basis for drug modification and clinical trial design.

Thermodynamic analysis of the reaction of phosphoramide mustard with protector thiols.

The systemic use of thiol-containing uroepithelial protecting agents, e.g., N-acetylcysteine (NAC) or mesna, in conjunction with the alkylating agent cyclophosphamide is predicated on the assumption that the toxic metabolic by-products will be consumed by thiol without diminishing the cytotoxicity of the active alkylating intermediate, phosphoramide mustard. Studies in murine tumor systems have been with either a single dose or two equally divided doses of thiol, administered within 30 min of the addition of cyclophosphamide, without an observed adverse effect on antitumor activity; however, the relatively short serum half-life of thiol relative to alkylating agent in humans weakens the clinical relevance of these results. This study presents a thermodynamic model for the chemical reaction of phosphoramide mustard with either NAC or mesna. The gas phase thermodynamic parameters for these reactions, enthalpy (H) and entropy (S), were calculated using the semiempirical quantum mechanical method AM1 and were used to predict the free energy (delta G) for these processes. For the reaction of phosphoramide mustard with NAC or mesna, delta G = +3.82 and 2.29 kcal/mol, respectively. In the absence of enzyme catalysis, these results suggest that such reactions are not favored. In order to assess the validity of this gas phase thermodynamic model, the cellular cytotoxicity of phosphoramide mustard in the presence or absence of either NAC or mesna was studied using CCRF-CEM cells in culture. In these experiments the 50% effective dose of phosphoramide mustard was 1.7 micrograms/ml; this result was unchanged in the presence of 10 micrograms/ml concentration of either thiol. This study supports the conclusion that phosphoramide mustard and protector thiols are compatible.

Radioiododestannylation: preparation and evaluation of radioiodinated thienyl alcohols.

Radioiododestannylations was employed to prepare a series of four specifically labeled thienyl alcohols: 1-(5-iodo-2-thienyl)-cyclopentan-1-ol and -cyclohexan-1-ol; 17 alpha-(5-iodo-2-thienyl)-17 beta-estradiol and -estradiol-3-O-methyl ether. The method utilized 5-(trimethylstannyl)thienyl intermediates which had been prepared in good yields from 2,5-bis(trimethylstannyl)thiophene and the appropriate cyclic ketones. The trimethylstannyl substrates reacted with no-carrier-added Na125I in the presence of 30% H2O2-acetic acid (2:1) at pH 4.5 at ambient temperature for 5-15 min to give, after HPLC separation, the desired 5-[125I]iodothien-2-yl products in greater than 90% isolated yields. Although tissue distribution studies in rats were uneventful, the methodology employed to give the labeled products possesses distinct advantages compared to alternative methods for preparing radioiodinated aryl moieties.

Radiohalodestannylation: synthesis of 125I-labeled 17 alpha-E-iodovinylestradiol.

17 alpha-E-Iodovinylestradiol and its iodine-125-labeled analog were prepared by halodestannylation. The synthesis of the unlabeled compound was achieved from estrone in two steps with an overall yield of 30%. The tributylstannylvinylestradiol intermediate reacted with sodium [125I]iodide (specific activity = 2200 Ci/mmol) in the presence of hydrogen peroxide and acetic acid to give the 17 alpha-E-[125I]iodovinylestradiol in 40-60% yield after isolation by reversed phase column chromatography. The radiochemical purity was greater than 98% and no other u.v. active components could be detected by HPLC. The ease of preparation and isolation of this radioligand suggests that radiohalodestannylation may be the method of choice for this and structurally similar compounds.

E-17 alpha[125I]iodovinylestradiol: an estrogen-receptor-seeking radiopharmaceutical.

Through the use of radioiododestannylation, the specifically labeled E-17 alpha-[125I]iodovinylestradiol [(I-125)VE2] was synthesized rapidly and in high yield from the stable precursor E-17 alpha-tributylstannylvinylestradiol (SnVE2), and its biodistribution was determined in immature female rats. The agent accumulated in the uterus, achieving a peak uptake of 0.465% ID-kg/g at 2 hr. Uterus-to-blood ratios of 19 and 16 occurred at 1 and 2 hr, respectively, declining to 7 by 4 hr after injection. The uptake of (I-125)VE2 by the uterus at 2 hr was reduced 58--65% by pretreatment of the immature rats with estradiol (5 micrograms) or tamoxifen (100 micrograms), and compared with 16 alpha-[125I]iodoestradiol, (I-125)VE2 showed greater uterine uptake and similar uterus-to-blood ratios. The ease of preparation of the radioligand represents an advantage over the synthetic procedures for other estrogen-receptor-seeking agents.