Peptide agonist of the thrombopoietin receptor as potent as the natural cytokine.

Two families of small peptides that bind to the human thrombopoietin receptor and compete with the binding of the natural ligand thrombopoietin (TPO) were identified from recombinant peptide libraries. The sequences of these peptides were not found in the primary sequence of TPO. Screening libraries of variants of one of these families under affinity-selective conditions yielded a 14-amino acid peptide (Ile-Glu-Gly-Pro-Thr-Leu-Arg-Gln-Trp-Leu-Ala-Ala-Arg-Ala) with high affinity (dissociation constant approximately 2 nanomolar) that stimulates the proliferation of a TPO-responsive Ba/F3 cell line with a median effective concentration (EC50) of 400 nanomolar. Dimerization of this peptide by a carboxyl-terminal linkage to a lysine branch produced a compound with an EC50 of 100 picomolar, which was equipotent to the 332-amino acid natural cytokine in cell-based assays. The peptide dimer also stimulated the in vitro proliferation and maturation of megakaryocytes from human bone marrow cells and promoted an increase in platelet count when administered to normal mice.

5-Ethynyluracil (776C85): effects on the antitumor activity and pharmacokinetics of tegafur, a prodrug of 5-fluorouracil.

We studied the effects of 5-ethynyluracil (776C85 and 776C), a potent mechanism-based inactivator of dihydropyrimidine dehydrogenase, on the antitumor efficacy and pharmacokinetics of tegafur (FT), a prodrug of 5-fluorouracil (5-FU), in rats with large s.c. colon carcinoma. Rats were dosed p.o. once daily for 7 days with either FT, FT and uracil in a 1:4 molar ratio (UFT), FT 1 h after 776C (776C/FT), or UFT 1 h after 776C (776C/UFT). 776C, which was dosed at 1 mg/kg, had neither intrinsic antitumor activity nor toxicity. The rank order in antitumor efficacy at the maximal tolerated dose of the FT (mg/kg/day) component was 776C/FT (5 mg/kg/day) > or = UFT (80 mg/kg/day) = 776C/UFT (5 mg/kg/day) >> FT (200 mg/kg/day). One-hundred % of rats treated with 776C/FT had complete and sustained tumor regression with no severe toxicity. The area under the plasma 5-FU concentration versus the time curve generated from UFT, FT, and 776C/FT at their maximum tolerated dose was 140, 50, and 27 microM.h, respectively. The area under the concentration in plasma versus time curve did not correlate with the rank order of antitumor efficacy. The vast majority of 5-FU derived from FT (alone) appeared to be rapidly catabolized. Furthermore, plasma exposure of 5-FU derived from UFT was more variable than that from 776C/FT. Each therapy also produced different levels of plasma uracil. Endogenous plasma uracil levels (1-3 microM) were not affected by FT but increased to 100 microM after dosing with 776C. Plasma uracil from UFT was 800 microM 1 h after dosing. These results suggest that moderately elevated uracil (776C/FT) may be beneficial, whereas uracil that is greatly elevated during the first 5 h (UFT) and 5-FU catabolites (FT alone) may interfere with antitumor efficacy. 776C, coadministered with FT, could provide once-a-day oral therapy for cancer patients.

2,4-Diamino-5-benzylpyrimidines and analogues as antibacterial agents. 12. 1,2-Dihydroquinolylmethyl analogues with high activity and specificity for bacterial dihydrofolate reductase.

Twelve 2,4-diamino-5-[(1,2-dihydro-6-quinolyl)methyl]pyrimidines containing gem-dimethyl or fluoromethyl substituents at the 2-position of the dihydroquinoline ring were prepared by condensations of dihydroquinolines with 2,4-diamino-5-(hydroxymethyl)pyrimidine. The dihydroquinolines were produced from the reaction of anilines with mesityl oxide or fluoroacetone. In some cases, 1-aryl-2,4-dimethylpyrroles were obtained as byproducts. Most of these pyrimidines were highly inhibitory to Escherichia coli dihydrofolate reductase (DHFR) and also had high specificity for the bacterial enzyme. 2,4-Diamino-5-[[1,2-dihydro-2,4-dimethyl-3-fluoro-2-(fluoromethyl)-8- methoxy-6(1H)quinolyl]methyl]pyrimidine had an apparent Ki value for E. coli DHFR 13 times lower than that of the control, trimethoprim (1), and was 1 order of magnitude more selective for the bacterial enzyme. It had outstanding activity against Gram-positive organisms in vitro, as well as broad-spectrum antibacterial activity equivalent to that of 1. The results of in vivo testing will be reported elsewhere. The gem-dimethyl substituents of the dihydroquinoline derivatives are considered to be responsible for the high selectivity, as well as contributing to potent bacterial DHFR inhibition. Molecular models are presented which suggest the probable interactions with the bacterial enzyme.

2,4-Diamino-5-benzylpyrimidines as antibacterial agents. 7. Analysis of the effect of 3,5-dialkyl substituent size and shape on binding to four different dihydrofolate reductase enzymes.

A group of trimethoprim (TMP) analogues containing 3,5-dialkyl(or halo)-4-alkoxy, -hydroxy, or -amino substitution were analyzed in terms of their inhibitory activities against four dihydrofolate reductase (DHFR) isozymes. Although selectivities were lower than with TMP, the activities against vertebrate DHFR were usually at least 2 orders of magnitude less than against enzymes from microbial sources. However, the profiles of activity were remarkably similar for rat, Neisseria gonorrhoeae, and Plasmodium berghei enzymes in all three series, although somewhat different for Escherichia coli DHFR, leading to the conclusion that the hydrophobic pockets are similar for the first three isozymes. Optimal substitution was reached with 3,5-di-n-propyl or 3-ethyl-5-n-propyl groups. Branching of chains at the alpha-carbon, which resulted in increased substituent thickness, was detrimental to E. coli DHFR inhibition in particular. MR is an inadequate parameter for use in correlating such substituent effects. Conformational changes of the more bulky inhibitors can be invoked to explain some differences in inhibitory pattern. Although log P explains simple substituent effects with the vertebrate DHFRs very well, it is insufficient in the more complex cases described here, where shape is clearly involved as well. Solvent-accessible surface areas were measured for TMP in E. coli and chicken DHFRs, where the coordinates are now known. The environment is more hydrophobic in the latter case; this can also be postulated for rat DHFR, which has a very similar activity profile. As with the mammalian isozymes, N. gonorrhoeae DHFR contains an active site phenylalanine replacing Leu-28 of E. coli DHFR, thus creating a more hydrophobic pocket. A similar replacement may also occur in the P.berghei isozyme. Selectivity for bacterial DHFR is dependent on the nature of the 4-substituent, being low for polar 4-hydroxy compounds but high for polar 4-amino analogues, possibly as a result of solvation differences. With complex substituents, the environment of each atom in the active site must be taken into account to adequately explain structure-activity relationships.

Selective inhibitors of Candida albicans dihydrofolate reductase: activity and selectivity of 5-(arylthio)-2,4-diaminoquinazolines.

The recent increase in fungal infections, especially among AIDS patients, has resulted in the need for more effective antifungal agents. In our search for such agents, we focused on developing compounds which inhibit fungal dihydrofolate reductase (DHFR). A series of 25 5-(arylthio)-2,4-diaminoquinazolines were synthesized as potentially selective inhibitors of Candida albicans DHFR. The majority of the compounds were potent inhibitors of C. albicans DHFR and much less active against human DHFR. High selectivity, as defined by the ratio of the I50 values for human and C. albicans DHFR, was achieved by compounds with bulky and rigid 4-substituents in the phenylthio moiety. For example, 5-[(4-morpholinophenyl)thio]-2,4-diaminoquinazoline displayed a selectivity ratio of 540 and was the most selective inhibitor synthesized to date. Substitution in the 2- or 3-position of the 5-phenylthio group provided only marginal selectivity. 6-Substituted-5-[(4-tert-butylphenyl)thio]-2,4-diaminoquinazolines showed potent activity against the C. albicans enzyme but were equally active against human DHFR. Most of the selective compounds were also good inhibitors of C. albicans cell growth, with minimum inhibitory concentration values as low as 0.05 microgram/ mL.

Inhibition of uridine phosphorylase: synthesis and structure-activity relationships of aryl-substituted 5-benzyluracils and 1-[(2-hydroxyethoxy)methyl]-5-benzyluracils.

A series of 1-[(2-hydroxyethoxy)methyl]-5-benzyluracils were synthesized and tested for inhibition of murine liver uridine phosphorylase (UrdPase). Inhibitors of UrdPase are reported to enhance the chemotherapeutic utility of 5-fluoro-2'-deoxyuridine and 5-fluorouracil and to ameliorate zidovudine-induced anemia in animal models. We prepared a series of 5-aryl-substituted analogues of 5-benzylacyclouridine (BAU), a good inhibitor of UrdPase (IC50 of 0.46 microM), to develop a compound with enhanced potency and improved pharmacokinetics. The first phase of structure-activity relationship studies on a series of 32 aryl-substituted 5-benzyluracils found several 5-(3-alkoxybenzyl) analogues of 5-benzyluracil with enhanced potency. The acyclovir side chain, the (2-hydroxyethoxy)methyl group, was substituted on the more potent aryl-substituted 5-benzyluracils. The two most potent compounds, 10y (3-propoxy) and 10dd (3-sec-butoxy), were inhibitors of UrdPase with IC50s of 0.047 and 0.027 microM, respectively. Six compounds were tested in vivo for effects on steady-state concentrations of circulating uridine in rats. Plasma uridine levels were elevated 3-9-fold by compound levels that ranged from 8 to 50 microM.

X-Ray crystal structures of Candida albicans dihydrofolate reductase: high resolution ternary complexes in which the dihydronicotinamide moiety of NADPH is displaced by an inhibitor.

X-ray crystallographic analysis of 5-(4'-substituted phenyl)sulfanyl-2,4-diaminoquinazoline inhibitors in ternary complex with Candida albicans dihydrofolate reductase (DHFR) and NADPH revealed two distinct modes of binding. The two compounds with small 4'-substituents (H and CH3) were found to bind with the phenyl group oriented in the plane of the quinazoline ring system and positioned adjacent to the C-helix. In contrast, the more selective inhibitors with larger 4'-substituents (tert-butyl and N-morpholino) were bound to the enzyme with the phenyl group perpendicular to the quinazoline ring and positioned in the region of the active site that typically binds the dihydronicotinamide moiety of NADPH. The cofactor appeared bound to DHFR but with the disordered dihydronicotinamide swung away from the protein surface and into solution. This unusual inhibitor binding mode may play an important role in the high DHFR selectivity of these compounds and also may provide new ideas for inhibitor design.

Inhibition of uridine phosphorylase. Synthesis and structure-activity relationships of aryl-substituted 1-((2-hydroxyethoxy)methyl)-5-(3-phenoxybenzyl)uracil.

Structure-activity relationship studies on a series of 1-((2-hydroxyethoxy)methyl)-5-(3-(substituted-phenoxy)benzyl)uracils as inhibitors of murine liver uridine phosphorylase have led to compounds with IC50s as low as 1.4 nM. The two most potent compounds, 10j (3-cyanophenoxy) and 11f (3-chlorophenoxy) were tested in vivo for effects on steady-state concentrations of circulating uridine in mice and rats. Both compounds were substantially more efficacious than BAU (5-benzylacyclouridine) both in vitro and in vivo.

Methotrexate analogues. 30. Dihydrofolate reductase inhibition and in vitro tumor cell growth inhibition by N epsilon-(haloacetyl)-L-lysine and N delta-(haloacetyl)-L-ornithine analogues and an acivicin analogue of methotrexate.

Analogues of methotrexate (MTX) with strong alkylating activity were prepared by replacing the L-glutamate side chain with N omega-haloacetyl derivatives of L-lysine and L-ornithine. Haloacetylation was accomplished in 30-40% yield by reaction of the preformed L-lysine and L-ornithine analogues of MTX with p-nitrophenyl bromoacetate or chloroacetate in aqueous sodium bicarbonate at room temperature. All four haloacetamides were potent inhibitors in spectrophotometric assays measuring noncovalent binding to purified dihydrofolate reductase (DHFR) from L1210 cells. In experiments designed to measure time-dependent inactivation of DHFR from L1210 cells and Candida albicans, the N epsilon-(bromoacetyl)-L-lysine and N delta-(bromoacetyl)-L-ornithine analogues gave results consistent with covalent binding, whereas N epsilon- and N delta-chloroacetyl analogues did not. The N delta-(bromoacetyl)-L-ornithine analogue appeared to be the more reactive one toward both enzymes. Amino acid analysis of acid hydrolysates of the L1210 enzyme following incubation with the bromoacetamides failed to demonstrate the presence of a carboxymethylated residue, suggesting that alkylation had perhaps formed an acid-labile bond. In growth inhibition assays with L1210 cultured murine leukemia cells, the four haloacetamides were all more potent than their nonacylated precursors but less potent than MTX. The greater than 40,000-fold MTX-resistant mutant cell line L1210/R81 was only partly cross-resistant to the haloacetamides. An analogue of MTX with acivicin replacing glutamate was a potent inhibitor of DHFR from chicken liver and L1210 cells but was 200 times less potent than MTX against L1210 cells in culture.

2,4-Diamino-5-benzylpyrimidines and analogues as antibacterial agents. 9. Lipophilic trimethoprim analogues as antigonococcal agents.

Lipophilic analogues of trimethoprim (1) bearing 3,5-dialkyl-4-hydroxy substituents in the benzene ring are much more active in vitro against Neisseria gonorrhoeae than is 1. The 3,5-diisopropyl-4-hydroxy derivative (2) was selected as a candidate for clinical evaluation as an antigonococcal agent, and as part of the preliminary evaluation it was submitted to extended pharmacokinetic and metabolism studies in dogs. Although the compound was not extensively conjugated by metabolic enzymes, one of the methyl groups was metabolized to produce a 3-isopropyl-4-hydroxy-5-(alpha-carboxyethyl)benzyl derivative (43), which was rapidly excreted. Related analogues were likewise extensively metabolized.

2,4-Diamino-5-benzylpyrimidines as antibacterial agents. 13. Some alkenyl derivatives with high in vitro activity against anaerobic organisms.

A series of 2,4-diamino-5-(3,5-dialkenyl-4-methoxy- or -4-hydroxybenzyl)pyrimidines was prepared from [(allyloxy)benzyl]pyrimidines by Claisen rearrangements, and the resulting allyl phenols were further modified by methylation and rearrangement to 1-propenyl analogues. Analogous 3,4-dimethoxy-5-alkenyl derivatives were prepared by similar techniques. High in vitro antibacterial activity was obtained against certain anaerobic organisms, such as Bacteroides species and Fusobacterium, which was equal to or better than the control, metronidazole, in several cases. The profile was similar against Neisseria gonorrhoeae and Staphylococcus aureus. The 3,5-bis(1-propenyl)-4-methoxy derivative 8 was 1 order of magnitude more active against Escherichia coli dihydrofolate reductase than its saturated counterpart, and it was also more active than trimethoprim, 1. However, it was considerably less active in vitro against the Gram-negative organisms. The 3,4-dimethoxy-5-alkenyl, -5-alkyl, and -5-alkoxy analogues had very high broad-spectrum antibacterial activity. However, pharmacokinetic studies of four of the compounds in dogs and rats and in vivo studies with an abdominal sepsis model in rats showed no advantages over trimethoprim.

Receptor-based design of dihydrofolate reductase inhibitors: comparison of crystallographically determined enzyme binding with enzyme affinity in a series of carboxy-substituted trimethoprim analogues.

By the use of molecular models of Escherichia coli dihydrofolate reductase (DHFR), analogues of trimethoprim (TMP) were designed which incorporated various 3'-carboxyalkoxy moieties in order to acquire ionic interactions with positively charged active-site residues. Certain of these compounds have shown exceptionally high affinity for this enzyme. For example, the 3'-(carboxypentyl)oxy analogue was found to be 55-fold more inhibitory than TMP toward E. coli DHFR (Ki = 0.024 nM vs. 1.32 nM for TMP). X-ray crystallographic studies of E. coli DHFR in binary complexes with TMP and two members of this acid-containing series of compounds defined the binding of these inhibitors and showed the carboxyl group of the latter two inhibitors to be ionically bound to Arg-57. These observations were in agreement with postulated binding modes that were based on receptor modeling.

High-affinity inhibitors of dihydrofolate reductase: antimicrobial and anticancer activities of 7,8-dialkyl-1,3-diaminopyrrolo[3,2-f]quinazolines with small molecular size.

A series of 7,8-dialkylpyrrolo[3,2-f]quinazolines were prepared as inhibitors of dihydrofolate reductase (DHFR). On the basis of an apparent inverse relationship between compound size and antifungal activity, the compounds were designed to be relatively small and compact. Inhibitor design was aided by GRID analysis of the three-dimensional structure of Candida albicans DHFR, which suggested that relatively small, branched alkyl groups at the 7- and 8-positions of the pyrroloquinazoline ring system would provide optimal interactions with a hydrophobic region of the protein. The compounds were potent inhibitors of fungal and human DHFR, with K(i) values as low as 7.1 and 0.1 pM, respectively, and were highly active against C. albicans and an array of tumor cell lines. In contrast to known lipophilic inhibitors of DHFR such as trimetrexate and piritrexim, members of this series of pyrroloquinazolines were not susceptible to P-glycoprotein-mediated multidrug resistance and also showed significant distribution into lung and brain tissue. The compounds were active in lung and brain tumor models and displayed in vivo activity against Pneumocystis carinii and C. albicans.

Kinetics of methotrexate binding to dihydrofolate reductase from Neisseria gonorrhoeae.

The kinetics of methotrexate inhibition of dihydrofolate reductase from Neisseria gonorrhoeae have been investigated. Methotrexate was shown to be a tight-binding inhibitor (Kt = 13 pM) competitive with dihydrofolate. However, "stoichiometric" or "pseudoirreversible" inhibition could not be demonstrated. Progress curves of inhibited assays quickly attained steady state regardless of the order of substrate addition, indicating that methotrexate association and dissociation processes were rapid. Kinetic techniques were used to measure the rate of methotrexate dissociation from the enzyme-NADPH-methotrexate ternary complex. At 30 degrees, the first-order off-rate constant (koff) was calculated to be 0.56 min-1. This value is approximately 40-fold greater than the dissociation rate constant of methotrexate for Escherichia coli dihydrofolate reductase. At lower temperatures, progress curves of methotrexate-inhibited gonococcal enzyme assays displayed marked increases in both curvature and the time to reach steady state. At 9 degrees, the methotrexate dissociation rate was slow enough (koff = 0.04 min-1) so that initial velocities of the reaction could be measured, and under these conditions methotrexate inhibition was shown to be "stoichiometric".

Species-dependent differences in the biochemical effects and metabolism of 5-benzylacyclouridine.

The pharmacokinetics and biochemical effects of the uridine phosphorylase (UrdPase) inhibitor 5-benzylacyclouridine (BAU) were investigated in the mouse, rat and monkey. Following i.p. administration of BAU (30 mg/kg) in the mouse and i.v. administration in the rat and monkey, initial BAU plasma half-life values were 36, 36 and 25 min, and the areas under the plasma BAU concentration versus time curves (AUC) were 127, 80 and 76 microM.hr, respectively. Rats were also dosed p.o. and i.v. with BAU at 90 mg/kg, and a comparison of the AUC values showed an oral bioavailability of 70%. Analyses of plasma samples by HPLC indicated that the metabolism of BAU differed in these species. A major BAU metabolite was observed in monkeys. Its concentration was greater than or equal to that of BAU in almost every plasma sample, and its elimination paralleled that of BAU. Urinary recovery of the metabolite was 10-fold higher than the recovery of unchanged drug. The compound was identified as the ether glucuronide of BAU by its UV absorption spectrum, its co-elution with BAU after incubation with beta-glucuronidase, and liquid chromatography/mass spectrum analysis. A different metabolite was detected in rat plasma; its maximum concentration was 15% of the BAU level, and its elution position on the HPLC chromatogram was not affected by the action of beta-glucuronidase. BAU had equivalent potency against UrdPase in liver extracts from the three species, with Ki values of about 0.17 microM. However, the in vivo effects of BAU on plasma uridine concentrations were species dependent. In mice, a 30 mg/kg i.p. dose of BAU increased the plasma uridine concentration to 11 microM from a control level of 1.8 microM. In the rat, a 30 mg/kg i.v. dose of BAU increased plasma uridine to 2.1 from 1.1 microM control levels, and a 300 mg/kg oral dose resulted in a peak plasma uridine concentration of only 6 microM. In the monkey, BAU (30 mg/kg, i.v.) had no effect on plasma uridine despite the presence of 10-100 microM BAU levels in plasma for 1.5 hr. These data show that there are significant differences in the biochemical effects and metabolism of BAU in CD-1 mice, CD rats and cynomolgus monkeys.

5-Ethynyluracil (776C85): protection from 5-fluorouracil-induced neurotoxicity in dogs.

5-Ethynyluracil (776C85) is a potent mechanism-based inactivator of dihydropyrimidine dehydrogenase (DPD), the enzyme that catalyzes the rapid catabolism of 5-fluorouracil (5-FU). Because catabolism is the major route for 5-FU clearance, we studied the effect of 5-ethynyluracil on the pharmacokinetics and toxicity of continuous i.v. 5-FU infusion in the dog. 5-FU at 40 mg/kg/24 hr resulted in a steady-state plasma 5-FU concentration of 1.3 microM and was fatal with dogs dying from apparent neurotoxicity. 5-Ethynyluracil lowered the total clearance of 5-FU from 9.9 to 0.2 L/hr/kg and enabled 1.6 mg/kg/24 hr 5-FU to achieve a steady-state plasma 5-FU concentration of 2.4 microM with no apparent toxicity. 5-FU at 4 mg/kg/24 hr achieved a steady-state plasma 5-FU concentration of 5.3 microM and produced only mild gastrointestinal disturbances in 5-ethynyluracil-treated dogs. Thus, a catabolite of 5-FU appears to be responsible for the 5-FU-induced neurotoxicity in dogs.

alpha-fluoro-beta-alanine: effects on the antitumor activity and toxicity of 5-fluorouracil.

We have shown previously that (R)-5-fluoro-5,6-dihydrouracil (FUraH(2)) attenuates the antitumor activity of 5-fluorouracil (FUra) in rats bearing advanced colorectal carcinoma. Presently, we found that alpha-fluoro-beta-alanine (FBAL), the predominant catabolite of FUra that is formed rapidly via FUraH(2), also decreased the antitumor activity and potentiated the toxicity of FUra. In rats treated with Eniluracil (5-ethynyluracil, GW776), excess FBAL, in a 9:1 ratio to FUra, produced similar effects when administered 1 hr before, simultaneously with, or 2 hr after FUra. FBAL also decreased the antitumor activity of FUra in Eniluracil-treated mice bearing MOPC-315 myeloma at a 9:1 ratio with FUra, but not at a 2:1 ratio. FBAL did not affect the antitumor activity of FUra in mice bearing Colon 38 tumors. We also evaluated the effect of thymidylate synthase (TS) and thymidine kinase (TK) from tumor extracts after FUra +/- Eniluracil +/- FBAL treatment. The activity of TK was similar among the three groups at both 18 and 120 hr. There was also no difference in TS inhibition ( approximately 35%) at 18 hr. However, significantly more TS inhibition was observed in the Eniluracil/FUra group than in the FUra-alone group at 120 hr. FBAL did not alter the effect of Eniluracil/FUra in TS inhibition. Neither FUraH(2) nor FBAL affected the IC(50) of FUra in culture. Thus, the effect of FBAL did not result from direct competition with FUra uptake or immediate anabolism. Either another downstream catabolite that is not formed in cell culture is the active agent, or the effect requires the complexity of a living organism or an established tumor.

Dihydropyrimidine dehydrogenase inactivation and 5-fluorouracil pharmacokinetics: allometric scaling of animal data, pharmacokinetics and toxicodynamics of 5-fluorouracil in humans.

UNLABELLED: The pharmacokinetics of 5-fluorouracil (5-FU) in different animal species treated with the dihydropyrimidine dehydrogenase (DPD) inactivator, 5-ethynyluracil (776C85) were related through allometric scaling. Estimates of 5-FU dose in combination with 776C85 were determined from pharmacokinetic and toxicodynamic analysis. METHOD: The pharmacokinetics of 5-FU in the DPD-deficient state were obtained from mice, rats and dogs treated with 776C85 followed by 5-FU. The pharmacokinetics of 5-FU in humans were then estimated using interspecies allometric scaling. Data related to the clinical toxicity for 5-FU were obtained from the literature. The predicted pharmacokinetics of 5-FU and the clinical toxicity data were then used to estimate the appropriate dose of 5-FU in combination with 776C85 in clinical trials. RESULTS: The allometric equation relating total body clearance (CL) of 5-FU to the body weight (B) (CL = 0.47B0.74) indicates that clearance increased disproportionately with body weight. In contrast, the apparent volume of distribution (Vc) increased proportionately with body weight (Vc = 0.58 B0.99). Based on allometric analysis, the estimated clearance of 5-FU (10.9 l/h) in humans with DPD deficiency was comparable to the observed values in humans lacking DPD activity due to genetic predisposition (10.1 l/h), or treatment with 776C85 (7.0 l/h) or (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVdUrd, 6.6 l/h). The maximum tolerated dose (MTD) of 5-FU in combination with 776C85 was predicted from literature data relating toxicity and plasma 5-FU area under the concentration-time curve (AUC). Based on allometric analysis, the estimated values for the MTD in humans treated with 776C85 and receiving 5-FU as a single i.v. bolus dose, and 5-day and 12-day continuous infusions were about 110, 50 and 30 mg/m2 of 5-FU, respectively. DISCUSSION: The pharmacokinetics of 5-FU in the DPD-deficient state in humans can be predicted from animal data. A much smaller dose of 5-FU is needed in patients treated with 776C85.

Substrate-inhibitor cooperative interactions with microbial dihydrofolate reductases.

Cooperativity in the binding of two substrates to an enzyme is a now well-established phenomenon. The x-ray crystallographic structure of the E. coli DHFR binary TMP complex compared with the ternary enzyme-NADPH-TMP complex suggests without too imaginative extrapolation, that the conformational changes resulting from the binding of one ligand aid in favorably positioning potential binding sites for the second ligand. Of greater importance is the fact that the extent to which inhibitor binding is enhanced by the binding of NADPH varies from species to species. To a significant extent, for example, the selectivity of TMP is enhanced by the increase in its binding to the E. coli enzyme when NADPH is present as compared with several mammalian enzymes. The reverse, negative cooperativity (a decrease in binding of a substance when moving from the binary to a ternary complex), is perhaps less common and certainly less well studied. The present paper deals with one such enzyme, the DHFR from C. albicans, and by reference to another, that from S. cerevisiae, where it is shown that the binding of substrates exhibit strong negative cooperativity. It was of interest also to determine the relationship between inhibitor/NADPH cooperativity and the relative insensitivity of N. gonorrhoeae to TMP. Equilibrium studies show that the binding of TMP in binary complex with this enzyme is exceedingly poor and that a 2,200-fold cooperative effect brings the gonococcal enzyme Ki within one order of magnitude of the E. coli enzyme Ki. Even so, it takes synergism of another sort (with sulfamethoxazole) and high doses to make co-trimoxazole therapy feasible for treating gonorrhoeae. The comparative results on the gonococcal enzyme for a family of near relatives of TMP are of interest also for the reason that the structure-activity relationships with this enzyme are quite different from those of the E. coli and other microbial enzymes. Finally, it should be pointed out that although the negative cooperativity found for the candida and saccharomyces enzymes is relatively large, it is the values of the substrate Michaelis constants that are physiologically relevant. The Km values of the yeast enzymes are within the range for other DHFR and therefore the intracellular activity of the enzymes should not be compromised.

The solubility of (14)C-labelled barium carbonate in aqueous systems.

The solubility of (14)C-labelled barium carbonate has been determined in basic aqueous solutions with and without added Ba(2+) present and in aqueous solutions containing no added Ba(2+) or OH(-) ions. By use of activity coefficients from the literature, K(sp) has been determined to be 4.0 x 10(-10) +/- 0.5 x 10(-10) at 25 degrees .