Biomedical subjects
Publications and source records attributed to J A Kepler.
[structure: see text]. Replacing the 17alpha-acetoxy substituent in an antiprogestational 17beta-acetyl-11beta-arylestra-4,9-dien-3-one by 3-hydroxypropyl significantly diminished glucocorticoid receptor binding with little effect on progestin receptor binding.
Dolastatin 10 is a highly cytotoxic antimitotic peptide in phase II clinical trials. Its cytotoxicity has been as much as 50-fold greater than that of vinblastine, despite quantitatively similar effects of the two drugs on tubulin polymerization. We compared uptake and efflux of radiolabeled dolastatin 10 and vinblastine in human Burkitt lymphoma CA46 cells to gain an understanding of the greater cytotoxicity of the peptide. In the Burkitt cells, dolastatin 10 was 20-fold more cytotoxic than vinblastine (IC(50) values, 50 pM and 1.0 nM). When drug uptake at 24 h was compared at IC(50) values of the two drugs, the intracellular concentrations were almost identical (50-100 nM). The accumulation factor observed for dolastatin 10 was 900 to 1800 versus 60 to 100 for vinblastine. The two drugs showed very divergent uptake kinetics, however. Vinblastine and dolastatin 10 reached maximum intracellular concentrations after 20 min and 6 h, respectively. Depletion of cellular ATP content did not alter the uptake of either drug, indicating passive uptake of both. When drug-preloaded cells were transferred to drug-free medium, there was no loss of dolastatin 10 for at least 2 h, whereas vinblastine exited the cells rapidly (approximate intracellular half-life, 10 min), with less than 10% of the initial drug remaining in the cells after the 2-h incubation. The potency of dolastatin 10 probably derives from its tenacious binding to tubulin, a property that in cells becomes translated into prolonged intracellular retention of the drug. Optimal clinical use of dolastatin 10 may require administration by infusion rather than by bolus.
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The antimitotic depsipeptide cryptophycin 1 (CP1) was compared to the antimitotic peptide dolastatin 10 (D10) as an antiproliferative agent and in its interactions with purified tubulin. The potent activity of CP1 as an inhibitor of cell growth was confirmed. The agent had an IC50 of 20 pM against L1210 murine leukemia cells versus 0.5 nM for D10. Both drugs were comparable as inhibitors of the glutamate-induced assembly of purified tubulin, with D10 being slightly more potent. CP1, like D10, was a noncompetitive inhibitor of the binding of [3H]vinblastine to tubulin (apparent Ki, 3.9 microM); and the depsipeptide was a competitive inhibitor of the binding of [3H]D10 to tubulin (apparent Ki, 2.1 microM). CP1 was less potent than D10 as an inhibitor of nucleotide exchange on tubulin, but the two drugs were equivalent in stabilizing the colchicine binding activity of tubulin. CP1, like D10, caused the formation of extensive structured aggregates of tubulin when present in stoichiometric amounts relative to the protein. Whereas at lower concentrations the drugs were equivalent in causing formation of small oligomers detected by gel permeation high-performance liquid chromatography, there were notable differences in the aggregation reactions induced by the two drugs. The electron micrographic appearance of the D10-induced aggregate differed substantially from that of the CP1-induced aggregate. With D10, but not CP1, aggregate morphology was greatly altered in the presence of microtubule-associated proteins. Finally, although CP1 caused the formation of massive aggregates, as did D10, there was little turbidity change with the depsipeptide as opposed to the peptide.
In vitro binding studies have shown that epibatidine and its norchloro analogues have high affinities for the cholinergic nicotinic receptor. In this study, the in vivo binding characteristics of [3H](+)norchloroepibatidine (NCEPB) and [3H](-)NCEPB in mice were examined. After injection of [3H](+)NCEPB, radioactivity levels in all brain regions examined increased and then decreased, with different regions accumulating different levels of radioactivity. The regional distribution of radioactivity at later times paralleled the distribution of nicotinic receptor binding in vitro. The ratio of radioactivity in the superior colliculus to that in the cerebellum, a reflection or estimate of total: non-specific binding, was strikingly high at 8 h (about 35). Drugs known to bind to nicotinic receptors reduced [3H](+)NCEPB binding, while atropine and spiperone, muscarinic and dopaminergic drugs respectively, did not. [3H](-)NCEPB had a similar regional distribution to [3H](+)NCEPB. Pretreatment with increasing doses of non-radioactive (-)NCEPB resulted in saturation of in vivo binding of [3H](-)NCEPB. These data indicate that [3H](+) and (-)NCEPB are useful in vivo binding ligands for the cholinergic nicotinic receptor.
The highly cytotoxic, sponge-derived, antimitotic macrolide polyether spongistatin 1 has been previously shown to inhibit microtubule assembly, the binding of vinblastine and GTP to tubulin, and displacement of GDP bound in the exchangeable site of tubulin. We have now examined in detail inhibition by spongistatin 1 of both [3H]vinblastine and [3H]dolastatin 10 binding to tubulin. We found spongistatin 1 to be a noncompetitive inhibitor of the binding of both radiolabeled drugs to tubulin, in contrast to competitive patterns obtained with vincristine versus [3H]vinblastine and with a chiral isomer of dolastatin 10 versus [3H]dolastatin 10. Since dolastatin 10 is itself a noncompetitive inhibitor of vinca alkaloid binding to tubulin, this implies at least three distinct binding sites for the structurally complex and diverse natural products that interfere with each others binding to tubulin and with nucleotide exchange. Spongistatin 1, in contrast to both vinca alkaloids and peptide antimitotic agents like dolastatin 10, does not induce formation of a GTP-independent, morphologically distinctive polymer ("aggregate"). We also examined eight compounds closely related structurally to spongistatin 1 (spongistatins 2-9). The most distinctive in their properties were spongistatins 6 and 8. These two compounds, despite activity comparable to spongistatin 1 as inhibitors of tubulin polymerization and [3H]vinblastine binding, had much reduced activity as inhibitors of nucleotide exchange and [3H]dolastatin 10 binding. Spongistatins 1 and 6 were compared for effects on dolastatin 10-induced aggregate formation in conjunction with effects on [3H]dolastatin 10 binding. Spongistatin 6 was about 4-fold less active than spongistatin 1 as an inhibitor of aggregation and over 20-fold less active as an inhibitor of dolastatin 10 binding.
We have prepared [3H]dolastatin 10 and examined its interactions with tubulin. Binding kinetics appeared to be biphasic, with a rapid initial reaction that could not be accurately measured, followed by a slower second reaction. Bound drug was stable in centrifugal gel filtration, column gel filtration, and high performance liquid chromatography gel filtration, but the bound drug could be displaced by an active isomer of dolastatin 10. Scatchard analysis of binding data was consistent with two classes of binding sites. However, dolastatin 10 induced an aggregation reaction upon binding to tubulin, complicating analysis of the data, and incorporation of [3H]dolastatin 10 into large aggregates was readily demonstrated. The chromatographic properties of the smallest radiolabeled species that could be documented were most consistent with a complex consisting of two molecules of alpha/beta-tubulin dimer and two molecules of [3H]dolastatin 10. The coexistence of an aggregation reaction with a binding reaction at a single site probably underlies the biphasic binding kinetics and the biphasic Scatchard plot. Of peptides that strongly inhibit tubulin polymerization (dolastatin 10, dolastatin 10 isomers, segments, and analogs, dolastatin 15, and phomopsin A), only those previously shown to be strong inhibitors of vinblastine binding and nucleotide exchange also strongly inhibited [3H]dolastatin 10 binding and induced tubulin aggregation (dolastatin 10 itself, two chiral isomers of dolastatin 10, and phomopsin A). The morphology of dolastatin 10-induced aggregates was compared with that of vinblastine-induced aggregates under a variety of reaction conditions. With both drugs the aggregates had a more organized appearance when microtubule-associated proteins were included in the reaction.
A number of 1,2,4-trioxanes were prepared and tested for antimalarial activity in search of a simplified analogue of the naturally occurring antimalarial qinghaosu. The compounds were assayed in an in vitro system for antimalarial activity against chloroquine-susceptible and chloroquine-resistant strains of Plasmodium falciparum. The most active compounds were methyl 2-(2,4a-epidioxy-4a,5,6,7,8,8a-hexahydro-5,5,8a-trimethyl-2H-1-benzop yra n-2-yl) acetate (3b), which showed IC20's of 96 and 39 ng/mL, respectively, and 2,4a-epidioxy-3,4,4a,5,6,7,8,8a-octahydro-2-[2-(benzoyloxy)propyl]-5,5,8 a- trimethyl-2H-1-benzopyran (12), which showed IC50's of 24 and 99 ng/mL, respectively. For comparison, qinghaosu exhibits an IC50 of 1 ng/mL for both strains.
Three peptide derivatives of primaquine were synthesized. The compounds were tested for radical curative antimalarial activity against Plasmodium cynomolgi in rhesus monkeys and blood schizonticidal antimalarial activity against Plasmodium berghei in mice. All three peptide derivatives showed activity against P. cynomolgi greater than that expected for the primaquine content of each prodrug. The toxicity of one of the peptide derivatives was less than that of primaquine in mice.
A number of mono- and bicyclic endoperoxides were prepared and tested for antimalarial activity in search of a simplified analogue of the 5-oxygen-substituted 1,2,4-trioxane ring structure of the naturally occurring antimalarial qinghaosu. The compounds were assayed in an in vitro system for antimalarial activity against chloroquine-susceptible and chloroquine-resistant strains of P. falciparum. The most active compound in this assay was 2-[((butyloxy)-carbonyl)oxy]-1,1,10-trimethyl-6,9-epidioxy-delta 7-octalin (17a), which showed an IC50 of 100 and 57 ng/mL, respectively. For comparison, qinghaosu exhibits a mean IC50 less than 3.4 ng/mL.
Diethylaminoethyl acetate, an acetylcholine analog, was formed upon the incubation of diethylaminoethanol and acetyl-CoA with bovine brain choline acetyltransferase (acetyl-CoA: choline O-acetyltransferase; EC 2.3.1.6). The new product co-chromatographed with authentic diethylaminoethyl acetate on thin layer plates, and its formation was proportional to the duration of incubation and enzyme concentrations. When tested on guinea-pig ileum, diethylaminoethyl acetate was found to be an agonist with an ED50 of 1.3 X 10(-4) M, compared to an ED50 of 2.0 X 10(-7) M for acetylcholine. The contraction of guinea-pig ileum induced by diethylaminoethyl acetate was blocked by atropine. Moreover, diethylaminoethyl acetate induced a secretion of alpha-amylase from isolated pancreatic acini cells; this effect was also blocked by atropine. It is entirely possible that diethylaminoethyl acetate can be a false cholinergic transmitter generated in vivo when drugs such as aprophen or procaine are administered to animals, since either of these drugs can undergo enzymatic hydrolysis to generate diethylaminoethanol. A method for the synthesis of radioactive diethylamino [1,2-14C]ethyl acetate was also described.
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Caffeine was analyzed in human plasma and saliva by a simple, rapid, and sensitive radioimmunoassay procedure. Immunization of rabbits with an antigen prepared by coupling 7-(5-carboxypentyl)-1,3-dimethylxanthine to bovine serum albumin resulted in the formation of antibodies selective for caffeine as opposed to various mono- and dimethylxanthines, mono-, di-, and trimethyluric acids and a variety of common drugs. The radioligand used for competitive binding studies was 7-(2,3-3H2-propyl)-1,3-dimethylxanthine. The procedure permits direct analysis of caffeine in plasma or saliva without extraction. Comparison with a high pressure liquid chromatography method for the analysis of caffeine gave satisfactory results and showed no evidence for interference by metabolites. A caffeine half-life of 4.0 hours determined by the radioimmunoassay was in agreement with previous work. Comparison of human plasma and saliva levels by the radioimmunoassay procedure indicated approximately equal concentrations in the two fluids.
Antisera to theophylline (T) have been obtained by immunizing rabbits with a conjugate of 8-(3-carboxypropyl)-1,3-dimethylxanthine and bovine serum albumin. Comparison of 50% displacement values indicated good selectivity for T vs. a number of other xanthine derivatives. An analytical procedure using this antiserum can measure 200 pg of T and direct analysis of 0.1 mug/ml in plasma or 0.02 mug/ml in saliva is feasible.
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