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Biomedical subjects

R L Chan

Publications and source records attributed to R L Chan.

At least 37 records · Page 2Linked to original sources
PubMed1988

Disposition of detirelix, a potent luteinizing hormone-releasing hormone antagonist, in rats and monkeys.

The disposition of detirelix, a potent luteinizing hormone-releasing hormone (LHRH) antagonist, was studied in rats and monkeys. After a single 300-microgram/kg intravenous dose in rats, the plasma elimination t1/2 was 1.6 hr and the plasma clearance was 3.3 ml/min/kg. In the monkey, the mean t1/2 and plasma clearance were 7.1 hr and 1.3 ml/min/kg, respectively, after an 80-microgram/kg intravenous dose. Long plasma t1/2 values of 18.7 and 31.6 hr were observed after single 0.2- and 1.0-mg/kg subcutaneous doses in the monkey, suggesting the possibility of subcutaneous depot formation at the injection site. Biliary excretion was the predominant route of elimination of detirelix in both species. Less than 10% of the detirelix was excreted renally. A major metabolite, isolated from the rat bile, was the 1-4 tetrapeptide fragment of detirelix. This metabolite was formed by enzymatic hydrolysis of the Ser4-Tyr5 bond, one of the only two peptide bonds in detirelix not containing a D-amino acid.

Animals↗
PubMed1988

Absorption and metabolism of nafarelin, a potent agonist of gonadotropin-releasing hormone.

Nafarelin, a potent gonadotropin-releasing hormone (GnRH) agonist, was absorbed rapidly into systemic circulation (time to reach peak concentration, 20 to 40 minutes) after intranasal but not after sublingual or vaginal administration. Serum elimination half-life was about 2 hours. Nasal absorption of nafarelin was increased by increasing the concentration of the drug in the dose solution and incorporating sodium glycocholate into the nasal formulation. An optimal formulation providing maximum nasal absorption of nafarelin was one containing 1.75 mg nafarelin per ml and 2% sodium glycocholate. Bioavailability of this nasal formulation relative to a single subcutaneous dose averaged 21%. The metabolism and excretion of nafarelin were determined in three subjects after subcutaneous administration of [14C]-nafarelin. Radioactivity was excreted in approximately equal amounts in urine and stool. Six metabolites accounted for most of the radioactivity in urine. Four metabolites were short peptide fragments of nafarelin and the other metabolites were naphthylalanine and 2-naphthylacetic acid.

Administration, Intranasal↗
PubMed1987

Immunological studies of the binding protein for chloroplast ferredoxin-NADP+ reductase.

Monospecific rabbit antibodies against the ferredoxin-NADP+ reductase binding protein of spinach thylakoids were obtained and characterized. The immunoglobulin G (IgG) fraction gave single precipitation arcs with the purified antigen or with Triton X-100 extracts of thylakoids or the reductase binding protein complex. Antibodies against the flavoprotein behave similarly. Both antibodies agglutinated thylakoids and precipitated the diaphorase activity of a Triton X-100 extract of these membranes. Isolated Fab fragments of the IgG anti-binding protein inhibited NADP+ photoreduction in a time- and Fab concentration-dependent manner. The presence of ferredoxin diminished the rate of inhibition. In the light, the inactivation rate was higher than in dark and this effect was abolished in the presence of uncouplers. These results suggest that the binding protein is protruding from the thylakoids and could be sensing the proton gradient.

Carrier Proteins↗
PubMed1987

Suppression of pituitary-gonadal function by a potent new luteinizing hormone-releasing hormone antagonist in normal men.

LHRH antagonists compete with endogenous LHRH for binding to receptors on pituitary gonadotrophs and thereby inhibit gonadal function by suppressing gonadotropin secretion. We studied the effects of a recently developed LHRH antagonist on the pituitary-gonadal axis in man. The antagonist Detirelix [( N-Ac-D-Nal(2)1, D-pCl-Phe2,D-Trp3, D-hArg(Et2)6, D-Ala10]LHRH) was given as a single sc injection to nine normal men at three dose levels (5, 10, and 20 mg) at intervals of at least 7 days. Serum FSH, LH, and testosterone levels were measured before treatment, at frequent intervals for 48 h, and 72, 96, and 168 h after administration of the antagonist. Mean serum FSH levels decreased (P less than 0.001) from 6.9 +/- 0.5 (+/- SEM) mIU/mL to nadirs of 4.4 +/- 1.1, 3.6 +/- 0.9, and 4.1 +/- 0.9 after the 5-, 10-, and 20-mg doses, respectively. Serum LH levels decreased (P less than 0.001) from 6.2 +/- 0.3 mIU/mL to nadirs of 3.3 +/- 0.4, 2.8 +/- 0.3, and 2.7 +/- 0.3 after all three doses. Serum testosterone levels decreased (P less than 0.001) in a dose-dependent fashion from 5.1 +/- 0.2 ng/mL to nadirs of 1.3 +/- 0.3, 0.9 +/- 0.3, and 0.6 +/- 0.1 after the same doses. After the initial testosterone decrease, however, escape occurred 12-28 h after the lower doses. The area under the response curve, describing hormone concentrations as a function of time during the study, diminished by 23 +/- 2%, 36 +/- 4%, and 36 +/- 3% for FSH, by 14 +/- 6%, 30% +/- 6%, and 34 +/- 5% for LH, and by 41 +/- 5%, 58 +/- 6%, and 68 +/- 4% for testosterone with the same doses, respectively. The apparent plasma disappearance half-life of Detirelix by RIA was at least 41 h after all three doses. Detirelix elicited only a minor local reaction; no systemic side-effects were observed within the dose range used. These results indicate that this LHRH antagonist is a safe, highly potent inhibitor of the human pituitary-gonadal axis with an exceptionally long duration of action.

Adult↗
PubMed1985

Plasma binding of LHRH and nafarelin acetate, a highly potent LHRH agonist.

The binding of LHRH and [6-(3-(2-naphthyl)-D-Ala]LHRH (nafarelin acetate), a highly potent LHRH agonist, to plasma proteins was investigated in vitro by equilibrium dialysis at 4 degrees C with fresh plasma from normal human subjects, female rhesus monkeys, and female rats. Over a wide range of concentrations (10(-8) to 10(-5) M), 78-84% of nafarelin acetate and only 22-25% of LHRH were bound to undiluted plasma. With 10% plasma, the extent of binding was 31-37%, and 0.9-4.2% for nafarelin acetate and LHRH, respectively. Albumin was shown to play a predominant role in the plasma binding of the two compounds. The considerable differences in the extent of binding of nafarelin acetate and LHRH to plasma may contribute to some of the differences in pharmacokinetic parameters observed for the two compounds.

Animals↗
PubMed1985

Isolation and sequencing of an active-site peptide from spinach ferredoxin-NADP+ oxidoreductase after affinity labeling with periodate-oxidized NADP+.

Spinach ferredoxin-NADP+ oxidoreductase was inactivated by treatment with 2',3'-dialdehyde NADP+ (periodate-oxidized NADP+), which selectively modifies a lysine residue at the nucleotide-binding domain of the enzyme. The identity of the derivatized residue was ascertained by thin-layer chromatography of the protein hydrolysate. Reductase that had been labeled with periodate-oxidized NADP+ and NaB3H4 was treated with trypsin, and samples of the tryptic digest were subjected to reverse-phase high-performance liquid chromatography. The radioactivity profiles showed modification of one specific peptide. The primary structure of this peptide was found to be Gly-Glu-Lys*-Met-Tyr-Ile-Gln-Thr-Arg, where Lys* represents the derivatized lysine. The sequence obtained corresponds to residues 242-250 in the primary structure of spinach ferredoxin-NADP+ reductase recently reported [Karplus et al. (1984) Biochemistry 23, 6576-6583].

Affinity Labels↗
PubMed1984

Affinity labeling of spinach ferredoxin-NADP+ oxidoreductase with periodate-oxidized NADP+.

Periodate-oxidized NADP+ (dialdehyde-NADP+) inactivated soluble ferredoxin-NADP+ oxidoreductase and combined covalently to the enzyme. This inactivation was first order with respect to dialdehyde-NADP+ and followed saturation kinetics, indicating that the enzyme initially forms a reversible complex with the inactivator. NADP+ afforded complete protection against inactivation, while spinach ferredoxin was uneffective. In the presence of exogenous ferredoxin and illuminated thylakoids, the nucleotide analog functioned as a coenzyme for the reductase, although with rather lower efficiency than NADP+. It also acted as a competitive inhibitor with respect to NADPH in diaphorase activity. Incorporation of radioactivity from periodate-oxidized [3H]NADP+ gave a stoichiometry of 0.85 mol of reagent/mol of reductase, indicating that the modification of a single residue in the flavoprotein is responsible for the loss of enzymatic activity.

Affinity Labels↗
PubMed1984

A fast and sensitive micromethod for the manual sequencing of peptides using O-phthalaldehyde as derivatizing reagent.

A general procedure for the manual sequencing of peptides using the fluorogenic reagent O-phthalaldehyde (OPA) is described. The method can be applied in two different ways. One of them involves back hydrolysis of the anilinothiazolinones resulting from the Edman degradation of the peptide and subsequent detection of the free amino acids as OPA derivatives. The other is a subtractive analysis in which the amino acid composition of the remaining peptide is determined after each degradation cycle. The direct procedure can be coupled to the subtractive one in order to assure the accuracy of the sequence analysis. The method is fast and simple, and allows determination of 10 pmol of amino acid per cycle using standard reagents and instrumentation. Sensitivity can be greatly enhanced provided that ultrapure chemicals are employed. Small peptides (8-10 residues) were sequenced from 200 pmol sample, using a high-performance liquid chromatography assembly coupled to a fluorescence detector.

Aldehydes↗
PubMed1984

Conformationally restricted retinoids.

A series of conformationally restricted retinoids was synthesized and screened in two assays used to measure the ability of retinoids to control cell differentiation, namely, the reversal of keratinization in tracheal organ culture from vitamin A deficient hamsters and the inhibition of the induction of mouse epidermal ornithine decarboxylase by a tumor promoter. These compounds had bonds corresponding to selected bonds of the E-tetraene chain of retinoic acid (1) held in a planar cisoid conformation by inclusion in an aromatic ring. The meta-substituted analogue 3 of 4-[(E)-2-methyl-4-(2,6,6-trimethylcyclohexenyl)-1,3-butadienyl+ ++]benzoic acid (2) was far less active than 2 in both assays. In contrast, the vinyl homologue of 2 (4) and the 7,8-dihydro and 7,8-methano analogues (5 and 6) had activity comparable to that of 2. Analogues of 4-[(E)-2-(1,1,4,4-tetramethyl-1,2,3,4-tetrahydro-6-naphthyl)propenyl] benzoic acid (7) were also screened. Replacement of the tetrahydronaphthalene ring of 7 by a benzonorbornenyl group (9) significantly reduced activity, as did removal of the vinylic methyl group from 9 (10). Replacement of the propenyl group of 9 by a cyclopropane ring (12) also reduced activity. Replacement of the tetrahydronaphthalene ring of 7 by 4,4-dimethyl-3,4-dihydro-2H-1-benzopyran and -benzothiopyran rings (13 and 14) also decreased activity. Inclusion of the 7,9 double bond system of 1 in an aromatic ring (15 and 16) reduced activity, whereas inclusion of the 5,7 double bond system in an aromatic ring enhanced activity (7 and 19). Inclusion of the 11,13 and 9,11,13 double bond systems in aromatic rings (2 and 18) also reduced activity below that of 1. Retinoic acid, 7, 13, 14, and 19 inhibited papilloma tumor formation in mice. Toxicity testing indicated that 7 was more toxic than 1, 13, 14, and 19, 19 was more toxic than 1, and 13 and 14 were less toxic than 1.

Animals↗
PubMed1984

Relationship between binding affinities to cellular retinoic acid-binding protein and biological potency of a new series of retinoids.

Binding affinities of a new and unusual series of retinoic acid analogues to cellular retinoic acid-binding protein, a possible mediator of their biological function in the control of differentiation and tumorigenesis, and to serum albumin, their plasma transport protein, were determined. Also, biological activity of these retinoids in the reversal of keratinization in hamster tracheal organ cultures was assessed and compared with their binding affinities. Analogues that possessed high biological activity showed high binding efficiency to cellular retinoic acid-binding protein. Those that were biologically less active were poor binders to the binding protein. Three retinoids, 4657-57, 3920-59, and 4445-75, which showed 90 to 100% binding efficiency of that of retinoic acid for cellular retinoic acid-binding protein expressed high biological activity detectable in the range of 10(-10) M as against 10(-11) M for retinoic acid. The correlation noticed in these two activities not only enhances the confidence in the two assay procedures but also paves the way for design and development of potential chemopreventive agents. No apparent differences were observed in the binding affinities of the retinoids to binding proteins of a normal tissue or a tumor tissue. No correlation existed between the binding affinities of these retinoids to serum albumin and their biological activity. Structure-activity relationships of the retinoids in relation to their binding affinities and biological activities have been discussed.

Animals↗
PubMed1983

Synthesis and pharmacological activity of 6-[(E)-2-(2,6,6-trimethyl-1-cyclohexen-1-yl)ethen-1-yl]- and 6-(1,2,3,4-tetrahydro-1,1,4,4-tetramethyl-6-naphthyl)-2-naphthalenecarboxylic acids.

6-[(E)-2-(2,6,6-Trimethyl-1-cyclohexen-1-yl)ethen-1-yl]- and 6-(1,2,3,4-tetrahydro-1,1,4,4-tetramethyl-6-naphthyl)-2-naphthalenecarboxylic acids (4 and 8) have been synthesized and show significant activity in reversing the keratinization process in hamster tracheal organ culture and in inhibiting the induction of ornithine decarboxylase in mouse skin by 12-O-tetradecanoylphorbol-13-acetate, two assays used to measure retinoid activity. The 2-naphthalenecarboxylic acid 8 was more active than 4.

Animals↗
PubMed1981

Aromatic retinoic acid analogues. Synthesis and pharmacological activity.

Aromatic analogues of (all-E)- and 13(Z)-retinoic acids have been synthesized as potential chemopreventive agents for the treatment of epithelial cancer. In the E series, (1E,3E)-1-(4-carboxyphenyl)-2-methyl-4-(2,6,6-trimethyl-1-cyclohexen-1-yl)-1,3- butadiene (7a), its ethyl ester 5a, and the epoxy ethyl ester 14 displayed excellent activity in the assay for the inhibition of tumor promotor-induced mouse epidermal ornithine decarboxylase, while (1E,3E)-1-(4-carboethoxy-3-methylphenyl)-2-methyl-4-(2,6,6-trimethyl-1-cyclohexen-1-yl)-1,3-butadiene (5b) was inactive. The 13(Z) analogues, (E)-1-(2-carboxyphenyl)-4-methyl-6-(2,6,6-trimethyl-1-cyclohexen-1-yl)-1,3,5-he xatriene (19) and (E)-1-(2-hydroxyphenyl)-4-methyl-6-(2,6,6-trimethyl-1-cyclohexen-1-yl)-1,3,5-he xatriene (27), had minimal activity.

Animals↗
PubMed1981

Retinoic acid analogues with ring modifications. Synthesis and pharmacological activity.

Analogues of retinoic acid that have their major modifications in the 5,6 double bond and 4-methylene group regions of the beta-cyclogeranylidene ring have been synthesized as potential agents for the treatment and prevention of epithelial cancer. These modifications were intended to reduce retinoid toxicity by lowering the effective treatment dose because the major metabolic deactivation pathway would be inhibited. Ethyl (E)-3,7-dimethyl-9-(exo-2-bicyclo[2.2.1]-heptyl)-2,4,6,8-nonatetraenoate (7), ethyl (E)-3,7-dimethyl-9-(2,2,6-trimethylbicyclo[4.1.0]hept-1-yl)-2,4,6,8-nonatetraen oate (18), (E)-1-(4-carbethoxyphenyl)-2-methyl-4-(2,2,6-trimethylbicyclo[4.1.0]hept-1-yl)- 1,3-butadiene (28), (E)-retinoic acid-4,4,18,18,18-d5 (39), and ethyl (E)-3,7-dimethyl-9-(3,3-ethano-2,6,6-trimethyl-1-cyclohexen-1-yl)-2,4,6,8-nonatetraenoate (47) displayed moderate to excellent activity in an assay for the inhibition of tumor promoter-induced mouse epidermal ornithine decarboxylase.

Animals↗
PubMed1979

Chloroquine resistance in malaria: accessibility of drug receptors to mefloquine.

The process of mefloquine accumulation was studied in mouse erythrocytes infected with either Plasmodium berghei CS (chloroquine susceptible) or P. berghei CR (chloroquine resistant). In both cases, mefloquine was accumulated by a saturable process with an apparent dissociation constant of 2.5 x 10(-6) M and an apparent maximal capacity of 700 mumol per kg of erythrocyte pellet; uninfected mouse erythrocytes accumulated more than half as much mefloquine as infected erythrocytes. The process of accumulation was not stimulated by providing glucose as a substrate, and it was not inhibited in infected erythrocytes by azide, iodoacetate, or incubation at 2 degrees C. Although mefloquine was accumulated more effectively than chloroquine by uninfected erythrocytes and by erythrocytes infected with P. berghei CR, competition between chloroquine and mefloquine was observed, raising the possibility that the same process of accumulation serves both drugs. Chloroquine competitively inhibits mefloquine accumulation, with an apparent inhibitor constant of 1.7 x 10(-3) M, and mefloquine competitively inhibits chloroquine accumulation, with an apparent inhibitor constant of 2 x 10(-6) M. The same process of accumulation and the same group of receptors could serve both drugs if mefloquine has greater access than chloroquine to the receptors. Regardless of whether the same process serves both drugs, undiminished accumulation by erythrocytes infected with P. berghei CR provides an explanation for the superiority of mefloquine in treating chloroquine-resistant malaria.

Animals↗
PubMedDate not supplied

Disposition of RS-26306, a potent luteinizing hormone-releasing hormone antagonist, in monkeys and rats after single intravenous and subcutaneous administration.

The metabolic disposition of RS-26306, a new potent luteinizing hormone-releasing hormone antagonist, was studied in rats and monkeys after single i.v. and sc administration with the 3H-labeled compound. Plasma pharmacokinetics after iv administration were: CLs = 2.5 ml/min/kg, Vd beta = 0.29 liter/kg, t1/2 = 1.4 hr (rats), and CLs = 0.8 ml/min/kg, Vd beta = 0.32 liter/kg, t1/2 = 5.1 hr (monkeys). Cmax and Tmax in rats were 0.53 micrograms/ml and 4 hr after the 1 mg/kg sc dose, and were 1.07 micrograms/ml and 12 hr after the 10 mg/kg sc dose. AUC0-infinity after the 10 mg/kg sc dose in rats was seven times that after the 1 mg/kg sc dose. Apparent plasma disappearance t1/2 in rats were 3.6 and 15.2 hr, respectively, after the 1 and 10 mg/kg sc doses. An average of 12 and 4% of dose radioactivity remained at the injection site in rats 3 and 10 days, respectively, after a 10 mg/kg sc dose. In monkeys, Tmax after a 1 mg/kg sc dose was 0.5 hr for three animals but was 24 hr for the fourth animal, although plasma of this monkey contained substantial levels of RS-26306 between 15 min and 24 hr. Apparent plasma t1/2 in monkeys after a 1 mg/kg sc dose was at least 19 hr. Our data suggest depot formation after sc doses. In vitro plasma binding amounted to 82-84%. Excretion was mainly biliary: 12-25 and 55-84% of dose radioactivity was recovered in urine and feces, respectively, in both species. The biological samples contained only traces of 3H2O. Three metabolites, which were truncated peptides of the parent decapeptide, were identified in the rat bile. One of these was also present in the monkey plasma. The restricted enzymatic degradation of RS-26306, extensive plasma binding, and long circulating t1/2 of RS-26306 contribute to its prolonged activity in animal models and in humans.

Animals↗
PubMedDate not supplied

Disposition of nafarelin acetate, a potent agonist of luteinizing hormone-releasing hormone, in rats and rhesus monkeys.

Nafarelin acetate, [6-(3-(2-naphthyl)-D-Ala)]-luteinizing hormone-releasing hormone (LHRH), is a synthetic LHRH agonist. In the suppression of estrus in female rats, nafarelin acetate was about 200 times as potent as LHRH. We have studied the distribution and excretion of radiolabeled nafarelin acetate administered iv to rats and rhesus monkeys and have also compared the pharmacokinetics of nafarelin acetate to LHRH in these two species. Distribution of a single 100-micrograms dose [14C]nafarelin acetate into tissues in male rats was rapid and of the 13 tissues assayed, highest concentrations of radioactivity were observed in the kidneys, liver, and intestines. The urine to feces ratio of nafarelin-associated 14C was 4:1 in rhesus monkeys and 1:3 in rats. Nafarelin acetate and LHRH given iv to female rhesus monkeys in approximately equimolar doses (6.5 nmol/kg) had terminal plasma t1/2 values of 120 min and 33 min, and systemic clearance values of 2.7 ml/min/kg and 31.4 ml/min/kg, respectively. In female rats, the plasma t1/2 was 33.6 min for nafarelin acetate and 6.7 min for LHRH, and the systemic clearance values for nafarelin acetate and LHRH were 12.0 ml/min/kg and 57.6 ml/min/kg, respectively, after approximately equimolar doses.

Animals↗