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L K Cheng

Publications and source records attributed to L K Cheng.

24 records · Page 2Linked to original sources
PubMed1978

Effects of L-cysteine, L-cysteine derivatives and ascorbic acid on lead excretion in rats.

Urinary and fecal excretion of lead in rats were compared after intravenous administration of L-cysteine and a number of its derivatives. In terms of increasing total lead excretion, L-cysteine ethyl ester was about half as effective as D-penicillamine, whereas L-cysteine and all other derivatives tested were only marginally effective. Interestingly, L-cysteine ethyl ester appeared to increase lead depletion mainly through biliary excretion. This compound may, therefore, have lower renal toxicity than other chelating agents which promote lead excretion principally by increasing urinary elimination. Dietary supplements containing 1% cysteine, 1% ascorbic acid of 1% of both substances did not dramatically increase lead elimination.

Animals↗
PubMed1976

Dose-dependent pharmacokinetics of laevodopa and its metabolites in the rat.

1. Plasma levels of total radioactivity, unchanged laevodopa, dopamine, 3-O-methyldopa and non-amino phenolic acids were measured in rats treated orally with [3H]laevodopa at total laevodopa doses of 1, 10, 20 and 100mg/kg while the radioactive dose was maintained at 100muCi/kg for all animal groups. 2. Within the dosage range studied, non-linear pharmacokinetics were observed for unchanged laevodopa, non-amino phenolic acids and dopamine but not for 3-O-methyldopa. 3. The area under the plasma concentration-time curve for laevodopa increased exponentially with linear increments in the oral dose. This non-linear increase in laevodopa bioavailability is consistent with the hypothesis that high oral doses of laevodopa are required to saturate gastro-intestinal metabolism of the drug. 4. At the lower doses (1 and 20 mg/kg), only 2-3% of total radioactivity could be attributed to dopamine. At 100mg/kg, the dopamine metabolite fraction amounted to about 1/3 of total radioactivity in the plasma.

Animals↗
PubMed1975

Fluorescence reactions of fluorescamine with levodopa and its derivatives: fluorescence assay of 3-methoxy-4-hydroxyphenylalanine in levodopa dosage forms.

A simple fluorometric method for the quantitation of 3-methoxy-4-hydroxyphenylalanine, either alone or in the presence of levodopa, is presented. Fluorescence is developed by reaction with fluorescamine. The interactions of this reagent with levodopa and a number of its derivatives under various experimental conditions were studied. Negligible fluorescence was obtained with levodopa, dopamine, levodopa benzyl ester, and 6-hydroxydopamine.

Chemical Phenomena↗
PubMed1975

Effects of chronic oral administration on the disposition of laevodopa and its major metabolites in the plasma of the rat.

1. Plasma levels of total radioactivity , unchanged laevodopa, catecholamines, 3-O-methyldopa and non-amino phenolic acids were measured in rats treated orally with [3H]laevodopa (100 mg/kg, 100 muCi/kg) for different periods of time. 2. Chronic oral administration of laevodopa for two months significantly delayed the appearance of peak plasma levels and decreased the overall plasma concentrations of the unchanged drug as compared to controls. The extent of absorption of an oral dose of [3H]laevodopa was not significantly impaired on chronic treatment and/or ageing. The decrease in the levels of unchanged laevodopa appeared to be due to an increase in the apparent volumes of distribution of the drug and its metabolites upon ageing. 3. The plasma levels of the catecholamine metabolite fraction, which included dopamine, noradrenaline, etc., were much higher in chronically dosed animals. 4. These findings may be relevant in the understanding of such clinical observations as decreasing therapeutic effects and increasing toxicity after long-term laevodopa treatment in human patients.

3,4-Dihydroxyphenylacetic Acid↗