Half-life Determinations

A nonlinear fit weights the initial data points more heavily and gives a better description of the decline in oxamyl residues during the critical period when the residues are a concern in the evaluation of worker safety. The nonlinear curve fitting approach has been accepted by regulatory agencies for the determination of pesticide half-life determinations in soil when the decline data do not fit a linear first-order curve. 

Eastwood, T. A., Brown, F., and Crocker, I. H., "A krypton-81 half-life determination using a mass separator", Nucl. Phys., 1964, 58, 328. 

Buffers used for preincubation of the enzyme aliquots at the various pH values were glycolate/NaOH (pH 3.5 and 4.0), acetate/NaOH (pH 5.0), MES/NaOH (pH 6.0), MOPS (pH 7.0), Tris (pH 8.0) and boric acid/NaOH (pH 9.0). The preincubation buffers were adjusted at room temperature (20 C) to the values indicated above the effect of temperature on the pH was then measured over the range from 20 C to 75 C, and the activity half-life determined at each temperature was reported at the actual pH of the preincubation. 

The half-life determines the time it will take to achieve steady state and is useful for determining a dosing regimen. However, it does not give any clue to the processes involved in handling the drug, so that knowledge of the half-hfe alone cannot be used to make predictions about factors which are likely to affect the rate of elimination. 

The foscarnet terminal half-life determined by urinary excretion was 87.5 41.8 hours, possibly because of release of foscarnet from bone. Postmortem data provide evidence that foscarnet does accumulate in bone in humans. 

Half-life determination (liver micros., human)... 

Chlorine fluoride, 21 235, 236, 238, 240, 242, 244, 246, 247, 249 geometry of, 18 320-322 oxygenation of, 18 349 oxygen-containing, 21 245 Chlorine fluoride oxide radicals, 18 385, 386 Chlorine hydroxide, 5 219 Chlorine isotope, half-life determination, 2 ... 

Decatriene complexes with gold, 12 348 Decay method, of isotope half-life determination, 2 325... 

SPARC (Secreted Protein Acidic and Rich in Cysteine), 46 484-485 Specific activity method, of isotope half-life determination, 2 326-327 Specific interaction theory, application, 43 19-21... 

Discontinuation of a hypnotic after a month of continued use can cause a rebound of REM (rapid eye movement) sleep. The duration of action of a hypnotic is determined not only by the half-life of the mother substance but especially by their biological half-life determined by the half-life of the mother substance and the biological active metabolites. On this basis the benzodiazepines can be divided in four different groups ultra short-acting with a half-life < 6 hours such as midazolam and triazolam, short-acting with half-lives between 6 and 12 hours like lormetazepam, loprazolam, oxazepam and temazepam. Alprazolam, bromazepam... 

The rather variable results of half-life determinations made by direct alpha-counting of samples mounted on metallic supports may be due to... 

In patients with heart failure, lidocaine s volume of distribution and total body clearance may both be decreased. Thus, both loading and maintenance doses should be decreased. Since these effects counterbalance each other, the half-life may not be increased as much as predicted from clearance changes alone. In patients with liver disease, plasma clearance is markedly reduced and the volume of distribution is often increased the elimination half-life in such cases may be increased threefold or more. In liver disease, the maintenance dose should be decreased, but usual loading doses can be given. Elimination half-life determines the time to steady state. Thus, although steady-state concentrations may be achieved in 8-10 hours in normal patients and patients with heart failure, 24-36 hours may be required in those with liver disease. Drugs that decrease liver blood flow (eg, propranolol, cimetidine) reduce lidocaine clearance and so increase the risk of toxicity unless infusion rates are decreased. With infusions lasting more than 24 hours, clearance falls and plasma concentrations rise. Renal disease has no major effect on lidocaine disposition. 

In order to rapidly achieve a steady-state level of the compound in the plasma so that the organism is exposed to a fairly constant level, the dosage interval and half-life should be similar (Fig. 3.30). For steady-state conditions, the half-life determines the plasma level. That is, substrates with a long half-life attain a higher steady-state plasma level than compounds with shorter half-lives. It is obviously important to measure this plasma concentration for an assessment of chronic toxicity (Fig. 3.30). 

The yield of this type of recoil collection was about 10 higher yield (i.e. up to nearly 50 ) could be obtained in vacuum without the necessity of an electric field. An adaptation of this technique made possible half-life determination on isotopes with values ranging from 30 seconds to. about 20 mseconds (175) ... 

After obtaining rate constants at 20 °C for the degradation in light and darkness in the presence of oxygen, half-life determinations were made ... 

The isomerization reaction A B is found to go to completion at 65°C. The reaction is found to be of % order in A on the basis of two measurements of the half-life determined on initial concentrations of A differing by a factor of 2. The analytical accuracy is 2 per cent, the error due to time is negligible, and the temperature control is 0.1 C, with Boat = 20 Kcal. (a) What is the expected precision of measurement of the half-life (6) Do you believe that the precision of these measurements is sufficient to tell whether the rate is % order or % order Explain. 

Neumann W. and Huster E. (1976) Discussion of Rb-87 half-life determined by absolute counting. Earth Planet. Sci. Lett. 33(2), 277-288. 

Clinically, the half-life determines the interdosing interval, how long a pharmacological or toxic effect will persist and drug withdrawal times in food-producing animals and performance horses (Martinez 1998b). The plasma concentration of... 

To determine the Cl-nt of compound X, we are able to use the in vitro half-life method, which is simpler than finding all the component Cl nt values. When the substrate concentration is much smaller than Km, the Michaelis-Menten equation simplifies from velocity (V) = Vmax([S])/(Km + [S]), because [S] (substrate concentration) becomes negligible. Furthermore, under these conditions, the in vitro half-life (7) 2 = 0.693/Xel) can be measured, and this, in turn, is related to the Michaelis-Menten equation through the relationship velocity (V) = volume x Kel (where volume is standardized for the volume containing 1 mg of microsomal protein). When both V and Vmax are known, then Km is also found. Although simpler than finding a complicated Cint, one caveat of the in vitro half-life method is that one assumes that the substrate concentration is much smaller than Km. It may be necessary to repeat the half-life determinations at several substrate concentrations, and even model the asymptote of this relationship, because very low substrate concentrations that are beneath biochemical detection may be needed to fulfill the assumptions needed to simplify the Michaelis-Menten equation. 

Jenkins, T., Balinsky, D. and Patient, D.W. Cholinesterase in plasma first reported absence in the Bantu half-life determination. Science, 156, 1748, 1967. 

Three kinetic constants determine the dosage requirements of patients. Clearance determines the maintenance dose (MD = ClCss), volume of distribution determines the loading dose (LD= VdCss) and half-life determines the time to steady state and the dosage interval. Several methods are available to compute these parameters. 

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