6.1: Compound half-life

Figure 2 Mean plasma (Cp), CSF (CCSF), and brain (Cb) compound concentration-time profiles (graph) and matrix-specific neuropharmacokinetic parameters (table) of a compound in rats following subcutaneous administration [42]. Abbreviations Cmax, maximal compound concentration Tmax, time of Cmax tV2, compound half-life.

Compounds Half-life in saline ) Inhibitory effects on human platelet aggregation in vitro (PGEi =... 

Compound Half-life Relative reactivity Classification ... 

Compound half life in trout (hr) half life In plants (hr)... 

Chemical class Generic name Compound Half-life (t f ) Usual adult dose... 

PROBABLE FATE photolysis direct photochemical degradation in the atmosphere or in the upper layers of surface waters should not be an important fate process half-life for the atmospheric reaction with photochemically produced hydroxyl radicals 10 hrs oxidation could occur, but too slow to be important hydrolysis gradual hydrolysis of carbon-chlorine bond is a probable principle fate mechanism, can be expected in comparison to other chlorine containing compounds, half-life for this pH independent process 0.5-2 yrs volatilization not important, volatilization from water should be a slow process half-life from a model pond 1 lyrs, volatilization from the soil to the atmosphere might occur, but will be a slow process, volatilization from moist soil should not be an important fate process sorption possible importance as catalyst for hydrolysis biological processes biodegradation not expected to be an important fate process, but there is not enough data to draw a conclusion... 

Compound Half-Life at 15 °C (Years) Half-Life at 100 °C (Days) pH Effect Hydrolysis Product Reference... 

Group of substances Individual compounds Half-life Organism/organ... 

Thirty isotopes are recognized. Only one stable isotope, 1271 is found in nature. The artificial radioisotope 1311, with a half-life of 8 days, has been used in treating the thyroid gland. The most common compounds are the iodides of sodium and potassium (KI) and the iodates (KIOs). Lack of iodine is the cause of goiter. 

The rate of the uncatalysed reaction in all four solvents is rather slow. (The half-life at [2.5] = 1.00 mM is at least 28 hours). However, upon complexation of Cu ion to 2.4a-g the rate of the Diels-Alder reaction between these compounds and 2.5 increases dramatically. Figure 2.2 shows the apparent rate of the Diels-Alder reaction of 2.4a with 2.5 in water as a lunction of the concentration of copper(II)nitrate. At higher catalyst concentrations the rate of the reaction clearly levels off, most likely due to complete binding of the dienophile to the catalyst. Note that in the kinetic experiments... 

Evidence from the viscosities, densities, refractive indices and measurements of the vapour pressure of these mixtures also supports the above conclusions. Acetyl nitrate has been prepared from a mixture of acetic anhydride and dinitrogen pentoxide, and characterised, showing that the equilibria discussed do lead to the formation of that compound. The initial reaction between nitric acid and acetic anhydride is rapid at room temperature nitric acid (0-05 mol 1 ) is reported to be converted into acetyl nitrate with a half-life of about i minute. This observation is consistent with the results of some preparative experiments, in which it was found that nitric acid could be precipitated quantitatively with urea from solutions of it in acetic anhydride at —10 °C, whereas similar solutions prepared at room temperature and cooled rapidly to — 10 °C yielded only a part of their nitric acid ( 5.3.2). The following equilibrium has been investigated in detail ... 

The existence of the XeCHg [34176-86-8] cation has been estabtished ia the gas phase. The Xe—C bond energy of the XeCHg cation has been estimated to be 180 A 33 kJ/mol (112) and more recently, 231 A 10 kJ/mol (113) by ion cyclotron resonance. The compound Xe(CF3)2 [72599-34-9] is reported to be a waxy white sotid having a half-life of ca 30 min at room temperature (114). The synthesis iavolved the addition of XeF2 to a tritiuoromethyl plasma, but the characterization of this compound is limited and has not been iadependently confirmed. 

Initiators (1) and (2) have 10-h half-life tempeiatuies of 237°C and 201°C, respectively. It has been reported that, unlike organic peroxides and ahphatic azo compounds, carbon—carbon initiators (1) and (2) undergo endothermic decompositions (62). These carbon—carbon initiators are useful commercially as fire-retardant synergists in fire-resistant expandable polystyrenes (63). 

Heptylphysostigmine (eptastigmine) (17) has been shown to be as active as physostigmine in AChE inhibition, but superior to physostigmine in terms of oral bioavadabihty and half-life (118—120). However, further clinical evaluation of this compound has been halted because of dmg-related hematological toxicity. 

Alkyl mercury compounds in the blood stream are found mainly in the blood cehs, and only to a smah extent in the plasma. This is probably the result of the greater stabhity of the alkyl mercuric compounds, as well as their pecuflar solubiUty characteristics. Alkyl mercury compounds affect the central nervous system and accumulate in the brain (17,18). Elimination of alkyl mercury compounds from the body is somewhat slower than that of inorganic mercury compounds and the aryl and alkoxy mercurials. Methylmercury is eliminated from humans at a rate indicating a half-life of 50—60 d (19) inorganic mercurials leave the body according to a half-life pattern of 30—60 d (20). Elimination rates are dependent not only on the nature of the compound but also on the dosage, method of intake, and the rate of intake (21,22). 

The first SRS-A antagonist, FPL-55712 (26) (149), was discovered before the stmctures of the leukotrienes were detemiined. Although this compound is relatively weak as an antagonist and suffers from a very short half-life in vivo, it played an important role both in leukotriene stmcture elucidation and as a model for later antagonists. In work stmcturaHy related to FPL-55712, LY-171883 was developed (27) (150). LY-171883 was evaluated in several clinical trials before development was stopped. Orally adrninistered, LY-171883 blocked slightly the response to aerosol LTD improved pulmonary function (FEV ) in mild asthmatics (151), decreased the sensitivity of asthmatics to cold air-induced bronchoconstriction (152), and significantly reduced the bronchoconstrictor response to inhaled antigen (153). However, in all these studies the beneficial effects were minimal. 

A variation on ofloxacin is mfloxacin (20) this compound lacks the methyl group on the 1,8-bridge and contains a sulfur in place of the oxygen attached to the 8-position. Rufloxacin, although less potent than ofloxacin, is well absorbed and has longer half life than does ofloxacin (44—46). 

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