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PubMedDate not supplied

Kinetics and mechanism of degradation of some 2-sulfanilamidopyrimidine derivatives. Part VI. The use of Hammett equation for kinetic investigation of 2-sulfanilamidopyrimidine derivatives hydrolysis.

General, k, and specific, k1 and k2, first-order rate constants for the parallel reaction of hydrolysis catalized by H+ ions were estimated for sulfadiazine (I), sulfamerazine (II), sulfadimidine (III), sulfaperine (IV) and sulfamethoxydiazine (V), hydrolyzed in 1 mole/dm3 HCl at 333, 343, 355 and 363 K. General first-order rate constants for the spontaneous hydrolysis of I--V in borate buffer pH 9.20 at 403, 411 and 418 K were also determined. Thermodynamic parameters of the reaction (delta Ha, deltaH not equal to, deltaS not equal to, deltaG not equal to and log A) were calculated. The effect of substituents in positions 4, 5 and 6 of the pyrimidine ring on the rate of hydrolysis was interpreted in terms of Hammett equation.

Chemical Phenomena↗
PubMed1977

Kinetics of drug decomposition Part 51: Kinetics of indometacine photodegradation.

Indometacine aqueous solution degradation was studied under the influence of UV light. Spectrophotometric readings of absorbances at lambdamax = 270 nm were taken in order to follow changes of the indometacine solutions as a function of irradiation time. It was found that the degradation of indometacine under influence of UV light proceeds according a sequential reaction A k2 leads to B k2 leads to C k3 leads to D in which each individual step is a zero-order process Individual zero-order rate constants were calculated for indomatacine photodegradation under anaerobic and aerobic conditions. They indicate the first step (A leads to B) to be oxygen independent and the other processes (B leads to C leads to D) to be oxygen catalyzed.

Aerobiosis↗
PubMed1982

Metabolite kinetics: formation of acetaminophen from deuterated and nondeuterated phenacetin and acetanilide on acetaminophen sulfation kinetics in the perfused rat liver preparation.

The role of hepatic intrinsic clearance for metabolite formation from various precursors on subsequent metabolite elimination was was investigated in the once-through perfused rat liver preparation. Two pairs of acetaminophen precursors: [14C] phenacetin-d5 and [3H] phenacetin-do, [14C] acetanilide and [3H] phenacetin were delivered by constant flow (10 ml/min/liver) either by normal or retrograde perfusion to the rat liver preparations. The extents of acetaminophen sulfation were compared within the same preparation. The data showed that the higher the hepatocellular activity (intrinsic clearance) for acetaminophen formation, the greater the extent of subsequent acetaminophen sulfation. The findings were explained on the basis of blood transit time and metabolite "duration time." Because of blood having only a finite transit time in liver, the longer the drug requires for metabolite formation, the less time will remain for metabolite sulfation and the less will be the degree of subsequent sulfation. Conversely, when the drug forms the primary metabolite rapidly, a longer time will remain for the metabolite to be sulfated in liver to result in a greater degree of metabolite sulfation. Finally, the effects of hepatic intrinsic clearances for metabolite formation and zonal distribution of enzyme systems for metabolite formation and elimination in liver are discussed.

Acetaminophen↗
PubMedDate not supplied

Kinetics of drug decomposition. Part 67. Kinetics of degradation of dicloxacillin in aqueous solution.

The rates of hydrolysis of beta-lactam ring of dicloxacillin (DC) in the pH range 1.10-9.65 at 293, 303 and 313 K were determined. Specific rate constants were calculated for: H degree ions, undissociated phosphoric acid and H2PO4- ions catalysis of hydrolysis of undissociated DC molecule; for H+, OH-, HPO4(-2) and CH3COO- ions and undissociated phosphoric and acetic acids catalysis of hydrolysis of dissociated DC species and spontaneous hydrolysis of dissociated DC species. For each process the thermodynamic parameters were determined.

Dicloxacillin↗
PubMed1981

Kinetics of drug decomposition. Part 66. Kinetics of the hydrolysis of carphecillin in aqueous solution.

The rates of hydrolysis of the beta-lactam ring in the pH range 1.77--9.22 at 294, 303, 313 and 233 K and of the ester bond in the pH range 0.43--8.78 at 273, 283, 294, 303, 313 and 323 K for carphecillin have been investigated. The rate constants were determined for the reactions catalyzed by H+ and OH- ions and moreover for the hydrolysis of the beta-lactam ring catalyzed by undissociated acids and anionic bases. The thermodynamic parameters were calculated for particular reactions. In the acidic medium carphecillin is significantly more stable than carbenicillin. In the alkaline medium the rate of inactivation of carphecillin and carbenicillin is the same because carbenicillin is formed from carphecillin as the result of the very fast hydrolysis of the ester bond.

Carbenicillin↗
PubMedDate not supplied

Kinetics of drug decomposition. Part 61. Kinetics of various ampicillin forms degradation in solid phase.

Ampicillin sodium salt degradation in solid state relies on a sequential reaction consisting of three pseudo first-order processes. The above salt is much more susceptible to degradation than acidic forms of ampicillin. The anhydrous acidic form of ampicillin is very stable in solid phase. Therefore, it can be recomended to be formulated in peroral ampicillin pharmaceutical preparations. Ampicillin trihydrate, now used in pharmaceutical formulations, is less stable than the last mentioned compound. It degrades according to Prout-Tompkins' model.

Ampicillin↗