The kinetics and mechanism of the hydrolysis of cyclopentolate hydrochloride in alkaline solutions

Cyclopentolate hydrochloride (Cy · HC1) is an ester of a substituted benzeneacetic acid, having N,N-dimethyl-aminoethanol as the alcohol moiety. A reversed-phase HPLC assay was employed to investigate the kinetics of degradation of Cy · HCI. The influence of pH, buffers, and temperature was studied in alkaline solutions. The degradation follows (pseudo) first-order kinetics at 50°C. Results indicate that it degrades very rapidly at higher pH values. Phenylacetic acid and a-(1-hydroxycyclopentyl)benzeneacetic acid were isolated and identified as the degradation products. The reaction mechanism appears to follow a parallel scheme where phenylacetic acid and a-(1-hydroxycyclopentyl)benzeneacetic acid are formed simultaneously. It is proposed that -(1-hydroxycyclopentyl)benzeneacetic acid is formed by normal ester hydrolysis. Phenylacetic acid is formed via a six-membered transition state and its formation requires the assistance of the hydroxyl group from the adjacent cyclopentanol moiety.     

盐酸贝那普利大鼠在体小肠吸收动力学

目的:研究盐酸贝那普利在大鼠小肠的吸收情况。方法:采用大鼠在体小肠回流实验装置,测定盐酸贝那普利的吸收动力学与透过速率常数,研究在不同浓度下盐酸贝那普利的吸收机制。结果:小肠是盐酸贝那普利的吸收最佳部位。盐酸贝那普利在低、中、高3种不同浓度下,透过速率常数分别为0.789,0.810,0.766,无显著差异。结论:盐酸贝那普利吸收机制为被动扩散。吸收动力学为一级吸收。     

Release kinetics of procaine hydrochloride (PrHy) from pH-responsive nanogels: theory and experiments

pH-responsive nanogels consisting of methacrylic acid–ethyl acrylate (MAA–EA) cross-linked with di-allyl phthalate (DAP) were synthesized via emulsion polymerization. Drug release studies were conducted under different pHs, drug loading and concentration gradient difference. The drug loading capacity depended on the cross-link density and MAA–EA molar content, where a lower cross-link density and higher MAA–EA molar content resulted in higher loading capacity. A drug selective electrode was used to directly measure the concentration of procaine hydrochloride (PrHy) released from MAA–EA nanogels. More than 50 data points were acquired, where the mathematical fitting to the Berens and Hopfenberg model allowed the parameters describing the contributions of chain relaxation and diffusion process to be determined. The release rate increased with pH and concentration gradient difference due to a reduction in diffusion barrier and higher concentration gradient driving force, respectively, but it decreased with drug loading as the nanogel could not relax from the compact structure as evident from the contribution of Fickian diffusion, _F, and chain relaxation, _R. A balance between chain relaxation and Fickian diffusion process controlled the release of drugs from these pH-responsive nanogels. Exponential relationships could be established between diffusion coefficient, characteristic relaxation time and various physical parameters, where the drug release kinetics could be predicted in a quantitative manner.     

基因毒性杂质甲磺酸酯稳定性及水解反应动力学研究

目的:在痕量水平基因毒性杂质甲磺酸酯(甲磺酸甲酯、甲磺酸乙酯、甲磺酸丙酯和甲磺酸异丙酯)的稳定性及水解动力学研究.方法:以乙腈-水(80∶20,v/v)为溶剂配制甲磺酸酯混合溶液,分别置于298、313及323 K温度下,定时取样并加入碘化钠衍生化溶液,分别得到相应的碘代烷烃(碘甲烷、碘乙烷、碘丙烷和碘代异丙烷),采用...     

Kinetics of hydrolysis of polyoxyethylene (20) sorbitan fatty acid ester surfactants

The kinetics of the hydrolysis of the oleate ester of polyoxyethylene (20) sorbitan (Tween 80) in aqueous buffers were studied at an initial concentration of 0m?020% (w/v) and over the pH range of 1m?10 to 10m?28. The hydrolysis appears to be specific acid-catalysed at pH values below 3 and specific base-catalysed at pH values greater than 7m?6. The pseudo first-order rate constants for hydrogen and hydroxyl ion catalysis were determined, and the temperature and ionic strength dependence of the acid-catalysed reaction was studied. Both the initial, acid- and base-catalysed hydrolysis of Tween 80 exhibited an unusual initial, micellar surfactant concentration-rate dependence, opposite to that previously reported for the hydrolysis of anionic surfactants of the n-alkyl sulphate-type. Specifically, as the initial concentration of Tween 80 was increased above its reported critical micellar concentration, there was a progressive, marked decrease in the rate of the reaction, with the rate eventually reaching a plateau value between 0m?100 and 1m?00% (w/v). It is suggested that this behaviour is due to alterations in the micellar state of the surfactant as its concentration is increased. The influence of the chemical structure of the Tween surfactant on the acid-catalysed hydrolysis reaction at 80° was also examined using the oléate (Tween 80), stéarate (Tween 60), and palmitate (Tween 40) esters of polyoxyethylene (20) sorbitan.     

盐酸度洛西汀大鼠离体肠吸收动力学

目的考察盐酸度洛西汀在大鼠各肠段的吸收特征。方法采用离体外翻肠囊法,以HPLC法测定不同浓度的盐酸度洛西汀在大鼠各肠段的吸收量,并分别计算吸收速率常数（ka）和表观渗透系数（Papp）;同法考察了P-糖蛋白抑制剂（维拉帕米和环孢素A）对盐酸度洛西汀肠道吸收的影响。结果药物浓度对大鼠各肠段ka没有明显影响;ka按空肠、回肠、十二指肠、结肠依次减小,分别为（2．64±0．17）、（2．15±0．11）、（1．24±0．04）和（0．88±0．09）·h^-1,药物吸收符合一级动力学特征,吸收机制为被动扩散;P。按空肠、回肠、十二指肠和结肠依次减小,分别为（5．59±1．69）×10^-5、（4．88±1．38）×10^-5、（3．08±1．05）和（2．66±0．70）×10^-5cm·8^-1;P-糖蛋白抑制剂对吸收没有明显影响。结论盐酸度洛西汀吸收窗比较长,适合制成肠溶制剂,也提示可以制成在肠道中缓释的制剂。     

Nefopam hydrochloride degradation kinetics in solution

A stability-indicating reversed-phase high performance liquid chromatographic method was developed for the detection of nefopam hydrochloride and its degradation products under accelerated degradation conditions. The degradation kinetics of nefopam hydrochloride in aqueous solutions over a pH range of 1.18 to 9.94 at 90 +/- 0.2 degrees C was studied. The degradation of nefopam hydrochloride was found to follow apparent first-order kinetics. The pH-rate profile shows that maximum stability of nefopam hydrochloride was obtained at pH 5.2-5.4. No general acid or base catalysis from acetate, phosphate, or borate buffer species was observed. The catalytic rate constants on the protonated nefopam imposed by hydrogen ion and water was determined to be 7.16 X 10(-6) M-1 sec-1, and 4.54 X 10(-9) sec-1, respectively. The pKa of nefopam hydrochloride in aqueous solution was determined to be 8.98 +/- 0.33 (n = 3) at 25 +/- 0.2 degrees C by the spectrophotometric method. The catalytic rate constant of hydroxyl ion on the degradation of nefopam in either protonated or nonprotonated form was determined to be 6.63 X 10(-6) M-1 sec-1 and 4.06 X 10(-6) M-1 sec-1, respectively. A smaller effect of hydroxyl ion on the degradation of nonprotonated than on the degradation of protonated nefopam was observed.(ABSTRACT TRUNCATED AT 250 WORDS)     

Hydration and dehydration kinetics of xylazine hydrochloride

From the experiments where mixture of xylazine hydrochloride hydrate H and anhydrous X were held at constant conditions, the stable form of xylazine hydrochloride can be found out. To determine equilibrium relative humidity, the unstable form of xylazine hydrochloride was inserted in thermostated humidity chamber and its weight was recorded by weighing the sample outside the chamber. The kinetic model and the rate constant for each condition were determined. The rate constants give information regarding the speed of the process at every experimentally used relative humidity. Thus using the data in coordinates k – p for each temperature it is possible to determine the water vapor pressure of the equilibrium. With this method the phase boundary for xylazine hydrochloride was determined and hydration enthalpy was calculated. The hydration rates of xylazine polymorphs A and X were investigated.     

Degradation kinetics of phentolamine hydrochloride in solution

The degradation kinetics of phentolamine hydrochloride in aqueous solution over a pH range of 1.2 to 7.2 and its stability in propylene glycol- or polyethylene glycol 400-based solutions were investigated. The observed rate constants were shown to follow apparent first-order kinetics in all cases. The pKa determination for phentolamine hydrochloride was found to be 9.55 +/- 0.10 (n = 5) at 25 +/- 0.2 degrees C. This indicated the protonated form of phentolamine occurs in the pH range of this study. The pH-rate profile indicated a pH-independent region (pH 3.1-4.9) exists with a minimum rate around pH 2.1. The catalytic effect of acetate and phosphate buffer species is ordinary. The catalytic rate constants imposed by acetic acid, acetate ion, dihydrogen phosphate ion, and monohydrogen phosphate ion were determined to be 0.018, 0.362, 0.036, and 1.470 L mol-1 h-1, respectively. The salt effect in acetate and phosphate buffers followed the modified Debye-Huckel equation quite well. The ZAZB value obtained from the experiment closely predicts the charges of the reacting species. The apparent energy of activation was determined to be 19.72 kcal/mol for degradation of phentolamine hydrochloride in pH 3.1, 0.1 M acetate buffer solution at constant ionic strength (mu = 0.5). Irradiation with 254 nm UV light at 25 +/- 0.2 degrees C showed a ninefold increase in the degradation rate compared with the light-protected control. Propylene glycol had little or no effect on the degradation of phentolamine hydrochloride at 90 +/- 0.2 degrees C; however, polyethylene glycol 400 had a definite effect.