Rapid analysis of phentolamine by high-performance liquid chromatography

A rapid liquid chromatographic method is validated for the quantitative analysis of phentolamine. Phentolamine exists in three forms for this investigation: as a mesylate salt, hydrochloride salt, and free base. In solution, phentolamine dissociates from its salt and is chromatographed as free phentolamine. This validation confirms the analysis of each form, which is simply based upon molar mass differences encountered in weighing. As such, both the United States Pharmacopeia hydrochloride and mesylate standards are used throughout this validation to demonstrate this equivalency. The validation demonstrates that this method may be used to quantitate phentolamine, regardless of its salt form.

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.

An assay for phentolamine using high performance liquid chromatography with electrochemical detection

A new method is presented for the detection of phentolamine by high performance liquid chromatography with electrochemical detection. The electrochemical detector was used in the oxidative mode at +900 mV potential versus Ag/AgCl reference. The on-column detection limit for phentolamine using this method was 3 ng, and detector response was linear for 3-1000 ng injected on column. The coefficient of variation for replicate injections was 2.4%. The measurement of phentolamine in biological samples was accomplished using yohimbime as the internal standard; retention time for yohimbine was 3.0 min while phentolamine eluted at 4.75 min. Biological samples were buffered to pH 9.2 and extracted with diethyl ether, followed by back extraction into 0.1 N HCl. The extraction efficiency for this method was 99.4% for phentolamine in serum and 59.3% in liver tissue. The detection limit for phentolamine was 5 ng/ml for 1.0-ml serum samples, and was 10 ng/ml for 1.0-ml liver homogenate samples. The disappearance of phentolamine from serum and liver after administration of a single ip dose of phentolamine to mice was determined using this method. Absorption from the ip route was rapid, with peak phentolamine concentrations achieved in 15 min or less. The elimination half-life of phentolamine in serum was approximately 50 min and was paralleled by disappearance of phentolamine in the liver.

Stability of papaverine hydrochloride and phentolamine mesylate in injectable mixtures

The stability of papaverine hydrochloride and phentolamine mesylate combined in a single vial was studied. Injectable mixtures (10 mL) of papaverine hydrochloride 300 mg and phentolamine mesylate 5 mg (from two sources) were prepared by adding the contents of one vial of lyophilized phentolamine mesylate to the contents of one vial of papaverine hydrochloride injection. The vials were stored at 5 degrees C and 25 degrees C. Duplicate aliquots of the mixtures were obtained, and the concentrations of papaverine hydrochloride and phentolamine mesylate remaining at time 0 and after 1, 2, 5, 10, 20, and 30 days were determined in triplicate by a stability-indicating high-performance liquid chromatographic assay. The concentration of papaverine hydrochloride stored in the vials remained constant (less than 1% loss) over the 30-day period at both 5 degrees C and 25 degrees C. Phentolamine mesylate was less stable than papaverine but still retained more than 97% of its original concentration after 30 days at 5 degrees C and more than 95% of its original concentration at 25 degrees C. Papaverine hydrochloride and phentolamine mesylate are stable in injectable mixtures when stored for up to 30 days at 5 degrees C or 25 degrees C.

13C-NMR spectra of alpha-adrenergic blocking agents

The natural abundance 13C-NMR spectra of five alpha-adrenergic blocking agents, tolazoline, dibenamine, azapetine, phenoxybenzamine, and phentolamine, are reported. The chemical shifts of various carbon resonances were assigned on the basis of chemical shift theory, multiplicities observed in single-frequency off-resonance-decoupled spectra, relaxation times, and comparisons with the chemical shifts of model compounds.