Rapid, high-sensitivity method for measurement of morphine in guinea-pig serum

High-performance liquid chromatographic assay for morphine in small plasma samples: Application to pharmacokinetic studies in rats

In order to perform a reliable pharmacokinetic study of morphine during subchronic treatment in rats, an easy, rapid, sensitive and selective analytical method was proposed and validated. The analyte and internal standard (naloxone) were extracted from plasma samples (100 microL) by a single solid-phase extraction method prior to reverse-phase high performance liquid chromatography (HPLC) along with electrochemical detection (ED). Standard calibration graphs were linear within a range of 3.5-1,000 ng/mL (r=0.999). The intra-day coefficients of variation (CV) were in the range of 5.8-8.5% at eight concentration levels (7-1,000 ng/mL) and the inter-day coefficient of variation ranged from 7.4 to 8.8%. The intra-day assay accuracy was in the range of -5-10% and the inter-day assay accuracy ranged from 3.0 to 9.3% of relative error (RE). The limit of quantification was 3.5 ng/mL using a plasma sample of 100 microL (15.8% of CV and 12.8% of RE). Plasma samples were stable for 2 months at -20 degrees C. This method was found to be suitable for pharmacokinetic studies in rats, after subcutaneous administration of morphine (5.6 mg/kg per day) in subchronic treatment for 6 and 12 days.

A pharmacokinetic-pharmacodynamic model of tolerance to morphine analgesia during infusion in rats

A pharmacokinetic-pharmacodynamic (PK-PD) model was constructed to describe the kinetics of tolerance development to morphine-induced antinociception. Tail-flick latencies in response to hot water (50 degrees C) were assessed in male Sprague-Dawley rats exposed to a 12-hr iv infusion of either morphine (1.4 to 3.0 mg/kg per hr) or saline. Morphine-induced antinociception, expressed as the percentage of maximum possible response (% MPR), peaked after 120 min of infusion and decreased thereafter despite sustained systemic morphine concentrations. Both the rate and extent of tolerance development increased with increasing concentrations; an overall residual effect of approximately 24% MPR was observed at the end of the infusion regardless of the steady-state morphine concentration. The kinetics of tolerance offset were examined in a separate experiment by assessing tail-flick latency 15 min after morphine iv bolus (2 mg/kg) in tolerant and control rats. Recovery of response neared completion 18.5 days after a 12-hr exposure to morphine (2.0 mg/kg per hr). A PK-PD model was constructed to account for the delay in onset of antinociceptive effect and tolerance development relative to the blood concentration-time profile. According to this model, both the extent and the rate of tolerance development were modulated by the kinetics of the drug in the central compartment. Accumulation of a hypothetical "inhibitor" acting either as a reverse agonist, a competitive or noncompetitive antagonist, or a partial agonist could potentially account for the loss of pharmacologic effect in the presence of an agonist. The rate of tolerance development predicted from the PK-PD model varied widely (28-fold) depending on the type of pharmacologic interaction selected to account for the loss of effect. Using the rate of tolerance offset to discriminate between the different models (t1/2 offset 5.4 days), onset and offset of tolerance was described accurately by postulating that the inhibitor behaves as a partial agonist with low intrinsic activity (5.5% MPR) and high binding affinity for the receptor (IC50 15.0 ng/ml).

Enterohepatic recirculation and renal metabolism of morphine in the rat

Morphine (2.5 mg/kg) was administered iv to intact (I), bile duct-cannulated (BC), and bile duct-cannulated--renal-ligated (BC-RL) rats (n = 4 per group) to investigate the extent of enterohepatic recirculation and renal metabolism of the drug. A decrease in the serum area under the concentration-time curve (AUC) was observed for the BC in comparison with I rats. From these AUC values, it was determined that approximately 16% of the administered dose was subject to enterohepatic recirculation. In addition, a statistically significant (p less than 0.05) decrease in the systemic clearance of morphine was observed in the BC-RL rats compared with the BC animals (55.2 +/- 17.2 versus 31.4 +/- 8.5 mL/min/kg). This decrement in systemic clearance appeared to be the result of a significant decrease in the formation clearance of morphine glucuronide after ligation of the renal pedicles (23.2 +/- 4.8 versus 10.9 +/- 5.0 mL/min/kg). Renal metabolic clearance was calculated as 15.7 mL/min/kg, accounting for 28.5% of the systemic clearance of morphine. Hepatic clearance (31.4 +/- 8.5 mL/min/kg) accounted for 56.8% of total systemic clearance.

Voltammetric assay of heroin in illicit dosage forms

The oxidation of heroin on a carbon paste electrode has been studied by using voltammetric techniques under both semi-infinite linear diffusion and hydrodynamic conditions. By employing a simple and rapid in situ pre-treatment of the electrode, excellent reproducibility of the current signal was obtained. Subsequently, the current was measured and the concentration of total heroin present could be determined. The method compared favourably with instrumental methods that are more commonly used for the quantification of heroin in clinical laboratories such as chromatographic, spectrophotometric or radiometric techniques. Moreover, the proposed method showed good selectivity and was superior with respect to cost and time of analysis, permitting the determination of heroin in seized drug samples over a wide concentration range.

Effect of chronic morphine administration on measured levels and chromatographic characterization of six neuropeptides in guinea-pig ileum

Guinea-pigs were chronically treated with morphine by subcutaneous implantation of a compounded morphine pellet. After 7 days, the animals were sacrificed, and the small intestine was removed and extracted for the assay of 6 neuropeptides (substance P, vasoactive intestinal peptide, galanin, dynorphin A (1-8), dynorphin A (1-17) and alpha-neo-endorphin). The neuropeptide content of the extracts was measured by radioimmunoassay, and the extracts were subjected to analysis by reversed-phase high pressure liquid chromatography. The measured level of each neuropeptide was not significantly different from that in a control group of animals, and the chromatographic elution pattern obtained for each peptide was also similar in control and morphine-treated animals. Furthermore, the major peak of immunoreactivity for each peptide occurred with approximately the same retention time as synthetic standards having the porcine sequence for that peptide. It is concluded that chronic morphine treatment does not alter the tissue concentration or molecular form of at least 5 of the 6 neuropeptides studied. In the case of a alpha-neo-endorphin, a peak of immunoreactivity was discovered that did not correspond to the synthetic peptide. The amount of immunoreactive material present in this peak varied with oxidation and so its contribution to the assay results could not be calculated.