Insights into the Chemical Discovery of Remifentanil.

Design, Synthesis, and Pharmacological Evaluation of Ultrashort- to Long-acting Opioid Analgetics. By Feldman PL, James MK, Brackeen MF, Bilotta JM, Schuster SV, Lahey AP, Lutz MW, Johnson MR, Leighton HJ. J Med Chem 1991; 34:2202-8. Copyright 1991 American Chemical Society. Reprinted with permission.In an effort to discover a potent ultrashort-acting µ-opioid analgetic that is capable of metabolizing to an inactive species independent of hepatic function, several classes of 4-anilidopiperidine analgetics were synthesized and evaluated. One series of compounds displayed potent µ-opioid agonist activity with a high degree of analgesic efficacy and an ultrashort to long duration of action. These analgetics, 4-(methoxycarbonyl)-4-[1-oxopropyl)phenylamino]-1-piperidinepropanoic acid alkyl esters, were evaluated in vitro in the guinea pig ileum for µ-opioid activity, in vivo in the rat tail withdrawal assay for analgesic efficacy and duration of action, and in vitro in human whole blood for their ability to be metabolized in blood. Compounds in this series were all shown to be potent µ agonists in vitro, but depending upon the alkyl ester substitution, the potency and duration of action in vivo varied substantially. The discrepancies between the in vitro and in vivo activities and variations in duration of action are probably due to different rates of ester hydrolysis by blood esterase(s). The [structure-activity relationships] with respect to analgesic activity and duration of action as a function of the various esters synthesized is discussed. It was also demonstrated that the duration of action for the ultrashort-acting analgetic, 8, does not change upon prolonged infusion or administration of multiple bolus injections.

The effect of concentration, reconstitution solution and pH on the stability of a remifentanil hydrochloride and propofol admixture for simultaneous co-infusion.

BACKGROUND: There are scenarios where pre-mixing and infusing analgesic and anaesthetic agents as a single intravenous (IV) solution is highly desirable; however, it is important to ensure the agents are compatible when mixed. As such, the long-term stability of a remifentanil-propofol mixture, and means of improving this, were assessed across a range of remifentanil concentrations, diluents, and time points. METHODS: Remifentanil was reconstituted with ultrapure water, 0.9% saline, 20% saline, or 8.4% sodium bicarbonate solution (the latter two chosen for their pH characteristics, rather than their use in pharmaceutical reconstitution) and then mixed with propofol (1%) or further diluted with water to derive concentrations of 10-50 µg mL- 1. Remifentanil and propofol concentrations were determined initially and then periodically for up to 24 h using high performance liquid chromatography (HPLC). Mass spectrometry (MS) was used to detect degradation products in solutions containing 30 µg mL- 1 of remifentanil. Statistical analysis was performed using ANOVA and Student's t-test, with a significance value of 0.05. RESULTS: Isolated remifentanil (pH < 4) and propofol (pH 7.35) did not degrade significantly when reconstituted with water or saline solution over 24 h, while remifentanil reconstituted with sodium bicarbonate degraded significantly (P < 0.001, pH 8.65). Mixing with propofol substantially increased the pH of the mixture and resulted in significant remifentanil degradation for all reconstitution solutions used, while propofol remained stable (pH 6.50). The amount of degradation product detected in samples containing isolated remifentanil and a mixture of the drugs was proportional to the remifentanil degradation observed. CONCLUSIONS: Remifentanil stability is affected by both the reconstitution solution used and when mixed with propofol, with pH appearing to be a contributing factor to degradation. If the pH of the solution and concentration of remifentanil are correctly controlled, e.g. through the use of a more acidic diluent, an admixture of remifentanil and propofol may be useful clinically.

Predosing Chemical Stability of Admixtures of Propofol, Ketamine, Fentanyl, and Remifentanil.

Admixtures of propofol-ketamine, propofol-ketamine-fentanyl, and propofol-ketamine-remifentanil were subjected to various clinically relevant conditions to study their chemical stability. A novel high-performance liquid chromatography-mass spectrometry method revealed no degradation of any compound by incubation at 37°C, constant mixing, or table-top storage for 6- and 24-hour time periods, except variable recovery of both propofol and fentanyl in the admixtures of propofol-ketamine-fentanyl suggesting possible degradation.

Quality control and stability of ketamine, remifentanil, and sufentanil syringes in a pediatric operating theater.

BACKGROUND: Transforming a drug from its commercial form into a ready-to-use drug is common practice, especially in pediatrics. However, the risk of compounding error is real and data on drug stability in practice are not always available. AIMS: The aim of this study was to assess, in real conditions, both the error rate and stability of three drugs: ketamine, remifentanil, and sufentanil. METHODS: A new rapid and easy-to-use high-performance liquid chromatography method with a diode array detector has been developed and validated to quantify these drugs and detect their degradation products. Over a 1-month period, 151 syringes were collected in the postanesthesia care unit. Seventy-three were stock solution syringes containing a 10-fold dilution of commercial drugs and 78 were serial dilution syringes made from successive dilutions of stock solutions. A comparison between real and expected concentrations as well as the detection of possible degradation products was carried out on these samples. RESULTS: All stock solution syringes had good chemical stability throughout the working day. A 4-µg/mL remifentanil serial dilution syringe, however, had to be discarded as a degradation peak was detected. Overall, 15.3% (95% CI, 9.5-21.1%) of syringes had a drug concentration outside the ±10% acceptability range, that is, 11.0% (95% CI, 3.7-18.2%) and 19.5% (95% CI, 10.6%-28.4%) of stock and diluted syringes respectively, with drug amounts ranging from -25.3% to 22.0%. The highest error rates were observed with sufentanil syringes: 20% and 28% for stock solution and serial dilution, respectively. CONCLUSION: The study shows that stock solution syringes prepared in advance are chemically stable throughout the day, unlike certain serial dilution syringes, indicating that the latter should be prepared just before administration to ensure chemical stability. Our results show that the error rate for serial dilution syringes is twice that of stock solution. Different safety measures are under discussion and have to be further studied.