Does bilirubin change the free concentration of propofol?

Is indirect hyperbilirubinemia a useful biomarker of reduced propofol clearance in neonates?

AIM

Large interindividual variability in neonatal propofol clearance is documented which, in part, can be explained by postmenstrual age (PMA) and postnatal age (PNA). We aimed to document whether indirect bilirubin, instead of or in addition to PNA, could improve predictability of propofol clearance and serve as a useful biomarker of reduced propofol clearance in neonates.

METHODS

Indirect serum bilirubin was introduced as a dichotomous or continuous variable (both age-normalized) in a previously developed three-compartment pharmacokinetic model, based on 235 concentration-time points obtained in 25 neonates after single bolus administration of propofol. For pharmacokinetic analysis, nonlinear mixed effect modeling 6.2 was used.

RESULTS

The covariates PMA and PNA explained 67% of the interindividual variability compared with 45% in the model with PMA and bilirubin.

CONCLUSION

Age, reflected by PMA and PNA, is a more relevant clinical predictor of neonatal propofol clearance compared with PMA and raised indirect hyperbilirubinemia.

Green Urine Discoloration due to Propofol Infusion: A Case Report

We present a 19-year-old man who excreted green urine after propofol infusion. The patient was admitted to our hospital for injuries sustained in a traffic accident and underwent surgery. After starting continuous infusion of propofol for postoperative sedation, his urine became dark green. Serum total bilirubin and urine bilirubin were both elevated. We believe that the green discoloration of the urine was caused by propofol infusion and was related to impaired enterohepatic circulation and extrahepatic glucuronidation in the kidneys.

Monitoring free drug concentrations: challenges

Measurement of drug concentrations in biological samples is of utmost importance in many research areas. The information about the amount of drug in a biological sample can be given as either total concentration, which ignores the interaction of the drug with the sample matrix, or as free concentration, which shows the portion of molecules able to diffuse through membranes and exert biological activity. Although the historical trend has been towards determining total concentrations, measurement of free concentrations is becoming more important since it correlates better with pharmacological and toxicological effects. This review will discuss the most popular experimental approaches for monitoring free drug concentrations, based on the type of sample to be investigated and the kind of information to be collected. It is shown that the current challenges in measuring free concentrations are: convenience, accuracy, precision, wide applicability, availability of accurate and precise reference methods, ruggedness, and standardized sample conditions.