The Role of Human Lungs in the Biotransformation of Propofol

After obtaining approval from the University Ethics Committee and consent from patients, samples were taken from 55 patients of American Society of Anesthesiologists grade 1–3. Two groups of patients were studied: (1) group I: 47 patients (26 women and 21 men; age, 45 ± 14 yr; weight, 70 ± 13 kg) scheduled for elective intracranial procedures; and (2) group II: 8 patients (2 women and 6 men; age, 60 ± 8 yr; weight, 65 ± 3 kg) scheduled for pulmonectomy.

All patients were premedicated with diazepam (Polfa, Warszawa, Poland) 10 mg administered orally 2 h before anesthesia. Before induction, the patients received a bolus dose of fentanyl (Polfa) 0.2 mg and were preoxygenated with 100% oxygen for 10 min. Propofol (Diprivan; AstraZeneca, Caponago, Italy) was first injected as a bolus dose (2 mg/kg) and then continuously infused. The following infusion rates were used 10,11: 12 mg · kg⁻¹· h⁻¹for the first 15 min, 9 mg · kg⁻¹· h⁻¹for the next 25 min, and 6 mg · kg⁻¹· h⁻¹thereafter. Endotracheal intubation was facilitated by pancuronium bromide (Organon Teknika, Boxtl, Holandia) 0.05 mg/kg. After tracheal intubation, the lungs were ventilated to normocapnia with an oxygen–air mixture (fraction of inspired oxygen = 0.33). In addition to the continuous infusion of propofol, anesthesia was maintained with repeated doses of fentanyl and pancuronium. The infusion of propofol was stopped immediately after the surgical procedure. Neuromuscular block was antagonized with atropine (Polfa; 0.5 mg) and neostigmine bromide (Pliwa, Kraków, Poland; 2.5 mg).

Before induction of anesthesia, a 17-gauge cannula was inserted in the large vein of left forearm and was used solely for the infusion of propofol. After induction of anesthesia, two additional cannulas were placed: one (20-gauge) in the radial artery in the contralateral forearm (for blood pressure monitoring and blood sampling), and the other (7 French × 20) in the right subclavian vein (for central venous pressure measurement and blood sampling). The position of the cannula was localized in the right atrium by radiograph.

In group I, blood samples were collected simultaneously from the right atrium and radial artery (venous blood and arterial blood) 90 min after the induction of anesthesia, when the rate of propofol infusion was 6 mg · kg⁻¹· h⁻¹. In addition, a second pair of samples was taken from 31 of 55 patients 20 min after termination of propofol infusion. In 60 pairs of blood samples (36 collected during infusion, 24 collected after infusion), the concentration of propofol was determined by high-performance liquid chromatography (HPLC). In the remaining samples, the levels of propofol and its metabolite, 2,6-diisopropyl-1,4-quinol, were estimated by gas chromatography–mass spectroscopy.

In group II, samples were taken simultaneously from the pulmonary and radial arteries (mixed venous blood and arterial blood) just before the removal of the lung; time of sampling was 117 ± 28 min after the beginning of propofol infusion. Propofol concentrations were measured by HPLC.

All chemicals used, except those mentioned separately, were obtained from the Polish Factory of Chemical Reagents-POCh (Gliwice, Poland) and were of analytical grade. A mixture of 67% acetonitrile and 33% double-distilled water (pH 2.0) was used as mobile phase. The desired pH was achieved by the addition of trifluoracetic acid (1 ml acid to 400 ml water). Propofol was obtained from ICI-Pharma (Goteborg, Sweden). Stock solutions of thymol and propofol in acetonitrile (1 mg/ml) were each prepared and stored at 4°C. Tetramethylammonium hydroxide (25% in methanol; Aldrich, Steinheim, Germany) was diluted with 2-propanol (3:37). Trifluoroacetic acid was purchased from Fluka (Buchs, Switzerland).

Thymol, phosphate buffer (1 ml of 0.1 m NaH²PO⁴, pH 4.2) and cyclohexane (5 ml) were added to the samples of blood. The samples were vigorously shaken for 15 min. After centrifugation (1,200 g  for 5 min), an aliquot of the cyclohexane layer (4.5 ml) was transferred to a clean tube to which tetramethylammonium hydroxide solution (20 μl) was subsequently added. The solvent was evaporated to dryness with a stream of nitrogen. The residue was redissolved in the mobile phase and injected into the chromatographic column.

A Gilson liquid chromatograph (Middleton, WI) consisting of two high-pressure pumps (Models 305 and 306), a Model 805 manometric module, and a Model 811 C dynamic mixer were used. The ultraviolet–visible variable wavelength detector working at 270 nm was a Model 119 (Gilson). Chromatographic separations were conducted using a 250 × 4 mm ID column packed with homemade C18 silica gel. The samples (20 μl) were injected into the column by a Model 7125 injection valve (Rheodyne, Cotati, CA). The intraday and interday relative recovery was 100% with a coefficient of variation (C^V) < 6%. The limit of sensitivity of the assay was 0.1 μg/ml.

The analyses were performed on gas chromatography–mass spectroscopy with an ion trap (ITS-40; Finnigan MAT, San Jose, CA). The samples (2 μl) were injected into a DB-5 (J&W, Folson, CA) fused silica capillary column (30 m × 0.25 mm ID; 0.25-μm film thickness) by splitless injection. The temperature program was as follows: isotherm 40°C (2 min), temperature program 8°C/min to 230°C, temperature program 25°C/min to 300°C, and final isotherm 300°C (5 min). Helium as carrier gas with the flow of 1.5 ml/min was used. Mass spectroscopy conditions were as follows: electron impact (70 eV); manifold temperature, 220°C; mass range, 35–400 atom unit mass. At the absence of an authentic metabolite standard, quantitative analysis was performed on the basis of the extracted ion chromatograms with total ion current for propofol, 2,6-diisopropyl-1,4-quinol, and thymol, which was used as an internal standard. The number of ions is proportional to the concentration of the corresponding compounds in blood.

The precision of the assay was found to have a C^Vless than 5.5%, and the accuracy expressed by the relative recovery was 100%.

Standard solutions of propofol with thymol were prepared in blood. Using this set of solutions and following the described extraction procedure and HPLC analysis, the ratios of the peak area of propofol to thymol (A ^P /A^T) were plotted against the concentration of propofol in blood.

The concentration of propofol in the examined samples was calculated using the regression parameters obtained from the calibration curves by means of UniPoint System Software (Gilson, Middleton, WI).

The data are expressed as mean value ± SD. The differences between blood vessels in relation to propofol and 2,6-diisopropyl-1,4-quinol were compared with the Student t  test for dependent samples. The comparison of ratios between propofol and 2,6-diisopropyl-1,4-quinol was analyzed by the Wilcoxon matched-pairs test. Differences were considered significant at P  less than 0.05.

Data for group I, the propofol concentrations determined for the blood samples taken simultaneously from the right atrium and the radial artery (venous blood and arterial blood) by means of HPLC, are presented in fig. 1. The higher propofol concentration in blood from the right atrium than from the radial artery is observed both during propofol infusion (6.17 ± 2.15 μg/ml and 3.49 ± 1.38 μg/ml, P < 0.001) and after termination of infusion (1.41 ± 0.48 μg/ml and 0.93 ± 0.36 μg/ml, P < 0.001). The mean veno-arterial differences and ratios of concentrations are shown in table 1.

The results of measurements of propofol concentration for group II performed in blood samples collected simultaneously from the pulmonary artery and the radial artery (mixed venous blood and arterial blood) are shown in figure 2. The higher propofol concentration is observed in mixed venous blood than in arterial blood (4.74 ± 1.30 μg/ml and 3.78 ± 1.34 μg/ml, P < 0.005).

The levels of propofol and 2,6-diisopropyl-1,4-quinol for group I determined by gas chromatography–mass spectroscopy in blood samples taken simultaneously from the right atrium and the radial artery (venous blood and arterial blood) expressed by the total number of ions are presented in figures 3 and 4. Both during propofol infusion (fig. 3) and after termination of infusion (fig. 4), the propofol level is higher in venous blood than in arterial blood (P < 0.001), whereas the level of 2,6-diisopropyl-1,4-quinol is higher in blood from the radial artery than from the right atrium (P < 0.01).

The presented results show that not only the liver but also the lungs are responsible for propofol metabolism in human organism; the decrease of propofol concentrations and the increase of 2,6-diisopropyl-1,4-quinol concentrations in blood after its passage through lungs are observed. As far as the liver is concerned, the clearance of propofol exceeds its capacity in respect of its total blood flow. 1,6,12Moreover, no significant changes of propofol pharmacokinetics were observed elsewhere in patients with the liver function abnormalities or alterations in the liver blood flow, which suggested the presence of extrahepatic metabolism. 13At the same time, Mather et al.  4reported a substantial regional propofol blood concentration gradient across lamb’s lungs but failed to detect propofol in lung tissues. To explain the role of human lungs in the biotransformation of propofol, first the propofol concentration was examined across human lungs during the drug infusion to determine whether veno-arterial differences occurred. It was found that the concentration of propofol measured by HPLC in group I during infusion of propofol was higher in blood collected from the right atrium than from the radial artery (venous blood and arterial blood in fig. 1); the ratio of drug concentrations for these blood vessels was 1.82 ± 0.39, and the difference was 2.68 ± 1.18 μg/ml (table 1). One possible interpretation of the results is that the observed differences result from the uptake of propofol by tissues or from the drug metabolism. However, a closer consideration of the obtained values indicates these changes can be a result of the dissolution of propofol in blood; the incomplete mixing of propofol in the blood stream was one of the explanations for the influence of sample site on blood concentration of the drug offered by Major et al.  14The collected data were obtained using blood samples taken during the continuous infusion of propofol. Therefore, the diminution of propofol concentration in blood can possibly result from the continuous mixing of propofol with blood during the flow of blood in blood vessels, known as the streaming effect. Moreover, it should also be remembered that the blood from superior vena cava and from inferior vena cava, mixing in the right atrium, can contain various amounts of propofol.

To exclude the possible streaming effect resulting from the continuous propofol infusion, the second series of investigations of propofol concentration in blood was conducted. This time, blood samples for analysis were taken from the same sites as before but 20 min after the termination of propofol infusion. The 20-min period of blood circulation in human organism seems to be sufficiently long to eliminate propofol concentration gradient resulting from the incomplete mixing when propofol was infused, if such effect really existed. The results of HPLC measurements for group I after termination of propofol infusion (fig. 1) show the same relation for propofol blood concentration as observed during infusion, namely, higher propofol concentration in venous blood and lower in arterial blood; the veno- arterial ratio was 1.58 ± 0.44, and the difference was 0.48 ± 0.33 μg/ml (table 1). Therefore, the decrease of the propofol level was caused by the uptake of propofol, its metabolism, the incomplete mixing effect in the right atrium, or any of these factors operating together.

To further exclude the incomplete mixing effect, blood samples were collected from pulmonary and radial arteries (patients in group II). In group II (fig. 2), higher propofol concentration in blood taken from pulmonary artery (mixed venous blood) than from radial artery (arterial blood) was observed, and the veno-arterial ratio and veno-arterial difference were 1.33 ± 0.35 and 0.97 ± 0.51 μg/ml, respectively. There are many reports showing the essential role of the lungs in the process of drug elimination. 15–17The data gathered for group II confirm the contribution of the lungs to the process of propofol diminution, which may result from uptake of propofol or its metabolism.

The results discussed thus far are consistent with the aforementioned remarks in the literature and suggest the role of the lungs in propofol clearance. 4,5,18However, they do not explain the role of the lungs in detail, because the process responsible for the observed differences still remains unknown.

The metabolism of propofol is very rapid. 1–3,12Approximately half of the drug is excreted in the form of propofol glucuronide conjugates. A significant part of propofol is transformed into 2,6-diisopropyl-1,4-quinol and removed from the organism in the form of sulfate or glucuronide complexes. Only 0.3% of propofol leaves the organism in the unchanged form. The decrease in propofol level in blood after its passing through the lungs observed in group II can be explained by the uptake of the drug by this organ or by the transformation of the drug into one of its metabolites.

Gas chromatography–mass spectroscopy investigations for group I confirmed the presence of 2,6-diisopropyl-1,4-quinol in the blood samples both during and after propofol infusion (figs. 3 and 4). Moreover, the higher level of this metabolite in arterial compared with venous blood was observed (for mean veno-arterial ratio and difference, see table 1). The increase of 2,6-diisopropyl-1,4-quinol concentration in blood after its passage through the lungs and the simultaneous decrease of propofol concentration (figs. 3 and 4) suggest that a part of the drug is transformed by the lungs into a quinol derivate. In addition, during total intravenous anesthesia, the mean ratio of propofol amount in venous blood to that in arterial blood is almost the same as the mean ratio of 2,6-diisopropyl-1,4-quinol amount in artery blood to that in venous blood (1.77 ± 0.37 and 1.81 ± 0.59, P = 0.99) and nearing the ratio from HPLC measurements (1.82 ± 0.39).The same conclusion can be offered considering the analogous ratios after total intravenous anesthesia: 1.50 ± 0.25, 1.36 ± 0.22 (P = 0.176), and 1.58 ± 0.44.The similarities of the ratios for propofol support the validity and reliability of the obtained data, and the similarities of the ratios for propofol and its quinol derivate further prove that the lungs metabolize propofol.

All of the aforementioned observations indicate that the lungs are as much involved in the transformation of propofol as the liver. Of course, the transformation of propofol into 2,6-diisopropyl-1,4-quinol may not be the only process that the drug undergoes in the lungs. Veroli at al.  5showed the presence of propofol glucuronide complex in human organism at the absence of the liver, whereas Matot at al. 18reported the ability of cats’ lungs to take up propofol. Therefore, partial uptake of propofol by human lungs or a formation of the other mentioned metabolites in the lungs are also possible. Further investigations are required to describe in detail the role of the lungs in the process of clearing human organism of propofol.