Increasing doses from 4 to 144 mg/kg in pigs produced the dose responses shown in Figure 2; onset and duration data are presented in Table 1. No effect was observed in the pigs injected with 4 mg/kg. The 14 mg/kg dose produced ataxia. Propofol blood concentrations at 60 min were small: 0.13 and 0.14 μg/mL for the 4 and 14 mg/kg doses, respectively. At the 72 mg/mL PP dose, the propofol concentration reached 1.3 μg/mL at 6.7 ± 5.4 min and remained >1 μg/mL for approximately 60 min (Fig. 3). Somnolence and good muscle relaxation were induced in the pigs injected with 72 and 84 mg/kg. These pigs recovered within 4 h, appearing awake and normal, and eagerly returned to their normal feeding schedule. The 144 mg/kg PP dose produced somnolence, with good muscle relaxation, adequate sedation, and normal reflexes 1 min after infusion, but induced bradycardia and apnea at approximately 15.5 min, in addition to a brief loss of corneal reflex. This pig was manually resuscitated (chest compressions without intubation) for 125 min. The pig first regained normal corneal reflex, followed by recovery of auditory reflex and spontaneous breathing, and became awake but recumbent at 270 min. The pig could not stand on her own 5 h after infusion but appeared normal when observed 24 h after injection. At the 144 mg/kg PP dose, propofol blood concentrations were consistently above the waking threshold of 1.0 ± 0.3 μg/mL (Fig. 3, Table 2) for 4 h. Because of an inability to intubate and support lung function, further injections at this dose were not attempted.
The onset and duration of sedation data for mice, rats, rabbits, and pigs are presented in Table 1. The onset of sedation was dose dependent in all species. In mice, the onset values varied from 6.1 ± 1.7 min to 2.7 ± 0.4 min postinjection for the small and large doses, respectively. In rats, the onset values ranged from 10.6 ± 2.9 min to 4.4 ± 1.0 min for the small and large doses, respectively. The onset and duration of sedation could not be studied in rabbits because of the lack of adequate sedation. In pigs, the onset values ranged from 3.0 ± 0.5 min to 1.0 min for the small and large doses, respectively. The duration of the effect was also dose dependent and correlated well with increasing doses (correlation coefficient ≥0.7;P ≥ 0.007) for all species except of rabbits.
On the administration of PP in rats, rabbits, and pigs, propofol blood concentration was followed in time at selected doses and is presented in Figure 3. Some estimates of pharmacokinetic variables are shown in Table 2. The pharmacokinetic profiles of PP appeared different from those described in the literature for propofol (20–23) for the respective species. On the administration of comparable doses in rats, rabbits, and pigs, propofol blood concentrations ranged from 1.0 to 10 μg/mL, with peak (Cmax) values of 9.6 ± 2.1 μg/mL, 7.7 ± 3.2 μg/mL, and 2.9 μg/mL, respectively. The propofol Cmax values were reached in times (Tmax) ranging up to 16.1 min, with propofol production half-lives [T1/2(α)] ranging up to 5.3 min. Propofol elimination half-lives [T1/2(β)] were comparable in rats and rabbits (24 ± 12 min versus 21 ± 16 min) but were much larger in pigs (225 ± 16 min and 275 min) at both doses. Propofol blood concentrations at waking were 0.9 ± 0.8 μg/mL and 1.0 ± 0.3 μg/mL in rats and pigs, respectively.
The economic and clinical pressure to provide water-soluble anesthetics has renewed interest in designing analogs and prodrugs of the most popular one, propofol (3,12–14,24,25). We have developed a water-soluble prodrug of propofol, PP (Fig. 1), and evaluated it in several studies. PP is formulated in water as the sodium salt (3.1% wt/vol; pH 7.4) and sterilized by filtration (0.2 μm), and then it is ready for IV administration; this is in contrast to a current propofol formulation (Diprivan and other generic formulations) that requires oil, solubilizing agents, and an emulsification process that is hard to control (26).
PP was administered IV in mice, rats, rabbits, and pigs and was shown to be converted to hypnotically active propofol. In mice, the PP HD50 was found to be 640 μmol/kg and was approximately 10 times larger than the HD50 values reported for propofol: 66.64 μmol/kg (18) and 68 μmol/kg (12). The PP HD50 value implies that achieving the same levels of hypnosis as with propofol will require the administration of approximately 10 times larger doses of PP. The PP LD50 in mice was found to be 2.3 mmol/kg, which is, again, approximately 10 times larger than the LD50 value of 0.23 mmol/kg reported for propofol (18) and is suggestive of less toxicity than that of propofol. Subsequently, the SI in mice remained comparable to that of propofol—3.6 vs 3.40 (18) and 3.5 (19) —simply because both the HD50 and LD50 values for PP were changed by the same factor of 10, resulting in the SI, expressed as a ratio of HD50/LD50, remaining unchanged. In rats, PP seems also less potent and less toxic than propofol. A PP dose of 30 mg/kg (116 μmol/kg), which is equimolar to 20 mg/kg (116 μmol/kg) of propofol, produced no effect (Table 1). A 130 mg/kg (499 μmol/kg) dose, which is 18 times larger than the reported propofol HD50 of 5 mg/kg (28 μmol/kg) (18) and 3 times larger than the propofol LD50 of 30.28 mg/kg (169.9 μmol/kg) (18), was well tolerated (n = 5) and produced sedation without a single lethality. In rabbits, lethality (3/3) was encountered at a 150 mg/kg (581 μmol/kg) PP dose, but this was not surprising because of the earlier report describing the same lethal outcome in rabbits (6/6) that achieved light anesthesia of very short duration and little reflex depression at the 3 studied propofol doses of 5, 10, and 15 mg/kg (18). In pigs, PP seemed less potent and less toxic. The PP dose of 4 mg/kg (15 μmol/kg), which is nearly equimolar to 2.5 mg/kg (14 μmol/kg) of propofol, had no hypnotic effect, and the propofol blood concentration was less than 1 μg/mL. The PP dose of 14 mg/kg (54 μmol/kg), which is nearly equimolar to the largest propofol dose reported in the literature of 10 mg/kg (56 μmol/kg) (18), produced only ataxia and propofol blood concentrations again less than 1 μg/mL. Results in pigs at both doses indicate decreased potency, consistent with the observations in mice and rats. PP 72 mg/kg doses, which were five times larger than the largest dose of propofol described in the literature, produced sedation and propofol blood concentrations more than 1 μg/mL and were well tolerated. The 144 mg/kg dose of PP, administered to 1 pig, was 10 times larger than the largest propofol dose reported and produced an anesthetic level of sedation and no lethality. It would seem appropriate to say that in pigs, PP might also be less toxic, although very small numbers of pigs were studied, and caution should be exercised in generalizing this conclusion for observations in pigs. Certainly, propofol at 100 mg/kg (10 times the largest reported dose of propofol in pigs) would produce lethal results. Thus, at least in mice, rats, and pigs, because of a combination of PP’s lower hypnotic activity and lower toxicity, it is possible to administer large enough doses of PP to achieve the same level of sedation as is possible with propofol.
Studies with propofol showed a correlation between propofol dose and the duration of sleep (18). In our studies, a similar correlation between PP dose and sleep duration was established, thus leading to the conclusion that, in a given species, the pharmacokinetic profile is the major contributing factor to performance of this drug, i.e., onset, duration, and recovery. All the species showed dose-dependent onset, depth of sedation (Fig. 2), and duration of sedation (Table 1). The onset of sedation ranged from a minute to several minutes and was much slower than that of propofol. The onset of sedation for propofol is typically <10 seconds in mice and rats (18). The onset value seemed to plateau at large doses, possibly because of enzymatic saturation. At saturation, propofol cannot be produced any faster, and, therefore, the onset value reaches its limits in a given species. Depth of sedation ranged from mild ataxia at small doses, to deep sedation with loss of reflexes and no response to painful stimuli at intermediate doses, to apnea, bradycardia, and lethality at large doses. Duration of sedation correlated well with dose (linear regression, r2 ≥ 0.7;P < 0.05) in mice, rats, and pigs. In addition, the duration of sedation induced with PP seemed longer than with propofol. In mice, the duration of sedation induced with PP at a 2 × HD50 (330 mg/kg) dose was much longer than that with propofol at the equivalent dose (2 × HD50 [propofol] = 26 mg/kg): 86.3 ± 32.4 minutes versus 5.6 ± 1.3 minutes (18). In rats, at a 130 mg/kg (499 μmol/kg) dose, the sedation was induced and maintained for 92.7 ± 33.5 minutes without any lethality. Such a long duration could not be achieved by bolus injection of propofol, because even at a dose of 30 mg/kg (a dose close to the propofol LD50 of 30.28 mg/kg), the duration was only 26.36 ± 6.28 minutes (18). The increase in duration of propofol- induced sedation, however, could be achieved by infusion of additional amounts of propofol at precisely determined use rates (18). The duration was not studied in rabbits and could not be compared in pigs because of lack of propofol data in this species. The recovery was uneventful, with the exception of rabbits. A lethal effect was obtained in rabbits and, at large doses, in rats when respiratory arrest occurred and lung function was not supported. A similar lethal outcome in rabbits was noted in the original propofol article (18). Gross necropsy revealed some lung inflammation; the other organs were normal. No signs of any delayed toxic effect were seen in the remaining animals, which all recovered and were alert and keen to drink and eat shortly after regaining the ability to walk.
Propofol blood concentrations after the administration of PP were determined over time, and the results are presented in Figure 3 and Table 2. The onset, duration, and recovery can be explained in pharmacokinetic terms. The propofol concentration has to reach above a certain threshold value in blood to elicit the onset of sedation, persist above that threshold to produce duration, and decrease below that threshold to initiate recovery. In the initial propofol production and distribution phase (α), the propofol concentration was increasing with the estimated T1/2(α) (in minutes) for all the species (Table 2). These half-lives were comparable to those found in rabbits and pigs on the administration of propofol but were increased in rats. In the second elimination phase (β), after reaching the Cmax, the propofol concentration decreased at slightly slower rates when propofol was produced from PP than those of propofol on the administration of propofol. T1/2(β) values in rats and rabbits were comparable to the published values for propofol (18,20). The T1/2(β) in pigs was much higher than the published value for propofol: 224.8 ± 55.7 minutes versus 57.4 ± 4 minutes (19). The higher T1/2(β) in pigs implies slower recovery, which could be advantageous for long-term maintenance of sedation and anesthesia. At 72 and 144 mg/kg in pigs, the T1/2(β) values were similar. With a slow clearance effect in pigs, the propofol concentration remained above the waking threshold for several hours and decreased gradually, in contrast to the fast clearance in rats and rabbits (Fig. 3). The gradual decrease is possibly due to a combination of the two factors: propofol production and elimination rates. In the initial α phase, the propofol production rate exceeds the elimination rate, and, therefore, increasing levels of propofol are observed. At Tmax, when the propofol concentration reaches Cmax in the blood, the rate of propofol production equals the rate of propofol elimination. Shortly after that time, the propofol production rate starts decreasing, mainly because of a gradually decreasing concentration of PP, and the elimination rate starts gradually exceeding the propofol production rate; therefore, a very gradual decline in propofol concentration is observed over an extended period of time. Eventually, the pool of PP will be depleted, the propofol production will cease, and the elimination rate will decrease to the normal values that were observed in rats and rabbits. Unfortunately, the sampling period was too short and the doses too large to reach a normal propofol elimination phase, and more detailed studies may be necessary at several smaller doses to establish the dose effect on the propofol elimination rate and other pharmacokinetic variables. The enzymatic saturation seen in pigs was not observed in rats and rabbits at the doses for pharmacokinetic studies; however, enzymatic saturation would be expected at larger doses. Waking propofol blood concentrations after the administration of PP were smaller in rats but were comparable in pigs to the propofol waking values after propofol administration in the corresponding species (21).
Overall, the SI of the water-soluble prodrug of propofol, PP, was comparable to that of propofol. The decreased potency and decreased toxicity of PP allowed the administration of larger PP doses to attain the same level of sedation as with propofol. At large enough PP doses, propofol was released at anesthetic concentrations, resulting in anesthetic-level sedation. Future studies in humans are required to better understand the clinical utility of PP. These studies suggest that PP can be used to induce and maintain sedation and anesthesia in laboratory animals; however, extrapolation to the clinical situation must be done cautiously, because pharmacokinetic/pharmacodynamic variables in different species may vary significantly.
We thank Dr. Michael Peck and Dr. Charles Lollo, for helpful discussions; Bob Sargent, E. Gouveia, and D. Bassett, for animal handling; and D. P. Wu, for formulation of PP for the pig study.
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© 2002 International Anesthesia Research Society
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