Anaesthesia, 1990, Volume 45, pages 189-197

Pregnanolone emulsion A preliminary pharmacokinetic and pharmacodynamic study of a new intravenous anaesthetic agent

P. CARL, S. H0GSKILDE, J. W. NIELSEN, M. B. SORENSEN, M. LINDHOLM, B. KARLEN T. BACKSTR0M

AND

Summary

Pregnanolone emulsion, a new steroid anaesthetic agent, was administered intravenously as bolus doses to six young healthy male volunteers in a preliminary pharmacokinetic and pharmacodynamic study. The plasma concentration-time curves fitted a twocompartment model. The elimination harf-life was between 0.9 and 1.4 hours, volume of central compartment between 0.95 and 2.10 litreslkg, volume of distribution between 3.75 and 5.58 litreslkg and total body clearance between 1.80 and 3.07 (litresl hour)/kg. The excretion in urine of unchangedpregnanolone was less than 0.1 %. The pharmacodynamic properties werefound to he similar to those of Althesin, with immediate induction of anaesthesia of short duration. The anaesthetic affected haemodynamics only slightly; minor depression of ventilation, with an increase in Paco2, occurred in several of the subjects. Excitation of short duration occurred in one subject during induction of sleep and slight involuntary muscle movements in another subject during sleep. It is impossible to draw any clear conclusions of the clinical efficacy and tolerance from this limited normal subject trial, but pregnanolone emulsion seems worthy of further clinical trial. Key words Anaesthetics, intravenous; pregnanolone. Pharmacokinetics.

Figdor et al.' demonstrated in 1957 that a series of structurally related pregnanes, without notable endocrine action, were anaesthetically active when administered to mice. The most active of these pregnanes was pregnanolone (3ahydroxy-5/l-pregnane-20-one), which is a naturally occurring metabolite of progesterone, first isolated from urine of pregnant women in 1937.' The anaesthetic effect of pregnanolone has been demonstrated in several animal speciesZ3 but its lack of water solubility prevented its development in clinical practice. Pregnanolone is now prepared in a stable emulsion which may prove to be suitable for clinical use. It is solubilised in soya bean oil and emulsified in a similar way to diazepam in the form of Diazemuls. A wide species variation in toxicity and tolerance of pregnanolone emulsion was observed in preclinical studies. Administration of a single bolus injection of 40 mg/kg did not cause any (or only marginal) mortality in the mouse and the rat. No organ toxicity was noted after repeated

daily intravenous injections in several species: rat 16 mglkg for 4 days and 7 mg/kg for 28 days; dog 12 mg/kg for 4 days and 3 mg/kg for 28 days; monkey 28 mg/kg for 14 days and 21 mg/kg for 28 days. Hepatic lesions restricted to the biliary system, manifested as morphological changes and increased serum enzymes, occurred after daily administration of 40-64 mg/kg for 4 days and after 14 mg/kg for 28 days in the rat. In the dog the threshold dose for hepatobiliary changes at repeated doses was about 7 mg/kg. The liver damage was clearly related to the dose, but also to the duration of the treatment, and was reversible at least in its initial state (toxicological report, not published). The anaesthetic and anticonvulsive properties of pregnanolone in animals are encouraging and very similar to those of Althesin.This study reports the findings of a preliminary and limited clinical investigation with pregnanolone emulsion in male volunteers.

P. Carl, MD, S. Hsgskilde, MD, J.W. Nielsen, MD, Senior Registrars, M.B. Ssrensen, MD, PhD, Chief Anaesthetist, Department of Anaesthesia, The Municipal Hospital of Copenhagen, 1399 Copenhagen K, Department of Anaesthesiology and Intensive Care, Hvidovre Hospital, Kettegird Alle 30, 2650 Hvidovre, Denmark, M. Lindholm, MD, PhD, Chief Anaesthetist, Associate Professor, Department of Anaesthesia, Huddinge University Hospital, Karolinska Institute, Stockholm, B. Karlen, PhD, Professor, Department of Toxicology, Karolinska Institute, Stockholm, T. Backstrom, MD, PhD, Assistant Professor, Department of Obstetrics and Gynaecology, University of UmeB, Sweden. Correspondence should be addressed to Dr P. Carl, Ericaparken 20, 2820 Gentofte, Denmark. Accepted 25 September 1989. 0003-2409/90/030189 + 09 rS03.00/0

@ 1990 The Association of Anaesthetists of Gt Britain and Ireland

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P. Carl et al.

Methods Six healthy male volunteers with no past history of earlier anaesthetic complication, allergy to drugs or abuse of alcohol or drugs took part In the study. The trial was conducted in accordance with the Helsinki I1 Declaration and approved by the local ethics committee. The experiment was explained to the volunteers and preliminary consent obtained. The subjects were then submitted to a complete physical examination and laboratory tests, in which a variation of 10% above or below the normal range of the laboratory data was allowed. The subjects were asked to report on the morning of the study fasted and completely rested. The radial artery was cannulated percutaneously using a 20-gauge catheter. Arterial cannulation was performed in the nondominanf limb and only in the presence of a competent ulnar artery. A systemic arterial catheter was connected via a pressure transducer calibrated with water standards to a, two-channel multiparameter monitor, and the systemic arkrial blood pressure was recorded continuously. A peripheral vein was cannulated in the contralateral arm for electrolyte solution and drug administration. Continuous electrocardiogram (ECG) using chest electrodes, pulse monitoring and central (rectal) and skin (toe) temperature were instituted. These variables were also charted continuously. Respiratory rate was measured at one-minute intervals. The subjects were then allowed to rest for one hour in the supine position after which baseline recordings were made. Pregnanolone emulsion (PE) was formulated as pregnanolone 4 mg, soya bean oil 200 mg, acetyl triglycerides 70 mg, egg yolk phosphatides 18 mg, glycerol 17 mg and with distilled water to a volume of 1 ml. The emulsion was isotonic, stable and had a pH of about 7.5. The mean particle size was 0.2-0.5 pm and less than 3% of particles were larger than I .O pm. The volunteers received two bolus doses of PE, with an interval of at least 2 weeks between the first and the second doses. The initial dosage of 0.05 mg/kg was chosen from consideration of data obtained in toxicological studies. The study was sequential, i.e. increasing doses in consecutive subjects until anaesthesia was induced for about 10 minutes or until a maximum dose of 1.6 mg/kg or unacceptable side effects were observed. Pregnanolone concentration was determined in arterial blood before injection and then at 5, 10, 20, 30, 40 and 50 minutes and at I , 1.5, 2, 2.5, 3 and 4 hours after the start of injection of the anaesthetic. The blood samples were centrifuged (1000 rpm for 5 minutes), frozen at -20°C and kept at this temperature until analysis. Urine samples were collected for 0-12 and 12-24 hours after administration of PE 0.3 mg/kg and higher doses, and then stored at -20°C. The following analyses were performed on blood samples to assess the effects on hepatic function and haematology: haemoglobin, sodium, potassium and creatinine concentrations, white cell count and platelet count (prior to induction and at 8, 12 and 24 hours); bilirubin, alkaline phosphatase and aspartate amino transferase (ASAT) concentrations (prior to induction and at 8, 12 and 24 hours); triglyceride, cholesterol and free fatty acid concentrations (prior to induction, immediately after administration, and at 1 and 24 hours); arterial blood gas tensions (prior to induction and at 2, 5 , 10, 15, 30 and 60 minutes).

Observations Induction time. At the start of injection, the subjects started to count and the times to stop counting, drop a syringe of water held 90" from the body and lose the eyelash reflex were noted. Seeping time. The time from loss to return of eyelash reflex was noted. Recovery from anaesthesia. This was assessed by measurement of the times from the end of the injection to return of eyelash reflex, spontaneous opening of the eyes on command (waking time), first verbal response, full orientation in time and space and a subjective experience of being unaffected by pregnanolone. Undesirable effects. Involuntary muscle movements, excitation, hiccough, cough, laryngospasm, upper airway obstruction, apnoea, and necessity for artificial ventilation or oxygen supplementation were noted. Efects on blood vessels used for injection. Pain on injection, redness, tenderness, induration and thrombophlebitis were sought and noted. The length of redness and tenderness was expressed in cm. The site of inkction was examined 24 hours and one week after injection of PE. A follow-up visit was scheduled one week after the study for a further physical examination, laboratory tests and detection of any delayed problems. Analytical technique The analytical method for assaying blood and urine concentrations was based on combined gas chromatographymass spectrometry (Hewlett Packard mass selective detector 5979B). The gas chromatograph (HP 5890) was equipped with a capillary column injection system and interfaced with an HP data acquisition system. Separation was performed on an SE-54 fused silica capillary column (Hewlett Packard, 25 m, 0.2 mm i.d., 0.1 pg phase thickness). The injector was operated in the splitless mode for 30 seconds at 280°C. The temperature of the oven at the start was 120"C, programmed to increase at 5O"C/minute to 280°C and then held isothermal for 2.5 minutes. The carrier gas was helium (1 ml/minute) and the column was directly interfaced to the ion source. The mass spectrometer was operated in single ion mode at 12 cycles/second, with a 0.25 second dwell time, monitoring m/z 300 for both pregnanolone and the internal standard. The method comprised extraction from 0.01-1 ml plasma with 3 ml diethyl ether/hexane (1/2) at pH 13 (3 ml 0.2 mol/litre aqueous NaOH) of unconjugated pregnanolone directly and of total (unconjugated and conjugated) pregnanolone after previous hydrolysis with glucuronidase (sulphatase). The dried (anhydrous Na,SO,) and evaporated extract was derivatised by 50 pl N, 0-bis (trimethylsilyl) triflouroacetamide and 10 pl dry triethylamine at 85°C for 15 minutes to give the 3-hydroxy-trimethylsilyl derivatives of pregnanolone and the internal standard 38hydroxy-5a-pregnanolone-(20).The derivatives were chromatographed by capillary gas chromatography, and on electron impact of the eluent peaks the m/z ion 300 trace was monitored and quantified for both compounds. Accuracy (recovery) tested on spiked serum samples in the range 4400 pg/litre varied between 95 and 106%. Between-day variation estimated for one sample and using different

Pregnanolone emulsion

calibration curves was 3.9% at a concentration of 444 pg/ litre. Limit of sensitivity was set to 5 pg/litre. The assay method also allows for analysis of the total amount of pregnanolone after previous hydrolysis of conjugates with glucuronidase. An estimate of conjugated pregnanolone was calculated by subtracting the amount of unconjugated pregnanolone obtained on direct extraction of the sample. The amounts are expressed as pregnanolone equivalents in pg mass.

Table 1. Details of subjects and intravenous doses of pregnanolone

administered as an emulsion formulation (mg/kg and total dose in ml). Subject number

Weight

Dose

Total dose

Administration

(kg)

(mg/kg)

(ml)

1

70

0.05 0.50 0.15 0.40 0.40 0.30 0.60 0.50 0.60 0.60

0.9 8.8 2.9 7.7 7.0 5.3 11.1 9.3 12.9 13.8

2 1

80

2 1

Kinetic analysis

70

2

Each plasma pregnanolone-time profile using an unweighted nonlinear regression programme was individually fitted to the following two-compartment model: C = A.e-" I + B.gb1

C = pregnanolone plasma concentration (pg/litre) at any time (t) in hours; A = intercept at t = 0 for distribution phase (pg/litre); c( = distribution rate constant (l/hour); B = intercept ai, t = 0 for elimination phase (pg/litre); fi = elimination rate constant (l/hour). The area under the concentration-time curve to 4 hours (AUC,) was calculated by the trapezoidal method, and residual area (AUC,,) by dividing the fitted concentration at 4 hours by p. CI,,,, the total body clearance [(litre/hour)/ kg)], was obtained by dividing the dose by AUC,,; Vc (volume of the central compartment in litre/kg) was obtained by dividing the dose by A + B. The volume of distribution (Vd; litre/kg) was obtained by dividing CI,,, by

B. Results

Dosage of PE covered a range of 0.05-0.6 mg/kg. Induction of sleep was achieved at a dose of 0.4 mg/kg and the protocol was changed according to Table 1 which shows the weight of the subjects and dosages employed. Subject number 5 received only one dose of PE because of paraesthesia in the hand several days after removal of the arterial cannula. Subject number 6 was excluded from study period 2 after admitting that he had been working through the previous night.

loo0

4

1

74

2 1 1

5 6

86 92

Pharrnacokinetics

Plasma concentrations of total and conjugated pregnanolone are shown in Tables 2 and 3. Very low concentrations were expected after doses up to 0.3 mg/kg, and plasma was sampled only 5 and 8 minutes after administration of PE in subjects 1, 2 and 3 in study period 1. Thus no pharmacokinetics could be derived from these treatments. Figure 1 shows the serum concentration-time curve of unconjugated and conjugated pregnanolone (subject 4) after injection of PE 0.5 mg/kg. The pharmacokinetic variables computed from the plasma pregnanolone concentrations are listed in Table 4. The two-compartment model fitted the data well and suggests a distribution phase with t,, varying between 9 and 15 minutes and the corresponding elimination half lives (tIs) between 0.9 and 1.4 hours. The volume of the central compartment (V,) varied between 0.95 and 2.10 litres/kg and that of the total compartment (V,) between 3.75 and 5.58 litres/kg. Total body clearance (V,,,,) was calculated between 1.80 and 3.07 (litre/hour)/kg; excretion in urine of unconjugated pregnanolone accounted for negligible amounts Uess than 0.1 YO). The total amounts of conjugated pregnanolone excreted in urine corresponded to about 10% of the dose (range 7.9-16.270).

r

I

0

I

50

I

I I50 Time (minutes)

I00

191

I

I

200

250

Fig. 1. Plasma concentration of pregnanolone in one subject after intravenous pregnanolone emulsion 0.5 mg/kg. 0-0, unconjugated pregnanolone; 0-0, conjugated pregnanolone.

192

P . Carl et al. Table 2. Total serum concentrations of pregnanolone (pg/litre) in subjects 1 4 before and after various doses of pregnanolone emulsion (4 mg/ml). Subject number Administration Dose (mg/kg) Time (minutes) 0

5 10 20 30 40 50 60

1 1

0.05 0 25 18

90 120 150 180' 240

1 2 0.5

2 1 0.15

2 2 0.4

3 1 0.4

3 2 0.3

4 1 0.6

4 2 0.5

0 292 183 I20 86 65 55 42 23 17 13 9 6

0 101 72

0 331 206 109 74 46 42 34 23 18 14 9 7

0 155 93

0 234

0 459 215 179

0 309 173 117 98 58 56 50 33 19 17

111

79 56 39 36 24 14 10 10 6 2

101

87 64

61 35 22 17 14 8

10

7

5

6

1

1

0.6

0.6

0 467 377 287 209 119 90 80 43 36 26 23 13

0 392 186 135 103 88 73 56 38 27 20 12 9

Table 3. Concentrations (pgllitre) of conjugated (total-unconjugated) pregnanolone in serum in subjects 1 4 before and after various doses of pregnanolone emulsion (4 rngiml). Subject number Administration Dose (mg/kg) Time (minutes) 0 5 10 20 30 40 50 60 90 120 I50 180 240

1 1

0.05 0 9 6

1

2 0.5 0 247 171 76 55 53 49 35 28 27 13 I1 9

5

2 1 0.15

2 2 0.4

3 1 0.4

3 2 0.3

4 1 0.6

4 2 0.5

0.6

6 I 0.6

0 155 117

0 375 194 222 74

0 206 166

0 125 75 32 20 28 8 10 9 8 9 11 6

0 300 309 I43 128 80 70 57 41 29 19 15 12

0 375 238 151 84 74 46 34 29 25 10 13 9

0 242 193 117 101 62 69 62 39 52 47 16 10

0 325 172 217 92 59 50 42 29 25 21 17 15

64

48 43 33 29 24 15 10

1

Table 4. Pharmacokinetic variables of pregnanolone in subjects 1-6. Subject number Administration Dose (mg/kg) A (pg/litre) a (lihour) B (pg/litre) B (lihour) tlS (hours) V, (litre/kg) V, (litre/kg) AUC,, [otg/ hour)/litre] Cl,,, [(litre/ hour)/kgl

2 0.5

2 2 0.4

3 2 0.3

4 1 0.6

4 2 0.5

0.6

6 1 0.6

203 2.49 49 0.55 1.26 1.98 5.58

342 4.60 51 0.53 1.31 I .02 4.74

168 3.94 50 0.76 0.91 1.38 3.9 1

293 2.77 65 0.54 1.28 1.68 5.01

167 2.81 71 0.59 1.17 2.10 4.8 1

540 3.27 93 0.48 1.44 0.95 3.75

229 3.18 89 0.61 1.14 1.89 4.70

163

I60

101

222

176

333

209

1

3.07

2.51

2.97

2.70

2.84

5 1

I .so

2.87

Pregnanolone emulsion Table 5. Onset of anaesthesia in five subjects following intravenous

doses of pregnanolone. Subject number Administration

3 1 0.4

Dose (mgike) Injection time (seconds)

30

Time (seconds) to: Loss of eyelash reflex Stop counting Drop syringe

75 20 20

4 1 0.6 45

4 2 0.5 38

75

75

15

39 63

5 1 0.6 45 75 33 45

6 1 0.6 45 90

35 68

Table 6. Recovery data after intravenous doses of pregnanolone.

Subject number Administration Dose (mg/kg) Time (minutes) to: Return of eyelash reflex Spontaneous opening of eyes on command (waking time) First verbal response Full orientation in time and space Subjectively unaffected by pregnanolone

Phurmacodynumics

The subjects who received PE a t a dose of 0.6 mg/kg fell asleep, whereas this was the case with only one of two subjects who received doses of 0.4 mg/kg and 0.5 mg/kg, respectively. Time to loss of eyelash reflex varied from 75 to 90 seconds and time to stop counting from 20 to 39 seconds, followe~shortlyby dropping of the syringe (Table 5). The subjects opened their eyes on command 5.0 minutes after P E 0.4 mg/kg, 7.3 minutes after PE 0.5 mg/kg, and 7.4, 11.2 and 15.2 minutes after P E 0.6 mg/kg (Table 6). All subjects were fully orientated in time and space within a few minutes of regaining consciousness, but were still a little drowsy and a further period elapsed before they were clear-headed (Table 6). Figures 2 and 3 show the changes in arterial blood pressure and pulse rate after induction doses of PE. These results differed from subject to subject without a uniform pattern. No ECG arrhythmia was observed. A short period of hypoventilation was observed after induction of anaesthesia in five of six subjects as indicated by an increase of Paco, and a decrease of Pao, (Fig. 4).

3

4

4

5

1

1

2

1

1

0.4

0.6

0.5

0.6

0.6

4.2

8.0

7.4

7.1

11.2

5.0 5.0

11.2 13.0

7.3 9.2

7.4 15.2 7.6 16.2

8.0

15.2 13.4 14.4 17.2

50

40

45

P

-

.'1 I20 0

c

In %

v)

ZII

I

E

E

I00

2

I

u)

e

a

.-0 &

40

6

35

These alterations were transient and within the normal ranges of Pacoz (4.3-6.0 kPa) and of Pao, (9.4-14.2 kPa). A significant increase in Pao, above normal limits was observed in one subject (dose 0.6 mg/kg). The Pacoz remained elevated for more than 60 minutes (the last measurement of blood gas tensions was made 60 minutes after injection of PE), whereas Pao, returned t o the baseline value within 60 minutes. The subject was totally unaffected by the increased Pacoz. p H and bicarbonate concentration were unchanged during the study period. Several recurrent apnoeic periods of about 10 seconds during 2 minutes were observed 20 minutes after injection of P E in subject number 4 (dose 0.5 mg/kg). At that time the subject had been fully awake, but he became drowsy if not stimulated. This lasted for about 5 minutes. This subject had earlier received P E 0.6 mg/kg without any periods of apnoea. No respiratory assistance or administration of oxygen was needed. Airway obstruction was not observed.

I40

1

193

80

c I

a 'I' 0 r

._ 60 n

I

I

I

I

I

I

0

2

4

6

8

I0

Time (minutes)

Fig. 2. Changes in systolic and diastolic arterial blood pressure after induction of anaesthesia with pregnanolone. Subject 3,0.4 mg/kg (0-0); subject 4,0.6 mg/kg (U-0); subject 4,O.S mg/kg (A-A); subject 5, 0.6 mg/kg (0-0); subject 6, 0.6 mg/kg (+-+).

194

P. Carl et al.

I

120r c 0)

2 1100 00

.

.-

E

u)

+

z

80

2

e

+

i

I

60. 60

2

0

4

6

10

8

Time (minutes)

Fig. 3. Changes in heart rate after induction of anaesthesia with pregnanolone. Subject 3, 0.4 mg/kg (0-0); subject 4, 0.6 mg/kg (0-U); subject 4, 0.5 mg/kg (A-A); subject 5, 0.6 mg/kg (0-0); subject 6, 0.6 mg/kg (+-+).

other subjects the temperature followed the normal diurnal fluctuation' of 0.5-1 .O"C. Excitatory side effects were seen only in one subject (dose 0.6 mg/kg) about 2 minutes after administration of PE. The excitation was not pronounced and lasted for about 3 minutes. Another subject had slight involuntary muscle movements about 4 minutes after injection of PE, but no excitation was observed. Pain at the injection site and venous sequelae after administration of PE were not seen. No subject complained of nausea or other side effects

The peripheral temperature increased by a few degrees in almost all of the subjects immediately after administration of PE, but in two of the subjects the increase in temperature was marked (up to 7.5"C 12 minutes after administration of PE). The core temperature was unchanged in all but one subject (subject number 3, dose 0.3 mg/kg) in whom core temperature increased by 1.3"C (from 37.6"C to 38.3"C) 1&11 hours after injection of PE. The temperature remained increased for about 2 hours. No leucocytosis was observed in relation to the temperature increase. In the

I

I

I

I

I

1

0

10

20

30

40

50

60

20

30

40

50

60

1

1.0

0

-a r

6.0

0

2 5.0

0

I0

Time (minutes)

Fig.4. Changes in Pao, and Paw2 after induction of anaesthesia with pregnanolone. Subject 3, 0.4 mg/kg (0-0); subject 4, 0.6 mg/kg (0-0); subject 4, 0.5 mg/kg (A-A); subject 5, 0.6 mg/kg (0-0); subject 6.0.6 mg/kg (+-+).

Pregnanolone emulsion 12

7

I0

-

195

? + ._ 2

\

0 x

8-

+ c

3 0 0

t

>

60 1

1

?a

I

1

0

1

I

1

I

1

5

10

I5

20

25

Time (hours)

Fig. 5. Changes in leucocyte count after intravenous injection of pregnanolone. Subject 1, 0.05 mg/kg subject 4, 0.6 mg/kg (0-0); subject 5 , 0.6 mg/kg (+-+); subject 6, 0.6 mg/kg ( x - x ) ; subject I , 0.5 mg/kg (0-0); subject 2, 0.4 mg/kg subject 3, 0.3 mg/kg (A-A);subject 4,0.5 mg/kg (+-+).

(0-0); subject 2, 0.15 mg/kg (0-0); subject 3,0.4 mg/kg (A-A);

(.-a);

during the recovery period. All the subjects reported that they felt comfortable during and after injection of PE. An increased white cell count occurred in nine of 10 subjects 8 hours after administration of PE (the first measurement after baseline value). In one case this exceeded 10% above the baseline value (Fig. 5); the white cell count was normal after 24 hours. Serum triglycerides increased as expected in all subjects after injection of PE. The increase exceeded 10% above the baseline value in four subjects (doses 0.5 and 0.6 mg/kg). Serum cholesterol was not affected. The concentration of free fatty acids increased in the same way as triglycerides. The other laboratory tests revealed no evidence that P E affected hepatic, renal o r haematological parameters adversely. Physical examination, biochemical and haematological tests revealed no abnormality one week after administration. Neither phlebitis nor thrombosis was detected a t the injection site in any subject. Discussion

The shapes of the concentration-time curves of unconjuga!ed and conjugated pregnanolone were very similar, with equal disposition half lives. This implies very fast conjugation of the parent drug. It also implies that the disposition kinetics (distribution and elimination) of the parent drug are rate limiting for the kinetics of the metabolite. The fact that the curves are superimposed with equal A U C suggests either that most of the parent drug forms the conjugate and the two have similar clearance or, more likely, that the clearance of the conjugate and fraction of drug forming the conjugate are both smaller. Elimination in the liver of a highly cleared drug (extraction ratio approaching I ) is limited by the blood flow in this organ (1.5 (litres/hour)/kg). Thus, pregnanolone (total body clearance between 1.80 and 3.07 (litre/hour)/kg) must also be cleared by other t i ~ s u e s . ~ . ~ Our results show that about 10% of pregnanolone is excreted as conjugates in the urine and very little (less than 0.lY0) as unchanged drug in 24 hours. The total body

clearance of the drug is large and it can be calculated that less than 8% (between 3 and 8Yo) of the unchanged drug remains in the body after 4 hours. Data obtained on excretion of pregnanolone in pregnant womenlo and in rats given radioactive pregnanolone* suggest that faecal excretion is a n important route of elimination of both conjugated and unconjugated pregnanolone. The pharmacokinetic profile of pregnanolone suggests that there should be a lower cumulative effect than any of the barbiturate intravenous agents; this should enable easily controlled and flexible intravenous sedative regimens to be implemented as well as ensuring rapid recovery without 'hangover'. However, major changes in liver blood flow and, perhaps less importantly, liver metabolism, may have marked effects on the disposition of the drug in the body and therefore on its clinical effects, although organs other than the liver possibly participate in the metabolism of p r e g n a n o l ~ n e . ~ . ~ Plasma concentrations of alphaxalone after a single injection of 0.7 mg/kg (induction dose of Althesin)" are almost identical to the concentrations of pregnanolone found in this study after a n induction dose of P E 0.6 mg/ kg. Total body clearance of P E is greater (1.5-2.5 times) than the clearance of Althesin (1.2 (litres/hour)/kg) and the volume of distribution (V,) would appear to be 2 to 4 times that of Althesin (1.37 litre/kg)." The half lives of PE were much larger than those reported for Althesin (ti%= 1.6 minutes; ti, = 34 minutes)," probably reflecting the larger volume of distribution. Experimental results from animal studies have indicated that the pharmacodynamic properties of PE are similar to those of AlthesimG The present study confirms the similarity in pharmacodynamic properties: induction of anaesthesia with P E was rapid and with similar speed of onset to Althesin,l*." sleep could be produced in one arm-brain circulation time, and recovery time and cardiorespiratory effects were similar to those associated with Althesin.lSi6 The body temperature was elevated to 38.3"C 11 hours after injection of PE in one subject (dose 0.3 mg/kg) without simultaneous leucocytosis, but the temperature

196

P . Carl e t al.

increase was transient and could be ascribed to reasons other than a reaction to PE. However, several studies have described the pyrogenic action of steroids in man; pregnanolone (dissolved in small amounts of 95% ethyl alcohol) has been described as particularly p y r o g e n i ~ . ” - ~The ~ steroid pyrogens are effective in producing fever characterised by a peak temperature elevation, which may reach 4°C or more above normal, 9 to 12 hours after injection. Steroid pyrogen fever is associated with a significant leucocytosis; the leucocyte count may reach levels 2 to 4 times normal, and usually regresses as temperature subsides. Repeated injection of steroid pyrogens does not result in development of tolerance. Studies with isotopically labelled etiocholanolone indicate that the steroid pyrogen is rapidly conjugated, and cleared from the blood largely via urine excretion; neither @peak blood nor cerebrospinal fluid concentrations of steroid coincide with the period of maximum temperature elevation. A significant portion of injected steroid pyrogen has already been excreted by the time the temperature elgvation occurs and that portion of administered steroid found in plasma when thermogenesis begins is nearly all in the form of inert physiological conjugates. Thus it remains uncertain whether free steroid acts directly on thermoregulatory centres, whether a derivative of injected steroid is active proximally, whether a prolonged stimulus of appropriate central nervous system centres by free steroid (or derivative) is essential to thermogenesis, or whether some specific product of steroid-tissue interaction at the site of local injection is the critical event in steroidinduced fever.22 A maximum dose of 25 mg pregnanolone has been used either intramuscularly o r i n t r a v e n o u ~ l y ’ ~inJ ~ studies on pyrogenic steroids in man. Both smaller and larger doses of pregnanolone were injected in the present study. Large doses of steroids may elicit pharmacological effects which counteract those produced by small doses in a manner comparable to the effects on the body temperature which may be induced by small and large doses of bacterial pyrogen.” Rapid injection of pregnanolone required for anaesthetic purposes may be ineffective in provoking fever, as suggested by Kappas e t af.‘*in studies with the highly potent steroid pyrogen 3a-hydroxy-pregnane- 1 1, 20-dione. It is also possible that the effect of steroid pyrogens on cerebral thermoregulatory centres is blocked during anaesthesia. The peripheral temperature increased by a few degrees in almost all of the subjects immediately after administration of PE, but in two of them the increase in temperature was marked (up to 7.5”C).These subjects were stressed by the situation and were cold peripherally after insertion of the arterial line. The only side effects seen after injection of PE were slight respiratory depression of short duration, excitation in one subject (the subject was nervous before injection of P E and there had been problems with insertion of the radial artery catheter) and involuntary muscle movement in another subject. Involuntary muscle movements were seen frequently with Althesin, especially when higher doses were used for induction of a n a e ~ t h e s i aand ~ ~ premedication with opioids tended to decrease the frequency of abnormal muscle movements. N o cases of venous sequelae were seen. The incidence of thrombophlebitis after Althesin was similar to that seen after dilute solutions of barbiturates.zs An intense phlebitic and periphlebitic inflammation was

describedZoafter prolonged intravenous administration of pregnanolone dissolved in small amounts of 95% ethyl alcohol. It is impossible to draw any clear conclusions of the clinical efficacy and tolerance from this limited normal subject trial, but P E appears to be worthy of further trial in man. References I . FICDORSK, KODETMJ, BLOOMBM, ACNELLO EJ, PAN SY, LAUBACH GD. Central activity and structure in a series of water-soluble steroids. Journal of Pharmacology and Experimental Therapeutics 1957; 119 299-309. 2. RUSSELE, MARKER R, KAMM0. Isolation of pregnanolone from human pregnancy urine. Journal of the American Chemical Society 1937; 5 9 13734. 3. GYERMEK L. Pregnanolone: a highly potent, naturally occurring hypnotic-anestheticagent. Proceedings ofthe Society for Experimental Biology and Medicine 1967; 125: 1058-62. 4. HOGSKILDE s, NIELSENJW, C A R L P, SBRENSEN MB. Pregnanolone emulsion. A new steroid preparation for intravenous anaesthesia: an experimental study in mice. Anaesthesia 1987; 4 2 586-90. 5. HWSKILDE s, WAGNERJ, CARLP, SBRENSEN MB. Anaesthetic properties of pregnanolone emulsion. A comparison with alphaxalone/alphadolone, propofol, thiopentone and midazolam in a rat model. Anaesihesia 1987; 4 2 1045-50. 6. HBGSKILDE S , WAGNER J, CARLP, ANKERN, ANGELO HR, S~RENSEN MB. Anti-convulsive properties of pregnanolone emulsion compared to Althesin and thiopentone in mice. British Journal of Anaesthesia 1988; 61: 462-7. 7. GANONG WF. Neural centers regulating visceral function. In: GANONG WF, ed. Review of medical physiology. California: Lange Medical Publications, 1969: 173-92. L, CAMPBELL P, LONGRJ. Distribution 8. SOYKA LF, GYERMEK and hepatic metabolism of pregnanolone in the tat. Biochemical Pharmacology 1972; 21: 657-71. 9. MICKAN H, ZANDERJ. Pregnanolones, pregnenolone and progesterone in human fetal tissues of early and midtrimester pregnancy. Journal of Steroid Biochemistry 1979; 11: 1467-70. 10. MARTINF, PELTONEN J, LAATIKAINEN T, PULKKINEN M,

11.

12.

13.

14.

ADLERCREUTZ H. Excretion of unconjugated and conjugated progesterone metabolites and estriol in pregnancy urine during ampicillin administration. CIinicaI Chimica Acta 1974; 55: 71-80. SIMPSON ME. Pharmacokinetics of Althesin+omparison with lignocaine. British Journal of Anaesthesia 1978; 5 0 1 2 3 1 4 , SCOTTDF, VIRDEN S. Comparison of the effect of Althesin with other induction agents on electroencephalographic patterns. Postgraduate Medical Journal 1972; 48 (Suppl. 2): 93-6. CAMPBELL D, FORRESTER AC, MILLER DC, HUTTONI, KENNEDY JA, LAWRIE TDV, LORIMER AR, MCCALLD. A preliminary clinical study of CTI 341-a steroid anaesthetic agent. British Journal of Anaesthesia 1971; 43: 14-24. SWERDLOWM. Studies with Althesin-a new steroid anaesthetic. Postgraduate Medical Journal 1972; 48 (Suppl. 2): 105-8.

15. HOPECE, WILSON TW, WYANTGM. Some pharmacological properties of Althesin (CT1341) in man. Canadian Anaesthetists’ Society Journal 1975; 22: 572-86. 16. SAVEGE TM, CALVIN MP, BLOCC CE, SIMPSON JC, WEAVER EJM. Cardiorespiratory effects of some commonly used intravenous induction agents. Acta Anaesthesiologica Belgica 1974; 25: 316-29. 17. WOODWB. Studies on the cause of fever. New England Journal of Medicine 1958; 2 5 8 1023-31. TF. 18. KAPPASA, SOYBELW, FUKUSHlMA DK, GALLAGHER Studies on pyrogenic steroids in man. Transactions of the Association of American Physicians 1959; 72: 54-61. 19. KAPPAS A, SOYBEL W, GLICKMAN P, FLUKUSHIMA DK. Feverproducing steroids of endogenous origin in man. Archives of Internal Medicine 1960; 105: 701-8. 20. KAPPAS A, GLICKMAN PB, PALMER RH. Steroid fever studies: Physiological differences between bacterial pyrogens and

Pregnanolone emulsion endogenous steroid pyrogens of man. Transactions of the Association of American Physicians 1960; 13: 176-85. 21. PALMER RH, KAPPASA. Fever-producing action of steroids. Medical Clinics of North America 1963; 41: 101-12. 22. KAPPAS A, PALMER R H . Thermogenic properties of steroids. R, ed. Methods in hormonal research. New York: In: DORFMAN Academic Press, 1965; 4: 1-19, 23. BENNETT IL, BEESON PB. The properties and biologic effects of bacterial pyrogens. Medicine 1950; 2 9 365400.

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24. CARSONIW. Group trial of Althesin as an intravenous anaesthetic. Posigraduate Medical Journal 1972; 48 (Suppl. 2): 108-10.

25. CARSON IW, ALEXANDER JP, HEWITTJC, DUNDEE JW. Clinical studies of induction agents. XLI: Venous sequelae following the use of the steroid anaesthetic agent, Althesin. Briiish Journal of Anaesthesia 1912; 44: 1311-3.