Water-soluble propofol analogues with intravenous anaesthetic activity

Article abstract of DOI:10.1016/S0960-894X(01)00088-9

Propofol (2,6-diisopropylphenol) is a widely used intravenous anaesthetic that is formulated as an emulsion since it lacks water solubility. We report a range of water-soluble analogues of propofol, containing a para-alkylamino substituent, which retain good intravenous anaesthetic activity in rodents.

Full text of DOI:10.1016/S0960-894X(01)00088-9

Bioorganic & Medicinal Chemistry Letters 11 (2001) 927–930
Water-Soluble Propofol Analogues with
Intravenous Anaesthetic Activity
Andrew Cooke,* Alison Anderson, Kirsteen Buchanan, Alan Byford, David Gemmell,
Niall Hamilton, Petula McPhail, Susan Miller, Hardy Sundaram and Peter Vijn
Departments of Medicinal Chemistry and Pharmacology, Organon Laboratories Ltd., Newhouse, Scotland ML1 5SH, UK
Received 27 November 2000; accepted 9 February 2001
Abstract—Propofol (2,6-diisopropylphenol) is a widely used intravenous anaesthetic that is formulated as an emulsion since it lacks
water solubility. We report a range of water-soluble analogues of propofol, containing a para-alkylamino substituent, which retain
good intravenous anaesthetic activity in rodents. # 2001 Elsevier Science Ltd. All rights reserved.
Introduction
Chain-extended para-aminoalkyl and aminoacyl analo-
gues were also prepared as shown in Scheme 1. Com-
pound 6 was prepared by a Friedel–Crafts acylation of
propofol using aluminium trichloride and bromoacetyl-
bromide in dichloromethane. Reaction of this a-
bromoketone intermediate with the desired amine in di-
chloromethane using triethylamine as base afforded
products 7 and 8. The carbonyl group in compound 7
was reduced to the alcohol intermediate using lithium
aluminium hydride in tetrahydrofuran. Subsequent
removal of the alcohol with potassium borohydride and
potassium hydroxide in an ethanol/water mixture afforded
compound 9.
Propofol (2,6-diisopropylphenol) is a widely used intra-
venous anaesthetic. Its mechanism of action involves the
positive allosteric modulation of the neurotransmitter
g-aminobutyric acid (GABA) at GABAA receptors.1
The main advantages of propofol are favourable oper-
ating conditions and a rapid recovery but disadvantages
include cardiovascular side-effects and pain on
injection.2À4 A number of approaches have been used to
overcome some of the drawbacks associated with the
formulation of propofol as an emulsion, and these have
been reviewed.2 Recent evidence suggests that propofol
analogues containing a para substituent still retain good
activity at the GABAA receptor.5,6 Our aim was to
produce water-soluble analogues of propofol, contain-
ing a para-alkylamino substituent, which retained
intravenous anaesthetic activity.
Replacement of the phenolic hydroxyl group in com-
pounds 1–3 with an aniline function gave compounds
10–12. This was achieved by reaction of 2,6-diisopropyl-
aniline with aqueous foramldehyde and the desired
amine in a refluxing ethanol/water mixture for several
days to afford the desired products (Fig. 2).8
Experimental
A number of analogues of compound 10 were synthesised
to explore SARs around the aniline moiety. Removal of
the aniline function was achieved by diazotisation of
compound 10 using sodium nitrite in hypophosphorus
Several propofol analogues were prepared containing a
para-aminomethyl substituent to aid water-solubility.
Reaction of propofol with aqueous formaldehyde and the
secondary amine of choice in a refluxing ethanol/water
mixture afforded the desired products (1–5; Fig. 1).7
*Corresponding author. Tel.: +44-1698-736136; fax: +44-1698-
736187; e-mail: a.cooke@organon.nhe.akzonobel.nl
Figure 1.
0960-894X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(01)00088-9

928
A. Cooke et al. / Bioorg. Med. Chem. Lett. 11 (2001) 927–930
acid (50% aqueous) and gave compound 13. Compound
10 was converted to acetamide 14 by reaction with ace-
tyl chloride in dichloromethane using triethylamine as
the base. The N,N-dimethyl aniline analogue 15 was
prepared by reaction of aniline 10 with aqueous form-
aldehyde and formic acid. Conversion of aniline 10 to
the N-methyl aniline analogue 16 was achieved by a two
step process: firstly compound 10 was converted to the
intermediate formamide by reaction with formic acid
and acetic anhydride and this product was then reduced
to compound 16 using lithium aluminium hydride in
tetrahydrofuran (Scheme 2).
The anaesthetic potency of compounds was determined
upon their intravenous administration to mice. In each
case the dose required to cause a loss of righting reflex
for a minimum period of 30 s in 50% of treated mice
after iv injection over 10 seconds was determined by
probit analysis. This dose is termed the HD50 (hypnotic
dose50). Propofol was injected as the commercial veter-
inary product (RapinovetTM, Schering-Plough Animal
Health, UK) at 10 mg mLÀ1. Compounds 1–5 and 8–16
(hydrochloride salts) were administered as 10 mg mLÀ1
solutions in distilled water. The in vitro effect of the
compounds at GABAA receptors was also assessed, by
determination of their ability to inhibit [35S]-tert-butyl-
bicyclophosphorothionate ([35S]TBPS) binding to rat
whole brain membranes. In each case the concentration
of drug required to inhibit 50% binding of this radio-
ligand was determined (TBPSIC 50). The in vitro and in
vivo results are shown in Table 1. All in vitro and in
vivo studies were carried out as described by Anderson
et al. except male MF-1 mice were used instead of male
CFLP mice for the in vivo studies.9
Results and Discussion
All final compounds (1–5 and 8–15) were converted to
the hydrochloride salt by passing hydrogen chloride gas
through a solution of the free amine in diethyl ether.
The hydrochoride salts that precipitated were filtered to
give water-soluble white solids, which were used for
pharmacological testing.
The in vivo anaesthetic potency of compounds (1–5 and
8–15) was determined upon their intravenous adminis-
tration to mice. Analogues containing a para-methyl-
amino substituent (1–5) compared very favourably with
propofol (HD50=68 mmol kgÀ1), the most potent being
compound 3 (HD50=19 mmol kgÀ1). Surprisingly, this
observation differs from the SAR for simple propofol-
like phenols where introduction of a para-methyl or
ethyl substituent resulted in lower hypnotic activity in
mice.10 To further explore the SAR around the para-
substituent the chain extended analogue 9 was synthe-
sised and was also found to retain good anaesthetic
potency (HD50=35.8 mmol kgÀ1).
Scheme 1. (i) Amine, Et3N, CH2Cl2; (ii) LiAlH4, THF; (iii) KBH4,
KOH, EtOH, H2O.
Previous studies had reported the importance of the
phenolic hydroxyl group for anaesthetic activity but
replacement with an aniline moiety had never been
investigated.10 All the aniline derivatives 10–12 exhib-
ited good hypnotic potency, especially compound 12
(HD50=14.4 mmol kgÀ1). To explore how important the
hydrogen bond donating moiety was for activity in this
Figure 2.
Table 1. Anaesthetic activity and GABAA receptor modulatory
effects of final compounds
Compound
HD50 mmol kgÀ1a
TBPSIC 50 (mM)b
Propofol
1
2
68.0
32.0
25.5
19.0
18
>100
27.6
3
31.0
4
5
8
9
10
11
12
13
14
15
16
38.8
25.1
68.5
35.8
36.5
22.9
14.4
>100
>10 (n=2)
>10
>100
>100
>100
na
>30
>10
>30
>100
Convulsant
>282
83.0
51.0
Scheme 2. (i) Hypophosphorus acid (50% aqueous), NaNO2; (ii)
acetyl chloride, Et3N, CH2Cl2; (iii) aqueous formaldehyde, formic
acid; (iv) formic acid, acetic anhydride; (v) LiAlH4, THF, reflux.
aMale MF-1 mice were used in all studies (n=8).
bValues are means of three experiments (na=not active).

A. Cooke et al. / Bioorg. Med. Chem. Lett. 11 (2001) 927–930
929
aniline series, compounds 13–16 were synthesised.
Complete removal of the aniline substituent gave com-
pound 13, which was convulsant. Conversion of the
aniline to the amide 14, which still contains a weak
hydrogen bond donor, dramatically reduced potency.
The mono-methyl aniline derivative 16, which has a
stronger hydrogen bond donor, retained some hypnotic
activity (HD50=51.0 mmol kgÀ1), while the dimethylaniline
derivative 15 was less active.
and the remaining six with 47.7 mmol kgÀ1 propofol.
The burst suppression ratio (BSR) was extracted from
the EEG as previously described.11 The following para-
meters were determined: the time to the first 0.25 s burst
suppression epoch (onset of anaesthesia), time to the
maximum BSR, time to 50% decrease in the BSR (offset
time) and the time to gain of the righting reflex (recovery
time). Compound 1 (hydrochloride salt) was dissolved in
distilled water at 20 mg mLÀ1. Propofol was injected as
the commercial veterinary product (RapinovetTM
Schering-Plough Animal Health, UK) at 10 mg mLÀ1
,
.
The in vitro effect of the compounds at GABAA recep-
tors was assessed by determination of their ability to
inhibit [35S]TBPS binding to rat whole brain mem-
branes. Most of the compounds were inactive or very
weakly active in this assay suggesting that their in vivo
anaesthetic activity is mediated by a non-GABAergic
mechanism.
The BSR profiles for propofol (33.7 mmol kgÀ1) and
compound 1 (hydrochloride salt, 51.0 mmol kgÀ1) are
shown in Figure 3. At these doses similar sleep times
were observed (approximately 4 min) and the gain of
righting reflex occurred around the same time as the
BSR returned to pre-drug levels. However, compound 1
(hydrochloride salt) produced more burst suppression
than propofol (55% and 39%, respectively). Another
comparison is to have two doses that produced the same
maximum burst suppression, unfortunately this did not
occur. However, it can be achieved by estimating the
dose producing 55% BSR for propofol based on linear
interpolation between the two doses as shown in Table
2. Compared to propofol, compound 1 (hydrochloride
salt) was slightly less potent and was slower in onset.
However, recovery from what is a reasonable depth of
anaesthesia was slightly faster with compound 1
(hydrochloride salt). Respiratory depression was a pro-
minent feature during the induction phase with both
compounds, but otherwise the sleeps were uneventful.
The anaesthetic effects of compound 1 and propofol
were also studied in male Wistar rats by extraction of
EEG parameters. Seventeen male Wistar rats (218–420 g
at the time of injection), previously implanted with
extradural electrodes, with a minimum of seven days
recovery period, were injected intravenously (10 s) via a
tail vein using a computer controlled infusion pump (Har-
vard 44, Harvard Apparatus). The animals were lightly
restrained by hand during induction of anaesthesia. Six
animals were injected with 51.0 mmol kgÀ1 compound 1
(hydrochloride salt), five with 33.7 mmol kgÀ1 propofol
Conclusion
A number of water-soluble derivatives of the intrave-
nous anaesthetic propofol were prepared and found to
retain good anaesthetic activity in mice. The anaesthetic
potency did not correlate with the ability of the com-
pounds to displace [35S]TBPS binding to rat whole brain
membranes, though compound 1 (hydrochloride salt)
was active in an EEG study of anaesthesia in rats. While
the anaesthetic activity may in part be related to the
ability of the compounds to allosterically modulate
GABAA receptors, the mechanism of action by which
these compounds mediate their anaesthetic effects is not
clear.
Figure 3. Burst suppression ratio (BSR) profiles for 33.7 mmol kgÀ1
propofol and 51.1 mmol kgÀ1 compound 1 (hydrochloride salt). The
lines represent the means of five and six rats, respectively. The triangles
represent the time to gain of the righting reflex (recovery time).
Table 2. Comparison between the anaesthetic profiles of compound 1
(hydrochloride salt) and propofol at 55% BSRa
Acknowledgements
Drug
Compound 1
(hydrochloride salt)
Propofol
39.2
The authors would like to thank the Analytical
Department, Organon, Newhouse.
Dose producing 55% BSR
)
51.0
(mmol kgÀ1
Onset time (s)
16.2Æ0.9
54.9Æ10.3
40Æ2.4
13.2Æ0.3
55
Maximum BSR (%)
Time to max. BSR (s)
Offset time (min)
30Æ2
References and Notes
1.7Æ0.9
1.4Æ0.2
Recovery time (min)
3.8Æ0.9
5.2Æ0.4
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aThe values for propofol are estimates based on linear interpolation
between the two doses tested. The values represent the mean and
SEM.

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A. Cooke et al. / Bioorg. Med. Chem. Lett. 11 (2001) 927–930
3. Trapani, G.; Altomare, C.; Sanna, E.; Biggio, G.; Liso, G.
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line hydrochloride 10: To a solution of 2,6-diisopropylaniline
(21 mL, 0.1 mol) in an ethanol (90 mL)/water (50 mL) mixture
was added morpholine (26.16 mL, 0.3 mol) and formaldehyde
(37 wt% in water, 24.35 g, 0.3 mol). The reaction mixture was
heated to reflux for 3 days and allowed to cool to room tem-
perature. The solution was partitioned between ethyl acetate
and water. The organic layer was separated, dried (Na2SO4)
and the solvent removed under reduced pressure. Chromato-
graphy on silica gel (gradient eluent: 2:1 petroleum ether/
EtOAc to EtOAc) gave the free amine of the title compound
as a brown oil. Conversion to the hydrochloride salt was
achieved by passing hydrogen chloride gas through a solution
of the free amine in diethyl ether. The hydrochoride salt that
precipitated was filtered to give the title compound as a white
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nol hydrochloride 1: To a solution of 2,6-diisopropylphenol
(17.83 g, 0.1 mol) in an ethanol (70 mL)/water (30 mL) mixture
was added morpholine (8.72 mL, 0.1 mol) and formaldehyde
(37 wt% in water, 8.12 g, 0.1 mol). The reaction mixture was
heated to reflux for 2 h and allowed to cool to room tempera-
ture. The solution was partitioned between ethyl acetate and
water. The organic layer was separated, dried (Na2SO4) and
the solvent removed under reduced pressure. Chromatography
of the crude product on silica gel (gradient eluent: dichloro-
methane to EtOAc) gave the free amine of the title compound
as an orange oil. Conversion to the hydrochloride salt was
achieved by passing hydrogen chloride gas through a solution
of the free amine in diethyl ether. The hydrochoride salt that
precipitated was filtered to give the title compound as a white
1
solid (20 g, 64%). Positive ion ESI (M+H)+ 277.2. H NMR
(CDCl3); d 1.26–1.28 (12H, d), 2.42–2.46 (4H, m), 2.89–2.98
(2H, septet), 3.42 (2H, s), 3.69–3.73 (4H, m), 6.95 (2H, s).
Elemental analysis: (C H N Cl) Found; C 57.67, H 8.61, N
7.61, Cl 18.25%. Calculated; C 57.86, H 8.81, N 7.94, Cl
18.59% (1.85 mol Cl and 0.5 mol H2O). Compounds 11 and
12 were synthesised in a similar manner.
9. Anderson, A.; Boyd, A. C.; Byford, A.; Campbell, A. C.;
Gemmell, D. K.; Hamilton, N. M.; Hill, D. R.; Hill-Venning,
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Peters, J. A.; Rees, D. C.; Stevenson, D.; Sundaram, H. J.
Med. Chem. 1997, 40, 1668.
1
solid (19 g, 61%). Positive ion ESI (M+H)+ 278.5. H NMR
(CDCl3+d5-pyridine); d 1.27–1.29 (12H, d), 2.96–3.10 (4H, br
s), 3.26–3.33 (2H, septet), 4.02–4.15 (6H, m), 7.24 (2H, s).
Elemental analysis: (C H N Cl) Found; C 65.01, H 8.93, N
4.35, Cl 10.54%. Calculated; C 65.06, H 9.09, N 4.46, Cl
10.17% (0.9 mol Cl and 0.2 mol H2O). Compounds 2–5 were
synthesised in a similar manner.
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415.