0022-3565/98/2853-0995$03.00/0
THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS Vol. 285, No. 3
Copyright © 1998 by The American Society for Pharmacology and Experimental Therapeutics Printed in U.S.A.
JPET 285:995 1004, 1998
Structure-Activity Relationships of Indole- and Pyrrole-Derived
Cannabinoids1
JENNY L. WILEY, DAVID R. COMPTON, DONG DAI, JULIA A. H. LAINTON, MICHELLE PHILLIPS, JOHN W. HUFFMAN
and BILLY R. MARTIN
Department of Pharmacology and Toxicology, Virginia Commonwealth University, Medical College of Virginia Campus, Richmond, Virginia
(J.L.W., D.R.C., B.R.M.) and Department of Chemistry, Clemson University, Clemson, South Carolina (D.D., J.A.H.L., M.P., J.W.H.)
Accepted for publication February 13, 1998 This paper is available online at http://www.jpet.org
ABSTRACT
Early molecular modeling studies with 9-tetrahydrocannabinol alepsy in mice and discriminative stimulus effects in rats. Re-
( 9-THC) reported that three discrete regions which interact ceptor affinity and potency of these novel cannabinoids were
with brain cannabinoid (CB1) receptors corresponded to the related to the length of the carbon chain. Short side chains
C-9 position of the cyclohexene ring, the phenolic hydroxyl and resulted in inactive compounds, whereas chains with 4 to 6
the carbon side chain at the C3 position. Although the location carbons produced optimal in vitro and in vivo activity. Pyrrole-
of these attachment points for aminoalkylindoles is less clear, derived cannabinoids were consistently less potent than were
the naphthalene ring, the carbonyl group and the morpholin- the corresponding indole derivatives and showed pronounced
oethyl group have been suggested as probable sites. In this separation of activity, in that potencies for hypomobility and
study, a series of indole- and pyrrole-derived cannabinoids was antinociception were severalfold higher than potencies for hy-
developed, in which the morpholinoethyl group was replaced pothermia and ring immobility. These results suggest that,
with another cyclic structure or with a carbon chain that more whereas the site of the morpholinoethyl group in these canna-
directly corresponded to the side chain of 9-THC and were binoids seems crucial for attachment to CB1 receptors, the
tested for CB1 binding affinity and in a battery of in vivo tests, exact structural constraints on this part of the molecule are not
including hypomobility, antinociception, hypothermia and cat- as strict as previously thought.
WIN 55,212, the prototypic aminoalkylindole cannabinoid, al., 1995a, b). Further, the pharmacological effects of 9-THC
is related structurally to pravadoline, a novel cyclooxygenase and WIN 55,212 are blocked by the cannabinoid antagonist,
inhibitor originally developed as an alternative to nonsteroi- SR 141716A (Pério et al., 1996; Rinaldi-Carmona et al., 1994;
dal anti-inflammatory drugs (Haubrich et al., 1990). Al- Wiley et al., 1995b), and chronic administration results in
though pravadoline is a weak anti-inflammatory agent, it
cross-tolerance to the hypomobility, hypothermia, antinoci-
possesses potent antinociceptive activity that apparently is
ceptive and cataleptic effects of these structurally distinct
unrelated to its inhibition of cyclooxygenase or to opioid
cannabinoids (Fan et al., 1994; Pertwee et al., 1993).
mechanisms. WIN 55,212 shares these antinociceptive ef-
Given the structural diversity of classical, bicyclic, anand-
fects with pravadoline (Compton et al., 1992). Because WIN
amide and aminoalkylindole cannabinoids, it is difficult to
55,212 and related aminoalkylindoles bind to brain cannabi-
imagine how these classes of drugs might bind to an identical
noid receptors (CB1), it has been suggested that these drugs
receptor. Enantiomer selectivity has been demonstrated in
produce their antinociceptive effects via cannabinoid mecha-
structure-activity relationship studies of classical and bicy-
nisms (D Ambra et al., 1992). Indeed, these drugs produce a
clic cannabinoids (Martin et al., 1981), which suggests that a
profile of behavioral effects that resemble those of 9-THC
minimum of three sites of attachment are required for recep-
and other classical and bicyclic cannabinoids, including sup-
tor binding and activation. The original three-point attach-
pression of spontaneous activity, antinociception, decreased
ment model proposed the following sites for 9-THC and
rectal temperature and ring immobility in mice (Compton et
similar classical tricyclic and bicyclic cannabinoids: (1) the
al., 1992) and cannabimimetic discriminative stimulus ef-
C-9 position of the cyclohexene ring, (2) a phenolic hydroxyl
fects in rats and rhesus monkeys (Pério et al., 1996; Wiley et
and (3) a nonpolar side chain at the C3 position (Binder and
Franke, 1982; Edery et al., 1971; Razdan, 1986; Thomas et
Received for publication December 16, 1997.
1 al., 1991). Although the discovery of anandamide (Devane et
Research supported by National Institute on Drug Abuse Grants DA-
03672 (B.R.M.) and DA-03590 (J.W.H.). al., 1992) and increased recognition of the importance of the
ABBREVIATIONS: DD, drug discrimination; MPE, maximal possible antinociceptive effect; RI, ring immobility; RT, rectal temperature; SA,
spontaneous activity; 9-THC, 9-tetrahydrocannabinol; BSA, bovine serum albumin; EDTA, ethylenediaminetetraacetic acid.
995
996 Wiley et al.
Vol. 285
geometry of the C-9 substituent (e.g., Reggio et al., 1989), as weight range by restricted postsession feeding. Rodents were drug
naive at the beginning of the study. Separate mice were used for
well as subsequent findings (Huffman et al., 1996; Martin et
testing each drug dose in the in vivo behavioral procedures. Brain
al., 1995), have eroded the validity of these specific putative
tissue for binding studies was obtained from male Sprague-Dawley
sites of attachment, the model still serves as an excellent
rats (150 200 g) obtained from Dominion Laboratories (Dublin, VA),
template for making structural comparisons between classi-
which were maintained on a 14:10 hr light/dark cycle and received
cal cannabinoids and aminoalkylindoles. Huffman et al.
food and water ad libitum.
(1994) suggested that the structure of the aminoalkylindole
Apparatus. Measurement of spontaneous activity in mice oc-
cannabinoids might conform to a three-point attachment
curred in standard activity chambers interfaced with a Digiscan
model with points of attachment at the naphthalene ring at
Animal Activity Monitor (Omnitech Electronics, Inc., Columbus,
the C7 position, the carbonyl group and the morpholinoethyl
OH). A standard tail-flick apparatus (described by Dewey et al.,
group (fig. 1). Eissenstat et al. (1995) proposed that the
1970) and a telethermometer (Yellow Springs Instrument Co., Yel-
morpholinoethyl group or another cyclic structure was re- low Springs, OH) were used to measure antinociception and rectal
quired for binding and cannabimimetic activity of aminoal- temperature, respectively. The ring immobility device (described by
Pertwee, 1972) consisted of an elevated metal ring (diameter, 5.5 cm;
kylindoles; however, classical cannabinoids and anandamide
height, 16 cm) attached to a wooden stand.
do not possess such a cyclic structure but rather have a
Drug discrimination training and testing used standard operant
carbon side chain at this location.
conditioning chambers (Lafayette Instruments Co., Lafayette, IN)
In the present study, a series of indole- and pyrrole-derived
housed in sound-attenuated cubicles. A pellet dispenser delivered
cannabinoids were developed in which a carbon chain of
45-mg BIO SERV (Frenchtown, NJ) food pellets to a cup located
varying lengths was substituted for the morpholinoethyl
between two response levers mounted on the front wall of the cham-
group. For purposes of comparison, selected compounds with
ber. Fan motors provided ventilation and masking noise for each
substitution of a saturated or unsaturated cyclic structure for
chamber. Four-watt houselights were located above each lever; both
the morpholinoethyl group of WIN 55,212 were synthesized,
were illuminated during training and testing sessions.
as were several compounds in which the carbon chain con-
Drugs. 9-THC (National Institute on Drug Abuse, Rockville,
tained at least one double bond. All compounds were tested in
MD) and CP 55,940 (Pfizer, Groton, CT) were suspended in a vehicle
vitro for displacement of CP 55,940 binding and, whenever of absolute ethanol, Emulphor-620 (Rhone-Poulenc, Inc., Princeton,
solubility allowed, they were tested in vivo in procedures in NJ) and saline in a ratio of 1:1:18. Novel indole- and pyrrole-derived
cannabinoids were synthesized in our laboratories (Clemson Univer-
which cannabinoids produce a characteristic profile of effects
sity, Clemson, SC) and also were mixed in a 1:1:18 vehicle. In mice,
in mice (Martin et al., 1991). Selected compounds also were
drugs were administered i.v. in the tail vein at a volume of 0.1
tested in rat cannabinoid discrimination procedures. Canna-
ml/10g. In rats, all drugs were administered i.p. at a volume of 1
binoid discrimination represents an animal model of the sub-
ml/kg.
jective effects of this class of compounds in humans (Balster
Membrane preparation and binding. [3H]CP 55,940 (KD 690
and Prescott, 1992). In addition, for classical cannabinoids,
nM) binding to P2 membranes was conducted as described elsewhere
potencies in these in vivo procedures with mice and rats show
(Compton et al., 1993), except whole brain (rather than cortex only)
strong positive correlations with binding affinity at CB1 re-
was used. The assays were performed in triplicate, and the results
ceptors (Compton et al., 1993). Synthesis procedures and
represent the combined data from three individual experiments.
preliminary pharmacological data for some of these com-
Detailed information on the membrane preparation and binding
pounds have been reported previously (Huffman et al., 1994;
assay are provided below.
Lainton et al., 1995). The methods for tissue preparation were those described by Dev-
ane et al. (1988). After decapitation and the rapid removal of the
brain, the cortex was dissected free with use of visual landmarks
Methods
following reflection of cortical material from the midline. The cortex
was immersed in 30 ml of ice-cold centrifugation solution (320 mM
Subjects. Adult male Sprague-Dawley rats (290 350 g), obtained
sucrose, 2 mM TrisEDTA, 5 mM MgCl2). The process was repeated
from Charles River (Wilmington, MA), were housed individually.
until the cortices of five rats were combined. The cortical material
Male ICR mice (25 32 g), obtained from Harlan (Dublin, VA), were
was homogenized with a Kontes Potter-Elvehjem glass-Teflon grind-
housed in groups of five. All animals were kept in a temperature-
ing system (Fisher Scientific, Springfield, NJ). The homogenate was
controlled (20 22°C) environment with a 12-hr light-dark cycle
centrifuged at 1600 g for 10 min, and the resulting pellet was
(lights on at 7 A.M.). Rats were maintained within the indicated
termed P1. The supernatant was saved and combined with the two
subsequent supernatants obtained from washing of the P1 pellet. The
combined supernatant fractions were centrifuged at 39,000 g for
15 min, resulting in the P2 pellet. This pellet was resuspended in 50
ml of buffer A (50 mM TrisHCl, 2 mM TrisEDTA, 5 mM MgCl2, pH
7.0), incubated for 10 min at 37°C, then centrifuged at 23,000 g for
10 min. The P2 membrane was resuspended in 50 ml of buffer A,
incubated again except at 30°C for 40 min, then centrifuged at
11,000 g for 15 min. The final wash-treated P2 pellet was resus-
pended in assay buffer B (50 mM TrisHCl, 1 mM TrisEDTA, 3 mM
MgCl2, pH 7.4) to a protein concentration of approximately 2 mg/ml.
The membrane preparation was divided into four equal aliquots and
quickly frozen in a bath solution of dry ice and 2-methylbutane
Fig. 1. Chemical structures of WIN 55,212 2 and 9-THC with pre-
(Sigma Chemical Co., St. Louis, MO), then stored at 80°C for no
sumed three points of attachment marked by letters for each compound,
more than 2 weeks. Before performing a binding assay an aliquot of
respectively: (a) naphthalene ring and cyclohexene ring, (b) carbonyl
frozen membrane was thawed rapidly and protein values were de-
group and phenolic hydroxyl and (c) morpholinoethyl group and carbon
side chain at C3. termined by the method of Bradford (1976) with Coomassie brilliant
1998
Indole-Derived Cannabinoids 997
blue dye (Bio-Rad, Richmond, CA) and BSA standards (fatty acid the ratio requirement on the correct lever. The schedule of daily
free, Sigma Chemical Co., St. Louis, MO) prepared in assay buffer. injections for each rat was administered in a double-alternation
The methods for ligand binding were essentially those described sequence of drug and vehicle. Both groups of rats were injected and
by Devane et al. (1988), except that the assay described here is a returned to their home cages for 30 min until the start of the
filtration assay. Binding was initiated by the addition of 150 gof P2 experimental session. Acquisition training occurred during 15-min
membrane to test tubes containing [3H]CP 55,940 (79 Ci/mmol), a sessions 5 days a week (Monday through Friday) until the rats had
cannabinoid analog (for displacement studies) and a sufficient quan- met three criteria during 10 consecutive sessions: (1) first completed
tity of buffer C (50 mM TrisHCl, 1 mM TrisEDTA, 3 mM MgCl2, 5 FR-10 on the correct lever; (2) percentage of correct-lever responding
mg/ml BSA) to bring the total incubation volume to 1 ml. The con- 80%; and (3) response rate 0.5 responses/sec.
centration of [3H]CP 55,940 in displacement studies was 1 nM. Substitution tests with novel indole- and pyrrole-derived canna-
Nonspecific binding was determined by the addition of 1 M unla- binoids (CP 55,940 and 9-THC groups, respectively) were conducted
beled CP 55,940. CP 55,940 and all cannabinoid analogs were pre- on Tuesdays and Fridays with continued training during sessions on
pared by suspension in buffer C from a 1 mg/ml ethanolic stock. Mondays, Wednesdays and Thursdays. During test sessions, consec-
After incubation at 30°C for 1 hr, binding was terminated by utive responses on either lever delivered reinforcement according to
addition of 2 ml of ice-cold buffer D and vacuum filtration through a FR-10 schedule. To be tested, rats must have met the three acqui-
pretreated filters in a 12-well sampling manifold (Millipore, Bedford, sition criteria (see above) during at least one of the vehicle training
MA). Reaction vessels were washed once with 2 ml of ice-cold buffer sessions and at least one of the drug training sessions occurring
D (50 mM TrisHCl, 1 mg/ml BSA), and the filters were washed twice within the week before testing. Test sessions lasted 15 min. All rats
with 4 ml of ice-cold buffer D. Filters were placed into 20-ml plastic were tested with their training drug ( 9-THC or CP 55,940) before
scintillation vials (Packard, Downer Grove, IL) with 1 ml of distilled being tested with any of the other compounds. Control tests with
water and 10 ml of Budget-Solve (RPI Corp., Mount Prospect, IL). vehicle and 9-THC (3 mg/kg) or CP 55,940 (0.1 mg/kg) were con-
After shaking for 1 hr the quantity of radioactivity present was ducted before each dose-effect curve determination. A within-sub-
determined by liquid scintillation spectrometry. jects design was used to test all indole (CP 55,940 group)- and pyrrole
Assay conditions were performed in triplicate, and the results ( 9-THC group)-derived cannabinoids, such that each rat received all
represent the combined data of three to six independent experi- doses of each test compound presented in ascending order.
ments. All assays were performed in siliconized test tubes, which Data analysis. Based on data obtained from numerous previous
studies with cannabinoids, maximal cannabinoid effects in each pro-
were prepared by air drying (12 hr) the inverted borosilicate tubes
cedure were estimated as follows: 90% inhibition of spontaneous
after two rinses with a 0.1% solution of AquaSil (Pierce, Rockford,
activity, 100% MPE in the tail-flick procedure, 6°C change in rectal
IL). The GF/C glassfiber filters (2.4 cm, Baxter, McGaw Park, IL)
were immersed before use in a 0.1% solution of pH 7.4 polyethyleni- temperature and 60% ring immobility. ED50 values were defined as
the dose at which half-maximal effect occurred. For drugs that pro-
mine (Sigma Chemical Co., St. Louis, MO) for at least 6 hr.
duced one or more cannabinoid effect, ED50 values were calculated
Mouse behavioral procedures. Before testing in the behavioral
procedures, mice were acclimated to the experimental setting (am- separately by least-squares linear regression on the linear part of the
dose-effect curve for each measure in the mouse tetrad, plotted
bient temperature, 22 24°C) overnight. Preinjection control values
against log10 transformation of the dose.
were determined for rectal temperature and tail-flick latency (in
For each rat drug discrimination test session, percentage of re-
seconds). Five minutes after i.v. injection with drug or vehicle, mice
were placed in individual activity chambers and spontaneous activ- sponses on the drug lever and response rate (responses/sec) were
calculated. When appropriate, ED50 values (with 95% confidence
ity was measured for 10 min. Activity was measured as total number
of interruptions of 16 photocell beams per chamber during the 10- intervals) were calculated separately for each drug by least-squares
linear regression on the linear part of the dose-effect curves (Tal-
min test and expressed as percent inhibition of activity of the vehicle
group. Tail-flick latency was measured at 20 min postinjection. Max- larida and Murray, 1987) for percentage of drug-lever responding,
plotted against log10 transformation of the dose. Because rats that
imum latency of 10 sec was used. Antinociception was calculated as
percent of maximum possible effect {%MPE [(test control la- responded 10 times or less during a test session did not press either
lever a sufficient number of times to earn a reinforcer, their lever
tency)/(10 control)] 100}. Control latencies typically ranged from
selection data were excluded from data analysis. For the purposes of
1.5 to 4.0 sec. At 1.5 hr postinjection, each mouse was placed on the
potency comparison, potencies were expressed as micromoles per
ring immobility apparatus for 5 min, during which the total amount
of time (in seconds) that the mouse remained motionless was mea- kilogram for both rat and mice data.
Pearson product-moment correlation coefficients (with associated
sured. This value was divided by 300 sec and multiplied by 100 to
significance tests) were calculated between binding affinity (ex-
obtain a percent immobility rating. The criterion for ring immobility
pressed as log Ki) and in vivo potency for each measure (expressed as
was the absence of all voluntary movement, including snout and
whisker movement. Rectal temperature was expressed as the differ- log ED50 in micromoles per kilogram) for all cannabinoid compounds
ence between control temperature (before injection) and tempera- that bound to the CB1 receptor with a Ki less than 10,000 nM. In
addition, multiple linear regression was used to calculate the overall
tures after drug administration ( °C). During the course of this
degree of relationship between binding affinity and potency in the
extended study, the ring immobility test was discontinued and the
mouse tetrad measures.
time at which rectal temperature was measured was changed. For
compounds that were tested in the ring immobility assay, rectal
temperature was measured at 60 min postinjection; for compounds
Results
that were not tested in this procedure, rectal temperature was mea-
sured at 30 min postinjection. Different mice (n 5 6/dose) were
Binding affinities. Tables 1 and 2 contain binding affin-
tested for each dose of each compound. Each mouse was tested in
ities for the indole series with and without a methyl at the
each of the three or four procedures.
2-position of the indole and the pyrrole series (all without a
Rat drug discrimination procedure. Two groups of rats were
methyl at the 2-position of the pyrrole). In each series, ma-
trained to press one lever after injection with 9-THC (3 mg/kg; n
nipulation of the length of the carbon chain resulted in an
10) or CP 55,940 (0.1 mg/kg; n 8) and to press another lever after
inverted U-shaped function for binding affinities. In all three
administration of vehicle to obtain food reinforcement under a fixed-
series, substitution of a methyl group for the morpholin-
ratio 10 (FR-10) schedule of food reinforcement. The position of the
oethyl substituent produced a compound that did not bind to
reinforced (correct) lever was determined by the type of injection the
rat received on a given day. A response on the incorrect lever reset CB1 receptors. In both indole series, binding affinity steadily
998 Wiley et al.
Vol. 285
TABLE 1
Binding affinity and in vivo potency of 9-THC, WIN 55,212-2 and indole-derived cannabinoids
For all tables, no max indicates that the compound produced only slightly greater than 50% of the presumed maximal effect. dose indicates an estimated ED50 because
the dose-effect curve was not linear. dose indicates that 50% activity was not achieved at this dose, which was the highest dose of the compound that was tested. NT
not tested. All ED50 values are expressed as micromoles per kilogram. SA, suppression of spontaneous activity; MPE, % maximum possible antinociceptive effect in the
tail-flick assay; RT, rectal temperature; RI, ring immobility; rat DD, drug discrimination in rats.
Compound Ki SA MPE RT RI Rat DD
nM
9-THC 41 0.92 2.7 2.5 NT 1.8
WIN 55,212-2 24 0.19 1.4 1.5 NT NT
RR Ki SA MPE RT RI Rat DD
nM
n-Methyl CH3 10,000 NT NT NT NT NT
n-Ethyl CH3 1180 44 NT NT NT NT NT
n-Propyl CH3 164 22 18.7 84.7 99.1 87.2 45.9
n-Butyl CH3 22 1.5 2.6 0.23 4.1 12.9 7.3
n-Pentyl CH3 9.5 4.5 0.70 0.25 4.3 1.9 8.5
n-Hexyl CH3 48 13 2.7 9.5 17.1 16.0 19.8
n-Heptyl CH3 10,000 117 261 261 261 261
n-Methyl H 10,000 35.1 35.1 35.1 NT NT
n-Ethyl H 1390 123 NT NT NT NT NT
n-Propyl H 1050 55 NT NT NT NT NT
n-Butyl H 8.9 1.8 0.34 1.3 3.3 NT NT
n-Pentyl H 9 5 0.44 0.09 1.7 3.9 NT
n-Hexyl H 9.8 2 0.96 0.73 1.5 NT NT
n-Heptyl H 128 17 56.9 17.6 81.3 NT NT
TABLE 2
Binding affinity and in vivo potency of nonmethylated pyrrole-derived cannabinoidsa
RR Ki SA MPE RT RI Rat DD
nM
n-Methyl H 10,000 106 No max 53.3 NT NT
n-Ethyl H 10,000 84.2 No max 77.2 NT NT
n-Propyl H 10,000 85.9 81.8 90.1 NT NT
n-Butyl H 666 77 No max No max 108.3 NT NT
n-Pentyl H 87 3 3.6 1.2 78.8 98.9 16.1
n-Hexyl H 399 109 8.8 9.6 62.8 67.8 22.3
n-Heptyl H 309 11 11.0 9.7 52.1 No max NT
a
See legend to table 1.
increased with the addition of each carbon until maximum Table 3 shows the results of manipulation of the placement
affinity was demonstrated for the 2-methyl-n-pentyl indole of a double bond in the carbon chain of selected methylated
and the nonmethylated n-butyl, n-pentyl and n-hexyl indoles, and nonmethylated indoles. Addition of a double bond by
each of which had approximately 2.5 times greater receptor substitution of an allyl group for propyl, E-2-pentenyl or
affinity than WIN 55,212 2 and 4 times greater affinity than 4-pentenyl for the pentyl of the methylated and nonmethyl-
9-THC (table 1). Similarly, optimal affinity for the pyrrole ated indoles resulted in compounds that had at least 3-fold
series was observed for the n-pentyl pyrrole (table 2); how- less affinity for the CB1 receptor than the corresponding
ever, the affinity of this compound for the CB1 receptor was parent compound in the indole series. Substitution of another
2 and 3.6 times less than affinities of WIN 55,212 2 and cyclic structure for morpholinoethyl group resulted in com-
9-THC, respectively. With only one exception, affinities of pounds that had 2- to 52-fold less affinity for the CB1 recep-
the pyrrole compounds for the CB1 receptor were consis- tor than did WIN 55,212 2 (table 3).
tently lower than the comparable compounds in both series of Structure-activity relationship in mice. 9-THC and
indoles. The exception is that n-heptyl pyrrole showed weak WIN 55,212 2 produced a characteristic cannabinoid profile
affinity for the CB1 receptor, whereas the 2-methyl-n-heptyl of in vivo effects in mice which included suppression of spon-
indole did not bind to this receptor. taneous activity, antinociception and hypothermia. Whereas
1998
Indole-Derived Cannabinoids 999
TABLE 3
Binding affinity and in vivo potency of indole-derived cannabinoids with placement of a double bond on the alkyl chain or substitution of a cyclic
structure for the morpholinoethyl groupa
RR Ki SA MPE RT RI Rat DD
nM
(E)-2-Pentenyl CH3 340 184 1.7 2.8 32.4 30.4 NT
4-Pentenyl CH3 38 13 3.5 0.34 5.6 9.9 NT
Allyl CH3 4518 187 Inactive Inactive Inactive Inactive NT
(E)-2-Pentenyl H 58 14 No max 3.1 Inactive NT NT
4-Pentenyl H 43 21 0.88 0.15 2.4 2.1 NT
2-Phenylethyl CH3 1250 250 No max No max No max No max NT
Cyclohexylethyl CH3 46 13 55.4 58.7 69.7 118 NT
Cyclopropylmethyl CH3 140 44 6.5 7.1 36.9 83.1 NT
a
See legend to table 1.
each drug produced antinociceptive and hypothermic effects moderately potent, in the spontaneous activity and tail-flick
with similar potencies across measures, both drugs were procedures, but produced slightly below half-maximal de-
more potent at decreasing spontaneous activity than they creases in rectal temperature at doses up to 30 mg/kg. The
were at producing the other two effects (table 1); however, remaining compounds in this series showed approximately
greater separation of locomotor and antinociceptive/hypo- equal affinities for CB1 receptors and each produced potent
thermic effects was obtained with WIN 55,212 2 than with cannabinoid effects on spontaneous activity, tail flick and
9-THC (7-fold vs. 3-fold difference, respectively). Consistent rectal temperature. The nonmethylated pentyl indole pro-
with its higher binding affinity at CB1 receptors, WIN duced 100% MPE at 1 mg/kg, but the dose-effect curve was
55,212 2 was more potent than 9-THC in all three proce- not linear; hence, an ED50 could not be calculated but was
dures. estimated as 0.03 mg/kg. As with the methylated indole
The methylated methyl and ethyl indole derivatives could derivatives, active nonmethylated indole derivatives were
not be dissolved in the vehicle at concentrations necessary for less potent at decreasing rectal temperature than at produc-
in vivo tests; hence, they were not tested. The 2-methyl-n- ing antinociception and hypomobility.
heptyl indole derivative was tested at doses up to 100 mg/kg Despite their lack of binding affinity at CB1 receptors,
(table 1). Although it decreased spontaneous activity at methyl and ethyl pyrroles were efficacious in the spontane-
higher concentrations, it was inactive in the rectal tempera- ous activity and rectal temperature assays, but produced
ture and ring immobility procedures at doses up to 100 mg/kg only 54 to 55% MPE at a dose of 56 mg/kg in the tail-flick
and produced a maximum of only 40 to 57% MPE across a procedure (table 2). The propyl pyrrole, which also did not
dose range of 30 to 100 mg/kg. In contrast, the methylated bind to CB1 receptors, was active in three of the in vivo
n-propyl, n-butyl, n-pentyl and n-hexyl indole derivatives procedures; however, none of these three pyrrole derivatives
produced characteristic cannabinoid effects in the mice on all with the shortest alkyl chains were very potent compared
four measures. For each compound, potencies for hypother- with nonmethylated indole or other pyrrole compounds (table
mia and ring immobility were lower than potencies for hypo- 2). In contrast, the butyl pyrrole showed greater binding
mobility and antinociception, although the magnitude of the affinity, but was not fully efficacious in any procedure. Up to
potency differences was variable across compounds. Rank doses of 30 mg/kg, this compound inhibited spontaneous ac-
order potencies for each of the in vivo effects corresponded tivity and %MPE to a maximum of 69% in a dose-related
with rank order binding affinities with two exceptions. Al- manner. Similar to corresponding compounds in both indole
though the 2-methyl-n-pentyl indole showed 2-fold higher series, pentyl and hexyl pyrroles were efficacious in all four
affinity for CB1 receptors than did the 2-methyl-n-butyl in- procedures and were less potent at producing hypothermia
dole, these two compounds were approximately equipotent in and ring immobility. In addition, each of these compounds
producing antinociception and hypothermia. Second, the was less potent at inducing hypomobility and antinociception
2-methyl-n-hexyl indole was more potent at decreasing spon- than were the corresponding nonmethylated indole com-
taneous activity than was the 2-methyl-n-butyl indole, even pounds. However, the reverse was true for the heptyl pyrrole
though the latter compound exhibited 2-fold higher affinity which showed greater potency at decreasing spontaneous
for CB1 receptors. activity and rectal temperature and at increasing antinoci-
In the nonmethylated indole series (table 1), none of the ception than did the methylated and nonmethylated heptyl
compounds was tested in the ring immobility task, and ethyl indoles. At doses up to 30 mg/kg, the nonmethylated heptyl
and propyl derivatives were not tested in any measure be- indole increased percent ring immobility only to approxi-
cause of their low solubility. Consistent with its lack of bind- mately half-maximal levels.
ing affinity to CB1 receptors, the nonmethylated methyl in- Substitution of an E-3-pentenyl or 4-pentenyl group for the
dole derivative was inactive in each of the three assays, n-pentyl group in each indole series resulted in active com-
although it was only soluble up to a dose of 10 mg/kg. The pounds with less affinity, but greater potency, than corre-
nonmethylated heptyl indole was fully efficacious, but only sponding analogs with a saturated substituent. In contrast,
1000 Wiley et al.
Vol. 285
an allyl group in place of the propyl group in the methylated F(4,12) 8.4, P .002). Individual correlations between log Ki
indole series resulted in an inactive compound at doses up to and log potency for each measure were 0.65, 0.58, 0.83 and 0.71
100 mg/kg. for hypomobility, antinociception, hypothermia and ring immo-
Substitution of a carbocyclic structure for the morpholin- bility, respectively (P .05 for all four correlations). Scatter-
oethyl group of the WIN series resulted in compounds with plots for each regression line are presented in figure 2.
less affinity and less potency than the parent compound. Drug discrimination in rats. As expected, CP 55,940
Substitution of a 2-phenylethyl group produced an inactive and 9-THC produced dose-dependent substitution for CP
compound whereas substitution of a 2-cyclohexylethyl or 55,940 (fig. 3, top left panel) with decreases in response rates
1-cyclopropylmethyl group resulted in compounds that were occurring at higher doses (fig. 3, bottom left panel). Although
maximally efficacious, but less potent than WIN 55,212 2. the indole-derived cannabinoids are structurally different
As with the indole- and pyrrole-derived compounds, poten- from both classical and bicyclic cannabinoids, four of the five
cies for producing hypothermia and ring immobility were less compounds, including the 2-methyl-n-propyl and n-butyl in-
than potencies for hypomobility and antinociception. doles (fig. 3, left panel) and the 2-methyl-n-pentyl and n-
Multiple regression analysis of binding affinity (Y log Ki) hexyl indoles (fig. 3, right panel), fully substituted for CP
and potency for each measure in the tetrad (X1 4 log ED50 55,940. With the exception of the 2-methyl-n-pentyl indole,
in mol/kg) confirmed that overall potency at producing the substitution was linear and dose-dependent; however, the
characteristic profile of cannabinoid effects was correlated 2-methyl-n-pentyl indole fully substituted at higher doses.
significantly with binding affinity at CB1 receptors (r 0.86; Decreases in response rates, if they occurred at all, were seen
Fig. 2. Scatterplots and regression lines of log Ki plotted
against log ED50 for each of the five in vivo tests (SA,
spontaneous activity; MPE, % maximum possible antino-
ciceptive effect; RT, change in rectal temperature; RI,
ring immobility; DD, rat drug discrimination).
1998
Indole-Derived Cannabinoids 1001
Fig. 3. Effects of CP 55,940, methylated indole-derived cannabinoids and 9-THC on percentage of CP 55,940-lever responding (upper panels) and
response rates (lower panels) in rats trained to discriminate CP 55,940 (0.1 mg/kg) from vehicle. Points above VEH and CP represent the results of
control tests with vehicle and 0.1 mg/kg CP 55,940 conducted before each dose-effect curve determination. For all dose-effect curve determinations,
each value represents the mean ( S.E.M.) of five to eight rats, except as indicated on the graph. ED50 values were 0.02 mg/kg for CP 55,940, 1.2 mg/kg
for 9-THC, 15 mg/kg for n-propyl, 2.5 for n-butyl, 1 mg/kg for n-pentyl, 7.3 mg/kg for n-hexyl and 100 mg/kg for n-heptyl. Values in table 1 are
listed in units of micromoles per kilogram.
only at higher doses (fig. 3, bottom panels). In contrast, the ring (corresponding to the cyclohexene ring of 9-THC and
2-methyl-n-heptyl indole-derived cannabinoid did not have the polyolefin loop of anandamide); (2) the carbonyl group
any effect on percentage of CP 55,940-lever responding or on (corresponding to the phenolic hydroxyl of 9-THC and the
response rates at doses up to 100 mg/kg (fig. 3, right panels). ethanol hydroxyl group of anandamide); and (3) the morpho-
In rats trained to discriminate 9-THC from vehicle, 9- linoethyl group (corresponding to the carbon side chain at C3
THC produced dose-dependent substitution with response of 9-THC and the five terminal carbons of anandamide)
rate decreases occurring at the higher doses (fig. 4). Similar (Huffman et al., 1994; Thomas et al., 1991, 1996). Previous
to the results with the corresponding 2-methyl indoles, pen- investigations of the structure-activity relationships of ami-
tyl and hexyl pyrrole-derived cannabinoids produced full noalkylindoles have confirmed the importance of the naph-
dose-dependent substitution for 9-THC without decreasing thalene or similar ring structure (e.g., benzofuryl derivatives)
response rates (fig. 4). Throughout both drug discrimination at position a (see fig. 1) for in vitro and in vivo activity
studies, rats responded predominantly on the injection-ap- (Compton et al., 1992; Eissenstat et al., 1995). All the com-
propriate lever during control tests with vehicle, 0.1 mg/kg pounds included in the present study contain this naphtha-
CP 55,940 and 3 mg/kg 9-THC (figs. 3 and 4). Rank order lene ring structure at position a. Manipulation of the car-
potencies were consistent with the rank order of binding bonyl group of the WIN series has not been examined
affinities of each indole and pyrrole compound (tables 1 and (position b, fig. 1).
2, respectively). For drug discrimination, the correlation be- In the present study, the importance of the morpholin-
tween log Ki and log ED50 was 0.71 (P .18) (fig. 2). oethyl group of aminoalkylindoles was investigated. Eissen-
stat et al. (1995) proposed that, in the aminoalkylindole se-
ries, the morpholinoethyl group or another cyclic structure in
Discussion
the same position (position c, fig. 1) is also required for
Molecular modeling studies have suggested that, similar to activity. Although indole-derived cannabinoids with substi-
classical cannabinoids and anandamide, aminoalkylindole tution of cyclohexylethyl and cyclopropylmethyl, but not phe-
cannabinoids have at least three discrete regions that inter- nylethyl, for the morpholinoethyl group of WIN 55,212 retain
act with the CB1 receptor upon binding: (1) the naphthalene reasonable CB1 affinity and in vivo activity, a cyclic struc-
1002 Wiley et al.
Vol. 285
variables. Increasing bulk at the C2 position in aminoalky-
lindole cannabinoids greatly decreased affinity for CB1 re-
ceptors (Eissenstat et al., 1995).
Because high CB1 affinity was seen with the pentyl indole-
derived compound in both indole series, we chose this chain
length for the addition of a double bond into the side chain.
(E)-2-Pentenyl and 4-pentenyl analogs of methylated and
nonmethylated pentyl indoles were investigated, as was sub-
stitution of an allyl group for the propyl of the corresponding
methylated indole. All these indole derivatives with more
rigid alkyl chains were less active in vitro and in vivo than
their corresponding parent compounds in the original indole
series. These results suggest that, although the ability of this
alkyl chain to rotate freely is not necessary for cannabimi-
metic activity, it is important in predicting potency of indole-
derived cannabinoid compounds. The largest decrease in
binding affinity was observed in the allyl and (E)-2-pentenyl
analogs, although in vivo potency was less affected by the
latter manipulation. The systematic exploration of the effect
of increasing the rigidity of the carbon side chain of classical
cannabinoids has not yet been reported.
The benzenoid ring attached to the nitrogen-containing
group of the indole portion of aminoalkylindole compounds
does not correspond to any of the three hypothesized points of
attachment and, theoretically, should be unnecessary for
cannabimimetic activity. In an attempt to test this hypothe-
sis, a series of pyrrole analogs of the nonmethylated indole
series was prepared (Lainton et al., 1995). One of the effects
of this manipulation was to eliminate receptor binding of
compounds with short alkyl chains; hence, whereas ethyl and
Fig. 4. Effects of 9-THC and pyrrole-derived cannabinoids on percent-
propyl nonmethylated indoles had measurable binding affin-
age of 9-THC-lever responding (upper panel) and response rates (lower
panel) in rats trained to discriminate 9-THC (3 mg/kg) from vehicle.
ity at CB1 receptors, the corresponding ethyl and propyl
Points above VEH and THC represent the results of control tests with
pyrrole compounds did not, although they were weakly active
vehicle and 3 mg/kg 9-THC conducted before each dose-effect curve
in some of the in vivo tests. For longer alkyl chains (butyl to
determination. For all dose-effect curve determinations, each value rep-
resents the mean ( S.E.M.) of seven to ten rats, except as indicated on
heptyl), the pyrrole series showed severely decreased affinity
the graph. ED50 values were 0.6 mg/kg for 9-THC, 4.7 mg/kg for n-pentyl
for the CB1 receptor (9 74-fold) and usually a decrease in vivo
and 6.8 mg/kg for n-hexyl. Values in table 2 are listed in units of micro-
potency, although there were minor exceptions. Similar to both
moles per kilogram.
indole series, highest binding affinity and potency was observed
ture is unnecessary, in that it may be replaced with an alkyl for the n-pentyl pyrrole compound. Cannabimimetic pyrroles
chain that corresponds more directly in structure to the li- were approximately equipotent in decreasing locomotor activity
pophilic portion of 9-THC and other classical cannabinoids. and producing antinociception; however, a consistent and pro-
As with classical and bicyclic cannabinoids and anandamide nounced separation of activity was observed between potencies
(Compton et al., 1993; Ryan et al., 1997; Seltzman et al., for these two measures and their 5- to 37-fold lower potencies
1997), the length of this alkyl chain is important in prediction for producing hypothermia and ring immobility. A similar sep-
of binding affinity and in vivo potency of both methylated and aration of activity was observed with a few of the indole-derived
nonmethylated indoles (table 1). Indoles with short chain cannabinoids [e.g., 2-methyl-n-pentyl and n-(E)-2-pentenyl in-
lengths (methyl or ethyl) either did not bind to the CB1 doles and the n-cyclopropylmethyl indole], although active com-
receptor or showed only weak cannabimimetic affinity and pounds from both indole and pyrrole series were fully effica-
activity. Maximal displacement of [3H]CP 55,940 and in vivo cious in all procedures in most instances (exceptions noted on
potency occurred with butyl through hexyl indole derivatives. tables).
A similar pattern of length-dependent activity was observed Despite the structural diversity of these indole- and pyr-
in previous studies in which some of the methylated indoles role-derived cannabinoids, overall potency at producing the
were tested in 9-THC discrimination in rhesus monkeys characteristic profile of cannabinoid effects in mice was signif-
(Wiley et al., 1995a) and in an isolated vas deferens assay in icantly correlated with binding affinity at CB1 receptors across
mice (Pertwee et al., 1995). In the indoles lacking a 2-methyl all series (r 0.86; P .05). Although the overall correlation
group, the net effect of substitution of hydrogen for the between potency in the tetrad measures and binding affinity for
methyl group was to increase the number of carbons needed the indole- and pyrrole-derived cannabinoids was similar to
for maximal binding affinity and potency as compared with those found for classical and bicyclic cannabinoids (Compton et
corresponding methylated indoles; that is, whereas propyl al., 1993), individual correlations between binding affinity and
through hexyl 2-methyl indoles showed reasonable affinity potency in single measures were lower for these novel com-
and in vivo potency, butyl through heptyl indoles lacking a pounds. There are a few possible explanations of this discrep-
2-methyl group showed the greatest activity in the measured ancy. First, although a more compounds were included in cal-
1998
Indole-Derived Cannabinoids 1003
Acknowledgments
culations of the correlations for traditional cannabinoids,
greater structural diversity was represented in the present The authors thank Renée Jefferson, Kari LaVecchia, Jonathan
study, because data for both indole- and pyrrole-derived com- McElderry, Allison Sistare and Ramona Winckler for their excellent
technical assistance.
pounds were included. Second, previous research found differ-
ences, as well as similarities, between the pharmacological ef-
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