J. Med. Chem. 1994,37, 1214-1219
1214
Structural Features Important for 6 1 Receptor Binding Richard A. G1ennont.J Seth Y. Ablordeppey,t Abd M. Ismaie1,t Mahmoud B. El-Ashmawy,t James B. Fischer; and Kathleen Burke Howiet Department of Medicinal Chemistry, School of Pharmacy, Medical College of Virginia, Virginia Commonwealth University, Richmond, Virginia 23298-0540, and Cambridge Neuroscience, One Kendall Square, Cambridge, Massachusetts Received November 29, 199P
Two problems that have hampered u receptor research are (i) a lack of high-affinity agents and (ii) the recent identification of multiple populations of u receptors (i.e., u1 and 6 2 sites). Recently, several high-affinity u ligands have been identified, and the term superpotent u ligands has been coined to describe agents with Ki values of lOO-fold selectivity for u1 versus u2 sites.
1 R = -(&CHIC& 2R
-CI+CH4(CH&
We have found that DTG binds nearly equally well at both populations of sites (DTG: u1 Ki = 41 nM; u2 Ki = 49 nM), whereas (+)-pentazocine binds with considerable difference (a1 Ki = 1.7 nM; u2 Ki = 860 nM).9 Consequently, much of our earlier work using r3H1DTG as radioligand probably reflects a composite of binding at both populations of sites. The clinical relevance of u1 and u2 receptors is unknown at this time. This is due, in part, to a lack of high-affinity selective ligands. As a prelude to the design of such agents, more needs to be known about the structural requirements of each of the u subpopulat Virginia Commonwealth University. 4
Cambridge Neuroscience. Abstract published in Advance ACS Abstracts, March 15, 1994.
Table 1. Physicochemical Properties of Novel Sigma Ligands yield compd method (%) RSa
mp/bp (“C) (torr)
mol formula
E 185-187 CisHzsN*CzHaO4 (+)6 A/T 87-88 CieHz&C4H404 (+)7 c 82-85 CzaHzeN (-)8 D 34 M/A 154-156 CzOHzeNI 15 D 23 M/A 161-163 CzzHszNI (-)16 E 60 E/W 155-157 CieH2aNGH204 17 E 23 PIT 81-82’’ C21HaN.HCl (-)l8 E 65 E/W 141-143 C2oHnNGHz04 31 F 67 E/T 140-142 CzoHnN-CzHzOr 37 A 47 M/A 28&282‘(dec) 40 F 74 M 167-169 CizHisN.CzHz04 41 F 50 E 143-145 CiaHzsN*C2Hz04 46 F 24 E 225-227 CuHz2Nz*2CzHz04 Recrystallization solvents: E = absolute EtOH, A = EtOAc, T = anhydrous EhO, M = MeOH, W = HzO, and P = 2-propanol. b Crystallized with 0.5 mol of H20. Lit.16 mp 269-272 OC. d Previously reported as free basel6 but no melting point provided. (-)5
A B
80 58 49
tions. We report now on the structure-affinity relationships of a series of phenylalkylamine u ligands with respect to their binding at a1 sites. The synthesis (and “overall a-binding data”) of many of the compounds used in the present investigation has been already reported; however, several new compounds were synthesized to further examine structure-affinity relationships at u1 sites. Chemistry Standard synthetic procedures were used to obtain the new compounds prepared for this study. @)-(-)-Amphetamine and phenylacetaldehyde were condensed under reductive alkylation conditions, and subsequent N-alkylation of the resulting secondary amine under EschweilerClarke conditions afforded (-15 (method A, Table 1). Compound 37 was prepared in a similar manner. To obtain compounds (+)6, (-)16, 17, and (-)18, acylation of the appropriate amine followed by reduction of the intermediate amide using LiAlH4afforded the desired compounds (methods B and E). The first step of method C involved reductive alkylation using (&-(+)-amphetamine and hydrocinnamaldehyde; the second step was a direct alkylation of the resulting secondary amine with benzyl bromide to obtain (+)-7. The quaternarycompounds (-)8 and 15 were prepared by direct alkylation of the requisite amine with methyl iodide (method D).Compounds 31,40,41, and 45
0022-2623/94/1837-1214$04.50/00 1994 American Chemical Society
Journal of Medicinal Chemistry, 1994, Vol. 37, No. 8 1215
Structural Features Important for a1 Receptor Binding Table 2. Radioligand-Binding Data for Derivatives of (-)-3 at Sites
xa
a1
Table 3. Binding Data for Several Fused Derivatives of (-)3
X-(CH,),-CH(R)-N(R’)-(CH,),-Y
\
R‘
compd (-)4 (+)4 (-)5 (+)5 (33 (+)3 (+)6
(+)7 (-)8 9 10
(-)11 12 13 14 15 (-)16 (+)16 17 (-)18 (+)18 a
X H €3 H H H H H H H 3-Br 4-Br 4-1 3-CF3 4-OH 4-OEt H H H H H H
R Me (-) Me(+) Me (-) Me(+) Me (-) Me(+) Me(+) Me(+) Me (-) Me(&) Me(&) Me (-) Me(*) Me(&) Me(&) Pr (i) Me (-1 Me(+) Pr(*) Me (-) Me(+)
R’ H H Me Me H H Me Bz Me2+ H H H
H H H Me2+ H H H H H
y
2 2 2 2 3 3 3 3 3 3 3 3 3 3 3 3 4 4 4 5 5
Ki,nM (SEM) 43.7 (2.1) 13.6 (1.8) 16.4 (1.8) 5.8(0.7) 10.8 (2.1) 39.0 (9.9) 1.3 (0.2) 130(25) 135 (6) 9.7 (2.4) 12.0 (2.9) 17.7 (3.6) 8.8 (4.0) 25.7 (6.3) 3.4 (0.3) 335(27) 7.4 (1.6) 19.4 (5.0) 2.7 (0.6) 0.5 (0.2) 0.9 (0.2)
X r R R’ y Y Ki,nM(SEM) rep 1-NPb 1 Me (A) H 3 phenyl 9.3 (0.8) 5 31.0 (13.0) 5 2-NP 1 Me (i) H 3 phenyl 2-NP 2 H Me 3 2-NP 30.0(11.1) 4 8.6 (1.4) 13 phenyl 1 Me (-) H 3 1-NP 5.7 (1.7) 13 phenyl 1 Me (-) H 3 2-NP a Literature reference for synthesis and overall o-binding data. NP = naphthyl.
compd
y-(Wb-Ph
rep 3 3 3 3 3
5 5 5 5 5 5 3 3
Literature reference for synthesis and overall a-binding data.
were prepared by alkylation of the corresponding secondary amines using the appropriate alcohol tosylates (method F). Reference is made in the other tables to the appropriate literature for previously synthesized compounds. Results and Discussion Binding data for N-substituted phenylaminopropane derivatives are shown in Table 2. The 3-phenylpropyl derivative (R)(-)PPAP [(-I31 was the first compound reported in this series;1° as such, it has served as an arbitrary standard to which other derivatives have been compared. The present investigation began with an examination of derivatives of (-)3.
3
Stereochemistry. Comparing (-)3 (Ki= 10.8nM) and its enantiomer (+I3 (Ki = 39.0 nM), it appears that .stereochemistry plays only a small role (SIR-enantiomeric potencyrativ = 3.6) (Table 2). However,with the 2-carbon alkyl-chain pair, the S-(+)-isomer (+I4 (Ki= 13.6 nM) binds with slightly more than 3 times the affinity of its R-(-)-enantiomer (-14 (Ki= 43.7 nM), and similar results were obtained comparing (+)5 with (-15. With the 4-carbon and 5-carbon alkyl-chain pairs [i.e., (-1 16 versus (+)16 and (-)18 versus (+)MI, there is little difference in the affinity of the R- and S-isomers (Table 2), but, similar to what was seen with 3, the (-)-isomers bind with about twice the affinity of the (+)-isomers. Although the enantioselective reversal seen for the binding of 3,16,and 18 relative to 4 and 5 may be real and although we have previously raised the possibility of reversed modes of binding for certain phenylalkylamines at u receptors: for the isomeric pairs examined, the role of stereochemistry appears minimal when the a-substituent is a methyl group.
19 20 21 (-)22 (-)23
Nevertheless, because 4 and 5 possess equivalent-length alkyl chains on both sides of the amine group, the reversed enantioselectivity may reflect somewhat greater tolerance by the receptor of an a-methyl group on one side of the amineversus the other. In contrast, when the substituents a to the amine are larger than methyl, such as with (+INANM (1; Ki = 150 nM)9 and its isomer (-)NANM (Ki = 3600 nMI9 or with (+)-pentazocine (Ki= 1.7 nMP and its isomer (-)-pentazocine (Ki= llOnM),9 stereochemistry plays a greater role. A similar trend was noted with the overall u binding of these agents, and it has been suggested that the rigid framework of the benzomorphans may impart directionality to the lone-pair nitrogen electrons or that the added steric bulk of the tricyclic ring system contributes to enantioselectivity.” N-Methylation. N-Methylation of (+)3 (Ki= 39 nM) to afford (+)6 (Ki= 1.3 nM) results in a 30-fold increase in affinity (Table 2), whereas N-methylation of (-)4 [i.e., (-)51 and (+)4 [i.e., (+)51 only doubles affinity. The effect of N-monomethylation in the 5-phenylpentylamine series is also variable (seebelow discussion). N,N-Dimethylation of (-)3 (Ki= 10.8 nM) to afford the quaternary amine derivative (-18 (Ki= 135 nM) decreases affinity by more than an order of magnitude. The quaternary amine analogue 15 also binds with low affinity. Evidently, tertiary and secondary amines are well tolerated, but quaternary amines are not readily accommodated by ul receptors. For the one case examined, the N-benzyl analogue of (+)3 [i.e., (+)7; Ki = 130 nM1 binds with only one-fourth the affinity of its parent and about one-tenth the affinity of (-)3. Alkyl Chain “y”. There is little effect (less than 3-fold) in increasing the length of the alkyl y-chain from 2 [(+)4; Ki = 13.6 nM1 to 3 [(+)3; Ki 39 nM1 to 4 [(+)16; Ki = 19.4 nM1 methylene units (Table 2). A parallel trend is seen with the (-)-isomers, but (-116 binds with nearly 6 times the affinity of (-)4. However, the 5-membered chain derivatives (+)18 and (-)18 (Ki = 0.9 and 0.5 nM, respectively) bind with significantly higher affinity (Table 2). This same trend is also seen with the a-desmethyl derivatives (described below). Aromatic Substituents. Comparing 9-14 (Table 21, it appears that aromatic substituents have relatively little effect on affinity with Ki values (3.4-25.7 nM) spanning about a 10-fold range. All of the substituted derivatives bind within several-fold of the affinity of the parent unsubstituted derivative (-)3 (Ki= 10.8 nM). Where the y-chain is held constant at 3 methylene units, replacement of the phenyl-A ring (19-211,the phenyl-B ring [(-)22, (-)23], or both (i.e., 21) with a 1-naphthyl or 2-naphthyl group has little effect on affinity (Kivalues range from 5.7 to 31 nM) (Table 3). Thus, although bulk appears to be tolerated, no affinity-enhancing effect is evident.
Glennon et al.
1216 Journal of Medicinal Chemistry, 1994, Vol. 37, No. 8
Table 5. Binding Data for 5-Phenylpentylamine-Related Derivatives
Table 4. Binding Data for Alkyl-Chain Variants
X-CH2-CH2-Z-CH2-CH2-Y compd
x
24 25 26 27 28 29 30 31 32 33 34 35 36
2 3 1 2 1
0
1
2 2 3 3 4 1 2
R H H H H H Me H Me H Me H H H
y
3 3 4 4 5 5 5
5 5 5 5 7 7
Ki,nM(SEM) 11.3 (1.0) 11.4 (1.8) 9.6 (2.9) 2.6 (0.3) 0.32 (0.10) 0.19 (0.04) 0.17 (0.0) 0.25 (0.06) 0.28 (0.03) 0.38 (0.14) 0.48 (0.05) 2.3 (1.0) 1.5 (0.6)
rep 4 4 4 4 4 14 4 4 14 4 4 4
Literature reference for synthesis and overall a-binding data.
a-Desmethyl Derivatives. The presence of the amethyl group makes only a negligible contribution to affinity; for example, the a-desmethyl counterpart of (-)3 (Ki= 10.8 nM) is 24 (Ki= 11.3 nM). For this reason, and to eliminate the need to examine pairs of optical isomers, a series of a-desmethyl derivatives was examined (see Table 4). With the x-chain held constant at 2 (i.e., an ethyl group), increasing the length of the y-chain from 3 (24; Ki = 11.3 nM) to 4 (27;Ki = 2.6 nM) to 5 (30;Ki = 0.17 nM) methylene units results in a progressive increase in affinity. The trend seems to terminate when the chain is longer than a pentyl group; for example, when the y-chain is 7 methylene units (36; Ki = 1.5 nM), affinity is decreased relative to that of 30. Nevertheless, compound 36 still binds at q sites with high affinity. Shorteningthe x-chain to 1(i.e., a methylene group) and lengthening the y-chain from 4 (26; Ki = 9.6 nM) to 5 (28; Ki = 0.32 nM) to 7 (35; Ki = 2.3 nM) methylene units provides the same trend. Via lengthening the x-chain to 3, again the 5-membered y-chain derivative (32; Ki = 0.28 nM) binds with higher affinity than its 3-membered chain counterpart (25; Ki = 11.4 nM). With an x-chain of 4, 34 binds with half the affinity of 32. Unlike what was observed above with (+)3 (Table 2), N-monomethylation of 28, 30, and 32 (i.e., 29, 31, 33, respectively) has no substantive effect on affinity. In contrast, N-monomethylationof pentylamine 40 (Ki= 418 nM; Table 5) to 39 (Ki= 11.7 nM) enhances affinity by about 30-fold. At this time, we can not satisfactorily explain the variable effect of N-methylation on ~1 binding. Desphenyl Derivatives. Evidently, and consistent with what we have previously reported: there is something unique about the 5-membered y-chain derivatives. Viewing these results from another perspective where the y-chain is held constant at 5 methylene units and the x-chain is varied from 1(28; Ki = 0.32 nM) to 2 (30; Ki = 0.17 nM) to 3 (32; Ki = 0.28 nM) to 4 (34; Ki = 0.48 nM) methylene units, it is evident that the length of the x-chain does not have much of an effect on binding. Because the x-chain serves as a spacer between the amine and the phenyl-A ring, the distance between these two features does not seem important and raises the question of whether the phenyl-A ring is necessary for binding. A comparison of 33 (Ki= 0.38 nM) with its desphenyl-A(i.e., N-n-propyl) counterpart 38 (Ki= 0.29 nM) reveals that the phenyl-A ring is not required for binding (see Table 5). However,
compd
X
Y
Ki,nM
(SEMI rep phenyl-CH~CHrNH- CH2 H 48.2 (3.4) 0.38 (0.14) 14 phenyl-CHnCH2CHz- CH2 phenyl N(CHn)38 0.29 (0.05) 14 CH3-CH2kH2-N(CH3)- CH2 phenyl 39 11.7 (3.5) 14 CH3-N(CH3)CH2 phenyl 40 418 (17) CH3-NHCH2 phenyl 41 0.76 (0.16) 14 1-pyrrolidinyl CH2 phenyl 42 0.48 (0.01) 7 1-piperidinyl CH2 phenyl 1-pyrrolidinyl -NH- phenyl 45 83 (15) 46 phenyl-CH2-NHCH2 cyclohexyl 0.81 (0.17) 14 47 CH3-N(CH3)CH2 cyclohexyl 0.26 (0.03) 14 48 14 CH3-NHCH2 cyclohexyl 6.8 (0.9) 49 14 H2NCH2 cyclohexyl 190 (17) Literature reference for synthesis and overall a-binding data. 2
37 33
Proton Donor Site Optimum binding (7-9 A)
u
Region tolerates Secondary Binding Site.Tolerates bulk
Primary Hydrophobic site
small groups only
Figure 1. Receptor features presumed to be important for the binding of ligands at u1 receptors.
shortening the residual n-propyl substituent to methyl (39; Ki = 11.7 nM) reduces affinity by 40-fold. It would seem that the x-chain functions not as a simple spacer but that it interacts in a productive manner with a binding feature of the receptor (see Figure 1). This (presumably hydrophobic) interaction is also evident with the shortest possible x-chain derivative .....the N-benzyl derivative 28 (Ki= 0.32 nM), where the phenyl-A group seemingly interacts with the same binding feature utilized by a portion of the n-propyl chain of 38 (see site A, Figure 1). An adjacent region of bulk tolerance allows for extension of the x-chain beyond the length of an n-propyl group, as with 34 (Ki= 0.48), with little to no effect on affinity. The presence of a region of bulk tolerance is also consistent with the finding that benz-fusion of the phenyl-A ring to naphthyl (e.g., 19, 20) has little impact on binding. In contrast, the desphenyl-B analogue 37 (Ki= 48.2 nM) binds with 280-fold lower affinity than its parent 30 (Ki = 0.17 nM), indicating the importance of the phenyl-B ring to binding. Thus, the alkyl chain length and/or the presence of a phenyl group (or reduced phenyl group; see below) may be determinants for specific modes of receptor interaction. Figure 1is a schematic representation of some features that appear to be important for q binding. Site A must be located at a distance from the amine site such that it optimally accommodates an n-propyl group (3.9 A). Comparing the progressively increasing affinities of 24, 27, 30, and 36 (all of which possess a common 2-phenylethyl substituent and bind with Ki values of
