Proc. Nat. Acad. Sci. USA Vol. 72, No. 5, pp. 1899-1903, May 1975 Phenothiazine Drugs: Structure-Activity Relationships Explained by a Conformation That Mimics Dopamine (catecholamine/neurotransmitter/receptor/chlorpromazine/schizophrenia) ANDREW P. FEINBERG AND SOLOMON H. SNYDER Departments of Pharmacology and Experimental Therapeutics and Psychiatry and the Behavioral Sciences, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205 Communicated by Seymour S. Kety, February 21, 1975 ABSTRACT The antischizophrenic activity of pheno- perphenazine, and fluphenazine were constructed with Corey- thiazine drugs and their tendency to elicit extrapyramidal Pauling-Koltun kits (Ealing Corp.; Cambridge, Mass.). To symptoms are thought to involve blockade of synaptic dopamine receptors in the brain. Space filling molecular confirm the apparent influence of Van der Waal's forces on the models show how favorable Van der Waal's interactions side chain conformation, we performed potential energy calcu- between the side chain amino of phenothiazines and the 2- lations on the following three compounds with differing ring substituent on ring A can promote a conformation mim- substituents but the same side chain: promazine, chlorpro- icking dopamine. These Van der Waal's attractive forces can explain (i) the greater potency of drugs with trifluoro- mazine, and triflupromazine. Calculations were performed on methyl rather than chlorine as a 2-substituent; (ii) the alkylamino side chain phenothiazines, since the great increase enhanced activity of phenothiazines with piperazine in- in conformational variables introduced by a flexible piperidine stead of alkylamino side chains; (iii) the increased po- or piperazine ring was beyond the practical capability of the tency associated with hydroxyethylpiperazines as con- trasted to piperazine side chains ; (iv) the greater potency computer. of cis rather than trans thioxanthenes; and (v) the crucial The calculations were performed on a PDP-12 computer location of the ring A substituent at carbon no. 2. Poten- with floating point processor and 8K of memory, using a form tial energy calculations support the observations with of the Buckingham 6-exp potential function proposed by molecular models and suggest an active conformation for A. I. Kitaygorodskii (8): the phenothiazines. V = 3.5 (8600 e-13Z- 0.04 Z-6) An abundance of recent research suggests that a major mechanism whereby antischizophrenic phenothiazine drugs where z is the ratio of the distance between atom centers and exert their therapeutic actions and extrapyramidal side effects G. N. Ramachandran's K2 values for Van der Waal's radii (9). observed involves a blockade of synaptic receptor sites for dopamine in This function has shown good agreement with x-ray the brain (1, 2). The activity of a dopamine-sensitive adenylate crystal structures (10). cyclase correlates with dopamine receptor activity (3). Rela- Two groups of computer programs, developed at Washing- tive potencies of several phenothiazine and related drugs as ton University Computer Systems Laboratory, were used. inhibitors of the dopamine-sensitive adenylate cyclase parallel The first group, CHEMAST (11, 12), was used to generate their antischizophrenic potency (4-6). Earlier we proposed a lists of atomic coordinates, atom types, connectivity and rota- molecular model wherein dopamine could be superimposed tion parameters, and stereoscopic images of the molecules on upon a portion of the chlorpromazine molecule (7). While ex- a point-plotting cathode ray tube. The second group, BUR- plaining some structure-activity characteristics of pheno- LESK (13, 14), was used to perform iterative rotations over thiazines, this model did not deal with important features the bonds indicated in Fig. 1 and the trifluoromethyl group such as the greater potencies of phenothiazines with piperazine in triflupromazine, computing the potential energy between rather than alkylamino side chains, nor did it explain the all atom pairs whose relative orientations were changed by the greater potency of trifluoromethyl than of chlorine ring sub- rotations; and to display the data on the cathode ray tube stituents. Moreover, it provided no explanation for the mech- as isoenergy curves. To obviate calculations on absurd con- anism whereby the A ring substituent caused the side chain formers, the programs made a "bump check", for which Van to tilt toward the A ring, nor did it explain why the A ring der Waal's radii were set at 90% of those used by Leach et al. substituent must be located in the number 2 position. (15). The data were compared to control data obtained with- In the present study, Corey-Pauling-Koltun molecular out a "bump check" to ascertain that no legitimate con- models and computer calculations support a model in which formers were excluded. phenothiazines assume a conformation that mimics that of A special connectivity table was prepared for the pheno- dopamine, explaining the role of trifluoromethyl, piperazine, thiazine ring system (16), and its bond lengths and bond and hydrox-yethvlpiperazine groups as well as the mechanism angles were obtained from the published crystal structure of whereby A ring substituents influence the side chain. thiethylperazine (17). It might have been more natural to use published data on chlorpromazine (18), but these yielded MATERIALS AND METHODS bizarre ring atom placements. Molecular models of promazine, chlorpromazine, triflu- For each molecule, between 16,000 and 150,000 conforma- promazine, prochlorperazine, trifluoperazine, thiethylperazine, tions were examined using 5-15 hr of computer time. In criti- 1899 Downloaded by guest on September 26, 2021 1900 Medical Sciences: Feinberg and Snyder Proc. Nat. Acad. Sci. USA 72 (1975) 4 5 19l 9 20 CH3 (ECH2\ 'qH2-C1 I*He 4 T3 162 CH3 19 T, a T(C,,-NO-C15"C16 ) T2=aT (NI0_CI5 -C6 -CI7 ) T3 T(Ci5-CI6-CI7-NIS) a T(C16-CI7 -N18 -C19) FIG. 1. Rotatable bonds of chlorpromazine, drawn with T1. =2 = T4 = 00 and T3 = 1800. The convexity of the pheno- thiazine ring system is upward. The torsion angle r(A-X-Y-B) is the dihedral angle between planes AXY and XYB, measured clockwise when viewed down the bond X-Y (25). cal regions, the data were compared to control data obtained C over a greater density of conformations to confirm their completeness. A Hewlett-Packard 7005B X-Y Recorder was used to draw the isoenergy curves on graph paper, and they were then re- traced. The figures of ball and stick models were made with a DECSYSTEM 10 time-sharing computer, AGT-30 ADAGE graphics system, and model 200 ZETA plotter, using the program package CHEM (19), and they were then redrawn by an artist. RESULTS AND DISCUSSION Space-filling models of chlorpromazine (Fig. 2) reveal that tilting of the side chain toward ring A permits favorable Van der Waal's interactions of the side chain with the chlorine substituent. These Van der Waal's attractive forces would be greatly lessened in the case of promazine, which lacks the chlorine substituent. This conformation permits the super- imposition of dopamine (Fig. 3), thus explaining how chlor- promazine can interact with dopamine receptors. The space- filling models provide a causal explanation for the folding of the side chain toward ring A. FIG. 2. Phenothiazines with the side chain "tilted" toward The notion that Van der Waal's attractions between the the A ring, drawn with relatively small atom size for ease of side chain and the ring A substituent could account for the viewing. ri and T2 are set at (-68°, 1350), the relative minimum of conformational zone no. 2 in Fig. 5. (a) Chlorpromazine (the ability of phenothiazines to assume the dopamine-like con- side chain N to Cl distance is 3.3 A); (b) triflupromazine formation allows several about the structure of (the predictions distance of the side chain N to the closest fluorines is 3.0-3.5 A); phenothiazines that might be expected to be more or less po- (c) trifluoperazine; (d) fluphenazine. tent in interactions with the dopamine receptors. For instance, this model requires that the ring A substituent be located at the chlorine substituent. Our model predicts that pheno- the number 2 position. A 1-substituent would sterically hinder thiazines with trifluoromethyl substituents should be more the ability of the side chain to approach ring A, while a 3- potent than those with chlorine substituents. Of the clinically substituent would be too distant from the side chain to pro- used phenothiazines, those with trifluoromethyl substituents vide Van der Waal's attractions of a significant degree. Exten- at position 2 of ring A are uniformly more potent than those sive experience with many phenothiazine structures has shown with chlorine substituents, both clinically and in their inhibi- that optimal neuroleptic activity occurs only when the ring A tion of the dopamine-sensitive adenylate cyclase (4-6, 20, 21). substituent is in the 2-position (20, 21). The nature of the atom in proximity to the side chain amine The nature of the 2-substituents in our model would be might also be anticipated to influence the conformation of the expected to influence the conformation of the side chain. A phenothiazine side chain. A piperazine side chain affords more trifluoromethyl substituent provides a greater number of Van der Waal's contacts with the 2-substituent than does an favorable Van der Waal's contacts with the side chain than alkylamino side chain (Fig. 2). The resultant predictions that Downloaded by guest on September 26, 2021 Proc. Nat. Acad. Sci. USA 72 (1975) Phenothiazines and Dopamine 1901 TABLE 1. Phenothiazine and thioxanthene drug structures and effects on a dopamine-sensitive adenylate cyclase Relative potency in inhibiting dopamine-sensitive adenylate cyclase of rat corpus striatum (chlorpromazine = 100) t Relative Results of Results of clinical Miller and Clement-Courmier Drug Ring R, R2 potency* Iversen (5) et al. (4) Ca-Flupenthixol thio. HEP CF3 1 4545 (aO6)-Flupenthixol thio.