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The synthesis and receptor binding affinities of DDD-016, a novel, potential,atypical antipsychotic†

Achintya Bandyopadhyaya,a Desikan R. Rajagopalan,a Nigam P. Rath,b Amy Herrold,c

Raghavan Rajagopalan,a T. Celesete Napier,c Clark E. Tedfordd and Parthasarathi Rajagopalan*a

Received 21st December 2011, Accepted 6th March 2012

DOI: 10.1039/c2md00311b

A new pentacyclic compound, DDD-016 (10d) was designed and synthesized as a potential

antipsychotic and screened for binding affinity in a broad panel of over 80 CNS and other receptors,

monoamine transporters, and ion channels. The resulting receptor binding profile of DDD-016 is

consistent with that of an improved atypical antipsychotic candidate.

The importance of serotonin (5-HT) and dopamine (D) receptors

in the discovery of central nervous system (CNS) drugs is well

recognized. Fourteen serotonin and five dopamine human

receptor subtypes have been cloned and expressed to date. As

these receptor subtypes are associated with numerous CNS and

peripheral diseases, they continue to be principal targets for the

development of new CNS medications.1 Schizophrenia is an

insidious and life-long illness that affects about 1% of the world

population.2 Characteristics of this disease include disturbances

in cognition, reality testing, and mood, which severely disrupt

interpersonal relations, and affect social and work functions.

Schizophrenic patients suffer from a loss of identity, individu-

ality, and mental capacity. It is a disease that usually affects

young people in their late teens or early twenties who are on the

threshold of adulthood all set to begin their productive lives.2

The tragedy of this cruel affliction is compounded by the more

than 10% suicide rate among the schizophrenics.3

The symptoms associated schizophrenia can be divided into

two categories. The negative symptoms consist of behavioural

problems such as blunting of emotions, language deficits and

lack of energy; and the positive symptoms comprise auditory and

visual hallucinations, delusions, and behavioural abnormalities.

In 1989, it was reported that treatment of schizophrenic patients

with the 5-HT2 receptor antagonist, haloperidol (1) in

aDaya Drug Discoveries, Inc., University of Missouri, St. Louis, OneUniversity Blvd., St. Louis, Missouri 63121, USA. E-mail:ramajayam30@yahoo.com; Fax: +1 314-516-5342; Tel: +1 636-541-5863bUniversity of Missouri, St. Louis, One University Blvd., St. Louis,Missouri 63121, USA. E-mail: nprath@umsl.edu; Fax: +1 314-516-5342;Tel: +1 636-541-5863/314-516-5331cDepartment of Pharmacology, Rush University Medical Center, 1735 W.Harrison St. Chicago, Illinois 60605, USA. E-mail: celeste_napier@rush.edu; Fax: +1 312-563-3552; Tel: +1 312-563-2416dPhoto Thera, 5925 Priestly Drive, Suite 120, Carlsbad, California 92008,USA. E-mail: ctedford@photothera.com; Fax: +1 760-496-0322; Tel:+1 760-692-4942

† Electronic supplementary information (ESI) available: Synthesis andX-ray crystallographic data of DDD-016. See DOI: 10.1039/c2md00311b

580 | Med. Chem. Commun., 2012, 3, 580–583

combination with the D2 receptor antagonist, ritanserin (2), was

found to help alleviate both the negative and positive symptoms

of the disease with concomitant reduction in adverse movement

disorders.4 This important observation provided the impetus for

the development of the so-called mixed ‘atypical antipsychotics’

This journal is ª The Royal Society of Chemistry 2012

Fig. 1 Energy-minimized 3D views of 10c and 10d.

Scheme 1

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such as olanzapine (3) which were primarily antagonists at both

the 5-HT2A and the D2 receptors. This ‘combination pharma-

cotherapy’ was based on the serotonin-dopamine antagonist

(SDA) hypothesis5 which postulates that moderate D2 receptor

blockade results in the attenuation of the positive symptoms, and

potent 5-HT2A receptor blockade leads to alleviation of the

negative symptoms and movement disorders.

In our effort directed toward the discovery of small molecules

for the treatment of CNS disorders, and in accord with the SDA

hypothesis referred to earlier, we have designed and synthesized

the novel, pentacyclic compound DDD-016 (10d) and subjected

it to extensive in vitro receptor binding studies and preliminary in

vivo tests for possible antipsychotic activity. This communication

describes: (i) rationale behind the design, synthesis, determina-

tion of structure of DDD-016; (ii) in vitro binding affinities of

DDD-016 to all of the serotonin, dopamine, and a number of off-

target receptors; (iii) comparison of receptor binding affinities of

DDD-016 with those of five atypical antipsychotics currently in

the market; and (iv) results of the preliminary in vivo pharma-

cological studies with DDD-016.

The structure of DDD-016 (10d) was based on that of

compound 10c, the rationale behind the design of which by one

of the authors (P.R.) over 25 years ago,6 was as follows: cloza-

pine (4) was at that time firmly established as a breakthrough,

effective, atypical antipsychotic. g-Carbolines of the type 8, had

also been reported by then to display antipsychotic activity

mediated by dopamine antagonism.7 However, we believed that

g-carbolines (8), which could be viewed as cyclic indolealkyl-

amines, would also display affinity to the serotonin receptors in

the brain. Therefore, we reasoned that the pentacyclic hybrid (9)

derived by the incorporation of the scaffolds of clozapine (4,

highlighted by red and blue colors) and of g-carboline (8, high-

lighted by blue and green colours) into its molecular frame work,

would elicit dopaminergic and serotoninergic activity in an

acceptable ratio that could be optimized for ideal atypical anti-

psychotic activity. Historically, 10c was synthesized prior to the

cloning and characterization of any of the dopamine or serotonin

receptor subtypes. Therefore, the choice of a candidate for

development as an antipsychotic then rested solely on the

conditioned avoidance response (CAR) and apomorphine

antagonism tests in rodents reflective of dopamine antagonist

activity and, inter alia, antipsychotic activity. Searching through

the literature for any prior publications pertaining to 10c, we

found that compound 10a, which could be construed as the

carbon isostere of 10c, was known and had been patented for

antidepressant, but not for antipsychotic activity.8 We adopted

the reported method for the preparation of 10a to the synthesis of

10b. Initially, we had planned to elaborate the lactam function of

compound 10b, when synthesized, to the N-methyl-piper-

azinoamidine moiety of clozapine present in the proposed hybrid

9. However, it turned out that it was not necessary because 10b

was prepared eventually and found to be almost on a par with

clozapine in its activity in both the CAR and the apomorphine

antagonism tests with mice.6 Further, compound 10c, obtained

by the reduction of 10b, was even more active than clozapine (4)

in the critical CAR test with mice indicative of potential anti-

psychotic activity in humans, and only weakly active as clozapine

in the apomorphine antagonism test suggestive of negligible

extrapyramidal side effects.6

This journal is ª The Royal Society of Chemistry 2012

However, the rationale for the design of compound 10c

mentioned above could not be extended to that of DDD-016

(10d) because the clozapine unit is not part of its structure and

was, therefore, based on the following reasons: First, the isosteric

substitution of the NH function in 10c by sulfur atom leading to

the target 10d would preserve the gross molecular geometry of

Med. Chem. Commun., 2012, 3, 580–583 | 581

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the pentacyclic scaffold as indicated by the two energy minimized

structures in Fig. 1 (ref. 9) and could, thereby, be expected to

display a pharmacological profile similar to that of 10c.

Secondly, such a transformation would likely eliminate any

toxicity associated with the secondary aniline function present in

10c.10,11 Thirdly, the presence of homophenothiazine unit with its

pendant three-carbon dialkylaminopropyl moiety branching off

the nitrogen of the indole and comprising the a and b bonds and

the methylene at the 3-position of the indole as well as the tet-

rahydro-g-carboline scaffold embedded in the structure of 10d

could result in both D2 and 5-HT2A antagonism.

Table 1 Crystal data and structure refinement for 10d

Empirical formula C19H18N2SFormula weight 306.41Temperature 100(2) KWavelength 0.71073 �ACrystal system OrthorhombicSpace group P212121Unit cell dimensions a ¼ 8.4860(8) �A, a ¼ 90�

b ¼ 11.7045(12) �A, b ¼ 93.01(3)�c ¼ 15.3316(16) �A, g ¼ 90�

Volume 1522.8(3) �A3

Z 4Density (calculated) 1.337 mg m�3

Absorption coefficient 0.210 mm�1

F(000) 648Crystal size 0.35 � 0.28 � 0.18 mm3

q Range for data collection 2.19 to 34.47�Index ranges �13 # h # 13, �17 # k # 18,

�24 # l # 22Reflections collected 49 374Independent reflections 6369 [R(int) ¼ 0.0319]Completeness to q¼ 25.00� 99.9%Absorption correction Semiempirical from equivalentsMax. and min. transmission 0.9635 to 0.9298Refinement method Full-matrix least-squares on F2

Data/restraints/parameters 6369/36/207Goodness-of-fit on F2 1.085Final R indices [I > 2s(I)] R1 ¼ 0.0344, wR2 ¼ 0.0928R indices (all data) R1 ¼ 0.0369, wR2 ¼ 0.0942Absolute structure parameter 0.01(6)Largest diff. peak and hole 0.415 and �0.354e �A�3

Fig. 2 Projection view of DDD-016 (10d).

582 | Med. Chem. Commun., 2012, 3, 580–583

The synthesis of 10d, as outlined in Scheme 1, was similar to

those that were reported for 10a and 10b.6,8 The nitroso deriva-

tive 12, derived from the known 11,12 was reduced with zinc and

acetic acid in the presence of 1-methyl-4-piperidone (13) to yield

the hydrazone (14) which underwent Fischer indole cyclization in

situ to furnish only one of the two possible regioisomers 10d and

15. The overall yield of the product (10d or 15) was low because

of the competing denitrosation of 12 to the parent amine 11.

However, the redeeming feature here is that the recovered 11 can

be recycled at least a couple of times to obtain more of 10d. That

the product from the final Fischer-indole cyclization consisted of

only one regioisomer was borne by its proton and carbon NMR

spectra which display only a single peak at d 4.02 ppm and

d 53.3 ppm respectively for the methylene protons adjacent to the

sulfur. If it were a mixture of both isomers then both the proton

and carbon spectrum would exhibit two signals for the methylene

group. However, the structure for this product could not be

assigned with any degree of certainty on the basis of NMR

spectra. Therefore, a suitable crystal of the product, obtained by

slow evaporation of a dilute solution of it in methanol, was

subjected to single crystal X-ray analysis (Table 1) which

unequivocally established its structure as represented by 10d

shown in Fig. 2. Interestingly, the 3-dimensional structure

derived from molecular modelling9 is identical to that of the

actual structure determined by X-ray crystallography.

DDD-016 (10d) was evaluated in a high-throughput screening

(HTS) panel consisting of about 80 transmembrane and soluble

receptors, ion channels, and monoamine transporters.13 This

Table 2 Selected off-target receptor binding data (pKi)

10d 3a 4a 5a 6a 7a

5-HT1A 7.4 <6 6–7 6–7 8–9 6–75-HT1B 6.5 6–7 6–7 <6 6–7 8–95-HT1D 7.2 <6 <6 <6 7–8 6–75-HT1E 5.6 <6 6–7 <6 <6 <65-HT2A 8.7 7–8 8–9 6–7 8–9 >95-HT2B 8.3 NAb NAb NAb NAb NAb

5-HT2C 8.1 8–9 7–8 <6 6–7 7–85-HT3 6.2 6–7 6–7 <6 6–7 <65-HT4 <6 NAb NAb NAb NAb NAb

5-HT5A 6.7 <6 <6 <6 6–7 6–75-HT6 7.8 8–9 7–8 <6 <6 <65-HT7 7.1 6–7 7–8 6–7 7–8 8–9D1 7.6 7–8 6–7 6–7 6–7 6–7D2 6.9 7–8 6–7 6–7 >9 8–9D3 7.5 7–8 6–7 6–7 8–9 7–8D4 6.7 7–8 7–8 <6 6–7 7–8D5 6.7 7–8 6–7 <6 <6 6–7a1A 6.3 6–7 8–9 7–8 7–8 8–9a1B <6 6–7 8–9 7–8 7–8 8–9a1D 6.1 NAb NAb NAb NAb NAb

a2A 7.2 6–7 6–7 <6 7–8 6–7a2B 6.7 7–8 7–8 6–7 6–7 6–7a2C 7.5 7–8 7–8 7–8 7–8 8–9H1 8.5 8–9 8–9 8–9 7–8 7–8H2 7.1 NAb NAb NAb NAb NAb

M1 <6 7–8 7–8 6–7 <6 <6M2 <6 7–8 7–8 <6 <6 <6M3 <6 7–8 7–8 <6 <6 <6M4 <6 6–7 7–8 6–7 <6 <6M5 6.1 8–9 7–8 <6 <6 <6

a Ref. 5 and 12. b Not available.

This journal is ª The Royal Society of Chemistry 2012

Table 3 Functional assay of DDD-016

% Inhibition of control(agonist activity)a

% Inhibition of control(antagonist activity)b

5-HT2A 4 101D2 0 95

a Agonist control, serotonin. b Antagonist control, ketanaserin.

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panel has been designed to provide information on target binding

affinities for drug candidate as well as off-target activity to

establish selectivity/specificity profiles. The pKi’s of the binding

affinities of DDD-016 to all of the dopamine and serotonin

receptors as well as some of the key off-target receptors are listed

in Table 2. Table 2 also displays receptor binding data pertaining

to five marketed antipsychotics: olanzapine (3), clozapine (4),

quetiapine (5), aripiprazole (6), and risperidone (7).14 The results

from HTS revealed that DDD-016 displayed high affinity and

selectivity only to the serotonin and dopamine receptors with

virtually no affinity to the rest of the constituents of the receptor

panel. In particular, DDD-016 displayed high affinity to the

5-HT2A receptor (pKi ¼ 8.7) and moderate affinity to the D2

receptor (pKi ¼ 6.9). Furthermore, in the functional assay,

DDD-016 was found to be a full antagonist at both of these

receptors (Table 3).

When compared to the pKi’s of five marketed antipsychotics

3–7, DDD-016 displayed a unique receptor binding profile

mostly similar to those but different from each of them in one or

more aspects. The salient features of its profile are as follows: The

affinity to the 5-HT2A receptor is optimal and higher than those

of olanzapine (3), clozapine (4), and quetiapine (5), comparable

to that of aripiprazole (6) and lower than that of risperidone (7),

all of which point to its efficacy in the alleviation of the negative

symptoms of schizophrenia with concomitant reduction in motor

side effects. The affinities to the 5-HT1A and D1 receptors are

balanced and higher than those of all the other agents except

aripiprazole (6) and olanzapine (3) suggestive of potential benefit

to the cognitive function.15 The affinities to the 5-HT3, 5-HT6,

and 5-HT7 receptors closely parallel those of clozapine (4) and

olanzapine (3). The D2 affinity is close to that of clozapine (4)

and lower than that of olanzapine (3) indicative of reduced

extrapyramidal side effects (EPS), which property is augmented

by its lower affinity to the D3, D4 and D5 receptors. The H1 and

a2C receptor affinities are similar to those of the five marketed

drugs. DDD-016 favours the 5-HT2A over 5-HT2B receptor

which factor coupled with D2 antagonism, increases the release

of dopamine preferentially in the frontal cortex and thus may be

beneficial in the treatment of cognitive deficits in schizophrenic

patients.15 Affinities to the muscarinic receptor subtypes are

weak. Overall, the receptor binding profile of DDD-016 is

consistent with that of an improved, atypical antipsychotic

compound.

In a preliminary in vivo pharmacological evaluation, DDD-

016 was highly active in the pre-pulse inhibition (PPI) test with

rats (i.p.) indicative of antipsychotic activity in humans with

negligible motor side effects at the effective doses. Further, in

This journal is ª The Royal Society of Chemistry 2012

a two-week oral dose-ranging study with rats, DDD-016 dis-

played no significant adverse effects, weight gain, abnormal

metabolic changes, elevation in liver enzyme levels or changes in

renal parameters compared to those of the controls. The absence

of weight gain in this study is noteworthy because almost all of

the marketed antipsychotics are best with this undesirable side

effect. The details of the in vivo studies will be reported elsewhere.

Conclusions

Our expectation that the novel, pentacyclic compound 10d we

had synthesized on the basis rational drug design would display

an improved, atypical antipsychotic profile was borne out by the

results that were obtained from the high throughput screen

(HTS) involving about 80 CNS and other receptors, monoamine

transporters, and ion channels, to which it was subjected. In

particular, DDD-016 displayed high affinity to the 5-HT2A and

moderate affinity to the D2 receptors and was an antagonist at

both of these receptors. Comparison of DDD-016 with five

leading antipsychotics in the market, particularly with respect to

their receptor binding affinities, indicates that DDD-016 is

mostly similar to them in its profile but different in some aspects.

In preliminary in vivo studies, DDD-016 elicited high degree of

antipsychotic-like activity, and displayed no significant weight

gain and other adverse effects.

Acknowledgements

This work was supported by a grant from the National Institute

of Mental Health (NIMH) (1R43084765). We thank University

of North Carolina Psychoactive Drug Screening Program

(PDSP) supported by NIMH for the receptor binding studies.

Funding from the National Science Foundation (MRI, CHE-

0420497) for the purchase of Apex II diffractometer is gratefully

acknowledged.

Notes and references

1 D. E. Nichols and C. D. Nichols, Chem. Rev., 2008, 108, 1614.2 N. C. Andreasen, Lancet, 1995, 346, 477.3 D. W. Black and T. J. Boffeli, Am. J. Psychiatry, 1989, 146, 1267.4 Y. G. Gelders, Br. J. Psychiatry, 1989, 155, S33.5 S. Kapur, R. Zipursky and G. Reminton, Am. J. Psychiatry, 1999,156, 286.

6 P. Rajagopalan, US Pat., 4438120, 1984.7 C. A. Harbert, J. J. Plattner, W. M. Welch, A. Weissman andB. K. Koe, Mol. Pharmacol., 1980, 17, 38.

8 H. A. Blumberg, US Pat., 4219550, 1974.9 Molecular modeling was carried out using ChemBio Ultra 3Dprogram using MM2 molecular mechanics software.

10 N. Yoshimi, S. Sugie, H. Iwata, K. Niwa, H. Mori, C. Hashida andH. Shimizu, Genetic Toxicology Testing, 1988, 206, 183.

11 M. A. Valentovic, J. G. Ball, D. K. Anestis, K. W. Beers, E. Madan,J. L. Hubbard and O. Rankin, Toxicology, 1992, 75, 121.

12 B. J. Margolis, J. J. Swidorski and B. N. Rogers, J. Org. Chem., 2003,68, 644.

13 The binding studies were conducted by the Psychoactive DrugScreening Program (PDSP) of University of North Carolina ChapelHill sponsored by the National Institute of Mental Health (NIMH).

14 H. A. Nasrallah, Mol. Psychiatry, 2008, 13, 27.15 A. R. Brennan and A. F. T. Amssten, Ann. N. Y. Acad. Sci., 2008,

1129, 236.

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