Full Paper Synthesis and Biological Evaluation of Novel 5,8- Disubstituted-2-methyl-2,3,4,5-tetrahydro-1H-pyrido[4,3-b] indoles as 5-HT6 and H1 Receptors Antagonists Alexandre V. Ivachtchenko1,2, Eugene B. Frolov2, Oleg D. Mitkin2, Volodymyr M. Kysil2, Alexander V. Khvat2, and Sergey E. Tkachenko2,3 1 Department of Organic Chemistry, Chemical Diversity Research Institute, Khimki, Moscow Reg, Russia 2 ChemDiv, Inc., San Diego, CA, USA 3 Department of Molecular Biology and High-Throughput Screening, Chemical Diversity Research Institute, Khimki, Moscow Reg., Russia Synthesis, biological evaluation, and structure-activity relationships (SAR) for a series of novel c- carboline analogues of Dimebon are described. Among the studied compounds, tetrahydro-c-car- boline 5b (2,8-dimethyl-5-[cis-2-pyridin-3-ylvinyl]-2,3,4,5-tetrahydro-carboline) has been identified as the most potent small molecule antagonist, in particular against histamine H1 and serotonin 5-HT6 receptors (IC50 a 0.45 lM and IC50 = 0.73 lM, respectively). A thorough comparative SAR study performed for the tested compounds has revealed significant correlations between the nature of side substituents and the related antagonistic activity. Keywords: Antagonists / Dimebon / Histamine receptors / Serotonin receptors / Tetrahydro-c-carbolines / Received: March 17, 2009 Accepted: August 11, 2009 DOI 10.1002/ardp.200900056 Introduction During the last few years, the wide therapeutic potential of 2,3,4,5-tetrahydro-c-carbolines (2,3,4,5-tetrahydro-1H- pyrido[4,3-b]indoles) and their bio-isosteric analogs have been attracting increasing attention among researchers [1���11]. Thus, in the 1950s and ���60s it was clearly shown that 2,3,4,5-tetrahydro-c-carbolines can be reasonably regarded as promising antihistaminic [1, 2, 4] and local anesthetic agents [5] as well as serotonin-release inhibi- tors [6]. They have also been shown to possess antidepres- sant [7] and anti-inflammatory [4, 8] activity. In addition, some compounds containing this core fragment were found to have an enhanced neuroleptic activity [9���11]. Indeed, Dimebon I (Fig. 1) (the generic name is dime- bolin, 2,8-dimethyl-5-[2-(6-methylpyridin-3-yl)ethyl]-2,3,4, 5-tetrahydro-1H-pyrido[4,3-b]indole dihydrochloride) ori- ginally synthesized in 1961 by A. N. Kost and colleagues [12, 13], is the best-known therapeutic agent within this group. It is currently being evaluated in advanced clinical trials as a promising small molecule drug-candidate for thetreatmentofvariousneurodegenerativedisorders[14��� 16], including Alzheimer9s disease [17] (it has successfully passedphaseII[18, 19]clinicalevaluationandisnowbeing investigated in phase III [20]) and Huntington9s disease [21, 22], as well as for different types of schizophrenia [23], andotherpathologicalconditions[24���26].Itshouldbepar- ticularly noted that in neuron cell lines, Dimebon effec- tively prevents the neurotoxic effects of ��-amyloids (ED50 = 25 lM)[27].Theobservedactionhasbeendirectlyassigned tothekeymechanismbywhichDimebonrevealsitsneuro- protectivepotential. Since 1983, people in Russia have been using this com- pound as an antihistaminic agent [28, 29]. Furthermore, a series of recent in-vivo studies has revealed the com- pound as a promising small molecule agent, having sig- nificantly suppressed NMDA receptors (ED50 = 42 mg/kg) [31]. In addition, Dimebon was found to strongly inhibit cholinesterase activity, in particular for AchE, IC50 = 45 lM and for BuChE, IC50 = 7.9 lM [32], as well as mito- Correspondence: Alexandre V. Ivachtchenko, ChemDiv, Inc., 6605 Nancy Ridge Drive, San Diego, CA 92121 USA. E-mail: av@chemdiv.com Fax: +1 858-794-4931 i 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 740 Arch. Pharm. Chem. Life Sci. 2009, 342, 740���747

Arch. Pharm. Chem. Life Sci. 2009, 342, 740���747 Synthesis and Activity of Novel Tetrahydro-c-carbolines 741 chondrial permeability transition pores [33]. It was also shown that Dimebon effectively blocked the L-type cal- cium channel activity with an IC50 of 57 lM [30]. How- ever, in turn, a recent proof-of-concept study performed by Medivation has revealed a novel mechanism of action for this compound. It has been tentatively associated with the mitochondrial signaling pathway [34]. Neverthe- less, a clear understanding of the non-trivial mechanism of action has not yet been achieved. Recently, in 2008, we have demonstrated that Dime- bon has a more extended therapeutic area. In particular, the field of its application as a small molecule inhibitor includes several types of adrenoceptors (a1A, a1B, a1D, a2A), dopamine (D1, D2L, D2S, D3), serotonin (5-HT2A, 5-HT2B, 5- HT2C, 5-HT6, 5-HT7) receptors, and other GPCR classes [35��� 37]. These findings clearly indicate that Dimebon has a very complex mechanism of action, therefore it can be reasonably regarded as an excellent example of biologi- cally active compound within the specific group of ���magic shotguns��� [38]. As briefly described above, the antagonistic potency of Dimebon against serotonin 5-HT6 and histamine H1 receptors is the most significant determinant for a broad spectrum of the related beneficial cognitive effects. Thus, comprehensive data show that in mammals, including humans, 5-HT6 serotonin receptor is thoroughly embedded in the CNS. The internal signaling cascades controlling the CNS activity are deeply implicated in the related processes of information perception, learning, and memory formation [39, 40]. It has also been shown that serotonin 5-HT6 receptors regulate several neuro- transmitter pathways including cholinergic, noradrener- gic, glutamatergic, and dopaminergic systems [41]. A number of recent studies have clearly pointed out that serotonin 5-HT6 receptor plays a critical role in the treat- ment of Alzheimer9s disease [42���44]. Thus, taking into account both the fundamental role of this signal trans- duction system in regular cognitive processes as well as their dysfunction caused by neurodegenerative disor- ders, it is therefore becoming evidently clear that seroto- nin 5-HT6 receptors represent extremely attractive tar- gets for the development of novel small molecule thera- peutics for the treatment of various neurodegenerative disorders [40]. The antihistamine activity of Dimebon was revealed long ago, and it was reasonable to compare the SAR profiles of new Dimebon analogs toward seroto- nin 5-HT6 and histamine H1 receptors. In this paper, we describe the syntheses, biological evaluations, and structure-activity relationships for a ser- ies of novel small molecule bioisosteric analogs of the template molecule I (Fig. 1). The evaluated compounds of the general formula III are closely related in structure to compounds II, which have also been prepared recently and tested in vitro using a similar biological assay in our laboratory [45]. The overall aim of the present study is to investigate the spectrum of biological activity and selec- tivity of the title compounds against a panel of serotonin receptors as well as against histamine H1 receptor. A com- parative study across all the analogs synthesized, in par- ticular from compounds II to III, has revealed a clear SAR profile. Jointly, these data may help scientists to more deeply understand the mechanism of action of Dimebon and its structural analogs. The second principal goal is to develop convenient synthetic routes to novel tetrahydro- c-carbolines which would be eminently suitable for a modern combinatorial format. Results and discussion During the initial step of our work, we have synthesized 8-substituted 2-methyl-5-pyridin- 3a���f and 2-methyl-5-pyr- imidin-5-yl-2,3,4,5-tetrahydro-1H-pyrido[4,3-b]indoles 3g, h using the synthetic approach developed by Antilla and colleagues [46]. Thus, the desired compounds 3a���h were readily obtained by the reaction of 8-substituted 2- i 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.archpharm.com Figure 1. Dimebon I, its close structural analogues II [45], and compounds described in this work III.