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Non-Naturally Occurring Small Molecule Microtubule-Stabilizing Agents: A Potential Tactic for CNS-Directed Therapies Carlo Ballatore,*,† Kurt R. Brunden,‡ John Q. Trojanowski,‡ Virginia M.-Y. Lee,‡ and Amos B. Smith, III*,§ †
Skaggs School of Pharmacy and Pharmaceutical Sciences, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093, United States ‡ Center for Neurodegenerative Disease Research, Institute on Aging, University of Pennsylvania, 3600 Spruce Street, Philadelphia, Pennsylvania 19104-6323, United States § Department of Chemistry, School of Arts and Sciences, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104-6323, United States ABSTRACT: Several independent studies indicate that microtubule (MT)-stabilizing agents hold considerable promise as candidate therapeutics for a wide spectrum of conditions of the central nervous system (CNS), from brain tumors to spinal cord injury, as well as a number of neurodegenerative diseases, including Alzheimer’s disease, frontotemporal lobar degeneration, Parkinson’s disease, and amyotrophic lateral sclerosis. Although the identification and development of candidate compounds for CNS-directed MT-stabilizing therapies has been a challenge in drug discovery for many years, a growing number of molecules have now been identified that exhibit both MT-stabilizing activity and brain penetration. In this Viewpoint, we will highlight the potential utility of MT-active triazolopyrimidines, phenylpyrimidines, and related classes of non-naturally occurring small molecules that exhibit favorable druglike properties, including brain penetration and oral bioavailability. The mode of action of these small molecules has not as yet been fully elucidated at the molecular level. However, based on all available data, compounds from these classes appear to act on MTs in a potentially unique manner. Further characterization of these molecules may have important ramifications for drug discovery, especially in the area of CNS diseases. KEYWORDS: Microtubule, small molecule microtubule-stabilizing agents, Alzheimer’s disease, brain tumor, traumatic brain injury, neurodegenerative disease, triazolopyrimidine, phenylpyrimidine
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Among these, several triazolopyrimidines,1 phenylpyrimidines,2 and other related heterocyclic molecules (Figure 1) appear to be potentially attractive starting points for CNS drug development due to their relatively simple structure and generally favorable pharmacokinetic properties, including brain penetration and oral bioavailability.3 These compounds were initially identified as antifungal agents and investigated as potential agrochemicals. The pharmaceutical company, Wyeth, later conducted an anticancer drug development program that resulted in the identification of a triazolopyrimidine clinical candidate, cevipabulin (1, Figure 1), which displayed favorable druglike properties, such as oral bioavailability and metabolic stability.1 In preclinical studies, the anticancer activity of 1 was found to be comparable to that of other well characterized MTstabilizing natural products, such as paclitaxel (Taxol, 2, Figure 1). However, unlike paclitaxel, cevipabulin retained antimitotic effects in paclitaxel-resistant cell lines, including cells that overexpress the P-glycoprotein (Pgp) active transporter. Several additional members of the triazolopyrimidine class, as well as phenylpyrimidines, exhibit marked anticancer activity in different mouse xenograft models, including the U87-MG cell model of glioblastoma and the A549 cell model of nonsmall cell
icrotubules (MTs), which comprise an essential component of the cytoskeleton in all eukaryotic cells, have long been recognized as a significant therapeutic target for the treatment of several forms of cancer due to the critical role that MTs play in chromosome segregation during cell division. As a result, MT-stabilizing drugs are an important class of therapeutics available to oncologists. However, in addition to the well-established role of MT-stabilizing agents in cancer chemotherapy, this highly heterogeneous group of molecules may have potential to treat other diseases, including nonproliferative diseases. Indeed, a series of independent studies revealed that the dysregulation of MT structure and dynamics in neurons may be a common phenomenon shared by several different diseases of the CNS and that stabilization of MTs in the brain may be a promising therapeutic strategy. Importantly, stabilization of MTs has proven to be an effective strategy in animal models of neurodegenerative tauopathies, a group of >20 diseases which include Alzheimer’s disease (AD). Moreover, preclinical studies demonstrated that MT-stabilizing agents may be beneficial to treat Parkinson’s disease (PD) and amyotrophic lateral sclerosis (ALS), as well as traumatic brain injury (TBI) and spinal cord injury (SCI). Until recently, the identification of CNS-active MTstabilizing agents has been challenging primarily due to issues related to limited brain penetration. However, in recent years, a growing number of molecules have been identified that exhibit both MT-stabilizing activity and excellent brain exposure. © XXXX American Chemical Society
Received: November 9, 2016 Accepted: November 11, 2016
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DOI: 10.1021/acschemneuro.6b00384 ACS Chem. Neurosci. XXXX, XXX, XXX−XXX
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ACS Chemical Neuroscience
Figure 1. (A) Structure of MT-stabilizing natural products, paclitaxel (2) and epothilone D (3). (B) Structures of representative triazolopyrimidines (1, 4, 5), phenylpyrimidines (6), and related pyridazines (7), pyridopyridazines (8), imidazoles (9), pyrazoles4 (10), and pyrazinones5 (11) with reported MT-stabilizing activity.
observed competition with [3H]-vinblastine results from overlapping binding sites or a distinct allosteric site. Either way, these findings suggest that the mode of action of these small molecules is completely distinct from other known MTstabilizing compounds. In this context, our recent studies6 with a series of brain-penetrant triazolopyrimidines and phenylpyrimidines revealed that the compounds can elicit substantially different outcomes on the MT structure in cells depending on the particular nature of the substituents decorating the heterocyclic scaffold. This phenomenon is evident when comparing closely related congeners such as 4 and 5 (Figure 1). Although the latter was found to be functionally similar to well characterized MT-stabilizing natural products, such as paclitaxel and epothilones, in producing a qualitatively similar increase in markers of stable MTs in cell cultures in a concentration-dependent manner, the former compound (4)
lung carcinoma. Interestingly, while the clinical candidate, cevipabulin, was found to exhibit limited brain uptake, several related triazolopyrimidine and phenylpyrimidine congeners were found in our laboratories to be brain-penetrant and to affect MTs in the brain of wild-type (WT) mice.3 Interestingly, although the triazolopyrimidines and related congeners appear to be functionally related to MT-stabilizing natural products, the mode of action of these heterocyclic compounds has not been completely defined at the molecular level. For example, competition binding studies using radioactive [3H]-labeled vinblastine, colchicine and paclitaxel demonstrate that these triazolopyrimidines and phenylpyrimidines do not effectively inhibit the binding of colchicine or paclitaxel. Somewhat surprisingly, these agents were instead found to inhibit the binding of the tubulin polymerization inhibitor, vinblastine.2 It is not clear, however, whether the B
DOI: 10.1021/acschemneuro.6b00384 ACS Chem. Neurosci. XXXX, XXX, XXX−XXX
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ACS Chemical Neuroscience
therapeutics for Alzheimer’s disease and related tauopathies. J. Med. Chem. 57, 6116−6127. (4) Dumeunier, R., Lamberth, C., and Trah, S. (2013) Synthesis of Tetrasubstituted Pyrazoles through Different Cyclization Strategies; Isosteres of Imidazole Fungicides. Synlett 24, 1150−1154. (5) Taggi, A. E., Stevenson, T. M., Bereznak, J. F., Sharpe, P. L., Gutteridge, S., Forman, R., Bisaha, J. J., Cordova, D., Crompton, M., Geist, L., Kovacs, P., Marshall, E., Sheth, R., Stavis, C., and Tseng, C.P. (2016) Tubulin modulating antifungal and antiproliferative pyrazinone derivatives. Bioorg. Med. Chem. 24, 435−443. (6) Kovalevich, J., Cornec, A. S., Yao, Y., James, M., Crowe, A., Lee, V. M., Trojanowski, J. Q., Smith, A. B., III, Ballatore, C., and Brunden, K. R. (2016) Characterization of brain-penetrant pyrimidinecontaining molecules with differential microtubule-stabilizing activities developed as potential therapeutic agents for Alzheimer’s disease and related tauopathies. J. Pharmacol. Exp. Ther. 357, 432−450.
was found to produce an unusual cellular response. This response is characterized by (a) a bell-shaped concentration− response effect on markers of MT stabilization; (b) a compound induced proteasome-dependent degradation of αand β-tubulin; and (c) altered MT morphology in cell cultures. These observations suggest the possibility that a different mode or modes of binding by these analogs may affect in substantially different ways the tubulin/MT structure. These different compound-induced effects on MT structure may provide an opportunity to develop MT-targeting strategies that are tailored for different indications, such as cancer and neurodegenerative diseases. For example, an examination of the published in vitro anticancer activity data suggest that triazolopyrimidines, such as 4 and cevipabulin, which alter MT-morphology and cause a proteasome-dependent degradation of α- and β-tubulin, are generally more cytotoxic to rapidly dividing cancer cell-lines than closely related congeners that stabilize MTs without apparent alterations in MT-morphology.1 This observation is consistent with the fact that cevipabulin was the preferred candidate compound for cancer treatment. Conversely, compounds such as 5, that are comparatively less cytotoxic and that do not grossly disrupt the MT morphology, may be more desirable for other indications like neurodegenerative disease where the goal is the preservation of MT integrity in neurons rather than alteration of the mitotic spindle in dividing cancer cells. Thus, the apparent ability to select the mode of action of the MT-active triazolopyrimidine and phenylpyrimidine congeners, combined with the generally favorable pharmacokinetic properties, may prove to be a desirable feature that could be exploited to develop MT-targeting agents for different clinical indications. In this regard, the continued development of these molecules would be greatly aided by further studies directed toward the elucidation of the binding site of the triazolopyrimidines and related heterocycles, and to a further understanding of the structure-MT-stabilizing activity relationship of these small molecules.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Carlo Ballatore: 0000-0002-2718-3850 Amos B. Smith III: 0000-0002-1712-8567 Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Zhang, N., Ayral-Kaloustian, S., Nguyen, T., Afragola, J., Hernandez, R., Lucas, J., Gibbons, J., and Beyer, C. (2007) Synthesis and SAR of [1,2,4]triazolo[1,5-a]pyrimidines, a class of anticancer agents with a unique mechanism of tubulin inhibition. J. Med. Chem. 50, 319−327. (2) Zhang, N., Ayral-Kaloustian, S., Nguyen, T., Hernandez, R., Lucas, J., Discafani, C., and Beyer, C. (2009) Synthesis and SAR of 6chloro-4-fluoroalkylamino-2-heteroaryl-5-(substituted)phenylpyrimidines as anti-cancer agents. Bioorg. Med. Chem. 17, 111− 118. (3) Lou, K., Yao, Y., Hoye, A. T., James, M. J., Cornec, A. S., Hyde, E., Gay, B., Lee, V. M., Trojanowski, J. Q., Smith, A. B., III, Brunden, K. R., and Ballatore, C. (2014) Brain-penetrant, orally bioavailable microtubule-stabilizing small molecules are potential candidate C
DOI: 10.1021/acschemneuro.6b00384 ACS Chem. Neurosci. XXXX, XXX, XXX−XXX
