Abstract
An efficient discovery strategy by combining diversity-oriented synthesis and converging cellular screening is described. By a three-round screening process, we identified novel tricyclic pyrido[2,3-b][1,4]benzothiazepines showing potent inhibitory activity against paclitaxel-resistant cell line H460TaxR (EC50 < 1.0 µM), which exhibits much less toxicity toward normal cells (EC50 > 100 µM against normal human fibroblasts). The most active hits also exhibited drug-like properties suitable for further preclinical research. This redeployment of antidepressing compounds for anticancer applications provides promising future prospects for treating drug-resistant tumors with fewer side effects.
Keywords: tricyclic thiazepine, H460TaxR, selective cytotoxicity, antidepressing, redeployment
Graphical abstract
INTRODUCTION
Non-small cell lung cancer (NSCLC) is the most frequent subtype (accounting for over 80%) of lung cancer, which remains the leading cause of cancer death worldwide.1 Despite improvements in multidisciplinary therapeutic approaches, the five-year survival rate for NSCLC patients is still disappointingly low at less than 20%.1–3 Multidrug resistance (MDR) has been a huge barrier for cancer therapy and is responsible for treatment failure in 90% of patients with metastatic cancer, including NSCLC.4–9 Novel anticancer agents that can overcome MDR are urgently needed. Therefore, a strategy combining efficient screening and chemical synthesis methods may facilitate the development of new agents and be of great interest in the field.
High-throughput screening (HTS) of chemical libraries, which may contain hundreds of thousands to millions of compounds, has become a routine effort for the discovery of tool compounds and drug leads.10 However, this method requires robotics and other sophisticated equipment to increase the scale and speed of assays. It still takes a highly specialized and expensive screening lab to run an HTS operation. Converging screening conducts screening in an iterative fashion instead of attempting to find all the desirable hits in a single round of screening. This approach has demonstrated its efficiency and flexibility in several lead identification projects.11
Tricyclic compounds have been widely used for antipsychotic drugs like tianeptine and anti-HIV or antiplatelet agents.12–14 These tricyclic derivatives were also reported to exhibit anticancer activity.15,16 In the last few years, we have developed synthetic methodologies to access a large library of novel diversified tricyclic derivatives.17 To minimize the effort required and maximize the information obtained, we have developed a strategy to apply converging screening to the discovery of anticancer agents from a library of heterocyclic compounds. As depicted in Figure 1, we initially selected 28 leukemia cell lines and 245 representative compounds from 23 novel heterocyclic scaffolds for initial screening. Then, EC50 values of compounds with anticancer activities were determined. Anticancer activities of compounds with the optimal scaffold against other solid tumor cell lines and normal human fibroblasts (NHFB) were investigated. After the second round of screening, the most active scaffold of compounds was paired with its target cancer cell line of interest. On the basis of the information obtained, a new round of design and synthesis was conducted, and the compounds were evaluated for their anticancer activity against the most active cell line and NHFB. With this iterative process, we have discovered novel anticancer agents with potent activity and low in vitro toxicity (EC50 > 100 µM against NHFB). Herein, details of the development of this strategy and its application in the discovery of novel tricyclic thiazepine derivatives17e (representative compound 1w, Figure 2) as anti-MDR cancer agents are reported.
Figure 1.
Discovery strategy.
Figure 2.
Structures of representative tricyclic thiazepine drugs and compound 1w.
RESULTS AND DISCUSSION
Selection of Representative Compounds
Our small-molecule compound library was designed and built with privileged structures to cover diverse chemical spaces.17,18 For ensuring the selection of high quality compounds, 23 scaffolds were selected based on novelty, availability, and empirical relevance to the current drugs (Figure 3). These scaffolds were classified into 6 monocyclic scaffolds, 5 bicyclic scaffolds, 11 tricyclic scaffolds, and 1 tetracyclic scaffold. All of these scaffolds can be accessed in large numbers from commercially available building blocks through parallel synthesis. Eventually, 245 representative compounds were selected based on structural diversity.
Figure 3.
Structures of selected scaffolds.
First Round of Screening
Leukemia is the most commonly diagnosed cancer in children, accounting for approximately 35% of all pediatric cancers. Over 80% of childhood leukemia cases are acute leukemia.19 Acute leukemia can potentially cause death within a few weeks without timely and appropriate treatment.20 In this work, we first selected 28 acute leukemia cell lines for screening (see Supporting Information for details).
All 245 selected compounds were evaluated against leukemia cell lines. The screening results of a tricyclic thiazepine scaffold XIV against the OP-1 cell line are summarized in Table 1 (see Supporting Information for the results of the other cell lines and compounds). As shown in Table 1, our data demonstrated that, among the 23 compounds tested, 5 compounds (1a, 1h, 1i, 1q, and 1w) exhibited antiproliferative activity with EC50 values below 100 µM. It was noteworthy that sulfone 1w (n = 2) showed a 23-fold increase of cytotoxicity compared with sulfide analogue 1a, indicating that the sulfone moiety is pivotal in the anticancer activity.
Table 1.
Cytotoxic Activity of the Active Scaffold XIV against Representative Cell Line OP-1
| cpd | X | Y | R1 | R2 | n | EC50 (µM) |
|---|---|---|---|---|---|---|
| 1a | N | CH | H | 4-MePh | 0 | 46.1 |
| 1b | N | CH | H | 4-MeOPh | 0 | >100 |
| 1c | N | CH | H | 4-FPh | 0 | >100 |
| 1d | N | CH | H | 4-NO2Ph | 0 | >100 |
| 1e | N | CH | H | 3-NO2Ph | 0 | >100 |
| 1f | N | CH | H | 4-F-3-NO2Ph | 0 | >100 |
| 1g | N | CH | H | furan-2-yl | 0 | >100 |
| 1h | N | CH | Me | Ph | 0 | 92.0 |
| 1i | N | CH | Me | 4-MePh | 0 | 46.6 |
| 1j | N | CH | Me | 4-MeOPh | 0 | >100 |
| 1k | N | CH | Me | 4-FPh | 0 | >100 |
| 1l | N | CH | Me | 3-NO2Ph | 0 | >100 |
| 1m | N | CH | Me | furan-2-yl | 0 | >100 |
| 1n | N | CH | MeO | Ph | 0 | >100 |
| 1o | N | CH | MeO | 3-MePh | 0 | >100 |
| 1p | N | CH | MeO | 4-FPh | 0 | >100 |
| 1q | N | CH | Cl | 4-MePh | 0 | 48.9 |
| 1r | N | CH | Cl | 3-NO2Ph | 0 | >100 |
| 1s | CH | N | Me | Ph | 0 | >100 |
| 1t | CH | N | Me | 4-MeOPh | 0 | >100 |
| 1u | CH | N | Me | 4-NO2Ph | 0 | >100 |
| 1v | N | CH | H | Ph | 1 | >100 |
| 1w | N | CH | H | 4-MePh | 2 | 2.0 |
Second Round of Screening
We evaluated the most active compound 1w from first round screening in several solid tumor cancers, including lung (the most common male cancer, causing 19.4% of all deaths from cancer), breast (the most common female cancer, 25.2% of cancers diagnosed among women), and colon cancer.21 Using CellTiter-Glo luminescent cell viability assay (Promega, Madison, WI, USA), compound 1w was further evaluated for its anticancer activity against HT29 (human colon adenocarcinoma cells), MCF-7/ADR (adriamycin-resistant human breast cancer cells), H460 (human non-small cell lung carcinoma cells), and H460TaxR (paclitaxel-resistant human non-small cell lung carcinoma cells) cell lines (Table 2). Compound 1w displayed the most potent inhibitory activity with EC50 values below 0.5 µM against H460 (entry 4) and H460TaxR (entry 5). It was noteworthy that compound 1w showed less toxicity to normal human fibroblasts NHFB (EC50 > 100 µM, entry 6). Taken together, compound 1w exhibits potent anticancer activities in both leukemia and solid tumor cancers.
Table 2.
Cytotoxic Activity of Compound 1w on Selected Cancer Cell Lines and Normal Human Fibroblasts (NHFB)
| entry | cell line | EC50 (µM) |
|---|---|---|
| 1 | OP-1 | 2.0 |
| 2 | HT29 | 0.9 |
| 3 | MCF-7/ADR | 1.2 |
| 4 | H460 | 0.4 |
| 5 | H460TaxR | 0.3 |
| 6 | NHFB | >100 |
Third Round of Screening and SAR Studies
Synthesis of Tricyclic Thiazepine Analogues
A series of novel tricyclic thiazepine derivatives were designed and synthesized using compound 1w as a lead (Figure 4). Compound 1w contains three appendage-diversification sites, namely, (1) the pyridine ring (A ring), (2) the benzene ring (C ring, R1), and (3) the p-MePh group (R2). Our design sought to explore these sites by altering the pyridine ring (A ring) or introducing other diverse groups (R1 and R2) to investigate the SAR. The key synthesis procedures included a cyclization reaction and sulfur oxidation (see Supporting Information for details). Pyridine analogues 2–15, pyrazine analogues 20–22, and benzene analogue 27 were prepared according to our previously reported method.17e Other analogues were synthesized following a similar procedure to that of quetiapine22 and additional sulfur oxidation.
Figure 4.
Design of tricyclic thiazepine analogues based on compound 1w.
SAR of Tricyclic Thiazepines
On the basis of the results in the second round screening, we decided to focus on evaluation of the cancer cell lines with the most potent activity. All newly synthesized compounds were examined for their antiproliferative activity against H460 and H460TaxR, which is highly resistant to several anticancer drugs, including paclitaxel (PTX).23–25 The EC50 values of the 27 tricyclic thiazepine derivatives are presented in Table 3. As a result, compounds 1w, 9–12, and 14 exhibited an EC50 < 1.0 µM against both H460TaxR and H460 cell lines while exhibiting minimal toxicity toward normal human fibroblasts (NHFB, EC50 > 100 µM) (Figure 5). The substituent on thiazepine (B ring) played an important role in cancer cell inhibition. Compound 1w with a 4-MePh group of R2 was more active than those with other substituents, including Ph (2), 4-MeOPh (5), 4-FPh (6), 4-CF3Ph (7), and furan-2-yl (8). The position of methyl in the MePh group was crucial because compounds 3 and 4 with 2- and 3-methyl, respectively, were both inactive in NSCLC cells. Clothiapine analogue 18 was not substantially toxic to either H460TaxR or H460 cells. In contrast, the substituents including F (9), Cl (10), Me (11), and MeO (12) on the benzene ring (C ring) were less important for the compounds’ cytoselective toxicity. Replacement of the pyridine ring (A ring) with other six-membered aromatic rings was also investigated. The corresponding pyridine analogue 15 (Y = N), pyrazine analogue 22, and benzene analogue 27 were all less potent than compound 1w (X = N). The positive control paclitaxel (PTX) has an EC50 value of 0.8 µM against the H460TaxR cell line but exhibited high toxicity toward NHFB (EC50 = 0.05 µM).
Table 3.
Cytotoxicity of 27 Tricyclic Thiazepine Derivatives against H460TaxR, H460, and NHFB Cell Linesa
| ||||||||
|---|---|---|---|---|---|---|---|---|
| EC50 (µM)
|
||||||||
| cpd | X | Y | Z | R1 | R2 | H460TaxR | H460 | NHFB |
| 1w | N | CH | CH | H | 4-MePh | 0.3 ± 0.1 | 0.4 ± 0.2 | >100 |
| 2 | N | CH | CH | H | Ph | >100 | >100 | ND |
| 3 | N | CH | CH | H | 2-MePh | >100 | >100 | ND |
| 4 | N | CH | CH | H | 3-MePh | >100 | >100 | ND |
| 5 | N | CH | CH | H | 4-MeOPh | >100 | >100 | ND |
| 6 | N | CH | CH | H | 4-FPh | 95.7 ± 10.3 | >100 | ND |
| 7 | N | CH | CH | H | 4-CF3Ph | 95.3 ± 10.7 | >100 | ND |
| 8 | N | CH | CH | H | furan-2-yl | >100 | >100 | ND |
| 9 | N | CH | CH | 8-F | 4-MePh | 0.7 ± 1.1 | 0.4 ± 0.3 | >100 |
| 10 | N | CH | CH | 8-Cl | 4-MePh | 0.7 ± 0.1 | 0.4 ± 0.2 | >100 |
| 11 | N | CH | CH | 8-Me | 4-MePh | 0.8 ± 1.2 | 0.8 ± 1.1 | >100 |
| 12 | N | CH | CH | 8-MeO | 4-MePh | 0.5 ± 0.2 | 0.5 ± 0.1 | >100 |
| 13 | N | CH | CH | 9-MeO | 4-MePh | 4.1 ± 0.4 | 3.0 ± 0.1 | ND |
| 14 | N | CH | CH | 10-MeO | 4-MePh | 0.6 ± 0.3 | 0.5 ± 0.1 | >100 |
| 15 | CH | N | CH | H | 4-MePh | 2.9 ± 0.1 | 3.6 ± 2.8 | ND |
| 16 | N | CH | CH | H | HO | >100 | >100 | ND |
| 17 | N | CH | CH | H | O(CH2CH2)2N | >100 | >100 | ND |
| 18 | N | CH | CH | H | CH3N(CH2CH2)2N | >100 | >100 | ND |
| 19 | N | CH | CH | H | (CH3)2N | >100 | >100 | ND |
| 20 | N | CH | N | H | Ph | >100 | >100 | ND |
| 21 | N | CH | N | H | 4-FPh | >100 | >100 | ND |
| 22 | N | CH | N | H | 4-MePh | 3.6 ± 0.6 | 3.3 ± 1.0 | ND |
| 23 | N | CH | N | H | HO | >100 | >100 | ND |
| 24 | N | CH | N | H | O(CH2CH2)2(O)N | >100 | >100 | ND |
| 25 | N | CH | N | H | (CH3)2N(CH2)2O | >100 | >100 | ND |
| 26 | N | CH | N | H | (CH3)2N | >100 | >100 | ND |
| 27 | CH | CH | CH | H | 4-MePh | 1.5 ± 1.8 | 4.3 ± 1.5 | ND |
| PTX | 0.8 ± 0.4 | 0.006 ± 0.002 | 0.05 ± 0.03 | |||||
The results represent the mean ± SD with n = 3. ND means not determined.
Figure 5.
Dose-dependent cytotoxicity of six hits against H460TaxR, H460, and NHFB are shown along with their chemical structures.
We also examined images of NSCLC and NHFB cells treated with compound 1w or DMSO. As shown in Figure S3, after treatment with DMSO, both NSCLC cell lines proliferated fast. After compound 1w treatment, NSCLC cells detached, became rounded, and did not grow. However, a growth inhibitory effect of compound 1w on NHFB cells was not observed. These results demonstrate that these tricyclic thiazepine derivatives exhibited selective cytotoxicity against H460TaxR and H460 cells.
Profiling the Most Active Compounds
To evaluate the potential for the active hits to be applied in vivo, we performed a panel of absorption, distribution, metabolism, and excretion (ADME) in vitro assays to simulate their in vivo behaviors. As shown in Table 4, six selected hits exhibited overall suitable drug-like properties. The parallel artificial membrane permeability assay (PAMPA) was performed to evaluate cell permeability of the active compounds. Generally, compounds with a Pe > 200 cm/s are classified as highly permeable. Representative compound 1w exhibited excellent potential cell permeability (1081 cm/s). The Caco-2 assay was performed in the P-gp-overexpressed Caco-2 cell line, which reflects the oral absorption ability of the compounds in vivo. With an efflux ratio greater than 2, a compound may be a substrate of P-gp efflux and subject to poor oral absorption. Compound 1w had an efflux ratio of 0.6, which predicted a good oral absorption for this compound. The half-lives (T1/2) of the six hits in human liver microsomes were determined, which reflect their metabolic stability in vivo. A very small T1/2 suggests quick metabolism in the liver, and a very long T1/2 (>24 h) may cause cumulative toxicity. Compound 1w exhibited a suitable T1/2 value (5.2 h) in human liver microsomes, suggesting its excellent metabolic stability in the liver. The high T1/2 value (>50 h) of compound 1w in human plasma indicated a good stability in vivo. Plasma protein binding influences the distribution of drugs into body tissues, and compounds with high plasma protein binding (>99%) are limited in terms of the amount of free compound that are available to act on the targeted tissue. Taken together, as indicated in Table 4, representative compound 1w exhibited favorable drug-like properties. These active tricyclic thiazepine derivatives may also maintain their potent anticancer activity in animals or patients.
Table 4.
Drug-like Properties Predicted by in Vitro ADME Assaysa
| hits | 1w | 9 | 10 | 11 | 12 | 14 |
|---|---|---|---|---|---|---|
| PAMPA Pe (10−6 cm/s) | 1081 ± 31 | 979 ± 63 | 1250 ± 278 | 1472 ± 290 | 1246 ± 309 | 686 ± 82 |
| Caco-2 Papp A/B (nm/s) | 345 ± 21 | 406 ± 86 | 125 ± 9 | 171 ± 83 | 233 ± 64 | 113 ± 20 |
| Caco-2 Papp B/A (nm/s) | 206 ± 71 | 155 ± 35 | 39 ±7 | 87 ± 20 | 118 ± 38 | 59 ± 15 |
| Efflux ratio (B2A/A2B) | 0.6 ± 0.2 | 0.4 ± 0.1 | 0.3 ± 0.1 | 0.6 ± 0.2 | 0.5 ± 0.1 | 0.5 ± 0.1 |
| T1/2 (h) in human liver microsome | 5.2 ± 0.7 | 1.2 ± 0.1 | 1.3 ± 0.1 | 0.7 ± 0.1 | 1.1 ± 0.1 | 2.3 ± 0.1 |
| T1/2 (h) in human plasma | >50 | >50 | >50 | >50 | >50 | >50 |
| Plasma protein binding (%) | 98.4 ± 0.2 | 98.2 ± 0.2 | 94.8 ± 0.3 | 99.1 ± 0.1 | 98.8 ± 0.1 | 97.8 ± 0.1 |
The results represent the mean ± SD with n ≥ 2.
CONCLUSIONS
In summary, we report an efficient strategy for the discovery of antidrug-resistant cancer agents by combining diversity-oriented synthesis and converging screening. By a three-round screening approach, we successfully identified novel tricyclic pyrido[2,3-b][1,4]benzothiazepines, which showed a potent inhibitory activity against paclitaxel-resistant cell line H460TaxR (EC50 < 1.0 µM) while exhibiting minimal toxicity toward normal human fibroblasts (EC50 > 100 µM). The active hits also exhibited suitable drug-like properties, which is key for application in vivo. This redeployment of clinical drugs for anticancer applications provides promising potential for the discovery of new anticancer agents.
Supplementary Material
Acknowledgments
We thank Drs. Guoqing Du and Lei Yang for their assistance in the early phase of this project.
Funding
This work was supported by a grant from the National Natural Science Foundation of China (No. 90713008) and grants from the Sci-Tech Development Project of Jilin Province in China (Nos. 20140309010YY and 20106039). Additional support was provided by Changchun Discovery Sciences, Ltd.
Footnotes
ASSOCIATED CONTENT
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscombsci.6b00010.
Scaffolds, cytotoxicity assays, cell line images, descriptions of biological assays, and NMR spectra of all of the products (PDF)
The authors declare no competing financial interest.
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