Introduction
Peripheral T-cell lymphoma (PTCL) is classified as a type of non-Hodgkin lymphoma and is associated with relatively poor survival compared with other lymphomas, such as diffuse large B-cell lymphoma and Hodgkin lymphoma [
1]. There are various subtypes of PTCL: anaplastic large cell lymphoma (ALCL) (either with or without anaplastic lymphoma kinase rearrangements), angioimmunoblastic T-cell lymphoma (AITL), PTCL not otherwise specified (PTCL-NOS), and cutaneous T-cell lymphoma (CTCL) [
2].
Chemotherapy regimens, such as the combination of cyclophosphamide, doxorubicin, vincristine, and prednisone (CHOP) with or without etoposide, have traditionally been the standard treatment for PTCL [
3]. Several new systemic agents, including histone deacetylase inhibitors (HDACis), folate analog metabolic inhibitors, and anti-CD30 antibody–drug conjugate, have been approved for the treatment of initial or relapsed PTCL [
4‐
6].
Brentuximab vedotin (BV) is an anti-CD30 monoclonal antibody linked to the microtubule-destabilizing agent monomethyl auristatin E (MMAE). MMAE is released into the cytosol of CD30-expressing cells and induces G2-/M-phase growth arrest and apoptosis [
7]. The expression of CD30 has been reported across PTCL subtypes. It is abundantly expressed in ALCL and at varying frequencies in other PTCLs [
8].
The clinical benefits of BV, either as monotherapy or in combination with other chemotherapies, have been well demonstrated in patients with PTCL in several clinical trials. In a phase 1 trial, patients with newly diagnosed CD30-expressing PTCL who received BV combined with CHOP or CHOP without vincristine (CHP) achieved an estimated 5-year progression-free survival (PFS) rate of 52%, and an estimated overall survival (OS) rate of 80% [
9,
10]. In a phase 3, randomized trial (ECHELON-2, NCT01777152), BV combined with CHP achieved superior PFS and OS rates compared with CHOP in patients with newly diagnosed CD30-expressing PTCL. BV combined with CHP demonstrated a 5-year PFS rate of 51% and a 5-year OS rate of 70% in the ECHELON-2 trial, and this is now the first-line treatment option in this disease setting [
11,
12].
The clinical benefits of BV have also been demonstrated, as monotherapy, in patients with relapsed or refractory (R/R) CD30-expressing PTCLs, including ALCL, PTCL-NOS, AITL, and CTCL [
6,
13,
14]. Based on these findings, BV monotherapy has been approved to treat a relatively wide range of patients with PTCL, including those with newly diagnosed PTCL and those with R/R disease, whereas drugs such as forodesine and pralatrexate are approved as monotherapy for R/R disease only. BV is now considered to be a key agent for the treatment of PTCL and has opened a new era in disease management.
Despite these advances, including BV and various combination chemotherapies, the survival time of patients with PTCL has not been improved much and still remains very poor. The 5-year OS rate for PTCL has been reported at 58.4%, whereas patients with other lymphomas, such as diffuse large B-cell lymphoma and Hodgkin lymphoma, are reported to achieve survival rates of 63.2% and 85.7%, respectively [
15].
Our research group has been exploring new BV combination partners using non-clinical approaches, to try and improve survival times for patients with PTCL. In the current study, we identified a synergistic partner of BV using a matrix concentration screening test, which is a high-throughput system that rapidly and quantitatively tests the effects of different drug combinations on cells. The effects of BV combined with this partner drug were then further investigated by in vitro and in vivo methods using xenograft mice. In addition, gene expression changes evoked by combination treatment were analyzed using next-generation sequencing.
Materials and methods
Cell lines
Human cancer cell lines (HH [CTCL], ATCC_CRL-2105 and MOLT-4 [acute lymphoblastic leukemia], ATCC_CRL-1582) were purchased from the American Type Culture Collection (Manassas, VA, USA). DND-41 (T-cell acute lymphoid leukemia) was purchased from DSMZ-German Collection of Microorganisms and Cell Cultures GmbH (Braunschweig, Germany). All cell lines were cultured in Roswell Park Memorial Institute-1640 medium supplemented with 10% fetal bovine serum at 37 °C in 5% CO2.
Reagents
BV was obtained from Takeda Pharmaceutical Company Limited (Tokyo, Japan). IgG was purchased from GeneTex (Irvine, CA, USA). Fourteen candidate drugs were assessed as potential synergistic partners of BV. Belinostat, romidepsin, and darinaparsin were purchased from Toronto Research Chemicals Inc. (North York, ON, Canada). Pralatrexate, chidamide, deoxycoformycin, dexamethasone, lenalidomide, gemcitabine, doxorubicin, etoposide, and MK2206 were purchased from MedChemExpress (Monmouth Junction, NJ, USA). Selinexor was purchased from Ark Pharm (Arlington Heights, IL, USA). Nelarabine was purchased from Sigma-Aldrich (St Louis, MO, USA).
Cell viability assays
To assess cell viability, HH (4 × 103 cells/well), DND-41 (4 × 104 cells/well), and MOLT-4 (3 × 104 cells/well) cells were seeded and cultured for 24 h and then treated with BV (0.0068, 0.020, 0.068, 0.20, 0.68, 2.0, 6.8, 20 and 68 nM) for 72 h. Adenosine 5′-triphosphate content was detected using the CellTiter-Glo Luminescent Cell Viability Assay (Promega, Madison, WI, USA) and luminescence was measured by ARVO X light (PerkinElmer, Waltham, MA, USA). Half-maximal inhibitory concentrations (IC50) were calculated using GraphPad Prism 6 software (GraphPad Software, San Diego, CA, USA).
Flow cytometry analysis
Cells were collected and incubated for 30 min at 4 °C with phycoerythrin (PE)-labeled mouse anti-human CD30 antibodies (#130-098-686, Miltenyi Biotec, Bergisch Gladbach, Germany), with isotype-matched control antibodies (#130-092-213, Miltenyi Biotec), or without antibodies. After washing with phosphate-buffered saline, cell-associated fluorescence was detected by the BD LSRFortessa Cell Analyzer (Becton Dickinson, Franklin Lakes, NJ, USA), and CD30 expression levels were analyzed using FlowJo software (Becton Dickinson).
Matrix concentration screening test
HH cells were seeded and treated with BV or one of the 14 other anticancer drugs, either as single agents or in combination, using a 7 × 10 concentration matrix for 72 h. Cell viabilities were detected and evaluated as described in the ‘
Cell viability assays’ subsection. The concentration ranges were 0.012–4.1 nM for BV and 0.001–10 µM for each of the 14 other anticancer drugs and the reproducibility study for chidamide was conducted using proper concentration ranges (0.004–1.2 nM for BV and 0.001–10 µM for chidamide). Commercially obtained IgG (0.004–1.2 nM) was used as the negative control for BV. Optimal molar concentration ratios (BV:chidamide = 1:2500 and 1:25,000) were used for isobologram analysis. The anticancer effects of each drug pair were scored by the R package SynergyFinder 1.8.0. [
16,
17], in which the difference between the actual effect and the expected effect was calculated based on the Bliss model and expressed as a Bliss score. The expected effect represented the additive effect estimated from the anticancer activity of each drug (A and B). For example, Bliss score C = A + B − A × B. A, B, and C were percentage fractional inhibitions; thus, they represent the magnitude of synergistic or antagonistic effects, corresponding to positive or negative values, respectively. Combination indices (CIs) were calculated using CalcuSyn Version 2.0 (BIOSOFT, Cambridge, UK) based on the Chou–Talalay method [
18]. This provided quantitative definitions for an additive effect (CI = 1), synergism (CI < 1), and antagonism (CI > 1) of drug combinations. To plot an isobologram, fraction-affected levels were predicted using CalcuSyn Version 2.0.
DNA fragmentation assay
DNA fragmentation in cells was detected using the Cell Death Detection ELISAPLUS (Sigma-Aldrich) and a Viento XS plate reader (BioTek, Winooski, VT, USA), according to the manufacturer’s instructions, following 24-h incubation with BV, chidamide, or the combination of BV and chidamide (Sample size, n = 2).
Caspase 3/7 assay
Cells were seeded and treated with BV or chidamide as single agents or in combination for 24 h. Caspase 3/7 activities were measured using the Caspase-Glo 3/7 Assay System (Promega) according to the manufacturer’s instructions (Sample size, n = 2). Luminescence was detected by the ARVO X light.
Western blotting
Cells were lysed in buffer containing 62.5 mM Tris hydrochloride, 10% glycerin, 2% sodium dodecyl sulfate, protease inhibitor (Sigma-Aldrich), and phosphatase inhibitor (Sigma-Aldrich). Xenograft tumors were disrupted using TissueLyser II (Qiagen, Hilden, Germany) in the same buffer.
Protein concentrations in the whole cell or tissue lysate were measured using the BCA Protein Assay Kit (Thermo Fisher Scientific, Waltham, MA, USA). Proteins (5 µg/lane) were separated by sodium dodecyl sulfate–polyacrylamide gel electrophoresis, transferred onto polyvinylidene fluoride (PVDF) membranes and blocked with PVDF blocking buffer (TOYOBO, Osaka, Japan). Membranes were incubated with the following primary antibodies diluted in Can Get Signal™ solution 1 (TOYOBO) overnight at 4 °C: acetyl-histone H3 (#9649, Cell Signaling Technology [CST], Danvers, MA, USA), histone H3 (#9715, CST), poly (adenosine diphosphate-ribose) polymerase (PARP; #9532, CST), cleaved PARP (#5625, CST), cleaved caspase 3 (#ab214430, Abcam, Cambridge, UK), caspase 3 (#ab179517, Abcam), Bim (#2933, CST), Mcl-1 (#ab28147, Abcam), Bcl-2 (#15071, CST), survivin (#af886, R&D Systems, Minneapolis, MN, USA), and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) (#3683, CST). Membranes were then incubated with anti-rabbit or anti-mouse immunoglobulin G secondary antibodies (#7074 or #7076, CST) diluted in Can Get Signal Solution 2 (TOYOBO). Protein–antibody interactions were detected with ECL Select (Amersham, Buckinghamshire, UK) according to the manufacturer’s instructions. Signal intensity was measured using ImageQuant LAS 4000 mini (GE Healthcare, Chicago, IL, USA) and analyzed with ImageQuant TL software (GE Healthcare).
In vivo mouse xenograft study
Five million HH cells were mixed with BD Matrigel (BD Biosciences, Franklin Lakes, NJ, USA) and inoculated in the right flank of 6-week-old female mice with severe combined immunodeficiency (CLEA, Tokyo, Japan). No sex difference in apoptosis was assumed. Mice were randomly assigned to four groups of five mice each and treated with either vehicle (0.2% carboxymethylcellulose saline and 0.1% Tween 80 for chidamide, and saline for BV), BV (0.1 mg/kg once a week, intravenously), chidamide (15 mg/kg once a day, oral gavage), or BV combined with chidamide (at the same dose and frequency as those used for single-agent treatment). For the negative control experiments shown in Fig.
4e and f, mice were administrated either combination of IgG (0.1 mg/kg once a week, intravenously) and vehicle, BV and vehicle, chidamide and IgG, or chidamide and BV. Drugs were administered when tumors reached an average volume of 100 mm
3. Tumors were measured twice a week using digital calipers, and volumes were calculated as [L × (W × W)]/2, in which L is the longest diameter (in mm) and W is the shortest diameter (in mm). All animal experiments were approved by the Institutional Animal Care and Use Committee (IACUC) of Shonan Health Innovation Park accredited by the Association for the Assessment and Accreditation of Laboratory Animal Care International (AAALAC).
Next-generation sequencing
Total RNA was extracted using the RNeasy Mini kit (Qiagen) according to the manufacturer’s instructions. Amplicon multiplex sequencing experiments were performed using the Ion AmpliSeq Transcriptome Human Gene Expression kit (Thermo Fisher Scientific) according to the manufacturer’s instructions. Briefly, target transcripts were amplified by polymerase chain reaction (PCR) from complementary DNA libraries synthesized from 1 ng of total RNA. Reactants were ligated to adapters and pooled at equal concentrations; multiplex sequencing at over 8 million reads per sample was performed using Ion Proton high-throughput sequencers (Thermo Fisher Scientific). Before identification of differentially expressed genes, genes were selected to satisfy the condition that they were expressed at a minimum of one read per million in the sample with the greatest expression levels among all the compared samples. Differentially expressed genes with a p value below 0.05 and an absolute log2-fold change above 0.5 were identified using the voom function in the limma package in R.
Gene pathway analysis
Gene pathway analysis was performed using the R packages ReactomePA 1.26.0, with an adjusted
p value cut off below 0.05, and GO Function, with the false-discovery rate cut off below 0.5 [
19]. The intersection of all assigned genes in AmpliSeq panel and human annotated genes in reference databases were selected as background. Principal component analysis was used to reduce the number of variables.
Quantitative reverse transcription PCR assays
In total, 4000 HH cells were harvested, and CDC45 messenger RNA (mRNA) expression was determined by quantitative reverse transcription (qRT) PCR using a FastLane Cell Probe Kit (Qiagen) according to the manufacturer’s instructions. Cycling parameters were 50 °C for 30 min and 95 °C for 15 min, followed by 40 cycles at 95 °C for 15 s and 60 °C for 1 min. Data were normalized using GAPDH as an internal control, and relative mRNA expression levels were calculated using the 2
−ΔΔCt method [
20].
Statistical analysis
Data were expressed as mean values and standard errors. Statistical significance was calculated using Student’s t-test. p < 0.05 was considered statistically significant.
Discussion
In this study, we aimed to identify the optimal partner for BV from drugs that are available for the treatment of PTCL in clinical practice or that are under development. Although other analyses typically examined the synergistic effect of multiple drugs based on their mechanism of action [
33,
34], we took a unique approach. All candidate drugs were tested quantitatively for synergistic effects with BV using a matrix concentration screening method. Further experiments were performed to determine the underlying mechanisms of the synergistic effects. This approach allowed for the exploration of unknown pathways evoked by drug combinations, which are not always able to be predicted based on the mechanism of action of either drug alone.
Chidamide (also known as tucidinostat) was identified as the most synergistic partner of BV based on the Bliss score, and in vitro and in vivo experiments further confirmed this synergism. Chidamide is an HDACi that specifically inhibits HDAC1, 2, 3, and 10, and has been approved for the treatment of R/R PTCL [
35].
Based on our results and previous reports, we suggest three key molecular mechanisms that may explain the efficient apoptosis induced by the combination of BV and chidamide: (1) reduction of Bcl-2 expression; (2) simultaneous cell-cycle arrest at M and G1 phases; and (3) downregulation of CDC45. These mechanisms are discussed in turn below, and a schematic of the potential combined anticancer effects of BV and chidamide is presented in Supplementary Fig. E3.
Reduced Bcl-2 expression levels and increased cleaved caspase 3 expression levels were observed following treatment with BV, regardless of the addition of chidamide. This result was consistent with the findings reported by Wang et al. [
36], where reduced Bcl-2 was observed in human Burkitt lymphoma cell lines (Daudi cells and Ramos cells) upon induction of apoptosis using anti-CD20 monoclonal antibody (Rituximab) conjugated with MMAE. Downregulation of Bcl-2 could be suggested as a reliable marker for microtubule disruption caused by MMAE and MMAE-conjugated compounds. Importantly, chidamide does not appear to impede the downregulation of Bcl-2 by BV. Chao et al. previously reported similar alterations of Bcl-2 and caspase 3 levels in MOLT-4 cells treated with the combination of a microtubule-disrupting agent (vincristine) and an HDACi (vorinostat) [
37]. Cyrenne et al. reported that the combination of a Bcl-2 inhibitor and HDACi synergistically killed CTCL cells [
38]. Thus, downregulation of Bcl-2 is thought to be a key factor for efficient apoptosis in cells treated with the combination of BV and chidamide.
It is known that BV induces G2-/M-phase arrest in a CD30-expressing cell line and that HDACi induces the cyclin-dependent kinase inhibitor p21, causing cell-cycle arrest at the G1 phase [
7,
24,
25,
28,
39]. Vincristine combined with vorinostat also showed a synergistic effect on M-phase arrest and an increase of cell numbers in the sub-G1 phase [
37]. Simultaneous cell-cycle arrest at the M and G1 phases could thus be a benefit of the combined use of a microtubule-destabilizing agent and a HDACi.
DEG analysis revealed that downregulation of
CDC45 occurred when HH cells were treated with chidamide combined with BV. CDC45 is a component of the CMG (Cdc45/Mcm2–7/GINS) helicase complex and is a rate-limiting factor of DNA unwinding at replication origin in the S phase [
40].
CDC45 knockdown promotes S-phase arrest and induction of apoptosis in human cancer cells [
41]. Highly proliferative cells express CDC45 abundantly throughout the cell cycle, whereas terminally differentiated and senescent cells lack CDC45. Thus, CDC45 has also been a target of cancer therapies. In cancer therapy, downregulation of
CDC45 might be a positive indicator of successful anticancer effects that change the cell status from proliferative to non-proliferative, further enhancing the apoptosis pathway following S-phase arrest.
In summary, we believe our results provide a reasonable basis for clinical investigation of BV in combination with chidamide as a potential treatment for PTCL. Whether the synergistic effects of BV are specific to chidamide or common to other HDACis is also a question of interest. In the present study, two other HDACis (romidepsin and belinostat) were tested. Both of them provided Bliss scores slightly below 0 when considering the relatively wide concentration range. However, at concentration points within the narrow range, these two drugs yielded Bliss scores above 0 (data not shown). To further ensure and confirm their synergistic or additive effects on BV, we employed another traditional method-isobologram analysis that has been mathematically proven (Supplementary Fig. E2). In this method, the combination effects of two drugs are assessed using pre-defined concentrations within the established effective range. All three HDACis exhibited synergistic effects on BV at the tested concentration points. Therefore, it can be inferred that synergistic anti-cancer effects in combination with BV may be common, at least among the three tested HDACis. Among them, chidamide demonstrated significant synergistic effects and showed effectiveness over a broader range of concentrations. Regarding the differences among these three inhibitors that may potentially contribute to the most notable synergistic effects of chidamide on BV, two factors are known at this point: (1) they target different HDACs as shown in Supplementary Fig. E1., and (2) they have distinct chemical structures. Chidamide belongs to the benzamide class of compounds, romidepsin is a cyclic tetrapeptide, and belinostat is a hydroxamic acid class compound [
42].
Further investigation will be required to determine whether the aforementioned differences could potentially explain the widest effective concentration range observed in chidamide.
Acknowledgements
The authors would like to thank Tomoko Yanai, MD, PhD, from Takeda Pharmaceutical Company Limited, and Saomi Murai, PhD, from Axcelead Drug Discovery Partners, Inc., for scientific advice. Submission assistance was provided by MIMs Japan Limited. Medical writing support was provided by Steven Inglis, PhD, and Angelica Papanicolaou, PhD, of Oxford PharmaGenesis, Melbourne, Australia, which has been funded by Takeda Pharmaceutical Company Limited in accordance with Good Publication Practice 3 (GPP3) guidelines.
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.