The Synergistic Effect of PARP Inhibitors and Irinotecan in Small Cell Lung Cancer Cells

Songji Oh; Soyeon Kim; Bhumsuk Keam; Jeonghwan Youk; Tae Min Kim; Dong-Wan Kim; Miso Kim

doi:10.4143/crt.2024.728

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Original Article Lung and Thoracic cancer The Synergistic Effect of PARP Inhibitors and Irinotecan in Small Cell Lung Cancer Cells: Songji Oh^1,2, Soyeon Kim¹, Bhumsuk Keam^1,3, Jeonghwan Youk^1,3, Tae Min Kim^1,3, Dong-Wan Kim^1,2,3, Miso Kim^1,3; Cancer Research and Treatment : Official Journal of Korean Cancer Association 2025;57(4):1040-1050.
DOI: https://doi.org/10.4143/crt.2024.728
Published online: January 7, 2025

¹Cancer Research Institute, Seoul National University, Seoul, Korea

²Integrated Major in Innovative Medical Science, Seoul National University College of Medicine, Seoul, Korea

³Department of Internal Medicine, Seoul National University Hospital, Seoul National University College of Medicine, Seoul, Korea

Correspondence: Miso Kim, Department of Internal Medicine, Seoul National University Hospital, 101 Daehak-ro, Jongno-gu, Seoul 03080, Korea
Tel: 82-2-2074-4035 E-mail: misokim@snu.ac.kr

• Received: August 1, 2024 • Accepted: January 6, 2025

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Abstract

Purpose
This study focused on combining irinotecan with poly(ADP-ribose) polymerase (PARP) inhibitors to explore the potential for novel combination therapeutics in small cell lung cancer (SCLC).
Materials and Methods
We selected 10 different SCLC cell lines with diverse mutational backgrounds in DNA damage response (DDR) pathway genes to evaluate the efficacy of the combination of three PARP inhibitors and irinotecan. After the cells were exposed to the drugs for 7 days, cell viability was measured, and a combination index was calculated. Apoptotic signaling was assessed via western blot, and DNA damage was evaluated using an alkaline comet assay.
Results
We assessed the synergistic effects of PARP inhibitors and irinotecan in in vitro SCLC models, which revealed increased sensitivity, particularly in cells harboring BRCA mutations. However, even in cells lacking mutations in DDR pathway genes, the combination of the two drugs exhibited a synergistic effect. When treated with 50 nM irinotecan, the IC₅₀ fold changes for PARP inhibitors were as follows: olaparib, 1,649±4,049; talazoparib, 25±34.21; venadaparib, 336±596.01. This combination enhanced apoptosis signaling and increased p-chk1 and p-p53 protein levels. Additionally, the treatment of PARP inhibitor with irinotecan increased DNA damage, as visualized by the alkaline comet assay.
Conclusion
This study provides preclinical evidence of the potential clinical benefits of combining irinotecan with PARP inhibitors in SCLC. Further clinical investigations are warranted to validate these findings for the development of more effective and personalized therapeutic strategies for SCLC patients.
Key words: Small cell lung carcinoma, Poly(ADP-ribose) polymerase inhibitors, Irinotecan

Introduction

Small cell lung cancer (SCLC) accounts for approximately 15% of all bronchogenic carcinomas. SCLC responds well to chemotherapy and radiation, but its high recurrence rate and frequent late-stage diagnosis pose challenges in achieving a cure [1]. Despite efforts to classify SCLC into subtypes and tailor therapeutic approaches, there has been limited progress in the treatment of SCLC [2-4]. The introduction of immune checkpoint inhibitors represents a recent milestone in the treatment of extensive-stage SCLC [5]. However, the efficacy of immunotherapy in SCLC is notably less pronounced compared to other cancer types. For platinum-resistant disease, cytotoxic chemotherapy drugs such as lurbinectedin, topotecan, or irinotecan are recommended as subsequent treatments [6-8]. Despite these therapeutic options, advancements in SCLC treatment have been relatively limited.

Recent research has identified novel pathway targets in SCLC, including those involved in the DNA damage repair (DDR) pathway [9]. Among these targets, poly(ADP-ribose) polymerase 1 (PARP1) was identified as a gene that is highly expressed in SCLC according to reverse-phase protein arrays [10]. PARP inhibitors were first introduced for the treatment of breast and ovarian cancer with DDR pathway defects, but their application has gradually expanded to include prostate, pancreatic, and lung cancer [11,12]. PARP inhibitors usually bind to PARP1 and compete with nicotinamide (NAD⁺) for the catalytically active site of PARP molecules (i in Fig. 1A). PARP trapping, another mechanism of action of PARP inhibitors, can induce the formation of a triple complex of DNA-PARP enzyme-PARP inhibitors, which prevents DNA repair and leads to cell death (ii in Fig. 1A) [13]. Different enzymatic activities of known PARP inhibitors affect cells and induce DNA damage to varying degrees [14].

PARP inhibitors have been evaluated for their efficacy as monotherapies and in combination with other agents, such as immunotherapy [15]. The evaluation of such combinations provides insights into potential synergies that could enhance therapeutic outcomes in SCLC patients.

The combination of PARP inhibitors and chemotherapy in SCLC has been studied in early-phase clinical trials. Given that the resistance to temozolomide is mediated through the PARP-dependent base excision repair pathway, the combination of the PARP inhibitor veliparib and low-dose temozolomide has been studied in SCLC, showing significant improvement in survival in Schlafen family member 11 (SLFN11)–positive SCLC patients [16]. The scientific basis for combining PARP inhibitors with irinotecan is that PARP1 can interact with tyrosyl-DNA phosphodiesterase 1, which facilitates the resolution of topoisomerase I cleavage complexes (iii in Fig. 1A) [17,18]. Here, we evaluated the combination of irinotecan with three different PARP inhibitors and discussed opportunities for drug combination therapy in SCLC.

Materials and Methods

1. Cell culture, reagents, and cell viability assays

The SCLC cell lines NCI-H146 (RRID:CVCL_1473), NCI-H209 (RRID:CVCL_1525), NCI-H211 (RRID:CVCL_1529), NCI-H841 (RRID:CVCL_1595), NCI-H1048 (RRID:CVCL_1453), NCI-H1341 (RRID:CVCL_1463), NCI-H1694 (RRID:CVCL_1489), NCI-H2029 (RRID:CVCL_1516), NCI-H2227 (RRID:CVCL_1542), and DMS 79 (RRID:CVCL_1178) were purchased from ATCC. The cells were maintained in RPMI-1640 or Dulbecco’s modified Eagle’s medium/F-12 medium supplemented with 10% fetal bovine serum and 1× pen/strep (100 units/mL penicillin and 100 μg/mL streptomycin; cat. No. 15140-122, Gibco). Fresh medium was supplied every 2-3 days.

SCLC cells were seeded in 96-well plates at a proper density and allowed to stabilize for 24 hours. Drugs including olaparib (cat. No. S1060, Selleckchem), talazoparib (cat. No. S7048, Selleckchem), venadaparib (provided by Idience Co., Ltd.), and irinotecan (cat. No. S1198, Selleckchem), were used from stock solutions dissolved in dimethyl sulfoxide and added at screening concentrations up to 10 μM. The cells were incubated for an additional 7 days without further drug treatment. Following drug treatment, the reagent (Chromo-CK, Chromogen) was applied and stabilized at 37°C for 30 minutes to 4 hours before measuring absorbance at 450 nm using a spectrophotometer. The half-maximal inhibitory concentration (IC₅₀) was determined using SigmaPlot 12.0 (RRID:SCR_003210). The results are reported as the mean±standard deviation of eight independent experiments. The two-drug combination index was calculated from the drug cytotoxicity using CalcuSyn software (RRID:SCR_020251). We performed data visualization tasks using R v.4.0.3 (R Software, R Foundation for Statistical Computing).

2. Western blot

After treatment with the drugs, the cells were cultured for up to 24 hours before being collected and lysed using cell lysis buffer (cat. No. 9803, Cell Signaling) supplemented with the protease inhibitor phenylmethylsulfonyl fluoride and the phosphatase inhibitor cocktail PhosSTOP (cat. No. 04906845001, Roche). Following a 20-minute incubation on ice, the cellular lysates were centrifuged at 13,000 rpm for 15 minutes at 4°C to precipitate cellular debris. The resulting supernatant was separated on a 4%-12% gradient bis-Tris acrylamide gel (cat. No. NP0336, Invitrogen). The following antibodies were used for western blotting: p-ATR (RRID: AB_2290281), ATR (RRID:AB_2798347), p-ATM (RRID:AB_10835213), ATM (RRID:AB_2062659), p-Chk1 (RRID:AB_331-212), Chk1 (RRID:AB_3662851), p-Chk2 (RRID:AB_2080501), Chk2 (RRID: AB_2080793), p-p53 (RRID:AB_331741), p-H2A.X (RRID:AB_2118009), cleaved caspase-9 (RRID:AB_331424), caspase-3 (RRID:AB_331439), PARP (RRID:AB_2160739), survivin (RRID:AB_2063948), and GAPDH (RRID:AB_2756824).

3. FACS analysis of apoptosis

After the cells were exposed to the drug for a specified duration, they were collected. The mixture was centrifuged at 1,500 rpm for 5 minutes, after which the supernatant was discarded. The washing process was repeated twice with DPBS, and the cells were resuspended in an appropriate amount of annexin V binding buffer. Annexin V-FITC (RRID:AB_2665412) was added to the sample, mixed thoroughly, and incubated for 10 minutes in the dark. 7-Aminoactinomycin D (7-AAD; cat. No. 00-6993-50, Invitrogen) was incubated for 5 minutes and analyzed via FACS machine.

4. Alkaline comet assay

Cells were combined at 2×10⁵ cells/mL with molten LMAgarose (at 37°C) at a 1:10 (v/v) ratio. Immediately pipette 50 μL onto the CometSlide. The slides were incubated at 4°C in the dark for 30 minutes. The slides were immersed in a Lysis Solution (cooled at 4°C for at least 20 minutes before use) overnight at 4°C. The excess buffer was drained and immersed in an Alkaline Unwinding Solution for 1 hour at 4°C in the dark. An appropriate volume of Alkaline Electrophoresis Solution was added, the slides were placed in an electrophoresis tray, and the power supply was set to 30 volts for 30 minutes. After electrophoresis, the samples were immersed twice in dH₂O for 5 minutes each and then in 70% ethanol for 5 minutes. The samples were dried at 42°C for 30 minutes, bringing the cells to a single plane for observation. Next, 100 μL of diluted SYBR Gold (cat. No. S11494, Invitrogen) was added to each dried agarose circle, which was stained for 30 minutes at room temperature in the dark, tapped to remove excess SYBR solution, rinsed briefly in water, and allowed to dry completely at 37°C. The slides were viewed with LSM 800 (RRID:SCR_015963) using a fluorescein filter with SYBR Gold maximum excitation/emission at 496 nm/522 nm. Tail moment analysis was performed using Comet Analysis IV software (Perceptive Instruments).

5. Statistical analyses

p-values were calculated by unpaired t tests between two groups using GraphPad Prism 8 software (RRID:SCR_002798).

Results

1. Characteristics of SCLC

PARP1 was significantly increased in SCLC tissues compared to adjacent normal or normal patient tissues in the GSE40275, GSE44447, GSE60052, and GSE149507 datasets (Fig. 1B). Because sensitivity to PARP inhibitors is thought to vary depending on homologous recombination deficiency (HRD) status, drug responses were observed in 10 SCLC cell lines with varying HRD backgrounds and PTEN status (Fig. 1C). Total PARP expression by western blot in 10 SCLC cell lines was similarly elevated across all cells (Fig. 1D). Interestingly, the activities of two drugs that induce DNA damage, talazoparib (a PARP inhibitor) and irinotecan, were positively correlated (r=0.82, p=3e-16) in SCLC cells according to the SCLC-CellMiner database [3] (Fig. 1E).

2. PARP inhibitors and irinotecan have synergistic effects on an in vitro SCLC model

We evaluated sensitivity to PARP inhibitors with or without irinotecan for 7 days (Fig. 2). To establish an appropriate concentration for the combination treatment, we first screened the effect of irinotecan as a single agent across all 10 cell lines (S1 Fig.). Based on these screening results, 10 nM and 50 nM irinotecan did not significantly reduce cell viability on their own, allowing us to more clearly observe the synergistic effects when combined with PARP inhibitors (S1 Fig.). In our experiments, talazoparib had the most potent effect at low concentrations, consistent with the finding that talazoparib has the strongest PARP trapping potency [19,20]. However, olaparib and venadaparib had more pronounced synergistic effects (average fold change in IC₅₀ [nM] with 50 nM irinotecan: olaparib, 1,649±4,049; talazoparib, 25±34.21; venadaparib, 336±596.01).

The combination of talazoparib or venadaparib with irinotecan resulted in a reduction in the IC₅₀ value to less than 10 nM in seven of the 10 tested cell lines. In the presence of BRCA1 or BRCA2 mutations, although the response rates to PARP inhibitors differ, the combination of irinotecan and PARP inhibitors can reduce cell viability (Fig. 2). Although the use of PARP inhibitors alone in cells without mutations in the DDR gene showed a relatively high IC₅₀ value, the application of PARP inhibitors in combination with irinotecan demonstrated a clear synergistic effect in two of the four cell lines (NCI-H146 and NCI-H2227) (Fig. 2).

The cell viability and corresponding combination index values of NCI-H146 and NCI-H1048 cells are shown in Fig. 3. NCI-H146 had no homologous recombination DNA damage repair gene mutations, and NCI-H1048 had a BRCA2 deletion mutation. Both cell lines strongly synergized with the PARP inhibitors and irinotecan. However, some combination index values at high concentrations of talazoparib or venadaparib exceeded 1, indicating lack of synergy as the single agents were already effective in cell killing. Cell viability decreased with relatively low concentrations of talazoparib (Fig. 3B), while the impact of combination treatment with irinotecan was more pronounced in olaparib and venadaparib, as observed by the extent of graph width change (Fig. 3A and C).

We developed an NCI-H146 cancer spheroid model to confirm that the synergistic effect of the PARP inhibitors with irinotecan is maintained in a 3D environment (Fig. 3D). In this model, the combination treatment significantly reduced cell viability compared to single-agent treatments, demonstrating that the synergy observed in 2D conditions is preserved in 3D models as well (Fig. 3E).

3. The combination of a PARP inhibitor with irinotecan increases the extent of apoptosis

We examined the protein expression of DNA damage pathway after the combination of PARP inhibitor and irinotecan. The combined use of both drugs increased the p-chk1 and p-p53 proteins levels and promoted apoptosis signaling (Fig. 4A). NCI-H841 and NCI-H1341 cells were relatively resistant to PARP inhibitors, whereas NCI-H146 and NCI-H1048 cells were sensitive to PARP inhibitors and exhibited synergistic effects with irinotecan (Fig. 2). PARP inhibitor-resistant cells did not induce cleaved caspase-9, cleaved caspase-3 and cleaved PARP molecules or the DNA damage indicator gamma-H2A.X upon treatment with PARP inhibitors and irinotecan (Fig. 4A). In contrast, PARP inhibitor-sensitive cells induced apoptotic signaling molecules and decreased survivin protein levels (Fig. 4A). Additionally, venadaparib alone did not induce DNA damage but significantly increased apoptosis when combined with irinotecan, even at very low concentrations (Fig. 4).

We also evaluated the apoptotic cell population using annexin V and 7-AAD staining (Fig. 4B and C). The cells were treated with the indicated drugs at constant concentrations for 24 hours (Fig. 4B) or 48 hours (Fig. 4C). Compared with the single agents, all three PARP inhibitors increased apoptosis in the group treated with irinotecan (Fig. 4B). After 48 hours, the combination of venadaparib and irinotecan significantly elevated the percentage of apoptotic cells (54.5%) despite minimal damage caused by the single agents (control, 23.9%; venadaparib, 28.1%; irinotecan, 24.1%) (Fig. 4C).

4. Treatment with a PARP inhibitor and irinotecan increases DNA damage

The extent of DNA damage was visualized using an alkaline comet assay (Fig. 5). The tail moment was measured after exposure to the labeled drug at a specific concentration for 6 hours (Fig. 5A) or 20 hours (Fig. 5B). Under both conditions, the tail moment was significantly greater in the group treated with the combination of irinotecan than in the group treated with the PARP inhibitor alone (Fig. 5). In the 20-hour drug treatment group, the frequency of comets with a higher amount of DNA in the tail compared to the head increased significantly in the combination group treated with irinotecan, indicating a significant increase in DNA damage with the combination of a PARP inhibitor and irinotecan (Fig. 5B).

Discussion

SCLC is a recalcitrant cancer with limited treatment advancements in recent decades. In this study, we investigated the potential efficacy of combining PARP inhibitors with irinotecan in SCLC cells. Notably, some SCLC cells showed sensitivity to the combination therapy despite being primarily resistant to PARP inhibitor monotherapy, suggesting its potential to overcome intrinsic resistance to PARP inhibitors. By analyzing 10 SCLC cell lines with diverse DDR gene mutations, we observed synergistic effects of PARP inhibitors and irinotecan and a marked apoptosis induction. These findings suggest that particular subgroups of patients with SCLC could potentially derive benefit from this combination therapy.

PARP inhibitors trigger cancer cell death by disrupting their DNA repair process. PARP inhibitors are being investigated as monotherapies or in combination with DNA damaging chemotherapy, immunotherapy, or targeted therapy to overcome resistance to PARP inhibitors or to enhance their therapeutic efficacy. Combination of temozolomide and veliparib or talazoparib showed promising but limited outcomes in SCLC patients [16,21]. Although SLFN11 expression levels have been proposed as predictive biomarkers for PARP inhibitors [16], there has been no prospective validation of SLFN11, and it is not yet implemented in clinical practice. BRCA mutations in SCLC patients only under the 3% [22], and they do not predict PARP inhibitors response in preclinical SCLC models [23]. Therefore, further exploration is needed to identify patient groups who are expected to respond well.

A comprehensive assessment of 10 diverse SCLC cell lines with various mutational backgrounds in DDR pathway genes and PTEN deficiency, which is known to cause HR defects in tumors [24], demonstrated the efficacy of the combination therapy of PARP inhibitors and irinotecan. Given PARP1’s role in promoting the disassembly of the topoisomerase I cleavage complexes [17,18], the combination of PARP inhibitors and irinotecan might induce supplementary DNA damage, resulting in a synergistic effect even in the absence of mutations in DDR pathway genes. Our results showed that the synergistic effect persisted even in cases where no mutations in the DDR pathway genes were present, expanding the applicability of the combined treatment. We observed no clear correlation between PARP1 protein expression levels and the cytotoxic effects of PARP inhibitors across the tested SCLC cell lines (Fig. 2). This suggests that the therapeutic efficacy of PARP inhibitors may not solely depend on PARP expression but rather involve alternative mechanisms such as PARP trapping, increased DNA damage accumulation, and activation of the DNA damage response [25]. These findings underscore the complexity of PARP inhibitor mechanisms and highlight the importance of exploring multifactorial determinants of treatment response. Among the three PARP inhibitors, talazoparib showed the highest potency as a single agent, but greater synergy effect was observed with olaparib and venadaparib. The observed increase in apoptotic signaling, evidenced by elevated cleaved caspase-9 and cleaved PARP levels and elevated p-chk1 and p-p53 levels, underscores the potential of combination therapy to induce programmed cell death. The lack of apoptotic signaling despite increased p-p53 (NCI-H841 in Fig. 4A) suggests that p-p53 may not act as a driver of apoptosis in these cells. Instead, it may function in DNA damage response, stress signaling, or other non-apoptotic pathways. Additionally, the notable increase in DNA damage revealed by the alkaline comet assay suggested a profound impact on cancer cells. These findings advocate for the evaluation of the synergistic effect observed in in vivo studies of the combined use of PARP inhibitors and irinotecan in clinical trials.

This study has the limitation of not being able to elucidate the differences between cell lines that exhibited synergism with the combination of PARP inhibitors and irinotecan, nor were we able to identify predictive biomarkers. This underscores the necessity for further research to understand the underlying mechanisms and to discover reliable biomarkers for predicting treatment response.

Electronic Supplementary Material

Supplementary materials are available at Cancer Research and Treatment website (https://www.e-crt.org).

crt-2024-728_S1_Fig.pdf

NOTES

Author Contributions

Conceived and designed the analysis: Oh S, Kim TM, Kim M.

Collected the data: Oh S, Kim M.

Contributed data or analysis tools: Oh S.

Performed the analysis: Oh S.

Wrote the paper: Oh S, Kim M.

Writing - review and editing, resources: Kim S, Keam B, Youk J, Kim TM, Kim DW.

Conflicts of Interest

M Kim received advisory or consulting fees from MSD, BMS/Ono Pharmaceutical, Ipsen, Roche, Janssen, Merck, Astellas, Eisai, Bayer, Pfizer, Boehringer Ingelheim, Boryung, and Yuhan outside the submitted work. T.M. Kim reported consulting or advisory roles outside the submitted work from Amgen, AstraZeneca/MedImmune, Boryung, Daiichi-Sankyo, HK inno.N, IMBDx Inc., Janssen, Novartis, Regeneron, Roche/Genentech, Samsung Bioepis, Takeda, and Yuhan. D.W. Kim reported research funding to his institution from Alpha Biopharma, Amgen, Astrazeneca/Medimmune, Boehringer-Ingelheim, Bridge BioTherapeutics, Chong Kun Dang, Daiichi-Sankyo, GSK, Hanmi, InnoN, IQVIA, Janssen, Merck, Merus, Mirati Therapeutics, MSD, Novartis, ONO Pharmaceutical, Pfizer, Roche/Genentech, Takeda, TP Therapeutics, Xcovery, and Yuhan; involvement in medical writing assistance for Amgen, AstraZeneca, Boehringer-Ingelheim, Bridge BioTherapeutics, Chong Keun Dang, Daiichi-Sankyo, GSK, Janssen, Merus, Mirati Therapeutics, MSD, Merck, Novartis, Pfizer, Roche, Takeda, and Yuhan; and participation in uncompensated consultation or advisory roles for Amgen, AstraZeneca, BMS/ONO Pharmaceuticals, Daiichi-Sankyo, GSK, Janssen, Meck, MSD, Oncobix, Pfizer, SK Biopharm, and Takeda. The other authors declare no potential conflicts of interest.

Funding

This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2021R1A2C2095456) and the Sudang Foundation.

Acknowledgments

The authors thank Sujung Huh and Daye Paek for their experimental support. We received venadaparib from Idience Co., Ltd. (Korea) for the purpose of this research.

Fig. 1.

Characteristics of small cell lung cancer (SCLC) cells. (A) Mechanism of action of poly(ADP-ribose) polymerase (PARP) inhibitors. (B) Expression values of PARP1 in the GSE40275, GSE44447, GSE60052, and GSE149507 datasets. Two-tailed t test, *p < 0.05, ***p < 0.001, ****p < 0.0001. (C) Genetic events associated with homologous recombination DNA damage response genes in selected SCLC cell lines. (D) Western blot analysis of total PARP in 10 SCLC cell lines. (E) Drug activity of irinotecan and talazoparib in the SCLC cell line database. CNA, copy number alteration; GAPDH; glyceraldehyde 3-phosphate dehydrogenase; HR-DDR, homologous recombination DNA damage repair; TDP1, tyrosyl-DNA phosphodiesterase 1.

Fig. 2.

Heatmap showing the IC₅₀ values of 10 small cell lung cancer cell lines treated with poly(ADP-ribose) polymerase (PARP) inhibitors and irinotecan for 7 days. The values obtained indicate the IC₅₀ (nM) of the PARP inhibitor in the presence or absence of irinotecan. GAPDH, glyceraldehyde 3-phosphate dehydrogenase.

Fig. 3.

Poly(ADP-ribose) polymerase (PARP) inhibitors in combination with irinotecan had synergistic effects on in vitro small cell lung cancer models. Cell viability graphs (left) and corresponding combination indices (right) for NCI-H146 and NCI-H1048 cells treated with 50 nM irinotecan and PARP inhibitors. (A) Olaparib. (B) Talazoparib. (C) Venadaparib. (D) Bright-field image of NCI-H146-CO cancer spheroid generated from the NCI-H146 cell line. (E) 3D spheroids viability was assessed 7 days after NCI-H146-CO treatment with 10 nM of PARP inhibitor with or without 10 nM irinotecan.

Fig. 4.

Poly(ADP-ribose) polymerase (PARP) inhibitors combined with irinotecan increase apoptotic signaling in small cell lung cancer (SCLC) cells. (A) Western blot analysis of four SCLC cell lines was used to verify changes in the expression of proteins associated with the apoptotic pathway following treatment with PARP inhibitors and irinotecan. (B, C) Apoptotic cell population stained with annexin V and 7-aminoactinomycin D (7-AAD) and exposed to the same concentration for 24 hours (B) and 48 hours (C), respectively. GAPDH, glyceraldehyde 3-phosphate dehydrogenase; O, olaparib; T, talazoparib; V, venadaparib.

Fig. 5.

Poly(ADP-ribose) polymerase (PARP) inhibitors combined with irinotecan increase DNA damage. The effects of PARP inhibitors and irinotecan as single agents and in combination on the DNA damage response in NCI-H1048 cells were determined by an alkaline comet assay at 6 hours (A) or 20 hours (B). The results quantifying DNA damage are presented using a violin plot with an emphasis on the mean values. The data from each group were analyzed using a two-tailed t test. Representative comet images are shown on the right of each figure, revealing an increase in the frequency of comets with more DNA in their tails than in their heads at 20 hours. O, olaparib; T, talazoparib; I, irinotecan; V, venadaparib. **p < 0.01, ***p < 0.001, ****p < 0.0001.

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The Synergistic Effect of PARP Inhibitors and Irinotecan in Small Cell Lung Cancer Cells

Cancer Res Treat. 2025;57(4):1040-1050. Published online January 7, 2025

DOI: https://doi.org/10.4143/crt.2024.728

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Abstract
Introduction
Materials and Methods
Results
Discussion
Electronic Supplementary Material
NOTES
REFERENCES

The Synergistic Effect of PARP Inhibitors and Irinotecan in Small Cell Lung Cancer Cells

Fig. 1. Characteristics of small cell lung cancer (SCLC) cells. (A) Mechanism of action of poly(ADP-ribose) polymerase (PARP) inhibitors. (B) Expression values of PARP1 in the GSE40275, GSE44447, GSE60052, and GSE149507 datasets. Two-tailed t test, *p < 0.05, ***p < 0.001, ****p < 0.0001. (C) Genetic events associated with homologous recombination DNA damage response genes in selected SCLC cell lines. (D) Western blot analysis of total PARP in 10 SCLC cell lines. (E) Drug activity of irinotecan and talazoparib in the SCLC cell line database. CNA, copy number alteration; GAPDH; glyceraldehyde 3-phosphate dehydrogenase; HR-DDR, homologous recombination DNA damage repair; TDP1, tyrosyl-DNA phosphodiesterase 1.

Fig. 2. Heatmap showing the IC50 values of 10 small cell lung cancer cell lines treated with poly(ADP-ribose) polymerase (PARP) inhibitors and irinotecan for 7 days. The values obtained indicate the IC50 (nM) of the PARP inhibitor in the presence or absence of irinotecan. GAPDH, glyceraldehyde 3-phosphate dehydrogenase.

Fig. 3. Poly(ADP-ribose) polymerase (PARP) inhibitors in combination with irinotecan had synergistic effects on in vitro small cell lung cancer models. Cell viability graphs (left) and corresponding combination indices (right) for NCI-H146 and NCI-H1048 cells treated with 50 nM irinotecan and PARP inhibitors. (A) Olaparib. (B) Talazoparib. (C) Venadaparib. (D) Bright-field image of NCI-H146-CO cancer spheroid generated from the NCI-H146 cell line. (E) 3D spheroids viability was assessed 7 days after NCI-H146-CO treatment with 10 nM of PARP inhibitor with or without 10 nM irinotecan.

Fig. 4. Poly(ADP-ribose) polymerase (PARP) inhibitors combined with irinotecan increase apoptotic signaling in small cell lung cancer (SCLC) cells. (A) Western blot analysis of four SCLC cell lines was used to verify changes in the expression of proteins associated with the apoptotic pathway following treatment with PARP inhibitors and irinotecan. (B, C) Apoptotic cell population stained with annexin V and 7-aminoactinomycin D (7-AAD) and exposed to the same concentration for 24 hours (B) and 48 hours (C), respectively. GAPDH, glyceraldehyde 3-phosphate dehydrogenase; O, olaparib; T, talazoparib; V, venadaparib.

Fig. 5. Poly(ADP-ribose) polymerase (PARP) inhibitors combined with irinotecan increase DNA damage. The effects of PARP inhibitors and irinotecan as single agents and in combination on the DNA damage response in NCI-H1048 cells were determined by an alkaline comet assay at 6 hours (A) or 20 hours (B). The results quantifying DNA damage are presented using a violin plot with an emphasis on the mean values. The data from each group were analyzed using a two-tailed t test. Representative comet images are shown on the right of each figure, revealing an increase in the frequency of comets with more DNA in their tails than in their heads at 20 hours. O, olaparib; T, talazoparib; I, irinotecan; V, venadaparib. **p < 0.01, ***p < 0.001, ****p < 0.0001.

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The Synergistic Effect of PARP Inhibitors and Irinotecan in Small Cell Lung Cancer Cells