PARP Inhibitors Research Articles

PARP Inhibitors in Colorectal Malignancies: A 2023 Update.

Colorectal carcinoma (CRC) is one of the most common malignancies in the Western world, and metastatic disease is associated with a dismal prognosis. Poly-ADpribose polymerase (PARP) inhibitors gain increasing attention in the field of medical oncology, as they lead to synthetic lethality in malignancies with preexisting alterations in the DNA damage repair (DDR) pathway. As those alterations are frequently seen in CRC, a targeted approach through PARP inhibitors is expected to benefit these patients, both alone and in combination with other agents like chemotherapy, immunotherapy, antiangiogenics, and radiation. This review article aims to better clarify the role of PARP inhibitors as a treatment option in patients with metastatic CRC with alterations in the DDR pathway. We used the PubMed database to retrieve journal articles and the inclusion criteria were all human studies that illustrated the effective role of PARP inhibitors in patients with metastatic CRC with homologous repair deficiency (HRD) and the correct line of therapy. Current evidence supports the utilization of PARP inhibitors in CRC subgroups, as monotherapy and in combination with other agents. Up to now, data are insufficient to support a formal indication, and further research is needed. Efforts to precisely define the homologous repair deficiency (HRD) in CRC - and eventually the subgroup of patients that are expected to benefit the most - are also underway. DOI: dx.doi.org/10.17504/protocols.io.dm6gp3ey8vzp/v1.

PARP1 expression predicts PARP inhibitor sensitivity and correlates with metastatic potential and overall survival in melanoma.

Metastatic melanoma is still a difficult-to-treat cancer type owing to its frequent resistance mechanisms to targeted and immunotherapy. Therefore, we aimed to unravel novel therapeutic strategies for melanoma patients. Preclinical and clinical studies show that melanoma patients may benefit from a treatment with poly (ADP-ribose) polymerase (PARP) inhibitors (PARPi). In this study, we focus on PARP1 as a potential biomarker to predict the response of melanoma cells to PARPi therapy. We found that melanoma cells with high basal PARP1 expression exhibit significantly increased cell death after PARPi treatment owing to higher PARP1 trapping compared with melanoma cells with low PARP1 expression. In addition, we could demonstrate that PARP1 expression levels are low in nonmalignant skin cells, and metastatic melanomas show considerably higher PARP1 levels compared with primary melanomas. Most strikingly, we found that high PARP1 levels correlate with worse overall survival of late stage metastasized melanoma patients. In conclusion, we show that PARP1 might act as a biomarker to predict the response to PARPi therapy, and that in particular the late stage metastasized melanoma patients are especially sensitive to PARPi therapy owing to elevated PARP1 expression. Our data suggest that the PARPi cytotoxicity primarily will affect the high PARP1 expressing melanoma cells, rather than the low PARP1 expressing nonmalignant skin cells resulting in only low side effects.

A 15-Inflammation-Related Gene Signature Predicts the Prognosis of Patients With Pancreatic Ductal Adenocarcinoma

Chronic inflammation promotes the development of pancreatic ductal adenocarcinoma (PDAC) and PDAC-related inflammatory tumor microenvironment facilitates tumor growth and metastasis. Thus, we aimed to study the association between inflammatory response and prognosis in patients with PDAC. We conducted the whole transcriptomic sequencing using tissue samples collected from patients diagnosed with PDAC (n = 106) recruited from Shandong Cancer Hospital. We first constructed a prognostic signature using 15 inflammation-related genes in The Cancer Genome Atlas (TCGA) cohort (n = 177) and further validated it in an independent International Cancer Genome Consortium (ICGC) cohort (n = 90) and our in-house cohort. PDAC patients with a higher risk score had poorer overall survival (OS) (P < 0.001; HR, 3.02; 95% CI, 1.94-4.70). The association between the prognostic signature and OS remained significant in the multivariable Cox regression adjusting for age, sex, alcohol exposure, diabetes, and stage (P < 0.001; HR, 2.91; 95% CI, 1.73-4.89). This gene signature also robustly predicted prognosis in the ICGC cohort (P = 0.01; HR, 1.94; 95% CI, 1.14-3.30) and our cohort (P < 0.001; HR, 2.40; 95% CI, 1.45-3.97). Immune subtype C3 (inflammatory) was enriched and CD8+ T cells were higher in patients with a lower risk score (P < 0.05). Furthermore, PDAC patients with higher risk scores were more sensitive to chemotherapy, immunotherapy, and PARP inhibitors (P < 0.05). In sum, we identified a novel gene signature that was associated with inflammatory response for risk stratification, prognosis prediction, and therapy guidance in PDAC patients. Future studies are warranted to validate the clinical utility of the signature.

Development and Characterisation of a New Patient-Derived Xenograft Model of AR-Negative Metastatic Castration-Resistant Prostate Cancer

As the treatment landscape for prostate cancer gradually evolves, the frequency of treatment-induced neuroendocrine prostate cancer (NEPC) and double-negative prostate cancer (DNPC) that is deficient for androgen receptor (AR) and neuroendocrine (NE) markers has increased. These prostate cancer subtypes are typically refractory to AR-directed therapies and exhibit poor clinical outcomes. Only a small range of NEPC/DNPC models exist, limiting our molecular understanding of this disease and hindering our ability to perform preclinical trials exploring novel therapies to treat NEPC/DNPC that are urgently needed in the clinic. Here, we report the development of the CU-PC01 PDX model that represents AR-negative mCRPC with PTEN/RB/PSMA loss and CTNN1B/TP53/BRCA2 genetic variants. The CU-PC01 model lacks classic NE markers, with only focal and/or weak expression of chromogranin A, INSM1 and CD56. Collectively, these findings are most consistent with a DNPC phenotype. Ex vivo and in vivo preclinical studies revealed that CU-PC01 PDX tumours are resistant to mCRPC standard-of-care treatments enzalutamide and docetaxel, mirroring the donor patient’s treatment response. Furthermore, short-term CU-PC01 tumour explant cultures indicate this model is initially sensitive to PARP inhibition with olaparib. Thus, the CU-PC01 PDX model provides a valuable opportunity to study AR-negative mCRPC biology and to discover new treatment avenues for this hard-to-treat disease.

A novel α,β-unsaturated ketone inhibits leukemia cell growth as PARP1 inhibitor.

Leukemia is a malignant disease of the hematopoietic system, in which clonal leukemia cells accumulate and inhibit normal hematopoiesis in the bone marrow and other hematopoietic tissues as a result of uncontrolled proliferation and impaired apoptosis, among other mechanisms. In this study, the anti-leukemic effect of a compound (SGP-17-S) extracted from Chloranthus multistachys, a plant with anti-inflammatory, antibacterial and anti-tumor effects, was evaluated. The effect of SGP-17-S on the viability of leukemic cell was demonstrated by MTT assay, cell cycle, and apoptosis were assessed by flow cytometry using PI staining and Annexin V/PI double staining. Combinations of network pharmacology and cellular thermal shift assay (CETSA) with western blot were used to validate agents that act on leukemia targets. The results showed that SGP-17-S inhibited the growth of leukemia cells in a time- and dose-dependent manner. SGP-17-S blocked HEL cells in the G2 phase, induced apoptosis, decreased Bcl-2 and caspase-8 protein expression, and increased Bax and caspase-3 expression. In addition, CETSA revealed that PARP1 is an important target gene for the inhibition of HEL cell growth, and SGP-17-S exerted its action on leukemia cells by targeting PARP1. Therefore, this study might provide new solutions and ideas for the treatment of leukemia.

Geniposide ameliorates atherosclerosis by restoring lipophagy via suppressing PARP1/PI3K/AKT signaling pathway

BackgroundAtherosclerosis (AS) is the leading cause of global death, which manifests as arterial lipid stack and plaque formation. Geniposide is an iridoid glycoside extract from Gardenia jasminoides J.Ellis that ameliorates AS by mediating autophagy. However, how Geniposide regulates autophagy and treats AS remains unclear. PurposeTo evaluate the efficacy and mechanism of Geniposide in treating AS.Study design and methods: Geniposide was administered to high-fat diet-fed ApoE−/− mice and oxidized low-density lipoprotein-incubated primary vascular smooth muscle cells (VSMCs). AS was evaluated with arterial lipid stack, plaque progression, and collagen loss in the artery. Foam cell formation was detected by lipid accumulation, inflammation, apoptosis, and the expression of foam cell markers. The mechanism of Geniposide in treating AS was assessed using network pharmacology. Lipophagy was measured by lysosomal activity, expression of lipophagy markers, and the co-localization of lipids and lipophagy markers. The effects of lipophagy were blocked using Chloroquine. The role of PARP1 was assessed by Olaparib (a PARP1 inhibitor) intervention and PARP1 overexpression. ResultsIn vivo, Geniposide reversed high-fat diet-induced hyperlipidemia, plaque progression, and inflammation. In vitro, Geniposide inhibited VSMC-derived foam cell formation by suppressing lipid stack, apoptosis, and the expressions of foam cell markers. Network pharmacological analysis and in vitro validation suggested that Geniposide treated AS by enhancing lipophagy via suppressing the PI3K/AKT signaling pathway. The benefits of Geniposide in alleviating AS were offset by Chloroquine in vivo and in vitro. Inhibiting PARP1 using Olaparib promoted lipophagy and alleviated AS progression, while PARP1 overexpression exacerbated foam cell formation and lipophagy blockage. The above effects of PARP1 were weakened by PI3K inhibitor LY294002. PARP1 also inhibited the combination of the ABCG1 and PLIN1. ConclusionGeniposide alleviated AS by restoring PARP1/PI3K/AKT signaling pathway-suppressed lipophagy. This study is the first to present the lipophagy-inducing effect of Geniposide and the binding of ABCG1 and PLIN1 inhibited by PARP1.

Blocking lipid synthesis induces DNA damage in prostate cancer and increases cell death caused by PARP inhibition.

Androgen deprivation therapy (ADT) is the primary treatment for prostate cancer; however, resistance to ADT invariably develops, leading to castration-resistant prostate cancer (CRPC). Prostate cancer progression is marked by increased de novo synthesis of fatty acids due to overexpression of fatty acid synthase (FASN), making this enzyme a therapeutic target for prostate cancer. Inhibition of FASN results in increased intracellular amounts of ceramides and sphingomyelin, leading to DNA damage through the formation of DNA double-strand breaks and cell death. We found that combining a FASNi with the poly-ADP ribose polymerase (PARP) inhibitor olaparib, which induces cell death by blocking DNA damage repair, resulted in a more pronounced reduction in cell growth than that caused by either drug alone. Human CRPC organoids treated with a combination of PARP and FASNi were smaller, had decreased cell proliferation, and showed increased apoptosis and necrosis. Together, these data indicate that targeting FASN increases the therapeutic efficacy of PARP inhibitors by impairing DNA damage repair, suggesting that combination therapies should be explored for CRPC.

Long-term survival after neoadjuvant therapy for triple-negative breast cancer under different treatment regimens: a systematic review and network meta-analysis

BackgroundTriple-negative breast cancer (TNBC) is a life-threatening subtype of breast cancer with limited treatment options. Therefore, this network meta-analysis (NMA) aimed to evaluate and compare the effect of various neoadjuvant chemotherapy (NCT) options on the long-term survival of patients with TNBC.MethodsPubMed, Embase, Medline, Cochrane Library, Web of Science, and major international conference databases were systematically searched for randomized controlled trials (RCTs) on the efficacy of various NCT options in patients with TNBC. Searches were performed from January 2000 to June 2023. Study heterogeneity was assessed using the I2 statistic. Hazard ratios (HRs) and 95% confidence intervals (CIs) were used to evaluate disease-free survival (DFS) and overall survival (OS). Odds ratios (ORs) and 95% CIs were used to evaluate the pathologic complete response (pCR). The primary outcome was DFS.ResultsWe conducted an NMA of 21 RCTs involving 8873 patients with TNBC. Our study defined the combination of anthracyclines and taxanes as the preferred treatment option. On this basis, the addition of any of the following new drugs is considered a new treatment option: bevacizumab (B), platinum (P), poly-ADP-ribose polymerase inhibitors (PARPi), and immune checkpoint inhibitor (ICI). Based on the surface under the cumulative ranking curve (SUCRA) values, the top three SUCRA area values of DFS were taxanes, anthracycline, and cyclophosphamide (TAC; 89.23%); CT (84.53%); and B (81.06%). The top three SUCRA area values of OS were CT (83.70%), TAC (62.02%), and B-containing regimens (60.06%). The top three SUCRA area values of pCR were B + P-containing regimens (82.7%), ICI + P-containing regimens (80.2%), and ICI-containing regimens (61.8%).ConclusionsThis NMA showed that standard chemotherapy is a good choice with respect to long-term survival. Moreover, B associated with P-containing regimens is likely to be the optimal treatment option for neoadjuvant TNBC in terms of pCR.

Long-Acting Poly(ADP-ribose) Polymerase Inhibitor Prodrug for Humans.

Poly(ADP-ribose) polymerase inhibitors (PARPi) have been approved for once or twice daily oral use in the treatment of cancers with BRCA defects. However, for some patients, oral administration of PARPi may be impractical or intolerable, and a long-acting injectable formulation is desirable. We recently developed a long-acting PEGylated PARPi prodrug, PEG∼talazoparib (TLZ), which suppressed the growth of PARPi-sensitive tumors in mice for very long periods. However, the release rate of TLZ from the conjugate was too fast to be optimal in humans. We prepared several new PEG∼TLZ prodrugs having longer half-lives of drug release and accurately measured their pharmacokinetics in the rat. Using the rates of release of TLZ from these prodrugs and the known pharmacokinetics of free TLZ in humans, we simulated the pharmacokinetics of the macromolecular prodrugs and released TLZ in humans. From several possibilities, we chose two conjugates that could be administered intravenously every 2 weeks and maintain TLZ within its known therapeutic window. We describe situations where the PEG∼TLZ conjugates would find utility in humans and suggest how the intravenously administered long-acting prodrugs could in fact be more effective than daily oral administration of free TLZ.

Discovery of Potent Isoindolinone Inhibitors that Target an Active Conformation of PARP1 Using DNA-Encoded Libraries.

Inhibition of poly (ADP-ribose) polymerase-1 (PARP1), a DNA repair enzyme, has proven to be a successful strategy for the treatment of various cancers. With the appropriate selection conditions and protein design, DNA-encoded library (DEL) technology provides a powerful avenue to identify small molecules with the desired mechanism of action towards a target of interest. However, DNA-binding proteins, such as PARP1, can be challenging targets for DEL screening due to non-specific protein-DNA interactions. To overcome this, we designed and screened a PARP1 catalytic domain construct without the autoinhibitory helical domain. This allowed us to interrogate an active, functionally-relevant form of the protein resulting in the discovery of novel isoindolinone PARP1 inhibitors with single-digit nanomolar potency. These inhibitors also demonstrated little to no PARP1-DNA trapping, a property that could be advantageous in the clinic.

TP53-specific mutations serve as a potential biomarker for homologous recombination deficiency in breast cancer: a clinical next-generation sequencing study

Abstract Background TP53 mutations and homologous recombination deficiency (HRD) occur frequently in breast cancer. However, the characteristics of TP53 pathogenic mutations in breast cancer patients with/without HRD are not clear. Methods Both tumor and paired blood DNA from 119 breast cancer patients were performed clinical next-generation sequencing (NGS) by a 520-gene panel. Mutations, tumor mutation burden (TMB), and genomic HRD scores were assessed from NGS data. Results All TP53 pathogenic mutations in patients were the somatic origin, which was associated with the protein expression of estrogen receptor and progestogen receptor. Compared to patients without TP53 pathologic mutations, patients with TP53 pathologic mutations had higher levels of HRD scores and different genomic alterations. The frequency of TP53 pathologic mutation was higher in the HRD-High group (HRD score≥42) relative to that in the HRD-Low group (HRD score&amp;lt;42). TP53 has different mutational characteristics between the HRD-Low and HRD-High groups. TP53-specific mutation subgroups had diverse genomic features and TMB. Notably, TP53 pathogenic mutations predicted the HRD status of breast cancer patients with an AUC of 0.61. TP53-specific mutations, namely HRD-Low mutation, HRD-High mutation, and HRD common mutation, predicted the HRD status of breast cancer patients with AUC values of 0.32, 0.72, and 0.58, respectively. Interestingly, TP53 HRD-High mutation and HRD common mutation combinations showed the highest AUC values (0.80) in predicting HRD status. Conclusions TP53-specific mutation combinations predict the HRD status of patients, indicating that TP53 pathogenic mutations could serve as a potential biomarker for PARP inhibitors in breast cancer patients.

Mutational signature-based identification of DNA repair deficient gastroesophageal adenocarcinomas for therapeutic targeting

Homologous recombination (HR) and nucleotide excision repair (NER) are the two most frequently disabled DNA repair pathways in cancer. HR-deficient breast, ovarian, pancreatic and prostate cancers respond well to platinum chemotherapy and PARP inhibitors. However, the frequency of HR deficiency in gastric and esophageal adenocarcinoma (GEA) still lacks diagnostic and functional validation. Using whole exome and genome sequencing data, we found that a significant subset of GEA, but very few colorectal adenocarcinomas, show evidence of HR deficiency by mutational signature analysis (HRD score). High HRD gastric cancer cell lines demonstrated functional HR deficiency by RAD51 foci assay and increased sensitivity to platinum chemotherapy and PARP inhibitors. Of clinical relevance, analysis of three different GEA patient cohorts demonstrated that platinum treated HR deficient cancers had better outcomes. A gastric cancer cell line with strong sensitivity to cisplatin showed HR proficiency but exhibited NER deficiency by two photoproduct repair assays. Single-cell RNA-sequencing revealed that, in addition to inducing apoptosis, cisplatin treatment triggered ferroptosis in a NER-deficient gastric cancer, validated by intracellular GSH assay. Overall, our study provides preclinical evidence that a subset of GEAs harbor genomic features of HR and NER deficiency and may therefore benefit from platinum chemotherapy and PARP inhibitors.

A novel dual ATM/DNA-PK inhibitor, XRD-0394, potently radiosensitizes and potentiates PARP and topoisomerase I inhibitors.

A majority of cancer patients receive radiation therapy as part of their treatment regimens whether using external beam therapy or locally-delivered radioisotopes. While often effective, some tumors are inadequately controlled with radiation and radiation therapy has significant short-term and long-term toxicities for cancer survivors. Insights into molecular mechanisms involved in cellular responses to DNA breaks introduced by radiation or other cancer therapies have been gained in recent years and approaches to manipulate these responses to enhance tumor cell killing or reduce normal tissue toxicity are of great interest. Here, we report the identification and initial characterization of XRD-0394, a potent and specific dual inhibitor of two DNA damage-response kinases, ATM and DNA-PKcs. This orally bioavailable molecule demonstrates significantly enhanced tumor cell kill in the setting of therapeutic ionizing irradiation in vitro and in vivo. XRD-0394 also potentiates the effectiveness of topoisomerase I inhibitors in vitro. Additionally, in cells lacking BRCA1/2 XRD-0394 shows single agent activity and synergy in combination with PARP inhibitors. A Phase Ia clinical trial (NCT05002140) with XRD-0394 in combination with RT has completed. These results provide a rationale for future clinical trials with XRD-0394 in combination with RT, PARP inhibitors and targeted delivery of topoisomerase I inhibitors.

Evaluating homologous recombination activity in tissues to predict the risk of hereditary breast and ovarian cancer and olaparib sensitivity.

Homologous recombination (HR) repairs DNA damage including DNA double-stranded breaks and alterations in HR-related genes results in HR deficiency. Germline alteration of HR-related genes, such as BRCA1 and BRCA2, causes hereditary breast and ovarian cancer (HBOC). Cancer cells with HR deficiency are sensitive to poly (ADP-ribose) polymerase (PARP) inhibitors and DNA-damaging agents. Thus, accurately evaluating HR activity is useful for diagnosing HBOC and predicting the therapeutic effects of anti-cancer agents. Previously, we developed an assay for site-specific HR activity (ASHRA) that can quantitatively evaluate HR activity and detect moderate HR deficiency. HR activity in cells measured by ASHRA correlates with sensitivity to the PARP inhibitor, olaparib. In this study, we applied ASHRA to lymphoblastoid cells and xenograft tumor tissues, which simulate peripheral blood lymphocytes and tumor tissues, respectively, as clinically available samples. We showed that ASHRA could be used to detect HR deficiency in lymphoblastoid cells derived from a BRCA1 pathogenic variant carrier. Furthermore, ASHRA could quantitatively measure the HR activity in xenograft tumor tissues with HR activity that was gradually suppressed by inducible BRCA1 knockdown. The HR activity of xenograft tumor tissues quantitatively correlated with the effect of olaparib. Our data suggest that ASHRA could be a useful assay for diagnosing HBOC and predicting the efficacy of PARP inhibitors.

Comprehensive machine learning boosts structure-based virtual screening for PARP1 inhibitors

Poly ADP-ribose polymerase 1 (PARP1) is an attractive therapeutic target for cancer treatment. Machine-learning scoring functions constitute a promising approach to discovering novel PARP1 inhibitors. Cutting-edge PARP1-specific machine-learning scoring functions were investigated using semi-synthetic training data from docking activity-labelled molecules: known PARP1 inhibitors, hard-to-discriminate decoys property-matched to them with generative graph neural networks and confirmed inactives. We further made test sets harder by including only molecules dissimilar to those in the training set. Comprehensive analysis of these datasets using five supervised learning algorithms, and protein–ligand fingerprints extracted from docking poses and ligand only features revealed one highly predictive scoring function. This is the PARP1-specific support vector machine-based regressor, when employing PLEC fingerprints, which achieved a high Normalized Enrichment Factor at the top 1% on the hardest test set (NEF1% = 0.588, median of 10 repetitions), and was more predictive than any other investigated scoring function, especially the classical scoring function employed as baseline.

Abstract LB427: UBA1 blockade induces homologous recombination deficiency and is synthetic lethal with PARP inhibition

Abstract Therapeutic strategies targeting DNA damage have revolutionized cancer treatment, yet their efficacy often comes at the cost of substantial adverse effects. The emergence of poly-ADP-ribose polymerase (PARP) inhibition as a targeted approach to disrupt DNA repair in tumors deficient in homologous recombination (HR) has shown promise. However, the limited response of most cancers to PARP inhibitors has posed a significant challenge. This study employs a genome-wide CRISPR knockout functional genomics screen, revealing that depletion of ubiquitin-activating enzyme E1 (UBA1) enhances sensitivity to PARP inhibition in HR-proficient ovarian cancer cells. Validation across different ovarian and breast cancer cell lineages, utilizing cell line and organoid models, establishes that pharmacological inhibition of UBA1, a pivotal player in ubiquitination, sensitizes cells to PARP inhibition and other DNA damaging therapies. In vivo experiments combining PARP and UBA1 inhibition demonstrate not only tolerability but also significant extension of survival in patient-derived xenografts compared to individual therapies. Mechanistic studies uncover that UBA1 inhibition not only impedes HR repair, sensitizing cells to PARP inhibition, but also induces widespread increases in PARylation, subsequently targeted by PARP inhibition. These findings underscore the multifaceted pathways through which UBA1 inhibition potentiates the efficacy of PARP inhibition. The demonstrated tolerability and substantial therapeutic gains establish the combined approach as a promising strategy to maximize the benefits of PARP inhibition in cancer therapy. Citation Format: Sharad Awasthi, Daniel J. McGrail, S. Stephen Yi, Nidhi Sahni. UBA1 blockade induces homologous recombination deficiency and is synthetic lethal with PARP inhibition [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr LB427.

Abstract CT169: First-in-human, phase 1/2 study of GSK4524101, an oral DNA polymerase theta inhibitor (POLQi), alone or combined with the poly(ADP-ribose) polymerase (PARP) inhibitor (PARPi) niraparib in adults with solid tumors

Abstract Background: Double-stranded DNA breaks (DSBs) in human cells are typically repaired via nonhomologous end joining (NHEJ) or homologous recombination (HR). Deficiency of HR-mediated DNA repair plays a role in the initiation and progression of many tumor types. In tumors with HR deficiency, PARP inhibition leads to generation of DSBs that cannot be effectively repaired because of the HR defect, resulting in synthetic lethality. This finding has led to the clinical development and approval of several PARPi for the treatment of various tumors including certain ovarian, breast, prostate, and pancreatic cancers that are prone to display HR deficiency (HRd). DNA polymerase theta (encoded by POLQ) mediates an alternative DNA repair mechanism, microhomology-mediated end joining (MMEJ). DNA polymerase theta is generally not detectable in normal tissues but is upregulated in many tumor types. In preclinical studies, POLQi plus PARPi treatment demonstrated superior efficacy to PARPi alone in preventing the growth of HRd tumors. To evaluate the clinical potential of combining POLQi and PARPi, this first-in-human study investigates treatment with GSK4524101, an investigational POLQi, with or without the PARPi niraparib, in patients with solid tumors. Methods: This open-label, multicenter, phase 1/2 study (NCT06077877) opened in October 2023 and comprises dose-finding (part 1, including a food-effect cohort) and dose-expansion (part 2) parts. This trial aims to assess the maximum-tolerated dose, pharmacokinetics, safety, and preliminary antitumor activity of oral GSK4524101 with or without niraparib. The primary endpoints are safety (part 1) and confirmed objective response rate (part 2). Secondary endpoints include pharmacokinetics, safety, progression-free survival (part 2), and response duration (part 2). Up to 135 patients may be enrolled. To be eligible, patients must be aged ≥18 years; have an advanced or metastatic solid tumor, an Eastern Cooperative Oncology Group performance status score of 0-2, and a life expectancy of ≥3 months; and have exhausted all standard treatment options. Individuals are ineligible if they have not recovered from chemotherapy-associated adverse events or have symptomatic uncontrolled brain or leptomeningeal metastases, a history of myelodysplastic syndrome or acute myeloid leukemia, uncontrolled hypertension, or a second malignancy that has progressed or required active treatment in the previous 2 years. The study is actively recruiting in the US and Canada; as of January 1, 2024, 1 patient has been dosed. Patients enrolled in part 1 will receive GSK4524101 alone or GSK4524101 plus niraparib; patients enrolled in part 2 will be randomized to either GSK4524101 plus niraparib or niraparib alone. Part 1 of this study is expected to complete in 2025. Citation Format: Vivek Samnotra, Veronica Moroz, Luda Shtessel, Mita Kuchimanchi, Patrick Hanafin, Malar Pannirselvam, Aishwarya Bhaskar, Andrew Liu, Haluk Yuzugullu, Minal Barve, David Sommerhalder, Timothy Yap, Michele Sanicola-Nadel. First-in-human, phase 1/2 study of GSK4524101, an oral DNA polymerase theta inhibitor (POLQi), alone or combined with the poly(ADP-ribose) polymerase (PARP) inhibitor (PARPi) niraparib in adults with solid tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr CT169.

Abstract LB273: Pharmacological synthetic lethality by co-inhibition of PARP and HDAC enzymes

Abstract Introduction: Inhibition of poly(ADP-ribose) polymerase (PARP) induces synthetic lethality in cells exhibiting defects in homologous repair (HR) pathways, such as BRCA1 and BRCA2-mutated breast or ovarian cancers, and 4 different PARP inhibitors (PARPi) are currently approved by the FDA. While HR-deficient cancers usually respond well to single agent PARPi, HR-proficient cancers are generally less sensitive to the treatment. Acquired resistance to PARPi due to activating mutations in BRCA genes are also not uncommon. In this work, we have examined strategies for enhancing PARPi activity in a HR-proficient context and identified histone deacetylases (HDACs) as targets for pharmacological synthetic lethality in the context of PARPi. Materials and methods: PARP1 and PARP2 activity were measured using Trevigen Universal Colorimetric PARP Assay Kit, BPS Bioscience PARP2 Colorimetric PARP2 Assay Kit, and PARylation assay. HDAC activity was measured using HeLa cell nuclear extracts and a fluorogenic peptide-based biochemical assay. Cell survival EC50 were determined using CellTiter Glo viability assay. Cell cycle analysis was performed by flow cytometry with propidium iodide staining. DNA damage was investigated by immunoblotting and immunofluorescence using gamma-H2AX antibodies, and comet assays for confirmation of double-strand breaks. Spheroid assays were performed using the Incucyte® S3 spheroid analysis module. In vivo metastasis formations were examined using the pulmonary metastasis assay (PuMA). Results and discussion: Combinations of PARP and HDAC inhibitors in HR-proficient tumor cells demonstrated that HDACi could enhance the efficacy of PARPi in vitro. This was associated with increased acute and sustained DNA-damage distinct cell cycle profiles showing activity in both S- and G2/M-phases. Importantly, dual single-molecule PARP-HDAC inhibitors showed potent inhibition of PARP1 and PARP2 activity and PAR synthesis, comparable to olaparib, as well as inhibition of HDAC with IC50 values in the low µM range. Dual PARP-HDAC inhibition decreased survival of HR-proficient cells in both 2D cultures and 3D spheroid assays with lower EC50 values than olaparib or vorinostat alone. Importantly, lung metastasis formation was significantly impaired by dual PARP-HDAC inhibition. Conclusion: Combination of PARP inhibition and HDAC inhibition showed increased efficacy compared to single agent PARP inhibition in HR-proficient cells. Development of bifunctional inhibitors may provide a novel therapeutic opportunity for tumors with limited response rates to single agent PARPi. Citation Format: Sarah Truong, Louise Ramos, Beibei Zhai, Fariba Ghaidi, Mona Marzban, Hans Adomat, Xiaoqi Chen, John Langlands, Dennis Brown, Jeffrey Bacha, Colin Collins, Poul H. Sorensen, Wang Shen, Mads Daugaard. Pharmacological synthetic lethality by co-inhibition of PARP and HDAC enzymes [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr LB273.

Abstract LB181: First disclosure of XRD-0394, a novel dual ATM/DNA-PK inhibitor, that potently radiosensitizes and potentiates PARP and topoisomerase inhibitors

Abstract A majority of cancer patients receive radiation therapy (RT) as part of their treatment regimens, whether using external beam therapy or locally-delivered radioisotopes. While often effective, RT may not be efficacious for some tumors while exerting significant short-term and long-term toxicities. Thus, enhancing therapeutic effectiveness of radiation therapy remains an important goal. ATM (ataxia-telangiectasia, mutated) and DNA-PKcs (deoxyribonucleic acid [DNA]-dependent protein kinase, catalytic subunit) are related protein kinases and both are critical regulators of cellular responses to ionizing radiation. We report here the identification and initial characterization of XRD-0394, a potent, specific dual inhibitor of both ATM and DNA-PKcs, whose chemical structure will be disclosed. A dual ATM/DNA-PK inhibitor has practical and theoretical advantages over compounds which inhibit just one of these kinases: 1) blunting both DNA damage response (DDR) pathways enhances radiation-induced tumor cell kill compared to single target inhibition; and 2) simultaneous inhibition of two DDR pathways should overcome intrinsic or acquired resistance of tumor cells to either target alone. Notably, transient inhibition of both kinases by XRD-0394 has little adverse effects on cells in the absence of irradiation. XRD-0394 is orally bioavailable, with excellent pharmacological profiles, demonstrating significant in vitro and in vivo sensitizing activity in combination with ionizing radiation and in combination with immunotherapy and radiation. In addition, a CRISPR screen identified alterations in the Fanconi A/BRCA pathway as an opportunity for synthetic lethality in tumor cells exposed to XRD-0394, an insight that was validated in multiple tumor cell types. In vitro studies demonstrated synergy of XRD-0394 in combination with PARP inhibitors, particularly in BRCA-mutant cells. XRD-0394 also potentiates the effectiveness of topoisomerase inhibitors in vitro. Single doses of XRD-0394 were well-tolerated in combination with palliative RT in subjects with advanced cancer in a Phase Ia clinical trial (NCT05002140). No DLTs were observed. At a dose of 160 mg, plasma concentrations were well above the preclinical target plasma level of 530 ng/mL (1 µM) for up to 15 h and inhibition of ATM kinase activity was demonstrated by inhibition of radiation-induced pKAP1 in tumor tissue. Importantly, this concentration of drug is sufficient to induce potent radiosensitization in numerous preclinical models. The median tmax (2.3 h) and mean terminal t½ (11.1 h) at the 160 mg dose in subjects support coadministration of XRD-0394 and RT within a clinically feasible time frame. These results provide a rationale for future clinical trials with XRD-0394 in combination with RT, RT and immunotherapy, PARP inhibitors and targeted delivery of topoisomerase inhibitors. Citation Format: Tona M. Gilmer, Chun-Hsiang Lai, Kexiao Guo, Katherine Deland, Kathleen A. Ashcraft, Amy E. Stewart, Yaode Wang, Jianmin Fu, Kris C. Wood, Deborah A. Smith, Jonathan T. Yang, Christopher T. Chen, Michael F. Gensheimer, Paul B. Romesser, Steven H. Lin, David G. Kirsch, Michael B. Kastan. First disclosure of XRD-0394, a novel dual ATM/DNA-PK inhibitor, that potently radiosensitizes and potentiates PARP and topoisomerase inhibitors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr LB181.

Abstract LB269: XZP-6924: A potent and highly selective USP1 inhibitor with the potential to improve efficacy and overcome resistance to PARP inhibitors

Abstract Genomic instability exists in most cancer cells, making them more dependent on certain DDR repair pathways. Existing research shows that inhibition of the DNA damage response (DDR) pathway stresses cell replication and survival. Drugs targeting the DDR pathway are effective in the treatment of many types of cancers, such as the approved Poly (ADP-ribose) polymerase (PARP) inhibitors that cause synthetic lethality to the cancers harboring BRCA1/2 mutations and other homologous repair deficiencies (HRD). Although PARP inhibitors have good clinical performance, not all patients respond to PARP inhibitor treatments. In addition, the effectiveness of PARP inhibitors in clinics is also limited by the rise of treatment-related drug resistance. Ubiquitin-specific protease 1 (USP1) is involved in DNA damage repair processes, including translesion synthesis and Fanconi anemia pathways. It has been well documented that USP1 deficiency leads to replication fork instability and synthetic lethality to HRD. Combination of USP1 inhibitors with PARP inhibitors synergistically targets BRCA1/2 mutant cancers, thereby enhancing the antitumor effect and reducing drug resistance to PARP inhibitors.We have developed a potent and highly selective USP1 inhibitor, XZP-6924. The selective inhibition of USP1 by XZP-6924 results in the accumulation of mono-ubiquitinated USP1 substrates, for example, proliferating nuclear antigen (PCNA), leading to DNA double-strand breaks (DSBs) and widespread DNA damage accumulation that contributes to cell death. In vitro data reveal that XZP-6924, when combined with Olaparib (PARPi), displays significantly enhanced activities in multiple Olaparib-resistant HRD tumor cells. In CDX and PDX mouse tumor models, XZP-6924 at low doses demonstrates synergy and superior efficacy with sustained tumor regression when combined with a PARP inhibitor (Olaparib). Furthermore, XZP-6924 possesses favorable ADME properties and pharmacokinetic profiles in preclinical animal species. Preliminary toxicology study data indicate that XZP-6924 is well tolerated with a favorable safety margin.In conclusion, XZP-6924 is a potent and highly selective USP1 inhibitor that has the potential for the combination therapy with PARP inhibitors to increase efficacy and overcome some primary and acquired resistance to PARP inhibitors. Current data strongly support further development of XZP-6924. The IND submission is expected in the second half of 2024. Citation Format: Zhihui Kan, Shujin Bai, Kelin Ma, Bo Chen, Benke Jiang, Feng Wang, Xifeng Ma, Dapeng Zhou, Chunmin Shi, Yanfang Nie, Yunling Wang, Ruxiao Liu, Jun Sun, Huimin Zhou, Mei Xu, Jiakui Li, Bin Liu. XZP-6924: A potent and highly selective USP1 inhibitor with the potential to improve efficacy and overcome resistance to PARP inhibitors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr LB269.