Breast cancer resistance protein (BCRP/ABCG2) and P-glycoprotein (P-GP/ABCB1) restrict oral availability and brain accumulation of the PARP inhibitor rucaparib (AG-014699)

Minimal Involvement of P-gp and BCRP in Oral Absorption of Ensitrelvir, An Oral SARS-CoV-2 3C-like Protease Inhibitor, in a Non-Clinical Investigation

P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) are important transporters causing drug-drug interaction (DDI). Here, we investigated the involvement of P-gp and BCRP in the oral absorption of ensitrelvir in non-clinical studies and estimated the DDI risk mediated by P-gp and BCRP inhibition in humans. Although ensitrelvir is an in vitro P-gp and BCRP substrate, it demonstrated high bioavailability in rats and monkeys after oral administration. Plasma exposures of ensitrelvir following oral administration were comparable in wild type (WT) and Bcrp (-/-) mice. On the other hand, the area under the plasma concentration-time curve (AUC) ratio of ensitrelvir in the Mdr1a/1b (-/-) mice to the WT mice was 1.92, indicating that P-gp, but not BCRP, was involved in the oral absorption of ensitrelvir. Based on our previous retrospective analyses, such a low AUC ratio (<3) in the Mdr1a/1b (-/-) mice indicates a minimal impact of P-gp on the oral absorption in humans. In conclusion, our studies demonstrate that the involvement of both P-gp and BCRP in the oral absorption of ensitrelvir is minimal, and suggest that ensitrelvir has a low risk for DDIs mediated by P-gp and BCRP inhibition in humans.

PARP inhibitor-related acute renal failure: a real-world study based on the FDA adverse event reporting system database

BACKGROUND

Current clinical trial data on PARP inhibitors (PARPis)-related acute renal failure (ARF) are not entirely representative of real-world situations. Therefore, in this study, the US Food and Drug Administration Adverse Event Reporting System (FAERS) was used to evaluate PARPis-related ARF.

RESEARCH DESIGN AND METHODS

Data were obtained from 1 January 2015, to 30 September 2023. ARF event reports were analyzed based on four algorithms. The time-to-onset (TTO) and clinical outcomes of PARPis-associated ARF were assessed.

RESULTS

The total included cases were 2726. Significant signals were observed for olaparib, niraparib, and rucaparib (reporting odds ratio (ROR): 1.62, 95% confidence interval (CI): 1.49-1.78, 1.25, 95% CI: 1.19-1.32 and 1.59, 95% CI: 1.47-1.72 respectively). The median TTO of ARF onset was 57, 36, and 85 days for olaparib, niraparib, and rucaparib, respectively. The proportion of deaths with olaparib (9.88%) was significantly higher than for niraparib (2.52%) and rucaparib (2.94%) (p < 0.005). The proportion of life-threatening adverse events associated with niraparib (4.89%) was significantly higher than for rucaparib (0.98%) (p < 0.005).

CONCLUSIONS

ARF and PARPi were related, with the exception of talazoparib. More emphasis should be given to PARPis-related ARF due to the high proportion of serious AEs and delayed adverse reactions.

Synthetically accessible de novo design using reaction vectors: Application to PARP1 inhibitors

De novo design has been a hotly pursued topic for many years. Most recent developments have involved the use of deep learning methods for generative molecular design. Despite increasing levels of algorithmic sophistication, the design of molecules that are synthetically accessible remains a major challenge. Reaction-based de novo design takes a conceptually simpler approach and aims to address synthesisability directly by mimicking synthetic chemistry and driving structural transformations by known reactions that are applied in a stepwise manner. However, the use of a small number of hand-coded transformations restricts the chemical space that can be accessed and there are few examples in the literature where molecules and their synthetic routes have been designed and executed successfully. Here we describe the application of reaction-based de novo design to the design of synthetically accessible and biologically active compounds as proof-of-concept of our reaction vector-based software. Reaction vectors are derived automatically from known reactions and allow access to a wide region of synthetically accessible chemical space. The design was aimed at producing molecules that are active against PARP1 and which have improved brain penetration properties compared to existing PARP1 inhibitors. We synthesised a selection of the designed molecules according to the provided synthetic routes and tested them experimentally. The results demonstrate that reaction vectors can be applied to the design of novel molecules of biological relevance that are also synthetically accessible.

Small molecule agents for triple negative breast cancer: Current status and future prospects

Triple-negative breast cancer (TNBC) is a subtype of breast cancer with poor prognosis. The number of cases increased by 2.26 million in 2020, making it the most commonly diagnosed cancer type in the world. TNBCs lack hormone receptor (HR) and human epidermal growth factor 2 (HER2), which limits treatment options. Currently, paclitaxel-based drugs combined with other chemotherapeutics remain the main treatment for TNBC. There is currently no consensus on the best therapeutic regimen for TNBC. However, there have been successful clinical trials exploring large-molecule monoclonal antibodies, small-molecule targeted drugs, and novel antibody-drug conjugate (ADC). Although monoclonal antibodies have produced clinical success, their large molecular weight can limit therapeutic benefits. It is worth noting that in the past 30 years, the FDA has approved small molecule drugs for HER2-positive breast cancers. The lack of effective targets and the occurrence of drug resistance pose significant challenges in the treatment of TNBC. To improve the prognosis of TNBC, it is crucial to search for effective targets and to overcome drug resistance. This review examines the clinical efficacy, adverse effects, resistance mechanisms, and potential solutions of targeted small molecule drugs in both monotherapies and combination therapies. New therapeutic targets, including nuclear export protein 1 (XPO1) and hedgehog (Hh), are emerging as potential options for researchers and become integrated into clinical trials for TNBC. Additionally, there is growing interest in the potential of targeted protein degradation chimeras (PROTACs), degraders of rogue proteins, as a future therapy direction. This review provides potentially valuable insights with clinical implications.

Druggable genomic landscapes of high-grade gliomas

Background

Despite the putatively targetable genomic landscape of high-grade gliomas, the long-term survival benefit of genomically-tailored targeted therapies remains discouraging.

Methods

Using glioblastoma (GBM) as a representative example of high-grade gliomas, we evaluated the clonal architecture and distribution of hotspot mutations in 388 GBMs from the Cancer Genome Atlas (TCGA). Mutations were matched with 54 targeted therapies, followed by a comprehensive evaluation of drug biochemical properties in reference to the drug's clinical efficacy in high-grade gliomas. We then assessed clinical outcomes of a cohort of patients with high-grade gliomas with targetable mutations reviewed at the Johns Hopkins Molecular Tumor Board (JH MTB; n = 50).

Results

Among 1,156 sequence alterations evaluated, 28.6% represented hotspots. While the frequency of hotspot mutations in GBM was comparable to cancer types with actionable hotspot alterations, GBMs harbored a higher fraction of subclonal mutations that affected hotspots (7.0%), compared to breast cancer (4.9%), lung cancer (4.4%), and melanoma (1.4%). In investigating the biochemical features of targeted therapies paired with recurring alterations, we identified a trend toward higher lipid solubility and lower IC50 in GBM cell lines among drugs with clinical efficacy. The drugs' half-life, molecular weight, surface area and binding to efflux transporters were not associated with clinical efficacy. Among the JH MTB cohort of patients with IDH1 wild-type high-grade gliomas who received targeted therapies, trametinib monotherapy or in combination with dabrafenib conferred radiographic partial response in 75% of patients harboring BRAF or NF1 actionable mutations. Cabozantinib conferred radiographic partial response in two patients harboring a MET and a PDGFRA/KDR amplification. Patients with IDH1 wild-type gliomas that harbored actionable alterations who received genotype-matched targeted therapy had longer progression-free (PFS) and overall survival (OS; 7.37 and 14.72 respectively) than patients whose actionable alterations were not targeted (2.83 and 4.2 months respectively).

Conclusion

While multiple host, tumor and drug-related features may limit the delivery and efficacy of targeted therapies for patients with high-grade gliomas, genotype-matched targeted therapies confer favorable clinical outcomes. Further studies are needed to generate more data on the impact of biochemical features of targeted therapies on their clinical efficacy for high-grade gliomas.

Poly(ADP-ribose) polymerase inhibitors in the treatment landscape of triple-negative breast cancer (TNBC)

OBJECTIVE

Chemotherapy is the mainstay for triple-negative breast cancer (TNBC) patients. Over the years, the use of chemotherapy for these patients has demonstrated many adversities, including toxicity and resistance, which suggested the need to develop novel alternative therapeutic options, such as poly(ADP-ribose) polymerase inhibitors (PARPi). Herein, we provide an overview on PARPi, mechanisms of action and the role of biomarkers in PARPi sensitivity trials, clinical advances in PARPi therapy for TNBC patients based on the most recent studies and findings of clinical trials, and challenges that prevent PARP inhibitors from achieving high efficacy such as resistance and overlapping toxicities with other chemotherapies.

DATA SOURCES

Searching for relevant articles was done using PubMed and Cochrane Library databases by using the keywords including TNBC; chemotherapy; PARPi; BRCA; homologous recombination repair (HRR). Studies had to be published in full-text in English in order to be considered.

DATA SUMMARY

Although PARPi have been used in the treatment of local/metastatic breast malignancies that are HER2 negative and has a germline BRCA mutation, several questions are still to be answered in order to maximize the clinical benefit of PARP inhibitors in TNBC treatment, such as questions related to the optimal use in the neoadjuvant and metastatic settings as well as the best combinations with various chemotherapies.

CONCLUSIONS

PARPi are emerging treatment options for patients with gBRCA1/2 mutations. Determining patients that are most likely to benefit from PARPi and identifying the optimal treatment combinations with high efficacy and fewer side effects are currently ongoing.

Dramatic, durable response to therapy in gBRCA2-mutated pancreas neuroendocrine carcinoma: opportunity and challenge

Poorly differentiated pancreatic neuroendocrine tumors (PDNEC), are a subtype of pancreatic cancer encompassing both small cell and large cell neuroendocrine carcinoma subtypes, and are characterized as distinct in terms of biology and prognosis compared to the more common pancreatic adenocarcinoma. Until recently, there has been a paucity of data on the genomic features of this cancer type. We describe a male patient diagnosed with PDNEC and extensive metastatic disease in the liver at diagnosis. Genomic analysis demonstrated a germline pathogenic variant in BRCA2 with somatic loss-of-heterozygosity of the BRCA2 wild-type allele. Following a favorable response to platinum-based chemotherapy (and the addition of immunotherapy), the patient received maintenance therapy with olaparib, which resulted in a further reduction on follow-up imaging (Fig. 1). After seventeen months of systemic control with olaparib, the patient developed symptomatic central nervous system metastases, which harboured a BRCA2 reversion mutation. No other sites of disease progression were observed. Herein, we report an exceptional outcome through the incorporation of a personalized management approach for a patient with a pancreatic PDNEC, guided by comprehensive genomic sequencing.

OGG1 competitive inhibitors show important off-target effects by directly inhibiting efflux pumps and disturbing mitotic progression

One of the most abundant DNA lesions induced by Reactive oxygen species (ROS) is 8-oxoG, a highly mutagenic lesion that compromises genetic instability when not efficiently repaired. 8-oxoG is specifically recognized by the DNA-glycosylase OGG1 that excises the base and initiates the Base Excision Repair pathway (BER). Furthermore, OGG1 has not only a major role in DNA repair but it is also involved in transcriptional regulation. Cancer cells are particularly exposed to ROS, thus challenging their capacity to process oxidative DNA damage has been proposed as a promising therapeutic strategy for cancer treatment. Two competitive inhibitors of OGG1 (OGG1i) have been identified, TH5487 and SU0268, which bind to the OGG1 catalytic pocket preventing its fixation to the DNA. Early studies with these inhibitors show an enhanced cellular sensitivity to cytotoxic drugs and a reduction in the inflammatory response. Our study uncovers two unreported off-targets effects of these OGG1i that are independent of OGG1. In vitro and in cellulo approaches have unveiled that OGG1i TH5487 and SU0268, despite an unrelated molecular structure, are able to inhibit some members of the ABC family transporters, in particular ABC B1 (MDR1) and ABC G2 (BCRP). The inhibition of these efflux pumps by OGG1 inhibitors results in a higher intra-cellular accumulation of various fluorescent probes and drugs, and largely contributes to the enhanced cytotoxicity observed when the inhibitors are combined with cytotoxic agents. Furthermore, we found that SU0268 has an OGG1-independent anti-mitotic activity-by interfering with metaphase completion-resulting in a high cellular toxicity. These two off-target activities are observed at concentrations of OGG1i that are normally used for in vivo studies. It is thus critical to consider these previously unreported non-specific effects when interpreting studies using TH5487 and SU0268 in the context of OGG1 inhibition. Additionally, our work highlights the persistent need for new specific inhibitors of the enzymatic activity of OGG1.

[1,2,4]Triazolo[3,4-b]benzothiazole Scaffold as Versatile Nicotinamide Mimic Allowing Nanomolar Inhibition of Different PARP Enzymes

We report [1,2,4]triazolo[3,4-b]benzothiazole (TBT) as a new inhibitor scaffold, which competes with nicotinamide in the binding pocket of human poly- and mono-ADP-ribosylating enzymes. The binding mode was studied through analogues and cocrystal structures with TNKS2, PARP2, PARP14, and PARP15. Based on the substitution pattern, we were able to identify 3-amino derivatives 21 (OUL243) and 27 (OUL232) as inhibitors of mono-ARTs PARP7, PARP10, PARP11, PARP12, PARP14, and PARP15 at nM potencies, with 27 being the most potent PARP10 inhibitor described to date (IC₅₀ of 7.8 nM) and the first PARP12 inhibitor ever reported. On the contrary, hydroxy derivative 16 (OUL245) inhibits poly-ARTs with a selectivity toward PARP2. The scaffold does not possess inherent cell toxicity, and the inhibitors can enter cells and engage with the target protein. This, together with favorable ADME properties, demonstrates the potential of TBT scaffold for future drug development efforts toward selective inhibitors against specific enzymes.

Targeted therapy and immunotherapy: Diamonds in the rough in the treatment of epithelial ovarian cancer

Currently, for ovarian cancer, which has the highest mortality rate among all gynecological cancers, the standard treatment protocol is initial tumor cytoreductive surgery followed by platinum-based combination chemotherapy. Although the survival rate after standard treatment has improved, the therapeutic effect of traditional chemotherapy is very limited due to problems such as resistance to platinum-based drugs and recurrence. With the advent of the precision medicine era, molecular targeted therapy has gradually entered clinicians' view, and individualized precision therapy has been realized, surpassing the limitations of traditional therapy. The detection of genetic mutations affecting treatment, especially breast cancer susceptibility gene (BRCA) mutations and mutations of other homologous recombination repair defect (HRD) genes, can guide the targeted drug treatment of patients, effectively improve the treatment effect and achieve a better patient prognosis. This article reviews different sites and pathways of targeted therapy, including angiogenesis, cell cycle and DNA repair, and immune and metabolic pathways, and the latest research progress from preclinical and clinical trials related to ovarian cancer therapy.

Predicting disruptions to drug pharmacokinetics and the risk of adverse drug reactions in non-alcoholic steatohepatitis patients

The liver plays a central role in the pharmacokinetics of drugs through drug metabolizing enzymes and transporters. Non-alcoholic steatohepatitis (NASH) causes disease-specific alterations to the absorption, distribution, metabolism, and excretion (ADME) processes, including a decrease in protein expression of basolateral uptake transporters, an increase in efflux transporters, and modifications to enzyme activity. This can result in increased drug exposure and adverse drug reactions (ADRs). Our goal was to predict drugs that pose increased risks for ADRs in NASH patients. Bibliographic research identified 71 drugs with reported ADRs in patients with liver disease, mainly non-alcoholic fatty liver disease (NAFLD), 54 of which are known substrates of transporters and/or metabolizing enzymes. Since NASH is the progressive form of NAFLD but is most frequently undiagnosed, we identified other drugs at risk based on NASH-specific alterations to ADME processes. Here, we present another list of 71 drugs at risk of pharmacokinetic disruption in NASH, based on their transport and/or metabolism processes. It encompasses drugs from various pharmacological classes for which ADRs may occur when used in NASH patients, especially when eliminated through multiple pathways altered by the disease. Therefore, these results may inform clinicians regarding the selection of drugs for use in NASH patients.

Solid phase radiosynthesis of an olaparib derivative using 4-[18F] fluorobenzoic acid and in vivo evaluation in breast and prostate cancer xenograft models for PARP-1 expression

INTRODUCTION

Solid-phase synthesis and conjugation reactions of acids and amines using coupling reagents are common in organic synthesis, but rare in 18F radiochemistry. 4-[18F]Fluorobenzoic acid (FBA) is a useful building block, but is seldom used directly with coupling reagents for the preparation of 18F radiopharmaceuticals. To overcome the inconveniences associated with using [18F]FBA in conjugation reactions, we have developed a non-covalent solid-phase synthesis (SPS) strategy for the radiosynthesis of [18F]PARPi, a derivative of olaparib as a Poly (ADP-ribose) polymerase-1 (PARP-1) radioligand.

METHODS

Fluoro-, bromo- and iodo-benzoic derivatives of olaparib were synthesized, and their PARP-1 affinities were measured using a recently developed cell culture-based competitive assay. To produce [18F]PARPi, [18F]FBA was radiosynthesized and purified using a cation-exchange cartridge, and then trapped by an anion-exchange resin cartridge, on which the solid-phase radiosynthesis was carried out to produce the desired product. [18F]PARPi was evaluated in vivo in breast and prostate xenograft tumor models by microPET imaging, biodistribution and autoradiography.

RESULTS

The best derivatives of olaparib were identified as compound 4, 7 and 8. [18F]4 ([18F]PARPi) was radiosynthesized in high radiochemical yield, high molar activity and high radiochemical purity using this SPS strategy. The in vivo evaluation of [18F]PARPi demonstrates the PARP-1 specific uptake of [18F]PARPi in the animal models.

CONCLUSIONS

This method is simple and efficient, having great potential for the synthesis of radiopharmaceuticals starting from [18F]FBA or other radiolabeled aromatic acids. Using [18F]PARPi prepared by this method, we demonstrated the promise of [18F]PARPi in the nuclear imaging of PARP-1 expression.

Omics Analysis of Chemoresistant Triple Negative Breast Cancer Cells Reveals Novel Metabolic Vulnerabilities

The emergence of drug resistance in cancer poses the greatest hurdle for successful therapeutic results and is associated with most cancer deaths. In triple negative breast cancer (TNBC), due to the lack of specific therapeutic targets, systemic chemotherapy is at the forefront of treatments, but it only benefits a fraction of patients because of the development of resistance. Cancer cells may possess an innate resistance to chemotherapeutic agents or develop new mechanisms of acquired resistance after long-term drug exposure. Such mechanisms involve an interplay between genetic, epigenetic and metabolic alterations that enable cancer cells to evade therapy. In this work, we generated and characterized a chemoresistant TNBC cell line to be used for the investigation of mechanisms that drive resistance to paclitaxel. Transcriptomic analysis highlighted the important role of metabolic-associated pathways in the resistant cells, prompting us to employ 1H-NMR to explore the metabolome and lipidome of these cells. We identified and described herein numerous metabolites and lipids that were significantly altered in the resistant cells. Integrated analysis of our omics data revealed MSMO1, an intermediate enzyme of cholesterol biosynthesis, as a novel mediator of chemoresistance in TNBC. Overall, our data provide a critical insight into the metabolic adaptations that accompany acquired resistance in TNBC and pinpoint potential new targets.

Prostate Cancer Brain Metastasis: Review of a Rare Complication with Limited Treatment Options and Poor Prognosis

Brain metastases (BM) are perceived as a rare complication of prostate cancer associated with poor outcome. Due to limited published data, we conducted a literature review regarding incidence, clinical characteristics, treatment options, and outcomes of patients with prostate cancer BM. A literature analysis of the PubMed, MEDLINE, and EMBASE databases was performed for full-text published articles on patients diagnosed with BM from prostate cancer. Eligible studies included four or more patients. Twenty-seven publications were selected and analyzed. The sources of published patient cohorts were retrospective chart reviews, administrative healthcare databases, autopsy records, and case series. BM are rare, with an incidence of 1.14% across publications that mainly focus on intraparenchymal metastases. Synchronous visceral metastasis and rare histological prostate cancer subtypes are associated with an increased rate of BM. Many patients do not receive brain metastasis-directed local therapy and the median survival after BM diagnosis is poor, notably in patients with multiple BM, dural-based metastases, or leptomeningeal dissemination. Overall, prostate cancer BM are rare and associated with poor prognosis. Future research is needed to study the impact of novel prostate cancer therapeutics on BM incidence, to identify patients at risk of BM, and to characterize molecular treatment targets.

Where Are We Now and Where Might We Be Headed in Understanding and Managing Brain Metastases in Colorectal Cancer Patients?

OPINION STATEMENT

Compared to liver and lung metastases, brain metastases (BMs) from colorectal cancer (CRC) are rare and remain poorly investigated despite the anticipated rise in their incidence. CRC patients bearing BM have a dismal prognosis with a median survival of 3-6 months, significantly lower than that of patients with BM from other primary tumors, and of those with metastatic CRC manifesting extracranially. While liver and lung metastases from CRC have more codified treatment strategies, there is no consensus regarding the treatment of BM in CRC, and their management follows the approaches of BM from other solid tumors. Therapeutic strategies are driven by the number and localisation of the lesion, consisting in local treatments such as surgery, stereotactic radiosurgery, or whole-brain radiotherapy. Novel treatment modalities are slowly finding their way into this shy unconsented armatorium including immunotherapy, monoclonal antibodies, tyrosine kinase inhibitors, or a combination of those, among others.This article reviews the pioneering strategies aiming at understanding, diagnosing, and managing this disease, and discusses future directions, challenges, and potential innovations in each of these domains.

HIGHLIGHTS

• With the increasing survival in CRC, brain and other rare/late-onset metastases are rising. • Distal colon/rectal primary location, long-standing progressive lung metastases, and longer survival are risk factors for BM development in CRC. • Late diagnosis and lack of consensus treatment strategies make BM-CRC diagnosis very dismal. • Liquid biopsies using circulating tumor cells might offer excellent opportunities in the early diagnosis of BM-CRC and the search for therapeutic options. • Multi-modality treatment including surgical metastatic resection, postoperative SRS with/without WBRT, and chemotherapy is the best current treatment option. • Recent mid-sized clinical trials, case reports, and preclinical models show the potential of unconventional therapeutic approaches as monoclonal antibodies, targeted therapies, and immunotherapy. Graphical abstract.

P-glycoprotein Mediates Resistance to the Anaplastic Lymphoma Kinase Inhiitor Ensartinib in Cancer Cells

Ensartinib (X-396) is a promising second-generation small-molecule inhibitor of anaplastic lymphoma kinase (ALK) that was developed for the treatment of ALK-positive non-small-cell lung cancer. Preclinical and clinical trial results for ensartinib showed superior efficacy and a favorable safety profile compared to the first-generation ALK inhibitors that have been approved by the U.S. Food and Drug Administration. Although the potential mechanisms of acquired resistance to ensartinib have not been reported, the inevitable emergence of resistance to ensartinib may limit its therapeutic application in cancer. In this work, we investigated the interaction of ensartinib with P-glycoprotein (P-gp) and ABCG2, two ATP-binding cassette (ABC) multidrug efflux transporters that are commonly associated with the development of multidrug resistance in cancer cells. Our results revealed that P-gp overexpression, but not expression of ABCG2, was associated with reduced cancer cell susceptibility to ensartinib. P-gp directly decreased the intracellular accumulation of ensartinib, and consequently reduced apoptosis and cytotoxicity induced by this drug. The cytotoxicity of ensartinib could be significantly reversed by treatment with the P-gp inhibitor tariquidar. In conclusion, we report that ensartinib is a substrate of P-gp, and provide evidence that this transporter plays a role in the development of ensartinib resistance. Further investigation is needed.

Role of the Efflux Transporters Abcb1 and Abcg2 in the Brain Distribution of Olaparib in Mice

Olaparib is a first-in-class poly (ADP-ribose) polymerase oral inhibitor used to treat various tumors. In this study, we clarified the roles of ABCB1/Abcb1 and ABCG2/Abcg2 transporters in restricting olaparib distribution to the brain. Olaparib was efficiently transported by human ABCG2, human ABCB1, and mouse Abcg2 in vitro. In the in vivo disposition study of olaparib using single or combination knockout mice, the systemic exposure of olaparib did not differ significantly between the strains over an 8-h period. However, the brain-to-plasma unbound concentration ratio of olaparib increased 5.6- and 8.1-fold in Abcb1a/1b and Abcb1a/1b;Abcg2 knockout mice, respectively, compared with wild-type mice. The Abcg2 single knockout mice exhibited a similar brain-to-plasma unbound concentration ratio to wild-type mice. Moreover, the brain distribution of olaparib could be modulated by the ABCB1/ABCG2 dual inhibitor elacridar to reach a similar degree of inhibition to Abcb1a/1b^-/-. These findings suggest that olaparib is actively transported by both human and mouse ABCB1/Abcb1 and ABCG2/Abcg2; while Abcb1a/1b is a major determinant of olaparib brain penetration in mice, Abcg2 is likely to be a minor contributor. Concomitant treatment with temozolomide slightly increased the brain distribution of olaparib in mouse, but the clinical impact of the interaction was expected to be limited.

Design and synthesis of benzodiazepines as brain penetrating PARP-1 inhibitors

The poly (ADP-ribose) polymerase (PARP) inhibitors play a crucial role in cancer therapy. However, most approved PARP inhibitors cannot cross the blood-brain barrier, thus limiting their application in the central nervous system. Here, 55 benzodiazepines were designed and synthesised to screen brain penetrating PARP-1 inhibitors. All target compounds were evaluated for their PARP-1 inhibition activity, and compounds with better activity were selected for further assays in vitro. Among them, compounds H34, H42, H48, and H52 displayed acceptable inhibition effects on breast cancer cells. Also, computational prediction together with the permeability assays in vitro and in vivo proved that the benzodiazepine PARP-1 inhibitors we synthesised were brain permeable. Compound H52 exhibited a B/P ratio of 40 times higher than that of Rucaparib and would be selected to develop its potential use in neurodegenerative diseases. Our study provided potential lead compounds and design strategies for the development of brain penetrating PARP-1 inhibitors.

HIGHLIGHTS

Structural fusion was used to screen brain penetrating PARP-1 inhibitors.

55 benzodiazepines were evaluated for their PARP-1 inhibition activity.

Four compounds displayed acceptable inhibition effects on breast cancer cells.

The benzodiazepine PARP-1 inhibitors were proved to be brain permeable.

PARP Inhibitors in Glioma: A Review of Therapeutic Opportunities

Gliomas are the most common malignant primary brain tumor in adults. Despite advances in multimodality therapy, incorporating surgery, radiotherapy, systemic therapy, tumor treating fields and supportive care, patient outcomes remain poor, especially in glioblastoma where median survival has remained static at around 15 months, for decades. Low-grade gliomas typically harbor isocitrate dehydrogenase (IDH) mutations, grow more slowly and confer a better prognosis than glioblastoma. However, nearly all gliomas eventually recur and progress in a way similar to glioblastoma. One of the novel therapies being developed in this area are poly(ADP-ribose) polymerase (PARP) inhibitors. PARP inhibitors belong to a class of drugs that target DNA damage repair pathways. This leads to synthetic lethality of cancer cells with coexisting homologous recombination deficiency. PARP inhibitors may also potentiate the cytotoxic effects of radiotherapy and chemotherapy, and prime the tumor microenvironment for immunotherapy. In this review, we examine the rationale and clinical evidence for PARP inhibitors in glioma and suggest therapeutic opportunities.

The roles of the human ATP-binding cassette transporters P-glycoprotein and ABCG2 in multidrug resistance in cancer and at endogenous sites: future opportunities for structure-based drug design of inhibitors

The ATP-binding cassette (ABC) transporters P-glycoprotein (P-gp) and ABCG2 are multidrug transporters that confer drug resistance to numerous anti-cancer therapeutics in cell culture. These findings initially created great excitement in the medical oncology community, as inhibitors of these transporters held the promise of overcoming clinical multidrug resistance in cancer patients. However, clinical trials of P-gp and ABCG2 inhibitors in combination with cancer chemotherapeutics have not been successful due, in part, to flawed clinical trial designs resulting from an incomplete molecular understanding of the multifactorial basis of multidrug resistance (MDR) in the cancers examined. The field was also stymied by the lack of high-resolution structural information for P-gp and ABCG2 for use in the rational structure-based drug design of inhibitors. Recent advances in structural biology have led to numerous structures of both ABCG2 and P-gp that elucidated more clearly the mechanism of transport and the polyspecific nature of their substrate and inhibitor binding sites. These data should prove useful helpful for developing even more potent and specific inhibitors of both transporters. As such, although possible pharmacokinetic interactions would need to be evaluated, these inhibitors may show greater effectiveness in overcoming ABC-dependent multidrug resistance in combination with chemotherapeutics in carefully selected subsets of cancers. Another perhaps even more compelling use of these inhibitors may be in reversibly inhibiting endogenously expressed P-gp and ABCG2, which serve a protective role at various blood-tissue barriers. Inhibition of these transporters at sanctuary sites such as the brain and gut could lead to increased penetration by chemotherapeutics used to treat brain cancers or other brain disorders and increased oral bioavailability of these agents, respectively.

Phase 1A/1B dose-escalation and -expansion study to evaluate the safety, pharmacokinetics, food effects and antitumor activity of pamiparib in advanced solid tumours

BACKGROUND

Pamiparib, a PARP1/2 inhibitor, demonstrated antitumor activity in preclinical models.

METHODS

This Phase 1A/1B dose-escalation/dose-expansion study enrolled adults (≥18 years) with advanced/metastatic cancer. The dose-escalation phase evaluated the recommended Phase 2 dose (RP2D), maximum tolerated dose (MTD), and pharmacokinetics; the dose-expansion phase evaluated the antitumor activity and food effects.

RESULTS

Patients (N = 101) were enrolled in dose-escalation (n = 64) and dose-expansion (n = 37). During BID dose-escalation, dose-limiting toxicities were Grade 2 nausea (n = 1, 40 mg; n = 1, 80 mg); Grade 2 nausea and Grade 2 anorexia (n = 1, 120 mg), Grade 2 nausea, Grade 3 fatigue and Grade 3 paraesthesia (n = 1, 120 mg); MTD was 80 mg BID and RP2D was 60 mg BID. Common adverse events (AEs) were nausea (69.3%), fatigue (48.5%) and anaemia (35.6%); the most common Grade ≥3 AE was anaemia (24.8%). There was a dose-proportional increase in pamiparib exposure; no food effects on pharmacokinetics were observed. In the efficacy-evaluable population (n = 77), objective response rate (ORR) was 27.3% (95% CI, 17.7-38.6%). Median duration of response was 14.9 months (95% CI, 8.7-26.3). In the epithelial ovarian cancer (EOC)-evaluable population (n = 51), ORR was 41.2% (95% CI, 27.6-55.8%).

CONCLUSIONS

Pamiparib was tolerated with manageable AEs, and antitumor activity was observed in patients with EOC. CLINICALTRIALS.

GOV IDENTIFIER

NCT02361723.

Role of PARP in TNBC: Mechanism of Inhibition, Clinical Applications, and Resistance

Triple-negative breast cancer is a combative cancer type with a highly inflated histological grade that leads to poor theragnostic value. Gene, protein, and receptor-specific targets have shown effective clinical outcomes in patients with TNBC. Cells are frequently exposed to DNA-damaging agents. DNA damage is repaired by multiple pathways; accumulations of mutations occur due to damage to one or more pathways and lead to alterations in normal cellular mechanisms, which lead to development of tumors. Advances in target-specific cancer therapies have shown significant momentum; most treatment options cause off-target toxicity and side effects on healthy tissues. PARP (poly(ADP-ribose) polymerase) is a major protein and is involved in DNA repair pathways, base excision repair (BER) mechanisms, homologous recombination (HR), and nonhomologous end-joining (NEJ) deficiency-based repair mechanisms. DNA damage repair deficits cause an increased risk of tumor formation. Inhibitors of PARP favorably kill cancer cells in BRCA-mutations. For a few years, PARPi has shown promising activity as a chemotherapeutic agent in BRCA1- or BRCA2-associated breast cancers, and in combination with chemotherapy in triple-negative breast cancer. This review covers the current results of clinical trials testing and future directions for the field of PARP inhibitor development.

PARP inhibitor resistance in ovarian cancer: Underlying mechanisms and therapeutic approaches targeting the ATR/CHK1 pathway

Ovarian cancer (OC) constitutes the most common cause of gynecologic cancer-related death in women worldwide. Despite consistent developments in treatment strategies for OC, the management of advanced-stage disease remains a significant challenge. Recent improvements in targeted treatments based on poly(ADP-ribose) polymerase (PARP) inhibitors (PARPi) have provided invaluable benefits to patients with OC. Unfortunately, numerous patients do not respond to PARPi due to intrinsic resistance or acquisition of resistance. Here, we discuss mechanisms of resistance to PARPi that have specifically emerged in OC including increased drug efflux, restoration of HR repair, re-establishment of replication fork stability, reduced PARP1 trapping, abnormalities in PARP signaling, and less common pathways associated with alternative DNA sensing and repair pathways. Elucidation of the precise mechanisms is essential for the development of novel strategies to re-sensitize OC cells to PARPi agents. Additionally, novel potential concepts for preventing and combating resistance to PARPi under development and relevant clinical reports on treatment strategies have been reviewed, with emphasis on the exploitation of the ATR/CHK1 kinase pathway in sensitization to PARPi to overcome resistance-induced vulnerability in ovarian cancer.

Research progress on the association between environmental pollutants and the resistance mechanism of PARP inhibitors in ovarian cancer

The occurrence and progression of ovarian cancer are closely related to genetics and environmental pollutants. Poly(ADP-ribose) polymerase (PARP) inhibitors have been a major breakthrough in the history of ovarian cancer treatment. PARP is an enzyme responsible for post-translational modification of proteins and repair of single-stranded DNA damage. PARP inhibitors can selectively inhibit PARP function, resulting in a synthetic lethal effect on tumor cells defective in homologous recombination repair. However, with large-scale application, drug resistance also inevitably appears. For PARP inhibitors, the diversity and complexity of drug resistance mechanisms have always been difficult problems in clinical treatment. Herein, we mainly summarized the research progress of DNA damage repair and drug resistance mechanisms related to PARP inhibitors and the impact of environmental pollutants on DNA damage repair to aid the development prospects and highlight urgent problems to be solved.

The evolving role of PARP inhibitors in advanced ovarian cancer

The field of ovarian cancer has been revolutionized with the use of poly (ADP-ribose) polymerase (PARP) inhibitors, which present greater inhibition effect in epithelial subtype due to high rates of homologous recombination deficiency. PARP inhibition exploits this cancer pitfall by disrupting DNA repair, leading to genomic instability and apoptosis. Three PARP inhibitors (olaparib, niraparib, and rucaparib) are now approved for use in women with epithelial ovarian cancer, while others are under development. Among women with BRCA1/2 mutations, maintenance PARP therapy has led to a nearly fourfold prolongation of PFS, while those without BRCA1/2 mutations experience an approximately twofold increase in PFS. Differences in trial design, patient selection and primary analysis population affect the conclusions on PARP inhibitors. Limited OS data have been published and there is also limited experience regarding long-term safety. With regard to toxicity profile, there are no differences in serious adverse events between the experimental and control groups. However, combining adverse event data from maintenance phases, a trend towards more events in the experimental group, compared with controls, has been shown. The mechanisms of PARP-inhibitor resistance include restoration of HR through reversion mutations in HR genes, leading to resumed HR function. Other mechanisms that sustain sufficient DNA repair are discussed as well. PARP inhibitors play a pivotal role in the management of ovarian cancer, affecting the future treatment choices. Defining exactly which patients will benefit from them is a challenge and the need for HRD testing to define ‘BRCA-ness’ will add additional costs to treatment.

Characterization of a hyaluronic acid and folic acid-based hydrogel for cisplatin delivery: Antineoplastic effect in human ovarian cancer cells in vitro

We successfully prepared and characterized a hyaluronic acid- and folic acid-based hydrogel for the delivery of cisplatin (GEL-CIS) with the aim to induce specific and efficient incorporation of CIS into ovarian cancer (OC) cells, improve its antineoplastic effect and avoid CIS-resistance. The slow and controlled release of the drug from the polymeric network and its swelling degree at physiologic pH suggested its suitability for CIS delivery in OC. We compared here the effects of pure CIS to that of GEL-CIS on human OC cell lines, either wild type or CIS-resistant, in basal conditions and in the presence of macrophage-derived conditioned medium, mimicking the action of tumor-associated macrophages in vivo. GEL-CIS inhibited OC cell growth and migration more efficiently than pure CIS and modulated the expression of proteins involved in the Epithelial Mesenchymal Transition, a process playing a key role in OC metastatic spread and resistance to CIS.

Moving beyond PARP Inhibition: Current State and Future Perspectives in Breast Cancer

Breast cancer is the most frequent and lethal tumor in women and finding the best therapeutic strategy for each patient is an important challenge. PARP inhibitors (PARPis) are the first, clinically approved drugs designed to exploit synthetic lethality in tumors harboring BRCA1/2 mutations. Recent evidence indicates that PARPis have the potential to be used both in monotherapy and combination strategies in breast cancer treatment. In this review, we show the mechanism of action of PARPis and discuss the latest clinical applications in different breast cancer treatment settings, including the use as neoadjuvant and adjuvant approaches. Furthermore, as a class, PARPis show many similarities but also certain critical differences which can have essential clinical implications. Finally, we report the current knowledge about the resistance mechanisms to PARPis. A systematic PubMed search, using the entry terms “PARP inhibitors” and “breast cancer”, was performed to identify all published clinical trials (Phase I-II-III) and ongoing trials (ClinicalTrials.gov), that have been reported and discussed in this review.

PARP1: Structural insights and pharmacological targets for inhibition

Poly(ADP-ribose) polymerase 1 (PARP1, also known as ADPRT1) is a multifunctional human ADP-ribosyltransferase. It plays a role in multiple DNA repair pathways, including the base excision repair (BER), non-homologous end joining (NHEJ), homologous recombination (HR), and Okazaki-fragment processing pathways. In response to DNA strand breaks, PARP1 covalently attaches ADP-ribose moieties to arginine, glutamate, aspartate, cysteine, lysine, and serine acceptor sites on both itself and other proteins. This signal recruits DNA repair proteins to the site of DNA damage. PARP1 binding to these sites enhances ADP-ribosylation via allosteric communication between the distant DNA binding and catalytic domains. In this review, we provide a general overview of PARP1 and emphasize novel potential approaches for pharmacological inhibition. Clinical PARP1 inhibitors bind the catalytic pocket, where they directly interfere with ADP-ribosylation. Some inhibitors may further enhance potency by "trapping" PARP1 on DNA via an allosteric mechanism, though this proposed mode of action remains controversial. PARP1 inhibitors are used clinically to treat some cancers, but resistance is common, so novel pharmacological approaches are urgently needed. One approach may be to design novel small molecules that bind at inter-domain interfaces that are essential for PARP1 allostery. To illustrate these points, this review also includes instructive videos showing PARP1 structures and mechanisms.

Overcoming PARPi resistance: Preclinical and clinical evidence in ovarian cancer

Ovarian cancer is the fifth cause of cancer-related deaths in women with high grade serous carcinoma (HGSOC) representing the most common histological subtype. Approximately 50 % of HGSOC are characterized by deficiency in homologous recombination (HR), one of the main cellular pathways to repair DNA double strand breaks and one of the well-described mechanisms is the loss of function of the BRCA1 or BRCA2 genes. Inhibition of the poly-ADP-ribose polymerase (PARP) is synthetic lethal with HR deficiency and the use of PARP inhibitors (PARPi) has significantly improved the outcome of patients with HGSOC with a greater benefit in patients with BRCA1/2 deficient tumors. However, intrinsic or acquired resistance to PARPi inevitably occurs in most HGSOC patients. Distinct heterogeneous mechanisms underlying the resistance to PARPi have been described, including a decrease in intracellular drug levels due to upregulation of multidrug efflux pumps, loss of expression/inactivating mutations in the PARP1 protein, restoration of HR and the protection of the replicative fork. Deciphering the molecular mechanisms of resistance to PARPi is of paramount importance towards the development of new treatment strategies and/or novel pharmacological agents to overcome this chemoresistance and optimize the treatment regimen for individual HGSOC patients. The current review summarizes the mechanisms underlying the resistance to PARPi, the available preclinical and clinical data on new combination treatment strategies (with chemotherapy, anti-angiogenic agents and immune checkpoint inhibitors) as well as agents under investigation which target the DNA damage response.

Drugging DNA Damage Repair Pathways for Trinucleotide Repeat Expansion Diseases

DNA damage repair (DDR) mechanisms have been implicated in a number of neurodegenerative diseases (both genetically determined and sporadic). Consistent with this, recent genome-wide association studies in Huntington’s disease (HD) and other trinucleotide repeat expansion diseases have highlighted genes involved in DDR mechanisms as modifiers for age of onset, rate of progression and somatic instability. At least some clinical genetic modifiers have been shown to have a role in modulating trinucleotide repeat expansion biology and could therefore provide new disease-modifying therapeutic targets. In this review, we focus on key considerations with respect to drug discovery and development using DDR mechanisms as a target for trinucleotide repeat expansion diseases. Six areas are covered with specific reference to DDR and HD: 1) Target identification and validation; 2) Candidate selection including therapeutic modality and delivery; 3) Target drug exposure with particular focus on blood-brain barrier penetration, engagement and expression of pharmacology; 4) Safety; 5) Preclinical models as predictors of therapeutic efficacy; 6) Clinical outcome measures including biomarkers.

Poly(ADP-ribose) polymerase inhibition: past, present and future

The process of poly(ADP-ribosyl)ation and the major enzyme that catalyses this reaction, poly(ADP-ribose) polymerase 1 (PARP1), were discovered more than 50 years ago. Since then, advances in our understanding of the roles of PARP1 in cellular processes such as DNA repair, gene transcription and cell death have allowed the investigation of therapeutic PARP inhibition for a variety of diseases - particularly cancers in which defects in DNA repair pathways make tumour cells highly sensitive to the inhibition of PARP activity. Efforts to identify and evaluate potent PARP inhibitors have so far led to the regulatory approval of four PARP inhibitors for the treatment of several types of cancer, and PARP inhibitors have also shown therapeutic potential in treating non-oncological diseases. This Review provides a timeline of PARP biology and medicinal chemistry, summarizes the pathophysiological processes in which PARP plays a role and highlights key opportunities and challenges in the field, such as counteracting PARP inhibitor resistance during cancer therapy and repurposing PARP inhibitors for the treatment of non-oncological diseases.

Pamiparib is a potent and selective PARP inhibitor with unique potential for the treatment of brain tumor

Pamiparib, an investigational Poly (ADP-ribose) polymerase (PARP) inhibitor in clinical development, demonstrates excellent selectivity for both PARP1 and PARP2, and superb anti-proliferation activities in tumor cell lines with BRCA1/2 mutations or HR pathway deficiency (HRD). Pamiparib has good bioavailability and is 16-fold more potent than olaparib in an efficacy study using BRCA1 mutated MDA-MB-436 breast cancer xenograft model. Pamiparib also shows strong anti-tumor synergy with temozolomide (TMZ), a DNA alkylating agent used to treat brain tumors. Compared to other PARP inhibitors, pamiparib demonstrated improved penetration across the blood brain barrier (BBB) in mice. Oral administration of pamiparib at a dose as low as 3 mg/kg is sufficient to abrogate PARylation in brain tumor tissues. In SCLC-derived, TMZ-resistant H209 intracranial xenograft model, combination of pamiparib with TMZ overcomes its resistance and shows significant tumor inhibitory effects and prolonged life span. Our data suggests that combination of pamiparib with TMZ has unique potential for treatment of brain tumors. Currently, the combination therapy of pamiparib with TMZ is evaluated in clinical trial [NCT03150862].

Overexpression of ABCB1 and ABCG2 contributes to reduced efficacy of the PI3K/mTOR inhibitor samotolisib (LY3023414) in cancer cell lines

LY3023414 (samotolisib) is a promising new dual inhibitor of phosphoinositide 3-kinase (PI3K) and mammalian target of rapamycin (mTOR). Currently, multiple clinical trials are underway to evaluate the efficacy of LY3023414 in patients with various types of cancer. However, the potential mechanisms underlying acquired resistance to LY3023414 in human cancer cells still remain elusive. In this study, we investigated whether the overexpression of ATP-binding cassette (ABC) drug transporters such as ABCB1 and ABCG2, one of the most common mechanisms for developing multidrug resistance, may potentially reduce the efficacy of LY3023414 in human cancer cells. We demonstrated that the intracellular accumulation of LY3023414 in cancer cells was significantly reduced by the drug efflux function of ABCB1 and ABCG2. Consequently, the cytotoxicity and efficacy of LY3023414 for inhibiting the activation of the PI3K pathway and induction of G0/G1 cell-cycle arrest were substantially reduced in cancer cells overexpressing ABCB1 or ABCG2, which could be restored using tariquidar or Ko143, respectively. Furthermore, stimulatory effect of LY3023414 on the ATPase activity of ABCB1 and ABCG2, as well as in silico molecular docking analysis of LY3023414 binding to the substrate-binding pockets of these transporters provided additional insight into the manner in which LY3023414 interacts with both transporters. In conclusion, we report that LY3023414 is a substrate for ABCB1 and ABCG2 transporters implicating their role in the development of resistance to LY3023414, which can have substantial clinical implications and should be further investigated.

PARP inhibitor cyanine dye conjugate with enhanced cytotoxic and antiproliferative activity in patient derived glioblastoma cell lines

We describe the synthesis and in vitro activity of drug-dye conjugate 1, which is a combination of the PARP inhibitor rucaparib and heptamethine cyanine dye IR-786. The drug-dye conjugate 1 was evaluated in three different patient-derived glioblastoma cell lines and showed strong cytotoxic activity with nanomolar potency (EC₅₀: 128 nM), which was a 780 fold improvement over rucaparib itself. We also observe a synergistic effect of 1 with temozolomide (TMZ), the standard drug for treatment for glioblastoma even though these cell lines were resistant to TMZ treatment. We envisage such conjugates to be worth exploring for their utility in the treatment of various brain cancers.

Transcription factor-mediated regulation of the BCRP/ABCG2 efflux transporter: a review across tissues and species

Introduction: The breast cancer resistance protein (BCRP/ABCG2) is a member of the ATP-binding cassette superfamily of transporters. Using the energy garnered from the hydrolysis of ATP, BCRP actively removes drugs and endogenous molecules from the cell. With broad expression across the liver, kidney, brain, placenta, testes, and small intestines, BCRP can impact the pharmacokinetics and pharmacodynamics of xenobiotics.Areas covered: The purpose of this review is to summarize the transcriptional signaling pathways that regulate BCRP expression across various tissues and mammalian species. We will cover the endobiotic- and xenobiotic-activated transcription factors that regulate the expression and activity of BCRP. These include the estrogen receptor, progesterone receptor, peroxisome proliferator-activated receptor, constitutive androstane receptor, pregnane X receptor, nuclear factor e2-related factor 2, and aryl hydrocarbon receptor.Expert opinion: Key transcription factors regulate BCRP expression and function in response to hormones and xenobiotics. Understanding this regulation provides an opportunity to improve pharmacotherapeutic outcomes by enhancing the efficacy and reducing the toxicity of drugs that are substrates of this efflux transporter.

Quantitative determination of niraparib and olaparib tumor distribution by mass spectrometry imaging

Rationale: Optimal intratumor distribution of an anticancer drug is fundamental to reach an active concentration in neoplastic cells, ensuring the therapeutic effect. Determination of drug concentration in tumor homogenates by LC-MS/MS gives important information about this issue but the spatial information gets lost. Targeted mass spectrometry imaging (MSI) has great potential to visualize drug distribution in the different areas of tumor sections, with good spatial resolution and superior specificity. MSI is rapidly evolving as a quantitative technique to measure the absolute drug concentration in each single pixel. Methods: Different inorganic nanoparticles were tested as matrices to visualize the PARP inhibitors (PARPi) niraparib and olaparib. Normalization by deuterated internal standard and a custom preprocessing pipeline were applied to achieve a reliable single pixel quantification of the two drugs in human ovarian tumors from treated mice. Results: A quantitative method to visualize niraparib and olaparib in tumor tissue of treated mice was set up and validated regarding precision, accuracy, linearity, repeatability and limit of detection. The different tumor penetration of the two drugs was visualized by MSI and confirmed by LC-MS/MS, indicating the homogeneous distribution and higher tumor exposure reached by niraparib compared to olaparib. On the other hand, niraparib distribution was heterogeneous in an ovarian tumor model overexpressing the multidrug resistance protein P-gp, a possible cause of resistance to PARPi. Conclusions: The current work highlights for the first time quantitative distribution of PAPRi in tumor tissue. The different tumor distribution of niraparib and olaparib could have important clinical implications. These data confirm the validity of MSI for spatial quantitative measurement of drug distribution providing fundamental information for pharmacokinetic studies, drug discovery and the study of resistance mechanisms.

Rucaparib: A Review in Ovarian Cancer

Rucaparib (Rubraca^®) is a small molecule poly(ADP-ribose) polymerase (PARP) inhibitor with potent activity against PARP-1, -2 and -3. It is approved in the USA and the EU for the treatment of adult patients with BRCA-mutated ovarian cancer who have been treated with two or more lines of chemotherapy. Rucaparib is also approved in the USA and the EU for use as maintenance therapy in adult patients with recurrent or relapsed ovarian cancer who are in a complete or partial response to platinum-based chemotherapy. Based on an analysis of patients across two phase II clinical trials, rucaparib displayed clinical activity as third- (or later-) line treatment of BRCA-mutated ovarian cancer, with rucaparib-treated patients having a confirmed objective response rate of 54%. Furthermore, as demonstrated in the randomized, placebo-controlled, phase III ARIEL3 trial, rucaparib significantly improved progression-free survival when used as maintenance treatment in patients with platinum-sensitive ovarian cancer. Rucaparib had an acceptable tolerability profile in clinical trials in women with ovarian cancer. Common adverse events were generally manageable with dose modification and/or supportive care. Thus, currently available data indicate that rucaparib is a useful addition to the options available to clinicians for the treatment of advanced ovarian cancer, in both the treatment and maintenance therapy settings.

Botryllamide G is an ABCG2 inhibitor that improves lapatinib delivery in mouse brain

Introduction: Transporters comprising the blood-brain barrier complicate delivery of many therapeutics to the central nervous system. The present study ascertained whether the natural product botryllamide G is viable for in vivo inhibition of ABCG2 using lapatinib as a probe for ABCB1 and ABCG2-mediated efflux from the brain. Methods: Wild-type and Mdr1a/Mdr1b (-/-) mice were treated with botryllamide G and lapatinib ("doublet therapy"), and while a separate cohort of wild-type mice was treated with botryllamide, tariquidar and lapatinib ("triplet therapy"). Results: Botryllamide G demonstrates biphasic elimination with a rapid distribution, decreasing below the in vitro IC50 of 6.9 µM within minutes, yet with a relatively slower terminal half-life (4.6 h). In Mdr1a/Mdr1b (-/-) mice, doublet therapy resulted in a significant increase in brain lapatinib AUC at 8 h (2058 h*ng/mL vs 4007 h*ng/mL; P = .031), but not plasma exposure (P = .15). No significant differences were observed after 24 h. Lapatinib brain exposure was greater through 1 h when wild-type mice were administered triplet therapy (298 h*pg/mg vs 120 h*pg/mg; P < .001), but the triplet decreased brain AUC through 24 h vs. mice administered lapatinib alone (2878 h*pg/mg vs 4461hr*ng/mL; P < .001) and did not alter the brain:plasma ratio. Conclusions: In summary, the ABCG2 inhibitor, botryllamide G, increases brain exposure to lapatinib in mice lacking Abcb1, although the combination of botryllamide G and tariquidar increases brain exposure in wild-type mice only briefly (1 h). Additional research is needed to find analogs of this compound that have better pharmacokinetics and pharmacodynamic effects on ABCG2 inhibition.

Mechanisms of resistance to PARP inhibitors - an evolving challenge in oncology

Poly-adenosine diphosphate ribose polymerase inhibitors (PARPi) lead to synthetic lethality when used in cancers harbouring a BRCA mutation or homologous recombination deficiency. There are now four PARPi approved by the Food and Drug Administration for therapeutic use is ovarian and breast cancer. In addition to this, there is data supporting its use in pancreatic adenocarcinoma and prostate cancer. However, development of resistance to PARPi limits the duration of response. Key mechanisms found to date include: (1) restoration of homologous recombination; (2) changes in PARP1; (3) suppression of non-homologous end joining; (4) replication fork protection; and (5) drug concentration. Gaining a better understanding of resistance mechanisms may guide combination therapies to overcome the resistance and improve the efficacy of PARPi. The purpose of this review is to describe the resistance mechanisms to PARPi and discuss their early detection.

Genome-scale CRISPR knockout screen identifies TIGAR as a modifier of PARP inhibitor sensitivity

Treatment of cancer with poly (ADP-ribose) polymerase (PARP) inhibitors is currently limited to cells defective in the homologous recombination (HR) pathway. Identification of genetic targets that induce or mimic HR deficiencies will extend the clinical utility of PARP inhibitors. Here we perform a CRISPR/Cas9-based genome-scale loss-of-function screen, using the sensitivity of PARP inhibitor olaparib as a surrogate. We identify C12orf5, encoding TP53 induced glycolysis and apoptosis regulator (TIGAR), as a modifier of PARP inhibitor response. We show that TIGAR is amplified in several cancer types, and higher expression of TIGAR associates with poor overall survival in ovarian cancer. TIGAR knockdown enhances sensitivity to olaparib in cancer cells via downregulation of BRCA1 and the Fanconi anemia pathway and increases senescence of these cells by affecting metabolic pathways and increasing the cytotoxic effects of olaparib. Our results indicate TIGAR should be explored as a therapeutic target for treating cancer and extending the use of PARP inhibitors.

Pingping Fang et al. report a CRISPR/Cas9-based genome-scale loss-of-function screen identifying TIGAR as a modifier of response to PARP inhibition. The authors find that knockdown of TIGAR increases intracellular reactive oxygen species levels, enhances more DNA damage after olaparib treatment, and induces a state of “BRCAness”, suggesting that TIGAR is a potential therapeutic target in ovarian cancer patients.

Impact of Q141K on the Transport of Epidermal Growth Factor Receptor Tyrosine Kinase Inhibitors by ABCG2

The ATP-binding cassette transporter ABCG2 is expressed in various organs, such as the small intestine, liver, and kidney, and influences the pharmacokinetics of drugs that are its substrates. ABCG2 is also expressed by cancer cells and mediates resistance to anticancer agents by promoting the efflux of these drugs. In the present study, we investigated the interactions between epidermal growth factor receptor tyrosine kinase inhibitors and ABCG2 by MTT assay, intracellular drug accumulation assay, and FACS. This study showed that four epidermal growth factor receptor tyrosine kinase inhibitors (EGFR TKIs) (gefitinib, erlotinib, lapatinib, and afatinib) were transported from tumor cells as substrates of ABCG2. Q141K is a common single-nucleotide polymorphism of ABCG2 in Asians. We demonstrated that the extracellular efflux of gefitinib, erlotinib, and lapatinib was reduced by Q141K, whereas afatinib transport was not affected. In addition, all four EGFR TKIs inhibited the transport of other substrates by both wild-type and variant ABCG2 at 0.1 μM concentrations. Accordingly, epidermal growth factor receptor tyrosine kinase inhibitors may induce interactions with other drugs that are substrates of ABCG2, and single-nucleotide polymorphisms of ABCG2 may influence both the pharmacokinetics and efficacy of these anticancer agents.

Coexpression of ABCB1 and ABCG2 in a Cell Line Model Reveals Both Independent and Additive Transporter Function

Although overexpression of multiple ATP-binding cassette transporters has been reported in clinical samples, few studies have examined how coexpression of multiple transporters affected resistance to chemotherapeutic drugs. We therefore examined how coexpression of ABCB1 (P-glycoprotein) and ABCG2 contributes to drug resistance in a cell line model. HEK293 cells were transfected with vector-encoding full-length ABCB1, ABCG2, or a bicistronic vector containing both genes, each under the control of a separate promoter. Cells transfected with both transporters (B1/G2 cells) demonstrated high levels of both transporters, and uptake of both the ABCB1-specific substrate rhodamine 123 and the ABCG2-specific substrate pheophorbide a was reduced when examined by flow cytometry. B1/G2 cells were also cross-resistant to the ABCB1 substrate doxorubicin, the ABCG2 substrate topotecan, as well as mitoxantrone and the cell cycle checkpoint kinase 1 inhibitor prexasertib, both of which were found to be substrates of both ABCB1 and ABCG2. When B1/G2 cells were incubated with both rhodamine 123 and pheophorbide a, transport of both compounds was observed, suggesting that ABCB1 and ABCG2, when coexpressed, can function independently to transport substrates. ABCB1 and ABCG2 also functioned additively to transport the common fluorescent substrates mitoxantrone and BODIPY-prazosin, as it was necessary to inhibit both transporters to prevent efflux from B1/G2 cells. ABCG2 expression was also found to decrease the efficacy of the ABCB1 inhibitor tariquidar in B1/G2 cells. Thus, ABCB1 and ABCG2 can independently and additively confer resistance to substrates, underscoring the need to inhibit multiple transporters when they are coexpressed.

Tailoring Ovarian Cancer Treatment: Implications of BRCA1/2 Mutations

High grade serous ovarian cancer (HGSOC) is the most common epithelial ovarian cancer, harbouring more than 20% germline or somatic mutations in the tumour suppressor genes BRCA1 and BRCA2. These genes are involved in both DNA damage repair process via homologous recombination (HR) and transcriptional regulation. BRCA mutation confers distinct characteristics, including an increased response to DNA-damaging agents, such us platinum chemotherapy and poly-ADP ribose polymerase inhibitors (PARPi). However, several mechanisms of resistance to these agents have been described, including increased HR capacity through reverse BRCA mutations, non-homologous end-joint (NHEJ) repair alterations and drug efflux pumps. Current treatments of ovarian cancer including surgery, chemotherapy, targeted treatment and maintenance strategies, as well as resistance mechanisms will be reviewed, focusing on future trends with respect to BRCA mutation carriers.

Tailoring Ovarian Cancer Treatment: Implications of BRCA1/2 Mutations

High grade serous ovarian cancer (HGSOC) is the most common epithelial ovarian cancer, harbouring more than 20% germline or somatic mutations in the tumour suppressor genes BRCA1 and BRCA2. These genes are involved in both DNA damage repair process via homologous recombination (HR) and transcriptional regulation. BRCA mutation confers distinct characteristics, including an increased response to DNA-damaging agents, such us platinum chemotherapy and poly-ADP ribose polymerase inhibitors (PARPi). However, several mechanisms of resistance to these agents have been described, including increased HR capacity through reverse BRCA mutations, non-homologous end-joint (NHEJ) repair alterations and drug efflux pumps. Current treatments of ovarian cancer including surgery, chemotherapy, targeted treatment and maintenance strategies, as well as resistance mechanisms will be reviewed, focusing on future trends with respect to BRCA mutation carriers.

Tailoring Ovarian Cancer Treatment: Implications of BRCA1/2 Mutations

High grade serous ovarian cancer (HGSOC) is the most common epithelial ovarian cancer, harbouring more than 20% germline or somatic mutations in the tumour suppressor genes BRCA1 and BRCA2. These genes are involved in both DNA damage repair process via homologous recombination (HR) and transcriptional regulation. BRCA mutation confers distinct characteristics, including an increased response to DNA-damaging agents, such us platinum chemotherapy and poly-ADP ribose polymerase inhibitors (PARPi). However, several mechanisms of resistance to these agents have been described, including increased HR capacity through reverse BRCA mutations, non-homologous end-joint (NHEJ) repair alterations and drug efflux pumps. Current treatments of ovarian cancer including surgery, chemotherapy, targeted treatment and maintenance strategies, as well as resistance mechanisms will be reviewed, focusing on future trends with respect to BRCA mutation carriers.

Contributions of Drug Transporters to Blood-Brain Barriers

Blood-brain interfaces comprise the cerebral microvessel endothelium forming the blood-brain barrier (BBB) and the epithelium of the choroid plexuses forming the blood-cerebrospinal fluid barrier (BCSFB). Their main functions are to impede free diffusion between brain fluids and blood; to provide transport processes for essential nutrients, ions, and metabolic waste products; and to regulate the homeostasis of central nervous system (CNS), all of which are attributed to absent fenestrations, high expression of tight junction proteins at cell-cell contacts, and expression of multiple transporters, receptors, and enzymes. Existence of BBB is an important reason that systemic drug administration is not suitable for the treatment of CNS diseases. Some diseases, such epilepsy, Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and diabetes, alter BBB function via affecting tight junction proteins or altering expression and function of these transporters. This chapter will illustrate function of BBB, expression of transporters, as well as their alterations under disease status.

How to make P-glycoprotein (ABCB1, MDR1) harbor mutations and measure its expression and activity in cell cultures?

Several polymorphisms have been identified in ABCB1, the gene encoding for the P-glycoprotein. This transporter alters the pharmacokinetics or effectiveness of drugs by excreting them from cells where it is expressed (e.g., blood-brain barrier, intestine or tumors). No consensus has been reached regarding the functional consequences of these polymorphisms in the transporter's function. The aim of this review was to describe a methodology that allows the assessment of P-gp function when harboring polymorphisms. We describe how to obtain cell lines with high expression levels of the transporter with polymorphisms and several tactics to measure its expression and activity. This methodology may help elucidate the contribution of polymorphisms in ABCB1 to drug pharmacokinetics, effectiveness and safety or to cancer chemotherapy failure.

ABCB1 Attenuates the Brain Penetration of the PARP Inhibitor AZD2461

Poly (ADP-ribose) polymerase (PARP) inhibitors are a relatively new class of anticancer agents that have attracted attention for treatment of glioblastoma because of their ability to potentiate temozolomide chemotherapy. Previous studies have demonstrated that sufficient brain penetration is a prerequisite for efficacy of PARP inhibitors in glioma mouse models. Unfortunately, however, most of the PARP inhibitors developed to date have a limited brain penetration due to the presence of P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) at the blood-brain barrier. AZD2461 is a novel PARP inhibitor that is unaffected by P-gp mediated resistance in breast cancer models and thus appears to have promising characteristics for brain penetration. We here use a comprehensive set of in vitro and in vivo models to study the brain penetration and oral bioavailability of AZD2461. We report that AZD2461 has a good membrane permeability. However, it is a substrate of P-gp and BCRP, and P-gp in particular limits its brain penetration in vivo. We show that AZD2461 has a low oral bioavailability, although it is not affected by P-gp and BCRP. Together, these findings are not in favor of further development of AZD2461 for treatment of glioblastoma.