The potential of PARP inhibitors in neuro-oncology

Brain metastases in patients with metastatic breast cancer and BRCA1/2 mutations in cell-free DNA

PURPOSE

Brain metastases (BM) in patients with metastatic breast cancer (MBC) cause significant morbidity/mortality. A relatively high prevalence of BM is seen in patients with germline BRCA1/2 mutations. Some patients with MBC have somatic BRCA1/2 mutations but the prevalence of BM in this setting is not known.

METHODS

Here, we evaluated the prevalence and clinical and genomic characteristics of BM in patients with MBC with somatic BRCA1/2 mutations in cell-free DNA (cfDNA) using the Guardant360 assay. Clinical and genomic features of patients with somatic BRCA1/2 mutations and brain metastases, and those without brain metastases were compared using a Chi-squared test for categorical variables and Wilcoxon rank-sum test for continuous variables.

RESULTS

Of 36 patients with MBC and somatic BRCA1/2 mutations, 9 (25%) developed BM. The median time to development of BM was 6.7 months after somatic BRCA detection by cfDNA testing. Among patients with BM, somatic BRCA mutations were commonly BRCA1, clonal, and present at a higher mutant allelic fraction. The coexisting genomic landscape in patients with BM commonly included PIK3CA, TP53, MYC, EGFR, CCNE1, and KIT mutations.

CONCLUSION

A relatively high prevalence of BM in patients with MBC harboring cfDNA somatic BRCA1/2 mutations was observed. CfDNA somatic BRCA1/2 mutations may help identify patients with MBC at risk for BM. To our knowledge, this is the first report linking cfDNA somatic BRCA mutations with BM, and requires further investigation in additional datasets and studies.

Advances in Treatment of Diffuse Midline Gliomas

PURPOSE OF REVIEW

Diffuse midline gliomas (DMGs) generally carry a poor prognosis, occur during childhood, and involve midline structures of the central nervous system, including the thalamus, pons, and spinal cord.

RECENT FINDINGS

To date, irradiation has been shown to be the only beneficial treatment for DMG. Various genetic modifications have been shown to play a role in the pathogenesis of this disease. Current treatment strategies span targeting epigenetic dysregulation, cell cycle, specific genetic alterations, and the immune microenvironment. Herein, we review the complex features of this disease as it relates to current and past therapeutic approaches.

Therapeutic Options for Brain Metastases in Gynecologic Cancers

OPINION STATEMENT

Brain metastases (BM) are rare in gynecologic cancers. Overall BM confers a poor prognosis but other factors such as number of brain lesions, patient age, the presence of extracranial metastasis, the Karnofsky Performance Status (KPS) score, and the type of primary cancer also impact prognosis. Taking a patient's whole picture into perspective is crucial in deciding the appropriate management strategy. The management of BM requires an interdisciplinary approach that frequently includes oncology, neurosurgery, radiation oncology and palliative care. Treatment includes both direct targeted therapies to the lesion(s) as well as management of the neurologic side effects caused by mass effect. There is limited evidence of when screening for BM in the gynecology oncology patient is warranted but it is recommended that any cancer patient with new focal neurologic deficit or increasing headaches should be evaluated. The primary imaging modality for detection of BM is MRI, but other imaging modalities such as CT and PET scan can be used for certain scenarios. New advances in radiation techniques, improved imaging modalities, and systemic therapies are helping to discover BM earlier and provide treatments with less detrimental side effects.

Medulloblastoma and the DNA Damage Response

Medulloblastoma (MB) is the most common malignant brain tumor in children with standard of care consisting of surgery, radiation, and chemotherapy. Recent molecular profiling led to the identification of four molecularly distinct MB subgroups – Wingless (WNT), Sonic Hedgehog (SHH), Group 3, and Group 4. Despite genomic MB characterization and subsequent tumor stratification, clinical treatment paradigms are still largely driven by histology, degree of surgical resection, and presence or absence of metastasis rather than molecular profile. Patients usually undergo resection of their tumor followed by craniospinal radiation (CSI) and a 6 month to one-year multi-agent chemotherapeutic regimen. While there is clearly a need for development of targeted agents specific to the molecular alterations of each patient, targeting proteins responsible for DNA damage repair could have a broader impact regardless of molecular subgrouping. DNA damage response (DDR) protein inhibitors have recently emerged as targeted agents with potent activity as monotherapy or in combination in different cancers. Here we discuss the molecular underpinnings of genomic instability in MB and potential avenues for exploitation through DNA damage response inhibition.

Perspective on the Use of DNA Repair Inhibitors as a Tool for Imaging and Radionuclide Therapy of Glioblastoma

Simple Summary

The current routine treatment for glioblastoma (GB), the most lethal high-grade brain tumor in adults, aims to induce DNA damage in the tumor. However, the tumor cells might be able to repair that damage, which leads to therapy resistance. Fortunately, DNA repair defects are common in GB cells, and their survival is often based on a sole backup repair pathway. Hence, targeted drugs inhibiting essential proteins of the DNA damage response have gained momentum and are being introduced in the clinic. This review gives a perspective on the use of radiopharmaceuticals targeting DDR kinases for imaging in order to determine the DNA repair phenotype of GB, as well as for effective radionuclide therapy. Finally, four new promising radiopharmaceuticals are suggested with the potential to lead to a more personalized GB therapy.

Abstract

Despite numerous innovative treatment strategies, the treatment of glioblastoma (GB) remains challenging. With the current state-of-the-art therapy, most GB patients succumb after about a year. In the evolution of personalized medicine, targeted radionuclide therapy (TRT) is gaining momentum, for example, to stratify patients based on specific biomarkers. One of these biomarkers is deficiencies in DNA damage repair (DDR), which give rise to genomic instability and cancer initiation. However, these deficiencies also provide targets to specifically kill cancer cells following the synthetic lethality principle. This led to the increased interest in targeted drugs that inhibit essential DDR kinases (DDRi), of which multiple are undergoing clinical validation. In this review, the current status of DDRi for the treatment of GB is given for selected targets: ATM/ATR, CHK1/2, DNA-PK, and PARP. Furthermore, this review provides a perspective on the use of radiopharmaceuticals targeting these DDR kinases to (1) evaluate the DNA repair phenotype of GB before treatment decisions are made and (2) induce DNA damage via TRT. Finally, by applying in-house selection criteria and analyzing the structural characteristics of the DDRi, four drugs with the potential to become new therapeutic GB radiopharmaceuticals are suggested.

Existing Evidence for the Repurposing of PARP-1 Inhibitors in Rare Demyelinating Diseases

Simple Summary

Poly (ADP-ribose) polymerase-1 (PARP-1) inhibitors are successful cancer therapeutics that impair DNA repair machinery, leading to an accumulation of DNA damage and consequently cell death. The shared underlying mechanisms driving malignancy and demyelinating disease, together with the success of anticancer drugs as repurposed therapeutics, makes the repurposing of PARP-1 inhibitors for demyelinating diseases a worthy concept to consider. In addition, PARP-1 inhibitors demonstrate notable neuroprotective effects in demyelinating disorders, including multiple sclerosis which is considered the archetypical demyelinating disease.

Abstract

Over the past decade, Poly (ADP-ribose) polymerase-1 (PARP-1) inhibitors have arisen as a novel and promising targeted therapy for breast cancer gene (BRCA)-mutated ovarian and breast cancer patients. Therapies targeting the enzyme, PARP-1, have since established their place as maintenance drugs for cancer. Here, we present existing evidence that implicates PARP-1 as a player in the development and progression of both malignancy and demyelinating disease. These findings, together with the proven clinical efficacy and marketed success of PARP-1 inhibitors in cancer, present the repurposing of these drugs for demyelinating diseases as a desirable therapeutic concept. Indeed, PARP-1 inhibitors are noted to demonstrate neuroprotective effects in demyelinating disorders such as multiple sclerosis and Parkinson’s disease, further supporting the use of these drugs in demyelinating, neuroinflammatory, and neurodegenerative diseases. In this review, we discuss the potential for repurposing PARP-1 inhibitors, with a focus on rare demyelinating diseases. In particular, we address the possible use of PARP-1 inhibitors in examples of rare leukodystrophies, for which there are a paucity of treatment options and an urgent need for novel therapeutic approaches.

MRI Detection of Changes in Tissue Sodium Concentration in Brain Metastases after Stereotactic Radiosurgery: A Feasibility Study

BACKGROUND AND PURPOSE

To date, treatment response to stereotactic radiosurgery (SRS) in brain metastases (BM) can only be determined by MRI evaluation of contrast-enhancing lesions in a long-time follow-up. Sodium MRI has been a subject of immense interest in imaging research as the measure of tissue sodium concentration (TSC) can give valuable quantitative information on cell viability. We aimed to analyze the longitudinal changes of TSC in BM measured with ²³ Na MRI before and after SRS for assessment of early local tumor effects.

METHODS

Seven patients with a total of 12 previously untreated BM underwent SRS with 22 Gy. In addition to a standard MRI protocol including dynamic susceptibility-weighted contrast-enhanced perfusion, a ²³ Na MRI was performed at three time points: (I) 2 days before, (II) 5 days, and (III) 40 days after SRS. Nine BMs were evaluated. The absolute TSC in the BM, the respective peritumoral edemas, and the normal-appearing corresponding contralateral brain area were assessed and the relative TSC were correlated to the changes in BM longest axial diameters.

RESULTS

TSC was elevated in nine BM at baseline before SRS with a mean of 73.4 ± 12.3 mM. A further increase in TSC was observed 5 days after SRS in all the nine BM with a mean of 86.9 ± 13 mM. Eight of nine BM showed a mean 60.6 ± 13.3% decrease in the longest axial diameter 40 days after SRS; at this time point, the TSC also had decreased to a mean 65.1 ± 7.9 mM. In contrast, one of the nine BM had a 13.4% increase in the largest axial diameter at time point III. The TSC of this BM showed a further TSC increase of 80.1 mM 40 days after SRS.

CONCLUSION

Changes in TSC using ²³ Na MRI shows the possible capability to detect radiobiological changes in BM after SRS.

A therapeutic update on PARP inhibitors: implications in the treatment of glioma

Central nervous system (CNS) cancers are among the most aggressive and devastating. Further, due to unavailability of neuro-oncologists and neurosurgeons, the specialized treatment options of CNS cancers are still not completely available in most parts of the world. Among various strategies of inducing death in cancer cells, inhibition of poly(ADP-ribose) polymerase (PARP) has emerged as a beneficial therapy when combined with other anticancer agents. In this review, we provide a detailed therapeutic update of PARP inhibitors that have shown clinical activity against glioma.

Combination of chemotherapy and radiotherapy: A thirty years evolution

Chemoradiotherapy is now considered the standard of care for many locally advanced diseases. Cytotoxic drugs have been largely evaluated in this setting, with cisplatin and 5FU the most often used drugs. A large amount of pre-clinical studies has demonstrated the synergy between both modalities. Concomitant administration seems the more beneficial in many diseases. Emergence of new approaches, combining targeted therapies and radiotherapy (RT) is now a reality. The main example is the association of cetuximab and RT in head and neck carcinomas, even if, 14 years after the initial publication, the best way to use it is still unknown. New compounds as inhibitors of DNA-repair or immune checkpoints are under investigation and showed early promising results.

Current Approaches and Challenges in the Molecular Therapeutic Targeting of Glioblastoma

Surgical resection continues to predominate as the primary treatment modality in glioblastoma (GBM). Effective chemotherapeutic/biologic agents capable of targeting GBM have yet to be developed in part because of the exceptionally heterogeneous nature and unique microenvironmental conditions associated with this malignant neoplasm. Temozolomide and bevacizumab represent the only U.S. Food and Drug Administration-approved agents for primary and recurrent GBM, respectively. Given the high therapeutic resistance of GBM to current therapies, as well as the failure of bevacizumab to prolong overall survival, new therapeutic agents are urgently warranted and are now in the preclinical and clinical phases of development. Accordingly, clinical trials evaluating the efficacy of immune checkpoint inhibition, chimeric antigen receptor T cell therapy, virotherapies, and tumor vaccination therapy are all under way in GBM. Herein, we review the application of current/novel therapeutics in GBM and in so doing attempt to highlight the most promising solutions to overcome current failures.

PARADIGM-2: Two parallel phase I studies of olaparib and radiotherapy or olaparib and radiotherapy plus temozolomide in patients with newly diagnosed glioblastoma, with treatment stratified by MGMT status

Glioblastoma has a dismal prognosis and molecular targeted agents have failed to improve outcomes to date. PARADIGM-2 is a phase I dose escalation study evaluating olaparib plus radiotherapy ± temozolomide in newly diagnosed glioblastoma, using MGMT methylation status to stratify patients and inform treatment schedules.

Selective Inhibition of Parallel DNA Damage Response Pathways Optimizes Radiosensitization of Glioblastoma Stem-like Cells

Glioblastoma is the most common form of primary brain tumor in adults and is essentially incurable. Despite aggressive treatment regimens centered on radiotherapy, tumor recurrence is inevitable and is thought to be driven by glioblastoma stem-like cells (GSC) that are highly radioresistant. DNA damage response pathways are key determinants of radiosensitivity but the extent to which these overlapping and parallel signaling components contribute to GSC radioresistance is unclear. Using a panel of primary patient-derived glioblastoma cell lines, we confirmed by clonogenic survival assays that GSCs were significantly more radioresistant than paired tumor bulk populations. DNA damage response targets ATM, ATR, CHK1, and PARP1 were upregulated in GSCs, and CHK1 was preferentially activated following irradiation. Consequently, GSCs exhibit rapid G2-M cell-cycle checkpoint activation and enhanced DNA repair. Inhibition of CHK1 or ATR successfully abrogated G2-M checkpoint function, leading to increased mitotic catastrophe and a modest increase in radiation sensitivity. Inhibition of ATM had dual effects on cell-cycle checkpoint regulation and DNA repair that were associated with greater radiosensitizing effects on GSCs than inhibition of CHK1, ATR, or PARP alone. Combined inhibition of PARP and ATR resulted in a profound radiosensitization of GSCs, which was of greater magnitude than in bulk populations and also exceeded the effect of ATM inhibition. These data demonstrate that multiple, parallel DNA damage signaling pathways contribute to GSC radioresistance and that combined inhibition of cell-cycle checkpoint and DNA repair targets provides the most effective means to overcome radioresistance of GSC.