PARP inhibitors and stratified treatment of prostate cancer

Dual targeting, a new strategy for novel PARP inhibitor discovery

As a hallmark for cancer treatment, PARP inhibitors can effectively kill tumor cells with a mechanism termed as synthetic lethality, and are used to treat various cancers including ovarian, breast, prostate, pancreatic and others with DNA repair defects. However, along with the clinical trials progressing, the limitations of PARP-1 inhibitors became apparent such as limited activity and indications. Studies have shown that a molecule that is able to simultaneously restrict two or more targets involving in tumors is more effective in preventing and treating cancers due to the enhancing synergies. In order to make up for the shortcomings of PARP inhibitors, reduce the development cost and overcome the pharmacokinetic defects, multiple works were carried out to construct dual targeting PARP inhibitors for cancer therapy. Herein, they were summarized briefly.

Silencing of carbonic anhydrase I enhances the malignant potential of exosomes secreted by prostatic tumour cells

We report results showing that the silencing of carbonic anhydrase I (siCA1) in prostatic (PC3) tumour cells has a significant impact on exosome formation. An increased diameter, concentration and diversity of the produced exosomes were noticed as a consequence of this knock‐down. The protein composition of the exosomes' cargo was also altered. Liquid chromatography and mass spectrometry analyses identified 42 proteins significantly altered in PC3 siCA1 exosomes compared with controls. The affected proteins are mainly involved in metabolic processes, biogenesis, cell component organization and defense/immunity. Interestingly, almost all of them have been described as ‘enhancers' of tumour development through the promotion of cell proliferation, migration and invasion. Thus, our results indicate that the reduced expression of the CA1 protein enhances the malignant potential of PC3 cells.

Impact of Phosphoproteomics in the Era of Precision Medicine for Prostate Cancer

Prostate cancer is the most common malignancy in men in the United States. While androgen deprivation therapy results in tumor responses initially, there is relapse and progression to metastatic castration-resistant prostate cancer. Currently, all prostate cancer patients receive essentially the same treatment, and there is a need for clinically applicable technologies to provide predictive biomarkers toward personalized therapies. Genomic analyses of tumors are used for clinical applications, but with a paucity of obvious driver mutations in metastatic castration-resistant prostate cancer, other applications, such as phosphoproteomics, may complement this approach. Immunohistochemistry and reverse phase protein arrays are limited by the availability of reliable antibodies and evaluates a preselected number of targets. Mass spectrometry-based phosphoproteomics has been used to profile tumors consisting of thousands of phosphopeptides from individual patients after surgical resection or at autopsy. However, this approach is time consuming, and while a large number of candidate phosphopeptides are obtained for evaluation, limitations are reduced reproducibility, sensitivity, and precision. Targeted mass spectrometry can help eliminate these limitations and is more cost effective and less time consuming making it a practical platform for future clinical testing. In this review, we discuss the use of phosphoproteomics in prostate cancer and other clinical cancer tissues for target identification, hypothesis testing, and possible patient stratification. We highlight the majority of studies that have used phosphoproteomics in prostate cancer tissues and cell lines and propose ways forward to apply this approach in basic and clinical research. Overall, the implementation of phosphoproteomics via targeted mass spectrometry has tremendous potential to aid in the development of more rational, personalized therapies that will result in increased survival and quality of life enhancement in patients suffering from metastatic castration-resistant prostate cancer.

Veliparib in combination with radiotherapy for the treatment of MGMT unmethylated glioblastoma

Background

The O⁶-methylguanine methyltransferase (MGMT) gene is frequently unmethylated in patients with glioblastoma (GBM), rendering them non-responsive to the standard treatment regime of surgery followed by concurrent radiotherapy (RT) and temozolomide. Here, we investigate the efficacy of adding a PARP inhibitor, veliparib, to radiotherapy to treat MGMT unmethylated GBM.

Methods

The inhibition of PARP with veliparib (ABT-888), a potent and orally bioavailable inhibitor in combination with RT was tested on a panel of patient derived cell lines (PDCLs) and patient-derived xenografts (PDX) models generated from GBM patients with MGMT unmethylated tumors.

Results

The combination of veliparib and RT inhibited colony formation in the majority of PDCLs tested. The PDCL, RN1 showed significantly reduced levels of the homologous repair protein, Mre11 and a heightened response to PARP inhibition measured by increased apoptosis and decreased colony formation. The oral administration of veliparib (12.5 mg/kg, twice daily for 5 days in a 28-day treatment cycle) in combination with whole brain RT (4 Gy) induced apoptosis (Tunel staining) and decreased cell proliferation (Ki67 staining) in a PDX of MGMT unmethylated GBM. Significantly longer survival times of the PDX treated with the combination treatment were recorded compared to RT only or veliparib only.

Conclusions

Our results demonstrate preclinical efficacy of targeting PARP at multiple levels and provide a new approach for the treatment of MGMT unmethylated GBM.

Defective DNA repair mechanisms in prostate cancer: impact of olaparib

The field of prostate oncology has continued to change dramatically. It has truly become a field that is intensely linked to molecular genetic alterations, especially DNA-repair defects. Germline breast cancer 1 gene (BRCA1) and breast cancer 2 gene (BRCA2) mutations are implicated in the highest risk of prostate cancer (PC) predisposition and aggressiveness. Poly adenosine diphosphate ribose polymerase (PARP) proteins play a key role in DNA repair mechanisms and represent a valid target for new therapies. Olaparib is an oral PARP inhibitor that blocks DNA repair pathway and coupled with BRCA mutated-disease results in tumor cell death. In phase II clinical trials, including patients with advanced castration-resistant PC, olaparib seems to be efficacious and well tolerated. Waiting for randomized phase III trials, olaparib should be considered as a promising treatment option for PC.