PARP inhibitor

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Model of the inhibitor olaparib (dark gray) occupying the NAD -binding site of PARP1. From PDB: 5DS3 . PARP1 binding olaparib 5DS3.png
Model of the inhibitor olaparib (dark gray) occupying the NAD -binding site of PARP1. From PDB: 5DS3 ​.

PARP inhibitors are a group of pharmacological inhibitors of the enzyme poly ADP ribose polymerase (PARP).

Contents

They are developed for multiple indications, including the treatment of heritable cancers. [1] Several forms of cancer are more dependent on PARP than regular cells, making PARP (PARP1, PARP2 etc.) an attractive target for cancer therapy. [2] [3] [4] [5] PARP inhibitors appear to improve progression-free survival in women with recurrent platinum-sensitive ovarian cancer, as evidenced mainly by olaparib added to conventional treatment. [6]

In addition to their use in cancer therapy, PARP inhibitors are considered a potential treatment for acute life-threatening diseases, such as stroke and myocardial infarction, as well as for long-term neurodegenerative diseases. [7]

Medical uses

Approved for marketing

Combination with radiotherapy

The main function of radiotherapy is to produce DNA strand breaks, causing severe DNA damage and leading to cell death. Radiotherapy has the potential to kill 100% of any targeted cells, but the dose required to do so would cause unacceptable side effects to healthy tissue. Radiotherapy therefore can only be given up to a certain level of radiation exposure. Combining radiation therapy with PARP inhibitors offers promise, since the inhibitors would lead to formation of double strand breaks from the single-strand breaks generated by the radiotherapy in tumor tissue with BRCA1/BRCA2 mutations. This combination could therefore lead to either more powerful therapy with the same radiation dose or similarly powerful therapy with a lower radiation dose. [13]

Mechanism of action

DNA is damaged thousands of times during each cell cycle, and that damage must be repaired, including in cancer cells. Otherwise the cells may die due to this damage. [14] Chemotherapy and radiation therapy attempt to kill cancer cells by inducing high levels of DNA damage. By inhibiting PARP1 DNA repair, the effectiveness of these therapies can be increased. [15]

BRCA1, BRCA2 and PALB2 [16] are proteins that are important for the repair of double-strand DNA breaks by the error-free homologous recombinational repair, or HRR, pathway. When the gene for one of these proteins is mutated, the change can lead to errors in DNA repair that can eventually cause breast cancer. Mutations in these genes can also cause ovarian, endometrial, pancreatic and prostate cancers. [17] When subjected to enough damage at one time, the altered gene can cause the death of the cells.

PARP1 is a protein that is important for repairing single-strand breaks ('nicks' in the DNA). If such nicks persist unrepaired until DNA is replicated (which must precede cell division), then the replication itself can cause double strand breaks to form. [18] The main function of PARP (located in the cell nucleus) is to detect and initiate an immediate cellular response to metabolic, chemical, or radiation-induced single-strand DNA breaks (SSB) by signaling the enzymatic machinery employed in the SSB repair. Cancer cells that are already deficient in homologous recombination DNA repair (due to mutation in BRCA1, BRCA2, or PALP2) are sensitive to targeted inhibition of PARP, a key component of alternative backup repair pathways. [17] Identifying cancer patients with homologous recombination deficiency biomarkers indicates those patients likely to benefit from PARP inhibitor therapies. [17]

Drugs that inhibit PARP1 cause multiple double strand breaks to form in this way, and in tumours with BRCA1, BRCA2 or PALB2 [16] mutations, these double strand breaks cannot be efficiently repaired, leading to the death of the cells. Normal cells that don't replicate their DNA as often as cancer cells, and that lack any mutated BRCA1 or BRCA2 still have homologous repair operating, which allows them to survive the inhibition of PARP. [19]

PARP inhibitors lead to trapping of PARP proteins on DNA in addition to blocking their catalytic action. [20] This interferes with replication, causing cell death preferentially in cancer cells, which grow faster than non-cancerous cells.

Some cancer cells that lack the tumor suppressor PTEN may be sensitive to PARP inhibitors because of downregulation of Rad51, a critical homologous recombination component, although other data suggest PTEN may not regulate Rad51. [3] [21] Hence PARP inhibitors may be effective against many PTEN-defective tumours [4] (e.g. some aggressive prostate cancers).

Cancer cells that are low in oxygen (e.g. in fast growing tumors) are sensitive to PARP inhibitors. [22]

Excessive PARP-1 activity may exacerbate the pathogenesis of stroke, myocardial infarction, neurodegeneration, and a number of other disease conditions due to excessive inflammation. Thus, reduction of inflammation by PARP-1 inhibition can mitigate these conditions. [23] PARP inhibitors such as olaparib, under experimental conditions, appear to be beneficial in limiting atrial fibrillation and other DNA damage associated cardiovascular diseases. [24]

Research

Examples of clinical trials

Started Phase III:

Started Phase II:

Currently Discontinued:

Experimental:

Studies of PARP inhibitor resistance

Despite the clinical success of PARP inhibitors, their efficacy is limited by the development of resistance. Overcoming resistance has thus become a major focus within the PARP inhibitor research field, prompting comprehensive studies into resistance mechanisms. At present, reversion-driven HR restoration has been established as the most common resistance mechanism. Reversion-driven HR restoration is the result of secondary mutation events within BRCA1, BRCA2, or other HR-related factors, which restore protein function and, thus, HR proficiency. HR can also be re-established without reversion events. For example, loss of end-protection (e.g. via 53BP1 loss), has been shown to restore HR. Other resistance mechanisms include enhanced drug efflux, restoration of DNA replication fork protection, mutations in PARP1, and PARG downregulation. [41]

See also

Related Research Articles

<span class="mw-page-title-main">BRCA1</span> Gene known for its role in breast cancer

Breast cancer type 1 susceptibility protein is a protein that in humans is encoded by the BRCA1 gene. Orthologs are common in other vertebrate species, whereas invertebrate genomes may encode a more distantly related gene. BRCA1 is a human tumor suppressor gene and is responsible for repairing DNA.

<span class="mw-page-title-main">DNA repair</span> Cellular mechanism

DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA damage, resulting in tens of thousands of individual molecular lesions per cell per day. Many of these lesions cause structural damage to the DNA molecule and can alter or eliminate the cell's ability to transcribe the gene that the affected DNA encodes. Other lesions induce potentially harmful mutations in the cell's genome, which affect the survival of its daughter cells after it undergoes mitosis. As a consequence, the DNA repair process is constantly active as it responds to damage in the DNA structure. When normal repair processes fail, and when cellular apoptosis does not occur, irreparable DNA damage may occur. This can eventually lead to malignant tumors, or cancer as per the two-hit hypothesis.

<span class="mw-page-title-main">BRCA2</span> Gene known for its role in breast cancer

BRCA2 and BRCA2 are human genes and their protein products, respectively. The official symbol and the official name are maintained by the HUGO Gene Nomenclature Committee. One alternative symbol, FANCD1, recognizes its association with the FANC protein complex. Orthologs, styled Brca2 and Brca2, are common in other vertebrate species. BRCA2 is a human tumor suppressor gene, found in all humans; its protein, also called by the synonym breast cancer type 2 susceptibility protein, is responsible for repairing DNA.

<span class="mw-page-title-main">Poly (ADP-ribose) polymerase</span> Family of proteins

Poly (ADP-ribose) polymerase (PARP) is a family of proteins involved in a number of cellular processes such as DNA repair, genomic stability, and programmed cell death.

<span class="mw-page-title-main">Homologous recombination</span> Genetic recombination between identical or highly similar strands of genetic material

Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids.

<span class="mw-page-title-main">Oncogenomics</span> Sub-field of genomics

Oncogenomics is a sub-field of genomics that characterizes cancer-associated genes. It focuses on genomic, epigenomic and transcript alterations in cancer.

Triple-negative breast cancer (TNBC) is any breast cancer that either lacks or shows low levels of estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2) overexpression and/or gene amplification. Triple-negative is sometimes used as a surrogate term for basal-like.

<span class="mw-page-title-main">PARP1</span> Mammalian protein found in Homo sapiens

Poly [ADP-ribose] polymerase 1 (PARP-1) also known as NAD+ ADP-ribosyltransferase 1 or poly[ADP-ribose] synthase 1 is an enzyme that in humans is encoded by the PARP1 gene. It is the most abundant of the PARP family of enzymes, accounting for 90% of the NAD+ used by the family. PARP1 is mostly present in cell nucleus, but cytosolic fraction of this protein was also reported.

<span class="mw-page-title-main">BARD1</span> Protein-coding gene in the species Homo sapiens

BRCA1-associated RING domain protein 1 is a protein that in humans is encoded by the BARD1 gene. The human BARD1 protein is 777 amino acids long and contains a RING finger domain, four ankyrin repeats, and two tandem BRCT domains.

<span class="mw-page-title-main">PALB2</span> Protein-coding gene in the species Homo sapiens

Partner and localizer of BRCA2, also known as PALB2 or FANCN, is a protein which in humans is encoded by the PALB2 gene.

Synthetic lethality is defined as a type of genetic interaction where the combination of two genetic events results in cell death or death of an organism. Although the foregoing explanation is wider than this, it is common when referring to synthetic lethality to mean the situation arising by virtue of a combination of deficiencies of two or more genes leading to cell death, whereas a deficiency of only one of these genes does not. In a synthetic lethal genetic screen, it is necessary to begin with a mutation that does not result in cell death, although the effect of that mutation could result in a differing phenotype, and then systematically test other mutations at additional loci to determine which, in combination with the first mutation, causes cell death arising by way of deficiency or abolition of expression.

<span class="mw-page-title-main">Olaparib</span> Chemical compound (cancer therapy drug)

Olaparib, sold under the brand name Lynparza, is a medication for the maintenance treatment of BRCA-mutated advanced ovarian cancer in adults. It is a PARP inhibitor, inhibiting poly ADP ribose polymerase (PARP), an enzyme involved in DNA repair. It acts against cancers in people with hereditary BRCA1 or BRCA2 mutations, which include some ovarian, breast, and prostate cancers.

<span class="mw-page-title-main">Veliparib</span> Chemical compound

Veliparib (ABT-888) is a potential anti-cancer drug acting as a PARP inhibitor. It kills cancer cells by blocking a protein called PARP, thereby preventing the repair of DNA or genetic damage in cancer cells and possibly making them more susceptible to anticancer treatments. Veliparib may make whole brain radiation treatment work more effectively against brain metastases from NSCLC. It has been shown to potentiate the effects of many chemotherapeutics, and as such has been part of many combination clinical trials.

<span class="mw-page-title-main">Rucaparib</span> Chemical compound

Rucaparib, sold under the brand name Rubraca, is a PARP inhibitor used as an anti-cancer agent. Rucaparib is a first-in-class pharmaceutical drug targeting the DNA repair enzyme poly-ADP ribose polymerase-1 (PARP-1). It is taken by mouth.

<i>BRCA</i> mutation Medical condition

A BRCA mutation is a mutation in either of the BRCA1 and BRCA2 genes, which are tumour suppressor genes. Hundreds of different types of mutations in these genes have been identified, some of which have been determined to be harmful, while others have no proven impact. Harmful mutations in these genes may produce a hereditary breast–ovarian cancer syndrome in affected persons. Only 5–10% of breast cancer cases in women are attributed to BRCA1 and BRCA2 mutations, but the impact on women with the gene mutation is more profound. Women with harmful mutations in either BRCA1 or BRCA2 have a risk of breast cancer that is about five times the normal risk, and a risk of ovarian cancer that is about ten to thirty times normal. The risk of breast and ovarian cancer is higher for women with a high-risk BRCA1 mutation than with a BRCA2 mutation. Having a high-risk mutation does not guarantee that the woman will develop any type of cancer, or imply that any cancer that appears was actually caused by the mutation, rather than some other factor.

<span class="mw-page-title-main">Hereditary cancer syndrome</span> Inherited genetic condition that predisposes a person to cancer

A hereditary cancer syndrome is a genetic disorder in which inherited genetic mutations in one or more genes predispose the affected individuals to the development of cancer and may also cause early onset of these cancers. Hereditary cancer syndromes often show not only a high lifetime risk of developing cancer, but also the development of multiple independent primary tumors.

<span class="mw-page-title-main">Talazoparib</span> Chemical compound

Talazoparib, sold under the brand name Talzenna, is an anti-cancer medication used for the treatment of breast cancer and prostate cancer. It is an orally available poly ADP ribose polymerase PARP inhibitor marketed by Pfizer for the treatment of advanced breast cancer with germline BRCA mutations. Talazoparib is similar to the first in class PARP inhibitor, olaparib.

<span class="mw-page-title-main">Niraparib</span> Anti-cancer medication

Niraparib, sold under the brand name Zejula, is an anti-cancer medication used for the treatment of epithelial ovarian, fallopian tube, or primary peritoneal cancer. It is taken by mouth. It is a PARP inhibitor.

<span class="mw-page-title-main">HRDetect</span>

HRDetect is a whole-genome sequencing (WGS)-based classifier designed to predict BRCA1 and BRCA2 deficiency based on six mutational signatures. Additionally, the classifier is able to identify similarities in mutational profiles of tumors to that of tumors with BRCA1 and BRCA2 defects, also known as BRCAness. This classifier can be applied to assess the implementation of PARP inhibitors in patients with BRCA1/BRCA2 deficiency. The final output is a probability of BRCA1/2 mutation.

<span class="mw-page-title-main">Breast and ovarian cancer</span>

Breast and ovarian cancer does not necessarily imply that both cancers occur at the same time, but rather that getting one cancer would lead to the development of the other within a few years. Women with a history of breast cancer have a higher chance of developing ovarian cancer, vice versa.

References

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