PARP inhibitor

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

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

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. 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.[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.[18]

PARP inhibitors lead to trapping of PARP proteins on DNA in addition to blocking their catalytic action.[19] 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][20] 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.[21]

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.[22] PARP inhibitors such as olaparib, under experimental conditions, appear to be beneficial in limiting atrial fibrillation and other DNA damage associated cardiovascular diseases.[23]

Research

Examples of clinical trials

Started Phase III:

Started Phase II:

Currently Discontinued:

Experimental:

See also

References
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  2. Pam Stephan. "PARP Inhibitor and DNA Polymerase Repair - PARP Inhibitor". About.com Health.
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  9. Zejula FDA Professional Drug Information.
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  23. Ramos KS, Brundel BJJM. DNA Damage, an Innocent Bystander in Atrial Fibrillation and Other Cardiovascular Diseases? Front Cardiovasc Med. 2020 Apr 28;7:67. doi: 10.3389/fcvm.2020.00067. PMID: 32411727; PMCID: PMC7198718
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