PARP inhibitors are a group of pharmacological inhibitors of the enzyme poly ADP ribose polymerase (PARP). They are developed for multiple indications; the most important is the treatment of cancer. Several forms of cancer are more dependent on PARP than regular cells, making PARP an attractive target for cancer therapy.  
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.
Mechanism of action
DNA is damaged thousands of times during each cell cycle, and that damage must be repaired.
BRCA1, BRCA2 and PALB2 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 either protein 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.
Drugs that inhibit PARP1 cause multiple double strand breaks to form in this way, and in tumours with BRCA1, BRCA2 or PALB2  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.
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. Hence PARP inhibitors may be effective against many PTEN-defective tumours (e.g. some aggressive prostate cancers).
Cancer cells that are low in oxygen (e.g. in fast growing tumors) are sensitive to PARP inhibitors.
Additional mode of action for PARP inhibitors
2012: Researchers at the National Cancer Institute have discovered a significant new mechanism of action for PARP inhibitors. They have also identified differences in the toxic capabilities of three drugs in this class, which are currently being tested in clinical trials. Prior to this study, PARP inhibitors were thought to work primarily by blocking PARP enzyme activity, thus preventing the repair of DNA damage and ultimately causing cell death. In this study, scientists established that PARP inhibitors have an additional mode of action: localizing PARP proteins at sites of DNA damage, which has relevance to their anti-tumor activity. The trapped PARP protein–DNA complexes are highly toxic to cells because they block DNA replication. When the researchers tested three PARP inhibitors for their differential ability to trap PARP proteins on damaged DNA, they found that the trapping potency of the inhibitors varied widely. The PARP family of proteins in humans includes PARP1 and PARP2, which are DNA binding and repair proteins. When activated by DNA damage, these proteins recruit other proteins that do the actual work of repairing DNA. Under normal conditions, PARP1 and PARP2 are released from DNA once the repair process is underway. However, as this study shows, when they are bound to PARP inhibitors, PARP1 and PARP2 become trapped on DNA. The researchers showed that trapped PARP–DNA complexes are more toxic to cells than the unrepaired single-strand DNA breaks that accumulate in the absence of PARP activity, indicating that PARP inhibitors act as PARP poisons. These findings suggest that there may be two classes of PARP inhibitors, catalytic inhibitors that act mainly to inhibit PARP enzyme activity and do not trap PARP proteins on DNA, and dual inhibitors that both block PARP enzyme activity and act as PARP poison.
Examples in clinical trials
Started Phase III:
- Iniparib (BSI 201) for breast cancer and squamous cell lung cancer. Failed trial for triple negative breast cancer. In 2012 iniparib was determined not be a true PARP inhibitor and its mechanism of action is believed to be through other means than PARP inhibition.
Started Phase II:
- Olaparib (AZD-2281) for breast, ovarian and colorectal cancer. AZ progressing it to phase III (published 20th september 2013).
- Rucaparib (AG014699, PF-01367338) for metastatic breast and ovarian cancer.
- Veliparib (ABT-888) for metastatic melanoma and breast cancer.
- CEP 9722 for non–small-cell lung cancer (NSCLC)
Started Phase I:
- MK 4827 Inhibitor of PARP1 and PARP2. Phase I trial on 59 patients.
- BMN-673 in trials for advanced hematological malignancies and for advanced or recurrent solid tumors.
- 3-aminobenzamide, a prototypical PARP inhibitor
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.
- http://www.cancernetwork.com/display/article/10165/1514773 "Development of PARP Inhibitors: An Unfinished Story " Jan 2010
- http://drugdiscoveryopinion.com/2009/09/parp-inhibitors-%E2%80%93-more-widely-effective-than-first-thought/ Sep 2009
- Graziani G, Szabó C (July 2005). "Clinical perspectives of PARP inhibitors". Pharmacol. Res. 52 (1): 109–18. doi:10.1016/j.phrs.2005.02.013. PMID 15911339.
- Buisson R, Dion-Côté A.M, et al. (2010). "Cooperation of breast cancer proteins PALB2 and piccolo BRCA2 in stimulating homologous recombination.". Nature Structural & molecular biology 17 (10): 1247–54. doi:10.1038/nsmb.1915. PMID 20871615.
- McGlynn, P. and Lloyd, B. "Recombinational Repair and Restart of Damaged Replication Forks." Nature Reviews, 2002, pp.859-870
- N Engl J Med 361:123
- N Engl J Med 361:189
- Gupta A, Yang Q, Pandita RK, et al. (July 2009). "Cell cycle checkpoint defects contribute to genomic instability in PTEN deficient cells independent of DNA DSB repair". Cell Cycle 8 (14): 2198–210. PMID 19502790.
- "NIH study uncovers new mechanism of action for class of chemotherapy drugs". National Cancer Institute. 1 Nov 2012.
- Murai, J; et al. (2012). "Differential trapping of PARP1 and PARP2 by clinical PARP inhibitors". Cancer Research. doi:10.1158/0008-5472.CAN-12-2753. PMID 23118055.
- http://www.nature.com/nbt/journal/v29/n5/full/nbt0511-373.html PARP inhibitors stumble in breast cancer. 2011
- Liu X, Shi Y, Maag DX, Palma JP, Patterson MJ, Ellis PA, Surber BW, Ready DB, Soni NB, Ladror US, Xu AJ, Iyer R, Harlan JE, Solomon LR, Donawho CK,Penning TD, Johnson EF, Shoemaker AR. Iniparib nonselectively modifies cysteine-containing proteins in tumor cells and is not a bona fide PARP inhibitor.Clin Cancer Res. 2012 Jan 15;18(2):510-23. doi: 10.1158/1078-0432.CCR-11-1973
- Anand G. Patel, Silvana B. De Lorenzo, Karen S. Flatten1, Guy G. Poirier, andScott H. Kaufmann. Failure of Iniparib to Inhibit Poly(ADP-Ribose) Polymerase In Vitro. Clin Cancer Res. 2012 Mar 15;18(6):1655-62. doi: 10.1158/1078-0432.CCR-11-2890
- "Study of CEP-9722 as Single-Agent Therapy and as Combination Therapy With Temozolomide in Patients With Advanced Solid Tumors".
- "PARP Inhibitors in Oncology. Chemosensitizers or Single-Agent Therapeutics?". July 2009.
- "PARP inhibitor, MK-4827, shows anti-tumor activity in first trial in humans". 17 Nov 2010.