The PARP Inhibitors: Down But Not Out
Published Online: Tuesday, July 31, 2012
Two views of the protein structure of a PARP1 inhibitor complex.
In spite of the setbacks, researchers retain a sense of cautious optimism and remain hopeful that a better understanding of the mechanism of action of these agents will lead to a renovation of PARP inhibitors for effective treatment of a wide variety of cancers.
What Are PARPs?PARPs are a family of enzymes implicated in a host of key cellular processes, including chromosome stability, regulation of apoptosis, cell division, and transcriptional regulation and differentiation. A particularly important role of PARPs is in repairing DNA damage that results from everyday environmental stresses and DNA replication errors.
First identified in the early 1960s, the substrates upon which they act and the precise cellular functions of most members of the PARP family remain unknown. The best studied are PARP1 and PARP2 and, to date, these are the only two PARPs known to be involved in DNA repair.
A diverse range of DNA repair pathways are utilized in the cell; which pathway depends on the type of damage and the type of repair required. Through their enzymatic activity, the PARPs coordinate the base excision repair (BER) pathway to repair single-strand breaks (SSBs) in DNA. If SSBs go unrepaired, they can lead to a more dangerous form of DNA damage, the double-strand break (DSB). PARP1 has also been implicated in the homologous recombination (HR) and nonhomologous end-joining (NHEJ) pathways of DNA repair, both of which are used to repair DSBs.
Major DNA Repair Pathways
PARP plays a central role in DNA repair, and researchers theorize that inhibiting PARP
would result in cancer cell death.
Inhibiting PARP to Treat Cancer: Synthetic Lethality and BeyondThe potential anticancer activity of PARP inhibitors was first elucidated in 2005. Currently, PARP inhibitors are designed to target the catalytic site of PARP1 and inhibit PARP1 by greater than 90%. They are thought to function in the treatment of cancer by exploiting the concept of synthetic lethality.
Synthetic lethality occurs if two genetic mutations are nonlethal when they occur individually, but lead to cell death when they occur in combination. If PARP1 activity is impaired, other DNA repair pathways take over to repair the resulting DSBs. However, if PARP inhibitors are used in tumors that already have defects in these other DNA repair pathways, then the combination becomes synthetically lethal, as the tumor cell cannot repair either SSBs or DSBs. The theory is that this should lead to selective tumor cell death without impacting normal cells.
This was initially tested, and proved to be true, in breast and ovarian cancers that commonly have a mutation in the breast cancer type 1 susceptibility (BRCA1) and breast cancer type 2 susceptibility (BRCA2) genes. It is believed that BRCA1/2-mutated tumors have a defective HR DNA repair pathway.
Research interest has been especially piqued in the use of PARP inhibitors to treat a particularly problematic form of breast cancer, triple-negative breast cancer (TNBC). TNBC displays clinical and pathological similarity to BRCA1/2- mutated breast cancers and has a very aggressive profile and poor prognosis.
PARP inhibitors also have significant potential for improving the efficacy of chemotherapeutic agents. These agents cause DNA damage, which can be repaired by PARP activity. Therefore, inhibiting PARP should increase the tumor cell-killing potential of chemotherapy. Research has shown that this is true only for certain types of chemotherapeutic agents, including alkylating agents such as temozolomide, and that the use of these combinations is limited by the risk of increased toxic side effects, particularly myelosuppression.
PARP Inhibitors Currently Under StudyThere are a number of PARP inhibitors currently undergoing clinical testing (Above). In 2009, a significant amount of excitement was generated when the PARP inhibitor iniparib (BSI-201; Sanofi) demonstrated extremely promising results in phase II trials in patients with TNBC. Furthermore, clinical testing of olaparib (AZD2281; AstraZeneca) showed promise in patients with serous ovarian cancer, with reported improvements in progression-free survival (PFS).
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