PARP Inhibitors in Oncology: a New Synthetic Lethal Approach to Cancer Therapy

Damage to DNA has emerged as a major culprit in cancer. Mammalian cells are continuously exposed to DNA damage, caused by exogenous toxins as well as endogenous activities such as DNA replication and cellular free radical generation. It is therefore essential that cells have DNA repair mechanisms in place to preserve its genomic integrity. Interestingly, cancer cells frequently harbour defects in DNA repair pathways, leading to genomic instability. This can foster tumorigenesis, but also provides a weakness in the tumour that can be exploited therapeutically. In this context, it has been shown that homologous recombination (HR)-deficient tumour cells--including those with defects in BRCA1/2--are highly sensitive to blockade of the base excision repair (BER) pathway via inhibition of the poly (ADP-ribose) polymerase (PARP) enzyme. This provides the basis for a novel 'synthetic lethal' approach to cancer therapy. Recent clinical trials have shown an enhancement of the cytotoxic effect of chemotherapy by adding a PARP inhibitor to the standard treatment. Still, clinical outcome may be even further improved if these drugs would be used as first-line therapy. In conclusion, it can be stated that an exciting new class of drugs has entered the arena of cancer therapy. However, additional clinical studies are needed before PARP inhibitors can definitely enter daily clinical practice.