Tesaro's poly(ADP-ribose) polymerase (PARP) inhibitor niraparib hit its primary end point in its first Phase III trial, lifting hopes for a class of anticancer drugs with a turbulent history.

Companies first started working on PARP inhibitors in the 1990s. Because PARP proteins facilitate the repair of DNA damage, researchers theorized that PARP inhibitors would lead to accumulation of DNA damage and therefore kill highly mutated cancer cells. The field stalled in 2011 with the Phase III failure of Sanofi's iniparib in triple-negative breast cancer, but made a comeback following the discovery that iniparib was not a bona fide PARP inhibitor (Nat. Rev. Drug Discov. 12, 725–727; 2013).

AstraZeneca secured a first approval for the drug class in 2014 with olaparib, but the newly released data have renewed interest in follow-on candidates.

In the Phase III trial of niraparib in patients with platinum-sensitive ovarian cancer, median progression-free survival for patients with germline BRCA mutations who received niraparib was 21 months compared with 5.5 months in the control arm. Although it is difficult to compare the results of two different trials — which can include patients with different baseline characteristics — investors interpreted these data as a win for Tesaro. In AstraZeneca's 2011 Phase II study of olaparib, median progression-free survival of patients with germline BRCA mutations was 11.2 months, compared with 4.1 months in the control arm.

Tesaro will present full data at the European Society for Medical Oncology (ESMO) congress in October. It also plans to file for regulatory approval in both the United States and the European Union in the fourth quarter of 2016.

Other PARP inhibitors in development include Clovis's rucaparib, which the company filed for FDA approval in February, Medivation's talazoparib, which is in Phase III trials for breast cancer and AbbVie's veliparib, which is in Phase III trials for breast cancer and non-small cell lung cancer.