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Tesaro (NASDAQ: TSRO) saw its shares increase 108% on Wednesday after announcing that its cancer drug, niraparib, extended progression-free survival -- a measure of how long it takes a tumor to start growing again -- in patients with ovarian cancer.

Niraparib belongs to a class of drugs that inhibit poly ADP-ribose polymerase (PARP), which repairs DNA damage. In the presence of a PARP inhibitor, the DNA damage isn't repaired, and the cell eventually dies.

Tesaro's results were quite impressive. In patients with BRCA mutations, which increase the amount of DNA damage, progression-free survival was extended from 5.5 months in the group receiving placebo to 21 months for those taking niraparib. Patients with tumors that were homologous recombination deficient, which also have trouble repairing DNA, saw progression-free survival increase from 3.8 months on placebo to 12.9 months on niraparib.

Tesaro plans to share the full data set at a medical meeting in October, but it's clear the doubling of the share price was clearly well deserved. It's not every day you see a drug tripling the progression-free survival of the control arm.

Missed the Tesaro double? Other opportunities

Tesaro isn't the only company with a PARP inhibitor. AstraZeneca (AZN 0.83%) gained FDA approval for its PARP inhibitor Lynparza in late 2014, but it's only approved for patients who have already received three or more chemotherapy treatments. Tesaro's trial, dubbed NOVA, tested niraparib in patients who were responding to platinum-based chemotherapy.

AstraZeneca is testing Lynparza in the same sort of maintenance-therapy setting in a trial dubbed SOLO2. Results from that trial and another one AstraZeneca is running -- called SOLO1, in untreated patients with BRCA mutations -- are expected soon.

In addition to ovarian cancer, other companies, including Tesaro, are testing PARP inhibitors in breast cancer where mutations in BRCA genes are also seen. Medivation (MDVN) and AbbVie (ABBV 0.69%) are both taking a two-pronged approach for their PARP inhibitors, talazoparib and veliparib, respectively, testing them in breast cancer patients who have BRCA mutations and in combination with chemotherapy drugs that work by creating DNA damage. Combining the two should, in theory, have a synergistic effect. AbbVie is also testing the veliparib-chemotherapy combination in lung cancer.

Finally, Clovis Oncology (CLVS) has a PARP inhibitor, rucaparib, that's already under review by the Food and Drug Administration through a rolling submission. In May, Clovis Oncology said it plans to finish up the FDA marketing application this quarter -- that ends today -- and submit an application to European regulators by the end of the year. If approved, rucaparib would be initially used for patients with advanced ovarian cancer who have BRCA mutations, but Clovis has an ongoing trial exploring the drug in earlier-stage ovarian cancer and plans for other trials, including one in prostate cancer.

Another double in the works?

For a clinical trial results or FDA approvals to double the value of a share, the company needs to be small enough that the results are substantially meaningful to its bottom line and/or unexpected.

That leaves out AstraZeneca and AbbVie. There's no possible way positive clinical trial results are going to double their value given those companies' size. Their PARP inhibitor programs should nevertheless increase their value if successful.

Medivation is too big to double as well, but it's an interesting case because Sanofi (SNY 0.52%) is in the middle of a hostile bid to buy Medivation. The Tesaro data gives Medivation ammunition to get Sanofi to raise its $9.3 billion buyout offer. Medivation is currently trading almost 10% higher than Sanofi's bid, so investors clearly think a higher bid is possible.

That leaves Clovis, which was up 22% yesterday after Tesaro announced its clinical trial results but has given some of that back today. At a market cap under $550 million, the biotech has plenty of room to run if it can show that rucaparib can compete with the rest of the PARP inhibitors.

Even if none of the companies experience an overnight double like Tesaro did, the PARP inhibitor space is clearly one investors should be watching.