AstraZeneca, Merck, Clovis and Tesaro, all vying for share, may all win on faster PARP growth

Ovarian cancer treatment has changed dramatically in the last few years, and with several new drugs now duking it out in that market, the battle for share is changing, too. If some recent numbers—and a new doctor survey—are any indication, Tesaro's Zejula is in the lead.

But it won't necessarily stay that way. To get a read on doctors' perceptions of PARP inhibiting drugs, Leerink checked in with a set of oncologists, and the word is that they don't see much difference among the three PARP therapies in ovarian cancer. "The survey largely confirmed our prior assumptions that the three PARP inhibitors are more similar than different, allowing for the possibility that share is evenly split between all three PARP inhibitors," Leerink analyst Seamus Fernandez wrote in a Wednesday note.

To hear Tesaro talk, though, Zejula has already scored a 60% share of the ovarian cancer field, with $39.4 million in third-quarter sales. The drug added 650 prescribers over that time period, and it's attracting women with BRCA mutations and those without—a key point, because BRCA-mutated cancers are obviously just one piece of the market.

RELATED: Clovis CEO deems new Rubraca data 'better' than its PARP rivals. Is it enough?

Clovis Oncology's Rubraca, on the other hand, reaped just $16.8 million in Q3 sales, plus another $4.4 million worth given away for free via the company's patient assistance program, Evercore ISI analyst Steven Breazzano said in a note after earnings were released.

As the first PARP drug approved for maintenance use in ovarian cancer—if not in ovarian cancer itself, a milestone that belongs to Lynparza—Zejula may well keep its lead there, Fernandez said. "A very realistic scenario for first-to-market Zejula is sustaining a 10 percentage point lead over competitors at steady-state in the U.S.," the analyst wrote.

But with new data, prospective combos and potential approvals on their way, that could change. One complicating factor right now is this: Zejula and AstraZeneca and Merck & Co.'s Lynparza are both approved as maintenance therapies, used in women who've responded to chemotherapy to prevent a recurrence down the road. Lynparza is also approved for women who've already undergone three other rounds of treatment.

ELATED: AstraZeneca, Merck team up on Lynparza combos in collaboration worth up to $8.5B

Rubraca only boasts the third-line nod, at least until the FDA speaks on its own maintenance therapy application early next year. That means Clovis can only tout its med for a smaller group of patients, and a smaller population equals smaller sales. In fact, Evercore estimates that 80% of ovarian cancer treatment lies in the maintenance group.

Future approvals are what could really shift the PARP landscape however, in ovarian cancer and beyond, with all three drugs aiming for the first-line maintenance setting. "[U]ltimately what will determine Clovis’ fate is the first-line ovarian strategy and landscape (not the currently 2L battle) and PARP inhibitor use beyond ovarian cancer," Breazzano wrote.

AstraZeneca is already powering beyond ovarian cancer; Lynparza is under FDA priority review for use in breast cancer now, the company announced last month. And a recent approval for Lynparza tablets—and planned switch to that formulation, which requires fewer doses—could help the therapy win over more doctors across all indications, partly because the approval extends to BRCA-negative patients. Tesaro and Clovis both are testing their PARP drugs in breast and prostate cancers, and Clovis is also stressing opportunities for Rubraca in bladder cancer. Clovis anticipates a filing in prostate cancer in 2019, Brazzeano notes.

RELATED: With Clovis team-up, Bristol-Myers' Opdivo eyes dynamic-duo attack on a trio of cancers

Plus, there are the combinations with PD-1/L1 immunotherapies. Rubraca is in testing alongside Bristol-Myers Squibb's Opdivo and Roche's Tecentriq, while AstraZeneca's partnership with Merck includes testing Lynparza with Keytruda, to name a few.

And the good news for everyone in the PARP field? The oncologists Leerink surveyed all expect doctors to pick up drugs in the class at a higher rate than the firm had anticipated. The physicians said they expected PARP use to grow to 82% of BRCA-positive patients and 70% of patients with unmutated tumors in three years. And for use earlier in the treatment line? The doctors showed "a high degree of enthusiasm regarding the potential use of PARP inhibitors in front-line maintenance ... particularly in BRCA+ patients."

Leerink sees PARP inhibitors selling in ovarian cancer to the tune of $2 billion at peak. 

Editor's note: This story was updated to reflect Lynparza's first-to-market status in ovarian cancer. Its recent new approval was for a tablet formulation, not capsules. The drug is not yet approved to treat breast cancer. It was also updated to correctly reflect approved indications for Rubraca and Lynparza.