Shares of Clovis Oncology (CLVS) - Get Clovis Oncology, Inc. Report are weak Thursday, which prompted reader Doug V. to email a question.

Doug asks, "I saw last night that Clovis reported better-than-expected sales with Rubraca so I thought the stock would react well today [Thursday.] Do you have an explanation for why Clovis is selling off?"

I do, and it involves some disappointment with the Clovis announcement Thursday regarding the timing for top-line results from the ARIEL-3 clinical trial of Rubraca for ovarian cancer patients undergoing maintenance therapy.

Recall, Rubraca is a PARP inhibitor. Clovis is up against two other PARP inhibitors: Tesaro's (TSRO) - Get TESARO, Inc. Report Zejula and AstraZeneca's (AZN) - Get Astrazeneca PLC Sponsored ADR Report  Lynparza. Clovis' ARIEL-3 study will deliver the first data for Rubraca that can be compared against Zejula and Lynparza in the ovarian cancer maintenance setting. That's why investors are so focused on it.

Back to Wednesday. Clovis said the target number of progression events in the ARIEL-3 study was reached in mid-April. Top-line results will be announced at the end of June.

For some investors, this timeline was basically in line with their expectations. Others were hoping to see more of a delay, meaning an announcement from Clovis that the triggering progression event needed to analyze the Rubraca ARIAL-3 study had not yet been reached.

A delay, meaning a clinical trial taking longer, would have suggested Rubraca might be performing better than Zejula and Lynparza as a maintenance therapy, based on how long their respective clinical trials ran.

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As it stands, from Clovis' statement Wednesday, it looks like the timing of ARIEL-3 is in line with the ovarian cancer maintenance trials already conducted by Tesaro and AstraZeneca.

If Rubraca isn't better than Zejula or Lynparza, being third to reach the ovarian cancer maintenance therapy market could be a challenge, Hence, Clovis shares are down 7% to $49.40.

You might be reading this and say to yourself, "For f--k's sake, these analysts and investors are idiots because they're just making wild guesses about Rubraca in the absence of actual clinical data!"

You'd be pretty much right to say that to yourself. For real, there is no way of knowing how Rubraca will perform in the ARIEL-3 study. We'll know in late June when Clovis announces the study results. And at that time, investors will be able to compare the ARIEL-3 results against the Zejula study (21 months median progression-free survival) and the Lynparza study (19 months median progression-free survival.)

Until then, it's all guess work.

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Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.