Clovis Oncology (CLVS - Get Report) shares are falling Friday after a clinical trial update on its PARP inhibitor in ovarian cancer revealed weaknesses against a similar, competing drug from AstraZeneca (AZN - Get Report) .

Takeover speculation that had fueled the recent rise in Clovis' stock price looks to be subsiding. The stock fell 18% to $29.42.

Clovis presented more detailed data Friday from two previously announced clinical trials of its PARP inhibitor rucaparib. These data have already been submitted to the U.S. Food and Drug Administration. Clovis is seeking approval for rucaparib as a treatment for advanced ovarian cancer in patients with BRCA-mutated tumors (germline and somatic) who have been treated previously with two or more chemotherapies.

The company had previously disclosed much of the rucaparib efficacy and safety data. None of the new data presented Friday necessarily dampen odds of rucaparib's approval in ovarian cancer.

Investors, instead, were reacting with disappointment to the drug's efficacy and safety profile, which could be inferior to Lynparza, an already approved PARP inhibitor marketed by AstraZeneca.

Specifically, investors keyed on a subset of ovarian cancer patients refractory to platinum chemotherapy. In these more sick patients, rucaparib was totally ineffective with a zero percent response rate.

By comparison, Lynparza, in a separate clinical trial, demonstrated a 14% response rate in ovarian cancer patients refractory platinum chemotherapy.

Comparing drugs across separate trials is challenging. In this case, there were only seven platinum-refractory patients in the Clovis trial. The enrollment eligibility requirements for the Clovis and AstraZeneca trials were also somewhat different.

None of these caveats stopped investors Friday from making a judgment about Clovis' rucaparib.

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.