What happened

Puma Biotechnology (NASDAQ:PBYI) ended the day down 13.8% after Roche (NASDAQOTH:RHHBY) reported that its rival breast cancer drug, Perjeta, had passed its phase 3 trial, dubbed "Aphinity."

Doctor looking at mammogram results

Image source: Getty Images.

So what

In Roche's trial, patients either took Perjeta and Herceptin with chemotherapy or just Herceptin with chemotherapy, and then took Perjeta and Herceptin, or just Herceptin, for an additional year. Roche didn't release the full data from the clinical trial, but it did say the triple combination reduced the risk of recurrence of invasive disease or death compared to Herceptin and chemotherapy alone.

The potential to establish a new standard of care where patients take Herceptin and Perjeta for a year could be problematic for Puma Biotechnology because its drug candidate, neratinib, was tested after just Herceptin use, the current standard of care.

Without any data, doctors will likely wonder whether neratinib helps patients that have received Herceptin and Perjeta. And the relapse rate for patients on the current standard of care is already quite low; if adding Perjeta decreases it further, doctors and their patients may decide taking another drug after that isn't worth it, especially given neratinib's side-effect profile.

Now what

Investors will have to wait for the full data from Aphinity -- perhaps at the American Society of Clinical Oncology meeting in June -- to know how much better Herceptin plus Perjeta is than Herceptin alone, and how that might affect neratinib's sales, assuming it's approved later this year.

Brian Orelli and The Motley Fool have no positions in any of the stocks mentioned. The Motley Fool has a disclosure policy.