What happened

Shares of Kura Oncology (NASDAQ:KURA) were down 19.3% at 1:35 p.m. EDT today, after the company presented phase 2 data on its drug candidate tipifarnib at the European Society for Medical Oncology's 2018 Congress.

The data weren't horrible -- tipifarnib appears to be working -- but Kura's investors seem worried about how many patients tipifarnib will ultimately be able to treat.

Doctor showing a report to a patient in a hospital bed

Image source: Getty Images.

So what

Tipifarnib works by inhibiting farnesyltransferase, which modifies a protein called HRAS that is mutated to become active in different types of cancer.

In this trial, Kura tested tipifarnib in two different tumor types: squamous cell carcinoma (SCC) and head and neck squamous cell carcinoma (HNSCC). All the tumors had mutations in HRAS, although at different frequencies.

For the HNSCC patients, five of 11 evaluable patients had a confirmed partial response by the Sept. 7 data cutoff; Kura noted that there was another partial response after the data cutoff, as well as an unconfirmed partial response for a patient being treated off-protocol.

For the SCC patients, one of two evaluable patients had a confirmed partial response.

Looking at the frequency of HRAS mutations, of the 13 patients with HNSCC or SCC who had a HRAS mutant allele frequency greater than 20%: There were six partial responses, one unconfirmed partial response, and one ongoing response. On the other hand, none of the seven patients with a HRAS allele frequency less than 20% had a response.

Now what

While tipifarnib has only been tested on a small number of patients, it appears the drug is working well on patients with a HRAS mutant allele frequency greater than 20%. To confirm the data, Kura plans to only enroll patients above that threshold in its next trial in HNSCC. If positive, the trial should be enough to gain approval from the Food and Drug Administration, given the unmet need.

The downside to stratifying the patients is that only 5% of HNSCC patients have an HRAS mutant allele frequency greater than 20%. Kura may be able to make up for the smaller patient population with a higher price for its drug, but only if the biotech can prove the drug is substantially helping patients.

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