What happened

Shares of ArQule (ARQL) fell over 28% today after the company released an abstract to be presented at an upcoming scientific conference. The abstract includes interim results from an ongoing phase 1 clinical trial for its lead drug candidate, ARQ 531, in patients with cancers affecting white blood cells. It appears that investors are disappointed with the number of partial responses to the experimental treatment.

A total of 10 patients achieved partial responses as of the abstract submission cutoff date of July 19. There were 40 evaluable patients, which suggests a relatively low response rate of just 25%. That's not quite the right takeaway from the abstract, however, as the design of the phase 1 trial must be taken into account. 

As of 11:43 a.m. EST, the small-cap stock had settled to a 14.4% loss.

A worried businessman holding up two pieces of paper.

Image source: Getty Images.

So what

While a response rate of 25% might be a disappointment, investors have to bear in mind that the results aren't apples to apples. The phase 1 study is looking at eight different daily doses of the drug candidate, from 5 mg all the way to 75 mg, and multiple types of white blood cell cancers. There may have been 40 evaluable patients at the cutoff date, but some of those patients were taking doses that likely have no clinical activity. Therefore, it's impossible to calculate a response rate for the treatment at this time.

That said, the updated results do appear to substantiate the promising results reported months ago. Of the six patients with relapsed or refractory (R/R) chronic lymphocytic lymphoma (CLL) who were initially dosed at 65 mg, five achieved a partial response. That's up from the four partial responses in six total patients at the last report. Meanwhile, a R/R CLL patient who started at 45 mg and graduated to a 65 mg dose also achieved a partial response, as did a patient who dropped from 75 mg to 65 mg.

ArQule also reported one partial response apiece in three separate (and more aggressive) types of blood cancer. All individuals in the trial have been heavily treated with other drugs in the past, making the partial response rate in R/R CLL relatively impressive and providing a basis for optimism that the reversible binding property of ARQ 531 might be providing benefit.

Now what

Assuming investors are disappointed with the purported partial response rate of ARQ 531, today's news is not much to be concerned about, because a single partial response rate cannot be calculated from eight different doses. That said, even with the updated interim results and promising report in R/R CLL patients, investors can't forget that the results need to be replicated in larger patient populations in mid- and late-stage trials. Future studies are likely to evaluate only the 65 mg dose (or an option to escalate to that dose), which is when interim updates will really get interesting for investors.