Way to go, Pharmacyclics (UNKNOWN:PCYC.DL) and Johnson & Johnson (NYSE:JNJ)! The companies said yesterday that they're ending a trial of their cancer drug Imbruvica early because of positive efficacy. 

It's not uncommon to see companies stopping clinical trials early after independent data monitoring committees conclude that the drug is working. At that point, it's unethical to continue the treatment in the inferior control arm.

What's remarkable about the latest trial that tested Imbruvica in patients with two different types of blood cancer is that patients taking the drug lived longer than those in the control arm.

When interim analyses are performed, they typically only show statistical significance in a surrogate endpoint called progression-free survival, a measurement of the time it takes for the disease to progress or for the patient to die, whichever comes first. Overall survival -- which just measures how long it takes for the patient to die -- may trend in the right direction, but it often doesn't reach statistical significance since most patients have measurable progression well before they die.

The Imbruvica trial analyzed progression-free survival -- it passed that measurement, too -- but the overall survival data is obviously much more important, because survival is what patients and their doctors care about.

Making the feat even more significant, Imbruvica beat GlaxoSmithKline's (NYSE:GSK) Arzerra, and the patients taking Arzerra were allowed to switch to Imbruvica once they progressed. That crossover won't affect the progression-free survival calculations, but for overall survival, Imbruvica was essentially competing against itself for some of the time patients were alive.

How good?
As is typical when companies stop clinical trials, Pharmacyclics and Johnson & Johnson didn't give any indication as to how much longer patients taking Imbruvica lived compared to those taking GlaxoSmithKline's Arzerra.

Companies like to withhold data to present at medical meetings, but when trials are stopped early, they can't even release top-line data, because there isn't time. It typically takes months to lock the database and do the statistical analysis, but companies don't have that kind of time when a clinical trial ends early. There are so many people involved in the trial that the ending of it would leak fairly quickly.

It's safe to assume, however, that Imbruvica extended survival substantially compared to Arzerra. To reach statistical significance, the difference would have to be fairly large given the low number of patients analyzed for the interim analysis.

Pharmacyclics and Johnson & Johnson would be best off releasing that data sooner rather than later. Imbruvica is already approved for the treatment of another blood cancer called mantle cell lymphoma, but doctors are allowed to prescribe drugs off-label for any disease they see fit. The companies could capture much of the late-stage chronic lymphocytic leukemia and small lymphocytic lymphoma markets before the FDA approves it for that indication if doctors knew how much better Imbruvica is than GlaxoSmithKline's Arzerra.