Abbott - whoops, pardon me, I mean AbbVie, damn that name - has been developing ABT-199, a selective Bcl-2-targeted oncology compound for CLL. Unlike some earlier shots in this area (ABT-263, navitoclax), it appeared to spare platelet function, and was considered a promising drug candidate in the mid-stage clinical pipeline.
Not any more, perhaps. Clinical work has been suspended after a patient death due to tumor lysis syndrome. This is a group of effects caused by sudden breakdown of the excess cells associated with leukemia. You get too much potassium, too much calcium, too much uric acid, all sorts of things at once, which lead to many nasty downstream events, among them irreversible kidney damage and death. So yes, this can be caused by a drug candidate working too well and too suddenly.
The problem is, as the Biotech Strategy Blog says in that link above, that this would be more understandable in some sort of acute leukemia, as opposed to CLL, which is the form that ABT-199 is being tested against. So there's going to be some difficulty figuring out how to proceed. My guess is that they'll be able to restart testing, but that they'll be creeping up on the dosages, with a lot of blood monitoring along the way, until they get a better handle on this problem - if a better handle is available, that is. ABT-199 looks too promising to abandon, and after all, we're talking about a fatal disease. But this is going to slow things down, for sure.
Update: I've had email from the company, clarifying things a bit: "While AbbVie has voluntarily suspended enrollment in Phase 1 trials evaluating ABT-199 as a single agent and in combination with other agents such as rituximab, dosing of active patients in ABT-199 trials is continuing. Previous and current trials have shown that dose escalation methods can control tumor lysis syndrome and we have every expectation that the trials will come off of clinical hold and that we will be able to initiate Phase 3 trials in 2013, as planned."