Advisory-panel briefing documents are like little windows into the FDA's mind-set on a drug. After peering into the agency's thinking on Ligand Pharmaceuticals (NASDAQ:LGND) and marketing partner GlaxoSmithKline's (NYSE:GSK) blood disorder treatment Promacta, an approvable letter (and its accompanying delay) seems all but certain.

Promacta is Ligand's lead pipeline candidate, undergoing testing as a treatment for idiopathic thrombocytopenic purpura (ITP), a disorder that causes low blood platelet counts. Yesterday, the FDA released its advisory-panel briefing documents ahead of the drug's Friday hearing, and Promacta's near-term future looks bleak.

There are several very strong arguments why the FDA will likely issue an approvable letter for Promacta when its PDUFA date comes up next month. Strike one: ITP is a chronic disease, but Glaxo and Ligand are only trying to get approval of Promacta as a short-term treatment for the condition -- even though the FDA's not yet convinced of the drug's short-term effectiveness. As the FDA's statistical reviewer succinctly notes, "The data provided in this NDA do not demonstrate statistically significant, robust (Promacta) treatment effect in decreasing bleeding events."

Strike two: the decrease in patients' blood platelet counts (compared to their initial baseline condition) after they stop taking the drug. In one study, about 30% of patients had their disease worsen after they stopped taking Promacta. Considering that Ligand and Glaxo only propose a short-term use label, and that the FDA wants Glaxo to try its hardest to limit the drug's use to short-term applications, it seems that virtually all patients are guaranteed to stop taking the drug at some point. If they're left worse off than when they started, doesn't that defeat the purpose of taking the drug to begin with?

Strike three: Ligand and Glaxo have ongoing studies of the drug that will provide more data on Promacta's long-term use. Since Glaxo is expected to file a New Drug Application for longer-term use this year, FDA reviewers might just hedge their bets and decide to wait for this data.

Ligand and Glaxo have a a June 19 PDUFA date scheduled for Promacta, but investors shouldn't be surprised if they get slapped with the same three-month review extension plaguing many drugmakers. Meanwhile, Ligand isn't the only drugmaker awaiting an FDA decision for an ITP treatment. Biopharma giant Amgen (NASDAQ:AMGN) has a PDUFA scheduled for July 23 for its candidate romiplostim (Nplate), which received a generally positive advisory panel hearing earlier in the year.

On the bright side for Ligand, the FDA never calls an advisory panel hearing unless it's at least on the fence about approving a drug. If it were completely unconvinced about Promacta, it would have rejected the drug outright.

Investors not sold on Promacta's potential, but wanting to get a piece of Ligand's multiple other late-stage drug candidates, just got their chance now that Ligand shares are down in the dumps. The bad news? Whatever the advisory panel says, an approvable letter seems inevitable for Promacta, at least until more of the ongoing long-term Promacta data is completed.