If you enjoy unscripted drama, you'll want to keep an eye on all three of these small- to mid-cap biotech stocks in the days and weeks ahead. Important trial results and an uncertain approval decision expected in the third quarter could cement their current paths to growth, or force them into long detours.

Here's what you need to know about the catalysts that could send these stocks soaring, or plummeting, in the third quarter.

Company Market Cap Incoming Catalyst
Spark Therapeutics Inc. (ONCE)  $3.09 billion Clinical trial results
Zogenix, Inc. (ZGNX) $1.56 billion Clinical trial results
Progenics Pharmaceuticals, Inc. (PGNX) $593 million PDUFA date

Data source: Yahoo! Finance.

Spark Therapeutics Inc: Once more?

The gene therapy train could pick up more steam when this company reports interim results from a closely watched study with a one-and-done therapy for Hemophilia A, a rare bleeding disorder. One-time genetic cures are Spark Therapeutics' claim to fame, and it could become a lot more popular soon.  

Early results from SPK-8011 wowed the crowd late last year, but we only saw data from four hemophilia patients that had been treated at least 12 weeks earlier. At their last observation, though, all were producing significant quantities of the clotting factor they previously lacked after receiving a single infusion of Spark's experimental gene therapy. 

Hemophilia is a lifelong disease that requires constant treatment at great expense. Spark and its big pharma partner, Pfizer, Inc. (PFE 0.29%), showed that SPK-8011 reduced these four patients' dependence on factor replacement infusions by 96% with a single dose. If these patients are still producing enough to get by without standard care at the next cutoff, without any safety issues, Spark Therapeutics stock could jump.

Three pill bottles with money in them.

Image source: Getty Images.

Zogenix, Inc.: Re-purposed drug of the year?

In May, this biotech finished enrolling 87 patients with a severe form of epilepsy known as Dravet syndrome into a pivotal trial, and top-line results are expected any day now. 

Unlike Spark, Zogenix doesn't have any significant sources of revenue at the moment. That puts a lot of pressure on its only drug candidate, a compound called ZX008 that you may recognize by its generic name, fenfluramine. Zogenix investors will want to keep an eye on potential competition, but results so far are highly encouraging.

Fenfluramine targets serotonin receptors in the brain that influence appetite, which is why it was part of a popular weight-loss treatment decades ago. Regulators yanked FenPhen from the market decades ago due to severe side effects that highlight the role unintended consequences play in drug development. It turns out that fenfluramine also acts on serotonin receptors that nobody realized were abundant in heart tissue, with disastrous consequences.

Fenfluramine's days as an appetite suppressant are long over, but it looks like it makes a difference for children and adults with a severe form of epilepsy called Dravet syndrome at a dosage low enough to avoid heart-related side effects. Zogenix has shown us that 70% of patients experienced half as many convulsive seizures after treatment with a 0.8 mg dose of ZX008, versus just 7.5% in the group given a placebo. 

If ZX008 can maintain its safety profile while putting up numbers like this, it could eventually become a popular drug for a large population of severely affected epilepsy patients that can't keep their seizure frequency low enough with existing drugs.

Doctor talking to a cancer patient.

Image source: Getty Images.

Progenics Pharmaceuticals, Inc.: Hope the second date goes smoother than the first

In return for a substantial fee, the FDA agrees to complete new drug application reviews on or before an action date 10 months from when it begins. At least that's what happens when everything's in order. In the days before electronic filing, new drug applications could fill a small office from floor to ceiling, which leaves a lot of chances to make mistakes.

Earlier this year, the FDA delayed the review of Azedra's application instead of delivering a decision on the first action date. Progenics' candidate showed a clear benefit for patients with ultra-rare hormone-secreting tumors during the study underpinning the submission, but FDA communications aren't subject to full disclosure. According to Progenics, the agency didn't take issue with safety or efficacy results, it just needed extra time to review info related to the drug's manufacturing process that the company had to submit after the review started.

Pheochromocytoma and paraganglioma are rare malignancies, but there aren't any therapies specifically approved to stop them from growing and secreting hormones that drive up patients' blood pressure. This might not become a chart-topping blockbuster, but peak sales could reach $300 million annually within several years. 

A favorite for the long haul

Progenics' first drug, Relistor for opioid-induced constipation, hasn't been a big seller, but Valeant Pharmaceuticals (BHC 0.81%) has taken over marketing of the drug in return for a reliable royalty stream. This will allow Progenics to focus on Azedra's potential launch and a prostate cancer imaging agent in late-stage development.

Zogenix and Spark's market caps have climbed to pretty tall perches in expectations of success, which gives them a long way to tumble if something doesn't work out. Progenics, on the other hand, could rise significantly if Azedra simply becomes a moderately successful cancer therapy. If I had to pick one of these stocks for the long run, this would be the one.