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Why Proteostasis Therapeutics Tumbled 23.2% on Friday

By Todd Campbell - Oct 19, 2018 at 5:41PM

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Some investors booked profits following the company's recent cystic fibrosis trial data.

What happened

After the company reported encouraging early-stage trial data for its cystic fibrosis therapies, shares in Proteostasis Therapeutics (PTI) skyrocketed yesterday, before retreating 23.2% on Friday. 

So what

Proteostasis didn't issue any press releases today, so it's likely that the sell-off in its shares was because of investors paring back their positions following its massive move on Thursday.

A man staring at a wall covered with drawings of bags of money and question marks.


The company's cystic fibrosis preliminary trial results were significant because the market for cystic fibrosis treatment is worth billions of dollars per year. For instance, Vertex Pharmaceuticals' (VRTX 3.77%) revenue from cystic fibrosis drugs eclipsed $750 million in the second quarter of 2018 alone.

In the study, patients were randomized into cohorts to determine the most effective dose of PTI-801 and PTI-808, a doublet combination therapy. 

The company reported that there was a dose-dependent response witnessed in the study, and that patients receiving 300 milligrams of PTI-801 and 300 milligrams of PTI-808 saw a statistically meaningful improvement in predicted forced expiratory volume for one second (ppFEV1) and sweat chloride concentration, a measure of the amount of chloride in a patient's sweat that's a gold standard for diagnosing this disease. Specifically, ppFEV1 was up 6.3% at day seven and 5.9% at day 14, and sweat chloride declined 13 millimoles per liter from the baseline.  

Those results compare well to Vertex Pharmaceuticals' currently available doublet therapies. Damian Downey, MD clinical senior lecturer  in respiratory medicine, Queen's University Belfast, said:

The current improvement in ppFEV1 for standard of care doublets is on average 3%-4% in the F508del homozygous patient population. ... At least a 6% improvement in ppFEV1, as observed in this study, exceeds efficacy seen with current dual CFTR modulator therapy and is starting to approach the efficacy of experimental triple combinations in this population. These results begin to confirm the exciting clinical effect of PTI's drug pipeline with more clinical data from both higher doses and proprietary triplet combinations to come.

Now what

There's no telling if Proteostasis will be able to duplicate the results it reported this week in larger trials, but the results were nevertheless intriguing, particularly because there's a big need for new treatment options. Current treatments only address about half of the cystic fibrosis population, and most patients pass away in their 30s.

The next thing to watch at Proteostasis is data coming early next year from two triplet combination studies in cystic fibrosis, including one that includes Vertex Pharmaceuticals' Symdeko as background therapy. Many researchers, including Vertex Pharmaceuticals, believe that triplet therapies are key to expanding treatment to most patients. Results from these trials are anticipated in the first quarter of 2019.


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Proteostasis Therapeutics, Inc. Stock Quote
Proteostasis Therapeutics, Inc.
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