WATERTOWN, Mass. (TheStreet) -- On Tuesday night, Tetraphase Pharmaceuticals (TTPH - Get Report) released data from the lead-in portion of a phase III study of eravacycline in which the experimental antibiotic is first given by intravenous infusion and then transitioned to an oral form. The study enrolled patients with complicated urinary tract infections (cUTI). Tetraphase only provided a summary of the IV-to-oral dosing data, but the company concluded oral eravacycline efficacy and tolerability were sufficient to move ahead with the second part of the phase III study in the fourth quarter.

I previewed these ervacycline data last week. Here's a look at what Tetraphase disclosed about the oral eravacycline data on Tuesday night:

The responder outcome (the FDA-mandated efficacy endpoint), for the IV-to-oral 200 mg, IV-to-oral 250 mg and levofloxacin (control) groups were 70.8%, 64.3% and 52.2%, respectively. A similar ordering was observed for microbiological response (European Medicines Agency endpoint), where the IV-to-oral 200 mg, IV-to-oral 250 mg and levofloxacin groups had response rates of 75.0%, 64.3% and 56.5%, respectively.

Both the low and high-dose oral eravacycline arms out-performed the levofloxacin control group, although the low dose of eravacycline demonstrated a better response than the high dose. What's causing this negative dose response? It could just be an artifact of a small trial, given each arm only enrolled 40 patients. Or, tolerability issues at the higher dose are leading to decreased efficacy. My guess is the former is more the cause.

In terms of adverse events, Tetraphase noted that only two patients discontinued from the study due to drug-related tolerability issues, one in each of the low and high dose arms. That's a relatively low 2.5% discontinuation rate. The most common side effects reported were nausea and vomiting, which were expected. The relatively low rate of discontinuation in the eravacycline study is encouraging given concerns about patients being able to tolerate oral dosing. 

Tetraphase shares closed Tuesday night at $13.08, ahead of the study announcement.

TTPH Chart

Sobek has no position in Tetraphase. 

David Sobek has been writing on biotech for a number of years through various outlets with a general focus on small cap oncology and antibiotics companies. He received his PhD in political science from Pennsylvania State University in 2003 and a BA in international relations from The College of William and Mary in 1997.