What happened

Shares of G1 Therapeutics (NASDAQ:GTHX) fell 37.2% in September, according to data from S&P Global Market Intelligence, after investors reacted to trial data for some of the biopharmaceutical company's cancer drug candidates.

So what

To be sure, shares plunged 33% on Monday, Sept. 30 alone following G1's weekend release of two separate updates. But that's not to say the data showed complete failure of those drug candidates.

Keeping in the mind the pullback came after a nearly 70% year-to-date pop for the stock, G1 first revealed the median overall survival for patients with metastatic triple-negative breast cancer who were treated with its trilaciclib drug candidate during a phase 2 trial was 20.1 months, compared with 12.6 months for patients who received only chemotherapy.

Blue, red, and white charts on a colorful display.

Image source: Getty Images.

Separately -- and perhaps lending more credence to the sell-off -- G1 also said that in a phase 1/2a dose-escalation study of patients with estrogen receptor-positive, HER2-negative (ER+, HER2-) breast cancer that its G1T48 drug candidate was "well tolerated and demonstrated evidence of anti-tumor activity in heavily pre-treated patients."

However, the clinical benefit rate (that is, those patients who saw a "complete response + partial response + stable disease for at least 24 weeks") across all doses was just 15.8%, or only three of the 19 patients treated by G1T48.

Now what

As such, G1 said it would begin a rolling new drug application submission for small cell cancer in the fourth quarter for trilaciclib, with a targeted completion in the second quarter of 2020. At the same time, the company began early discussions with the U.S. Food and Drug Administration for development of trilaciclib in triple-negative breast cancer.

As for G1T48, G1 Therapeutics selected the two most effective doses for further evaluation in the phase 2a portion of its study, and hopes to start a first-line phase 3 trial using that data for the drug candidate in 2020.

Still, these results were hardly indicative of runaway successes. And given G1 Therapeutics' considerable rise through the first eight months of 2019, it was no surprise to see shares give back some of those gains in response last month.