What happened

Shares of Mersana Therapeutics (NASDAQ:MRSN) were crashing 29.1% lower as of 12:47 p.m. EST on Tuesday. The steep decline came after the biotech provided a corporate and pipeline update earlier in the day. Investors were especially disappointed with new data from an early-stage clinical study evaluating XMT-1536 in treating ovarian cancer.

So what

Mersana announced that patients receiving XMT-1536 in its phase 1 study achieved an objective response rate (ORR) of 28%. ORR is the percentage of patients whose cancer shrank by a predetermined level after treatment. The problem with Mersana's latest ORR figure is that it's significantly lower than the 35% ORR reported in May 2020.

Man looking at falling red arrow plunging through floor

Image source: Getty Images.

Those earlier results included 20 patients. Mersana's latest ORR number included 47 patients. It's definitely not encouraging that the efficacy of XMT-1536 has declined with more patients.

The data looked somewhat better, though, in a group of patients with higher NaPi2b expression. NaPi2b is a sodium-dependent phosphate transporter that's expressed in several types of cancer, including ovarian cancer. Among 31 patients with higher NaPi2b expression, an ORR of 32% was achieved after receiving XMT-5136. 

Now what

Although today's results weren't great for Mersana, the biotech stock could have potential catalysts on the way. The company expects to begin a pivotal study of XMT-1536 in treating platinum-resistant ovarian cancer in the first quarter of this year. This study will include around 180 patients.

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