What happened

Shares of NextCure (NASDAQ:NXTC), a clinical-stage biopharmaceutical company developing new cancer drugs, exploded higher this morning. Updated clinical trial results from a handful of non-small cell lung cancer patients sent the biotech stock 225.8% higher as of 3:22 p.m. EST on Tuesday.

So what

NextCure made its stock-market debut in May. At the moment, a Siglec-15 (S15) antibody called NC318 is its only new drug candidate in clinical-stage testing.

Businessperson with a jetpack

Image source: Getty Images.

In March the company told us that NC318 shrank tumors for one lung-cancer patient treated in a proof-of-concept study. The stock took off like a rocket today when investors learned that one patient achieved complete remission, another experienced tumor shrinkage, and three more haven't gotten any worse.

The experimental cancer drug's target, S15, shuts down immune-system attacks against foreign cells before neighboring healthy cells get caught in the fray. This pathway is often exploited by cancer cells, which allows tumors to grow unchecked. If this sounds familiar, it's because blockbuster cancer therapies like Keytruda work similarly, by aiming at a different target called PD-L1.

Third-quarter sales of Merck & Co.'s (NYSE:MRK) Keytruda jumped 62% year over year to an annualized $12.4 billion, even though the drug only helps some patients. Expectations for NextCure and NC318 are soaring because S15-positive tumors tend to be PD-L1-negative, and NC318 could become as popular as Keytruda if clinical trial results continue to impress.

Now what

Before you run out and buy a heap of NextCure shares, it's important to understand just how difficult it is to draw any meaningful conclusions from the results we've seen so far. The ongoing proof-of-concept trial is testing six ascending dosage strengths, from 8 milligrams to 800 milligrams, in 43 patients with tumors that originated in five different organs.

Among the first 32 evaluable patients, NextCure has reported just two responses. If we only focused on the group of seven lung cancer patients who have been evaluated, out of 10 enrolled, two responses so far would be a great result. Two responses among 32 patients, however, is nothing to write home about.

On Nov. 9, at the Society for Immunotherapy of Cancer meeting, NextCure will present results for all 43 patients who have been treated with its lead candidate. It's probably best to wait for a little more confirmation that the pair of responses observed so far wasn't just a mirage.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.