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What: Immunomedics (NASDAQ:IMMU), a clinical-stage antibody-based therapeutics company developing treatments for cancer and autoimmune diseases, saw its shares bolt higher by 88% last month, according to data from S&P Capital IQ. The event that appears to have jump-started this small cap biotech stock is the news that the company's lead antibody-drug conjugate (ADC), sacituzumab govitecan, produced durable responses in patients with metastatic triple-negative breast (TNBC), small-cell (SCLC) and non-small-cell lung (NSCLC) cancers. 

So what: Immunomedics' stock is still in full rebound mode after its shares took a beating earlier this year due to the failure of epratuzumab to meet its primary endpoint in two late-stage trials for systemic lupus erythematosus. The basic issue is that epratuzumab's failure left the company's pipeline extremely thin in terms of advanced experimental product candidates, with investors hopes now pinned directly on 90Y-clivatuzumab tetraxetan's late-stage trial in the advanced pancreatic cancer setting. Sacituzumab govitecan's strong clinical showing of late thus helps to flesh out Immunomedics' overall pipeline and gives investors an all-important backup plan if 90Y-clivatuzumab tetraxetan flames out as well.

What now: Unfortunately, the history of ADCs is pockmarked with numerous late-stage failures. Roche and its research partner Immunogen (NASDAQ:IMGN), for instance, have hit a brick wall when attempting to expand Kadcyla's label beyond HER2-positive metastatic breast cancer. Last month, Immunogen disclosed that Kadcyla failed to show any clinical benefit as a second-line stomach cancer treatment, and the drug has also had trouble differentiating itself from other available treatments in the metastatic breast cancer space.

So, investors probably shouldn't get too excited about these preliminary results for sacituzumab govitecan, even if they do appear to be extremely promising. After all, ADCs have a strange way of pulling the rug out from underneath investors' feet.

George Budwell has no position in any stocks mentioned. The Motley Fool recommends ImmunoGen. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.