What happened

On Tuesday, shares of up-and-coming biotechs Marinus Pharmaceuticals (NASDAQ:MRNS), iBio (NYSEMKT:IBIO), and Arcturus Therapeutics Holdings (NASDAQ:ARCT) fell by 12%, 13%, and 19%, respectively.

One common catalyst seems to be the secondary-share issues announced by all three companies, although another important factor may be at play.

A medical professional using a microscope.

Image source: Getty Images.

So what

iBio and Marinus are making fairly sizable issues given their current market capitalizations. The former is seeking to raise gross proceeds of $35 million; its latest market-cap figure is nearly $239 million. Marinus would like to reap $70 million; it is currently valued at $421 million by investors. That means some notable share dilution for both biotechs.

Arcturus is a bit more of a mystery. It's seeking to raise "only" $150 million in its flotation, against a market cap approaching $2.5 billion.

But in the case of Arcturus and iBio, the share-dilution issue might be compounded by the perception that both are falling behind in the coronavirus vaccine race. Both have candidates that are in development but have not yet advanced as far as rivals in clinical testing.

Now what

While share dilution is a real concern, these companies still have good potential, so a "buy on weakness" should be seriously considered for all three. Marinus' ganaxolone pipeline epilepsy drug has tested well, and iBio and Marinus remain intriguing underdog coronavirus vaccine plays.

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.