So, Apellis Pharmaceuticals Inc. (APLS) is having a very good Tuesday morning. Its shares are up a staggering 136% in premarket trading after news broke that Biogen Inc. (BIIB) has agreed to buy the company for $41 per share in cash. That's a deal worth roughly $5.6 billion.

Think of it as Biogen writing a very large check to instantly bulk up its portfolio in immunology and rare diseases. The prize? Two FDA-approved drugs called Empaveli and Syfovre, which treat complement-driven diseases. For Biogen, this isn't just about adding products; it's about buying a growth engine. The combined net sales for these two drugs hit $689 million in 2025, and they're expected to keep growing at a healthy mid-to-high teens percentage rate through 2028.

But wait, there's a little extra for Apellis shareholders. The deal includes a contingent value right, or CVR, for each share. This is essentially a bonus coupon that pays out if the drugs hit certain sales targets. Specifically, shareholders could get two separate payments of $2 per share if those thresholds are met. It's a way for Biogen to share some of the future upside if the drugs really take off.

For Biogen, the math looks promising on the bottom line. The company says the acquisition will strengthen its revenue and earnings growth and should start adding to its adjusted earnings per share in 2027. Looking further out, it expects the deal to meaningfully boost its compounded annual growth rate for adjusted EPS through the end of the decade.

Now, $5.6 billion is not pocket change. Biogen plans to pay for this with a combination of cash on hand and new borrowings. The company is confident it can manage the new debt, projecting it can pay it all down by the end of 2027. That timeline is important because it would restore financial flexibility for Biogen to make other moves down the road.

The market's reaction tells two different stories. Apellis shareholders are, understandably, celebrating. The stock was trading at $40.36, just shy of the $41 offer price, and hit a new 52-week high. On the other side, Biogen's stock was down about 4.6%. That's a pretty common sight when a big acquirer announces a deal—investors sometimes worry about the price paid or the debt taken on, even if the strategic rationale is sound.

In the end, this is a classic biotech play: one company with a deep pipeline and commercial muscle buys another company with promising, commercial-stage assets to fuel its next phase of growth. Biogen is betting that Empaveli and Syfovre are that fuel.