Source: Merck.

Merck (NYSE:MRK) has won the race to get the first immunotherapy drug that blocks the PD-1 pathway on the U.S. market. Today, the FDA approved pembrolizumab, which will go by the brand name Keytruda, for patients with advanced or unresectable melanoma who are no longer responding to other drugs.

It's good news for sure, but the development of PD-1 drugs is like the Tour de France with multiple stages. Merck might be wearing the yellow jersey right now, but it's far from winning it all. Shares of the company hardly budged on the news, highlighting the relative insignificance of this approval (and the fact that it was widely expected).

Multiple indications
The PD-1 pathway allows the immune system to recognize cells as human and tells the immune system not to attack them.

Many tumor cells have enough mutations in them that the immune system would recognize them as foreign, but they still have the PD-1 pathway that shuts down the attack. PD-1 drugs block the pathway, allowing the immune system to do its job.

PD-1 isn't tumor-specific, so Merck is testing Keytruda in a wide variety of cancers. Beyond melanoma, there are clinical trials in everything from lung cancer to kidney cancer and even some blood cancers like multiple myeloma. In many cases, it can be used in combination with current targeted cancer drugs. For example, Merck is working with GlaxoSmithKline (NYSE:GSK) to test the combination of Keytruda with GlaxoSmithKline's Votrient in kidney cancer. 

In the grand scheme of things, advancing into those other cancers and getting approved as a first-line therapy for melanoma is more important than the current approval for melanoma patients using it as a last resort.

Of course, being on the market (for any indication) would allow the drug to be used by doctors off-label to treat other types of cancer, which is probably the main reason Merck pushed to get Keytruda approved.

The competition
Merck isn't the only company going after the PD-1 pathway. Bristol-Myers Squibb's (NYSE:BMY) Opdivo is already on the market in Japan. The company plans to submit an application to market Opdivo in the U.S. this quarter, theoretically putting it eight to nine months behind Keytruda, but Merck got its approval almost two months early, so the delay could be shorter.

For lung cancer, Bristol-Myers might actually be first to approval. The company is working on a rolling submission with the FDA, with plans to complete the submission by the end of the year. 

Roche and AstraZeneca (NYSE:AZN) also have drugs that target the PD-1 pathway, but they're further behind and not an immediate threat (at the moment).

A long race
We can't declare a winner because we're still waiting on the phase 3 data for lung cancer and later for other indications. How long the drugs are able to extend survival will ultimately determine the winner. Of course, there are so many opportunities here, there are likely to be niches for each of the drugs to fit into.

Brian Orelli and The Motley Fool have no position in any of the stocks mentioned. 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.