Because effective [cancer] treatments almost always require hitting more than one target at the same time, some very good and relatively safe cancer drugs show no evidence of effectiveness when used alone, and therefore, might not be approved by the FDA

See Frances Kelsey Syndrome from Marginal Revolution.

"Occasionally I have been told that FDA reform is something that only a few libertarian economists support. But in fact, there is strong support for reform in much of the medical community. See, for example, the survey that Dan Klein and I did on off-label prescribing and FDA reform or the many surveys of physicians done by CEI.

I mentioned Dr. Vincent DeVita in my post, Will Trump Appoint a Great FDA Commissioner? It’s worthwhile exploring DeVita’s views at greater length because he is a prominent figure in the field of oncology. Here, from Wikipedia, is some background on DeVita:

Vincent Theodore DeVita, Jr., MD is an internationally recognized pioneer physician in the field of oncology. DeVita spent the early part of his career at the National Cancer Institute (NCI). In 1980, the president of the United States appointed him as director of the NCI and the National Cancer Program, a position he held until 1988. While at the NCI, he was instrumental in developing combination chemotherapy programs that ultimately led to an effective regimen of curative chemotherapy for Hodgkin’s disease and diffuse large cell lymphomas. Along with colleagues at the NCI, he developed the four-drug combination, known by the acronym MOPP, which increased the cure rate for patients with advanced Hodgkin’s disease from nearly zero to over 70%.
DeVita was the Director of Yale Cancer Center from 1993 to 2003. He is currently the chair of the Yale Cancer Center advisory board and is professor of internal medicine and of epidemiology and public health at Yale’s medical school.
DeVita currently serves on the editorial boards of numerous scientific journals and is the author or co-author of more than 450 scientific articles. He is one of the three editors of the popular textbook (also available online) Cancer: Principles and Practice of Oncology and serves as the editor-in-chief of The Cancer Journal.

In his book, The Death of Cancer, DeVita has a chapter on the FDA. The title of that chapter? Frances Kelsey Syndrome. He writes:

The thalidomide episode sent the message to those who worked at the FDA that the way to do right by people was to say no. Saying yes would prove perilous–not only to patients, but to the careers of a reviewer. As a result, the agency tends to reward those who say no, not yes. (In fact, there’s an annual Frances Kelsey award. But there are no awards for getting a good drug quickly into the public domain.)

Exactly right. Later he discusses another problem that he sees with FDA evaluation procedures:

We are approaching what we might have considered nirvana years ago. We can design drugs that will hit a specific target, and by being so specific, they have fewer side effects. But because effective treatments almost always require hitting more than one target at the same time, some very good and relatively safe cancer drugs show no evidence of effectiveness when used alone.
What a dilemma. After spending millions of dollars developing a drug, a company may be forced to abandon it for lack of efficacy, when, if approved, it would be another exciting tool for clinical investigators who want to explore combinations of targeted therapies in post-market research trials. Compound that with the FDA’s insistence on testing them first on patients with very advanced, resistant disease, and many potentially useful drugs don’t look so good. As a result, drug companies are reluctant to invest money in new cancer drugs, because they might never make it past the FDA’s hurdles.

DeVita may be wrong but he’s certainly not uninformed."