President Biden's pick to lead the Food and Drug Administration (FDA) pledged to Sen. Ron Wyden (D-Ore.) that he would crack down on companies that take advantage of the agency's accelerated approval pathway.

But in a letter released Friday, the chairman of the Senate Finance Committee said that even though he extracted that promise from Robert Califf, he's still not satisfied.

Under the accelerated approval pathway, the FDA can accept lower quality evidence from manufacturers in order to expedite approval if the drug will address a serious unmet medical need.  

But Wyden said some companies have been taking advantage of the process by not following up with the required studies even after gaining approval, and the FDA has not been holding those companies accountable. 

The accelerated approval process was most notably used to approve Aduhelm, the controversial Alzheimer's drug from Biogen. 

The company gained accelerated approval for Aduhelm last summer, but the decision came despite mixed evidence at best that the drug worked. A committee of outside scientific advisers to FDA said the drug shouldn’t have been approved, and three members resigned.

Wyden said that during his meeting with Califf on Monday, Califf pledged to "take strong action" within his first 30 days to ensure the agency holds companies accountable for providing the necessary follow-up evidence.

But Wyden said he needed to know just how forceful Califf will be. In the letter, he asked Califf a series of questions, including how he will determine a company's failure to comply, and what authorities he will use to hold companies accountable. 

Califf previously led the FDA from Feb. 2016 to Jan. 2017 under former President Obama, but his confirmation this time is uncertain. At least five Democrats have already spoken out against him, and it's not yet clear if enough Republicans will support him to make up the difference.

Sen. Elizabeth Warren (D-Mass.) recently said she would vote to confirm Califf after he made significant ethics concessions, including a pledge not to seek employment or compensation from any drug or medical device companies he interacts with as commissioner.