AFAIK, the situation is like this. As part of the 2007 update to the Food and Drug Administration Act added the authority for the FDA to require drug manufacturers to implement a Risk Evaluation and Mitigation Strategy (REMS) to ensure that the benefits of a drug or biological product outweigh its risks. The theory was that some drugs might have serious enough side effects or complicated treatment plans that the FDA should require drug manufactures to make sure patients weren't harmed needlessly by taking these drugs in a way not supported by safety trials (aka elements to assure safe use or ETASU).
As an example, they could restrict wholesalers to sell the drug only to physicians or patients who attended training seminars, or only allow use for certain purposes and time-limit quantities to prevent certain side effects, make sure medicine is stored correctly and destroyed when expired, and they could require patients to be monitored for certain specific serious side effects, not allow the drug to be administer to otherwise healthy people etc... Seemed like a good idea at the time....
The unintended side effect of this is that Pharma companies have been crafting REMS to make it nearly impossible for generic manufacturers to obtain sufficient quantities of approved drugs for the required safety and equivalence trials. For example, a part of the ETASU might be that all patients must attend a company training seminar, or not allow the drug to be used on healthy people, but if you are doing a blind trial, that won't work.
To make the situation worse, even if the FDA didn't require REMS for a particular drug, the Pharma companies decided to "voluntarily" implement similar restrictions for their drugs on the wholesalers.
Wholesalers that don't comply with the Pharma's ETASUs would be violating both FDA rules and probably licensing restrictions and subject them to direct liability and thus will generally not sell product directly to these generic manufacturers. The only option remaining for generic manufacturers would be to purchase the product directly from the brand-name manufacturer. Under current law they are not required to sell drugs directly to their competitors and under strict interpretation of the FDA act, if a drug has a specific REMS, it is likely not technically legal.
Also even if the generic manufacturer decided to buy some of the drug on the "grey-market", they won't satisfy the requirements of the ANDA (abbreviated new drug application) which would require the same version available in the US market for demonstrating bio-equivalence.
FWIW, in 2012 there was an effort to amend the FDA act to allow the medical trials to bulk purchase of brand-name drugs at market prices and exempt REMS requirements, but it failed due to heavy lobbying...