###
I suspect that some policy work on setting Part B (injectible) drug prices to European levels did start under Obama (2012-2016) (here, here, here). But the major landmark came in the middle of the Trump administration, October 2018, when CMS released a Federal Register "advance notice of proposed rule-making." See webpage for "IPI International Price Indexing" Model here and Fed Reg here.
Under the updated title, "MFN Most Favored Nation" Model, CMS released a "interim final rule" on MFN on November 27, 2020 (here). This was later rescinded by a Biden-era final rule, December 27, 2021 (ibid).
In between, on December 28, 2020, one month after the interim final rule for MFN, the US District Court for Northen California issued an injunction (part of BIO v Azar, 3:20-cv-08603).
Summary
So whatever the earliest history of IPI/MFN ideas, there was major activity in the mid-Trump period (2018), and the interim final rule came out at the very end of the Trump administratoin (November 2020), and collided with courts by December 2020.
Using different concepts (negotiation), the IRA nflationi Reduction Act signed in August 2022 has been going into effect rapidly, lowering certain Medicare drug prices. It involves both Part B and Part D.