regarding patents - have read a few articles recen

regarding patents - have read a few articles recently about the BP drugs that are having patents expire, and im still in the dark as to when patent windows begin. not sure if keytruda falls into this category but its is widely known that BP especially will modify a successful drug in order to start a new patent clock, therefore extending the profit calendar. usually they slip in a novel delivery mechanism like a salt compound and are allowed to call it a new formulation, a new patent, and a new drug - resetting the clock and reaping billions in extra profits while denying the generic drug makers the right to produce these drugs.

the question of when patent calendars begin - when did keytruda begin, what modification were made to extend it, and when will patent expire? next question of course is when did the leronlimab patent calendar begin, when is it scheduled to end, and is there any possibility that it can be extended the same way the BPs cheat everyday? there has been talk that LLs patent window is closing soon, even though its not on the market...why does the patent calendar start before its on the market? should these windows start the day it is available to the public?