According to Kant, is it permitted under any circumstance to treat another rational being as merely a means to an end? Why or why not?

Immanuel Kant was a German philosopher in the 18th and early 19th centuries who had a profound affect on modern moral philosophy, especially through what many have dubbed the "Mere Means Principle." According to Kant, it is NEVER permissible for one human being to treat another as simply a means to end; rather the core of our humanity is always treating others as if they are the end. Kant makes clear there is absolutely no exception to this principle.
Such a principle spawned an entire branch of ethics called "Kantian ethics." Essentially, this belief system is based on deontological moral theory, which uses defined rules to judge the morality of an action, and espouses that actions are not deemed right or wrong based on their consequences or outcomes, but instead on whether we followed our obligations under an absolute moral authority.
Kant was raised in a devout Lutheran household that emphasized devotion to religious principle and a strict adherence to Bible truths. This upbringing clearly influenced his philosophical views and lead to him deeming principles like "Mere Means" as categorical imperatives, meaning they are an unconditional and irrevocable command by a higher authority than us. Therefore, we are not in a position to make exceptions; it's simply beyond our power.
http://faculty.philosophy.umd.edu/SKerstein/Kersteinmeremeans.pdf