Washington (CNN)The outgoing director of the National Institutes of Health said Sunday that he faced political pressure from then-President Donald Trump and other Republicans to endorse unproven Covid-19 remedies such as hydroxychloroquine and to fire Dr. Anthony Fauci, the head of NIH's National Institute of Allergy and Infectious Diseases.

       Dr. Francis Collins, whose last day as NIH director is Sunday, told CBS News that he got a "talking to" by Trump, but that he held his ground and would have resigned if Trump made him endorse remedies for Covid-19 that were not based in science.

       "I have done everything I can to stay out of any kind of political, partisan debates because it is really not a place where medical research belongs," he said. "I was not going to compromise scientific principles to just hold onto the job."

       Trump frequently touted hydroxychloroquine as a potential Covid-19 cure, and he claimed while in office to have used it himself even as medical experts and the US Food and Drug Administration questioned its efficacy and warned of potentially harmful side effects. In June 2020, the FDA revoked its emergency use authorization for both hydroxychloroquine and chloroquine for the treatment of Covid-19, saying it determined the drugs were unlikely to be effective in treating Covid-19 based on the latest scientific evidence.

       CNN has reached out to the office of the former President for comment on Collins' interview.

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       Collins also said he fought back calls from Republicans for him to fire Fauci, the nation's top infectious disease expert who now serves as President Joe Biden's chief medical adviser.

       "Can you imagine a circumstance where the director of the NIH, somebody who believes in science, would submit to political pressures and fire the greatest expert in infectious disease that the world has known, just to satisfy political concerns?" he said.

       Fauci has faced harsh criticism from Republicans, including Trump, during the pandemic, with the longtime public servant being assailed for what they see as an overly cautious approach to the crisis and his occasional reversal on some key issues, including mask-wearing.

       Collins said on Sunday that during his 12 years serving as NIH director, one thing he would have studied more carefully is hesitancy.

       "I did not imagine there would be 60 million people who, faced with compelling evidence of the life-saving nature of Covid vaccines, would still say, 'No, not for me,'" he said.