His case is now being handled by the US special envoy for hostage affairs.
At least 65 Americans were being unfairly detained abroad in 2022, according to a report by the James Foley Legacy Foundation.
The daughter of the human rights activist Andrei Sakharov, who became the first Soviet citizen to be awarded the Nobel Peace Prize, has described the treatment dealt out to journalists as "primitive, unjustifiable, and appalling".
Tatiana Yankelevich, who is a US-based scholar, has said in an article about her friend Mr Kara-Murza that he is unlikely to survive his 25-year jail term and that "there are many other lesser known but ever-so-courageous people resisting official lies and propaganda".