Presumably Nikita Kamaev died of a massive heart attack.
Former General Director of the Russian anti-doping Agency (RUSADA) Ramil Khabriev said that he died a former Executive Director Nikita Kamaev. Presumably Kamaev died of a massive heart attack after actively out for exercise.
Note that in the two weeks we have already lost two of the leaders of RUSADA. 3 February died the Chairman of Executive Council RUSADA Vyacheslav Sinev, who has headed the organization since its inception until 2010.
Recall that in December last year the management RUSADA resigned. It happened after he was disqualified the entire Russian athletics.