Nasse joined Huntsworth in September from his role as director of medical affairs at Manning Selvage & Lee, where he spent nine years (PRWeek, 31 August 2007).
A Huntsworth spokeswoman said: 'Guy wanted to return to international PR where his strength and heart lies. It is an entirely amicable separation and we wish him all the best.'
Nasse took on the newly created post of associate director at Huntsworth last year, managing PR and medical education accounts out of its Chiswick office.
Before joining MS&L, Nasse worked with Shandwick Health as account director for its global healthcare taskforce across neurology, infertility and medical device ranges.
Huntsworth also lost its client services director last week to specialist healthcare shop Aurora.
Sian Hurst joins the senior management team at Aurora as associate director and will report to MD Claire Eldridge.
Hurst leaves Huntsworth after 11 years at the agency, working in therapeutic areas including respiratory, oncology, gastroenterology and men's health.