Hughesman will start in September as the bureau's health practice leader, reporting to New York-based president of global healthcare Laura Schoen.
Her decision means Hughesman, who was unavailable to comment, will return to her native US.
It emerged more than a month ago that Hughesman had quit her Luton-based AZ job, with speculation at the time suggesting she was poised to join H&K in London (PRWeek, 6 May).
H&K UK chief executive Marie-Louise Windeler told PRWeek: 'We were looking forward to her joining us but I understand that her circumstances changed - we wish her well.'
Hughesman's AZ role, which saw her overseeing a three-strong team handling UK corporate messaging for drugs brands, is being covered on an interim basis by brand communications manager David Prager.
Pfizer's reassessment of the UK strategy behind Celebrex follows a period of negative stories about its class, Cox-2 inhibitors.
The European Medicines Agency (EMEA) warned in February that doctors should be 'cautious' about giving Cox-2s to patients who have risk factors for heart disease, following episodes such as Merck's withdrawal of Vioxx.
EMEA expects to issue updated guidance on Cox-2s later this month.
Shire Health London MD Matt de Gruchy described Celebrex as an 'incredibly important' account for the agency. It has held the business for three years.
Rotarix is an oral vaccine developed to protect against infection with rotavirus - a stomach infection that causes severe diarrhoea that in turn leads to dehydration and is particularly dangerous in under-twos.
Rotarix launched in Mexico earlier this year, and is now under review by the European Medicines Agency.
It is seen to have fewer complications than the previous rotavirus vaccine, which was pulled several years ago after being linked with bowel blockage.
B-M, which won a three-way pitch for the brief, has teams in London and Brussels on the job. B-M London co-practice chair Amie Malkin is leading the business in the UK, while director Elizabeth de Bony heads the Brussels team.
B-M London continues to promote other GSK brands, including Panadol.