Wall Street's biggest advisory banks and boutiques have missed out on the lion's share of a near-$250 million fee pot after pharma companies Pfizer and Allergan officially abandoned their $160 billion merger in the face of aggressive US rule changes to clamp down on so-called inversion deals.

The companies confirmed in separate statements on April 6 that their deal, which was agreed in November 2015 and would have been the second-biggest M&A deal yet, behind Vodafone's $171 billion acquisition of Mannesmann, which closed in 2000, had been terminated "by mutual agreement".