In a crossover study , dogs were randomly assigned ( in groups of @ ) to receive a single oral dose ( @ mg/kg ) of doxycycline hyclate without excipients ( control ) or extended-release formulations ( ERFs ) containing doxycycline , acrylic acid polymer , and polymethacrylate in the following proportions : @:@:@ ( ERF@ ) or @:@:@ ( ERF@ ) .