Federal charges against Derby nurse Heather Alfonso center on a powerful and addictive painkiller called Subsys, which has been heavily marketed by the Arizona-based manufacturer Insys Therapeutics, federal officials confirmed Wednesday. Alfonso, 42, of Middlebury, pleaded guilty Tuesday in U.S. District Court in Hartford to receiving $83,000 in kickbacks from January 2013 until March 2015 from a pharmaceutical company that makes a drug used to treat cancer pain.