WASHINGTON—Drugmakers hiked U.S. prices on 7 of the 10 costliest prescription drugs in 2020 without justification, increasing drug spending by $1.67 billion, a U.S. group that reviews the value of medicines said on Tuesday. AbbVie Inc.’s widely used Humira rheumatoid arthritis drug accounted for the majority of the spending increase, with a net price increase of 9.6 percent leading to an almost $1.4 billion increase in U.S. spending. The Boston-based Institute for Clinical and Economic Review (ICER) said in a report that after reviewing published studies and input from drug manufacturers it found no evidence of new clinical benefits to justify the price hikes. “Several of these treatments have been on the market for many years, with scant evidence that they are any more effective than we understood them to be years ago when they cost far less,” said ICER Chief Medical Officer David Rind. “The most extreme of these …