(Corrects spelling to Nina Varghese from Nin Varghese in paragraph 7)

By Caroline Humer

NEW YORK, Oct 8 (Reuters) - AbbVie Inc's arthritis drug Humira and Roche Holding AG's cancer drug Rituxan topped a list of seven treatments whose combined 2017 and 2018 price hikes accounted for a $5.1 billion increase in U.S. spending, a report released on Tuesday showed.

The price hikes were more than twice the rate of medical inflation and were not supported by any new clinical evidence, the Institute for Clinical and Economic Review (ICER) said in the analysis.

It was the first such annual report by the Boston-based research group, which assesses the cost-effectiveness of drugs.

Other top treatments by spending that were called out included Pfizer Inc's pain drug Lyrica, Gilead Sciences Inc's HIV drug Truvada, Amgen Inc's cancer drug Neulasta, Eli Lilly & Co's erectile dysfunction drug Cialis and Biogen Inc's multiple sclerosis treatment Tecfidera.

U.S. drug prices are tough to pin down. Companies may provide list prices, but they also negotiate discounts and after-market rebates with purchasers and their representatives such as pharmacy benefit managers, health insurers, employers, and government and state health coverage programs.

ICER evaluated the pricing in partnerships with SSR Health Inc, a research firm, which calculated the increases excluding discounts and after-market rebates.

Gilead spokesman Ryan McKeel disagreed with ICER's conclusion, saying it left out real-world and economic evidence. Biogen spokeswoman Nina Varghese said ICER's methodology was flawed and did not consider evidence it submitted such as observational studies.

Lilly spokesman Mark Taylor also questioned the methodology for the report and said that generic versions of Cialis were on the market for less than 90% of the retail price.

Pfizer spokesman Steve Danahy said the analysis did not include external factors that affect drug prices or take into account what an appropriate value-based price should be.

Roche spokeswoman Emmy Wang said that in pricing drugs, the company strived for the right balance between patient access and investing for breakthroughs in medicine.

Amgen and AbbVie did not have an immediate comment.

ICER acknowledged it was difficult to determine the actual increase in spending on the drugs, but said it was confident that the seven drugs cost a lot more.

Pricing drugs based on new benefits could help slow cost hikes, ICER Chief Medical Officer David Rind said.

"If manufacturers weren't raising prices if they haven't shown a new important benefit, I think that would help," he said.

Celgene's Revlimid and Gilead's Genovya were also large contributors to spending but were excluded from the list because of their clinical advancements, ICER said.

Humira's net price increase over 2017 and 2018 added $1.8 billion in spending while Rituxan added $806 million, the report said. (Reporting by Caroline Humer; Editing by Richard Chang and Bernadette Baum)