WASHINGTON - Makers of the top-selling drugs in the United States are costing patients billions of dollars and worsening a drug pricing crisis by abusing the American patent system to stifle competition and inflate prices, a consumer group said Thursday.

The New York-based Initiative for Medicines, Access & Knowledge (I-Mak) said in a report that three of the top 10 selling drugs in the US face no competition in the country.

"US prescription drug spending, which exceeds US$400 billion (S$563 billion) today, is projected to reach nearly a trillion dollars by 2030," said I-Mak, a non-profit group of lawyers and scientists partly funded by groups critical of the pharmaceutical industry.

I-Mak said cheaper generic and biosimilar versions of Bristol-Myers Squibb and Pfizer's blood clot preventer Eliquis, AbbVie's Humira and Amgen's Enbrel, both used to treat rheumatoid arthritis, were being blocked via a practice known as "patent thickets".

Drugmakers have used the practice of seeking multiple patents for minor variations on a single invention to stave off generic competition for decades.

"Patent abuse is not limited to a few bad actors. A growing body of evidence demonstrates that an essential part of the pharmaceutical industry's business model for top-selling drugs is now built on maintaining market control by exploiting an outdated patent system," the group said.

"Pharmaceutical companies secure hundreds of patents to block competition because they can," it said.

It called on lawmakers and government agencies to end patent thickets and modernise the patent system.

Makers of the top ten selling drugs in the US filed an average of 140 patent applications per drug, 66 per cent of which followed US Food and Drug Administration approval, I-Mak said.

An average of 74 patents were granted on each drug, four times as many patents than in Europe.

The Swiss competition commission on Thursday opened an investigation into Novartis over possible unlawful use of a patent to reduce competitive pressure. REUTERS