At a cost of more than $300,000 a year, the new cystic fibrosis drug, ivacaftor (Kalydeco), is one of the most expensive pills in American medicine.

This week three doctors who treat cystic fibrosis patients said that kind of extremely high pricing for new drugs that treat rare conditions threatens the U.S. health care system and can't be sustained.

In a viewpoint published online in the Journal of the American Medical Association, the doctors said drug companies have an obligation to put patient well-being on equal ground as profitability.

Similar concerns were voiced in May in a Journal Sentinel/MedPage Today investigation that revealed the huge profits reaped by executives of Vertex Pharmaceuticals, which makes Kalydeco, and the millions of dollars in drug royalties earned by the Cystic Fibrosis Foundation, which invested in the drug's development.

The story noted that there was also a substantial public investment by the National Institutes of Health and others that led to the development of the drug.

About 30,000 people in the U.S. have cystic fibrosis. When Kalydeco came on the market in 2012, it was approved to treat only about 4% of those patients who have a particular genetic mutation.

Originally, the drug, which comes in the form of two pills a day, was priced at $294,000 a year. Months later, Vertex increased its price to $307,000.

The charge to individual patients can be as high as $373,000, according to doctors writing in JAMA.

"(Kalydeco) will need to be taken for decades by individual patients with the potential total cost of many millions of dollars," the doctors wrote. "The vast majority of patients cannot afford this financial burden and transferring the cost to private or federal insurers does not obviate the underlying problem -- an unsustainable pricing structure."

Replying by email to a request for comment, Robert Beall, PhD, CEO of the Cystic Fibrosis Foundation said the JAMA viewpoint highlighted the need to find a balance between drug innovation to help those with rare diseases and the extremely high cost of new therapies.

"We are deeply committed to finding this balance," he said. "We are concerned that the current pricing models are unsustainable."

The Journal Sentinel/MedPage Today story noted that the cost of Kalydeco is borne by taxpayers through Medicaid and other government programs and by the workers and companies that finance employee health insurance plans.

The viewpoint was signed by Brian O'Sullivan, MD, of the University of Massachusetts Medical School; David Orenstein, MD, of the University of Pittsburgh School of Medicine; and Carlos Milla, MD, of Stanford University School of Medicine.

In an email response, Nikki Levy, a spokeswoman for Vertex, said the vast majority of patients pay between $15 and $50 a month out-of-pocket for Kalydeco.

"The price of Kalydeco reflects how well this medicine works, the time and cost it took to develop and our commitment to reinvest to help many more people with CF -- work that is highly expensive, risky and takes the dedication of hundreds of people over decades," Levy said.

In an interview, Orenstein, of the University of Pittsburgh, said he and his co-authors decided to publish their viewpoint because a previous effort to privately voice concern to Vertex only led to a $13,000 price increase.

He noted that cost of Kalydeco increased by 10 times the medical costs of the typical cystic fibrosis patient.

"It is going to place unbearable burden on the system," Orenstein said. "It just seems like a freight train running down the tracks without any brakes."

Last April, a group of more than 100 cancer experts jointly signed an editorial in the medical journal Blood, imploring drug companies to bring down prices for specialty drugs used to treat chronic myeloid leukemia.

They noted that 11 of the 12 new drugs approved last year to treat various cancers cost more than $100,000 per patient per year.

The authors of the JAMA viewpoint note that Vertex had access to a clinical trial network operated by the Cystic Fibrosis Foundation that helped the drug company win FDA approval for Kalydeco.

"Ironically, the patients who assumed the risk of participating in the clinical trials necessary to bring this drug to market and who devoted countless hours to raising money for the CF Foundation to underwrite early work are now being asked to pay, most often through their insurers, an exorbitant price for the product that resulted from their efforts," they wrote.