The United States Federal Drug Administration (FDA) has just approved a new treatment for a rare blood clotting disease, one with a hefty price tag.

Per dose, it'll cost US$3.5 million, making it the most expensive drug anywhere in the world.

At first glance, the price is gobsmacking, but a recent analysis on the cost-effectiveness of the drug suggests that's a relatively 'fair' expense for what the treatment achieves at least in the US.

The medicine, called Hemgenix, is a gene therapy treatment for hemophilia B, which is a rare genetic disease that causes reduced clotting of the blood. The most serious symptoms include spontaneous and repeated bleeding episodes that are difficult to stop.

Hemophilia B tends be more common in men than women, and while an exact number is hard to come by, estimates suggest nearly 8,000 men in the US suffer from the lifelong disease.

The main medication currently used to treat hemophilia B in the US bestows patients with a much-needed clotting factor, but its lifetime treatment costs are steep. In those with severe symptoms, a routine and expensive treatment regime is required, one that over time can begin to wane in effectiveness.

Today, researchers estimate the adult lifetime cost for every patient with moderate to severe hemophilia B is around US$21 to $23 million. Treatment costs in the UK are cheaper than in the US or elsewhere in Europe, but still add up to tens of millions of dollars per patient over their lifetime.

Hemgenix, on the other hand, is a one-time intravenous product given in a single dose at a fraction of the price. The product is carried into the body via a viral-based vector, which is engineered to deliver DNA to target cells in the liver. This genetic information is then replicated by cells, spreading the instructions for a clotting protein, known as Factor IX.

Two studies have so far tested the efficacy and safety of Hemgenix. In one study among 54 participants with severe or moderately severe hemophilia B, researchers found increased Factor IX activity levels, reducing the need for routine replacement therapies currently available to patients.

After receiving the gene therapy, the rate at which patients developed uncontrolled bleeds fell by over 50 percent compared to their baseline rate.

Side effects included headaches, flu-like symptoms, and enzyme elevations in the liver, all of which should be monitored carefully by physicians going forward.

"Gene therapy for hemophilia has been on the horizon for more than two decades. Despite advancements in the treatment of hemophilia, the prevention and treatment of bleeding episodes can adversely impact individuals' quality of life," says Peter Marks, the director of the FDA's Center for Biologics Evaluation and Research.

"Today's approval provides a new treatment option for patients with hemophilia B and represents important progress in the development of innovative therapies for those experiencing a high burden of disease associated with this form of hemophilia."

It's not yet clear whether this gene therapy treatment is a cure for hemophilia B, but the initial results are promising.

For serious, yet rare, disorders like hemophilia B, the FDA has a special designation to incentivize medical research. Hemgenix, for instance, is classified as an 'orphan drug' because it will only treat a small number of patients.

As part of this designation, the Hemgenix manufacturer, CSL Behring, has exclusive rights to the US marketplace for the next seven years.

The incentives employed by the US government to spur on scientific research is a useful way to boost innovation into rare diseases that would otherwise fall by the wayside, but this policy presents a double-edged sword.

It also means that the US market shoulders the cost of propping up drug monopolies, while other nations, who put restrictions on drug prices, reap the benefits of international research.

Today, the US pays between two and six times more for prescription drugs than other countries around the world.

The previous record holder for most expensive drug was another 'one shot' form of gene therapy for treating spinal muscular atrophy. At an estimated US$2 million per course, it also generated heated debate over the way drug companies fund their businesses.

While many drug manufacturers have taken advantage of the orphan drug status over the past few decades to create medicine monopolies, this latest product could be one case where the policy just might work for at least some people.

The upfront costs are certainly immense, but for those who might be fortunate enough to have a supportive insurance company, Hemgenix could save millions in medical costs, improving lives in immeasurable ways.

The European Medicines Agency and its drug regulator counterparts in the United Kingdom and Australia are now also reviewing the gene therapy treatment for use.

It will be interesting to see how much drug manufacturers are allowed to charge for Hemgenix in other parts of the world.