Some of the most expensive drugs on the market are used to treat cancer, and those drugs are getting increasingly costlier as they get more complex. Over the past five years, IMS Health reports that 70 new cancer drugs have been launched, and while their prices vary, the vast majority of them cost upwards of $100,000 per year. Because cancer drug prices are soaring, IMS Health thinks global spending on cancer drugs could climb from $100 billion in 2014 to $150 billion in 2020.

Here are five of the most expensive cancer drugs currently on the market.  

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No. 1: Oncaspar

Shire (NASDAQ: SHPG) added Oncaspar to its drug lineup when it acquired Baxalta last summer in a $32 billion merger. 

Oncaspar is used to treat acute lymphoblastic leukemia, and a study conducted last year by the insurance industry trade group America's Health Insurance Plans reports that its average wholesale price is $387,864.

Because of that lofty price tag, Shire stands to generate hundreds of millions of dollars in sales from Oncaspar, despite the fact that there are only 6,600 new cases of this cancer diagnosed in the U.S. every year. In the third quarter alone, Shire reported Oncaspar sales of $55 million, up 64% from one year ago.

No. 2: Revlimid

Celgene's (NASDAQ:CELG) Revlimid is more widely used than Oncaspar, so while its average wholesale price of $146,172 seems like a relative bargain, it represents a much larger share of the global spending on cancer medicine.

Revlimid is the most widely used first- and second-line multiple myeloma therapy in the world, and rising use of it has Celgene's predicting Revlimid sales of more than $8 billion in 2017. 

Given Revlimid's high-price and widespread use, it's probably not surprising that it's a big reason why Celgene's revenue has grown by a compounded annual rate of 20% since 2012.

No. 3: Adcetris

Lymphoma is among the most expensive cancer indications, but Seattle Genetics' (NASDAQ:SGEN) average wholesale price of $337,632 is truly staggering. 

Adcetris treats Hodgkin lymphoma (HL) and a rare lymphoma known as systemic anaplastic large cell lymphoma, and it's arguably one of the more complex medicines on the market today. The drug is an antibody-drug conjugate that relies on an antibody to target a protein on lymphoma cells known as CD30 to deliver a drug payload that can destroy the cancer cell. 

The drug is only approved for use in pre-treated patients who haven't responded to other therapies or have seen their disease return. Although that limits the drug's addressable patient population, that hasn't stopped Adcetris from becoming a top seller. In the third quarter, Seattle Genetics says Adcetris net sales in the U.S. and Canada climbed to $70.1 million, up from $59.1 million last year. The company also collected $12.2 million in revenue in the quarter from royalties tied to Adcetris sales overseas by licensing partner Takeda.

No. 4: Jevtana

Sanofi's (NASDAQ:SNY) Jevtana was supposed to be the successor to Sanofi's widely used prostate cancer drug Taxotere when taxotere lost patent protection in 2010, but it's become more of a niche drug following the launch of multibillion-dollar-per-year drugs Xtandi and Zytiga, which are more commonly used.

Prostate cancer, however, remains one of the most common cancers, so even though Jevtana's prescription volume pales in comparison to other drugs, it's still generating a nine-figure revenue for Sanofi every year.

Jevtana's average wholesale price of $187,236 is far north of Xtandi and Zytiga's $127,416 and $115,152, respectively, and as a result, Sanofi's Jevtana sales were $284 million through the first nine months of 2016, at current exchange rates.

No. 5: Xalkori

Pfizer's (NYSE:PFE) committing a lot of resources to its oncology research and development programs, and one of the company's big successes so far is Xalkori, an expensive drug that's used to treat ALK+ non-small cell lung cancer.

There are an estimated 224,390 new cases of lung cancer diagnosed in the U.S. annually and, sadly, an estimated 158,080 deaths per year are caused by it. With a five-year survival rate of less than 18%, there's undeniably a big unmet need for new treatment options for this devastating disease.

Approved in 2011, Xalkori, which carries a $193,908 average wholesale price, is designed to block enzymes that can stimulate cancer cell growth. Specifically, Xalkori blocks an enzyme called anaplastic lymphoma kinase (ALK). Unfortunately, only about 5% of lung cancer patients have overactive ALK enzyme activity that can benefit from Xalkori.

An eye on the future

Cancer is becoming more common as baby boomers get older and live longer, and while these drugs are expensive, they do offer many patients hope for a longer life.

How much drugs that target life-threatening diseases like cancer should cost is likely to become a much bigger question as cancer numbers climb and cancer spending surges. Nevertheless, advances in genetics mean that cancer drugs are getting more complex, and that, I'm afraid, is unlikely to lead to less expensive treatments. 

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.