Just five diseases caused the deaths of roughly 1.6 million Americans in 2015, the most recent year for which the Centers for Disease Control and Prevention (CDC) has statistics. Many more patients suffer from the diseases. 

Promising drugs could be on the way to treat these diseases, from drugmakers including Alnylam (NASDAQ:ALNY), AstraZeneca (NYSE:AZN), Biogen (NASDAQ:BIIB), Bristol-Myers Squibb (NYSE:BMY), Merck (NYSE:MRK), and The Medicines Company (NASDAQ:MDCO). Here are the five deadliest diseases in the U.S. -- and some of the top drugs that could be available for treatment in the not-too-distant future.

Scientists in lab

Image source: Getty Images.

1. Heart disease

Heart disease took the lives of more than 614,000 Americans in 2015. The most common type of heart disease, coronary heart disease (CHD), kills around 370,000 people in the U.S. each year. 

CHD is caused by the buildup of plaque inside the coronary arteries. The good news is that keeping your cholesterol in check can help prevent CHD. Even better, a new class of drugs known as PCSK9 inhibitors could help more people lower their cholesterol levels than ever before.

A couple of PCSK9 inhibitors have already received approval from the U.S. Food and Drug Administration (FDA), but another promising one could be on the way. The Medicines Company and partner Alnylam recently announced great results for inclisiran in a mid-stage clinical study. 

The experimental drug works by turning off the liver's synthesis of PCSK9, a protein that's important in regulating cholesterol levels in the body. PCSK9 inhibitors that are currently available require 12 to 24 injections each year. Inclisiran, however, requires only two shots per year. The Medicines Company and Alnylam will next evaluate the drug in a late-stage clinical trial, so it will be a few years before inclisiran could reach the market. 

2. Cancer

Nearly 592,000 people in the U.S. died from cancer in 2015. Lung cancer accounted for more than a third of those deaths. 

Two powerful lung cancer drugs have received FDA approval over the last couple of years for treating the most common type of lung cancer, non-small cell lung cancer (NSCLC). Bristol-Myers Squibb won approval for Opdivo in March 2015, while Merck got the green light for Keytruda in October 2016.  Both drugs are also approved for treating other types of cancer, including melanoma and classical Hodgkin lymphoma. 

The biggest opportunity for treating lung cancer could lie with combination therapies with Opdivo and Keytruda. Bristol-Myers Squibb has multiple combination studies in progress, with several reporting data in 2018. Merck awaits approval for a Keytruda/chemotherapy combo and is evaluating Keytruda in combination with other drugs targeting lung cancer as well. 

3. Chronic lower respiratory diseases

Chronic lower respiratory diseases caused more than 147,000 deaths in the U.S. in 2015. The most deadly chronic lower respiratory disease is chronic obstructive pulmonary disease (COPD), which includes emphysema and chronic bronchitis. 

Several drugmakers are developing COPD treatments. One leading the way is AstraZeneca. The big pharmaceutical company is evaluating two experimental COPD drugs in late-stage studies.

PT010 delivers a triple-whammy, combining three types of COPD treatments: a long-acting muscarinic antagonist (LAMA), a long-acting β2-agonist (LABA), and an inhaled corticosteroid. Benralizumab is a monoclonal antibody that inhibits interleukin-5 receptor (IL-5R), a signaling protein of the immune system. AstraZeneca expects to file for U.S. regulatory approval for both PT010 and benralizumab in 2018. 

4. Stroke

More than 133,000 Americans died in 2015 from stroke. There are several drugs currently approved for treating stroke, but a recent study suggests that the best of the bunch could be Bristol-Myers Squibb's Eliquis.

An analysis of Medicare data showed that Eliquis reduced stroke risk by 60% compared to warfarin, a commonly used drug for treating stroke. That result was much higher than any of the drug's main rivals. 

There aren't too many drugmakers working on the development of new stroke drugs right now, although several non-profit and government organizations are involved in research. However, Bristol-Myers Squibb is collaborating on additional studies of Eliquis in potentially preventing and treating stroke.  

5. Alzheimer's disease

Alzheimer's disease took over 93,000 American lives in 2015. Unfortunately, the landscape is full of once-promising drugs for the condition that didn't pan out.

Biogen hopes to change the story, though. In December, the biotech reported positive results from an early stage clinical study of aducanumab. Patients with early Alzheimer's disease who took the experimental drug experienced reduced rates of clinical decline and lower levels of amyloid plaque, which is thought to play a role in causing the disease.

These early stage results could bode well for Biogen's late-stage studies of aducanumab that are currently under way. Although it will be a few years before these studies wrap up, Biogen just might be able to succeed where others haven't. 

Cautious optimism

Drugs that seem promising can prove to be ineffective. Safety issues can also arise. Still, there is reason to be cautiously optimistic.

Significant progress has been made in treating all of these deadly diseases in recent years, with the notable exception of Alzheimer's disease. The drugs that are being studied today could be the game-changing treatments of the future. 

Keith Speights has no position in any stocks mentioned. The Motley Fool owns shares of and recommends Alnylam Pharmaceuticals and Biogen. The Motley Fool has a disclosure policy.