What Is Precision Medicine in Cancer?

In contrast to a one-size-fits-all approach to cancer treatment, precision medicine is an approach that looks at specific information about a person's tumor to diagnose and treat the disease. Historically, treatments for cancer varied depending primarily on the type of cancer cell seen under the microscope.

With further understanding of the human genome and immunology, many new therapies have been developed that are designed to target specific molecular changes and pathways in the growth of cancer, or ways that cancers have learned to evade the immune system. Gene profiling and next-generation sequencing can help doctors find subsets of people with these cancer types who may respond to therapies that directly target these changes.

It is now believed that between 40 and 50 percent of cancers may be treated with precision medicine.

The following details how precision medicine works, the testing that is required, as well as some examples of drugs that are being used in this way for cancer.

Definition

In the past, cancers were divided primarily by cell type, with perhaps two or three primary types of cancer arising in a particular organ such as the lungs. We now know that every cancer is unique. If 200 people in a room had lung cancer, they would have 200 unique types of cancer from a molecular standpoint. Unlike chemotherapy, a treatment that works to eliminate any rapidly dividing cells, precision medicine involves new treatments that target either the way cancer grows (targeted therapies) or the way it evades the immune system (immunotherapy drugs).

The National Cancer Institute defines precision medicine as a form of medicine that uses information about a person’s genes, proteins, and environment to prevent, diagnose, and treat disease.

With cancer, precision medicine uses specific information about a person’s tumor to help diagnose, plan treatment, find out how well treatment is working, or make a prognosis. Examples of precision medicine include using targeted therapies to treat specific types of cancer cells, such as HER2-positive breast cancer cells, or using tumor marker testing to help diagnose cancer.

Pharmacogenomics, in turn, is the branch of personalized medicine that focuses on finding medications to treat specific genetic changes in a tumor.

Precision vs. Personalized

The terms precision medicine and the somewhat older term personalized medicine are sometimes used interchangeably. The difference is that the older term implied that treatments were designed specifically for each person. In contrast, with precision medicine, treatments focus on abnormalities in tumors based on genetic factors, the environment, and lifestyle.

How Often Can It Be Used?

Whether precision medicine options are available and how many people they may affect can vary between different cancers. For instance, according to the International Association for the Study of Lung Cancer, around 60 percent of people with lung cancer have tumors with genetic traits that may have treatments available with precision medicine. As more is known, it's likely that these numbers will increase.

Though our focus here is on cancer, there are other areas of medicine in which precision medicine is used as well. A simple example is that of testing a person's blood before a blood transfusion.

Diagnostic Tests

Before a tumor can be treated with precision medicine therapies (pharmacogenomics), the molecular characteristics of that tumor need to be defined. Unlike conventional tests, such as looking at cancer cells under the microscope, tumors must be analyzed at the molecular level.

Molecular Profiling of Tumors

Molecular profiling looks for genetic changes in cancer cells such as a mutation or rearrangement that acts as cancer's biggest weakness. Specifically, this kind of profiling looks for mutations or other changes in genes that code for proteins which drive the growth of a tumor or signal tumor pathways.

Next-Generation Sequencing

Next-generation sequencing is more complex than molecular profiling. It looks for a large variety of genetic changes that may be associated with a wide range of cancers.

Testing for Familial Cancer Mutations

Talking about mutations in cancer cells can be very confusing, as there are two different types of mutations that are discussed:

  1. Acquired Mutations. These are the mutations that are detected with molecular profiling of tumors. They arise after birth in the process of a cell becoming a cancer cell. The mutation is present in only the cancer cell and not all the cells of the body, and are the "target" of the targeted therapies discussed here.
  2. Hereditary Mutations (Germ-Line Mutations). These are present from birth, and in some cases, can raise the risk of developing cancer. While these mutations are most often tested to learn if a person has a predisposition to cancer or if it runs in their family, they are not addressed with targeted therapies.

That said, we are learning that some hereditary mutations can affect the behavior of a tumor. Treatment of the tumor based on this information (including testing for familial mutations) thus falls under the heading of precision medicine.

PD-L1 and Tumor Mutation Burden Testing

Molecular profiling and next-generation sequencing look for genetic changes in tumor cells that may respond to targeted therapies. However, another major new form of therapy is immunotherapy, which are drugs that work simplistically by boosting the immune system.

For instance, with lung cancer, there are now four immunotherapy drugs that are approved for advanced disease. We know, however, these don't work for everybody.

Some people have a very dramatic response to immunotherapy drugs, whereas others do not seem to respond or their cancer even worsens.

While the science is young, researchers are looking for ways to determine who will respond to these drugs, which is something that can't be determined under the microscope. At the current time, there are two approaches to testing a patient's responsivity to immunotherapy, but further research is strongly needed:

  • PD-L1 testing can sometimes predict who will respond to immunotherapy, but it is not always accurate. Even people with low levels of PD-L1 (a protein that suppresses the immune system) sometimes respond very well.
  • Tumor Mutation Burden (TMB) has recently been evaluated as another method to predict response. TMB is a measure of the number of mutations present in a tumor, and those who have a higher TMB often respond better to the immunotherapy drugs. This makes sense, as the immune system is designed to attack foreign material (including cancer cells), and cells that have more mutations may appear more abnormal.

Benefits

The most obvious benefit of precision medicine is that it allows a doctor to tailor cancer treatment based on further information about cancer cells.

This both increases the chance that a person will respond to treatment, and reduces the chance that a person will have to cope with the side effects of a treatment that does not work.

One example that describes this is the use of the eGFR inhibitor called Tarceva (erlotinib). When this therapy was first approved for lung cancer, it was often prescribed with a one-size-fits-all mentality, meaning it was prescribed to many different cases. When used this way, only a small number of people (around 15 percent) responded.

Later on, gene profiling allowed doctors to determine which people had tumors with an eGFR mutation and which people did not. When Tarceva was given to people with the specific mutation, a much higher number of people responded (roughly 80 percent).

Since that time, further testing and drugs have been developed so that a different drug (Tagrisso) may be used to treat people with a particular type of eGFR mutation (T790M) that would not respond to Tarceva. Also, recently, Tagrisso has been shown to be a more potent drug than Tarceva in lung cancer tumors bearing eGFR mutations. With newer generations and more specific treatments, more patients respond positively to individualized treatment.

Challenges

Precision medicine can still be considered in its infancy, and there are many challenges that accompany it.

Eligibility. Even when mutations can be found in tumor cells (and it's likely that there are many more to be discovered), there are targeted medications available that address only a subset of these changes—either approved drugs or those available in clinical trials. In addition, even when these drugs are used to address a specific mutation, they don't always work.

Not everyone is tested. The science is changing so rapidly that many doctors are unaware of all the testing options available, such as next-generation sequencing. Without testing, many people are unaware that they have options. This is one of the reasons why it is so important to learn about your cancer and be your own advocate.

Resistance. With many targeted therapies, resistance develops in time. Cancer cells figure out a way to grow and divide to actually bypass being inhibited by a targeted drug.

Control doesn't mean cure. Most targeted therapies can control a tumor for a period of time until resistance develops—they do not cure cancer. Cancer can recur or progress when treatment is stopped. In some cases, however, benefits of some immunotherapy drugs may persist after the drug is stopped, and in some uncommon cases, may cure cancer (known as a durable response).

Lack of clinical trial participation. Therapies need to be tested before they are approved for everyone, and far too few people who qualify in a clinical trial are enrolled. Minority groups are also greatly underrepresented in clinical trials, so the results don't necessarily reflect how a drug performs across a diverse group of people.

Cost. Some health insurance policies fail to cover all or a portion of gene profiling tests. Some cover testing for only a few mutations, rather than a more comprehensive screen such as testing by Foundation Medicine (a company that performs genomic testing). These tests can be prohibitively expensive for those who must pay out-of-pocket.

Privacy. In order to move forward with precision medicine, data is needed from a large number of people. This can be challenging as more people fear the loss of privacy that might occur with genetic tests.

Timing. Some people who might qualify for these treatments are very sick at the time of diagnosis, and may not have the time that is needed to do the testing, wait for the results, and receive the medications.

Uses and Examples

HER2 in Breast Cancer

Breast cancer can be defined in categories based on the types of cells seen under the microscope, such as ductal carcinoma that arises in cells that line the breast ducts, and lobular carcinoma that arises in cells of the breast lobules.

Traditionally, breast cancers were treated as if they were one type of disease, with surgery, chemotherapy, and/or radiation. Precision medicine now involves testing the molecular characteristics of tumors.

For example, some breast cancers are estrogen receptor positive, whereas others may be HER2/neu positive. With HER2 positive breast cancers, the tumor cells have an increased number (amplification) of HER2 genes. These HER2 genes code for proteins that act as receptors on the surface of some breast cancer cells. Growth factors in the body then bind to these receptors to cause the growth of the cancer. HER2 targeted therapies, such as Herceptin and Perjeta target these proteins so that growth factors cannot bind and cause the growth of the cancer.

eGFR Mutations in Lung Cancer

Lung cancers may be broken down by the cell type under the microscope, such as non-small cell lung cancers and small cell lung cancers. Now, there are changes that can be detected on gene profiling which can be treated with precision medicine, including eGFR mutations, ALK rearrangements, ROS1 rearrangements, BRAF mutations, and more.

With EGFR positive lung cancer there are now several drugs that have been approved. Resistance develops for most people in time (due to acquired mutations), but changing to another drug in this category (for example, second or third-generation drugs) may be effective. For example, some people become resistant to Tarceva (erlotinib) when a T790M mutation develops, and may then respond the the drug Tagrisso (osimertinib).

The hope is that in time, by using targeted therapies such as these and switching to a next-generation drug when resistance develops, doctors will be able to treat some cancers as chronic diseases that require treatment but can be controlled.

NTRK Fusion Genes in Multiple Cancers

Most medications that fall under precision medicine primarily work on one type of cancer, but there are some that may work across cancers. The first drug shown to be effective in this way was the immunotherapy drug Keytruda (pembrolizumab) which works for a few types of cancer.

The medication Vitrakvi (larotrectinib) was approved as the first targeted therapy to work across cancers. It targets a specific molecular change, called the neurotrophic receptor tyrosine kinase (NRTK) fusion gene, and was effective in 17 diverse types of advanced cancers in clinical trials.

Side Effects

The side effects of precision medicine therapies vary depending on the treatment, but sometimes, they are significantly milder than chemotherapy drugs.

As noted, chemotherapy attacks all rapidly dividing cells, including hair follicles, cells in the gastrointestinal tract, and cells in the bone marrow—this results in the well-known side effects. Since targeted therapies work by targeting specific pathways in the growth of cancer cells, and immunotherapy drugs work to enhance the immune systems ability to fight cancer simplistically, they often have fewer side effects. An example is the medication Tarceva, which is used for eGFR positive lung cancer. It is usually well-tolerated with the exception of an acne-like rash and diarrhea.

A Word From Verywell

We know that every cancer is unique, and precision medicine takes advantage of addressing those unique characteristics. Most of the challenges relate to the newness of the science, but with further information and research, it will hopefully replace the one-size-fits-all approach to many cancers.

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Article Sources

  • What Is Precision Cancer Medicine? American Association for Cancer Research Foundation.

  • Precision Medicine. NCI Dictionary of Cancer Terms. National Cancer Institute.

  • Friedman AA, Letai A, Fisher DE, Flaherty KT. Precision medicine for cancer with next-generation functional diagnostics. Nature Reviews Cancer. 2015;15(12):747-756. DOI:10.1038/nrc4015