clinical trial Clinical trials are prospective biomedical or behavioral research studies on human participants designed to answer specific questions about biomedical or behavioral interventions, including new treatments (such as novel vaccines, drugs, diet ...

s, a surrogate endpoint (or surrogate marker) is a

measure of effect An outcome measure, endpoint, effect measure or measure of effect is a measure within medical practice or research, (primarily clinical trials) which is used to assess the effect, both positive and negative, of an intervention or treatment. Measures ...

of a specific treatment that may correlate with a ''real''

clinical endpoint Clinical endpoints or clinical outcomes are outcome measures referring to occurrence of disease, symptom, sign or laboratory abnormality constituting a target outcome in clinical research trials. The term may also refer to any disease or sign tha ...

but does not necessarily have a guaranteed relationship. The

National Institutes of Health The National Institutes of Health, commonly referred to as NIH (with each letter pronounced individually), is the primary agency of the United States government responsible for biomedical and public health research. It was founded in the late ...

(USA) defines surrogate endpoint as "a

biomarker In biomedical contexts, a biomarker, or biological marker, is a measurable indicator of some biological state or condition. Biomarkers are often measured and evaluated using blood, urine, or soft tissues to examine normal biological processes, p ...

intended to substitute for a clinical endpoint". Surrogate markers are used when the primary endpoint is undesired (e.g., death), or when the number of events is very small, thus making it impractical to conduct a clinical trial to gather a statistically significant number of endpoints. The

FDA The United States Food and Drug Administration (FDA or US FDA) is a federal agency of the Department of Health and Human Services. The FDA is responsible for protecting and promoting public health through the control and supervision of food ...

and other regulatory agencies will often accept evidence from

s that show a direct clinical benefit to surrogate markers.Alexandra Goho, "An Imperfect Substitute" ''CR Magazine'', Spring 2009
/ref> Surrogate endpoints can be obtained from different modalities, such as, behavioural or cognitive scores, or

s from Electroencephalography ( qEEG),

MRI Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to form pictures of the anatomy and the physiological processes of the body. MRI scanners use strong magnetic fields, magnetic field gradients, and radio waves ...

PET A pet, or companion animal, is an animal kept primarily for a person's company or entertainment rather than as a working animal, livestock, or a laboratory animal. Popular pets are often considered to have attractive appearances, intelligence ...

, or biochemical biomarkers. A correlate does not make a surrogate. It is a common misconception that if an outcome is a correlate (that is, correlated with the true clinical outcome) it can be used as a valid surrogate end point (that is, a replacement for the true clinical outcome). However, proper justification for such replacement requires that the effect of the intervention on the surrogate end point predicts the effect on the clinical outcome: a much stronger condition than correlation. In this context, the term ''Prentice criteria'' is used. The term "surrogate" should not be used in describing end points. Instead, descriptions of results and interpretations should be formulated in terms that designate the specific nature and category of variable assessed. A surrogate endpoint of a clinical trial is a laboratory measurement or a physical sign used as a substitute for a clinically meaningful endpoint that measures directly how a patient feels, functions or survives. Changes induced by a therapy on a surrogate endpoint are expected to reflect changes in a clinically meaningful endpoint.

Examples

Cardiovascular disease

A commonly used example is

cholesterol Cholesterol is any of a class of certain organic molecules called lipids. It is a sterol (or modified steroid), a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell mem ...

. While elevated cholesterol levels increase the likelihood for heart disease, the relationship is not linear - many people with normal cholesterol develop heart disease, and many with high cholesterol do not. "Death from heart disease" is the endpoint of interest, but "cholesterol" is the surrogate marker. A clinical trial may show that a particular drug (for example,

simvastatin Simvastatin, sold under the brand name Zocor among others, is a statin, a type of lipid-lowering medication. It is used along with exercise, diet, and weight loss to decrease elevated lipid levels. It is also used to decrease the risk of hear ...

(Zocor)) is effective in reducing cholesterol, without showing directly that simvastatin prevents death. Proof of Zocor's efficacy in reducing cardiovascular disease was only presented five years after its original introduction, and then only for

secondary prevention Preventive healthcare, or prophylaxis, consists of measures taken for the purposes of disease prevention.Hugh R. Leavell and E. Gurney Clark as "the science and art of preventing disease, prolonging life, and promoting physical and mental hea ...

. In another case,

AstraZeneca AstraZeneca plc () is a British-Swedish multinational pharmaceutical and biotechnology company with its headquarters at the Cambridge Biomedical Campus in Cambridge, England. It has a portfolio of products for major diseases in areas includi ...

was accused of marketing

rosuvastatin Rosuvastatin, sold under the brand name Crestor among others, is a statin medication, used to prevent cardiovascular disease in those at high risk and treat abnormal lipids. It is recommended to be used together with dietary changes, exercise, ...

(Crestor) without providing hard endpoint data, relying instead on surrogate endpoints. The company countered that rosuvastatin had been tested on larger groups of patients than any other drug in the class, and that its effects should be comparable to the other statins.

Cancer

Progression Free Survival Progression-free survival (PFS) is "the length of time during and after the treatment of a disease, such as cancer, that a patient lives with the disease but it does not get worse". In oncology, PFS usually refers to situations in which a tumor is p ...

is a prominent example in

Oncology Oncology is a branch of medicine that deals with the study, treatment, diagnosis and prevention of cancer. A medical professional who practices oncology is an ''oncologist''. The name's etymological origin is the Greek word ὄγκος (''� ...

contexts. There are examples of cancer drugs approved on the basis of progression-free survival failed to show subsequent improvements in overall survival in subsequent studies. In breast cancer, Bevacizumab (Avastin) initially gained approval from the

Food and Drug Administration The United States Food and Drug Administration (FDA or US FDA) is a List of United States federal agencies, federal agency of the United States Department of Health and Human Services, Department of Health and Human Services. The FDA is respon ...

, but subsequently had its license revoked. More patient focused surrogate endpoints may offer a more meaningful alternative such as

Overall Treatment Utility Overalls, also called bib-and-brace overalls or dungarees, are a type of garment usually used as protective clothing when working. The garments are commonly referred to as a "pair of overalls" by analogy with "pair of trousers". Overalls were ...

Infectious disease

In HIV/AIDS medicine, CD4 counts and viral loads are used as surrogate markers for drug approval for clinical trials. In hepatitis C medicine, the surrogate endpoint "Sustained Virological Response" has been used for the approval of expensive drugs known as Direct Acting Antivirals. The validity of this surrogate endpoint for predicting clinical outcomes has been challenged. For several vaccines (anthrax, hepatitis A, etc), the induction of detectable antibodies in blood is used as a surrogate marker for vaccine effectiveness, as exposure of individuals to an actual pathogen is considered unethical.

Criticism

There have been a number of instances when studies using surrogate markers have been used to show benefit from a particular treatment, but later, a repeat study looking at endpoints has not shown a benefit, or has even shown a harm. In 2021, the FDA came under heavy criticism for the approval of an alzheimer's drug called Aduhelm based on a surrogate endpoint that was later shown to be based on fraudulent data.

References