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Surrogate endpoint

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In clinical trials, a surrogate endpoint (or surrogate marker) is a measure of effect of a specific treatment that may correlate with a real clinical endpoint but does not necessarily have a guaranteed relationship. The National Institutes of Health (USA) defines surrogate endpoint as "a biomarker intended to substitute for a clinical endpoint".[1][2]

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 and other regulatory agencies will often accept evidence from clinical trials that show a direct clinical benefit to surrogate markers.[3]

Surrogate endpoints can be obtained from different modalities, such as, behavioural or cognitive scores, or biomarkers from Electroencephalography (qEEG), MRI, PET, 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 endpoint (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 endpoint predicts the effect on the clinical outcome: a much stronger condition than correlation.[4][5] In this context, the term Prentice criteria is used.[6]

The term "surrogate" should not be used in describing endpoints. Instead, descriptions of results and interpretations should be formulated in terms that designate the specific nature and category of variable assessed.[7]

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.[8]


Cardiovascular disease[edit]

A commonly used example is cholesterol. 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 (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.[9] In another case, AstraZeneca was accused of marketing rosuvastatin (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.[10]


Progression Free Survival is a prominent example in Oncology 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, but subsequently had its license revoked.[11][12] More patient focused surrogate endpoints may offer a more meaningful alternative such as Overall Treatment Utility.[13][14]

Infectious disease[edit]

In HIV/AIDS medicine, CD4 counts and viral loads are used as surrogate markers for drug approval for clinical trials.[15]

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.[16][17]

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.[18]

Alzheimer's disease[edit]

A recent study[19] showed that plasma biomarkers have the potential to be used as surrogate biomarkers in Alzheimer's disease (AD) clinical trials. More specifically, this study demonstrated that plasma p-tau181 could potentially be used to monitor large-scale population interventions targeting preclinical AD individuals.


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.[20] 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.[21][22]

See also[edit]


  1. ^ De Gruttola, Victor G; Clax, Pamela; DeMets, David L; Downing, Gregory J; Ellenberg, Susan S; Friedman, Lawrence; Gail, Mitchell H; Prentice, Ross; Wittes, Janet; Zeger, Scott L (2001). "Considerations in the Evaluation of Surrogate Endpoints in Clinical Trials". Controlled Clinical Trials. 22 (5): 485–502. doi:10.1016/S0197-2456(01)00153-2. ISSN 0197-2456. PMID 11578783.
  2. ^ Cohn JN (2004). "Introduction to Surrogate Markers". Circulation. 109 (25 Suppl 1): IV20–1. doi:10.1161/01.CIR.0000133441.05780.1d. PMID 15226247.
  3. ^ Alexandra Goho, "An Imperfect Substitute" CR Magazine, Spring 2009
  4. ^ Fleming, Thomas R. (1996). "Surrogate End Points in Clinical Trials: Are We Being Misled?". Annals of Internal Medicine. 125 (7): 605–613. doi:10.7326/0003-4819-125-7-199610010-00011. PMID 8815760. S2CID 12267404.
  5. ^ Prentice, Ross L. (1989). "Surrogate endpoints in clinical trials: Definition and operational criteria". Statistics in Medicine. 8 (4): 431–440. doi:10.1002/sim.4780080407. PMID 2727467.
  6. ^ O'Quigley, John; Flandre, Philippe (March 2006). "Quantification of the Prentice Criteria for Surrogate Endpoints". Biometrics. 62 (1): 297–300. doi:10.1111/j.1541-0420.2006.00538.x. PMID 16542258. S2CID 19927364.
  7. ^ Sobel, Burton E.; Furberg, Curt D. (1997). "Surrogates, Semantics, and Sensible Public Policy". Circulation. 95 (6): 1661–1663. doi:10.1161/01.CIR.95.6.1661. PMID 9118540.
  8. ^ Temple RJ. A regulatory authority's opinion about surrogate endpoints. Clinical Measurement in Drug Evaluation. Edited by Nimmo WS, Tucker GT. New York: Wiley; 1995.
  9. ^ Pedersen TR, Olsson AG, Faergeman O, et al. (1998). "Lipoprotein changes and reduction in the incidence of major coronary heart disease events in the Scandinavian Simvastatin Survival Study (4S)". Circulation. 97 (15): 1453–1460. doi:10.1161/01.cir.97.15.1453. PMID 9576425.
  10. ^ Horton, Richard (October 25, 2003). "The statin wars: why AstraZeneca must retreat". Lancet. 362 (9393): 1341. doi:10.1016/S0140-6736(03)14669-7. PMID 14585629. S2CID 39528790.
    McKillop T (November 1, 2003). "The statin wars". Lancet. 362 (9394): 1498. doi:10.1016/S0140-6736(03)14698-3. PMID 14602449. S2CID 5300990.
  11. ^ d'Agostino, Ralph B. (2011). "Changing End Points in Breast-Cancer Drug Approval — The Avastin Story". NEJM. 365 (2): e2. doi:10.1056/NEJMp1106984. PMID 21707384.
  12. ^ Lenzer, J. (2011). "FDA committee votes to withdraw bevacizumab for breast cancer". BMJ. 343: d4244. doi:10.1136/bmj.d4244. PMID 21729988. S2CID 206893438.
  13. ^ Handforth C, Hall PS, Marshall HC, Collinson M, Jones M, Seymour MT (2013). "Overall treatment utility: a novel outcome measure reflecting the balance of benefits and harms from cancer therapy". European Journal of Cancer. 49 (S2): 346.
  14. ^ Hall PS, Lord SR, Collinson M, Marshall H, Jones M, Lowe C, Howard H, Swinson D, Velikova G, Anthoney A, Roy R, Seymour M (2017). "A randomised phase II trial and feasibility study of palliative chemotherapy in frail or elderly patients with advanced gastroesophageal cancer (321GO)". British Journal of Cancer. 116 (4): 472–478. doi:10.1038/bjc.2016.442. PMC 5318975. PMID 28095397.
  15. ^ Epstien, Stephen (1998). Impure Science: AIDS, Activism, and the Politics of Knowledge. Berkeley: University of California Press. pp. 270–276. ISBN 0-520-20233-3.
  16. ^ Koretz, Ronald (January 13, 2015). "Is Widespread Screening for Hepatitis C Justified?". British Medical Journal. 350: g7809. doi:10.1136/bmj.g7809. PMID 25587052. S2CID 36816304.
  17. ^ "Expert: SVR does not equate to a cure in HCV". Helio.
  18. ^ "Table of Surrogate Endpoints That Were the Basis of Drug Approval or Licensure". Food and Drug Administration. 28 February 2022.
  19. ^ Ferreira PL, Ferrari-Souza JP, Tissot C, Bellaver B, Leffa D, Lussier FZ, et al. (March 2023). "Potential Utility of Plasma P-Tau and Neurofilament Light Chain as Surrogate Biomarkers for Preventive Clinical Trials". Neurology. 101 (1): 38–45. doi:10.1212/WNL.0000000000207115. PMC 10351303. PMID 36878697.
  20. ^ Psaty BM, Weiss NS, Furberg CD, et al. (1999). "Surrogate end points, health outcomes, and the drug approval process for the treatment of risk factors for cardiovascular disease". JAMA. 282 (8): 786–790. doi:10.1001/jama.282.8.786. PMID 10463718. S2CID 16582482.
  21. ^ "Three F.D.A. Advisers Resign Over Agency's Approval of Alzheimer's Drug". New York Times. 2 September 2021.
  22. ^ Glenza, Jessica (23 July 2022). "Critical elements of leading Alzheimer's study possibly fraudulent". The Guardian.