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AIDSinfo Drug Database

AIDSinfo Drug Database

Drugs by class



PSI-5004  Audio icon

Other Names: (+)/(-)-FTC, RCV, Racivir
Drug Class: Nucleoside Reverse Transcriptase Inhibitors
Molecular Formula: C8 H10 F N3 O3 S
Registry Number: 143491-54-7 (CAS)
Chemical Name: 4-amino-5-fluoro-1-[(2S,5R)-2-(hydroxymethyl)-1,3-oxathiolan-5-yl]pyrimidin-2-one
Chemical Class: Pyrimidine Nucleosides
Company: Pharmasset, Inc. (acquired by Gilead Sciences, Inc. in 2011)
Phase of Development: Phase II
Chemical Image:
Click image to enlarge
Molecular Weight: 247.249
(Compound details obtained from ChemIDplus Advanced,1 NIAID Therapeutics Database,2 Antiviral Chemistry & Chemotherapy journal article,3 and Gilead Sciences, Inc. Press Release4)

What is an investigational drug?

An investigational drug is one that is under study and is not approved by the U.S. Food and Drug Administration (FDA) for sale in the United States. Medical research studies are conducted to evaluate the safety and effectiveness of an investigational drug. These research studies are also called clinical trials. Once an investigational drug has been proven safe and effective in clinical trials, FDA may approve the drug for sale in the United States.

What is PSI-5004?

PSI-5004 (also known as Racivir) is an investigational drug that is being studied for the treatment of HIV infection. 

PSI-5004 belongs to a class (group) of HIV drugs called nucleoside reverse transcriptase inhibitors (NRTIs).2 NRTIs block an HIV enzyme called reverse transcriptase. (An enzyme is a protein that starts or increases the speed of a chemical reaction.) By blocking reverse transcriptase, NRTIs prevent HIV from multiplying and can reduce the amount of HIV in the body.

PSI-5004 is similar in chemical structure to the FDA-approved NRTIs lamivudine (brand name: Epivir) and emtricitabine (brand name: Emtriva).5

In vitro studies have shown that PSI-5004 is also active against hepatitis B virus (HBV).5 (In vitro studies are studies done in test tubes or other laboratory equipment and not on animals or humans.)

How are clinical trials of investigational drugs conducted?

Clinical trials are conducted in phases. Each phase has a different purpose and helps researchers answer different questions.6

  • Phase I trials: Researchers test an investigational drug in a small group of people (20–80) for the first time. The purpose is to evaluate its safety and identify side effects.
  • Phase II trials: The investigational drug is administered to a larger group of people (100–300) to determine its effectiveness and to further evaluate its safety.
  • Phase III trials: The investigational drug is administered to large groups of people (1,000–3,000) to confirm its effectiveness, monitor side effects, compare it with standard or equivalent treatments, and collect information that will allow the investigational drug to be used safely.6

In most cases, an investigational drug must be proven effective and must show continued safety in a Phase III clinical trial to be considered for approval by FDA for sale in the United States. Some drugs go through FDA’s accelerated approval process and are approved before a Phase III clinical trial is complete. After a drug is approved by FDA and made available to the public, researchers track its safety in Phase IV trials to seek more information about the drug’s risks, benefits, and optimal use.6

In what phase of testing is PSI-5004?

PSI-5004 has been studied in Phase II clinical trials.2

What are some studies on PSI-5004?

In a Phase II study (known as Study 201), the safety and effectiveness of PSI-5004 was compared to the safety and effectiveness of lamivudine. The study included HIV-infected participants who were already taking HIV medicines before entering the study (also called treatment-experienced) and who had the M184V HIV mutation (a mutation associated with drug resistance to the HIV medicine lamivudine). Before starting the study, all participants were on a failing antiretroviral therapy (ART) regimen that included lamivudine. (A failing ART regimen is one that is not effective at controlling a person's HIV.) Study participants were randomly assigned to receive either PSI-5004 in place of lamivudine or to continue with lamivudine as part of their ART.7-9

In this study, PSI-5004 was shown to decrease viral load (the amount of HIV in a blood sample) in the treatment-experienced adults who had the M184V resistance mutation and who had fewer than three thymidine analog mutations (HIV mutations associated with NRTI drug resistance). In terms of safety, no severe drug-related side effects occurred in either the PSI-5004 treatment group or the lamivudine treatment group.8

What side effects might PSI-5004 cause?

In a 14-day Phase Ib/IIa study of various doses of PSI-5004, the most common side effects reported were mild to moderate dizziness and headache, both of which may have been related to PSI-5004. Some other side effects that occurred in the study and may have been because of PSI-5004 included the following: difficulty concentrating, nausea, fatigue, common cold, and eye discomfort. Heartburn and vomiting were also noted by participants and may have been associated with PSI-5004.10

In Study 201, the Phase II study discussed under the previous question, no severe side effects were reported after 28 days of treatment with PSI-5004.8

Because PSI-5004 is still being studied, information on possible side effects of the drug is not complete. As testing of PSI-5004 continues, additional information on possible side effects will be gathered.

Where can I get more information about clinical trials studying PSI-5004?

More information about racivir-related research studies is available from the AIDSinfo database of study summaries. Click on the title of any trial in the list to see the trial summary and more information about the study.

I am interested in participating in a clinical trial of PSI-5004. How can I find more information about participating in a clinical trial?

Participating in a clinical trial can provide benefits. For example, a volunteer participant can benefit from new research treatments before they are widely available. Participants also receive regular and careful medical attention from a research team that includes doctors and other health professionals. However, clinical trials may also involve risks of varying degrees, such as unpleasant, serious, or even life-threatening side effects from the treatment being studied.6

Your health care provider can help you decide whether participating in a clinical trial is right for you. For more information, visit NIH Clinical Research Trials and You.


  1. United States National Library of Medicine. ChemIDplus Advanced. Available at: Last accessed on October 25, 2014.
  2. National Institute of Allergy and Infectious Diseases (NIAID). NIAID ChemDB, HIV Drugs in Development. Available at: Last accessed on October 25, 2014.
  3. Hurwitz SJ, Otto MJ, Schinazi RF. Comparative pharmacokinetics of Racivir, (+/-)-beta-2',3'-dideoxy-5-fluoro-3'-thiacytidine in rats, rabbits, dogs, monkeys and HIV-infected humans. Antivir Chem Chemother. 2005;16(2):117-27. Available at: Last accessed on October 25, 2014.
  4. Gilead Sciences, Inc.: Press Release, dated November 21, 2011. Gilead Sciences to Acquire Pharmasset, Inc. for $11 Billion. Available at: Last accessed on October 25, 2014.
  5. Sen S, Mathur AG, Gupta RM, Kapila K, Chopra GS. Investigational antiretroviral drugs. Recent Pat Antiinfect Drug Discov. 2008 Nov;3(3):199-205. Available at: Last accessed on May 29, 2013.
  6. National Institutes of Health (NIH). NIH Clinical Research Trials and You. Available at: Last accessed on October 25, 2014.
  7. Pharmasset. Randomized, Double-Blind, Placebo-Controlled, Multicenter Study Exploring the Safety, Tolerability, and Antiviral Effect of Substituting 600 mg Racivir for 3TC in HIV-Infected Subjects Who Have the M184V Mutation and Are Currently Failing on a HAART Regimen Containing Lamivudine. In: Bethesda (MD): National Library of Medicine (US). Registered on July 18, 2005. NLM Identifier: NCT00121979. Available at: Last accessed on October 25, 2014.
  8. Cahn P, Sosa N, Wiznia A, et al. Racivir Demonstrates Safety and Efficacy in Patients Harboring HIV with the M184V Mutation and <3 TAM. Abstract presented at: 14th Conference on Retroviruses and Opportunistic Infections (CROI); February 25-28, 2007; Los Angeles, CA. Abstract 488. Available at: Last accessed on May 29, 2013.
  9. Pharmasset, Inc: Press Release, dated September 17, 2008. Racivir – New HIV Drug. Available from National AIDS Treatment Advocacy Project (NATAP) at: Last accessed on October 25, 2014.
  10. Herzmann C, Arastèh K, Murphy RL, et al. Safety, pharmacokinetics, and efficacy of (+/-)-beta-2',3'-dideoxy-5-fluoro-3'-thiacytidine with efavirenz and stavudine in antiretroviral-naïve human immunodeficiency virus-infected patients. Antimicrob Agents Chemother. 2005 Jul;49(7):2828-33. Available at: Last accessed on October 25, 2014.

Last Reviewed: October 25, 2014

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