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The Body Covers: The 39th Annual Meeting of the Interscience Conference on Antimicrobial Agents and Chemotherapy
Slide Session 66.A: Drug Interactions in HIV Therapy

September 27, 1999

A note from TheBody.com: Since this article was written, the HIV pandemic has changed, as has our understanding of HIV/AIDS and its treatment. As a result, parts of this article may be outdated. Please keep this in mind, and be sure to visit other parts of our site for more recent information!

One of the most frequent problems encountered by people with HIV infection is erectile dysfunction. This may arise as a result of illness, medication side effects and other factors. For the past year, many patients have found relief from this problem with the use of ViagraTM (sildenafil citrate). A persistent question, however, has been what, if any, interaction there is between sildenafil and anti-HIV medications, especially protease inhibitors (PI).

Muirhead and colleagues studies the interaction of sildenafil with both ritonavir and saquinavir. They did this in 28 healthy HIV-negative volunteers. Subjects received 100 mg of silenafil on day 1 and then on days 2 to 8 they got either saquinavir 1200 mg three times per day or a dose escalation of ritonavir from 300 mg twice per day to 500 mg twice per day. Subjects then received either sildenafil or placebo.

The design of this study allowed researchers to observe the sildenafil levels in untreated patients, then allowed the PI to achieve steady state and then repeated the sildenafil with the PI on board. This allowed each subject to serve as his own control.

Saquinavir significantly increased Cmax, the peak concentration at 24 hours, the area under the curve (AUC), delayed the time of clearance and increased the half-life of the drug compared to placebo. Ritonavir changed the pharmacokinetic parameters of sildenafil to an even greater extent.

Change in pharmacokinetic parameters of Sildenafil with PIs
Saquinavir Cmax 2.4 fold increase
AUC 3 fold
Tmax 2.6 hr delay
T1/2 1 hour longer
Ritonavir Cmax 3.9 fold increase
AUC 11 fold
Tmax 3.1 hour delay
T1/2 1.8 hours longer

Sildenafil had no effect on the pharmacokinetics of either norvir or saquinavir. The incidence of adverse events was no different in the treatment arm and the placebo arm. The most common AEs were headache, facial flushing, dyspepsia and postural hypotension, all described previously.

The authors recommend making no change in the dose of Viagra used with saquinavir, but felt that the large changes in AUC, Cmax, Tmax and T1/2 with ritonavir made dose modification important. They recommend:

  • Decrease the initial dose of sildenafil used to 25 mg

  • Sildenafil should only be used at a maximum frequency of 25 mg every 48 hours.

A note from TheBody.com: Since this article was written, the HIV pandemic has changed, as has our understanding of HIV/AIDS and its treatment. As a result, parts of this article may be outdated. Please keep this in mind, and be sure to visit other parts of our site for more recent information!

Abstract: Pharmakokinetic (PK) Interactions between the HIV Protease Inhibitors Ritonavir (RTV) and Saquinavir (SQV) and VIAGRA® (Sildenafil Citrate) (Paper 659)
Authored by: G. J. Muirhead, et al.

See Also
More on HIV Medications
More Research on Saquinavir (Fortovase, Invirase)



  
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Please note: Knowledge about HIV changes rapidly. Note the date of this summary's publication, and before treating patients or employing any therapies described in these materials, verify all information independently. If you are a patient, please consult a doctor or other medical professional before acting on any of the information presented in this summary. For a complete listing of our most recent conference coverage, click here.

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