Coinciding with the approval of tadalafil, the Pulmonary Arterial Hypertension and Response to Tadalafil (PHIRST) trial is now published online May 26, 2009 in Circulation and shows that tadalafil 40 mg was well tolerated and improved exercise capacity and quality-of-life measures in patients with PAH. In addition, the drug reduced the time to clinical worsening, and side effects were reported as mild or moderate, the most common being headache.
On the basis of the favorable efficacy and safety data, lead investigator Dr Nazzareno Galiè (University of Bologna, Italy) and colleagues state that the drug "offers a clinically meaningful addition to the currently approved treatment options."
Improves Six-Minute Walk Test Distances
Currently, treatment of PAH includes the use of prostanoids, endothelin-receptor antagonists, and sildenafil, another FDA-approved phosphodiesterase type-5 inhibitor. Like sildenafil, tadalafil increases cyclic guanosine monophosphate (cGMP), which is the final mediator in the nitric-oxide pathway. Studies with sildenafil have shown that the drug, the active ingredient in erectile dysfunction drug Viagra (Pfizer), improves exercise capacity and hemodynamic parameters.
In the PHIRST study, investigators randomized 405 individuals with pulmonary arterial hypertension to treatment with tadalafil 2.5 mg, 10 mg, 20 mg, or 40 mg or to placebo for 16 weeks. Roughly half of subjects in each study arm were treated with background bosentan (Tracleer, Actelion), an endothelin-receptor antagonist.
At 16 weeks, tadalafil 10 mg, 20 mg, and 40 mg increased the distance walked in six minutes in a dose-dependent manner, although the 40-mg dose was the only dose that reached statistical significance compared with placebo. The time to clinical worsening and quality-of-life scores were all significantly improved with tadalafil 40 mg, but changes in WHO functional class were unaffected by treatment.
The placebo-adjusted change in six-minute-walk distance in the treatment-naive patients was 44 m, compared with 23 m for patients also receiving bosentan. Secondary end points also tended to be better in treatment-naive patients compared with bosentan-treated subjects.
These data, suggest investigators, "support the hypothesis that increases in exercise capacity may be blunted in patients already treated with PAH-targeted medications compared with treatment-naive subjects." The reasons for the effect are not clear but might be related to a "ceiling" reached that limits additional improvements in patients on background therapy, they write.
The most common side effects associated with use included headache, myalgia, and flushing, and most events were reported as mild or moderate. Regarding the FDA approval, tadalafil should be avoided in patients with severe renal and liver impairment and should not be used in patients taking nitrates, according to the label.
United Therapeutics will market tadalafil for PAH in the United States. The company obtained the marketing rights to sell the drug for PAH from Eli Lilly in 2008.
Galiè has served as a member of the PHIRST steering committee for Eli Lilly and has participated in advisory boards and/or served as a paid lecturer for Actelion, Pfiz
References
- Galiè N, Brundage BH, Ghofrani HA, et al. Tadalafil therapy for pulmonary arterial hypertension. Circulation 2009; DOI:10.1161/circulationaha.108.839274. Available at: http://circ.ahajournals.org. Abstract
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