No pharmaceutical opinion available for this interaction.
Atazanavir / ritonavir can inhibit the metabolism (CYP 3A4) and increase the plasma concentration of Sildenafil (Viagra).
3A4, P-gp/3A4 > 2D6, P-gp
Ritonavir: 1A2, 2C9, 2C19, UGT, 2B6, 3A4 (auto-induction)
3A4, UGT1A1, P-gp E et 2C8/3A4 > 2D6 > 2A6, 2E1, P-gp, MRP2
Possible increase of adverse effects.
Start at the lowest possible dose (12.5 mg) and increase up to a maximum of 25 mg per 48 hours according to response and adverse effects.
If already used, reduce the dose to a maximum of 25 mg per 48 hours.
Monitor closely clinical efficacy and appearance of adverse effects.
In patients already being treated with this combination and tolerating it, if deemed appropriate, keep actual treatment and exercise close monitoring of adverse effects.
Give extra caution to patients with hepatic or renal insufficiency, or older than 65 years.
Sildenafil toxicity: hypotension, tachycardia, headache, dizziness, flushing, visual changes (difficulty distinguishing blue and green) and syncope.
Ref #734 : Sildenafil AUC increased by 11-fold when using a 100 mg dose of sildenafil associated with ritonavir 500 mg BID and 3.1-fold with saquinavir 1200 mg TID, two potent inhibitors of cytochrome 3A4.
See ritonavir and saquinavir/ritonavir with sildenafil (Viagra).
Ref # 2193: Another study showed that sildenafil plasma levels are similar when a dose of 25 mg is combined with darunavir/ritonavir 400/100 mg BID compared to a dose of sildenafil 100 mg alone.
Caution: Liver impairment (Child Pugh A and B): increases AUC to 84% and kidney failure (Clcr < 30 ml/min): increases AUC to 100% and age > 65 years old can also increase the concentrations of sildenafil.
Contraindicated with sildenafil (Revatio). See sildenafil (Revatio).