Presentation on theme: "Phosphodiesterase V Inhibitors By Lydia Zou Doctor of Pharmacy Candidate Class of 2008."— Presentation transcript:
Phosphodiesterase V Inhibitors By Lydia Zou Doctor of Pharmacy Candidate Class of 2008
Goals: Historical treatments for Erectile Dysfunction before the PDE5 inhibitors were discovered General overview of the three drugs currently available today Discuss the mechanism of action Discuss the use of each drug (dosing, duration of action, etc) List the side effects common and unique to all three drugs Discuss the metabolism pathway(s) for each of the drugs Discuss the interactions seen
History: Penile Implants (1973): These are surgically implanted They are still used in patients who have failed on other therapies Overall patient-partner satisfaction is 90% Vascular Surgery (1973): Not done very much these days (its a last resort) Vacuum Pump (1983): They work by creating a negative vacuum pressure. There is a constriction ring that is placed at the base of the penis to prevent venous drainage and maintain rigidity. Good in theory but not in reality b/c it was clumsy to use
History Vasoactive Intracavernosal pharmacotherapy (1995): There are 2 products approved by the FDA Caverject (alprostadil injection) Edex (alprostadil injection) Transurethral alprostadil (1997): MUSE: Medicated Urethral System for Erection Its a small stick with aprostadil (PGE1) at the end that is stuck into the urethra. A release button is pushed to release the medication. Oral PDE-5 Inhibitors (1998)
PDE 5 Inhibitors Three drugs are currently approved: Sildenafil (Viagra) Doses: 25mg, 50mg, 100mg Most patients start at 50mg and it can be taken anytime from 1/2 to 4 hours before sexual activity Max frequency is once per day Maybe best to take it on an empty stomach Vardenafil (Levitra) Doses: 2.5mg, 5mg, 10mg, and 20mg Most patients start at 10mg taken ~60minutes before sexual activity High fat meals can reduce the plasma concentration by % (on an empty stomach is not a bad idea) Tadalafil (Cialis) Doses: 5mg, 10mg, 20mg Most patients start on 10mg and it can be taken without regard to food Maximum dosing frequency is once per day
Mechanism of Action They inhibit phosphodiesterase V, which is responsible for the breakdown of cGMP cGMP relaxes the smooth muscle and increases blood flow to the corpus cavernosum. Note: These drugs do not cause a chemical erection. Sexual stimulation is still needed to cause the initial release of nitric oxide, which stimulates the synthesis of cGMP.
Side Effects Sildenafil: Headache and flushing Dyspepsia Nasal congestion Abnormal vision (blue halo around lights)
Side Effects Vardenafil: Headache and flushing Rhinitis Dyspepsia Flu syndrome
Side Effects Tadalafil: Headache and flushing Dyspepsia Nasal congestion Back pain--unique to Cialis Myalgia Can be very bad (cant move). Pt will need to stop and use an alternative treatment.
Side Effects Non-arteritic Ischemic Optic Neuropathy This is the most common optic nerve disease for adults >50 years old The onset is sudden and patients usually experience decreased vision in one eye Occurs most frequently in the morning when first waking up Risk Factors: Ischemic heart disease HTN Hypercholesterolemia Diabetes Age
Metabolism Pathways Sildenafil: Major pathway: 3A4 (79%) Minor pathways: 2C9 (20%) 2C19 and 2D6 (<2%) Vardenafil: Major pathway: 3A4 Minor pathways: 3A5 and 2C9 Tadalafil: Major pathway: 3A4
Drug-Drug Interactions for the PDE 5 Inhibitors
Contraindications for PDE 5 Therapy Nitrates: Patients using nitrates regularly or intermittently should not use any of the PDE-5 Inhibitors. Applies for any form of nitrates (sublingual, transdermal, etc) Patients can get extreme hypotension Hypersensitivity to any of the components of the tablet.
Use with Caution: Alpha-Blockers: In the past these drugs were contraindications, but recent evidence suggests that they can be used together Patients should be stabilized on either the alpha-blocker or the PDE-5 inhibitor first before the other is added on at the lowest possible dose.
Sildenafil Drug-Class Interactions: Beta-Blockers: The product insert states that the AUC of the active metabolite, N-desmethylsildenafil, was increased by 102% when taken with non-specific beta-blockers Drug-Drug Interactions: Ambrisentan: A negative trial, cited in the product insert of ambrisentan, on healthy volunteers showed that sildenafil and ambrisentan did not have a clinically relevant effect on each others pharmacokinetics. No dose adjustments are needed when they are co- administered.
Vardenafil contd Drug-Drug Interactions: Nifedipine: According to the product insert, co- administration of vardenafil 20mg and slow- release nifedipine 30mg or 60mg daily did not affect the Cmax or AUC of nifedipine. Nifedipine also did not change vardenafils plasma levels. Patients did experience an additional systolic/diastolic BP decrease of 6/5 mmHg when on both medications.
Vardenafil contd Drug-Drug Interactions: Ranitidine: According to the product insert, there is no pharmacokinetic interaction between these 2 drugs. S-warfarin: According to the package insert, no pharmacokinetic reactions occur between these two drugs. Vardenafil does not seem to have an effect on prothrombin time or other pharmacokinetic factors when taken with warfarin
Vardenafil Drug-Drug Interactions: Digoxin: According to the product insert, there is no pharmacokinetic interaction between these two drugs. Glyburide: According to the product insert, there is no pharmacokinetic interaction between these two drugs Indinavir: According to the package insert, patients on indinavir (800mg TID) should not exceed 2.5mg of vardenafil per 24 hours. Indinavir increased vardenafils AUC by 16- fold, Cmax by 7-fold, and half-life by 2-fold.
Tadalafil Drug-Class Interactions: H2-Antagonists: According to the package insert, there was no significant pharmacokinetic effect on tadalafil when stomach pH was increased Drug-Drug Interactions: Alfuzosin: A randomized, double-blind, placebo-controlled, crossover study in 18 healthy normotensive adults studied the effects of tadalafil 20mg and alfuzosin 10mg daily. The mean standing and supine BP were slightly lower in those taking tadalafil compared to placebo, but these differences were not statistically significant
Tadalafil Drug-Drug Interactions: Ritonavir (initially): According to the PI, patients at steady state on ritonavir 500mg or 600mg twice per day had a 32% increase in exposure (AUC) and a 30% decrease in Cmax when given a single dose of tadalafil compared to tadalafil 20mg alone. Patients on ritonavir 200mg BID experienced a 124% increase in AUC with no change in Cmax. Based on these results, its suggested that patients on potent 3A4 inhibitors should take no more than 10mg of tadalafil per 72 hours.
The Genelex Database It lists the current drug interactions available in the literature. Accounts for individual genotypes when metabolizing drugs. Lists a severity rating. Predicts interactions based on metabolism information when studies are not available. It also lets users know when an interaction is based on literature and when it is based on the algorithm.
Accuracy of the Algorithm
When it was Right There were instances where the algorithm was correct but it didnt predict the right percentage of change.
When it was Wrong In many instances, the interaction I put in was a pharmacodynamic one so I wouldnt have expected the algorithm to get it since it focuses on pharmacokinetics. Although they were counted as part of the total Wrong, I dont think it was actually wrong. In actuality, the number of pharmacokinetic interactions that were Wrong was 7.
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