This is an old study, but that is because MUSE got crushed when Viagra came out and the company that makes it gave up on it. But it is a good product.

Please note these doses described below are high and we do not recommend them. Discuss the use of all medications with your physician before trying them. We are not doctors.

By the way, I predict that this is the future of ED treatments – combination therapy. Multiple mechanisms of action.

AUA MEETING: Viagra And Muse More Effective When Combined In Some Patients 

DALLAS, TX — May 4, 1999 — Researchers from the world-renowned Mayo Clinic presented results from the first-ever combination study of erectile dysfunction treatments — Janssen-Ortho Inc.’s Muse(R) (alprostadil) and Pfizer Canada Inc.’s Viagra(TM) (sildenafil citrate) — at the 94th annual meeting of the American Urological Association in Dallas today.
Patients who had been unable to achieve an erection sufficient for intercourse when treated with either drug individually were given a combination of the treatments that resulted in erections sufficient for intercourse for all patients. The results were based on patient questionnaires.
“Though this is a small study group, these results are very promising, especially for patients in a similar situation where they may not have had hope for using effective, non-invasive treatment options,” said Dr. Ajay Nehra, consultant, department of urology at the Mayo Clinic and lead investigator of the study.
Of the 16 patients enrolled in the open-label study 11 had suffered from erectile dysfunction as a result of a radical prostatectomy (surgery for prostate cancer) and five had a diagnosis of organic erectile dysfunction.
“By and large, this is the most difficult patient study group because of the extent of nerve damage, yet we were still able to generate a successful response which is very encouraging, because it means that future studies can examine combination therapy in a wider variety of patients,” Dr. Nehra said.
To enter the study, patients had to have failed an initial trial of either 100mg of Viagra and/or 1,000 micrograms of Muse. This is the highest dose for each drug. Combination therapy was initiated using 100mg of Viagra 60 minutes prior to intercourse and a lower dose of Muse consisting of 500 micrograms immediately before intercourse.
“All patients were able to generate tumescence [some rigidity] before combination therapy on both drugs,” Dr. Nehra said. “However, this tumescence was not sufficient for penetration.”
According to Dr. Nehra, the rationale for choosing to combine the treatments was straight forward. “First of all, none of these patients were interested in intracavernosal injection therapy,” Dr. Nehra explained. “In addition, we know that Muse has worked well in some patients who have had a radical prostatectomy and that Viagra is less efficacious in this subset of patients.
“Since the two drugs work through different pathways, it made sense to try them together.”
In addition to being administered differently, Muse and Viagra also have different mechanisms of action. Muse contains alprostadil, a synthetic version of prostaglandin E(1) which directly causes vasodilation of blood vessels and smooth muscle relaxation through a chemical messenger called cyclic AMP (cAMP), leading to an erection. Patients do not need to have intact nerve pathways from the brain in order for Muse to work.
This differs from Viagra which works by breaking down a chemical messenger called cyclic GMP (cGMP), a compound that causes smooth muscle relaxation. Cyclic GMP is made available through the release of the neurotransmitter nitric oxide, which only works if the neuropathways from the brain are intact and if they receive some sort of sexual stimulus to do so. As such, men must have some erectile function in order for Viagra to work.
“The failure of MUSE may be due to high doses of PGE(1) which can cause contraction of the smooth muscle [instead of relaxation] by interacting with other receptors,” he said. “The failure of Viagra may be due to the nerve damage in patients. However, since they act by two different pathways, together they provide enough relaxation for an erection.”
For Dr. Nehra, he plans to continue recruitment for the study. “Given the success rate at the six month duration, our plans are to expand this study to men with diverse causes of erectile dysfunction.”

Here is a link to the source: LINK