Evaluate the Effects of GPI 1485 on Erectile Function Following Bilateral Nerve-Sparing Prostatectomy

Inclusion Criteria:

1. Clinical diagnosis of localized prostate carcinoma and scheduled to undergo curative surgical resection via bilateral nerve sparing procedure.
2. Localized prostate cancer is defined as:

   * Gleason score \<=7 (\<=3 + \<=4)
   * PSA \<=10 off of finasteride (Proscar®) and dutasteride (Avodart®)
   * \<=T2a stage disease
3. Motivated males, 40 to 69 years of age inclusive, and in the opinion of the investigator, are currently in a monogamous, sexually active relationship.
4. Males that have erectile function (EF) (without the use of pharmacotherapy within 30 days prior to screening assessment).
5. EF is defined as a score of \>=26 on the EF domain of the IIEF questionnaire based on patient experiences over the 4 weeks prior to biopsy.
6. Body Mass Index (BMI) within the 18-34.9 kg/m2 range.
7. Able to swallow whole tablets equivalent to capsule size 0.
8. Available for protocol-specified visits and procedures.
9. Informed written consent must be provided prior to any study-specific procedures.

Exclusion Criteria:

1. Recent history, within 6 months before screening, of drug or alcohol abuse.
2. History of peripheral neuropathy.
3. History of acute or chronic depression that in the opinion of the investigator may interfere with protocol-specified efficacy measurements.
4. History of diabetes that requires use of insulin or oral hypoglycemic agents, myocardial infarction, or cerebrovascular accident.
5. History of spinal trauma or surgery to the brain or spinal cord.
6. Any medical disability or laboratory abnormality (e.g., serum creatinine \> 2.0 mg/dL) that, in the opinion of the Investigator, may interfere with the protocol-specified safety and efficacy measurements, present an unacceptable risk to the patient's well-being, or compromise the patient's ability to provide informed consent.
7. History of pelvic radiation therapy (external beam radiation or brachytherapy).
8. Concomitant use of hormonal therapy, vasodilators (nitrates), dutasteride (Avodart®) or finasteride (Proscar®) within the 30 days prior to biopsy and throughout the study.
9. Concomitant use of therapeutic agents to treat ED other than those specified in the protocol is not permitted starting at least 30 days prior to biopsy and continuing throughout the study.
10. Previous exposure to GPI 1485 (previously AMG-474-00).
11. Treatment with an investigational agent within the 30 days before screening or scheduled to receive an investigational agent other than that specified by this protocol during the course of this study.
12. Any contraindication to Viagra® use
13. Unable to stop the use of inducers or inhibitors of cytochrome P450 (CYP) 3A4 from Baseline until the end of the study.