Inclusion Criteria:

• Male patients aged ≥ 18 years with histologically documented adenocarcinoma of the prostate.
• Clinically refractory to hormone therapy (orchiectomy or luteinizing hormone-releasing hormone agonist/antagonist).
• Presence of metastatic prostate cancer that fulfills at least one evaluation category as listed: * Measurable Disease: Lesion(s) that can be accurately measured in at least one dimension with the longest diameter ≥ 20 mm using conventional techniques or ≥ 10 mm with spiral CT scan (or otherwise at least twice the reconstruction interval for CT or MRI scans). Previously irradiated lesions may be considered to be measurable provided: 1) there has been documented progression of the lesion(s) since completion of radiotherapy, and 2) the criteria for measurability as outlined above are met. *Non-measurable disease: Lesions noted on imaging studies (including metastatic bone lesions on bone scan) or other non-measurable lesions as defined by the modified RECIST criteria. *Progressive disease following a cytotoxic chemotherapy regimen for prostate cancer. Progression is defined as any of the following: (1) progression of measurable disease as defined by RECIST criteria; (2) progression of bone disease as defined by the appearance of new bone lesions on bone scan(s); (3) a minimum of two consecutive increases in PSA above a previous reference measurement. The measurements must be obtained at a single laboratory, at least 7 days apart, and the latest value must be ≥ 5 ng/mL.
• Previous treatment with at least one taxane-containing chemotherapy regimen. Patients may have received treatment with not more than 3 additional regimens of cytotoxic chemotherapy prior to study entry.
• Orchiectomy, or castrate levels of testosterone maintained by LHRH agonist/antagonist < 50 ng/mL.
• Predicted life expectancy > 12 weeks.
• ECOG PS ≤ 2.
• Adequate renal and hepatic function, defined as: *Total serum bilirubin ≤ 1.5 x ULN for the institution; *AST and/or ALT ≤ 3 x ULN for the institution (≤ 5 x ULN if liver metastases are present); *Serum albumin ≥ 2.5 g/dL; *Serum creatinine ≤1.5 x ULN for the institution (or a calculated creatinine clearance ≥ 50 mL/min/1.73m2)
• Adequate bone marrow function, defined as: *ANC ≥ 1.5 x 10^9/L; *Platelet count ≥ 100 x 10^9/L
• Serum cholesterol < 350 mg/dL and triglycerides < 400 mg/dL.
• Male patients who are not surgically sterile must agree to use reliable methods of birth control for the duration of the study until 30 days after the last dose of study drug.
• Able to understand and give written informed consent.

Exclusion Criteria:

• Presence of active or progressive brain metastases.
• Prior therapy with rapamycin, rapamycin analogues or tacrolimus.
• Prior non-hormonal anticancer treatment (chemotherapy, radiotherapy, immunotherapy, biological response modifiers, signal transduction inhibitors, etc.) within 4 weeks prior to the first dose of AP23573; the interval is ≥ 2 weeks for signal transduction inhibitors with a half-life known to be <24 hours, and is ≥ 6 weeks for nitrosourea or mitomycin. If treated with antiandrogens, a withdrawal period of ≥ 4 weeks for flutamide or ≥ 6 weeks for bicalutamide or nilutamide must have elapsed before study entry.
• Ongoing toxicity associated with prior anticancer therapy (except peripheral neuropathy of ≤ grade 1 by NCI toxicity criteria).
• Another primary malignancy within the past three years (except for non-melanoma skin cancer).
• Known or suspected hypersensitivity to drugs formulated with polysorbate 80 (Tween) or any other excipient contained in the study drug.
• Known Grade 3 or 4 hypersensitivity to macrolide antibiotics (e.g., clarithromycin, erythromycin, azithromycin).
• Significant uncontrolled cardiovascular disease.
• Active infection requiring systemic therapy.
• Known HIV infection.
• Treatment with any investigational agent within 4 weeks prior to the first dose of AP23573.
• Concurrent treatment with immunosuppressive agents other than prescribed corticosteroids at stable doses for ≥ 2 weeks prior to first planned dose of study drug.
• Inadequate recovery from any prior surgical procedure or having undergone any major surgical procedure within 2 weeks prior to the first dose of AP23573.
• Presence of any other life-threatening illness or organ system dysfunction which, in the opinion of the Investigator, would either compromise the patient's safety or interfere with evaluating the safety of the study drug.